SB2009 Proposal Document

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SB2009 Proposal Document



Release 1
.1


December 2009


ILC Global Design Effort



Prepared by the Technical Design Phase Project Managers and

the Accelerator Design and Integration
Team Leaders


Project Managers:


AD/I Team Leaders:

M. Ross, N. Walker, A. Yamamoto


C. Adolphsen, D. Angal
-
Kalinin,

W. Bialowons, A. Brachmann,

J. Carwardine, J. Clarke, A. Enomoto,

T. Himel, S. Fukuda, P. Garbincius,

R. Geng, C. Ginsburg,
S.Guiducci,

J. Kerby, V. Kuchler, T.Lackowski,

A. Latina, C. Nantista, J. Osborne,

M. Palmer, J.M. Paterson, M. Pivi,

A. Seryi, T. Shidara, N. Solyak, N. Toge,

J. Urakawa







Table of Contents


SB2009 Proposal Document

................................
................................
................................
....................

1

1 Introduction

................................
................................
................................
................................
..........

1

2

SB2009 Overv
iew

................................
................................
................................
................................
..

3

3 Layout

................................
................................
................................
................................
.................

16

4 SB2009 Proposal

................................
................................
................................
................................
.

20

4.1 Issues of Main Linac Accelerating Field

................................
................................
.......................

20

4.2 Electron Source

................................
................................
................................
...........................

22

4.3 Positr
on Source

................................
................................
................................
...........................

25

4.4 Damping Rings

................................
................................
................................
.............................

33

4.6 Main Linacs

................................
................................
................................
................................
..

43

4.7 Beam Del
ivery Systems

................................
................................
................................
...............

62

4.8 Conventional Facilities

................................
................................
................................
.................

67

5 Cost
Studies

................................
................................
................................
................................
........

80

6 Risk Analysis

................................
................................
................................
................................
.......

85

Appendix A.
Availability Task Force Report

................................
................................
...........................

89





1


SB2009 Proposal Document


1

Introduction


This report documents the proposal from the
ILC
Project Management Design Team for
major

changes to the published RDR
[Intro1]
baseline,
as a result of the effort
in accordance with the GDE
R&D Plan [
Intro2
]. It represents the culmination of approximately one
-
year of study and includes
new results from
the on
-
going risk
-
mitigating R&D, as well as a critical review of the existing
Reference Design.

At

the end of the Technical Design (TD) Phase in 2012 the GDE will publish a Technical Design Report
(TDR)
.
One of the primary goals of the TD Phase is to con
strain the VALUE estimate to the RDR value
of 6.6 BILCU.

The TDR

will contain a description of ILC design in
sufficient

detail to support an
updated VALUE estimate.
The updated reference design

will
incorporate

the results
from

the on
-
going risk
-
mitigating

R&D and the Accelerator Design and Integration (AD/I) studies, but will remain

strongly based
o
n

the existing published Reference Design (RDR).


With these goals
in

mind, the Project Management

considered it

most
beneficial

to
focus

the
available (and limited) design resources

on

a

re
-
examination of

the reference design

during

2008
-
2009
,

with a view to

establishing

a better

and

more cost
-
effective baseline
before
commencing

TD
Phase
2

(
Summer,
2010). The
intention is to use
the

modif
ied baseline

as
the basis for detailed design
and cost work foreseen for the TDR

in the two years to follow. During TD Phase 2

the baseline will be
maintained and evolved under strict change control.

The

adopted approach to re
-
baselining is based on an ass
umption that the RDR design


although
sound


is both conservative in many of its design decisions, relatively immature from a detailed
engineering standpoint, and is “performance
-
driven” as opposed to cost optimised. Conventional
Facilities and Siting (C
FS) was identified early on as a strong focus for design optimisation; in
particular the reduction of underground civil construction, achieved by a critical re
-
evaluation of the
criteria driven by the accelerator design assumptions.

With nearly all ILC wor
ldwide resources focused on risk mitigating R&D (most notably SCRF), a full
study
of system engineering
is not practical: Therefore, a strategic decision was made by the Project
Management to focus
the
available

design resources on a few top
-
level design e
lements which have
a large cost leverage (at the level of a few % each), and where alternative designs exist which are
both
feasible and
more cost
-
effective. The Project Management believe
s that

the resulting proposed
baseline is:



a simplification of the s
cope of the construction project

with

a potential for cost reduction of
up to one
-
billion ILCU

(
c
ost
-
reduction at this level will supply
a
margin
to accommodate

possible cost inflation incurred during TD Phase 2, most likely at the unit cost level);



achiev
able

while

maintaining a level of technical risk
which is
comparable with the RDR
baseline.

Further to the basic (and in many respects

generic) accelerator layout, the study has also
begun

to
address
t
he impact of potential site constraints

in a more reali
stic way
. Particular attention
is

paid to

allowing flexibility in design decisions in support of
future
possible
host site
s
. This aspect is
considered fundamental to the proposed approach
,

and will be reflected in the Project
Implementation Plan (to be pub
lished as part of the TDR). Several layout and design flaws with the

2


reference design have also been identified as a direct result of these studies; these have begun to be
addressed as part of this re
-
baseline proposal.
The Project Management believes thi
s process will
naturally lead to a better, more cost
-
optimised design, which in many respects is more complete
than the RDR.

The report is divided into six sections:



Section 1


this introduction.



Section 2 gives an overview of SB2009



Section 3 gives a bird's eye view description of the overall system layout.



Section 4 discusses the proposed design changes in more detailed, including additional studies
and R&D that remain to be done (TD Phase 2). This section is sub
-
divided into the fo
llowing sub
-
sections:



main linac (single tunnel solution, high
-
level RF, and accelerating gradient);



electron and positron sources;



damping rings;



bunch compressors;



beam delivery systems;



conventional facilities.



Section 5 summarises the impact of the pr
oposed modifications on the RDR VALUE estimate.



Section 6 gives a brief outline of risk analysis.

Finally,
a report on the system availability studies made is given in an Appendix
.

The following sections are not intended to be a design report
. Rather
,
t
hey

contain a relatively
conceptual description of the proposed modifications to the RDR baseline. It is acknowledged that
not all questions raised have been answered at this
time
, but the Project Management Team believe
s
that

enough of the key issues hav
e been addressed
so that

these modifications
are proposed
based
on sound judgement. Many of the remaining issues (details)


as well as new R&D required as a
result of the proposal


will feedback into the GDE R&D Plan
,
to be updated at the end of TD Phase

1.

This Proposal document will be submitted to the Project Director for analysis and review in early
2010, before
launching

TD Phase 2. In due course, reviews, high
-
level approval, and possible
refinements are expected before the establishment of the new
baseline configuration.


References

[Intro1] ILC Reference Design Report Volume 3
-

Accelerator, August, 2007,
http://www.linearcollider.org/cms/?pid=1000437

[Intro2] ILC Research and Developm
ent Plan for the Technical Design Phase,
http://ilcdoc.linearcollider.org/record/15442/files/TD_Phase_R%26D_Report.pdf?version=2





3


2

SB2009 Overview


In
this section, we provide a
n

overview of the key SB2009 elements and their rationale. Section 2.1
introduces the overall approach and goals of the Accelerator Design and Integration

(AD/I)
process.
Section 2.2 introduces the key proposed top
-
level baseline

modifications and then summarises each
of them.

Sections 2.3 and 2.4 briefly summarise the impact on cost and physics related performance
respectively.

2.1

Rationale

and Methodology

The
baseline modifications described in this document
were

arrived at

with s
everal specific
goals in

mind
:




Overall cost reduction

-

Any opportunities for cost reduction should be taken, in as much as
they do not
unacceptably impact

performance

or increase
technical risk.



