120-keV neutral-beam injection system development

The neutral-beam injection systems for the next generation of U.S. fusion experiments (TFTR, MX, Doublet III) have requirements that considerably exceed the present state of development. TFTR, for example, desires 20 MW of 120-keV deuterium atoms in pulses of 0.5-sec duration. The neutral-beam systems needed to meet these demands will be large and complex and will require much development effort. In order to develop components for these new systems, a new, large test facility has been constructed. The vacuum system, which has a volume in excess of 170,000 liters, will handle the pulsed gas loads by volume expansion; the gas will be pumped out in the one-minute intervals between pulses. The power-supply philosophy and specifications are discussed elsewhere. A mini-computer system will be used to monitor and control the power supplies and to diagnose the beam. Of the beam-line components, a plasma source and 120-kV accelerator structure which are under construction are discussed. A new long-pulse version of the 15-cm-diam high-current plasma source has been developed. Significant improvements have been made by re-orienting the filaments, so that they produce closed magnetic field lines near the wall, and by changing the shape and position of the anode. This new geometry has allowed considerable simplification in source construction. The accelerator structure is based on a computer-optimized design with four elements. The entrance-grid element is a 60%-transparent, 10- x 10-cm array of slots designed for an extraction-current density of approximately 0.25 A/cm2. In addition to providing long-pulse, high-voltage operating experience with a large-current system, this program will allow the investigation of the very difficult problems associated with beam dumps and beam calorimetry