Retarding field analysis of long pulse electron beams through combined bifurcated bifilar wiggler and guide magnetic fields.

Abstract

In the past several years, the free electron laser (FEL) has been used to produce frequency tunable coherent radiation in the millimeter to submillimeter wavelength range. The reasonably high efficiency and power levels already achieved have spurred further work to refine and improve the FEL, particularly for lower wavelength applications. In a free electron laser, the electron beam is given a substantial transverse velocity component by passing it through a transverse periodic magnetic field commonly referred to as a wiggler. Recent work includes analytical and experimental investigations of beam propagation dynamics in the wiggler field. The present investigation is an experimental and numerical study of a type of wiggler called the bifurcated bifilar wiggler. In the present investigation a retarding field energy analysis is performed upon a long-pulse, mildly relativistic electron beam after it passes through combined bifurcated bifilar wiggler and axial magnetic fields. A 5-$\mu$s, 75-kV, 0-20 A, square output pulse electron beam is generated by a 3-stage, crowbarred Marx generator with a 10-90% risetime of less than 1 $\mu$s. The electron gun contains a lanthanum hexaboride (LaB6) cathode which is heated by electron bombardment in a Pierce electron-gun geometry. The cathode is immersed in a collimating axial guide field of 1.1 kG which extends over the length of the beam. The variable pitch wiggler has an entrance pitch of 5.65 cm and is 30 cm long. Diagnostics include a resistive-divider beam voltage monitor, a Pearson coil cathode current monitor, and a post-wiggler current collector. An aperture in the current collector allows part of the beam to enter a newly-developed retarding potential velocity analyzer. This analyzer is able to operate at full beam voltage of up to 85 kV. Numerical methods were used to produce computer generated retarding potential curves for operation at various combinations of system parameters. It was shown by simulation that the analyzer gives good selection of axial velocity, and that it could operate properly when the electrons had large Larmor radii. A code was also used to compare three-dimensional realizable wiggler operation with existing one-dimensional idealized wiggler theory. The experimental and numerical retarding potential plots were compared and demonstrated good agreement. The wiggler demonstrated a high level of axial-to-transverse energy conversion. However, it also greatly increased the axial velocity spread of the beam as it passed through the wiggler.