career_version1 - Northwestern University

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25 Νοε 2013 (πριν από 3 χρόνια και 6 μήνες)

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1.

ABSTRACT

We propose a five year prog
ram, in which we will develop the next generation of
beam instrumentation necessary to operate future accelerators

with high current, low
emittance and high repetition rate
.

RF beam position monitors is on
e

of the detec
tors
to be developed

in order to measure the length of ver
y short electron bunches (r.m.s. ≤
1ps)
. In addition
, we will use these new and/or improved techniques to instrument the
facility needed to validate

the CLIC
design for
the high current drive beam

that will be
decelerated as
RF power
is

extracted to

accel
erate

a parallel

beams with
unprec
e
dente
d

high
gradients, that

is 150 eV/m. This will allow u
s to built multi
-
TeV
machine
s
.

This facility has to be
operational by 2008, and final results available by
2010
in order
to meet the schedule
imposed
by ….



2.

I
NTRODUCTION


1.
Vision of the future


2.
Funding Agencies meeting

3.
Expertice of the group


3.

DESCRIPTION OF CLIC
AND CTF3

As explained above, our group is very interested on the physics capabilities of a
high luminosity (10
34
-
10
35
/cm
2
/s)
Multi
-
TeV e
+
e
-

collider, a
nd we are of the opinion
that the ultimate machine design to achieve this goal will be based on the
so
-
called
“Two
-
Beam S
cheme


from CLIC, Compact LInear Collider, currently under
development at CERN.


The CLIC design aims for a maximum energy of 3 to 5 Te
V (centre of mass) to be
reached by acceleration with high gradients of 150 MV/m at 30 GHz with a RF pulse
length of 130 n
s.

In the two
-
beam scheme, t
he pulsed RF power (460 MW per metre
length of linac) to feed the acceleratin
g structures is produced by
extracting 30 GHz
power
from high
-
intensity/low
-
energy drive beams running parallel to the main beam.
These drive beams are generated in a centrally
-
located area and then distributed along
the main linac. The beams are accelerated using a low frequency (9
37 MHz) fully
-
loaded normal
-
conducting linac. Operating the linac in the fully
-
loaded condition
results in a very high RF
-
power
-
to
-
beam efficiency (~97%). Funnelling techniques in
combiner rings are then used to give the beams the desired bunch structure
with the
concomitant increase in intensity, in this process the bunch spacing is reduced in
stages from 64 cm to 2 cm, and the beam current is increased from 7.5 to 240 A.
The
RF power is extracted from the drive beam in Power Extraction and Transfer
Struc
tures (P
ETS)
.


An overall

CLIC

layout of the complex is shown in Fig.1. A single tunnel, housing
only the two linacs and the various beam transfer lines, results in a very simple, cost
effective and easily extendable configuration for energy upgrades.




Fig. 1: Overall layout of the CLIC complex.



The technical feasibility of two
-
beam acceleration has been demonstrated in CLIC
Test Facility 2 (CTF2). In this test, the energy of a single electron bunch was increased
by 60 MeV using a string of 30 GHz acc
elerating cavities powered by a high intensity
drive linac. Peak accelerating gradients of 190 MV/m have been obtained in CTF2
using molybdenum
-
irises in 30 GHz copper structures with rf pulse lengths of 16 ns.
This result has to be confirmed for the nomin
al CLIC pulse length (130 ns).

An
experimental demonstration of the principle of the bunch combination scheme has
been made at low charge using a modified layout of the former LEP Pre
-
Injector (LPI)
complex.


A number of CLIC specific items that need to be

understood have been listed by t
he
International Tech
nical Review Committee. For the

so
-
called R1 items we need
to
provide a feasibility proof, while the

R2 items

are those
which must be investigated
in order to arrive at a Conceptual Des
ign.