Improved cost balancing

-

Cost margins

created as part of t
he cost
-
reduction exercise
, can be
made available for
other
subsystems which
incur

increased
(estimated)
construction cost
s
.



Improved understanding of system functionality

-

Attempts at u
nderstanding of any
performance
impact

force a careful analysis of systems' f
unctionalities
, strength
s

and

vulnerabilities
; this

has a critical value on its own

beyond cost
-
reduction.



More complete
and robust
design

-

R
evisit
ing

many of the design and implementation
details

that

were
not compl
etely covered during the RDR
design phase.
T
hese efforts,
when

made

appropriately,

will

improve the overall robustness of the ILC systems design.



Re
-
optimised R&D plans

-

Improved understanding of the system functionalities and
performance issues
will

help

the Project Management
in

produc
ing

a re
-
optimi
s
ed and more
effective global R&D plan to pursue in T
D Phase 2
.


The key design elements for the studies were primarily identified by their cost impact (based on the
RDR VALUE estimate). The list of proposed

design

modifications were iterated and reviewed during
the second
-
half of 2008, resulting in a final set of proposals for study that were published in
January

200
9 [Over1]
. Further studies and refinements evolved into an integrated proposal by the Project
Management for a straw
-
man baseline (SB2009) in May 2009
[Over2]
. The baseline modifications
proposed in this document are the end result of iteration by the
Accelerator D
esign & Integration
(AD/I) Team (see below), as well as input from various external re
views and the broader ILC
community.


The AD/I Team, under the direct leadership of the Project Managers,
includes
:




A

set of key leaders who organized the efforts and
led

the overal
l

discussion and direction
.



I
ndividual

area


and

global


groups who are responsible for specific accelerator subsystems or
accelerator
-
wide facility systems
.



T
he costing group who are responsible for collecting and organizing the cost implications
.




A
d
-
hoc groups formed under the leadership of the Project Manag
ers to address the risks in
terms of technical development toward construction of the I
LC and the systems availability.


The work was orchestrated around several GDE workshops and special face
-
to
-
face focus meetings,
as well as numerous weekly teleconferen
ces.


4


2.2

Summary of Proposed Changes

Figures 2
.
1
shows

an app
roximate comparison of the over
all ILC layout as docume
n
ted in RDR and as
put forth in
the proposed new baseline
.



Figure 2
.
1: RDR layout (left) and the re
-
baseline layout in proposal (right).


Bot
h the RDR layout and the
propos
ed

new
layout
require

a site footprint of approximately 31km
length
, consistent with maximum operational beam energy of 250

GeV (500

GeV centre
-
of
-
mass
energy).

Exact lengths still remain to be made consistent, but any fut
ure adjustments are expected to
have a negligible impact on the estimated cost increments
.


T
he following changes are proposed for the ILC design baseline configuration

as described in the RDR,
are summarised in the following seven primary top
-
level
Workin
g Assumptions

(WA):


WA1.

A Main Linac length
consist
e
nt with an
average

accelerating gradient of 31.5

MV/m and
maximum operational beam energy of 250

GeV, together with a High
-
Level RF distribution
scheme which optimally supports a spread of individual cavity
gradients.


5


WA2.

A s
ingle
-
tunnel solution for the Main Linacs and RTML, with two possible variants for the
High
-
Level RF (HLRF):

a)

Klystron cluster scheme

(KCS);

b)

Distributed RF Source

scheme

(DRFS).

WA3.

Undulator
-
based positron

source located at the end of the electron Main Linac (250 GeV)
, in
conjunction with a
Quarter
-
wave transformer

as capture device.

WA4.

A lower beam
-
power parameter set with the number of bunches per
pulse

reduced by a
factor of two (
n
b

= 1312
), as compared to
the nominal RDR parameter set.

WA5.

Reduced circumference Damping Rings (~
3.2 km
)

at 5 GeV
with a
6

mm bunch length

WA6.

Single
-
stage bunch compre
ssor with a

compression
ratio

of 20
.

WA7.

Integration of the positron and electron

sources into a common “central region beam

tunnel”, together with the BDS, resulting in an overall simpli
fi
cation of civil construction in
the central region.


The above WA
s

are the reference for the recent design work, specifically for the CFS and cost efforts.
It is however important to note tha
t the WA
s

are not entirely independent of each other


particularly WA4 and WA5, and WA3 and WA7. These dependencies are explained in the following
sub
-
sections, which are organised more thematically than the above list.

The key top
-
level design parameters

for the SB2009 proposal are summarised and compared to the
RDR nominal parameters in
Table 2.1.





6


Table
2.
1
: Selected design parameters for SB2009 compared to the RDR nominal parameter set.




RDR
(nominal)

SB2009

Beam and RF Parameters









No. of bunches



2625

1312

Bunch spacing

ns

356

670

beam current

mA

9.0

4.8

Avg. beam power (250 GeV)

MW

10.8

5.4

Accelerating gradient

MV/m

31.5

31.5

Pfwd / cavity (matched)

kW

294

156

Qext (matched)



3.
5
E+06

6.6
E+06

Tfill

ms

0.6
0

1.12

RF pulse length

ms

1.
57

2.00

RF to beam efficiency
(matched)

%

61

4
4











IP Parameters









Norm. horizontal emittance

mm.mrad

10

10

Norm. vertical emittance

mm.mrad

0.040

0.035

bunch length

mm

0.3

0.3

horizontal β*

mm

20

11

horizontal beam size

nm

640

470







no trav.
focus

with trav.
focus

vertical β*

mm

0.4

0.48

0.2

vertical beam size

nm

5.7

5.8

3.8

Dy


19

25

38

δ
EBS/E

%

2

4

3.6

Avg.
P
BS

kW

260

200

194

Luminosity


cm
-
2 s
-
1

2.00E+34

1.50E+34

2.00E+34





7


2.2.1

Main linac


SCRF: choice of operational accelerating gradient (WA1)

The most highly leveraged R&D

that ha
s the greatest potential return

in terms of performance and
cost
is the development of cavity gradient. The cost impact is seen primarily in the linac length
required to achieve 500 GeV in the
c
entre

of

mass
. In 2005, a goal of an operational accelerating
gradient of 31.5

MV/m per cavity
with Q
0



0.8 x 10
9

in pulsed op
eration
was adopted for the
Refere
n
ce Design, with a cavity production specification of ≥35

MV/m

with Q
0



10
10

in a low
-
power
,
CW

vertical acceptance test
. These parameters were

adopted within the paradigm of deve
loping a
forward
-
looking design, and
assumed a worldwide R&D effort to routinely establish these
parameters. This R&D is currently on going and represents the largest fraction of the global ILC R&D
resources.

For the current SB2009 proposal


and in lieu of expected developments in the SCRF R
&D


these
specifications for

the accelerating gradient and Q
0

are left unchanged.

T
hus the global CFS
requirements (linac length, cryogenic requirements)
are left
as
per

the RDR.
This

is primarily to allow
the CFS groups to continue with their design and
cost plans. It is
noted

that any subsequent change
s

in accelerating gradient
,

arising from review of the R&D status will

(
to the first
-
order
)

result in a
scaling of the main linac which is relatively straight
-
forward, and will not require major re
-
evaluati
on
of the design

[Over3].

In contrast to the RDR baseline, however, we propose the HLRF systems support a spread of
operational gradients from cavity to cavity. This will allow lower performance cavities to be utilised,
assuming that high
-
performing ones m
aintain the average. The approach has impact on both the
definition

and accounting

of cavity production yield (
together with

acceptance criteria), as well as
required operational overhead in the accelerator (RF power and gradient). The global R&D groups
ar
e currently evaluating these issues. As of
this
writing every indication is that
support of a spread of
cavity gradients

will provide a better cost optimised solution, albeit at the expense of a more
complex HLRF distribution system and lower overall RF
-
to
-
beam power efficiency.

