Overview of the 100 mA average-current RF photoinjector

Abstract

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 high-duty-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 2 -cell, π-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 2 -cell injector can produce a 7 μm 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 6.5 μm 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.