LCLS-2 physics meeting - Electron Collimation

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Nov 15, 2013 (3 years and 8 months ago)

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Christoph Steier

SLAC


LCLS
-
2 accelerator physics meeting

Oct. 17, 2013

RECAP OF
NGLS

ELECTRON

COLLIMATION DESIGN STUDIES

NGLS Electron Collimation

Motivation

(Gun) Dark Current

Collimation System Layout

Injector Kicker

Energy Collimation

Betatron

Collimation

Simulation of Collimation Effectiveness

Dark Current

Touschek

Scattering

Gas Scattering

Collimator Hardware

Development Plan

Summary

Motivation


High duty factor accelerators have main beams with
considerable power (MW in our case)


Even small fractional losses can have substantial effects


Demagnetization of permanent magnet
undulators


Quenches of s/c cavities and s/c
undulators


Heat damage to vacuum envelope


Activation


Collimation system is essential


Needs to localize ‘routine’ losses away from sensitive areas


Needs to prevent equipment damage in case of malfunction
until MPS stops beam


For routine losses, experience elsewhere has shown gun dark
current to be dominant source


our calculations so far confirm
this

NGLS

Layout

APEX
-
based
e
-

injector (1 MHz,


= 0.6

m)

300 pC/bunch (0.3 mA max. current)

1.3
-
GHz
CW SRF
@ 16 MV/m (27 CM’s
)

Two bunch compressors + heater (500 A)

Beam spreader using RF deflectors (

9 FELs)

Three (initial) very diverse FEL designs

Diagnostics and collimation sections

720
-
kW
main beam
stops
(

3)

injector

linac

spreader

FELs (1
-
9)

beam stops

compressors

e
-

diagnostics

e
-

diagnostics

exp. halls

collimation

APEX: Injector
Dark current status

Use integrated Fowler
-
Nordheim
formula to fit with instantaneous
formula (E=E0*cos(ωt))

Fernando's Dark
Current measurements

Resulting current profile of dark
current

Longitudinal Phase Space at
Injector exit

Transverse Distribution of
D
ark
C
urrent in APEX

S
olenoid magnet

Single
-
cell RF cavity

Multi
-
cell
RF cavity


Laser
pulse

Gun

Buncher

Dark current “Hotspots


Butterfly shape due to large energy
spread

<750
keV
,

Identical to NGLS

>750
keV
,

Similaro

to NGLS,

Not superconducting

FLASH gun for
comparison

Dark current losses in injector

Simulated different initial distributions (spots or uniform)

After transport in the injector,

about 10% (spots) to 15% (uniform) of the dark current

survives.


C.
Papadopulos

Electron Collimation

CM01

CM2,3

CM04

CM09

CM10

CM27

BC1

215 MeV

R
56

=
-
94 mm

s
d

= 0.44%

BC2

720 MeV

R
56

=
-
76 mm

s
d

= 0.48%

GUN

0.75 MeV

Heater

94 MeV

R
56

=
-
5 mm

s
d

= 0.02%

L0

j



0

I
pk

= 47 A

s
z

= 0.85 mm

L1

j

=
-
20.0
°

I
pk

= 47 A

s
z

=
0.85
mm

Lh

j

=
180
°

V
0

= 0 MV

L2

j

=
-
23.2
°

I
pk

= 90 A

s
z

=
0.44
mm

L3

j

=
+
34.8
°

I
pk

= 500 A

s
z



0.08
mm

SPRDR

2.4 GeV

R
56

= 0

s
d



0.04%

300 pC;

2012
-
04
-
18 & 2012
-
07
-
02

3.9

CM01

CM2,3

CM04

CM09

CM10

CM27

BC1

215 MeV

s
d

= 0.44%

BC2

720 MeV

s
d

= 0.48%

GUN

0.75 MeV

Heater

94 MeV

s
d

= 0.02%

L0

I
pk

= 47 A

L1,
Lh

I
pk

= 47 A

L2

j

=
-
23.2
°

I
pk

= 90 A

L3

j

=
+
34.8
°

I
pk

= 500 A

SPRDR

2.4 GeV

s
d



0.04%

3.9

Energy

Collimator


1.5
mm


Energy

Coll.