The final choice of average operating gradient will be taken
during

TD Phase 2 after review of the
worldwide R&D status. (An interim review is currently on going.) The choice of parameters must
necessarily include an analysis of ca
vity and cryomodule production statistics, as well as results of
operational overhead under full beam
-
loading conditions. The choice will also be influenced by
production models, assumed yield distributions and must be cost
-
optimised and
be technically
jus
tified
. Finally,
prospects

in the R&D beyond the TDR

should also be taken into consideration
, given
the uncertainty in the
formal
start of
project
construction. The ongoing SCRF R&D will be tailored in
TD Phase 2 to address these critical cost and performa
nce issues.

For SB2009 it is acknowledged that working assumptions for several key margin and overhead
parameters still need to be addressed. The exact amount of gradient and RF power overhead
required for

operational margin
remains to be specified
.

The ch
oices made at this juncture should be
consist
e
nt with the goals proposed for the TD Phase 2 R&D Plans, but should maintain as realistically
as possible the ‘forward looking’ paradigm established during the RDR.

2.2.2

Main linac


Single tunnel configuration

(WA2)

Foremost among the proposed changes is the adaptation of the main linac tunnel configuration to
one with only a single, accelerator
-
enclosure tunnel,

thereby eliminating

the support equipment
tunnel proposed in the Reference Design. Several key studies and strategic initiatives make it
attractive

to propose this change
for the following reasons
:

1.

The single tunnel configuration is a simpler underground construction
, effec
tively removing
~26

km of underground (support equipment) tunnel.


8


2.

Safety studies commissioned in each region (Asia /Japan, Americas/US and Europe / CERN)
found

that
valid
strategies
could

be realized for
single
-
tunnel life safety egress
in each of the
regi
onal locations studied (allowing for
some variations according to
local regulations).

3.

Recent studies

on High Level RF power sources and distribution
have resulted in feasible new
concepts for these systems that are much more suited to a single
-
tunnel solut
ion than those
proposed in the RDR (namely the Klystron Cluster Scheme (KCS) and the Distributed RF
Source
S
cheme (DRFS)).

4.

Availability studies
of the Main Linac
on

the proposed
single
-
tunnel and
new HLRF systems
configurations
show
that
an acceptable perf
ormance

can be
achieved

with appropriate
engineering

of sub
-
system designs
.

The two proposed novel HLRF solutions (KCS, DRFS) are described in more detail in Section 4.
6
. The
se

different approaches
provide

options for specific sites where local constraints

may favour one
solution over the other. Allowing such flexibility in the designs (multiple configurations) goes beyond
the “generic site” approach used in the RDR. Dealing with the
reality

of the requirements of potential
national hosts and their proposed

host
-
sites (so
-
called “bid
-
to
-
host”) is an important aspect of the
Project Implemen
ta
tion Plan. However,
t
he desire to
maximally
maintain a single design in both
the
layout and the core technologies remains paramount
, in an attempt to make the most effici
ent use
of the worldwide available resources.

2.2.3

Reduced beam
-
power

parameter set

(WA4, WA5,
WA2
)

The proposed baseline change with largest anticipated cost saving
(both construction and operation)
is the
reduction of the beam
-
power by 50%. The beam
-
power scales as
N n
b
,

where N is the
number

of particles
per

bunch, and n
b

the number of bunches per pulse. Since the luminosity
L


N
2

n
b

it is
proposed to halve

n
b
, while

keeping
N

the same as the 2007 Referen
ce Design
.

A

factor
-
of
-
two
reduction in n
b

allows:



a reduction in the number of klystrons and modulators (peak current/power reduction), in
this case by ~50% over the RDR number;



the possibility to halve the circumference of the damping rings to approximat
ely 3.2

km.

The luminosity can be recovered i
n general
by
pushing the beam
-
beam parameters, which

are not
considered a major cost driver
. The more demanding parameters

a
re however a

potential
increase in
performance risk
,

and have potential issues for the

physics and detectors
.

The significant reduction in RF power source together with the smaller damping rings offer a
considerable cost

reduction and overall simplification
in terms

of
construction scope. For the RF
power source at least, the option is
attractive as it opens up the possibility to restore the “missing
klystrons” at a later stage
, allowing
operation with an increased beam current

as a possible
luminosity upgrade path. The amount of infrastructure
, such as
conventional facilities
,

included
during the construction phase to support this upgrade still requires study
to be examined for further

cost
-
optimisation. However, for the smaller circumference damping ring, the upgrade would require
increasing the current by up to a factor of two (beyond
the Reference Design specification); collective
effects in the damping rings (most notably e
-
cloud) may well be a bottleneck to increasing the
number of bunches, an issue which is currently under study (part of the on
-
going e
-
cloud R&D
programme). Hence an

upgrade along these lines represents an increased risk over the Reference
Design parameters.


9


The obvious cost and construction scope benefits for this option must be weighed against a
perceived higher risk in achieving the design luminosity due to the mor
e demanding beam
-
beam
parameters. The impact of the higher
B
eamstrahlung on the detectors and physics programme must
also be reviewed. Finally, the effective lower beam current reduces the overall RF to beam power
efficiency, and so the reduction in averag
e RF power is not a full factor
-
of
-
two but closer to 38%.

T
here has been a conscious decision to
maintain
the
same
power handling capability
as assumed in
the RDR for the

beam dumps, positron target and capture, etc, and other systems which might be
diffic
ult to upgrade later and after some time in operation.
The benefits of initially over
-
rating these
systems (reduced technical risk)
,

while supporting possible future beam
-
power upgrade
,

are
considered to out
-
weigh any possible cost saving.

2.2.4

Central Region
Integration

(WA7, WA3, WA5)

The motivation for the
changes in the
Central Region Integration
as proposed in this document

is the
simplification of the central region tunnelling and civil engineering. By combining
their
functions
within a single beam tunnel
, the complexity of the underground excavation in the central region can
be substantially reduced. A more detail
ed

description and historical evolution of the current scheme
is given in Section

3
.

The modifications compared to the Reference Design
include
:



The Damping Rings have been moved vertically into the same plane as the BDS and shifted
horizontally to avoid the Detector Hall (see Figure 2
.
1 right). This removes the need for the
long (
~2

km) vertically slop
ed

beam tunnels (so
-
called
escalator
), which
can be replaced by
much shorter horizontal transfer tunnels. The Damping Rings tunnel can now also share one
shaft with the Detector Hall.



Since the BDS magnets do not require a large amount of transverse tunnel space, it
is

feasible to house the electron
source and the 5 GeV injector linac in the same tunnel as the
positron BDS, thus removing the need for a separate beam tunnel.



Similarly, the undulator
-
based positron source and associated 5 GeV booster linac can be
more efficiently accommodated at the
exit of the main electron linac (electron BDS, see
Section
s 2.2.5,

4.3

and 4.7
for more details).



An additional ~450 m beam path length is maintained in the positron system for bunch
timing.



Finally, an additional 500

MeV linac can be incorporated into th
e e+ source region which,
when used in conjunction with the e+ source photon target, can facilitate a low charge
auxiliary source for commissioning and tuning etc.

In addition to the simplifications in civil construction and the associated cost saving, the

proposed
scheme further consolidates sources of high
-
radiation hazard into the central region, which is
considered beneficial with respect to environmental impact.

Although a significant simplification in the scope of the civil construction and overall la
yout, the need
to support multiple beamlines and their support infrastructure in a single tunnel is challenging.
Availability and operational issues (impact on personnel protection and access zoning) needs to be
reviewed. Use of 3D
-
CAD is essential in unde
rstanding both the topology of the accelerator
beamlines and issues such as installation, safety etc. Initial studies in support of this proposal indicate
that acceptable solutions exist.


