A 6 GeV Compact X-ray FEL (CXFEL) Driven by an X-Band Linac

1. A 6 GeV Compact X-ray FEL(CXFEL) Driven by an X-Band Linac Zhirong Huang, Faya Wang, Karl Bane and Chris Adolphsen SLAC

2. Compact X-Ray (1.5 Å) FEL

3. X-band Linac Driven Compact X-ray FEL Linac-1 250 MeV Linac-2 2.5 GeV Linac-3 6 GeV X BC1 BC2 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 x-band H60VG3R structure (<a>/l=0.18) after BC1 • LiTrack simulates longitudinal dynamics with wake and obtains 3 kA “uniform” distribution • Similar results for T53VG3R (<a>/l=0.13) with 200 pC charge

4. Design Issues • X-band Components • Cost • Performance • Emittance Preservation • Tolerance

5. Operation Beam Parameters * Possible for multi-bunch operation with separation > 10ns

6. Layout of Linac RF Unit 50 MW XL4 400 kV 1.6us 50 MW 1.5us

7. Two Accelerator Structure Types

8. RF Unit for Two Structure Types Operating at 70 MV/m* *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

9. NLC RF Component Costs (2232 RF Units) For RF unit quantities less than 50, assume the rf item cost will be 4 times the NLC cost

10. 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)

11. Gradient Optimization Assuming 1) Tunnel cost 25 k$/m, AC power + cooling power 2.5 $/Watt 2) Modulator efficiency 70%, Klystron efficiency 55%.

12. Structure Breakdown Rates with 150 ns Pulses At 70 MV/m, Rate Less Than 1/100hr at 120 Hz H60VG3R scaled at 0.2/hr for 65 MV/m,400 ns, 60Hz T53VG3R scaled at 1/hr for 70 MV/m, 480 ns, 60 Hz Assuming BDR ∞G26, ∞ τ6

13. Design Issues • X-band Components • Cost • Performance • Emittance Preservation • Tolerance

14. H60VG3 Dipole Wakes K. Bane, SLAC-PUB-9663, 2003

15. 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

16. 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, _z= 56 m; in Linac-3, _z= 7 m

17. Emittance Growth Strength parameter: Emittance growth due to injection jitter xo if  small: Chao, Richter, Yao (for ~E) • For CXFEL, eN= 250 pC, N= .4 m, = 0, and • Linac-2: E0= .25 GeV, Ef= 2.5 GeV, z= 56 m, l= 32 m, 0= 10 m (x0= 90 m) => = .14 • Linac-3: E0= 2.5 GeV, Ef= 6 GeV, z= 7 m, l= 50 m, 0= 10 m (x0= 29 m) => = .01 • For random misalignment, let x02-> xrms2/Mp • lcu= a2/2z= 1.6 m (Linac-3)—catch-up distance, estimate of distance to steady-state

18. Single Bunch Wake and Tolerance Summary • In both Linac-2 and Linac-3, << 1, => short-range, transverse wakefields in H60VG3 are not a major issue in that: An injection jitter of 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.

19. Dipole Mode Density Wakefield Damping and Detuning for Multibunch Operation Ohmic Loss Only Frequency (GHz) Detuning Only Measurements Wakefield Amplitude (V/pC/m/mm) Time of Next Bunch Damping and Detuning Time After Bunch (ns)

20. High Gradient Structure Development • Since 1999: • 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. Traveling-Wave Structure

21. 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 Eight-Pack From Station 2 3 dB From Station 1 3 dB 3 dB 3 dB 3 dB Beam

22. 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).