1. Juhao Wu LCLS FAC jhwu@SLAC.Stanford.edu 7 Apr. 2005 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 LCLS

2. Juhao Wu LCLS FAC jhwu@SLAC.Stanford.edu 7 Apr. 2005 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 Description of the Study

3. Juhao Wu LCLS FAC jhwu@SLAC.Stanford.edu 7 Apr. 2005 ‘Fowler-Nordheim’ on Cathode J. Schmerge J. Wang

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

5. Juhao Wu LCLS FAC jhwu@SLAC.Stanford.edu 7 Apr. 2005 Choosing cathode radius for dark current production particles surviving after ‘L0-a’ all particles at cathode use +5 mm radius for dark current production (better statistics) C. Limborg

6. Juhao Wu LCLS FAC jhwu@SLAC.Stanford.edu 7 Apr. 2005 Transverse Phase Space of Dark Current dump next bucket into main one run08_5mm_eth06_el117_400k.dat shift phase so that z = 0 is photo-beam nominal phase at “L0-a” exit

7. Juhao Wu LCLS FAC jhwu@SLAC.Stanford.edu 7 Apr. 2005 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);L3_10_50 (E=23.6 MV/m); 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 Quads deflect dark current effectively

8. Juhao Wu LCLS FAC jhwu@SLAC.Stanford.edu 7 Apr. 2005 Structure dark current Study approach: Use Mafia to get field map Use Mafia to get field map Use Mathematica (K. Bane’s code) to track through 3-m structure 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 A e ~ 350  m 2 Normalized according to measurement: 15 pC in 2  s pulse for 3 meter structure at 26 MV/m (J. Schmerge) --- fit  ~ 120, and A e ~ 350  m 2 Most capture in down stream Most capture in down stream Examples of K. Bane’s study for X-band. We then compute for S-band and X-band

9. Juhao Wu LCLS FAC jhwu@SLAC.Stanford.edu 7 Apr. 2005 Structure dark current Contribution of structure dark current: X-band gives the largest contribution, however, deflected X-band gives the largest contribution, however, deflected Structures withE~24 MV/m will give additional particle loss Structures with E~24 MV/m will give additional particle loss Green: difference Black: total Red: Gun DC only

10. Juhao Wu LCLS FAC jhwu@SLAC.Stanford.edu 7 Apr. 2005 Tracking and Collimation undulator ‘L0-b’start existing collimators (4 x and 4 y ) new energy collimators new  collimators BC1 coll. BC2 coll. ‘underground’

11. Juhao Wu LCLS FAC jhwu@SLAC.Stanford.edu 7 Apr. 2005 2-Phase, 2-Plane Und. Collimation, 1½ Times  x1x1x1x1 x2x2x2x2 x3x3x3x3 phase-1 phase-2 phase-1 again halo  70  (  2.5 mm)  40  (  2.2 mm) undulator beam pipe  45  edge scattering (also collimation in y and energy – see next slides) e  beam  40  (  2.2 mm)

12. Juhao Wu LCLS FAC jhwu@SLAC.Stanford.edu 7 Apr. 2005 Collimation in Linac-To-Undulator (LTU) E1E1E1E1 E2E2E2E2 y1y1y1y1 x2x2x2x2 y2y2y2y2 x3x3x3x3 y3y3y3y3 muon shielding undulator x1x1x1x1  - spoiler

13. Juhao Wu LCLS FAC jhwu@SLAC.Stanford.edu 7 Apr. 2005 Particle losses up to, and through BC1 BC1DL1L0-bL1 X-band 1-inch ID 7-mm ID 120 pC lost per pulse = 1.9 W @ 120 Hz, 135 MeV 300 pC lost per pulse = 9 W @ 120 Hz, 250 MeV

14. Juhao Wu LCLS FAC jhwu@SLAC.Stanford.edu 7 Apr. 2005 Particle losses through undulator and dump BC2undulator 2.6 pC/pulse 3.5 W (120 Hz, 11.3 GeV) 4 existing x -coll.’s 4 existing y -coll.’s  1.6 &  1.8 mm 2 new E -coll.  2. 5 mm (  =  2%) 3 new x -coll.’s 3 new y -coll.’s  2.2 mm… 1 new BC2 E -coll.  36-mm (  =  10%) 1 new BC1 E -coll.  45-mm (  =  20%) 0.1 pC/pulse 0.2 W (120 Hz, 13.6 GeV) BC1 0.7 pC/pulse 1.1 W (120 Hz, 13.6 GeV) underground  E / E of 1 dropped klystron =  1.7%

15. Juhao Wu LCLS FAC jhwu@SLAC.Stanford.edu 7 Apr. 2005 Undulator Protection (1) undulator vacuum chamber (at start of und.)

16. Juhao Wu LCLS FAC jhwu@SLAC.Stanford.edu 7 Apr. 2005 Undulator Protection (2) undulator length (undulator aperture limit) maximum particle extent

17. Juhao Wu LCLS FAC jhwu@SLAC.Stanford.edu 7 Apr. 2005 Transverse Wakefield Alignment Tolerances N = 6.25  10 9  N = 1.2  m a b  z << a xxxx [4] longitudinal wakes also checked (no problem) 0.5-mm tolerances

18. Juhao Wu LCLS FAC jhwu@SLAC.Stanford.edu 7 Apr. 2005 Collimator Gaps, Losses, and Alignment Tolerances

19. Juhao Wu LCLS FAC jhwu@SLAC.Stanford.edu 7 Apr. 2005 Shower calculation -- FLUKA 13.6 GeV electrons hitting front face of CX35 H.H. Vincke

20. Juhao Wu LCLS FAC jhwu@SLAC.Stanford.edu 7 Apr. 2005 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)