10


Because of the multiple beamlines


including the two superconducti
ng 5 GeV booster linacs


it has
been initially decided to retain a parallel service tunnel in this region to house the power supplies,
klystrons etc. As the 3D
-
CAD work evolves
into

more details

of the hardware implementation
, this
will be reviewed and co
st
-
optimised.

2.2.5

Changes to the u
ndulator
-
based e+ source

(WA3, WA7)

The motivation for moving the u
n
dulator
-
based positron source from the 150

GeV point to the end
(250

GeV) point in the electron Main Linac is primarily to consolidate technical systems and r
adiation
sources as already discussed briefly in Section 2.2.4. However, there are several additional impacts of
this particular modification that merit additional attention.

Several “system integration” aspects
should be noted as

direct benefits of moving

the source to the
high
-
energy end of the Main Linac:



Machine protection of the undulator is combined with machine protection of the Beam
Delivery system, resulting in a net beamline length reduction of about 400 m



The low
-
energy positron transport system
is shortened by several kilometres.



The positron source
, which represents

a high
-
radiation environment
,
moves within ±2.5 km
of the
centre

of the accelerator complex where other high
-
radiation sub
-
systems (e.g. the
main dumps) are located
. This

is a prefe
rred
feature
in

some potential site
s
,
since it
consolidat
es

most of the radiation hazard into a
single
“central campus area”.



The beam
-
line ‘chicane’
, which was

required in the RDR design can be replaced by a dogleg,
and better integrated into the BDS lattice. The narrow energy acceptance associated with the
RDR chicane is also moved to the BDS where there is already a natural bandwidth limit of a
few percent; th
is greatly facilitates beam
-
based alignment in the linac.



The RDR solution (i.e. Locating the source at the 150

GeV point in the electron Main Linac)
requires the upstream linac to perform
to its full

specification
s
. Failure to achieve these
highest perfor
mance goals during early stage commissioning could result in poor source
performance, as there is little or no safety margin. Initially achieving 80% full acceleration in
the main linac would still allow physics reach to 400

GeV centre
-
of
-
mass
:

However, th
is

would
come
at

a cost of reduced

positron yield below 1.0.

An additional proposed modification, which is independent of the central region integration, is the
adoption

of a simpler capture magnet (Quarter Wave Transformer, QWT) immediately after the
posi
tron
-
production target. This simpler magnet is
definitely

feasible, and represents a more
con
s
ervative (reduced
-
risk) option in comparison with the Flux Concentrator (FC)

assumed in the
RDR. However, use of a QWT reduces the capture yield by a factor of 2
when compared to the Flux
Concentrator


a difference that must be mitigate
d

by a significantly longer undulator, resulting in
higher incident power on the production target. For this reason, the FC is still a very desirable option
and continues to be a pr
iority R&D item.

One fundamental ramification of the relocation of the source is the need to run it at varying electron
energies, depending on the required centre
-
of
-
mass energy. Specifically, the impact on luminosity
below centre
-
of
-
mass energies of 250
-
3
00 GeV, where the positron yield drops rapidly. In this region,
alternate
-
pulsing schemes can be implemented to regain the luminosity (to within a factor of two) of
the 1/


scaling value (See section 4.3 for details). For detector calibration at the Z mas
s, the
proposed low
-
charge auxil
i
ary source will be sufficient.


11


At higher energies, the higher
-
energy electron beam becomes an advantage, offering very large
production margins (up to a factor of 5 at 250 GeV), giving further risk reduction at the higher
centre
-
of
-
mass energies where luminosity is likely to be at a premium. For the same reasons, this opens up
the potential for very high
-
polarised beams at these top
-
range energies.

Finally, it should be noted that the original RDR concept for E
cm
<

300

GeV w
as based on decelerating
the electron beam after the positron source. While conceptual feasible, this mode of operation is not
without challenges and still requires careful study.

2.2.6


Single Stage bunch compressor

(6 mm bunch length) (WA6, WA5)

Adoption of a

new lattice for the Damping Rings has allowed the extracted bunch length to be
reduced from the RDR value of 9

mm to 6

mm. (This modification has been implemented both for a
n
earlier

6.7 km ring and the current
ly

proposed 3.2 km ring.) The shorter bunch l
ength opens up the
possibility to adopt a simpler single
-
stage compressor with a compression ratio of 20, still achieving
the nominal RDR value of 0.3

mm at the Interaction Point.

The benefit of this is overall simplification of the
bunch
compressor system
, as well as a saving in cost
(mostly tunnel length). Beam dynamics studies have also shown that the emittance degradation is
reduced. The disadvantage is the loss of flexibility in bunch length tuning and the ability to reduce
the bunch length to 150


m.
These aspects can be seen as reducing the operational risk margin.

2.3

Cost Increment

One of the primary motivations for the proposed baseline modifications is the reduction in
the
overall cost. This is expected to help constrain the TDR updated VALUE estimate

to the RDR value of
6.6 billion ILC
U
, allowing some margin for incurring increased estimated costs during TD Phase 2.
During the early analysis of the proposed modifications, a ballpark estimate suggested a possible
total

saving of
approximately

15%.

Duri
ng

the last six
-
months

(May
-

December, 2009)
, a more detailed design and costing effort has
been made. The cost increments associated with the proposed modifications have been scaled
directly from the RDR VALUE estimate basis
. N
o attempt was made to provi
de new updated unit cost
estimates at this time, with the exception of those components that are new to SB2009 (notably the
HLRF system components). Thus
,

the increments reported here are direct comparisons to the
reported RDR estimate.

To date, the total
reduction of the modifications proposed in this document amount to
~13
% of the
RDR value estimate. It is expected that this will increase as further cost
-
driven iterations and
refinements are made during the co
u
rse of TDP
-
2.

A high
-
level breakdown and explanation of these increments is given in Section 5.

2.4

Availability studies

As for the published Reference Design, availability (and design for high
-
availability, HA) remains an
important issue for SB2009. To
-
date, the focus has

been on the removal of the Main Linac service
tunnel, where availability considerations have played a key role. An availability task force was set
-
up
by the Project Managers with a goal of finding a realistic and workable HA solutions for the Main
Linac s
ingle
-
tunnel configuration, including the two new proposed HLRF solutions (KCS and DRFS).
The studies were substantially based on availability modelling using the purpose
-
written Monte Carlo
tool AVAILSIM, which has been used to model and compare various c
onfigurations and to provide HA
specifications for the Main Linac components. To
-
date, the results indicate that satisfactory HA
engineering solutions exist for a single
-
tunnel, which add approximate 1% unscheduled downtime to

12


the overall availability duri
ng simulated luminosity runs at the maximum centre
-
of
-
mass energy of
500

GeV. (This seemingly small number actually corresponds to a doubling of the downtime of the
Main Linac as compared to the RDR two
-
tunnel solution.) Of primary importance is the energy

overhead required to maintain the top
-
most energy of the machine. The simulations show that a
3.5% overhead is typically required for both KCS and DRFS, providing the meantime between failure
(MTBF) specifications for the RF hardware are achieved. The ava
ilability simulations have provided
key input for the design of both HLRF systems, where acceptable HA solutions have been found
corresponding to the MTBF requirements.

Other aspects of SB2009


in particular the central region integration


have not as y
et been
modelled in detail. This remains to be done in TD Phase 2. However, early cursory examinations
indicate that no major differences with the RDR solutions exist.

It should be noted that the simulations and subsequent requirements on MTBF are based on

a set of
specific assumptions for the required uptime (15% budgeted), maintenance models (including
preventive maintenance) during operations as well as the impact of various failure modes. Achieving
the required component MTBFs based on these assumptions

remain in many cases an R&D
challenge. However, the differences between the RDR and SB2009 appear marginal in this respect.

