A METASTABLE HELIUM BEAM PROBE DIAGNOSTIC TECHNIQUE

A DIAGNOSTIC HELIUM (HE) BEAM IS PLANNED THAT APPLIES COLLISIONALLY INDUCED FLUORESCENCE OF THE BEAM ATOMS IN METASTABLE STATES TO MEASURE THE TIME EVOLUTION OF DENSITY FLUCTUATIONS AND THE PROFILES OF PLASMA DENSITY AND ELECTRONTEMPERATURE IN MAGNETIC CONFINEMENT EXPERIMENTS. THE METASTABLE HELIUM ATOMS ARE FORMED WHEN A BEAM OF HE IONS ISNEUTRALIZED BY RESONANT CHARGE EXCHANGE WITH ATOMS IN A GAS CELL. IN THE PROCESS OF NEUTRALIZATION, ABOUT 1 TO 10% OF THE BEAM ATOMS ARE PRODUCED IN THE METASTABLE 2(1)S AND 2(3)S LEVELS. BECAUSE THE EXCITATION RATE FROM THE METASTABLE LEVEL IS TYPICALLY A FACTOR OF 10(3) TO 10(4) LARGER THAN TRANSITIONS FROM THE GROUND STATE, COLLISIONALLYINDUCED FLUORESCENCE FROM THE BEAM PROPAGATING THROUGH A PLASMA IS SIGNIFICANTLY INCREASED BY THE ENHANCED POPULATIONOF METASTABLE STATES. THE FLUORESCENCE SIGNAL FROM THE PROBE BEAM IS PROPORTIONAL TO THE LOCAL DENSITY AND IS USED TO MEASURE THE DENSITY FLUCTUATIONS LOCALLY IN A PLASMA. BOTH TEMPORAL AND SPATIAL INFORMATION ABOUT THE FLUCTUATIONS SHOULD BE OBTAINABLE. IN ADDITION, THE EXCITATION RATES OF SOME SPECIFIC SPECTRAL TRANSITIONS OF HEARE VERY TEMPERATURE DEPENDENT. THEREBY, THE RATIO OF THE LINE INTENSITY OF THE HE TRANSITION THAT IS TEMPERATURE INDEPENDENT TO ONE THAT IS HIGHLY TEMPERATURE DEPENDENT COULD YIELD A SENSITIVE MEASUREMENT OF THE ELECTRON TEMPERATURE. THESE MEASUREMENTS COULD PROVIDE TEMPORAL AND SPATIAL DATA LEADING TO A BETTER UNDERSTANDING OF TOKAMAK PLASMA EQUILIBRIUM, STABILITY, AND TRANSPORT. PHASE I OF THIS PROJECT EVALUATES THIS DIAGNOSTIC TECHNIQUE AND PERFORMS A CONCEPTUAL DESIGN OF A DIAGNOSTIC SYSTEM FOR USE ON A TOKAMAK, SUCH AS THE POLOIDAL BEAM EXPERIMENT-MODIFIED,WHICH COULD BE IMPLEMENTED IN PHASE II.