Optical Guiding in a Raman Free-Electron Laser Observed Using Wave Profile Modification Measurements

Optical guiding in a Raman free-electron laser is studied experimentally, using an electron beam (800kV, 200A) enclosed in a waveguide (R _{g} = 9mm) contained in an undulator (period = 1.7cm), all of which will generate power at lambda ~ 2mm in our FEL facility. The effect of optical guiding on the profile of the electromagnetic radiation is examined in the "ringdown" region which follows the termination of the electron beam using a polyethylene plug. In the ringdown region, the fields show a characteristic periodic structure which results from the upstream guiding. The fields are detected using a specially-developed waveguide probe. Experimental results are compared with numerical simulations (done as part of another dissertation). The evidence supports validation of optical guiding as it occurs in the linear regime of exponential growth in our experimental geometry. Further measurements of gain are reported herein; an unsuccessful experiment to detect optical guiding in the strong signal, trapped-particle regime of slow growth along a tapered undulator is also reported.