A full report from the Availability Task Force can be found in the Appendix.

2.5

Risk assessment

An important aspect of evaluating the
soundness of any design is an assessment of the technical risk
associated with it. A Risk Register was produced for the Reference Design that catalogued the

top 50
or so critical design decisions, and attempted to rate them in terms of their perceived risk

to the
success of the project. Those design decisions with the highest risk (accelerating gradient, electron
cloud effects in the DR etc.) are being addressed with the highest priority by the on
-
going worldwide
R&D.

As we approach the end of TD Phase 2, i
t is important to review and re
-
assess the technical risk, in
view of progress in R&D, and in particular the design modifications proposed in this document. Such
a review and re
-
assessment requires a well
-
defined methodology (beyond what was done for the
R
DR).

At this time, it is not possible to provide a full quantitative and formal risk assessment, as this takes
time. Such an assessment is currently being planned and should be complete by the end of TD Phase
1 (mid
-
2010). A methodology is currently being
developed by the Project Management, which will be
applied during the assessment.

What is perhaps more pertinent to this proposal at this time is the perceived change in risk with
respect to the Reference Design. The ‘increased risk’ associated with the d
esign modifications are
documented throughout technical descriptions given in Section 4. In many cases, this perceived risk
is difficult to quantify, and by nature the assessment of whether the risk has increased (or decreased)
tends to be subjective.

In S
ection
6
, an attempt is made to summarise the key incremental risk elements of SB2009, as
compared to the Reference Design. A brief description of the methodology being developed is also
given.


13


2.6

Physics scope impact

While all of the proposed modifications
have at various levels an impact on the overall performance
risk of the machine, two working assumptions have a direct and quantifiable impact on the physics
-
relevant machine parameters:



WA3 Locating the undulator
-
based positron at the end of the main elec
tron linac



WA4 Reduced beam
-
power parameter set

In this section we briefly summarise the physics related parameters. More specific details can be
found in the relevant subsection of Section 4.

2.6.1

Operation at
500

GeV
C
entre
-
of
-
mass
Energy

All of the work
to
-
date, including the RDR phase, has been focused on an optimised parameter set at
the maximum energy of 500

GeV centre
-
of
-
mass. The basis for design during the RDR was the so
-
called “parameter plane” which attempted to give some r
e
dundant scope and safet
y margin to
achieve the design luminosity of 2×10
34

cm
-
2
s
-
1
. The requirements for the machine were based on
being able to accommodate the complete parameter ranges specified by this plane.

Table
2
.2
: RDR parameter plane ranges comp
ared to SB2009 specifications

(TF refers to Travelling Focus)
.


RDR

SB2009

min

nominal

max

no TF

with TF

Bunch population

x 10
10

1

2

2

2

2

Number of bunches


1260

2625

5340

1312

1312

Linac bunch interval

ns

180

369

500

530

530

R
M

bunch length


m

200

300

500

300

300

Normalized horizontal emittance at IP

mm
-
mr

10

10

12

10

10

Normalized vertical emittance at IP

mm
-
mr

0.02

0.04

0.08

0.035

0.035

Horizon
t
al beta function at IP

mm

10

20

20

11

11

Vertical beta function at IP

mm

0.2

0.4

0.6

0.48

0.2

RMS
horizontal beam size at IP

nm

474

640

640

470

470

RMS vertical beam size at IP

nm

3.5

5.7

9.9

5.8

3.8

Vertical disruption parameter


14

19.4

26.1

25

38

Fractional RMS energy loss to
beamstrahlung

%

1.7

2.4

5.5

4

3.6

Luminosity

x 10
34
cm
-
2
s
-
1

2

1.5

2


For 500

GeV centre
-
of
-
mass
operation

the most direct impact is from the WA4, the reduced beam
power. From simple scaling, a reduction in the number of bunches per pulse by a factor of 2 reduces
the luminosity by the same factor (luminosity per bunch crossi
ng remains the same).

The RDR parameter plane allows us to regain some if not all of this luminosity in principle. Table 2
.2

gives the ranges of key parameters from the RDR compared to SB2009.


The approach is to push harder on the beam
-
beam parameters to

increase the specific luminosity.
The largest modification is a reduction in the horizontal and vertical beta functions. These
modifications have impact on beamstrahlung as well as the required beam
-
halo collimation depth (or
conversely on the allowed ape
rtures within the IR).

However, all of these parameters (with the notable exception of the vertical disruption parameter for
the Travelling Focus parameters) are still within the parameter plane for which the BDS and IR was

14


designed. It is acknowledged, h
owever, that the risk margin in these parameters is decreased,
although the impact of this is difficult to quantify.

Currently SB2009 has two feasible parameter sets for 500

GeV centre
-
of
-
mass:



Pushing the beam
-
beam into a high
-
disruption regime (D
y

~ 25)
, which results in a luminosity
of approximately 1.5×10
34

cm
-
2
s
-
1



a reduction of ~25% over the RDR value. The reduction in
the peak luminosity (top 1% in

E/E) is slightly worse at ~37%. Beamstrahlung (and therefore
beam
-
beam backgrounds) at 4% is higher

than the RDR nominal value by a factor of 1.6 but is
still significant
l
y less than the maximum RDR specified value of 5.5%. The higher vertical
disruption parameter leads to tighter tolerances on stability at the collision point (beam
-
beam based feedback)
. The same va
l
ue was however proposed for TESLA, which has been
extensi
ve
ly studied. The approach requires virtually no additional hardware or modification
to the BDS/IR design.



A second approach relies on a technique know
n

as a travelling focus, where the

focus at the
interaction point is adjusted along the bunch, effectively compensating the hourglass effect
and allowing the vertical beta function to be reduced below the bunch length. In principle
such a scheme achieves higher luminosities, and can compen
sate completely for the factor
-
of
-
two reduction due to the lower bunch number. The beamstrahlung is higher than the RDR
nominal parameter value, but again within or close to limits of the RDR specification. The
very high disruption parameter does have impl
ications for luminosity stability and tuning
issues; these still remain to be resolved and will require further beam
-
beam simulation. The
technique also incurs a cost as additional hardware (crab
-
cavities) is foreseen to produce the
effect, however this co
st is relatively small.

The only direct impact of relocating the undulator
-
driven positron source to the end of the main
elect
r
on linac (WA3) for 500

GeV centre
-
of
-
mass running is an increase in the single
-
bunch RMS
energy spread. The RDR solution (Flux Co
ncentrator, 147

m undulator at 150

GeV) results in an RMS
energy spread of 0.12% at 250

GeV beam energy. Adoption of the more conservative quarter
-
wave
transformer (QWT) as capture device, requiring an increase in undulator length to 231

m, increase
the en
ergy spread at 250

GeV to 0.14%. With the undulator at the end of the linac (71

m undulator,
QWT), this increases further to 0.2% at 250

GeV
-

still well within the bandwidth of the BDS.

2.6.2

O
peration below 500

GeV

Centre
-
of
-
mass Energies

During the RDR phase

no attempt was made to produce optimised luminosity parameter sets for
centre
-
of
-
mass energies below 500

GeV. A basic assumed requirement was a 1/

scaling below
500

GeV down to 200 GeV centre
-
of
-
mass.

As of writing, there are still no formal optimised pa
rameters for specific intermediate energies,
although possible scaled parameter sets do exist. Generation of possible formal parameter sets for
key centre
-
of
-
mass energy scenarios still remains to be done in TD Phase 2.

