Laser photo-detachment measurements in the ECR driven H-negative ion source PROMETHEUS I
Résumé
Large interest is presently devoted to the development of high-power negative ion sources applicable to the International Thermonuclear Experimental Reactor (ITER) project. Hydrogen negative ions H- are efficiently used to generate neutral beams for heating magnetically confined fusion plasmas [1]. Despite the large scale of ITER project, dedicated bench-scale experiments try to optimize the different sources acting on the elementary processes at the basis of the production of negative ions [2]. Understanding the negative ion formation mechanisms requires diagnostic techniques which can determine the absolute density of the hydrogen negative ions in the bulk plasma of these sources [3]. Photo-detachment is possibly the most convenient method towards this direction [3,4]. In the frame of the above concept and as continuity of our recent research (e.g. [5,6]), a new H- negative ion source named PROMETHEUS I has been installed in the High Voltage Laboratory of the University of Patras. The source is based on a cubic [(240 mm)3] stainless steel high vacuum chamber (base pressure 1.5x10-6 Torr). Instead of traditional filaments, the power is delivered by five elementary plasma sources sustained at electron cyclotron resonance (ECR) [7], due to a combination of microwaves (2.45 GHz) and static magnetic fields (875 G). Furthermore, rather than conventional magnetrons, the microwaves are produced by PC-controlled solid state components. In this work, this new source is probed by means of single beam laser photo-detachment technique. The plasma is locally disturbed by photons of 1.2 eV energy and the electrons photo-detached from the H- ions are collected by a thin cylindrical electrostatic probe. The H- density is eventually calculated for various working pressures (up to 10 mTorr) and microwave powers (up to 1 kW), providing an efficient operational window of the source. The full process, including the calibration method in order to achieve full conversion of H- ions into photo-electrons along the laser beam path, is as well considered herein.