RECAP OF NGLS ELECTRON
COLLIMATION DESIGN STUDIES
Christoph Steier
SLAC – LCLS-2 accelerator physics meeting
Oct. 17, 2013
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
e- diagnostics
compressors
collimation
e- diagnostics
FELs (1-9)
injector
linac
spreader
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)
beam stops
exp. halls
APEX: Injector Dark current status
Fernando's Dark
Current measurements
Resulting current profile of dark
current
Use integrated Fowler -Nordheim
formula to fit with instantaneous
formula (E=E0*cos(ωt))
Longitudinal Phase Space at
Injector exit
Transverse Distribution of Dark
Current in APEX
>750 keV,
Similaro to NGLS,
Not superconducting
Buncher
<750 keV,
Identical to NGLS
Gun
Solenoid magnet
Single-cell RF cavity
Multi-cell RF cavity
Laser
pulse
FLASH gun for
Dark
current “Hotspots”
comparison
Butterfly shape due to large energy
spread
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
Lh
L1
L0
L1, Lh
L0
L2L2
L3 L3
j = -20.0°
 0A
= -23.2°
= +34.8°
Ipk = 47 Aj = 180°
Ipk =j 47
j =j -23.2°
j =j+34.8°
Ipk = 47 A V0 = 0 MV
Ipk = 47 A
I
=
90
A
I
= 500
Ipk =
Ipk =pk500
A A
pk 90 A
sz = 0.85 mm
s = 0.85 mm
sz = 0.44 mm BC2
sz  0.08 mm
Heater
BC1
GUN z
SPRDR
94
MeV
215
MeV
720
MeV
0.75 MeV
3.9
CM01
CM09
CM272.4 GeV
sd = 0.02%CM2,3
sd = 0.44% CM04
sd = 0.48% CM10
sd  0.04%
BC1
BC2
SPRDR
Heater
GUN
215
MeV
720
MeV
2.4
GeV
3.9
CM01 94 MeVCM2,3
CM09
CM27
CM04
CM10
0.75 MeV
R56 = -94 mm
R56 = -76 mm
R56 = 0
R56 = -5 mm
s
=
0.44%
s
=
0.48%
s
sd = 0.02%
d
d
d  0.04%
300 pC; 2012-04-18 & 2012-07-02
Dark
Current
Kicker
 10
Energy
Collimator
1.5 mm
Energy
Coll.
15 mm
Energy
Coll.
8 mm
4 -tron
Coll.’s
10 mm (x)
2 mm (y)
Energy
Coll.
2.5 mm
 Assumed 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
FLASH: F. Obier
 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 20 ns, >70 rad@1.9 GeV)
 Simulations: scaled version of ALS kicker
could reduce by factor of >10
ALS: S. Kwiatkowski
Dark Current Kicker Simulation
<750 keV, Warm
kicker
>750 keV, Cold
Buncher
collimator
Cryomodule
Gun
Plan to collimate in this region
Need to collimate kicked beam without
scraping main beam: Collimator R=10 mm
Condition
Dark current (μA)
@ injector exit
15 mm ap. across the inj.
(ie doing nothing)
1.228
10 mrad Kicker
10 mrad Kicker + 10 mm
coll.
15 mrad kicker + 10 mm
coll.
ALS kicker kicks >70 rad at 1.9 GeV - 180 mrad at 750 keV
Factor 2 shorter, factor 3 larger opening, about 30 mrad possible
0.695
0.345
0.057
Dark Current Deflector
H. Qian, S. de Santis, S.
Kwiatkowski
New shape
54 mm
64 mm
21 mm
153 mm
 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 20 ns, >70
rad@1.9 GeV)
 Simulations: scaled version of ALS kicker could reduce by factor of >10
Electron Collimation
Lh
L1
L0
L1, Lh
L0
L2L2
L3 L3
j = -20.0°
 0A
= -23.2°
= +34.8°
Ipk = 47 Aj = 180°
Ipk =j 47
j =j -23.2°
j =j+34.8°
Ipk = 47 A V0 = 0 MV
Ipk = 47 A
I
=
90
A
I
= 500
Ipk =
Ipk =pk500
A A
pk 90 A
sz = 0.85 mm
s = 0.85 mm
sz = 0.44 mm BC2
sz  0.08 mm
Heater
BC1
GUN z
SPRDR
94
MeV
215
MeV
720
MeV
0.75 MeV
3.9
CM01
CM09
CM272.4 GeV
sd = 0.02%CM2,3
sd = 0.44% CM04
sd = 0.48% CM10
sd  0.04%
BC1
BC2
SPRDR
Heater
GUN
215
MeV
720
MeV
2.4
GeV
3.9
CM01 94 MeVCM2,3
CM09
CM27
CM04
CM10
0.75 MeV
R56 = -94 mm
R56 = -76 mm
R56 = 0
R56 = -5 mm
s
=
0.44%
s
=
0.48%
s
sd = 0.02%
d
d
d  0.04%
300 pC; 2012-04-18 & 2012-07-02
Dark
Current
Kicker
 10
Energy
Collimator
1.5 mm
Energy
Coll.
