UKXFEL Strawman Output Parameters Electron Beam Parameters Neil

UK-XFEL ‘Strawman’ Output Parameters & Electron Beam Parameters Neil Thompson, June 7 2018

Photon Output Spec Parameter Value Comment Minimum photon energy 250 e. V To cover carbon K edge Maximum photon energy 25 ke. V For extreme materials study Minimum pulse duration 100 as case for < 100 as to be determined Typical pulse duration 50 fs Pulse energy > 1 m. J @ 25 ke. V Repetition Rate 1 MHz in SXR (250 e. V to 2 ke. V) 100 Hz in HXR (>2 ke. V) Polarisation Variable in SXR (250 e. V to 2 ke. V). Requirement @ > 2 ke. V TBD. Two pulse output Separation up to 60 fs direct from source Larger separation will use split and delay beam lines Two colour output Photon energy separation factor of 2 in SXR, factor of 1. 1 in HXR. For extreme materials This combination of parameters would enable UK-XFEL to match or better all other existing FEL facilities to date in terms of photon energy range, maximum photon energy, repetition rate, pulse energy and minimum pulse duration. SXR/HXR split at 2 ke. V given by optics – gratings in SXR, crystals in HXR

Electron Beam Spec • What is the minimum electron beam energy required? • We assume fairly aggressive undulator parameters – SXR – here variable polarisation is required, and high repetition rate -> choose a DELTA type undulator. – HXR – here we don’t need variable polarisation so choose the UK SCU. • The required beam energy then depends on: – the minimum gap – the smaller the better – set 5 mm – the maximum AND the minimum photon energy that are required at a fixed electron beam energy (min photon energy corresponds to minimum gap) – the smaller this range the lower the required beam energy, therefore as we will need several FEL beamlines to cover the full photon energy range, set the tuning range of the highest photon energy beamlines (at each electron beam energy) to be small. – The minimum aw at highest photon energy – set awmin = 0. 7

FEL Beamlines + Electron Beam Energy • With these assumptions, UK-XFEL beamlines could be this: E SX 1 SX 2 HX 1 HX 2 HX 3 • Notes: Rep Rate Undulator 2. 4 Ge. V MHz DELTA 5. 6 Ge. V 8. 5 Ge. V 100 Hz UK SCU Period Tuning (ke. V) 27 mm 0. 25 – 1 18 mm 1– 2 25 mm 2– 5 21. 5 mm 5 – 10 15 mm 10 – 25 – For the SCU have used the data for the minimum period I know is feasible – 15 mm. Gives a maximum aw = 1. 87 @ 10 ke. V thus minimum aw = 0. 89 at 25 ke. V. – For the DELTA, if we wanted a higher minimum aw (-> bigger rho parameter, more power, shorter saturation length) the beam energy would be higher. For example, for a w (min) = 1. 0, SX 1/2 would need 3 Ge. V, HX 1/2 would need 6 Ge. V. – If 5 mm minimum gap is increased, beam energy also has to go up. – If less aggressive undulators are used beam energy also has to go up. – If we can’t get the high pulse energies at 25 ke. V, beam energy has to go up. – Therefore, these really are minimums. The only way they could be lower is if we had no gap/field tuning, just energy tuning. For example, HX 1/2 could then use a beam energy of 4. 7 Ge. V if aw = 0. 7.

Emittance, Peak Current, Charge • A key driver is the requirement to get m. J pulse energies at 25 ke. V. • It is anticipated that tapering in the FEL will be required to maximise the peak power. • Taper Efficiency, the factor by which the power can be increased beyond nominal saturation, is therefore important • For SASE a factor of 4 seems reasonable, unless combined with some method for improvement of temporal coherence where much higher efficiencies are theoretically possible (~20? ) • The following plots show, based on a Ming Xie parameterisation of the FEL peak power for HX 3 at 25 ke. V (SCU, 8. 5 Ge. V, energy spread 1 e-4), the required CORE bunch charge as a function of peak current and emittance to achieve different pulse energies. . .

1 m. J Pulse Energy Normalised emittance (mm-mrad) Required core Q (p. C) with Taper Efficiency = 4 Peak Current (A)