However, WA3 and WA4 do have immedia
te implications for lower energy running. Specifically:



For the reduced bunch number parameters at lower centre
-
of
-
mass energies, the luminos
i
ty
compensating effects from the beam
-
beam become somewhat weaker, and so the
luminosity reduces faster than 1/





The generation of positrons from the undulator source reduces approximately quadratically
with the beam energy, and rapidly decreases to zero below a threshold at ~125 GeV
(250

GeV centre
-
of
-
mass).


15


Recent estimates have suggested an approximate luminosity

of 3×10
33

cm
-
2
s
-
1

at E
cm

= 250

GeV for
SB2009. Further studies and optimisations are expected in TD Phase 2.

The positron source is currently specified to produce a yield of 1.5 positrons per electron at a beam
energy of 150

GeV. (A yield of 1 is required
, and the specifications assume a 50% production
overhead margin.) Above this value, the yield increases rapidly, providing significant overhead and
safety margin. With these design parameters, the positron yield drops to ~1 (no production
overhead) at abo
ut 125

GeV. Should the physics demand high
-
luminosity running below 250

GeV
centre
-
of
-
mass, there are several conceptual schemes that could be considered. One such scheme is
to use alternate 2.5

Hz pulses for generating positrons and luminosity respectivel
y. These schemes
require additional hardware at the end of the linac (bypass lines, kickers, dumps etc). Although these
schemes require further detailed design (and cost) studies, there appears to be no showstoppers.
One such possible scheme is outlined in

Section 4.3.

Finally, we should note that the proposed auxiliary positron source should provide sufficient
luminosity (few ×10
32

cm
-
2
s
-
1
) for calibration of the detector at the Z
-
pole.


References

[Over1] ILC Minimum Machine Study Proposal, ILC
-
Report
-
20
09
-
019, January, 2009,
http://ilc
-
edmsdirect.desy.de/ilc
-
edmsdirect/file.jsp?edmsid=*865085

[Over2] Summary report of the first meeting on Accelerator Design and Integration, ILC
-
INT
-
2009
-
035,
June, 2009,
http://ilc
-
edmsdirect.desy.de/ilc
-
edmsdirect/file.jsp?edms
id=*879845

[Over3]
This is based on the assumption that the choice of gradient is unlikely to change by more
than the order of 10%.

[Over4]
In part, as the number of klystrons and modulators required depends on this option.




16


3

Layout


In this section we pre
sent an overview of the changes that are considered for the ILC layout. While
some of these changes are consequences of the proposed changes in the subsystem designs and are
integral part of the SB2009 proposal, some have
come about through continuing syst
em design and
development prior to the SB2009 re
-
baseline studies
.


Referring to Figure 2.1 in Section 2, t
he most
visible

changes are in the central region
,

specifically the
Damping Rings

(DRs)
.
In the Reference Design, the

6.4 km six
-
fold symmetric ring
s
[Layout1]

w
ere

located around the interaction region and elevated by 10 m above the Beam Delivery systems along
with the e
lectron

injector and
positron

KAS (keep alive source).
The arrangement

allowed some
independence in operation of the injectors and rin
gs from personnel access to the
Beam Delivery
Systems (
BDS
)
, but at a cost of a relatively complex layout of tunnels and shafts.

When considering the detail component layout of the six
-
fold ring
,

a different philosophy was
explored
, resulting in a proposal

for a

two
-
fold design

‘racetrack geometry’ with long straight
sections
to accommodate RF, wigglers as well as injection and extraction
. The lattice of the arc
sections was changed to a new very flexible one
,

where the emittance and momentum compaction
cou
ld be separately adjusted. These changes were accepted in 2008 before the SB2009 formal
process was begun. For further detail regarding DR component layouts, see
S
ection 4.
5
.

The

changes in geometry stimulated discussions regarding alternate layouts
for

the central region
.

The

racetrack design

has the beam

injection and extraction occu
r
r
ing

at
nearly
the same point in the
centre locations

of one
of the two
long straight section
s; this allows the

DR
to be located in the same
plane as the BDS, but
offset h
orizontally from
it (clear of the detector hall at the IR).

Short (in
-
plane)
transfer line tunnels can then connect the DRs to the main BDS tunnel.
Further layout considerations
led to the concept

of a compact and
efficient
central region
[Layout2]
,

which i
ncorporated all of the
e
±

injection systems, DRs, BDS and Interaction Region in a single central campus. The long linac tunnels
containing
both
the
main linac

and RTML
beamlines
would then extend outwards in either direction
without interruptions.

These i
deas were combined with
the construction scheme of the main linacs on the basis of the
s
ingle
-
tunnel
scheme
, the
l
ow
beam p
ower option and
s
ingle
-
s
tage
b
unch
c
ompression into the AD&I
s
tudies and the SB2009 Design.

The single
-
tunnel linac layout i
ncorporat
es the proposed single
-
stage bunch compressor
. This single
-
stage bunch compressor

ends with the beams at ~ 5GeV
as opposed to

15 GeV
in

the two
-
stage
design.

The overall design results in a net reduction of some ~300

m in comparable machine length
.
A

major

change
in the electron linac
is
the re
-
location of

entire undulator
-
based positron source from
the 150

GeV point
to the central region
.

Electron and positron main

linacs are
now

identical
, except

for an additional 4.1

GeV energy reach in the electron linac to drive the positron source (the
Reference Design has 3

GeV).
This
allows
changes in accelerating gradient, gradient distribution or
maximum design energ
y to be
easily accommodated. Changes of
gradient of the order of 10% would
require
little in the way of fundamental

changes in the layout or of infrastructure support from CF&S.

Fig
ure

3.
2

show
s

the concept of beamlines which share tunnels in
the central

region
.
Figure 3.3
shows a cross section
of the positron main linac near the beam delivery section.

Accurate to
-
scale

tunnel schematics can be found
in the ILC EDMS and
at
http://ilc.kek.jp/SB2009/TUNNEL%20DRAWING%
20SET%20(11
-
20
-
2009).pdf

.



17



Figure 3
.
2
:

Topological diagram of the beamlines in the ILC central area.


Figure 3.3: Cross section of the positron main linac near the beam delivery section.

Many of the design details and changes of these systems are described in
S
ection 4
.

T
he following is
a discussion of the general layout and
the
design philosophy.

The
SB2009 design
goal was to minimize the underground volume
,

while developing the individu
al
system designs and interfaces with CF&S beyond where they were at the
publication

of the RDR.
Since

the injectors and beam delivery systems are very varied in
their

support requirements (power
supplies, RF, instrumentation etc), it was decided at an ear
ly stage of the studies that a single support
tunnel
would be maintained
for the central region. Even
with

the

support tunnel
,

there is a reduction
of ~5 km out of ~14 km in tunnel length in central region,
not including

the DR’s.

The
reduced beam power

op
tion
,

with half the number of bunches
,

enables

(
but does not require
)

a
half
-
circumfe
rence damping ring. The central campus layout can
accommodate either
the
3.2 or 6.4
km
ring designs
, since both utilise the same
design

for the
1 km straight section
s, one

of which is
used for injection and extraction for both electron and positron rings are located (the other
accommodates the RF and damping wigglers).


18




Figure
3.
4
: Schematic Directional Beamline Layout


19


Much of the effort to date has been on the region where the
positron

production systems and the
beam delivery systems co
-
exist. In the
Reference Design
,

the
positron

production system
was
incorporated into main electron linac at the nominal 150

GeV locatio
n
.
A
t the
publication

of the RDR
,

t
here were
only
conceptual designs

for this system
,

whereas today the designs now address many
more practical engineering issues. For example
,

there are short sections of tunnel
requiring

“alcoves”
,

which will require diff
erent

tunnel cross
-
sections
,

the exact cost
-
optimised design of which
will be dependent on

local geology. Many of these issues were duplicated in the “Keep alive source”
(KAS) in the RDR
. The KAS

has now been replaced by an “Auxiliary Source”
,

which uses

a
n
independent electron

beam
in conjunction with

the same target, capture,
and
booster systems as the
gamma beam from the undulator.
As of writing,
the SB2009 design has
only one
vault, which is
designated to handle
high radiation
environment
.

