Experimental design of spectroscopic measurements of microwave electric field in MTX tokamak plasma

As part of the Microwave Tokamak Experiment (MTX), a method has been developed to measure the spatially resolved microwave electric field in plasmas. It combines laser-induced-fluorescence spectroscopy with a neutral particle beam (laser-aided particle probe spectroscopy). The RF electric field has been previously measured in small laboratory experiments. The MTX plasma, however, has a much higher density and temperature. One of the most difficult problems is to provide a sufficient density of the radiating atoms in the center of the (burned out) plasma. The authors propose that a sufficient density can be obtained with a small helium neutral-beam probe. A dye laser will be used to pump the helium atoms from the metastable level. The metastable helium atoms will be excited when they traverse a tiny metal-vapor chamber, or they will be produced by the collisional excitation in MTX plasma. The microwave electric field in the MTX plasma is expected to be several hundred kilovolts per centimeter. Therefore, the forbidden line will be strongly excited by the Stark effect. The intensity of the forbidden line emission will be measured to estimate the microwave electric field.<>