The

R1 is
sues are:

R1.1 Test of damped accelerating structure at design gradient and pulse length

R1.2 Validation of the drive beam generation scheme with a fully loaded linac

R1.3 Design and test of an adequately damped power
-
extraction structure,




which can be switched ON and OFF


The
R2 issues are:

R2.1 Validation of beam stability and losses in the drive beam decelerator,


and design of a machine protection system

R2.2 Test of a relevant linac sub
-
unit with beam

DETECTORS
624 m DRIVE BEAM
DECELERATOR

e
-
e -
FINAL
FOCUS
e
-
e
+

e
-
e
+

e +

MAIN LINAC

LASER
FINAL
FOCUS
e -


MAIN LINAC

LASER
DRIVE BEAM
GENERATION
COMPLEX
~ 460 MW/m
30 GHz RF POWER
MAIN BEAM
GENERATION
COMPLEX

o

All t
hese
R1 and R2

feasibility iss
ues can be demonstrated by
CLIC Test
Facility 3 (
CTF3
)
.



o

A total of 17.2 MCHF
≈XX USD


are needed
to
demonstrate the critical R1 and R2 issues before 2010.


o

Our

Northwestern

group
and the CTF3 collaboration
will like

us

to t
ake
the
responsibility
over the
instrumentation needed answer
R2.1 issues.
That will
required the construction of a
35 A Test Beam Line (TBL):


o

Project description:
Design, construction, installation,
exploitation and bench
-
marking simulation tests of a 2
0 m long,
well
-
instrumented test decelerator with typically 10
-
15 RF power
-
extracting structures (PETS), to validate the CLIC drive beam
stability and losses with the CTF3 beam.

o

Collaborators:
CERN, Northwestern Univ. and ??

o

Schedule:

Design 2005/2006,

re
ady for tests in 2008

o

Resources:

1
MCHF 8 man
-
years


OVERALL DESCRIPTION
OF THE CTF3/CLIC FAC
ILITY

Fig.2 shows the locations in the CTF3 facility where each of the key feasibility tests
will be performed:



the test of a damped accelerating structure at the

design gradient and pulse
length (R1.1) requires the linac
-
driven high
-
gradient test stand to be completed
(location 1)
;



t
he validation of the drive
-
beam generation scheme with a fully
-
loaded linac
(R1.2), and the design and test of an adequately damped p
ower
-
extraction
structure, which can be switched ON and OFF (R1.3), requires th
e complex
with the delay loop and
the combiner ring (location 2)
;



t
he validation of beam stability and losses in the drive
-
beam decelerator, and
design of a machine protection s
ystem (R2.1), and the test of a relevant linac
sub
-
unit with beam (R2.2), requires the
CTF3
-
CLEX experimental area
, which
consists of a high
-
power test stand (location 3), the Test Beam Line (TBL)
(location 4) and the probe beam with a relevant linac two
-
b
eam module
(location 5)
.



Fig. 2


Schematic layout of the CLIC Test Facility CTF3

Need plot from
Gunther.


Northwestern c
urrent project at the

CTF3/CLIC facility
--

Beam loss
monitor system

At
CT
F3 the control of beam losses is an important issue due t
o the

average high
power of the machine, 4kW.
Beam loss
es

must

be monitored all along the linac in
order to keep the radiation level and the activation as low as possible.

We have taken
advantage of our expertise on beam instrumentation, and taken the
resp
ons
i
bility

for

the
B
eam
L
oss
M
onitor (BLM)

system for the linac.
The linac, providing a 3.5A,
1.5

s electron beam pulse of 150MeV, is scheduled for co
mpletion by the end of
2004
, and our detector will be completely installed by then
.
The goa
l of our effor
t is to
provide
quantitative beam loss measurements

with a BLM system that will
be able to
detect losses corresponding to the ‰
level of the nominal beam current.
A
n intensive
simulation work has been
carried out

based on
Geant3.21

in order to predict

the
characteristics of the
electromagnetic

showers in a realistic accelerator envir
onment.
The simulation takes into account that the CTF3 linac is

composed of accelerating
modules that have a beam position and an intensity monitor followed by a set of three
q
uadrupoles and two 3 GHz accelerating cavities. Each module is 4m long and there
are a total of 9 consecutives modules along the linac.