This region w
ill require further detailed
development in TD Phase 2.

The SCRF Main Linacs have a relative large acceptance in both energy and transverse phase space as
compared to either the undulator source or the BDS. Hence the from one to the other require
protectio
n in the form of collimator and fast beam
-
abort systems.
Because
the undulator
is located

at the end of the linac leading into the BDS

i
n SB2009
,

a single section of collimation and abort
system will protect both in an integrated cost
-
effective fashion
.

Consolidation of the positron source
into the BDS also moves all the bandwidth limiting beamline sections to the central location, which
will simplify the main linac commissioning and tuning.

The
electron

injector easily fits into the SB2009 layout
,

whose

geometry
choice is primarily
determined by the
requirements from the
positron

injector side
.

It is desirable

(but not mandatory) that a bunch is

inject
ed

into (fills) an empty DR bucket that has
just been empt
i
ed by extraction.
This allows maximum flexibi
lity in the bunch patterns, but also

puts
tight
constraints on the difference in path lengths travelled by the electrons and positrons through
the
machine
.
The

issue was well known

a
t the time of the RDR
, but a fully worked
-
out lattice solution
was not pre
sented, because

at that time
the path length correction
had to be

kilometres. In the
SB2009 layout and with the
smaller
3.2 km rings,
the

correctio
n can now be a few hundred metre
s.

A

natural asymmetry in the length of

electron and positron

side
s

is

exploi
ted

to incorporate
this

delay
drift
length.

There are
other

examples where the SB2009 designs and layouts are more complete and detailed
than the RDR. This is due to both the continuing design work over the time since the RDR but also
driven by the AD&I st
udies of the SB2009 proposal. In some places this complicates comparisons.
The detail system designs and their pros and cons are based on these layouts and are discussed in
detail in the following sections.

Figure 3.
4

gives the schematic directional beaml
ine layout.


Re
fer
ences

[Layout1]

Adjusted from the 6.7

km circumference quoted in the reference design to accommodate
inconsistencies with bunch timing.


[Layout2]

Efficient
in this context refers to minimizing the necessary underground volume of
tunnels,

caverns and shafts.





20


4

SB2009 Proposal

4.1

Issues of Main Linac Accelerating Field

4.1.1

Status of Cavity Field Gradient
:

The expected production cavity gradient and yield
are two of

the most important parameters in the
ML
-
SCRF technical area to be
provided in the

evaluation process, as well as
in the studies of the
overall construction cost and plans in relationship with the Conventional Facilities & Siting Group
and
Accelerator System areas
.

In 2005, t
he
S0
performance goal for each of the 9
-
cell cavities to use

in
the ILC main linacs

was
defined as

≥35

MV/m
with

Q
0

≥ 8 x 10
9

in

low
-
power vertical acceptance
tests. This number included an estimated 10% for the ensuing assembly steps and operational
margin,
so as to ensure an average accelerating gradient of
31.5

MV/m
with

Q
0

≥ 1 x 10
10

in cavities
installed in horizontal cryomodules. The
world
-
wide R&D effort
ensued with these goals.

This R&D is
currently
still
ongoing
,

and
it
represents the largest fraction of the global ILC R&D
effort
.

As part of the TD Phase
-
1

baseline review and subsequent re
-
baselining effort (including SB2009
proposal
), it
was agreed

to review the choice of accelerating gradient
, in the light of present and
prospective outcome from the R&D programs, together with the specific system implemen
tations
and operational models of the main linacs
. Test results from approximately sixty 9
-
cell cavities
processed and vertically RF tested are expected to be included in the TDP
-
1 cavity gradient
evaluation.
One of the critical exercises, therefore, is t
o catalogue and analyse the production
statistics of the 9
-
cell cavities and their performance by putting together all the available data from
all three regions in a scientifically consistent fashion.

For this purpose
a task
force
was established
to
create
a global database for ILC cavity test
results.

The
mandate of
this taskforce
is

to:

1.

C
ollect and verify the global cavity R&D information;

2.

Cr
eate and maintain a global database for the ILC
-
related cavity performance;

3.

A
nalys
e the database to understand th
e global progress in improving cavity performance;

4.

A
ssist the Project Managers through analysis of these results and in re
-
evaluation and projection
of future cavity performance during TDP
-
II and the future production phase.


The team

produced
an interim,

progress
report

at the Linear Collider Workshop of the America
s
(ALCPG) and GDE meeting
held in Albuquerque, New Mexico
in the

fall

of 2009
.

Figure
4.1.1

shows
the current (Dec 2009) status of
,
cavity gradient performance in the first and second passes i
ncluding
the chemical process and cold vertical tests for the cavities manufactured by vendors and processors
who have currently reached the S0 goal with at least one cavity.


In addition, the database effort
allows tracking of
the rapid progress made
in
cavity processing at ANL/FNAL and KEK and cavity
manufacturing at new vendors
.

In addition to evaluations of vendor progress based on hard cuts in cavity performance, another
important production statistic is the scatter in production cavity performance, a
nd the effect on the
requirements of the RF systems. The cavity database will allow for a systematic means to complete
this evaluation. During actual machine construction, methods such as sorting or grouping cavities
before installation in a cryomodule m
ay be used to soften the range requirements on the RF system.



21



Figure
4.1.1:

Production yield as a function of gradient after the 1
st

(left) and 2
nd

(right) pass process
and vertical test as of December 2009. Note all cavities with a gradient exceeding 35MV/m meet the
ILC specification of Q
0

≥ 8 x 10
9

at 35MV/m.

4.1.2

Issues of Main Linac System Design

In conjunction with

the (GDE and AAP) review process i
n 2010, based on the current R&D results we
propose to keep the cavity gradient goals at 35MV/m in vertical test,S0, and 31.5MV/m in operation
in an installed cryomodule, S1. We note that as the R&D progresses, including horizontal testing of
individual d
ressed cavities, and full cryomodules and strings of cryomodules through the ILC S1 and
S2 programs, and efforts at the XFEL, a better understanding of the components of the margin
between the cavity gradient goals will emerge. Ultimately for the ILC accel
erator the gradient will be
reviewed based not only on this information, but also on the economics of mass production, and
production yield. Furthermore, as design of the RF systems progresses, full systems analyses of the
integrated power / cavity system
, allowing for appropriate variability in cavity gradient and system
operational overhead, will have to be completed.

We propose that TD Phase 2 we define the specific magnitude of the spread (+/
-

10


20%, pending
more discussion) of operational gradien
t to support with the HLRF systems. This allows lower
performance cavities to be utilised and increases the production yield, assuming that high
-
performing ones maintain the average. This approach has impact on both the definition of cavity
production yiel
d and acceptance criteria. Currently every indication is that this will provide a better
cost optimised system, albeit at the expense of a more complex HLRF distribution system and lower
overall RF
-
to
-
beam power efficiency. The exact amount of RF power an
d gradient overhead required
for

operational margin
remains to be determined
.

Within the TD Phase, the FLASH

beam tests with
high
-
gradient cryomodules will give us further details.


In preparation for industrial production, we will investigate the assumpti
on of a usable cavity fraction
of 80 % utilized in the RDR, with the possibility that it may be improved to be a level of 90 % including
all effects described above.

Based on the S0, S1, S2 and related R&D programs, the operational field gradient for the
ILC machine
will be confirmed in light of the experience and expectations related to margins required for
redundancy, operational margin, dynamic tuning, and availability.

