Temporal behavior of neutral particle fluxes in TFTR neutral beam injectors

• Published in: Review of scientific instruments 1989-12, Vol.60 (12), p.3721-3729
• Main Authors: Kamperschroer, J. H., Gammel, G. M., Roquemore, A. L., Grisham, L. R., Kugel, H. W., Medley, S. S., O’Connor, T. E., Stevenson, T. N., von Halle, A., Williams, M. D.
• Format: Article
• Language: English
• Subjects: 70 PLASMA PHYSICS AND FUSION TECHNOLOGY; 700208 - Fusion Power Plant Technology- Inertial Confinement Technology; APERTURES; BEAM CURRENTS; BEAM INJECTION; CALORIMETERS; COMPUTERIZED SIMULATION; CURRENTS; Exact sciences and technology; FLOW RATE; ION SOURCES; JET TOKAMAK; MEASURING INSTRUMENTS; NEUTRAL BEAM SOURCES; NEUTRAL PARTICLES; OPENINGS; Other topics in plasma physics and electric discharges; PARTICLES; Physics; Physics of gases, plasmas and electric discharges; Physics of plasmas and electric discharges; PLASMA; SIMULATION; TFTR TOKAMAK; THERMONUCLEAR REACTORS; TIME DEPENDENCE; TIMING PROPERTIES; TOKAMAK TYPE REACTORS
• ISSN: 0034-6748; 1089-7623
• DOI: 10.1063/1.1140482

Data from an E∥B charge exchange neutral analyzer (CENA), which views down the axis of a neutral beamline through an aperture in the target chamber calorimeter of the TFTR neutral beam test facility, exhibit two curious effects. First, there is a turn‐on transient lasting tens of milliseconds having a magnitude up to three times that of the steady state level. Second, there is a 720 Hz, up to 20% peak‐to‐peak fluctuation persisting the entire pulse duration. The turn‐on transient occurs as the neutralizer/ion source system reaches a new pressure equilibrium following the effective ion source gas throughput reduction by particle removal as ion beam. Widths of the transient are a function of the gas throughput into the ion source, decreasing as the gas supply rate is reduced. Heating of the neutralizer gas by the beam is assumed responsible, with gas temperature increasing as gas supply rate is decreased. At low gas supply rates, the transient is primarily due to dynamic changes in the neutralizer line density and/or beam species composition. Light emission from the drift duct corroborate the CENA data. At high gas supply rates, dynamic changes in component divergence and/or spatial profiles of the source plasma are necessary to explain the observations. The 720 Hz fluctuation is attributed to a 3% peak‐to‐peak ripple of 720 Hz on the arc power supply amplified by the quadratic relationship between beam divergence and beam current. Tight collimation by CENA apertures cause it to accept a very small part of the ion source’s velocity space, producing a signal linearly proportional to beam divergence. Estimated fluctuations in the peak power density delivered to the plasma under these conditions are a modest 3%–8% peak to peak. The effects of both phenomena on the injected neutral beam can be ameliorated by careful operation of the ion sources.