15 mm

Energy

Coll.


8

mm

Energy

Coll.


2.5
mm

Dark

Current

Kicker



10

4

-
tron

Coll.’s


1
0

mm (
x
)


㈠浭 (
y
)


A
ssumed apertures for machine


+/
-

18 mm radius pipe almost everywhere


No restriction (except collimator) in LH, BC1/2, FEL chicanes


Undulator

chamber +/
-

15 mm (x), +/
-

3 mm (y)

Motivation for Dark Current
Kicker


First dispersive place to collimate
-

laser heater chicane (100
MeV)


15% of 8

A at 100 MeV corresponds to 120 W


Anything not captured there quickly gains more energy
towards bunch compressor


FLASH stays below 100 W losses in bunch compressor due
to radiological concerns


Coordinating with EHS


cost implications for shielding


Some of the other 85% is lost in injector
cryo
-
module


XFEL guidance is <0.1W/m to avoid cavity quenches,
simulation shows about 1 W/m for uniform emission case


<0.1 W/m for other more realistic distributions



Gaining factor 10 safety margin necessary


Dark Current
Kicker

Dark Current Deflector


Dark current produced in every injector RF
bucket (186 MHz)


useful beam only 1 MHz


FLASH kicker reduces dark
current intensity
by factor
of
>3


NGLS:


kick main bunches and compensate with DC
magnet


high
repetition rate (1 MHz) and
fast
rise
and fall
times


Just
after the
gun (0.75 MeV).


Reference: ALS camshaft kicker (1.5
MHz,
rise/fall
times of 2
0 ns, >70

rad@1.9
GeV
)


Simulations: scaled
version of
ALS kicker
could reduce
by factor
of >
10

FLASH: F.
Obier

ALS: S. Kwiatkowski

Dark Current Kicker Simulation

Gun

Buncher

Cryomodule

>750
keV
, Cold

<750
keV
, Warm

Plan to collimate in this region

collimator

kicker

Need to collimate kicked beam without

scraping main beam:

Collimator R=10 mm


Condition

Dark current (
μ
A)

@ injector exit

15 mm
ap.
across the inj
.

(
ie

doing nothing)

1.228

10
mrad

Kicker

0.695

10
mrad

Kicker

+ 10 mm
coll
.

0.345

15
mrad

kicker

+ 10 mm
coll
.

0.057

ALS kicker kicks >70

rad

at 1.9
G
eV

-

180
mrad

at 750
keV

Factor 2 shorter, factor 3 larger opening, about 30
mrad

possible

Dark Current Deflector


Dark current produced in every injector RF bucket (186 MHz)


useful
beam only 1 MHz


FLASH kicker reduces dark
current intensity
by factor
of
>3


NGLS:


kick main bunches and compensate with DC magnet


high
repetition rate (1 MHz) and
fast
rise and fall
times


Just
after the
gun (0.75 MeV).


Reference: ALS camshaft kicker (1.5
MHz,
rise/fall
times of 2
0 ns, >70

rad@1.9
GeV
)


Simulations: scaled
version of
ALS kicker
could reduce
by factor
of >
10

New shape

153 mm

54 mm

21 mm

64 mm

H.
Qian
, S. de
Santis
, S.
Kwiatkowski

Electron Collimation

CM01

CM2,3

CM04

CM09

CM10

CM27

BC1

215 MeV

R
56

=
-
94 mm

s
d

= 0.44%

BC2

720 MeV

R
56

=
-
76 mm

s
d

= 0.48%

GUN

0.75 MeV

Heater

94 MeV

R
56

=
-
5 mm

s
d

= 0.02%

L0

j



0

I
pk

= 47 A

s
z

= 0.85 mm

L1

j

=
-
20.0
°

I
pk

= 47 A

s
z

=
0.85
mm

Lh

j

=
180
°

V
0

= 0 MV

L2

j

=
-
23.2
°

I
pk

= 90 A

s
z

=
0.44
mm

L3

j

=
+
34.8
°

I
pk

= 500 A

s
z



0.08
mm

SPRDR

2.4 GeV

R
56

= 0

s
d



0.04%

300 pC;

