Design of microwave electric field diagnostics (LAPPS) in MTX tokamak plasma

The Microwave Tokamak Experiment (MTX) at LLNL applies an ultra high (peak) power microwave (a few GW) into the Tokamak plasma for current drive, plasma heating, etc. One of the key issues of this MTX project is to understand the ray tracing and power deposition of the ultra high power microwaves in the Tokamak plasma. A new method has been developed to measure the spatially resolved microwave electric field in MTX Tokamak plasma. This method combines laser induced fluorescence spectroscopy with a neutral particle beam; Laser-Aided Particle Probe Spectroscopy (LAPPS). One of the most difficult problems is to provide a sufficient density of the radiating atoms in the center of the (burned out, and highly collisional) plasma. The authors' computer simulation calculations indicate that a sufficient density of radiating atoms can be obtained at the center of MTX plasma with a helium diagnostic neutral beam(energy = 50 keV, current (metastable triplet) = 80 mA). The metastable helium atoms will be produced when they traverse a Xe charge exchange chamber (about 10% conversion efficiency). A dye laser will be used to pump the helium atoms from the metastable level