Injector Physics C. Limborg-Deprey, D. Dowell ,Z. Li. ,H. Loos, J Injector Physics C.Limborg-Deprey, D.Dowell ,Z.Li*,H.Loos, J.Schmerge, L.Xiao* (*) RF Gun modifications Linac Sections modifications Risk Mitigation Plans Commissioning Safety Summary

RF Systems – November Review Presentations http://www-ssrl.slac.stanford.edu/lcls/photoinjector/reviews/2004-11-03_rf_review Recommendations Report from J.Wang et al. Initial Gun UCLA/BNL/SLAC  LCLS Gun version

Modifications RF gun Courtesy L.Xiao Gun fabricated at SLAC Dual Feed RF design complete Mechanical model in progress 120Hz heat calculations under way Dual Feed Suppresses the time dependent dipole mode Matching phase for 2 feeds by holding mechanical tolerances on both arms Z coupling (instead of -coupling) Pulsed heating reduced + easier machining Racetrack shape compensates for stronger quadrupole mode 15 MHz mode separation adopted Lower cathode voltage for the 0-mode “Suppresses” two degrees of freedom in parameter space Larger radius for coupling cell iris Reduces RF emittance Easy to accomodate elliptical curvature to reduce surface field Shaping of RF pulse for reducing average power 4kW -> 1.8 kW ; cooling channels designed for handling 4kW Reduce reflected power from gun LCLS-TN-05-3.pdf Courtesy L.Xiao

Reducing Pulsed Heating with RF Pulse Shaping 1.8 kW 4.0 kW Advantages Factor 2 less power dissipated in structure Reach higher peak fields due to shorter rf pulse length Less dark current Courtesy J.Schmerge

Study suggested by T.Smith 0 and Pi Mode 0-mode Accelerating field -mode Study suggested by T.Smith

0 and Pi Mode Separation  between 0 and PI mode for short klystron pulse Small and nearly independent of 

Geometry changes Modification of cell-to-cell iris for 15MHz mode separation Reduces the surface field could help reach more than120MV/m 9.525 12.4968 22.0472 14.85 12.025 13.025 19.050 Prototype Gun LCLS Gun Courtesy Z.Li

Racetrack Shape in Full Gun 2D- no port no racetrack (3D) all ports and racetrack shape (3D)

Linac: Dual Coupler at entrance cell rms head-tail trans. kick for 10ps bunch Head-tail dipole kick from single feed Generates emittance growth Operating point Kick is reduced by more than 4 times in output coupler nominal 1nC Ent. L0a Exit L0a Ent. L0b Exit L0b (/) at 0 single feed in % 1.8 0.4 12 0.6/0.5 With dual feed reduction head-tail kick reduced by 20 (/) at 0 dual feed in % 0.005 0.04 Dual feed at entrance cell BUT NOT at exit Quadrupole head-tail not a problem at exit cell

Linac Final Design Using standard WR284 waveguide – eliminate all tapers (flanges closer to body, to accommodate linac solenoid ) Coupler cell lengthened to match height of WR284 waveguide Racetrack parameters readjusted Courtesy J.Chan

Cathode QE improvement H-ion Cleaning Experiment QE at low voltage (No Shottky Enhancement) LCLS QE Spec. 6x10-5 @ 255nm After H-beam cleaning 1.2x10-4 Idt for H-ion beam %Carbon on surface initial 30 1H 0.0378 C 11 2H 0.124 12 3H 0.1818 10 4H 0.614 8 Courtesy D.Dowell, R.Kirby Surface unaltered by H-ion beam cleaning contrary to effect of laser cleaning

Schottky Enhancement of the QE QE improvement Schottky Enhancement of the QE Some Approximations to theory More tests planned More samples (process) Find optimal H-ion current, integration time and temperature Implement on LCLS gun? GTF (measured) LCLS Specifications LCLS minimum required Courtesy D.Dowell

“Beer Can” vs “3D ellipsoid”  = 1.02 mm.mrad;  80% = 0.95 mm.mrad = 0.57 mm.mrad ;  80% = 0.58 mm.mrad (standard “cathode” = 0.6 mm.mrad / mm radius - similar meshing parameters) Sensitivity much reduced and Tuning much easier Margin to 1mm.mrad very large Feasibility under study

Commissioning Preparation Gun-To-Linac Cathode emittance Line density Orthogonality of knobs: from minimum energy spread Beam–Based Alignment of Gun-Solenoid-Linac 135 MeV Straight Ahead Spectrometer Design modified More simulations on poor quality beam Commissioning Emittance measurement data analysis Comparison of various techniques in collaboration with DESY team Converge towards “agreed upon protocol” Software Import available software from GTF and DUVFEL List of all additional software to be written

Cathode emittance Image of divergence of source Assumes cathode = 0.6 mm.mrad Image of divergence of source At second screen + Vrf reduced Measure transverse momentum distribution

Orthogonality of knobs Detect rf  Vrf To Be updated with better resolution In region of interest

Longitudinal Uniformity at Gun Exit 8% modulation 1nC  RF 25 Resolves modulation at YAG1 location

Straight Ahead Spectrometer Modification of design (added pole face rotation) Uses matching lattice Leaves possibility of stealing pulses at ~1 Hz or less, in the future Dx ~ 1m x ~0.1 Same tuning

Longitudinal Phase Space resolution < 10 keV 3keV -> 40 keV will be easily measurable rms

Transported In Injector Safety Maximum credible beam (MCB) estimate needed for Laser tube penetration & personnel access shielding design, locations of beam loss Review previous estimate of MCB to linac (agrees with original value) Explosive electron emission from cathode is source of MCB Over-focused drive laser beam generates plasma at cathode surface Beam power limited by stored energy in gun and beam loading Large energy spread limits MCB transported to linac Documented in SLAC Memo April, 2005 & LCLS-TN-01-2 1210W 4MeV 2030W 44.5MeV 4740W 104MeV 1130W 135MeV Est. Max. Credible Beam Transported In Injector Courtesy D.Dowell

Conclusion Gun Linac Risk Mitigation Plans for 1nC Safety RF design completed Mechanical design under way (thermal analysis on-going) Linac Mechanical design in progress Risk Mitigation Plans for 1nC Cathode studies: H-ion cleaning for higher QE Tuning of 0.2nC completed (see P.Emma) On-going work on laser pulse shaping Safety Commissioning Preparation Laser steering stabilization Feedback for Laser Energy/ RF gun (P,) Commissioning Plans (detailed schedule + software)