Spectroscopic Diagnostics of Tokamak Plasma Heating by High Power Microwave

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1. We have developed a diagnostic system using laser-aided particle-probe spectroscopy (LAPPS) to measure the high power microwave (>1 GW) generated by a free electron laser (FEL) beam for the ECH study in the Microwave Tokamak Experiment (MTX) at the Lawrence Livermore National Laboratory. In this diagnostics we observe flourescences induced by the forbidden transitions due to the Stark effect from the helium metastable atoms to determine the electric field strength in the plasma.2. We designed the diagnostic apparatus after detailed discussions about the specifications of the apparatus. It consists of a neutral helium beam (50keV, 0.2A), a excimer-laser excited dye laser exciting the neutral beam, and a spectroscopic system with two channels of light detector.3. In the assembled apparatus, we have measured the triplet metastable atom density in the LAPPS helium beam. To avoid stray light due to the laser (389 nm, 2^3S*3^3P), we also observed a cascade flourescence (706 nm, 3^3S*2^3S) … More to measure the matastable atom density.4. We measured the vxB field by raising the forbidden transition (2^3S*3^3D) due to the motional Stark effect when the 45-keV helium beam crossed the MTX magnetic field (the calculated field was 96 kV/cm) to calibrate the FEL electric field measurements.5. We measured the FEL electric-field in vacuum by a similar way as for the vxB field measurement except that we had to extract large background light produced by a wide-wavelength-band light burst that occurred when the FEL pulse reached the MTX tokamak. This is the first time to measure the high power FEL electric-field in the center of tokamak chamber, and the measured result (200 kV/cm) is helpful as a standard to measure the FEL absorption in the plasma.6. Because of time constraints caused by long elapse for the development of FEL, the measurement of the FEL electric-field in the MTX plasma was not made. In conclusion, our Joint Research was fruitful in that we developed the diagnostic system as the first time to measure the electric field in the tokamak plasma and observed the high power microwave of an order of 1 GW. Less