A MEASUREMENT OF SPACE-CHARGE FIELDS IN A MICROWAVE FREE-ELECTRON LASER

We report on the measurement of the electrostatic fields between the s pace charge bunches, done in an operating Free Electron Laser (FEL) at Columbia University. The FEL uses a 600 kV, 100 A electron beam and a helical undulator with period of 4 cm to produce a few MW of radiatio n at 24 GHz (12.5 mm). The FEL is operated in the exponential gain reg ime, and amplifies a signal of several kW which is provided by a magne tron. A specially designed and calibrated probe, described herein, is used to determine the electrostatic fringing held near the electron be am, which is then corrected to give the field within the beam. A field similar to 60 kV/cm was measured when the FEL power was similar to 1 MW, and is in the range which is predicted by numerical simulation of the theory.