SLAC Accelerator Development Program: X-Band Applications

1. SLAC Accelerator Development Program: X-Band Applications Chris Adolphsen OHEP Accelerator Development Review January 24-26, 2011

2. X-band Development Objectives World’s Highest Gradient (80 MV/m) Accelerator in Routine Operation • Long Term: Provide Higher Energy Reach for a Linear Collider • CLIC assumes ~100 MV/m acceleration in X-band structure powered with a drive beam • Collaborating with them to use X-band klystrons to drive the structures as a first stage to reduce the technical risk and allow earlier construction • Short Term: Other Applications • X-band ideal for compact (50-100 MV/m gradient), low bunch charge (e.g. 250 pC) linacs, in particular light sources similar to LCLS • Allows developments from the US High Gradient program to be implemented on a larger scale • Immediate: Expand Vendor Base • Improve X-band klystron performance and have industry build them (see previous talk). SLAC Accelerator Development Program Page 2

3. CLIC Collaboration (CERN/SLAC/KEK) • High Gradient Characterization Prototype CLIC Linac Structures • T18 series – first optimized structure with ramped gradient design • T24 series – next iteration structure optimized for higher efficiency • Special Structures/Tests to Explore Breakdown Limits • Pulse heating study with the TD18 structure • Dual Mode Cavity • C10 structures • High Power Tests of Power Extraction Structures (PETS) • Nominally driven with 100 A beams, which are not available • Instead test rf-driven versions • Structure Simulations (see Arno Candel’s talk later today) • Dark current propagation within structure • Dipole mode propagation between the PETS and main accelerator structures SLAC Accelerator Development Program Page 3

4. CLIC T18-Disk Structure Require breakdown rate < 4e-7 /pulse/m with 230 ns pulses The gradient including beam loading will be 10-20 MV/m smaller Field Profile Along the Structure

5. L-Band TTF Couplers Lasers E-163 DLA Marx Modulator RF Testing Echo-7 XTA X-Band 2*300 MW S-Band Structure Installed in NLCTA for Testing X-band 2-Pack 500 MW L-band Big Pipe Also have two X-band rf stations in the Klystron Test Lab (ASTA) for structure/component testing

6. Second T18 Structure Tested at SLAC Processed structure by progressively lengthening the pulse at constant gradient (110 MV/m)

7. Gradients Achieved at a CLIC-Acceptable Breakdown Rate 1400 T18 by KEK/SLAC at SLAC #1 3900 T18 by KEK/SLAC at KEK T18 by KEK/SLAC at SLAC #2 280 T18 by CERN at SLAC 550 Total testing time [hr] TD18 by KEK/SLAC at SLAC 1100 HOM Damped TD18 by KEK/SLAC at KEK 3200 HOM Damped T24 by KEK/SLAC at SLAC 200 Unloaded Gradient [MV/m]

8. Pulsed Heating Effect in TD18 Cutaway view of 1/8 of the TD18 cell showing its surface magnetic field * It appears that for pulsed temperature increases above ~ 50 degC, the breakdown rate becomes strongly dependent on this heating SLAC Accelerator Development Program Page 8 * Phys. Rev. ST Accel. Beams 14, 010401 (2011)

9. Dual Mode Cavity for Heating/Field Study Top) electric field on the center conductor with 3.3 MW driving the TEM3 mode Bottom) center conductor magnetic field with and without 18.3 MW driving the TE011 mode - on the same scale with a peak of 1 MA/m Built rf cavity resonant in two modes, which when driven independently, allow the rf magnetic field to be increased on the region of highest electric field without affecting the latter So far have powered the TEM3 mode, achieving 200 MV/m surface fields SLAC Accelerator Development Program Page 9 * Phys. Rev. ST Accel. Beams 14, 010401 (2011)

10. Structure Performance vs Group Velocity 260 ns 130 ns RF At ASTA, have measured two 10 cell, constant impedance TW structures (C10), with group velocities of 1.35 % and 0.7 % of c. Breakdowns appear to be mostly on first regular cell – in process of redesigning the coupler SLAC Accelerator Development Program Page 10

11. Power Extraction Structures (PETS) 56 MV/m Max at 135 MW Large aperture (23 mm, vg = 46%c) structures that weakly couple to a 100 A drive beam to extract rf power Second rf-driven version tested at ASTA, which included HOM absorbers, met breakdown rate specs at 135 MW SLAC Accelerator Development Program Page 11

