Behavior of a free-electron laser in the vicinity of gyroresonance

The behavior of a coaxial wiggler free-electron laser (FEL) amplifier was examined in the vicinity of gyroresonance. This resonance between the wiggler induced and cyclotron motions of the beam is possible when a solenoidal field is present in addition to the transverse wiggler field. The result is an increase in the transverse motion of the beam which can enhance the FEL interaction. Orbits on Group I (orbits below gyroresonance) and Group II (above gyroresonance) exhibited very different behavior. Amplification was achieved much closer to gyroresonance on the Group II side. This increases the possible enhancement on the Group II side, but also makes these orbits much more sensitive to variations in voltage. Disadvantages of the existence of the solenoidal field include the appearance of cyclotron maser oscillations over a certain range of the solenoidal field. A scan of the parameters to map out where these oscillations occur was performed. The Coaxial Hybrid Iron (CHI) wiggler, by its very nature, includes a solenoidal field and the possibility for working close to gyroresonance. This is because the wiggler field itself is simply produced by ferromagnetic rings and spacers immersed in a solenoidal field. The simple design of the CHI wiggler makes it very easy to scale the wiggler down to small periods, which would lead to operation at low voltages and high frequencies without diminishing the periodic transverse velocity imparted to the electrons. The present experiment has a wiggler period of 0.64 cm and a center frequency of 35 GHz. The interaction bandwidth measured was on the order of a few percent for both Group I and Group II operation. Problems experienced with beam transmission prevented the device from operating at the design value of current. The linear gain was therefore limited to about 5 dB on the Group I side, and 7 dB on the Group II side, when wideband grazing intersection operation was achieved. For cutting intersection between the beam line and the TE...