The layout of a linac module
equipped with beam loss monitors is shown in Figure 1.


Figure 1:

(a)

Layout of the beam

loss system in a CTF3 linac module

and monitoring
chamber. (b)
Geometry simulated by Geant3.21
.




As
shown in Fig. 2, using the

full geometry and beam description we expected

that
a


loss of the beam requires a BLM system based on detectors capable of
measuring
10
10

to 10
12

particles/cm
2
/s
[
i
]
. In parallel to the simulations
,

a preliminary test of beam
loss monitoring was performed in November 2003 [
ii
] on the already existing part of
the accelerator.

From that exercise we learn that detectors based on se
condary
emission are fast enough to see the time structure of the beam
, and that the detector
that we had developed to monitor high flux hadron beams as those expected at high
power proton drivers and neutrino factories, could be used
.

Based on this

simula
tions
and cost considerations
, it was decided

to build a system with 4
detectors per module.








Figure 2
: Electrons / Positrons flux distribution at 100cm from the point of the beam
loss in the X/Y plane transverse to the beam line.




The individual


detector
s are
small gas sealed ceramic chamber sensitive to charged
particles and developed by Northwestern University, Fermilab and
Richardson
Electronics. It has
both a very good resistance to radiation and a high dynamic range
(>10
5
). The chamber can
be filled with helium and
be
used as an ionization chamber
(IC), or just under vacuum and
be
used as a Secondary Electron Monitor chamber
(SEM). Preliminary
test made at the Fermilab booster

abort area
,

show
s

that as
exp
e
cted,
the response of the IC chamb
ers will saturate around
10
10
particles/cm
2
/s
,
while the SEM continues to have a linear re
sponse even at high flux rate, s
ee Fig. 3.

The choice between an
IC or a SEM will be a

compromise between the time
resolution, which is faster for the SEM mode, and t
he sensitivity, which i
s 1000 higher
for the IC

mode. The output signal is amplified near the detector itself. Data
acquisition, based on 50MHz ADC’s is performed in a gallery, located just above the
accelerator tunnel.


The mechanical support for the dete
ctors allows an easy
modific
ations of their longitudinal

and transverse

positions depending on the
experimental needs.

The experience that we will acquire will be needed for the design of the beam loss
monitor system of the R2.1
-
TBL project.


Figure 3:
Le
ft
--

signal response follows the beam intensity. This chamber is in
vacuum, SEM
-
mode
.

Right
--

signal response for a chamber
filled with He at 1atm.
Saturation appears at the expected rate.


Upper curves are always for the chamber
response, while the low
er curve gives the beam intensity.






4.

PROPOSED PROJECT:
B
EAM INSTRUMENTATION
DEVELOPMENT
AND TEST OF PRIMARY
CONCERNS FOR THE CLI
C DRIVE BEAM AT
THE CTF3
-
TEST BEAM LINE

(R2.1
TBL

PROJECT
)

The goal
of the Test Beam Line (TBL) is
to demonstrate the feasib
ility of the CLIC
drive beam decelerators[1]

at CTF3
. The main concerns are whether or not this beam
can be operated with acceptable beam losses and sufficient beam stability.



As
already mentioned, these concerns

were classified

as a

R2 feasibility item
for
CLIC
technology in

the International Linear Collider


Technical Review Committee
(ILC
-
TRC) report [2]:



The very high power of the drive beam and its stability are serious concerns for
CLIC. The drive beam stability should be validated, and the

driv
e beam Machine
Protection

System, which is likely to be a complex system, should be designed to
p
rotect the

decelerator structures”


The TBL will be a scaled model of a CLIC drive beam
decelerator that will allow us to
test the operation procedure,
the
req
uired instrumentation and
the
feedback systems for
such a decelerator in order to guarantee the stability of the beam
.
In addition, the TBL
will be

used to benchmark the predictive power of

the

numerical simulation tool
which are used for its design.


