J. A. Casey

Plasma production and heating in a tandem mirror central cell by radio‐frequency waves in the ion cyclotron frequency range

Plasma production and heating in the central cell of the Tara tandem mirror [Nucl. Fusion 22, 549 (1982); Plasma Physics and Controlled Nuclear Fusion Research, 1986, Proceedings of the 11th International Conference, Kyoto, Japan (IAEA, Vienna, 1987), Vol. 2, p. 251] have been studied. Using radio‐frequency excitation by a slot antenna in the ion cyclotron frequency range (ICRF), plasmas with a peak β⊥ of 3%, density of 4×1012 cm−3, ion temperature of 800 eV, and electron temperature of 75–100 eV were routinely produced. The plasma radius decreased with increasing ICRF power, causing reduced ICRF coupling and saturation of the plasma beta. About 70% of the applied ICRF power can be accounted for in direct heating of both ions and electrons. Wave field measurements have identified the applied ICRF to be the slow, ion cyclotron wave. In operation without end plugging, the plasma parameters were limited by poor axial confinement and the requirements for maintenance of magnetohydrodynamic stability and micros...

Experimental study of nonlinear M = 1 modes in the Tara tandem mirror

The nature of a rigid, flutelike M=1 instability as seen in the Tara tandem mirror [Nucl. Fusion 22, 549 (1982); Plasma Physics and Controlled Nuclear Fusion 1984 (IAEA, Vienna, 1985), Vol. 2, p. 285] is discussed. Radial density and light emission profiles obtained by inverting chord measurements are compared to end loss radial profiles during the evolution of the mode to its nonlinear saturated state. This final state is characterized by a coherent, flutelike motion of the plasma as a whole about the machine axis.

Stability of plasmas sustained by ion cyclotron wave excitation in the central cell of the Tara tandem mirror

The stability of plasmas produced by radio‐frequency heating in the ion cyclotron frequency range (ICRF) has been studied in the central cell of the Tara tandem mirror [Nucl. Fusion 22, 549 (1982); Plasma Physics and Controlled Nuclear Fusion Research 1986, Proceedings of the 11th International Conference, Kyoto (IAEA, Vienna, 1987), Vol. II, p. 251]. Ion cyclotron wave excitation by a slot antenna provided stability against macroscopic plasma motions in an axisymmetric configuration. The maintenance of macroscopic stability depended on the ICRF power, gas fueling rate, ion cyclotron resonance location, and ω/ωci at the antenna location. The ICRF ponderomotive force model is consistent with many of the observed stability features and predicts that the E+ component of the ion cyclotron wave was responsible for the stabilization. The Alfven ion cyclotron microinstability was observed when the plasma β⊥ and anisotropy were sufficiently high. Magnetic probe measurements of the unstable mode identified it as a...

Construction of a scanning two‐dimensional Thomson scattering system for Alcator C‐Mod

A novel Thomson scattering diagnostic for the Alcator C‐Mod tokamak is nearing operation. A vertically oriented 1‐J Nd:YAG laser beam will be viewed along eleven (initially six) quasihorizontal chords simultaneously for one‐dimensional profiles every laser pulse. Each chord is viewed by a Wadsworth grating spectrometer. Laser operation at 50 pps gives a coarse time axis. A counterrotating prism arrangement steps the laser position by up to 2 cm between pulses, allowing full two‐dimensional profiles to be obtained over 160 ms (within the 14‐cm accessible plasma region) during a single shot. Design details, calibration results, and control system and alignment details are presented.

Observation of trapped particle modes in a tandem mirror

An instability with azimuthal mode number m≥3 localized to an axisymmetric end cell of the Tara tandem mirror [Nucl. Fusion 22, 549 (1982)] has been observed, most prominently during strong ion cyclotron resonance heating in the end cell. The instability, which causes enhanced radial losses, becomes either more stable or flutelike when the connection (passing fraction) between the central cell and end cell is increased, depending on whether sufficient stabilization is provided in the central cell. The beta is sufficiently low to rule out the possibility of magnetohydrodynamic ballooning modes. Based on the plasma parameters, the instability appears to be a collisionless curvature‐driven trapped particle mode that has been predicted to be unstable in linked minimum‐ and maximum‐B mirror devices.

