L. R. Grisham

Measurements of neutral beam species, impurities, spatial divergence, energy dispersion, pressure, and reionization using the TFTR U.S. Common Long Pulse Ion Source

Results are given from the first comprehensive and complementary measurements using the final production U.S. Common Long Pulse Ion Sources mounted on both the TFTR neutral beam test beamline and the TFTR neutral beam injection system, with actual tokamak experimental conditions, power systems, controls, and operating methods. The set of diagnostics included water calorimetry, thermocouples, vacuum ionization gauges, photodiodes, neutron, gamma‐ray, and charged particle spectroscopy, optical multichannel analysis, charge exchange spectroscopy, Rutherford backscatter spectroscopy, and implantation/secondary ion mass spectroscopy. These systems were used to perform complementary measurements of neutral beam species, impurities, spatial divergence, energy dispersion, pressure, and reionization. The measurements were performed either in the neutralizer region, where the beam contained both ions and neutrals, or in the region of the output neutral beam. The average of the neutral particle ratios in the range f...

High current source of He− ions

A negative helium ion beam of 70 mA at 10.5 kV has been produced by charge exchange in sodium. The production is studied as a function of sodium line density, beam energy and background helium gas density. The characteristics of this high current He− source are analyzed to determine the design requirements for He− beam generation in the range of tens to hundred of milliamperes.

TFTR neutral beam injected power measurement

Energy flow within TFTR neutral beamlines is measured with a waterflow calorimetry system capable of simultaneously measuring the energy deposited within four heating beamlines (three ion sources each), or of measuring the energy deposited in a separate neutral beam test stand. Of the energy extracted from the ion source on the well‐instrumented test stand, 99.5±3.5% can be accounted for. When the ion deflection magnet is energized, however, 6.5% of the extracted energy is lost. This loss is attributed to a spray of devious particles onto unmonitored surfaces. A 30% discrepancy is also observed between energy measurements on the internal beamline calorimeter and energy measurements on a calorimeter located in the test stand target chamber. Particle reflection from the flat plate calorimeter in the target chamber, which the incident beam strikes at a near‐grazing angle of 12°, is the primary loss of this energy. A slight improvement in energy accountability is observed as the beam pulse length is increased...

Neutralization of multi-MeV light negative ions by plasma neutralizers

A hollow cathode discharge fed plasma neutralizer has been built and was used to neutralize 3‐MeV Li−, C−, and Si− ions. Initial results indicate that the performance of an unoptimized plasma neutralizer is better than gas stripping. Since hollow cathodes produce plasma targets with high efficiency, this type of plasma neutralizer can improve the overall efficiency of a neutral beam line.

Doppler-shifted neutral beam line shape and beam transmission

Analysis of Doppler‐shifted Balmer‐α line emission from the Tokamak Fusion Test Reactor’s (TFTR) neutral beam injection systems has revealed that the line shape, which is a direct measure of the velocity distribution function, is well approximated by the sum of two Gaussians, or, alternatively, by a Lorentzian. For the sum of two Gaussians, the wide‐divergence part of the distribution contains 40% of the beam power and has a divergence five times that of the narrow part. Assuming a narrow 1/e‐divergence of 1.3° (based on fits to the beam shape on the calorimeter), the wide part has a divergence of 6.9°. The entire line shape is also well approximated by a Lorentzian with a half‐maximum divergence of 0.9°. Up to now, most fusion neutral beam modelers have assumed a single Gaussian velocity distribution, at the extraction plane, in each direction perpendicular to beam propagation. This predicts a beam transmission efficiency from the ion source to the calorimeter of 97%. Waterflow calorimetry data, however,...

