SLAC Accelerator Development Program XBand Applications Chris Adolphsen

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

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 p. C) linacs, in particular light sources similar to LCLS – Allows developments from the US High Gradient program to be implemented on a larger scale World’s Highest Gradient (80 MV/m) Accelerator in Routine Operation • Immediate: Expand Vendor Base – Improve X-band klystron performance and have industry build them (see previous talk). SLAC Accelerator Development Program Page 2

CLIC Collaboration (CERN/SLAC/KEK) • High Gradient Characterization Prototype CLIC Linac Structures – T 18 series – first optimized structure with ramped gradient design – T 24 series – next iteration structure optimized for higher efficiency • Special Structures/Tests to Explore Breakdown Limits – Pulse heating study with the TD 18 structure – Dual Mode Cavity – C 10 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

CLIC T 18 -Disk Structure Cells 18+input+output Filling Time: ns 36 Active Length: cm 17. 5 a/λ (%) 15. 5 ~ 10. 1 vg/c (%) 2. 6 - 1. 0 Phase Advance Per Cell 2 p/3 Power Needed <Ea> = 100 MV/m 55. 5 MW Es/Ea 2 Require breakdown rate < 4 e-7 /pulse/m with 230 ns pulses The gradient including beam loading will be 10 -20 MV/m smaller Field Profile Along the Structure

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

Second T 18 Structure Tested at SLAC Processed structure by progressively lengthening the pulse at constant gradient (110 MV/m)

Gradients Achieved at a CLIC-Acceptable Breakdown Rate 1400 T 18 by KEK/SLAC at KEK 3900 T 18 by KEK/SLAC at SLAC #2 280 T 18 by CERN at SLAC 550 TD 18 by KEK/SLAC at SLAC HOM Damped 1100 TD 18 by KEK/SLAC at KEK HOM Damped 3200 T 24 by KEK/SLAC at SLAC 200 60 70 80 90 Unloaded Gradient [MV/m] 100 110 Total testing time [hr] T 18 by KEK/SLAC at SLAC #1

Pulsed Heating Effect in TD 18 It appears that for pulsed temperature increases above ~ 50 deg. C, the breakdown rate becomes strongly dependent on this heating Cutaway view of 1/8 of the TD 18 cell showing its surface magnetic field * SLAC Accelerator Development Program Page 8 * Phys. Rev. ST Accel. Beams 14, 010401 (2011)

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 TEM 3 mode, achieving 200 MV/m surface fields Top) electric field on the center conductor with 3. 3 MW driving the TEM 3 mode Bottom) center conductor magnetic field with and without 18. 3 MW driving the TE 011 mode - on the same scale with a peak of 1 MA/m SLAC Accelerator Development Program Page 9 * Phys. Rev. ST Accel. Beams 14, 010401 (2011)

Structure Performance vs Group Velocity • At ASTA, have measured two 10 cell, constant impedance TW structures (C 10), 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 260 ns 130 ns RF SLAC Accelerator Development Program Page 10

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 56 MV/m Max at 135 MW SLAC Accelerator Development Program Page 11

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 Me. V to Many Ge. V Linacs – LLNL 250 Me. V linac for gamma-ray production (MEGa-ray) – LANL 6 -20 Ge. V linac for an XFEL source to probe dense matter (Ma. RIE) – Trieste 1 Ge. V linac extension – Alternative to SC for the proposed 2. 25 Ge. V NLS linac – 2. 6 Ge. V linac for a soft X-ray FEL facility at KVI, U. Groningen, NLD – SLAC study of a 6 Ge. V Linac for a Compact XFEL (CXFEL) source – X-band Gun (with LLNL) and Test Beamline SLAC Accelerator Development Program Page 12

X-Band Energy ‘Linearizer’ at LCLS Energy-Time Correlation sz = 840 mm X-Band Structure: 0. 6 m long, 20 MV After BC 1 sz = 227 mm sz = 200 mm Non-linearity limits compression… …and spike drives CSR

LLNL 250 Me. V X-band Linac for Compton Gamma Ray Production

LANL Ma. RIE Project: 50 ke. V XFEL 20 Ge. V, 70 MV/m X-band Linac (space limited)

Swiss. FEL Main Linac Building Block Hans Braun: “X-band was not considered because no commercial klystrons available” Recently issued bids to have two vendors each build a 50 MW XL 4 klystron C-band- Klystron 5. 7 GHz, 50 MW, 2. 5 μs, 100 Hz 40 MW 2. 5 μs SLED RF pulse compressor 120 MW 0. 5 μs 3 d. B 30 MW 30 MW 10 m C-band structures at 26 MV/m

X-band Linac Driven Compact X-ray FEL Linac-1 250 Me. V S BC 1 X Linac-2 2. 5 Ge. V BC 2 Linac-3 6 Ge. V X X RF Gun LCLS-like injector L ~ 50 m 250 p. C, gex, y 0. 4 mm Undulator L = 40 m undulator X-band main linac+BC 2 G ~ 70 MV/m, L ~ 150 m Use LCLS injector beam distribution and H 60 structure (a/l=0. 18) after BC 1 Li. Track simulates longitudinal dynamics with wake and obtains 3 k. A “uniform” distribution Similar results for T 53 structure (a/l=0. 13) with 200 p. C charge

Operation Parameters Units CXFEL NLC Beam Energy Ge. V 0. 25 -6 2 -250 Bunch Charge n. C 0. 25 1. 2 RF Pulse Width* ns 150 400 Linac Pulse Rate Hz 120 Bunch Length μm 56, 7 110 Linac Gradient MV/m 70 65 * 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

Layout of CXFEL Linac RF Unit Power and Field Levels Already Demonstrated !! 400 k. V 50 MW XL 4 100 MW 1. 5 us 12 m 480 MW 150 ns Nine T 53 Structures (a/l = 13%) or Six H 60 Structures (a/l = 18%)

5. 59 Cell X-Band Gun 200 MV /m at C athode

X-Band Gun Development (with LLNL) Emittance ~ 0. 5 micron for a 250 p. C Bunch, Longitudinal Emittance Less Than ½ of that at LCLS Comparison of 4 D emittance along the gun computed with Impact. T (‘instant’ space charge) and PIC 3 D (‘delayed’ space charge plus wakes with true geometry) at two bunch charges and three laser offsets

Optimization Using Stacked Lasers At NLCTA, will be able to run short laser pulses and stack two pulses. For 250 p. C 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

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 FY 11 funds directed at this project Cut-away view of an X-band Deflecting Cavity for bunch slice diagnostics

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-Te. V collider SLAC Accelerator Development Program Page 24