Thomson scattering in the Tara tandem mirror central cell

A Thomson scattering experiment is under construction for the central cell of the Tara tandem mirror. This experiment has been designed to measure electron temperature and density at densities as low as 3×1011 cm−3. The measurement is made at a single time point per shot using a Holobeam 10‐J Q‐switched ruby laser, folded back for two passes through the scattering volume. Detection is at 90° with f/5 optics. The optical table is mounted on linear bearings with a 10‐in. stroke, so that the radius of the scattering volume may be changed from shot to shot. Novel features of the experiment include a high‐throughput (>25%) commercial triple‐grating polychrometer using holographic gratings, and a directly coupled ten‐channel microchannel plate photodetector array.

First operation of the Alcator C‐mod scanning 2D Thomson scattering diagnostic (abstract)a)

The scanning Alcator C‐mod 2D Thomson scattering diagnostic has been installed. This diagnostic utilizes a single Nd:YAG laser system to generate a ∼1 J pulse at a 50 pulse per second repetition rate. The nearly vertically oriented laser beam is viewed along approximately horizontal directions. Collected plasma scattered light is coupled to multiple grating spectrometers used for spectral analysis in order to derive local temperature and absolutely calibrated density information. The vertical laser beam chord may be translated by up to 2 cm during the 20 ms period between laser pulses by a counterrotating prism technique, allowing full two‐dimensional profiles to be obtained over 160 ms (covering the 14 cm major radius region which is accessible) during a single tokamak discharge. Remote laser and scanner synchronization and control via a programmable logic controller and CAMAC has been installed and is fully functional. First experimental results with the initial spectrometer set will be presented.

Hα laser fluorescence diagnostic on the Tara Tandem Mirror experiment

A laser fluorescence diagnostic has been used for measuring the neutral hydrogen density in the central cell of the Tara thermal barrier tandem mirror. Experiments have been performed using laser‐induced, resonance fluorescence detection of Hα (6563‐A) radiation. Measurements were made at a number of radial positions with 1‐cm resolution, from the magnetic axis to near the plasma limiter. Stray laser light contributions to the signal were eliminated with a double‐pulse technique. For comparison, the chord‐averaged plasma Hα radiation was analyzed under the identical conditions for which laser fluorescence data were taken.

Atomic hydrogen density measurements in the Tara tandem mirror experiment

Neutral and plasma density have been measured in the north well of the central cell of the Tara tandem mirror [Nucl. Fusion 22, 549 (1982)]. The electron plasma density and temperature on the magnetic axis were measured by Thomson scattering to be about 3×1012 cm−3 and 70 eV, respectively. The corresponding axial neutral hydrogen density was found to be 1 ×109 cm−3, while near the plasma edge at r=15 cm it reached 1×1010 cm−3. The fill gas density at r≥22.5 cm was ≊1011 cm−3. Additional information from secondary electron detectors was used to estimate the radial ion temperature distribution, which was found to have about the same width, 12 cm, as the plasma density. The resulting ion pressure profile is peaked compared to the electron pressure profile. Charge exchange losses in the well are found to have a maximum at a radius equal to half the e‐folding distance of the plasma density and ion temperature distributions.

Pseudotomographic fitting algorithm for density profile reconstruction from a sparse 1-D interferometer array

An algorithm has been developed for fast analysis of a sparse multichord interferometer array, yielding peak density, Gaussian width, radial offset, ellipticity, and instability amplitude and frequency from line‐integrated density measurements. A nonlinear least‐squares fit is performed over many short time intervals, with independent variables reduced to one dimension. Assumptions of plasma rigidity and slowly evolving rotation rate are necessary to trade temporal resolution (on instability timescales) for spatial resolution across the moving plasma column. Results from the Tara tandem mirror experiment are shown.