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Notes: Charge exchange diagnostics in a hostile neutron and gamma radiation environment require substantial shielding against radiation-induced noise signal. Minimizing the dimensions of the apparatus significantly reduces the shielding complexity. Conceptual designs for compact analyzers are presented. The reduced size is achieved primarily by employing high-field-strength (≤8 kG) permanent rare earth magnets to simplify magnetic field generation. In addition, a carbon foil is used in place of a gas cell to eliminate the customary analyzer vacuum pumping systems. Alternative designs employing electric and magnetic fields for mass and energy analysis will be presented that afford a reduction in volume by an order of magnitude compared with the E(parallel)B analyzers currently in use on the tokamak fusion test reactor (TFTR).

Notes: Data from an E(parallel)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.

Notes: 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 from 80 to 114 keV is D0[E]:D0[E/2]:D0[E/3]=53(5):27(4):20(4), where the quantities in parentheses are the average experimental uncertainties.The corresponding neutral power ratio is P0[E]:P0[E/2]:P0[E/3]=72(9):19(3):9(2). The half widths (1/e) in the horizontal plane for the full-, half-, and third-energy components were 0.26°, 0.34°, and 0.42°, respectively. The dispersions of the full-, half-, and third-energy components were 1.20 keV, 2.35 keV, and 2.26 keV, respectively. The carbon impurity concentration in a 80 keV D0 beam was not greater than 2×10−4 per D0 beam particle, and exhibited an apparent acceleration state of C+. The oxygen impurity concentration was less than 5×10−4 per D0 beam particle, and exhibited an apparent acceleration state of O+. A variety of vacuum conditions were observed depending on the operating conditions. Typically, pressures in the transition ducts were in the range from 0.3 to 0.7×10−5 Torr at the beginning of injection pulses, and reionized power losses were in the range from 0.75% to 1.5% of incident power. At the end of injection pulses, pressures in the transition ducts were in the range from 0.6 to 2×10−5 Torr and reionized power losses were in the range from 2% to 6% of incident power. This work describes generic results, new apparatus, and advances in measurement techniques for the optimization of tokamak neutral beam heating operations and the analysis of neutral beam heated plasmas.

Notes: The efficiency of the stripping cell incorporated in a charge exchange analyzer has been measured over a pressure range of 1.8–130 mTorr and over an energy range of 1–130 keV/amu. In these measurements, a beam of hydrogen atoms was ionized by helium gas. As a by-product of the measurements, the scattering cross section was also measured. At the lowest pressure, the scattering cross section compared well with those measured previously; at high pressure, the cross section no longer followed an energy power law. The analyzer efficiency (defined as the fraction of the atoms that is ionized and not lost through scattering) can be modeled over the entire range of energy and pressure by including the measured scattering cross sections.

Notes: The ion source of the TFTR diagnostic neutral beam utilizes a magnetic cusp bucket from a PLT source modified with an internal rf antenna in place of filaments. The species mix of the beam has been measured after the neutralizer as a function of several parameters in two ways: spectroscopically and by an electrostatic charge exchange analyzer. Source and neutralizer gas feeds (both hydrogen) were varied. Some measurements were made with the bending magnet alternately on and off to obtain neutralizer efficiency for each component. Accelerator current was varied, at fixed acceleration voltage and gas feed rate, by varying rf power to the antenna. Relative beam divergence during this scan was monitored by both diagnostics, and an absolute divergence was obtained from a spectroscopic profile scan.

Notes: The high source strength of 2.45-MeV neutrons (∼1.2×1016 n/s) and neutron-induced gamma rays produced by the Tokamak Fusion Test Reactor (TFTR) have substantially degraded the performance of many diagnostics. In particular, data obtained by the charge exchange neutral analyzer (CENA) were seen to degrade at modest production levels of ∼1015 n/s. To reduce the radiation-induced noise to acceptable levels, massive shielding was required. A design and cost-effectiveness study was performed for the TFTR CENA which showed that a double-layered enclosure consisting of a 10-cm-thick inner housing of lead surrounded by a 23-cm-thick layer of 1.0% borated polyethylene was required to reduce the neutron and gamma noise level by two orders of magnitude. This design was recently implemented on TFTR and was shown to perform as predicted. The details of the enclosure will be presented and the contributing factors that led to the choice of this particular design will be reviewed.

Notes: A set of experiments are carried out to measure and understand the response of a free-burning atmospheric pressure carbon arc to applied transverse dc and ac magnetic fields. The arc is found to deflect parabolically for the dc field and assumes a growing sinusoidal structure for the ac field. A simple analytic two-parameter fluid model of the arc dynamics is derived, in which the arc response is governed by the arc jet originating at the cathode, with the applied J×B force balanced by inertia. Time variation of the applied field allows evaluation of the parameters individually. A fit of the model to the experimental data gives a value for the average jet speed an order of magnitude below Maecker's estimate of the maximum jet speed [H. Maecker, Z. Phys. 141, 198 (1955)]. An example industrial application of the model is considered. © 2000 American Institute of Physics.

Notes: Perturbative experiments on the Tokamak Fusion Test Reactor [Phys. Plasmas 4, 1736 (1997)] (TFTR) have investigated transport in reverse shear plasmas. On TFTR, reverse magnetic shear plasmas bifurcate into two states with different transport properties: reverse shear (RS) and enhanced reverse shear (ERS) with improved core confinement. Measurements of the 14 MeV t(d,n)α neutrons and charge-exchange recombination radiation spectra are used to infer the trace tritium and helium profiles, respectively. The profile evolution indicate the formation of core particle transport barriers in ERS plasmas. The transport barrier is manifested by an order-of-magnitude reduction in the particle diffusivity (DT,DHe) and a smaller reduction in the pinch within the reverse shear region. The low diffusivities are consistent with neoclassical predictions. Furthermore, DT and DHe(approximate)χeff, the effective thermal diffusivity. Although the measured coefficients imply no helium ash accumulation, the situation is uncertain in a reactor due to unknown χeff scaling.

Notes: Compact electrostatic charge exchange analyzers (18×18×13 cm) have been constructed for installation in areas of high magnetic fields and restricted access near tokamak fusion devices. The analyzers employed carbon stripping foils, and have been calibrated for proton and deuteron energies between 1 and 70 keV. They have been successfully used to study charge exchange losses in auxiliary-heated tokamak plasmas and were found to be ideally suited to supplement the larger and more complex gas-cell analyzers by providing additional spatial and temporal information at a modest effort and cost.

Notes: Short duration (20 msec) neutral deuterium beams are injected into the TFTR tokamak [Plasma Physics and Controlled Nuclear Fusion Research 1986 (IAEA, Vienna, 1987), Vol. I, p. 51]. The subsequent confinement, thermalization, and diffusion of the beam ions are studied with multichannel neutron and charge exchange diagnostics. The central fast-ion diffusion (〈0.05 m2/sec ) is an order of magnitude smaller than typical thermal transport coefficients.