2012
-
04
-
18 & 2012
-
07
-
02

3.9

CM01

CM2,3

CM04

CM09

CM10

CM27

BC1

215 MeV

s
d

= 0.44%

BC2

720 MeV

s
d

= 0.48%

GUN

0.75 MeV

Heater

94 MeV

s
d

= 0.02%

L0

I
pk

= 47 A

L1,
Lh

I
pk

= 47 A

L2

j

=
-
23.2
°

I
pk

= 90 A

L3

j

=
+
34.8
°

I
pk

= 500 A

SPRDR

2.4 GeV

s
d



0.04%

3.9

Energy

Collimator


1.5
mm


Energy

Coll.


15 mm

Energy

Coll.


8

mm

Energy

Coll.


2.5
mm

Dark

Current

Kicker



10

4

-
tron

Coll.’s


1
0

mm (
x
)


㈠浭 (
y
)


A
ssumed apertures for machine


+/
-

18 mm radius pipe almost everywhere


No restriction (except collimator) in LH, BC1/2, FEL chicanes


Undulator

chamber +/
-

15 mm (x), +/
-

3 mm (y)

Location of Collimators (LHS, BC1)


Based on low impedance version of ALS collimators (as
well as other places) 50 cm is reasonable length for
collimators


With safety margin for finalized mechanical design
(impedance calculation)


desirable to reserve 1 m


Enough space available in BC1, working to increase space
in LH, downstream of
undulator

Location of Collimators (BC2, MCS)


Enough space seems available in BC2


Generous space available in FODO section after main
LINAC (MCS, which is just after L3S)


MCS, SLS collimation section of NGLS design much more
compact than XFEL


No requirement to transport energy chirped
bunchtrains


No need for very high beta functions (
bunchtrain

power)


No separate need for R56 variability, …


Spreader angle and
achromats

in SLS provide natural place for energy collimation
with secondary showers kept away from
undulators


Location of Collimators (
SLSx
)


Current simulations are based on spreader lattice from October


Baseline change to RF spreader since then


General
achromat

layout and space similar


current collimator layout should work


will verify


Space at first collimator OK, at second one a little tight.


Beta functions at second collimator very small


better spaces later
in arc (need trade
-
off analysis of required MPS speed vs.
secondaries

escape rate)

Technical Details of Tracking


Started from CDR MAD file (
sharepoint
)


Translate (automated) with mad2elegant (does not accept matching
routines, but bare lattice)


Needed
to remove all CSR (just turning switch off is not enough)


otherwise dark current gets lost in first CSR element


Translate (automated) with mad2at


Added
beamline

apertures (see before) and collimators to resulting files


Will slowly add all apertures/collimators to baseline MAD files


Imported ASTRA distributions (astra2elegant,
Matlab
)



Need to carefully consider phase matching between different
distributions, energy scaling, …


Important to use elegant
fiducialization

correctly


In elegant always need to track two bunches (
fiducialization

reference +
dark current)


Tracked CDR beam (and
gaussian

approximation of it) to determine
collimator settings


No loss of nominal beam (or 6 sigma particles) + 10
-
20%

Collimator Location

+ Setting


LHEATCOL


|x|<1.5 mm


BC1COL


|x|<15 mm


BC2COL


|x|<8 mm


CXL3ED_1


|x|<10 mm


CXL3ED_2


|x|<10 mm


CYL3ED_1


|y|<2 mm


CYL3ED_2


|y|<2 mm


SPREADCOL1


|x|<2.5 mm


SPREADCOL2


|x|<5 mm


Tracking Gun Dark Current

Dark Current losses well controlled


Most losses on Laser Heater Collimator


Followed by BC1 and BC2


Remaining losses in warm section around laser heater


L
osses in
Linac

1 below XFEL quench
criterium

of 0.1 W/m


Dark current kicker will help


No losses beyond BC2 (and in
undulator
)

