LA-UR-16-6007 High-Brightness Electron Beam Challenges for the Los Alamos MaRIE XFEL Bruce Carlsten October 7, 016
We expect that energy spread constraint will lead to difficult trades DISCUSSION POINTS: MaRIE emittance and energy-spread requirements Our pre-conceptual baseline design (specifically the photoinjector design) has addressed the emittance requirement at the expense of the energy spread requirement High compression ratios exacerbates energy spread constraint Discussion of energy-spread contributors Space-charge potential depression (sets an absolute minimum) Higher-order space harmonics (only from the photoinjector) Intrabeam scattering (IBS) (impacts BC energies, compression ratios) Laser heater needed for microbunch instability (mbi) suppression Other odd ones Slide
4-keV X-rays with a 1-GeV accelerator is aggressive Both the emittance and energy spread requirements become harder at low energy n x < 4 ray max I I K 1 K 3/ xray 8n A JJ / Linac target: n 0.10 mm, / = 0.01% Photon requirements: 5x10 10 4-keV photons 0.0% bandwidth (plot overestimates flux by about 30% decrease from time dependent effects) XFEL target: E n < 0. mm,, / 0.015% (factor of decrease in flux) DE=1.8 MeV =3.5 Slide 3
MaRIE photoinjector design has very low emittance due to solenoid improvement and low current MaRIE Injector PITZ Injector Modified Solenoid Slide 4
Exquisite phase-space manipulation through first cryomodule leads to very low emittance Slide 5
Low emittance comes at cost of low initial beam current (7 A) implies 500:1 compression to achieve 3.5 ka at MaRIE undulator Property Energy Charge Thermal n (copper *) Residual n Total RMS n RMS Length Value 195 MeV 100 pc 41 nm 60 nm 73 nm 5.5 ps RMS Energy Spread 0.08% Current vs. Time therm total In our current design, we suppress the thermal emittance by keeping the RMS beam size very small, which makes a very long, low current bunch We might be able to have twice the current (and need only half the compression) at the same total emittance if the thermal emittance contribution vanished W mc thermal Excess energy from emission process residual Slide 6
Beam s potential depression provides a minimum longitudinal emittance Essentially negligible RMS energy spread from potential depression: rms I 48 0 c (only about 9 kv for 3.5 ka) Corresponding longitudinal emittance is: norm, z Q 48 mc 0 (about 0.15 mm for 100 pc) Slide 7
Higher-order RF space harmonics only affect the energy spread in the photoinjector Higher-order space harmonics exist in the first cell of the photoinjector because of its length RMS energy spread is about 146 ev from PIC simulations (due to the potential depression) using only the fundamental space harmonic Harmonic Amplitude 1.05-0.85 3 0.053 4-0.010 5 0.00 6-0.0003 RMS energy spread is about 480 ev with all the space harmonics (RMS energy spread grows to ~ 5 kev in the second cell if not focused) Only the contribution from the synchronous space harmonic survives for a ultrarelativistic beam at constant radius: j I krr j E r z t dz E e 0 0( ) g d E e g z,, ) 0 0 ( ) 1 0 g( ) f z I ( k a) c c ( ) ( jz e dz 0 r We ve studied the energy spread induced by radial variations, energy spreads of order 0.01% are only theoretically possible with pathological focusing Slide 8
CSR can lead to two forms of energy spread CSR noise leads to an energy diffusion (worse at high energy, few kev at 1 GeV) CSR geometry can leads to an energy smearing (can be up to 1 MeV from compression in BC, included in elegant) E CSR, proj ECSR, peak This is because particles in the same slice at different radii map to different values of s along their trajectory in a BC. Estimate of the energy spread (in volts) is: V CSR V, proj CSR, peak 4 3 uncompressed Slide 9
Intra-beam scattering may be a big issue IBS-induced energy spread formula: DE rms mc I A ec x rms t norm I I A 1/ log IBS-induced energy spread doesn t have great energy scaling, higher gradient and larger transverse size helps Sample numbers for BC1/BC: 500 MeV/ GeV, 0:1/5:1 ratios, 45 and 60 degrees off crest (before BC1 and BC respectively), 0.1 mm beam size and 0.1 mm emitttance Pre BC1: 950 kev (can get this down with bigger beam size) BC1-BC: 440 kev Post BC: 160 kev Slide 10
Effect from laser heater Liable to exceed our 1.8 MeV energy spread threshold The maximum uncompressed energy spread with a 500:1 compression is 3.6 kev ESASE scheme or other alternative scheme could eliminate BC and need for a laser heater Final D versus laser heater energy including microbunch instability, for a total compression of 194:1 (corresponding to a photoinjector current of 18 A). Slide 11
We can buy ourselves out of trouble by going to higher energy (e.g., 15 GeV) 1/3 1 K K JJ / X ray 4 /3 I I A 1/3 Pierce parameter written in terms of X-ray wavelength The Pierce parameter is maximized around K=1.414 and it increases gradually with beam energy. 10 ISR 3.810 B 3/ [T] L 1/ The magnetic field for a fixed X-ray wavelength scales as 1/ so the ISR-induced energy spread K1 K / B[T] decreases with increasing beam energy 0.93 X ray if K is fixed. [m] ISR-induced energy spread in the undulator [cm] Slide 1
We can buy ourselves out of trouble by going to higher energy (e.g., 15 GeV) It will cost us $$$ and length to buy our way out of trouble so we do not want to overdesign the machine Slide 13