High average power free-electron lasers (FELs) require high average current electron injectors capable of generating high quality, short duration electron bunches with a repetition rate equal to the frequency of the rf linac. In this paper we propose, analyze, and simulate an rf-gated, gridded thermionic electron gun for use in high average power FELs. Thermionic cathodes can provide the necessary high current, have long lifetimes, and require modest vacuums. In the proposed configuration the rf-gated grid is modulated at the fundamental and 3rd harmonic of the linac frequency. The addition of the 3rd harmonic on the grid results in shorter electron bunches. In this configuration, every rf bucket of the linac accelerating field contains an electron bunch. Particle-in-cell simulations indicate that this approach can provide the necessary charge per bunch, bunch duration, longitudinal and transverse emittance, and repetition rate for high average power FELs operating in the IR regime.

https://link.aps.org/pdf/10.1103/PhysRevSTAB.14.020702

High average current electron guns for high-power free electron lasers

Currently available, fully coherent (laser) light sources emit radiation only in a limited range of wavelengths in the infrared, visible and near-ultraviolet range, excluding their use in all the measurements needing tunable photons of energy higher than a few eV. Consequentially, the strong scientific need for tunable, coherent radiation sources from the vacuum ultraviolet (VUV) to the X-ray in the femtosecond and picosecond time domain has spurred international research efforts to develop a new generation of research facilities. Laser-driven light sources that use non-linear processes to create very high harmonics and the interaction between an ultra-short laser pulse and an electron bunch in a storage ring are both able to produce radiation pulses in the femtosecond time domain and in the soft EUV and X-ray region, but with a relatively low useful photon flux on the sample. In contrast Free-Electron Lasers (FELs) can produce light pulses with peak brilliance as much as ten orders of magnitude higher than the pulses generated in present third-generation synchrotron light sources and with photon energies ranging from the VUV to the hard X-rays, i.e. from about 10eV (120nm) to 10keV (0.12nm). This manuscript reviews the scientific fields that can be opened or extended by the new techniques along with the progress of FEL physics and the supporting accelerator physics and technology.

/pdf/vuv-and-x-ray-free-electron-lasers-the-technology-and-its-4l3q3bh9cg.pdf

VUV and X-ray free-electron lasers: The technology and its scientific promise

Electron bunches for high performance free electron lasers are subject to constraints on charge per bunch and pulse shape. A Diamond secondary emitter used in conjunction with a photocathode and drive laser has potential to enable a high brightness, high peak current photoinjector by increasing the effective quantum efficiency of the photocathode. A theoretical characterization of the bunches so produced has been heretofore absent. Using a combination of Monte Carlo and analytical models, the shape of the bunches, their transit time, and emission time constants are determined and shown to be sensitive to the accelerating field in the diamond flake, incident beam profile, doping, and surface conditions. Methods to allow for extension to regimes of technological interest in terms of diamond thickness, external field, and primary pulse shape are given.

Bunch characteristics of an electron beam generated by a diamond secondary emitter amplifier

Usable lifetime has long been a limitation of high efficiency photocathodes in high average current accelerator applications such as free electron lasers, where poor vacuum conditions and high incident laser power contribute to early degradation in electron beam emission. Recent progress has been made in adapting well known thermionic dispenser techniques to photocathodes, resulting in a dispenser photocathode whose photosensitive surface coating of cesium can be periodically replenished to extend effective lifetime. This article details the design and fabrication process of a prototype cesium dispenser photocathode and describes in detail the dominant factors affecting its performance: activation procedure, surface cleanliness, temperature, and substrate microstructure.

Factors affecting performance of dispenser photocathodes

Requirements and challenges for high-brightness electron injectors operating in a high-repetition-rate X-ray FEL are described. Schemes presently under development or study are reviewed, and their advantages and limitations are compared. Beam dynamics and engineering/technological aspects are addressed, with a particular emphasis placed on how the high-repetition-rate requirement impacts the choice of cathodes and of gun/accelerator technologies, and on how those choices consequently impact beam dynamics.

Schemes and challenges for electron injectors operating in high repetition rate X-ray FELs

High-average-power FELs require high-current, low-emittance and low-energy-spread electron beams. These qualities have been achieved with RF photoinjectors operating at low-duty factors. To date, a high-average-current RF photoinjector operating continuously at 100% duty factor is yet to be demonstrated. The principal challenges of a highduty-factor normal-conducting RF photoinjector are related to applying a high accelerating gradient continuously, thus generating large ohmic losses in the cavity walls, cooling the injector cavity walls and the high-power RF couplers, and finding a photocathode with reasonable Q.E. that can survive the poor vacuum of the RF photoinjector. We present the preliminary design of a normal-conducting 700 MHz photoinjector with solenoid magnetic fields for emittance compensation. The photoinjector is designed to produce 2.7 MeV electron beams at 3 nC bunch charge and 35 MHz repetition rate (100 mA average current). The photoinjector consists of a 2 1 -cell, p-mode, RF cavity with on-axis electric coupling, and a non-resonant vacuum plenum. Heat removal in the resonant cells is achieved via dense arrays of internal cooling passages capable of handling high-velocity water flows. Megawatt RF power is coupled into the injector through two tapered ridge-loaded waveguides. PARMELA simulations show that the 2 1 -cell injector can produce a 7mm emittance directly. Transverse plasma oscillations necessitate additional acceleration and a second solenoid to realign the phase space envelopes of different axial slices at higher energy, resulting in a normalized rms emittance of 6.5mm and 34 keV rms energy spread. We are developing a novel cesiated p-type GaN photocathode with 7% quantum efficiency at 350 nm and a cesium dispenser to replenish the cathode with cesium through a porous silicon carbide substrate. These performance parameters will be necessary for the design of the 100 kW FEL.

Overview of the 100 mA average-current RF photoinjector

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Overview of the 100 mA average-current RF photoinjector