Trajectories, Loss Power

Power densities [W/m] on right are for 8
A

dark current from gun:


10
-
100 W on collimators


Likely need for reduction
(deflector)


Up to 1 W/m around laser heater


Would like to reduce


10s
mW
/m in Linac1


Tesla used threshold 10
mJ
/cm
3

over 20
ms

for 25 MeV/m


extrapolating their shower
calculations this is safe by factor of >10

Removing collimators (start to end)


When removing collimators earlier in accelerator,
undulators

remain
protected from dark current (until very last energy collimator is
pulled)


Of course,
Linac

does not and loss power gets much higher (because
collimation does not occur at lowest possible energy)


Encouraging with regards to protection from
Touschek+Gas

Scattering in
Linac+Spreader

Post
Linac

Collimation (Gas
Scattering)


Test of post
linac

collimation by artificially increasing (20
-
50x)
divergence of beam at points along the
linac



In vertical plane, combination of two (90 degree apart) collimators
and energy collimators protects
undulators



Rough estimate of pressure requirements on next slide, plan to
quantify further with
monte

carlo

and tracking of scattered
p
articles

Estimate of gas scattering loss rates


For electrons one can simplify the formulas for gas Bremsstrahlung
lifetime (in the approximation of <
Z
2
> ~ 50):





In the same approximation, the elastic gas scattering lifetime
becomes:



For NGLS:


Assume 1% energy acceptance (logarithmic dependence)


relative losses of 10
-
9

for 100
nTorr

due to inelastic scattering over
full length


Assuming 7mm ID vacuum chamber


relative losses of 10
-
8

for
100
nTorr

due to inelastic
scattering


1
-
10
mW

for nominal beam power
(ALS total
beamloss

power about
30
mW
)


No concern




Post
Linac

Collimation (Gas,
Touschek

Scattering)


Test of post
linac

collimation by artificially increasing (20x)
energy spread of beam at points along the
linac



For energy error originating within LINAC (inelastic gas or
Touschek

scattering), very small
betatron

amplitudes


First momentum collimator in spreader effectively removes
scattered beam


very small amplitudes in
undulator

Touschek

losses


In Rings

-

Bruck’s

formula for
Touschek

lifetime


valid for flat beam


Only complicated part is to calculate
momentum aperture/acceptance


For NGLS with its round beams and
changing energy not sufficient


Multiple approaches: Monte
-
Carlo, …


We are using approach used by
Xiao/Borland for APS
-
ERL studies:
Based on analytic
Piwinski

formula:




Still needs calculation of momentum
acceptance


because of tight
collimator settings (dark current),
acceptance is pretty small in parts of
line.

Above: APS
-
ERL example


dependence of
Touschek

loss
-
rate in full
energy arcs
o
n Momentum Aperture

B
elow: Momentum Aperture of NGLS
with baseline collimation.

Touschek

losses (2)


Scattering rate based on
analytic
Piwinski

formula:




Scattering rates with NGLS
momentum acceptance +
design beam parameters:


Integrating local scattering
probability leading to loss
on a collimator of up to few
10
-
6
(<10 W on spreader
collimator)


Acceptable


Verified calculation on ALS
example


agree well with
measured lifetimes


Collimator Design


Main issues that determine space requirements
for each collimator (necessary for CDR):


Heat load / beam power / power density


<=1
ms

MPS
-
> similar to 3
rd

generation
light sources (kJ)


consistent with XFEL
scaling


Impedance heating
-
> similar to rings


Wake fields, effect on beam:


Need to not spoil beam quality


Radiation showers, secondary particle
transport, activation:


Use of collimator pairs where possible


Considered for local shielding and tunnel
wall thickness

XFEL collimator damage


In XFEL design collimator damage sets requirements for large
beta functions, one driver for length of collimation section
(energy acceptance, R56
tunability
, fixed (set of) collimator
apertures …)

Scaling of XFEL considerations to NGLS


Our assumption is 1
ms

MPS, i.e. 1000 bunches


XFEL was 80


90 bunches


We assume 0.3
nC
, XFEL is 1
nC


Gun (750
keV
)


