Stark-effect measurement of high FEL [free-electron laser] electric fields in MTX [Microwave Tokamak Experiment] by laser-aided particle-probe spectroscopy

We are constructing a diagnostic system to measure the electric field (>100 kV/cm) of a free-electron laser (FEL) beam when injected into the plasma of the Microwave Tokamak Experiment (MTX). The apparatus allows a crossed-beam measurement, with 2-cm spatial resolution in the plasma, involving the FEL beam (with 140-GHz, ∼1-GW ECH pulses), a neutral-helium beam, and a dye-laser beam. After the laser beam pumps metastable helium atoms to higher excited states, their decay light is detected by a collimated optical system. Because of the Stark effect due to the FEL electric field (rvec E), a forbidden transition can be strongly induced. The intensity of emitted light resulting from the forbidden transition is proportional to E2. Because photon counting rates are calculated to be low, extra effort is made to minimize background and noise levels. It is possible that the lower rvec E of an MTX gyrotron-produced ECH beam with its longer-duration pulses also can be measured using this method. Other applications may include measurements of ion temperature (using charge-exchange recombination), edge-density fluctuations, and core impurity concentrations. 11 refs., 2 figs., 2 tabs