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Dark Current, Beam Loss, and Collimation in the LCLS

LCLS. Dark Current, Beam Loss, and Collimation in the LCLS J. Wu , D. Dowell, P. Emma, C. Limborg, J. Schmerge, H. Vincke LCLS FAC Meeting April 7, 2005. Thanks to M. Borland for Elegant code changes in support of these studies. Description of the Study.

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Dark Current, Beam Loss, and Collimation in the LCLS

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  1. LCLS Dark Current, Beam Loss, and Collimation in the LCLS J. Wu ,D. Dowell, P. Emma, C. Limborg, J. Schmerge, H. Vincke LCLS FAC Meeting April 7, 2005 Thanks to M. Borland for Elegant code changes in support of these studies

  2. Description of the Study • Model dark current from cathode using Fowler-Nordheim and Parmela, but scaling charge from GTF measurements • Add dark current in critical RF structures along linac, based on K. Bane work in NLC (not significant) • Track dark current through entire linac up to and through undulator, using symplectic integration for every bend and quadrupole in Elegant (M. Borland, ANL) • Include aperture restrictions and collimators • Assess collimation scheme in terms of undulator protection and average power loss on each collimator • Evaluate wakefield effect of each collimator

  3. ‘Fowler-Nordheim’ on Cathode J. Wang J. Schmerge

  4. Longitudinal Distribution after ‘L0-a’ • GTF measurements: • 3 nC maximum (E = 120 MV/m)over 1-msec RF pulse (3000 buckets) at gun exit • Parmela Results: • 5-mm cathode radius for max. transmission (worst case) • ~75% transmission through gun: (400000  300000 particles) • 3 nC  300000 macro-particles (3 nC)(19160/300000) 200 pC/pulse at L0-b entrance RF crest Only 19160 particle remain after “L0-a” RF section (6% or 200 pC/pulse) dump next bucket into main one 360 º run08_5mm_eth06_el117_400k.dat (C. Limborg: Jan. 7, 2005)  head nominal laser pulse

  5. Choosing cathode radius for dark current production all particles at cathode particles surviving after ‘L0-a’ use +5 mm radius for dark current production (better statistics) C. Limborg

  6. Transverse Phase Space of Dark Current shift phase so that z = 0 is photo-beam nominal phase at “L0-a” exit run08_5mm_eth06_el117_400k.dat dump next bucket into main one

  7. Structure dark current • Critical RF structures: • L0b (E=23.8 MV/m); X1_Xband (E=31.7 MV/m);L2_10_50 (E=23.0 MV/m); and L3_10_50 (E=23.6 MV/m); • Quads deflect dark current effectively

  8. Structure dark current • Study approach: • Use Mafia to get field map • Use Mathematica (K. Bane’s code) to track through 3-m structure • Normalized according to measurement: 15 pC in 2 s pulse for 3 meter structure at 26 MV/m(J. Schmerge) --- fit  ~ 120, and Ae ~ 350 m2 • Most capture in down stream Examples of K. Bane’s study for X-band. We then compute for S-band and X-band

  9. Structure dark current • Contribution of structure dark current: • X-band gives the largest contribution, however, deflected • Structures withE~24 MV/m will give additional particle loss Green: difference Black: total Red: Gun DC only

  10. Tracking and Collimation new energy collimators new b collimators BC1 coll. BC2 coll. ‘under ground’ existing collimators (4 x and 4 y) ‘L0-b’ start undulator

  11. 2-Phase, 2-Plane Und. Collimation, 1½ Times p/2 ~p/2 70s (2.5 mm) 40s (2.2 mm) 40s (2.2 mm) 45s halo e- beam edge scattering undulator beam pipe x1 x2 x3 phase-1 again phase-2 phase-1 (also collimation in y and energy – see next slides)

  12. Collimation in Linac-To-Undulator (LTU) y1 y2 y3 E1 E2 x1 x2 x3 muon shielding undulator m-spoiler

  13. Particle losses up to, and through BC1 1-inch ID 7-mm ID L0-b DL1 L1 BC1 X-band 300 pC lost per pulse = 9 W @ 120 Hz, 250 MeV 120 pC lost per pulse = 1.9 W @ 120 Hz, 135 MeV

  14. Particle losses through undulator and dump 1 newBC2E-coll. 36-mm (d = 10%) 2 newE-coll. 2.5 mm (d = 2%) under ground BC1 BC2 undulator 4 existingx-coll.’s 4 existingy-coll.’s 1.6 & 1.8 mm 3 newx-coll.’s 3 newy-coll.’s 2.2 mm… 1 newBC1E-coll. 45-mm (d = 20%) 2.6 pC/pulse 3.5 W (120 Hz, 11.3 GeV) 0.7 pC/pulse 1.1 W (120 Hz, 13.6 GeV) 0.1 pC/pulse 0.2 W (120 Hz, 13.6 GeV) DE/E of 1 dropped klystron = -1.7%

  15. Undulator Protection (1) undulator vacuum chamber (at start of und.)

  16. Undulator Protection (2) undulator length (undulator aperture limit) maximum particle extent

  17. Transverse Wakefield Alignment Tolerances b sz << a a Dx [4] N = 6.25109 eN = 1.2 mm longitudinal wakes also checked (no problem) 0.5-mm tolerances

  18. Collimator Gaps, Losses, and Alignment Tolerances

  19. Shower calculation -- FLUKA • 13.6 GeV electrons hitting front face of CX35 H.H. Vincke

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

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