The

potential difficulties are due to:



the very high current of beam (damage potential),



the large total energy spread (up to 90%
), and



the presenc
e of considerable transverse wakefields.


In addition, the characteristics of this beam are very different from

any other
beam

previously build
. Therefore,
this facility will also allow us

to gain experi
e
nce on the
construction and installation procedure
.

DESCRIPTION OF THE TBL

The TBL will be
20 m long

scaled model of the
CLIC drive beam decelerator sector
to be

located at the end of CLEX, see Fig. 1. Table 1 gives a comparison of the
parameters for the drive beam of the CLIC and at the TBL. As shown, the beam
energy and the beam
current are a factor of 13 and
4.3 lower compared with a CLIC
decelerator. The FOD
O period length is the same as for CLIC, but the total length is
about 30 times smaller than a CLIC decelerator sector.
















TBL tentative parameters ?comparison with CLIC decelerator

20

2.23

35

0.15


TBL

624

Total length (m)

2.23

FODO period length (m)

147

Beam current (A)

2

Beam energy GeV

CLIC





A tentative layout of the TBL is shown in Fig. 2. Here we ass
ume
that there will

7
FODO modules, which is the minim
um number
required to perform
a
well thought
-
out
tests of the drive beam tuning procedures. In addition, there will be 10
-
16 RF power
-
extracting structures (PETS).



























Each TBL module corresponds to one full FODO cell equipped with:



two quadrupoles (including
movers for steering
),



two PETS (provided by WP7 along with the

power measurement and RF
load, and corresponding monitoring),



two BPM’s,



appropriat
e monitors for beam loss.


The PETS girders and the quadrupole supports have mo
tors for independent remote
control of their transverse position. The quadrupole currents are individually
controlled and the quadrupole


strength should be sufficient for a phase advance per
FODO cell of up to 120
0

with a 300 MeV beam (the maximum beam en
ergy available
from the drive beam accelerator and combiner ring with

minimum beam current).


D

F

D

F

D

F

D

F

D

F

D

F

D

F

D

F

DUMP

DUMP

3
5 A,
150MeV

beam from

combiner
ring

Instrumentation
section


Decelerator FODO
modules


D

F


PETS

BPM


PETS


BPM

2.23
m


20 m

An instrumentation section at the end of the TBL will allow us to determined the
mean energy, the energy spread and the emittance growth of the beam after passa
ge
though the TBL. All the beam instrumentati
on needs to be capable to make a time
resolved
mea
surements with a resolution of at least 10

ns
in order to observe the build
up of the instabilities along the 140 ns long bunch train.


Sophisticated software f
or automated steering and position feedback has to be
developed in close collaboration with the team working on the CLIC decelerator
design, to mimic with the TBL the tuning and operation procedures foreseen for
CLIC
.


PURPOSE, OPERATION PRINCIPLE, AND DE
VELOPMENT REQUIRED FOR
EACH SUBDETECTOR SYSTEM

Details about the required Instrumentation at the TBL:


The instrumentation need of the TBL can be summarized as follows



Measurement of Beam energy …to 30 GHz Line



B
eam position monitors B
PM



Beam loss



Beam Ha
lo and transverse emittance



Bunch Length conservation





DUDGET….






[1] H.H. Braun et al., “The CLIC RF power source, A Novel Scheme of Two Beam



Acceleration for e
±

Linear Colliders,” CLIC note 364, 1998

[2] G. Loew et al., “International Linear Collider Technical Review Committee, 2
nd

report”, SLAC
-
R
-
606, 2003





[
i
]
M. Wood,
CTF3 note 064
, (2004)

[
ii
] T. Lefevre et al, to be published in the proceeding of the 11
th

B
eam
I
nstrumentation
W
orkshop,
Knoxville,
2004
.