LCLS-II options: CuRF → SXR, VPU, HXR harmonics G. Marcus 5/13/2015

2 Outline CuRF → SXR VPU HXR harmonics (time permitting)

3 CuRF → SXR Basic question: What performance can we expect if we feed the LCLS-II SXR undulator with the e-beam from the CuRF linac? Assumptions: We can transport the e-beam to the SXR undulator. -Transport beamline designed after this study. X-rays can be transported to users. Main results: The production of high peak power and high peak flux soft X-ray FEL pulses is possible. The extremely impressive pulse characteristics can potentially be isolated using advanced slicing techniques.

4 Semi-analytic estimates H-D. Nuhn

5 “Start-to-end” performance study head I ~ 2.5 kA n ~ 0.31 mm-mrad σ E ~ 1.4 MeV

6 LCLS-II SXR Undulator ParameterValueUnit TypeHybrid PM, planar- Full gap heightVariable- Period39mm K0K Segment length3.4m Break length1.0m # segments21- Total length96m * Optimal taper for this case *

7 Gain curve for E γ = 1.5 keV E ~ 10.1 mJ E ~ 2.7 mJ

8 Power profile at the end of the tapered undulator, optimizing the taper on the FEL energy (10.1 mJ) Power (blue), Current (green), tapered FEL P ~ 1 TW

9 Zooming in on a typical spike You can think of schemes to slice up the e-beam to allow only a single spike to amplify Emittance spoiler Single-cycle laser modulation with tapering Differential heating Combinations of the above… Advanced schemes such as E-SASE and/or chirp and taper could potentially reduce the spike width further FWHM ~ 400 as E single spike ~ 280 μJ FWHM ~ 400 as E single spike ~ 280 μJ

10 CuRF → SXR: Cost estimate and conclusions Cost estimate: ~ $5M -Transport line only. Additional $$$ for advanced slicing techniques. Conclusions: Impressive SXR performance -High pulse energy across tuning range -High peak power (~ 1 TW) -Isolated pulses show sub-fs structure Transport beamline has been designed -Additional S2E simulations will be needed to evaluate the impact, if any, of this transport on the FEL performance

11 VPU Basic question: What is the impact of replacing a number of HPUs with VPUs for the efficient production of vertically polarized light? Assumptions: Can replace any number of undulators (1-32). Impact will be greatest on high end of the tuning range (E γ = 5 keV). Main results: At least half (16) of the undulators need to be VPU to recover single undulator polarization performance (significant 3D effects). Roughly two undulators are needed to recover energy from the pre-bunched e- beam at the undulator polarization transition. Mode quality is moderately impacted.

12 Start-to-end performance study at E γ = 5 keV head I ~ 350 A n,x ~ 0.15 mm-mrad σ E ~ 450 keV This 20 pC distribution has since been updated. New simulations are forthcoming.

13 LCLS-II HXR undulator ParameterValueUnit TypeHybrid PM, planar- Full gap heightVariable- Period26mm K0K Segment length3.4m Break length1.0m # segments32- Total length140m

14 5 keV performance ~ 8 μJ ~ 25 μJ Need to switch in linear growth region well before onset of saturation and large slice energy spread growth (see H: U20, V: U20) Takes ~ 1 L g to recover pulse energy after switch Some lag before vertical polarization returns to exponential growth (see H: U16, V: U16) ~ 100 m Saturation is slightly delayed But total pulse energy is recovered

15 Vertical polarization mode quality ~ 60% in fundamental mode

16 VPU: Cost estimate and conclusions Cost estimate: ??? Conclusions: At least half of the undulators need to be VPU to recover single undulator polarization performance at E γ = 5 keV. Roughly two undulator sections are needed to recover energy from a pre- bunched beam at the polarization transition. Mode quality is impacted. -Implications for downstream optics. Transition at the 16 th undulator leaves only two undulators to produce seeded bunching if running the beamline in self-seeded mode. -A more detailed study of the e-beam from the CuRF should be investigated if this option is pursued. The ability to taper when the beamline is tuned toward the lower end of the tuning range will be negatively impacted. In general expect better performance from a single undulator polarization.

17 HXR Harmonics Basic question: What is the expected performance of the harmonics from the HXR undulator when fed by the CuRF e-beam? Assumptions: Can produce harmonics through nonlinear harmonic generation (NHG) or harmonic lasing (HL). -The fundamental can be effectively suppressed for HL. Analytic estimates (M. Xie style) are valid. -Some benchmarking to S2E simulations. Main results: Some reasonable power can be expected from the harmonics. -This, of course, depends on the definition of ‘reasonable’.

18 LCLS-II HXR undulator

19 Nonlinear harmonic generation (100 pC e-beam) The power in the third harmonic begins to drop for large photon energies as the undulator parameter decreases and 3D effects become significant

20 S2E NHG electron beam head I ~ 720 A n,x ~ 0.35 mm-mrad n,y ~ 0.42 mm-mrad σ E ~ 450 keV

21 S2E NHG and HL performance NHG agrees well with analytic estimates HL simulations have fundamental interaction artificially suppressed. This is a best case scenario for HL performance, although more detailed simulation studies of HL have been done previously for LCLS-II. Post saturation tapering can further enhance performance. Can potentially push to 7 or maybe 8 keV photons.

22 HXR harmonics: Cost estimate and conclusions Cost estimate: ??? -Methods and materials for effective suppression of the fundamental using the HL concept are still being evaluated. Conclusions: S2E simulations and analytic estimates are in relatively good agreement. NHG yields between 0.5 – 1.0 % of fundamental power up to 9 keV 3 rd harmonic photons. HL may do a little better up to ~ 7 keV photons in the third harmonic.