dark current, beam loss, and collimation in the lcls

Juhao WuJuhao Wu

LCLS FACLCLS FAC [email protected]@SLAC.Stanford.edu7 Apr. 20057 Apr. 2005

Dark Current, Beam Loss, Dark Current, Beam Loss, and Collimation in the and Collimation in the LCLSLCLSJ. Wu ,J. Wu , D. Dowell, P. Emma, C. Limborg, J. Schmerge, H. VinckeD. Dowell, P. Emma, C. Limborg, J. Schmerge, H. Vincke

LCLSLCLS FAC Meeting FAC Meeting April 7, 2005April 7, 2005

Thanks to M. Borland for Thanks to M. Borland for ElegantElegant code changes in support of these studies code changes in support of these studies

LCLSLCLS

http://www-ssrl.slac.stanford.edu/

Juhao WuJuhao Wu


Model dark current from cathode using Model dark current from cathode using Fowler-NordheimFowler-Nordheim and and ParmelaParmela, but scaling charge from , but scaling charge from GTFGTF measurements measurementsAdd dark current in critical RF structures along linac, Add dark current in critical RF structures along linac, based on K. Bane work in NLC (not significant)based on K. Bane work in NLC (not significant)Track dark current through entire linac up to and through Track dark current through entire linac up to and through undulator, using symplectic integration for every bend and undulator, using symplectic integration for every bend and quadrupole in quadrupole in ElegantElegant (M. Borland, (M. Borland, ANLANL))Include aperture restrictions and collimatorsInclude aperture restrictions and collimatorsAssess collimation scheme in terms of undulator Assess collimation scheme in terms of undulator protection and average power loss on each collimatorprotection and average power loss on each collimatorEvaluate wakefield effect of each collimatorEvaluate wakefield effect of each collimator

Description of the StudyDescription of the Study


Juhao WuJuhao Wu


‘‘Fowler-Nordheim’ on CathodeFowler-Nordheim’ on Cathode

J. SchmergeJ. Schmerge

J. WangJ. Wang


Juhao WuJuhao Wu


Longitudinal Distribution after ‘L0-a’Longitudinal Distribution after ‘L0-a’

headhead

dump next dump next bucket into bucket into main onemain one

nominal laser pulsenominal laser pulse

Only 19160 particle Only 19160 particle remain after “L0-a” remain after “L0-a” RF section (6% or RF section (6% or

200 pC/pulse)200 pC/pulse)

(3 nC)(19160/300000) 200 pC/pulse at L0-b entranceGTFGTF measurements: measurements:• 3 nC maximum (3 nC maximum (E = E =

120 MV/m120 MV/m)) over 1-over 1-sec RF pulse sec RF pulse (3000 buckets) at (3000 buckets) at gun exitgun exit

ParmelaParmela Results: Results:• 5-mm cathode 5-mm cathode

radius for max. radius for max. transmission (worst transmission (worst case)case)

• ~75% transmission ~75% transmission through gun: through gun: (400000 (400000 300000 300000 particles)particles)

• 3 nC 3 nC 300000 300000 macro-particlesmacro-particles

run08_5mm_eth06_el117_400k.datrun08_5mm_eth06_el117_400k.dat(C. Limborg: Jan. 7, 2005)(C. Limborg: Jan. 7, 2005)

RF crestRF crest

360 360 ºº


Juhao WuJuhao Wu


Choosing cathode radius for dark current productionChoosing cathode radius for dark current production

particles particles surviving surviving

after ‘L0-a’after ‘L0-a’

all all particles particles

at cathodeat cathode

use +5 mm radius use +5 mm radius for dark current for dark current

production (better production (better statistics)statistics)

C. LimborgC. Limborg


Juhao WuJuhao Wu


Transverse Phase Space of Dark CurrentTransverse Phase Space of Dark Current

dump next bucket into main onedump next bucket into main one run08_5mm_eth06_el117_400k.datrun08_5mm_eth06_el117_400k.dat

shift phase so shift phase so that that z z = 0= 0 is is photo-beam photo-beam nominal phasenominal phase

at “L0-a” exitat “L0-a” exit


Juhao WuJuhao Wu


Structure dark currentStructure dark currentCriticalCritical RF structures: RF structures:• L0b (L0b (E=23.8 MV/mE=23.8 MV/m); X1_Xband (); X1_Xband (E=31.7 MV/mE=31.7 MV/m);); L2_10_50 (L2_10_50 (E=23.0 E=23.0

MV/mMV/m);); and L3_10_50 (L3_10_50 (E=23.6 MV/mE=23.6 MV/m);); • Quads deflect dark current effectivelyQuads deflect dark current effectively


Juhao WuJuhao Wu


Structure dark currentStructure dark currentStudy approach:Study approach:• Use Use MafiaMafia to get field map to get field map• Use Use MathematicaMathematica (K. Bane’s code) to track through 3-m structure (K. Bane’s code) to track through 3-m structure • Normalized according to measurement: Normalized according to measurement: 15 pC in 2 15 pC in 2 s pulse for 3 meter s pulse for 3 meter

structure at 26 MV/mstructure at 26 MV/m (J. Schmerge) --- fit (J. Schmerge) --- fit ~ 120 ~ 120, and , and AAe e ~ 350 ~ 350 mm22

• Most capture in down streamMost capture in down stream

Examples of K. Bane’s study for X-band. We then compute for S-band and X-band


Juhao WuJuhao Wu


Structure dark currentStructure dark currentContribution of structure dark current:Contribution of structure dark current:• X-bandX-band gives the largest contribution, however, deflected gives the largest contribution, however, deflected• Structures withStructures with E~24 MV/mE~24 MV/m will give additional particle loss will give additional particle loss

Green: difference

Black: total

Red: Gun DC only


Juhao WuJuhao Wu


Tracking and CollimationTracking and Collimation

undulatorundulator

‘‘L0-b’L0-b’startstart

existing existing collimatorscollimators

(4 (4 xx and 4 and 4 yy))

new energy new energy collimatorscollimators

new new collimatorscollimatorsBC1 coll.BC1 coll. BC2 coll.BC2 coll.

