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

2. X-band Development Objectives 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 World’s Highest Gradient (80 MV/m) Accelerator in Routine Operation

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 Field Profile Along the Structure Cells18+input+output Filling Time: ns36 Active Length: cm17.5 a/λ (%)15.5 ~ 10.1 v g /c (%)2.6 - 1.0 Phase Advance Per Cell 2  /3 Power Needed = 100 MV/m55.5 MW E s /E a 2 Require breakdown rate < 4e-7 /pulse/m with 230 ns pulses The gradient including beam loading will be 10-20 MV/m smaller

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

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

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

8. Pulsed Heating Effect in TD18 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) Cutaway view of 1/8 of the TD18 cell showing its surface magnetic field *

9. Dual Mode Cavity for Heating/Field Study 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) 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

10. Structure Performance vs Group Velocity 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 10 260 ns 130 ns RF

11. Power Extraction Structures (PETS) 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 56 MV/m Max at 135 MW

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

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. C-band structures at 26 MV/m 10 m C-band- Klystron 5.7 GHz, 50 MW, 2.5 μs, 100 Hz 120 MW 0.5 μs 40 MW 2.5 μs 30 MW 3 dB 30 MW SwissFEL Main Linac Building Block SLED RF pulse compressor Hans Braun: “X-band was not considered because no commercial klystrons available” Recently issued bids to have two vendors each build a 50 MW XL4 klystron

17. Linac-1 250 MeV Linac-2 2.5 GeV Linac-3 6 GeV BC1BC2 Undulator L = 40 m S RF Gun X undulator LCLS-like injector L ~ 50 m 250 pC,  x,y  0.4  m X-band Linac Driven Compact X-ray FEL X X X-band main linac+BC2 G ~ 70 MV/m, L ~ 150 m Use LCLS injector beam distribution and H60 structure (a/ =0.18) after BC1 LiTrack simulates longitudinal dynamics with wake and obtains 3 kA “uniform” distribution Similar results for T53 structure (a/ =0.13) with 200 pC charge

18. UnitsCXFELNLC Beam EnergyGeV0.25-62-250 Bunch ChargenC0.251.2 RF Pulse Width * ns150400 Linac Pulse RateHz120 Bunch Lengthμm56, 7110 Linac GradientMV/m7065 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 50 MW XL4 100 MW 1.5 us 400 kV 480 MW 150 ns 12 m Nine T53 Structures (a/ = 13%) or Six H60 Structures (a/ = 18%) Power and Field Levels Already Demonstrated !!

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 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).  = 0.76 ps

23. New Gun/Structure Beamline NLCTA 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 Cut-away view of an X-band Deflecting Cavity for bunch slice diagnostics

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