Referenc
e
s

[SRF1] C.Ginsburg, presentation at the GDE meeting, Albequerque, October, 2009,
http://ilcagenda.linearcollider.org/contributionDisplay.py?con
tribId=182&sessionId=29&confId=346
1

Electropolished 9-cell cavities
0
10
20
30
40
50
60
70
80
90
100
>10
>15
>20
>25
>30
>35
>40
max gradient [MV/m]
yield [%]
JLab/DESY first successful test of cavities from qualified vendors - ACCEL+ZANON+AES (30 cavities)
Electropolished 9-cell cavities
0
10
20
30
40
50
60
70
80
90
100
>10
>15
>20
>25
>30
>35
>40
max gradient [MV/m]
yield [%]
JLab/DESY (combined) up-to-second successful test of cavities from qualified vendors - ACCEL+ZANON+AES (25 cavities)

22


4.2

Electron Source

4.2.1

Overview:

The proposed changes in the SB2009 design have two notable implications to the electron source
system:



The low
-
P parameter set increases the bunch spacing by a factor ~2. Increased bunch spacin
g
will reduce the challenges for the source drive laser system, due to a more complete population
inversion of the Ti:Sapphire laser medium in

between pulses. However, the goal of the R&D
remains
unchanged and set to satisfy
the full RDR specification.



Cha
nge in the damping ring design, combined with the revised layout of the central injector
complex
, as
shown in Figure 4.2.1, will affect geometry of the transport lines, including their
lengths.

By far the most significant modification is the re
-
location of

the source from its own independent
underground housing (tunnel), into the tunnel system shared with the positron
and beam delivery
systems
. As explained in Section
s

2 and 3, this has been motivated primarily by cost considerations,
and a desire to reduce

the scope of the underground construction and maximise the use of common
shafts and utilities.


Figure
4.2.
1: Topological layout of the positron side of the central region, showing the approximate
location of the electron source (in green).

At the level
of subsystem design, however, t
he ILC SB 2009 results in
no
fundamental
changes to the
polarized electron source
. The source parameters are essentially the same
. They are
summarized in
T
able
4.2.
1
.

T
he fundamentals of the laser system design would
also
remain
essentially
the same.


23


Table
4.2.
1: Source parameters

Parameter

Symbol

Value

Unit

Comments

Electrons per bunch (at gun
exit)

n
e

4*10
10

Number

Same as
RDR

Electrons per bunch (at DR
injection)

n
e

2*10
10

Number

Same as
RDR

Number of bunches

N
e

~
1312

(was ~2625
in RDR)

Number

Low
-
P
parameter; new
baseline

bunch repetition rate

F
µb

1.5

(was 3 in
RDR)

MHz

Low
-
P
parameter

bunch train repetition rate

F
mb

5

Hz

Same as
RDR

bunch length at source

Δt

~ 1ns

ns

Same as
RDR

Peak

current in bunch at
source

I
avg

3.2

A

Same as
RDR

Energy stability

S

< 5

% rms

Same as
RDR

Polarization

Pe

80 (min)

%

Same as
RDR

Photocathode Quantum
Efficiency

QE

0.5

%

Same as
RDR

Drive laser wavelength

Λ

780
-
810
(tunable)

nm

Same as
RDR

single bunch laser energy

E

5

µJ

Same as
RDR


4.2.2

Design Work to Pursue during TDP2

Although the studies to date have shown that acceptable solutions exist for the shared tunnel layout,
f
urther optimisation
is needed in the beamline design during TD Phase 2
. For
instance, for
the
finalised tunnel design
,

a re
-
optimisation of the electron source transport lattice is required
.

One
critical item

to note

is the control of spin polarisation
.

C
orrect bend angles and arc radii for most
transport lines

have to be established. The resultant

constrain
t
s
have to be an integral part of
the
overall design of the central region
, including the
Source, RTML and BDS.


Figure
4.2.
2: Schematic view of Wien filter proposal

Recently, a new option for the electron spin rotation from longitudinal to tra
nsverse has been
discussed
, as shown in
F
igure
4.2.
2.
It utilizes

a Wien filter
, introduced

immediately downstream of
the electron gun
,
replac
ing

the two spin rotating superconducting solenoids in the electron to ring
transfer line. This would result in a small cost saving for the electron source. However, additional
space is required
immediately after capture solenoid (see Figure 4.2.2)
, and it is

desirable to keep the
total distance between the gun and the

sub
-
harmonic buncher system as short as possible
, so as

to

24


reduce the growth of longitudinal emittance due to space charge. Further simulations are needed to
verify the feasibility of this optio
n
, and this work is expected to continue during TD Phase 2
.

4.2.3

SB2009
-
specific
R&D Work to Pursue during TD

Phase 2

No additional work is required on the electron source system i
n addition to the
already ongoing
risk
-
mitigating R&D

such as those

in the laser
system, high
-
voltage gun and photocathode development,
and studies into cathode surface charge limits. Design work on layout and lattice optimisation will
continue naturally during TD Phase 2.




25


4.3

Positron Source

4.3.1

Overview

In addition to changes in some
aspects of the beam parameters, t
he SB2009 proposes
some
major
change
s

to the

system

implementation of the undulator
-
based positron source at the ILC.

Relocation of the undulator:

As part of the proposed central region integration (Section 3), the
complete

undulator
-
based positron source is re
-
located from the 150

GeV energy point to the exit of
the main electron linac
, and is
integrated with the up
-
stream part of the BDS. Th
is

modification
will
benefit several system integration issues:



The undulator sourc
e can be better integrated into the upstream area of the BDS, where
more tunnel space and more freedom of lattice design
are available
than in the main linac.



Machine protection systems for the smaller acceptance undulator and BDS beamlines can be
effectiv
ely combined into a single system
, located

immediately downstream of the main linac.



All sources of restricted energy bandwidth are now localised in the central region, leaving
both the main SCRF linac as
systems with nearly
identical contiguous high
-
bandw
idth
. This

greatly facilitat
es

their commissioning and tuning.



Moving the positron target and capture system to the end of the linac consolidates all the
high
-
radiation environment systems within the central area, which is expected to be
beneficial for cer
tain host sites (radiation safety and environmental impact).



A large energy overhead is available to drive the undulator source, which allows operational
margin for the early commissioning in the event that the maximum
-
performance of the main
linacs system

is not achieved (i.e. maximum gradient at full beam loading).

An important consequence of the re
-
location is that the

energy of the electron beam passing though
the undulator will vary with the required centre
-
of
-
mass operation.

In general the positron yi
eld from
the source scales approximately as


down to a beam energy of ~125

GeV, after which the yield
rapidly drops to zero. This clearly has ramifications for the physics related parameters (primarily
luminosity) at lower than 500

GeV centre
-
of
-
mass oper
ation. This important issue is discussed more
detail, together with the proposed solution, in the latter part of this section
.

Adoption
of a simpler capture magnet (Quarter Wave Transformer, QWT)
:

The QWT would be
implemented

immediately after the pair
-
pro
duction target. This magnet is
technically simpler and
more feasible than the flux concentrator that was previously considered during RDR. Therefore,

this
change contributes positively to reduction of the overall technical risk of the system. This is
accom
plished at the cost of reduced positron capture, however, and a subsequent increase in the
undulator length is introduced to make up for the loss. Our strategy is to maintain the more
conservative QWT
-
based scheme as the new baseline, while continuing the
R&D effort on the flux
con
c
entrator.
The flux concentrator
may

be adopted again in the future
,

once a feasible design has
been established.
It is noted that thi
s proposal is independent of the location of the positron source.

Removal of an independent Keep

Alive source:

The positron Keep Alive Source in the RDR
was based
on

an
independent

500MeV electron drive beam, target, capture RF, remote handling, etc., which
necessitated

rep
lication of

many subsystems.

For SB2009, it is proposed to replace the Refere