No concern, low power, very large beam


LH, BC


Beam is enlarged a lot due to dispersion


Post LINAC


2.4
GeV

vs. 20
GeV



total deposited energy is factor 2.2 higher in
XFEL


but shower is deeper


Normalized
emittance

(0.6
vs

1.4 mm
mrad
)


absolute
emittance

is factor 3.6 larger in NGLS


NGLS beta functions at collimators factor 10 below XFEL


Potentially worse in spreader


Overall seems similar
-
> Need detailed quantitative analysis


But faster MPS response possible (desirable?), i.e. current
solution is feasible

Protector absorbers between
cryomodules


At CD
-
0 design had distributed collimators along length of
LINAC and large beta functions to make them effective


Based on tracking of gun dark current and gas/
Touschek

scattering estimates we do not believe we need those


It was proposed (by reviewers) that local fixed absorbers might
be a good idea to localize most of losses (for fault conditions
like
quadrupole

PS trip, …) away from cavities


Also provides well defined spots for where to place discrete,
fast loss monitors for MPS


Marco incorporated those in new layout


However, looking at geometry in more detail, they naturally
appear just downstream of
cryomodule

(70
-
>35 mm)


Still need to verify that location is appropriate and consider
potential impact for designing transition

500

A,

0
.
6

um,

150

keV,

10

m

beta,

2
.
4

GeV,

3
.
3

m

segment,

4
.
4

m

break,

self
-
seeded

(
L
u
x
1
.
5
),

25
%

safety

factor

on

length

(
L
u
x
1
.
5
x
1
.
25
),

60
-
um

Nb
3
Sn

SCU

insulator

at

80
%

(
0
.
48

mm

diam
.
)

c
hamber gap is 2 mm less than magnetic gap

7.5 mm

6
.0 mm

Note that
10
-
mm gap
(XFEL) is
only ~
20 m
longer!

Smaller Magnetic Gap and Impact on
Undulator

Length

(Emma)

Self
-
seeded undulator with breaks, etc

Estimate of gas scattering loss rates


For electrons one can simplify the formulas for gas Bremsstrahlung
lifetime (in the approximation of <
Z
2
> ~ 50):





In the same approximation, the elastic gas scattering lifetime
becomes:



For NGLS:


Assume 1% energy acceptance (logarithmic dependence)


relative losses of 10
-
8

for 100
nTorr

due to inelastic scattering over
full length


Assuming
4
mm ID vacuum chamber


relative losses of 10
-
8

for
100
nTorr

due to inelastic
scattering


<
2
0
mW

for nominal beam power
(ALS total
beamloss

power about
30
mW
)


Still
no concern




Effect of smaller
undulator

gap on
darkcurrent

collimation


Smaller
undulator

gap means vertical collimation is necessary in
addition to energy
collimamation


Reducing YCOL from +/
-
2 mm to +/
-

1 mm is sufficient


Losses on YCOL get much bigger


too high ?


Also tighter tolerances on orbit, collimator position, …
-

probably OK




To do list
+

work in progress


Further characterize transverse dark current
distribution from APEX.
Refine models. Study how to reduce dark current and what final level
might be achievable
.


Study
secondary particles, escaped particles after the collimators.
Continue
s
tudy of sensitivity
to lattice errors, changes in initial
distribution, collimator misplacements,



Do trade
-
off study between cost for shielding/mitigation of activation
and complexity and operational impact of collimation system


Carry out tracking of scattered
particles (Monte Carlo of
gas/
Touschek
).
Potentially benchmark calculations with FLASH
measurements
.


Finish Collimator hardware reference design


Shower simulations, detailed thermal
simulations
.


calculate
short and long range
wakefields
.