12. High Power (Multi-MW) X-Band Applications • Short bunch FELs • Energy Linearizer: in use at LCLS, planned for BNL, PSI, Fermi/Trieste and SPARX/Fascati • Deflecting Cavity for Bunch Length Measurements • 100’s of MeV to Many GeV Linacs • LLNL 250 MeV linac for gamma-ray production (MEGa-ray) • LANL 6-20 GeV linac for an XFEL source to probe dense matter (MaRIE) • Trieste 1 GeV linac extension • Alternative to SC for the proposed 2.25 GeV NLS linac • 2.6 GeV linac for a soft X-ray FEL facility at KVI, U. Groningen, NLD • SLAC study of a 6 GeV Linac for a Compact XFEL (CXFEL) source • X-band Gun (with LLNL) and Test Beamline SLAC Accelerator Development Program Page 12

13. Energy-Time Correlation sz = 227 mm After BC1 Non-linearity limits compression… …and spike drives CSR X-Band Energy ‘Linearizer’ at LCLS sz = 840 mm X-Band Structure: 0.6 m long, 20 MV sz = 200 mm

14. LLNL 250 MeV X-band Linac for Compton Gamma Ray Production

15. LANL MaRIE Project: 50 keV XFEL 20 GeV, 70 MV/m X-band Linac (space limited)

16. SwissFEL Main Linac Building Block C-band- Klystron5.7 GHz, 50 MW, 2.5 μs, 100 Hz Hans Braun: “X-band was not considered because no commercial klystrons available” 40 MW 2.5 μs SLED RF pulse compressor Recently issued bids to have two vendors each build a 50 MW XL4 klystron 120 MW 0.5 μs 3 dB 3 dB 3 dB 30 MW 30 MW 30 MW 30 MW 10 m C-band structures at 26 MV/m

17. X-band Linac Driven Compact X-ray FEL Linac-1 250 MeV Linac-2 2.5 GeV Linac-3 6 GeV BC1 BC2 X X S Undulator L = 40 m X undulator RF Gun LCLS-like injector L ~ 50 m 250 pC, gex,y 0.4 mm X-band main linac+BC2 G ~ 70 MV/m, L ~ 150 m Use LCLS injector beam distribution and H60 structure (a/l=0.18) after BC1 LiTrack simulates longitudinal dynamics with wake and obtains 3 kA “uniform” distribution Similar results for T53 structure (a/l=0.13) with 200 pC charge

18. Operation Parameters * Allows ~ 50-70 ns multibunch operation CXFEL wakefield effects are comparable at the upstream end of the linac as the lower bunch charge and shorter bunch length offset the lower energy, however the bunch emittance is 25 times larger

19. Layout of CXFEL Linac RF Unit Power and Field Levels Already Demonstrated !! 400 kV 50 MW XL4 100 MW 1.5 us 12 m 480 MW 150 ns Nine T53 Structures (a/l = 13%) or Six H60 Structures (a/l = 18%)

20. 5.59 Cell X-Band Gun 200 MV/m at Cathode

21. X-Band Gun Development (with LLNL) Emittance ~ 0.5 micron for a 250 pC Bunch, Longitudinal Emittance Less Than ½ of that at LCLS Comparison of 4D emittance along the gun computed with ImpactT (‘instant’ space charge) and PIC 3D (‘delayed’ space charge plus wakes with true geometry) at two bunch charges and three laser offsets

22. Optimization Using Stacked Lasers = 0.76 ps At NLCTA, will be able to run short laser pulses and stack two pulses. For 250 pC bunches, emittance = 0.3 m (95% particles) with single Gaussian (500 fs FWHM) vs 0.25 m (95% particles) with stack of two Gaussians (300 fs FWHM each).

23. New Gun/Structure Beamline NLCTA Cut-away view of an X-band Deflecting Cavity for bunch slice diagnostics Adding a new beamline segment to NLCTA to characterize rf photocathode guns and to test high gradient structures Major portion of the FY11 funds directed at this project

24. Summary • Our X-band program builds on the 15 year effort at SLAC to develop this technology for a linear collider • Have extensive resources including several high power X-band stations and experts in rf and accelerator design • There has been a revival of interest in X-band in recent years with the • Adoption of the technology by CLIC • Use of X-band linearizers in existing FELs • Desire for compact linacs for future light sources • Our program is geared to meeting this growing demand, while in the long term, seeking a cost effective solution for a multi-TeV collider SLAC Accelerator Development Program Page 24