The Boeing photocathode accelerator magnetic pulse compression and energy recovery experiment

An 18 MeV photocathode accelerator, consisting of a two-cell RF photocathode injector (previously operated at 27 MHz micropulse repetition frequency, 5 nC micropulse charge, and 25% duty factor [D.H. Dowell et al., Appl. Phys. Lett. 63 (1993) 2035]) followed by four new multicell cavities, all operating at 433 MHz, is currently being tested for FEL applications. In the present experiment, the inclusion of a third harmonic RF linearizer (operating at 1300 MHz) will allow tailoring of flat-topped micropulse shapes with high peak current. This pulse shaping leads to optimal FEL efficiency and is discussed in another contribution to this conference [J.L. Adamski et al.]. The 1300 MHz structure can also be used in a simple experiment to investigate energy recovery for high power and high efficiency FELs. In this case, proper RF phasing of the drive laser micropulses will allow the 1300 MHz accelerator section to simultaneously accelerate and decelerate alternate micropulses. Longitudinal phase space preparation for both experimental modes is discussed.