Nanosecond Radio-Frequency Pulse Driven Photogun for Very Hard X-ray Free-electron Laser

TL;DR

This work introduces CUPID, a nanosecond rf-pulse photogun designed to reach high gradient operation ($\sim$500 MV/m) while mitigating rf breakdowns via ultrashort pulses at $11.424$ GHz. The photogun (1.6‑cell over-coupled cavity) is integrated with a superconducting solenoid and three S-band linacs to form a photoinjector capable of delivering beams with $\varepsilon_n \approx 58$ nm at 100 pC, enabling low-emittance, high-brightness pulses suitable for hard x-ray FELs. End-to-end simulations with the LCLS copper accelerator show CUPID can deliver $\gtrsim$0.1 mJ-level pulses up to $60$ keV, with a notable improvement over the LCLS nominal case (e.g., $\sim$0.32 mJ at 40 keV) and manageable limits due to existing FEL undulator layouts. The combination of high-gradient photogun technology, precise rf pulse shaping, and integration with established accelerator infrastructure positions CUPID as a path toward brighter, higher-energy x-ray FELs, with prototypes and high-power rf tests underway. $\mathcal{B}_{6D}$ brightness scales as $\mathcal{B}_{6D} \propto E^{\nu}$ for suitable beam geometries, and achieving $E$ in the hundreds of MV/m regime substantially boosts FEL efficiency and photon energies.

Abstract

One pathway to producing high brightness electron beams is to use a radio-frequency (rf) driven high field photogun to rapidly accelerate photoemitted electrons to the relativistic regime and preserve the brightness. However, the highest attainable field is limited by rf breakdowns of materials used in a photogun. Shortening rf pulse duration feeding into a photogun provides a viable pathway to achieve high field and prevent rf breakdowns. Here we propose and investigate Compressed Ultrashort Pulse Injector Demonstrator (CUPID), a nanosecond rf pulses driven photogun powered by a klystron and rf pulse compression system capable of achieving 300 MW at 20 ns duration, to produce bright electron beams with high electric field. We first introduce the design of the CUPID photogun and its expected rf performance at 500 MV/m driven by high power nanosecond rf pulses, followed by beam dynamics studies showing its capability for producing bright electron beams with 60 nm emittance when forming a photoinjector with a superconducting solenoid and downstream accelerating structures. Finally, we show a proof-of-concept start-to-end simulation of the CUPID photoinjector paired with the existing Linac Coherent Light Source (LCLS) copper accelerator free-electron laser (FEL) to demonstrate achievable mJ pulse energy very hard x-ray photons at 40 keV or higher.

Figures (20)

• Figure 1: X-ray FEL outputs for different photon energies with 8GeV and 15GeV 100pC 3.5kA beams with emittances at 100.0, 200.0, 300.0, 400.0 and 500nm respectively, calculated using Ming-Xie parameterization. Parameters taken from Fig. 1-3 of ref osti_1616511.
• Figure 2: Cut-away view of a solid model of the CUPID photogun. Copper parts are shown in varying shades of blue. The exit of the photogun (right) is designed as a 11.43mm radius waveguide to connect to a TM$_{01}$ mode launcher. Four tuning pins are available for frequency tuning of both cells. The copper backplate (left) of the photogun serves as the cathode. It has large aperture sizes to reach a very low $Q_l$.
• Figure 3: Photograph of fabricated test cavity for CUPID. This cavity will be used in upcoming high power tests to verify the ability to generate high electric fields with short pulses and low breakdown rate.
• Figure 4: Cut-away view of the relative spacing between the CUPID photogun, the connecting waveguide, and the TM10 mode launcher, with superimposed $\uppi$-mode field map. The relatively long waveguide connecting both components is a design choice to accommodate the physical size of a solenoid designed for CUPID photoinjector. Extended waveguide and solenoid are not shown in this view.
• Figure 5: Plot of simulated $S_{11}$ reflection parameter of the CUPID photogun. At 11.424GHz, the $S_\text{11}$ reflection parameter is -0.59dB ("$\times$" sign).