‘‘underunderground’ground’


Juhao WuJuhao Wu


2-Phase, 2-Plane Und. Collimation, 1½ Times2-Phase, 2-Plane Und. Collimation, 1½ Times

xx11 xx22 xx33

phase-1phase-1 phase-2phase-2 phase-1 phase-1 againagain

halohalo

7070((2.5 mm)2.5 mm)4040

((2.2 mm)2.2 mm)

undulator undulator beam pipebeam pipe

4545

edge edge scatteringscattering

(also collimation in (also collimation in yy and energy – see next slides) and energy – see next slides)

ee beam beam

4040((2.2 mm)2.2 mm)


Juhao WuJuhao Wu


Collimation in Linac-To-Undulator (LTU)Collimation in Linac-To-Undulator (LTU)EE11 EE22

yy11

xx22

yy22

xx33

yy33

muon muon shieldingshielding

undulatorundulator

xx11

--spoilerspoiler


Juhao WuJuhao Wu


Particle losses up to, and through BC1Particle losses up to, and through BC1

BC1BC1DL1DL1L0-bL0-b L1L1 X-bandX-band

1-inch ID1-inch ID 7-mm ID7-mm ID

120 pC lost per pulse120 pC lost per pulse= 1.9 W @ 120 Hz, 135 MeV= 1.9 W @ 120 Hz, 135 MeV

300 pC lost per pulse300 pC lost per pulse= 9 W @ 120 Hz, 250 MeV= 9 W @ 120 Hz, 250 MeV


Juhao WuJuhao Wu


Particle losses through undulator and dumpParticle losses through undulator and dump

BC2BC2 undulatorundulator

2.6 pC/pulse2.6 pC/pulse3.5 W (120 Hz,3.5 W (120 Hz,

11.3 GeV)11.3 GeV)

4 4 existingexisting xx-coll.’s-coll.’s4 4 existingexisting yy-coll.’s-coll.’s1.6 & 1.6 & 1.8 mm1.8 mm

2 2 newnew EE-coll-coll..22..5 mm (5 mm ( = = 2%)2%)

3 3 newnew xx-coll.’s-coll.’s3 3 newnew yy-coll.’s-coll.’s2.2 mm…2.2 mm…

1 1 newnew BC2BC2 EE-coll-coll..36-mm (36-mm ( = = 10%)10%)

1 1 newnew BC1BC1 EE-coll-coll..45-mm (45-mm ( = = 20%)20%)

0.1 pC/pulse0.1 pC/pulse0.2 W (120 Hz,0.2 W (120 Hz,13.6 GeV)13.6 GeV)

BC1BC1

0.7 pC/pulse0.7 pC/pulse1.1 W (120 Hz,1.1 W (120 Hz,

13.6 GeV)13.6 GeV)

underundergroundground

EE//EE of 1 dropped klystron = of 1 dropped klystron = 1.7%1.7%


Juhao WuJuhao Wu


Undulator Protection (1)Undulator Protection (1)

undulator undulator vacuum vacuum chamberchamber(at start (at start of und.)of und.)


Juhao WuJuhao Wu


Undulator Protection (2)Undulator Protection (2)

undulator lengthundulator length

(undulator aperture limit)(undulator aperture limit)

maximum particle extentmaximum particle extent


Juhao WuJuhao Wu


Transverse Wakefield Alignment TolerancesTransverse Wakefield Alignment Tolerances

NN = 6.25 = 6.25101099

NN = 1.2 = 1.2 mm

aabb zz << << aa

xx

[4][4]

longitudinal wakes also longitudinal wakes also checked (no problem)checked (no problem)0.5-mm tolerances0.5-mm tolerances


Juhao WuJuhao Wu


Collimator Gaps, Losses, and Alignment TolerancesCollimator Gaps, Losses, and Alignment Tolerances


Juhao WuJuhao Wu


Shower calculation -- FLUKAShower calculation -- FLUKA• 13.6 GeV electrons hitting front face of CX35

H.H. VinckeH.H. Vincke


Juhao WuJuhao Wu


SummarySummaryUndulator is protected from gun and structure Undulator is protected from gun and structure dark currentdark currentMaximum collimated beam power in above-Maximum collimated beam power in above-ground section is 0.2 Wground section is 0.2 WResults still look safe even for 10-times more Results still look safe even for 10-times more dark current (but already used worst-case dark current (but already used worst-case GTFGTF))Collimator wakefields should not be an issue Collimator wakefields should not be an issue (~0.5-mm alignment tolerances)(~0.5-mm alignment tolerances)Shower calculations were done (20 W/coll. was Shower calculations were done (20 W/coll. was assumed, now ~100-times smaller)assumed, now ~100-times smaller)


dark current, beam loss, and collimation in the lcls

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