A 6 GeV Compact X-ray FEL (CXFEL) Driven by an X-Band Linac Zhirong Huang, Faya Wang, Karl Bane and Chris Adolphsen SLAC
Compact X-Ray (1.5 Å) FEL Parameter symbol LCLS CXFEL unit Bunch Charge Q 250 250 pc Electron Energy E 14 6 GeV Emittance γε x,y 0.4-0.6 0.4-0.5 µm Peak Current I pk 3.0 3.0 ka Energy Spread σ E /E 0.01 0.02 % Undulator Period λ u 3 1.5 cm Und. Parameter K 3.5 1.9 Mean Und. Beta β 30 8 m Sat. Length L sat 60 30 m Sat. Power P sat 30 10 GW FWHM Pulse Length Τ 80 80 fs Photons/Pulse N γ 2 0.7 10 12
X-band Linac Driven Compact X-ray FEL Linac-1 250 MeV S X BC1 Linac-2 2.5 GeV X BC2 Linac-3 6 GeV X Undulator L = 40 m rf gun undulator LCLS-like injector L ~ 50 m 250 pc, γε x,y 0.4 µm X-band main linac+bc2 G ~ 70 MV/m, L ~ 150 m Use LCLS injector beam distribution and x-band H60VG3R structure (<a>/λ=0.18) after BC1 LiTrack simulates longitudinal dynamics with wake and obtains 3 ka uniform distribution Similar results for T53VG3R (<a>/λ=0.13) with 200 pc charge
Design Issues X-band Components Cost Performance Emittance Preservation Tolerance
Operation Beam Parameters Beam Parameters Units CXFEL 2004 NLC Final Beam Energy GeV 6 250 Bunch Charge nc 0.25 1.2 RF Pulse Width * ns 150 400 Linac Pulse Rate Hz 120 120 Beam Bunch Length μm 7 110 * Possible for multi-bunch operation with separation > 10ns
Layout of Linac RF Unit 50 MW XL4 50 MW 1.5us 400 kv 1.6us
Two Accelerator Structure Types Structure Type Units T53VG3R H60VG3R Constant E_surface Detuned Length cm 0.53 0.6 Filling time ns 74.3 105 Phase Advance/ Cell π 2/3 5/6 <a>/λ % 13.4 17.9 Power Needed for <Ea> = 70 MV/m MW 48 73
RF Unit for Two Structure Types Operating at 70 MV/m* Units T53VG3RA H60VG3R Average RF Phase Offset + Deg 2.6 2.6 Power Gain 4.83 4.83 Klystrons per Unit 2 2 Acc. Structures per Unit 9 6 RF Unit Length (Scaled to NLC) m 6.75 4.5 Total RF units 18 24 Main Linac Length m 122 108 Total Linac Length # m 192 178 *Assume 13% RF overhead for waveguide losses + scaled to NLC for single bunch loading compensation # including ML, 50m injector and 20 m BC2 at 2.5 GeV
NLC RF Component Costs (2232 RF Units) Per Item Cost (k$) LLRF 26.1 Modulator 83.7 Klystron 56.6 TWT 13.3 SLED-II 242.3 Structures 21.5 For RF unit quantities less than 50, assume the rf item cost will be 4 times the NLC cost
6 GeV X-Band Main Linac Cost Using Structure Type: T53VG3R, Total Cost = 56 M$ (3.1 M$ Per RF Unit) H60VG3R, Total Cost = 62 M$ (2.6 M$ Per RF Unit)
Gradient Optimization Relative 6GeV ML Cost 1.4 1.35 1.3 1.25 1.2 1.15 1.1 1.05 H60VG3R T53VG3R 1 50 60 70 80 90 100 Gradient (MV/m) Assuming 1) Tunnel cost 25 k$/m, AC power + cooling power 2.5 $/Watt 2) Modulator efficiency 70%, Klystron efficiency 55%.