15 mm
Energy
Coll.
8 mm
4 -tron
Coll.’s
10 mm (x)
2 mm (y)
Energy
Coll.
2.5 mm
 Assumed 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
• Losses 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
•
•
Up to 1 W/m around laser heater
•
•
Likely need for reduction (deflector)
Would like to reduce
10s mW/m in Linac1
•
Tesla used threshold 10 mJ/cm3 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 particles
Estimate of gas scattering loss rates
• For electrons one can simplify the formulas for gas Bremsstrahlung
lifetime (in the approximation of <Z2> ~ 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 on Momentum Aperture
Below: 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 3rd 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
Self-seeded undulator with breaks, etc
Smaller Magnetic Gap and Impact on Undulator Length
(Emma)
Note that
10-mm gap
(XFEL) is
only ~20 m
longer!
7.5 mm
6.0 mm
chamber gap is 2 mm less than magnetic gap
500 A, 0.6 um, 150 keV, 10 m beta, 2.4 GeV, 3.3 m segment, 4.4 m break, self-seeded (Lux1.5), 25%
safety factor on length (Lux1.5x1.25), 60-um Nb3Sn SCU insulator at 80% (0.48 mm diam.)
Estimate of gas scattering loss rates
• For electrons one can simplify the formulas for gas Bremsstrahlung
lifetime (in the approximation of <Z2> ~ 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 4mm ID vacuum chamber relative losses of 10-8 for 100
nTorr due to inelastic scattering
• <20 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 study 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
Lh
L1
L0
L1, Lh
L0
L2L2
L3 L3
j
=
180°
j
=
-20.0°
 0A
= -23.2°
= +34.8°
Ipk = 47 A
Ipk =j 47
j =j -23.2°
j =j+34.8°
Ipk = 47 A V0 = 0 MV
Ipk = 47 A
=
90
A
I
Ipk I=
90
A
I
=
500
A A
pk
pk pk = 500
s
=
0.85
mm
s = 0.85 mm
sz = 0.44 mm BC2
sz  0.08 mm
Heaterz
BC1
GUN z
SPRDR
94
MeV
215
MeV
720
MeV
0.75 MeV
3.9
CM01
CM09
CM272.4 GeV
sd = 0.02%CM2,3
sd = 0.44% CM04
sd = 0.48% CM10
sd  0.04%
BC1
BC2
SPRDR
Heater
GUN
215
MeV
720
MeV
2.4
GeV
94
MeV
3.9
CM01
CM2,3
CM09
CM27
CM04
CM10
0.75 MeV
R56 = -94 mm
R56 = -76 mm
R56 = 0
R56 = -5 mm
sd = 0.44%
sd = 0.48%
sd  0.04%
sd = 0.02%
300 pC; 2012-04-18 & 2012-07-02
Dark
Current
Kicker
 10
Energy
Collimator
1.5 mm
L0
j=*
V0 =94 MV
Ipk = 12 A
Lb = 2.0 mm
CM01
Energy
Coll.
15 mm
L2
j = -21°
V0 =1447 MV
Ipk = 50 A
Lb = 0.56 mm
L1
j =-21°
HL
V0 =223 MV
j
=-165°
Ipk = 12 A
Lb =2.0 mm V0 =55 MV
CM2,3
LH
GUN
E = 95 MeV
0.75 MeV R = -14.5 mm
56
sd = 0.05 %
3.9GHz
4 -tron
Coll.’s
10 mm (x)
2 mm (y)
Energy
Coll.
8 mm
CM04
BC1
E = 250 MeV
R56 = -55 mm
sd = 1.4 %
Energy
Coll.
2.5 mm
L3
j=0
V0 =2409 MV
Ipk = 1.0 kA
Lb = 0.024 mm
CM15
CM16
BC2
E = 1600 MeV
R56 = -60 mm
sd = 0.46 %
100-pC machine layout: Oct. 8, 2013; v21 ASTRA run; Bunch length Lb is FWHM
CM35
LTU
E = 4.0 GeV
R56 = 0
sd  0.016%
2-km
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
 Betatron collimation (and post linac energy collimation)
effective in stopping Touschek+Gas scattered particles
before undulators
 Space requirements for collimators are workable 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
State of the art
25 MV/m BCP cavities
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
ALS Routine Stored Beam Losses
• New scrapers
localize losses
away from
beamline
source points
and undulators
• Installed+work
very well
JH Scrapers
Sector 1
JH Scrapers
Sector 3
DESY – FLASH / XFEL