Observation of Doppler shifted Tα emission from TFTR tritium neutral beams

195 tritium ion source shots were injected into Tokamak Fusion Test Reactor (TFTR) high power plasmas during December 1993–March 1994. In addition, four highly diagnosed pulses were fired into the calorimeter. Analysis of the Doppler shifted Tα emission of the beam in the neutralizer has revealed that the extracted ion compositions for deuterium and tritium are indistinguishable: 0.72±0.04 D+; 0.22±0.02 D+2; 0.07±0.01 D+3 compared to 0.72±0.04 T+; 0.23±0.02 T+2; 0.05±0.01 T+3. The resultant tritium full‐energy neutral fraction is higher than for deuterium due to the increased neutralization efficiency at lower velocity. To conserve tritium, it was used only for injection and a few calorimeter test shots, never for ion source conditioning. When used, the gas species were switched to tritium only for the shot in question. This resulted in an approximately 2% deuterium contamination of the tritium beam and vice versa for the first deuterium pulse following tritium. Data from the calorimeter shots indicate th...

Measurement of thermal ion profiles in TFTR neutral beamlines

A technique is described whereby the ion dumps inside the TFTR Neutral Beam Test Stand were used to measure thermal profiles of the full‐, half‐, and third‐energy ions. 136 thermocouples were installed on the full‐energy ion dump, allowing full beam contours. Additional linear arrays across the widths of the half‐ and third‐energy ion dumps provided a measure of the shape, in the direction parallel to the grid rails, of the half‐ and third‐energy ions, and, hence, of the molecular ions extracted from the source. As a result of these measurements, it was found that the magnet was more weakly focusing, by a factor of 2, than expected, explaining past overheating of the full‐energy ion dump. Hollow profiles on the half‐ and third‐energy ion dumps were observed, suggesting that extraction of D+2 and D+3 is primarily from the edge of the ion source. If extraction of half‐energy ions is from the edge of the accelerator, a divergence parallel to the grid rails of 0.6°±0.1° is deduced. It is postulated that a non...

Operation of a TFTR ion source with a ground potential gas feed into the neutralizer

TFTR long pulse ion sources have been operated with gas fed only into the neutralizer. Gas for the plasma generator entered through the accelerator rather than directly into the arc chamber. This modification has been proposed for tritium beam operation to locate control electronics at ground potential and to simplify tritium plumbing. Source operation with this configuration and with the nominal gas system that feeds gas into both the ion source and the center of the neutralizer are compared. Comparison is based upon accelerator grid currents, beam composition, and neutral power delivered to the calorimeter. Charge exchange in the accelerator can be a significant loss mechanism in both systems at high throughput. A suitable operating point with the proposed system was found that requires 30% less gas than used presently. The extracted D+, D+2, and D+3 fractions of the beam were found to be a function of the gas throughput; at similar throughputs, the two gas feed systems produced similar extracted ion fr...

Low Z impurity ion extraction from Tokamak Fusion Test Reactor ion sources

Tokamak Fusion Test Reactor (TFTR) deuterium neutral beams have been operated unintentionally with significant quantities of extracted water ions. Water has been observed with an optical multichannel analyzer. These leaks were thermally induced with the contamination level increasing linearly with pulse length. Up to 6% of the beam current was attributed to water ions, corresponding to an instantaneous value of 12% at the end of a 1.5 s pulse. A similar contamination is observed during initial operation of ion sources exposed to air. Operation of new ion sources typically produces a contamination level of ∼2%, with cleanup to undetectable levels in 50–100 beam pulses. Approximately 90% of the water extracted from ion sources with water leaks was deuterated, implying that there is the potential for tritiated water production during TFTR’s forthcoming DT operation. It is concluded that isotope exchange in the plasma generator takes place rapidly, most likely as the result of surface catalysis. The primary c...

Diagnostics for TFTR neutral beam species measurement

Five diagnostic systems were used for initial species measurements during tokamak fusion test reactor (TFTR) neutral beam test stand operations involving four ion sources, as well as several different configurations and operating conditions. Initial results were obtained for total neutral species fractions at the beamline input and the beamline output, differential radial profiles of species fractions, angular divergences of species components, species radial power density profiles, and beam impurity components for various conditions.