Structure Breakdown Rates with 150 ns Pulses Breakdown Rate at 150 ns 120 Hz (#/hr) 10 0 10-1 10-2 10-3 10-4 H60VG3R T53VG3R 10-5 60 65 70 75 80 Gradient (MV/m) 1) H60VG3R scaled at 0.2/hr for 65 MV/m,400 ns, 60Hz 2) T53VG3R scaled at 1/hr for 70 MV/m, 480 ns, 60 Hz 3) Assuming BDR G 26, τ 6 At 70 MV/m, Rate Less Than 1/100hr at 120 Hz
Design Issues X-band Components Cost Performance Emittance Preservation Tolerance
H60VG3 Dipole Wakes K. Bane, SLAC- PUB-9663, 2003
Fitted Equation K. Bane, SLAC- PUB-9663, 2003 Fit equation for wakefield of disk loaded structure. For average cell of h60vg3, a= 4.7 mm, g= 6.9 mm, L= 10.9 mm
Wake Averaged over a Gaussian Bunch Linac-3 Linac-2 Average wake for Gaussian bunch as function of bunch length. Note that in Linac-2, σ_z= 56 µm; in Linac-3, σ_z= 7 µm
Emittance Growth Strength parameter: Chao, Richter, Yao (for β~ E ζ ) Emittance growth due to injection jitter xo if Υ small: For CXFEL, en= 250 pc, ε N =.4 µm, ζ= 0, and Linac-2: E 0 =.25 GeV, E f = 2.5 GeV, σ z = 56 µm, l= 32 m, β 0 = 10 m (σ x0 = 90 µm) => Υ=.14 Linac-3: E 0 = 2.5 GeV, E f = 6 GeV, σ z = 7 µm, l= 50 m, β 0 = 10 m (σ x0 = 29 µm) => Υ=.01 For random misalignment, let x 02 -> x rms2 /M p l cu = a 2 /2σ z = 1.6 m (Linac-3) catch-up distance, estimate of distance to steady-state
Single Bunch Wake and Tolerance Summary In both Linac-2 and Linac-3, Υ<< 1, => short-range, transverse wakefields in H60VG3 are not a major issue in that: An injection jitter of σ x0 yields 1% emittance growth in Linac-2 and.003% in Linac-3 Random misalignment of 1 mm rms, assuming 50 structures in each linac, yields an emittance growth of 1% in Linac-2, 0.1% in Linac-3 With the T53VG3R structure, the jitter and misalignment tolerances are about three times smaller for the same emittance growth. The wake effect is weak mainly because the bunches are very short.
Wakefield Damping and Detuning for Multibunch Operation Dipole Mode Density Ohmic Loss Only Wakefield Amplitude (V/pC/m/mm) Time of Next Bunch Measurements Detuning Only Frequency (GHz) Damping and Detuning Time After Bunch (ns)
High Gradient Structure Development Since 1999: Traveling-Wave Structure - Tested about 40 structures with over 30,000 hours of high power operation at NLCTA. - Improved structure preparation procedures - includes various heat treatments and avoidance of high rf surface currents. - Found lower input power structures to be more robust against rf breakdown induced damage. - Developed NLC/GLC Ready design with required wakefield suppression features.
RF Unit Test in 2003-2004 Powered eight accelerator structures in NLCTA for 1500 hours at 65 MV/m with 400 ns long pulses at 60 Hz: the structure breakdown rate was less than 1 per 10 hours. Also accelerated beam. From Station 1 From Station 2 From Eight-Pack 3 db 3 db 3 db 3 db 3 db Beam
RF System Readiness (Black Comments from 2004, Red - Comments from 2010) Accelerator Structures - Continue efforts to improve high gradient performance (now includes the US High Gradient program and the CERN/SLAC/KEK collaboration). - Well developed fabrication procedures to achieve wakefield and energy performance. - Three production groups churning out structures (still three with CERN replacing FNAL). System Integration - Accelerating beam with eight (three) structures at NLCTA. Summary - Ready for industrialization (still ready see X-band Workshop tomorrow) - Plan to expand NLCTA and GLCTA (now Nextef) to test industrially-built components (hope to build an improved 8-structure rf unit at NLCTA aimed at light source applications).