An experimental analysis of the waveguide modes in a high-gain free-electron laser amplifier

The presence, growth, and interaction of transverse waveguide modes in high-gain free-electron laser (FEL) amplifiers has been observed and studied. Using the Electron Laser Facility at Lawrence Livermore National Laboratory, a 3 MeV, 800 A electron beam generated by the Experimental Test Accelerator was injected into a planar wiggler. Power was then extracted and measured in the fundamental (TE{dollar}sb{lcub}01{rcub}{dollar}) and higher-order modes (TE{dollar}sb{lcub}21{rcub}{dollar} and TM{dollar}sb{lcub}21{rcub}{dollar}) under various sets of operating conditions. Horizontal focusing through the wiggler was provided by external quadrupole magnets. There was no axial guide field. The input microwave signal for amplification was generated by a 100 kW magnetron operating at 34.6 GHz. Power measurements were taken for both flat and tapered wigglers, for two sizes of waveguide, and for both fundamental and higher mode injection. Mode content was determined by sampling the radiated signal at specific points in the radiation pattern. For the flat wiggler and with the large waveguide (2.9 cm x 9.8 cm) the power in the higher modes was comparable to power in the fundamental. Both exhibited gains greater than 30 dB/m prior to saturation and both reached powers in excess of 80 MW. Choice of injection mode had little effect on the operation of the system. Operation with the smaller guide (WR-229) provided much better mode selectivity. The fundamental mode continued to show optimum gain in excess of 30 dB/m while the higher-mode gain was of order 20 dB/m. As expected, power output increased significantly with the tapered wigglers. The relative mode content depended on the specific taper used. Operation with a linear taper resulted in both modes showing significant power increases at 3 m to about 400 MW each. However, an optimized non-linear taper resulted in an increase in power in the fundamental to 750 MW while power in the higher-order modes was unchanged.