Simulations of the TJNAF FEL with tapered and inversely tapered undulators

Abstract

Experiments using the TJNAF FEL have explored the operation with both tapered and inversely tapered undulators. We present here numerical simulations using the TJNAF experimental parameters, including the effects of taper. Singlemode simulations show the effect of taper on gain. Multimode simulations describe the evolution of short optical pulses in the far infrared, and show how taper affects single-pass gain and steady-state power as a function of desynchronism. A short optical pulse presents an ever-changing field strength to each section of the electron pulse so that idealized operation is not possible. Yet, advantages for the recirculation of the electron beam can be explored. r

Simulations of the 100kW TJNAF FEL using a step-tapered undulator

“…For linear taper ðD ¼ 0Þ; we considered different values of d ¼ 7p; 72p; 74p corresponding to a change DK=K ¼ 70:9%; 71:9%; 73:8%; respectively. We found that negative taper, d ¼ Àp; À 2p gave the highest efficiency ZE0:9% at desynchronism d ¼ 0:04: This is similar to our simulations of the TJNAF 10 kW FEL [8]. For these cases, the full-width induced energy spread was Dg=g ¼ 3%; which is below the limit for safe recirculation.…”