Tom Schultheiss

The polarized SRF gun experiment

RF electron guns are capable of producing electron bunches with high brightness, which outperform DC electron guns and may even be able to provide electron beams for the ILC without the need for a damping ring. However, all successful existing guns for polarized electrons are DC guns because the environment inside an RF gun is hostile to the GaAs cathode material necessary for polarization. While the typical vacuum pressure in a DC gun is better than 10{sup -11} torr the vacuum in an RF gun is in the order of 10{sup -9} torr. Experiments at BINP Novosibirsk show that this leads to strong ion back-bombardment and generation of dark currents, which destroy the GaAs cathode in a short time. The situation might be much more favorable in a (super-conducting) SRF gun. The cryogenic pumping of the gun cavity walls may make it possible to maintain a vacuum close to 10{sup -12} torr, solving the problem of ion bombardment and dark currents. Of concern would be contamination of the gun cavity by evaporating cathode material. This report describes an experiment that Brookhaven National Laboratory (BNL) in collaboration with Advanced Energy Systems (AES) is conducting to answer these questions.

CW RF Cavity Design for High-Average-Current Photoinjector for High Power FEL

This project is a coordinated effort among NAVSEA, LANL and AES, to develop a key enabling technology for high-power FEL called for by the High Energy Laser Joint Technology Office: a high current RF photoinjector capable of producing continuous average current greater than 100 mA. The specific aim is a π-mode, normal-conducting RF photoinjector, 5 nC of bunch charge, 100 mA of current (at 21.88 MHz bunch repetition rate) and emittance less than 10 mm-mrad. This level of performance will enable robust 100-kW-class FEL operation with electron beam energy <100 MeV, thereby reducing the size and cost of the FEL. This design is scalable to the MW power level by increasing the electron bunch repetition rate from 21.88 MHz (the 32nd sub-harmonic of 700 MHz) to a higher value. The major challenges are emittance control and high heat flux within the CW 700 MHz RF cavities. Preliminary results of RF cavity designs and cooling schemes are presented, including both high-velocity water and liquid-nitrogen cooling options. PACS codes: 41.60. Cr, 29.17+w, 29.27. Bd, 41.75. Fr