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A measurement of space-charge fields in a microwave free electron laser

We report on the measurement of the electrostatic fields between the space 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 radiation at 24 GHz (12.5 mm). The FEL is operated in the exponential gain regime, and amplifies a signal of several kW which is provided by a magnetron. A specially designed and calibrated probe, described herein, is used to determine the electrostatic fringing field near the electron beam, which is then corrected to give the field within the beam. A field ∼60 kV/cm was measured when the FEL power was ∼1 MW, and is in the range which is predicted by numerical simulation of the theory.