Differences NGLS vs. LCLS
-
2

CM01

CM2,3

CM04

CM09

CM10

CM27

BC1

215 MeV

R
56

=
-
94 mm

s
d

= 0.44%

BC2

720 MeV

R
56

=
-
76 mm

s
d

= 0.48%

GUN

0.75 MeV

Heater

94 MeV

R
56

=
-
5 mm

s
d

= 0.02%

L0

j



0

I
pk

= 47 A

s
z

= 0.85 mm

L1

j

=
-
20.0
°

I
pk

= 47 A

s
z

=
0.85
mm

Lh

j

=
180
°

V
0

= 0 MV

L2

j

=
-
23.2
°

I
pk

= 90 A

s
z

=
0.44
mm

L3

j

=
+
34.8
°

I
pk

= 500 A

s
z



0.08
mm

SPRDR

2.4 GeV

R
56

= 0

s
d



0.04%

300 pC;

2012
-
04
-
18 & 2012
-
07
-
02

3.9

CM01

CM2,3

CM04

CM09

CM10

CM27

BC1

215 MeV

s
d

= 0.44%

BC2

720 MeV

s
d

= 0.48%

GUN

0.75 MeV

Heater

94 MeV

s
d

= 0.02%

L0

I
pk

= 47 A

L1,
Lh

I
pk

= 47 A

L2

j

=
-
23.2
°

I
pk

= 90 A

L3

j

=
+
34.8
°

I
pk

= 500 A

SPRDR

2.4 GeV

s
d



0.04%

3.9

Energy

Collimator


1.5
mm


Energy

Coll.


15 mm

Energy

Coll.


8

mm

Energy

Coll.


2.5
mm

Dark

Current

Kicker



10

4

-
tron

Coll.’s


1
0

mm (
x
)


2 浭m(
y
)

CM01

CM2,3

CM04

CM15

CM16

CM35

BC1

E

= 250 MeV

R
56

=
-
55 mm

s
d

= 1.4 %

BC2

E

= 1600 MeV

R
56

=
-
60 mm

s
d

= 0.46 %

GUN

0.75 MeV

LH

E

= 95 MeV

R
56

=
-
14.5 mm

s
d

= 0.05 %

L0

j

=

*

V
0

=
94 MV

I
pk

= 12 A

L
b

= 2.0 mm

L1

j

=
-
21
°

V
0

=223
MV

I
pk

= 12 A

L
b

=2.0 mm

HL

j

=
-
165
°

V
0

=55 MV

L2

j

=
-
21
°

V
0

=1447
MV

I
pk

= 50 A

L
b

= 0.56 mm

L3

j

= 0

V
0

=2409
MV

I
pk

= 1.0 kA

L
b

= 0.024 mm

LTU

E

= 4.0 GeV

R
56

= 0

s
d



0.016%

2
-
km

100
-
pC

machine layout: Oct. 8, 2013; v21 ASTRA run; Bunch length
L
b
is FWHM

3.9GHz

Summary


Have completed tracking with energy +
betatron

collimators in
CDR lattice


Energy collimators sufficient to protect superconducting
cavities +
undulators

from gun dark current


Dark current kicker appears necessary to minimize activation
of collimators and protect injector s/c cavities


B
etatron

collimation (and post
linac

energy collimation)
effective in stopping
Touschek+Gas

scattered particles
before
undulators


Space requirements for collimators are
w
orkable within
current layout


No apparent show
-
stoppers remain for CD
-
1, will finish work in
progress


main area is detailed collimator hardware design


Thanks to Hiroshi Nishimura
, Christos
Papadopoulos, Fernando
Sannibale
, et al.


Backup Slides

ΔE = 13.5 MV/m

State of the art

25 MV/m BCP cavities

State of the art

35 MV/m EP cavities

Elegant Tracking

AT / Elegant comparison


At first, results did not agree at all … Doubted my AT
modifications


However, reason turned out to be intricacies of how elegant
tracks (
fiducialization
, no reference particle) and how data
from
astra

was transferred


Now good agreement


small remaining discrepancies are
different modeling of apertures, small differences in import of
large energy offset coordinates from ASTRA


JH Scrapers

Sector 1

JH Scrapers

Sector 3


ALS Routine
Stored Beam Losses


New scrapers
localize losses
away from
beamline
source points
and undulators


Installed+work
very well

DESY


FLASH / XFEL