Ultimate bunch length and emittance performance of an MeV ultrafast electron diffraction apparatus with a dc gun and a multicavity superconducting rf linac

We present the design of a high repetition rate MeV energy ultrafast electron diffraction instrument based on a dc photoelectron gun and an superconducting rf (SRF) linac with multiple independently controlled accelerating and bunching cavities. The design is based on the existing Cornell photoinjector, which can readily be applied to the presented findings. Using particle tracking simulations in conjunction with multiobjective genetic algorithm optimization, we explore the smallest bunch lengths, emittance, and probe spot sizes achievable. We present results for both stroboscopic conditions (with single electrons per pulse) and with ${10}^5\mathrm{electrons}/\mathrm{bunch}$ which may be suitable for single-shot diffraction images. In the stroboscopic case, the flexibility provided by the many-cavity bunching and acceleration allows for longitudinal phase space linearization without a higher harmonic field, providing sub-fs bunch lengths at the sample. Given low emittance photoemission conditions, these small bunch lengths can be maintained with probe transverse sizes at the single micron scale and below. In the case of ${10}^5$ electrons per pulse, we simulate state-of-the-art 5D brightness conditions: rms bunch lengths of 10 fs with 3-nm normalized emittances, while now permitting repetition rates as high as 1.3 GHz. Finally, to aid in the design of new SRF-based ultrafast electron diffraction machines, we simulate the trade-off between the number of cavities used and achievable bunch length and emittance.

Figure 1

(a) 3D model of the apparatus. (b) Electric and magnetic field profiles along the beamline. The dc gun field is shown in blue, solenoid fields in green, rf cavities in orange, and the sample location is shown via a red line.

Figure 2

Dependence of achievable bunch length on initial bunch length at the cathode. An initial transverse spot size of $2\mu \mathrm{m}$ was used for these simulations.

Figure 3

(a) Dependence of achievable bunch length on final beam size in the absence of space charge forces for three different initial laser sizes: 2, 10, and $25\mu \mathrm{m}$ (dots), compared to a simple model (lines). Here the MTE is 35 meV, and so these laser sizes correspond to normalized source emittances of 0.5, 2.6, and 6.5 nm, respectively. (b) Comparison between optimizations using the default cavity field maps and ones without radial dependence of the accelerating fields. In both cases, an initial laser size of $10\mu \mathrm{m}$ was used.

Figure 4

Correlation between time and transverse momentum at the sample location. Examples are shown for both (a) typical and (b) minimal bunch lengths.

Figure 5

(a) Beam width and (b) bunch length and energy throughout the injector at near zero charge for an example solution on the Pareto front. The sample (and clipping aperture) is located at $z=8.228\mathrm{m}$.

Figure 6

(a) Influence of collimation on the achievable bunch length and emittance with a final bunch charge of 16 fC. (b) Effects of the cathode mean transverse energy (MTE) on the final bunch length and emittance at the same charge.

Figure 7

(a) Beam width and (b) bunch length and energy throughout the injector at a final charge of ${10}^5\mathrm{electrons}/\mathrm{pulse}$. After the sample (and clipping) aperture at $z=8.228\mathrm{m}$, the bunch had an emittance of 2.6 nm, beam width of $5\mu \mathrm{m}$, and bunch length of 10 fs.

Figure 8

Bunch transverse distribution and longitudinal phase space before and after going through the aperture at the sample location at a final charge of ${10}^5\mathrm{electrons}/\mathrm{pulse}$. After the aperture, the bunch had an emittance of 2.6 nm, beam width of $5\mu \mathrm{m}$, and bunch length of 10 fs. (a) Transverse distribution before aperture. (b) Transverse distribution after aperture. (c) Longitudinal phase space before aperture. (d) Longitudinal phase space after aperture.

Figure 9

Dependence of transverse brightness on the clipping aperture’s radius. The dashed line indicates the radius used in optimization.

Figure 10

(a) Trade-off between achievable temporal resolution and emittance for different numbers of SRF cavities. (b) Temporal resolution and bunch length for different numbers of SRF cavities at a constant emittance of 4 nm.