TW FEL “Death-Ray“ Studies

1. Josef Frisch, Zhirong Huang, Yi Jiao TW FEL “Death-Ray“ Studies

2. FEL Optimized for Imaging • Many imaging experiments are similar • Hard X-rays ~1 Angstrom, minimum focus size, maximum flux. • Liquid or gas target injection • Pump laser • CCD arrays surrounding sample • Use a special purpose imaging chamber designed for rapid change of samples • FEL optimized for maximum flux at 1 Angstrom, ~10-30fsec • Conversations with experimenters suggest that 2TW in 10fs is required for imaging of single bio-molecules

3. Self Seeding Improves FEL Efficiency Energy extraction taper Seeding undulator Gain Undulator Diamond crystal band-stop filter Minimal mirrors (maybe just KB set) Narrow band seed power Band stop filter produces narrow band signal in tail Chicane delays beam so that it interacts with tail of filtered pulse A “notch” filter produces a narrow band peak in the transmitted spectrum Idea invented by E. Saldin at DESY

4. High Energy Operation - Estimates • Vary electron voltage while keeping operating wavelength fixed at 1 Angstrom. • Assume that peak current increases as the 4th root of beam energy. • Undulator wavelength changes, keep same peak B field as LCLS undulator so "K" changes • Roughly matches what we can use in LCLS_I, is probably conservative • Assume 0.6um slice emittance and 1MeV energy spread (before spontaneous energy spread is added). • Run Ming Xie formula with spontaneous energy spread (Saldin et al, “Design Formulas for VUV and X-RAY FELs”) calculated for 20 gain lengths (iterate). • Allow Beta function to optimize from 10 to 100 Meters

5. No Taper ~60GW at 28 GeV

6. 0.4 emittance, higher peak current, more agressive parameters 90GW at 28 GeV Usable to 45 GeV

7. 500eV FEL can use high beam energy as well !

8. ~25 GeV beam with existing LCLS_I undulator should work at 25 KeV

9. TW FEL Genesis Simulations by Zhirong Huang 27 GeV, 0.6um Emittance, 5KA peak current, 1.4 MeV espread, 4.5cm Undulator, K=4.95, 30M beta function 4TW (!!) 700M undulator(!!) NOT OPTIMAL!

10. Need Simulations • The simulation above is based on a guess at beam parameters from Ming-Xi calculations. It may be VERY far from optimal • 700 M undulator is huge – but not completely insane • Undulators are $200K/M, this is $140M for an undulator, LCLS_II is $400M, XFEL is >$1B • E-beam parameters are reasonable, but R&D might improve emittance or peak current. • Energies up to 45 GeV in principal available from the SLAC linac. (probably can’t use that much energy) • High K helical undulators (Superconducting) may be possible. • Want to explore the design phase space.

11. Simulations • Use Genesis in CW beam mode to evaluate the performance of various accelerator and undulator configurations • Slice simulation takes ~15 seconds on a PC – can afford to run a LOT of simulations. • Use various electron beam energies, emittance, peak current. • Use linear and helical undulators, set for 1 angstrom output wavelength with maximum practical K at each undulator wavelength. • Optimize output power at various undulator lengths (each 100M) for beta function and K (2 parameters). • Note: need to apply physics to constrain parameters and to aid optimization. • Create a large “library” of simulations to explore the phase space • >106 simulation possible – probably 104 optimized conditions. • Generate an empirical model or software tool to predict the performance of an FEL give input e-beam parameters (energy, emittance, peak current), and undulator parameters (length, max B-field for PM, or SC).

12. Automatically optimizing • Varying one input parameter while keep others constant • Generate the corresponding input file for Genesis steady-state simulation • Read the Genesis output, record interested quantities, e.g. power, transverse radiation size, etc. • Find out the optimal input parameter • All these jobs are integrated into a MATLAB code.

13. Scan sx, sy, at the undulator entrance and quadrupole gradient for a higher radiation power Before scanning : sx,= 11.6 mm, sy = 9.1 mm, K = 26.7 T/m, power = 2.63 TW After scanning : sx,= 9.3 mm, sy = 9.3 mm, K = 22.4 T/m, power = 2.83 TW

14. Other Issues • Transverse mode quality may degrade for long tapered undulators • Juhao is working on this? • Full 3-d effects may be important • Will need full 3-d genesis runs for some interesting cases • Do we trust Genesis in this regime? • Need to cross-compare against other FEL simulations • Experiments • Want to compare prediction with the HXRSS seeding experiment at LCLS late this year. • Need results soon enough to predict, not post-dict! • Need to do this quickly! • SLAC is designing LCLS_II, working on Sector 0 test facility. • Will soon determine what parts of the LINAC are available in the future • Knowing possible future uses of the LINAC and undulator hall may affect decisions now! • Around the world $5B of FEL projects proposed or under construction – these results might affect their decisions.

15. Benefit to SLAC • Imaging experiments are an important application of FELs. • Provides a unique capability at SLAC: No other lab will build a 30 GeV accelerator for an FEL. • Not made obsolete by XFEL or any other proposed FEL project LCLS Publicity image shows single molecule imaging, but this is NOT POSSIBLE with present day FELs!