A. Mase

Control of the radial electric field and of turbulent fluctuations in a tandem mirror plasma

The characteristics of low frequency waves in the presence of × rotation of a tandem mirror plasma are investigated using the Fraunhofer diffraction method. The observed dispersion relations are in good agreement with those of drift waves including a Doppler shift due to the × rotation velocity. The effect of the radial electricfield on the drift waves is studied quantitatively by applying a bias voltage to the end plates of the tandem mirror. The fluctuation level is observed to depend on the radial electric field Er. The fluctuation has a maximum value when Er 0 and decreases with increasing Er, regardless of its sign. The radial confinement time estimated from the particle balance equation decreases as the fluctuation level increases. The dependence of the fluctuation level agrees with that evaluated from the quasi-linear theory of drift wave turbulence.

Thermal barrier potential and two‐dimensional space‐potential measurements with gold neutral‐beam probes in GAMMA 10

Gold neutral‐beam probes are used for the space‐potential measurements in the barrier and in the central cells of the GAMMA 10 tandem mirror. The thermal barrier potential depth is directly observed by simultaneous measurements of the on‐axis space potentials at both locations. Two‐dimensional space potentials in the barrier region are measured by sweeping the probing beams with different beam energies.

Characteristics of hot ions with strong radiofrequency heating in the GAMMA 10 tandem mirror

Radiofrequency waves in the ion cyclotron range of frequency (ICRF) are mainly used for plasma production and heating in the central cell of the GAMMA 10 tandem mirror. GAMMA 10 has minimum-B anchor cells with a non-axisymmetric magnetic field configuration. The ICRF heating system in the central cell has been improved to create a more axisymmetric plasma. A high ion temperature is attained with the system, and high energy ions with energies of more than 50 keV are detected both parallel and perpendicular to the magnetic field lines. Strong temperature anisotropy is observed and strong Alfven ion cyclotron (AIC) modes are excited due to the anisotropy. With the AIC modes, the number of high energy ions detected at the end of the mirror increase and the number of high energy ions with a pitch angle in the central cell of nearly 90° decrease.

Two-dimensional microwave scattering by fluctuations of plasma density and magnetic field

A system of 2D full wave equations is solved for the case of microwave scattering by plasma density and magnetic field fluctuations near the cut-off layer. The 2D field structure in ordinary-to-ordinary scattering (O to O), extraordinary-to-extraordinary scattering (X to X) and O to X, X to O mode conversion processes is studied as well as the influence of refraction, side scattering, diffraction on both X- and O-mode reflection near the cut-off layer. For the case of relatively large antenna size the expressions obtained reveal high sensitivity of the scattered field to the plasma disturbance wavenumber in the transverse direction. For the horn size of 1-2 wavelengths the sensitivity is weak. Numerical examples exhibit multisource spatial distribution of the scattered field. Resonance features of modes scattered by wave-shaped fluctuations were found to be similar to the 1D case.

Millimeter-wave phase-imaging interferometer for the GAMMA 10 tandem mirror

A millimeter‐wave phase‐imaging interferometer has been developed for the study of density profiles of the GAMMA 10 tandem mirror. The interferometer uses a 70‐GHz klystron oscillator and a quasi‐optical transmission system. The probe beam is expanded so as to fill an orthogonal view of a plasma cross section. The view is imaged onto a detector array. The detector array consists of beam‐lead GaAs Schottky barrier diodes bonded to antennas fabricated using photolithographic techniques on a fused‐quartz substrate. Two types of antennas, bow‐tie and Yagi–Uda antennas have been used in order to provide an effective matching to millimeter‐wave beams, and compared for the performance of an imaging system. The interferometers have been applied to the central‐cell and plug‐cell plasmas of GAMMA 10.

Enhancement of the thermal barrier depth and two-dimensional potential profiles in GAMMA 10

Gold neutral beam probes are used for the determination of the space potentials at the thermal barrier cells and the central cell of the GAMMA 10 tandem mirror. The thermal barrier potential is determined by simultaneous measurements of the on-axis space potentials at both locations. Equipotential contours over a cross-section at the barrier are measured by sweeping the probing beams with different beam energies. The enhancement of the thermal barrier depth and of the neoclassical radial ion transport in relation with two-dimensional space potential profiles has been studied in GAMMA 10 for the case of low density experiments.

Detection of fusion neutrons from a deuterium‐proton mixed plasma in the GAMMA 10 Tandem Mirror

Neutron diagnostics are applied to a deuterium plasma mixed with protons in the central cell of the GAMMA 10 tandem mirror [Inutake et al., Phys. Rev. Lett. 55, 939 (1985)]. The deuterium ions are heated with a slow ion cyclotron wave tuned to the fundamental deuterium resonance near the mid‐plane of the central cell while the plasma is sustained with the fundamental resonance heating of protons in the minimum‐B anchor cells. The measurement is based on in situ calibration to determine the transmission efficiency through machine walls and the counting efficiency of the detection system for neutrons emitted from 252Cf with energy close to that in the deuterium–deuterium (D–D) fusion reaction. The observation shows that the count rate of neutrons increases with diamagnetism, and this relation is accounted for in terms of fusion reaction between deuterium ions with a transverse temperature exceeding 10 keV. Discrepancies among ion temperatures determined with different diagnostics are mostly attributed to insufficient knowledge of the profiles of plasma parameters. The results indicate that the neutron measurement can be added as a powerful diagnostic tool for hot ions if combined with more detailed profile measurements.

Space‐resolving flat‐field vacuum ultraviolet spectrograph for plasma diagnostics

A spatial imaging vacuum ultraviolet (VUV) spectrograph has been constructed for simultaneous observation of spatial and spectral distributions of plasma radiation in the wavelength range 150–1050 A. The spectrograph consists of an entrance slit of limited height which provides spatial resolution, an aberration‐corrected concave grating with varied spacing grooves which gives a flat‐field spectral output plane, and an image‐intensified two‐dimensional detector system. The basic characteristics of the spectrograph have been investigated by ray‐tracing calculations. The expected performance has been confirmed through experiments using a dc glow discharge source on the reciprocal dispersion, the spatial resolution, or the incident angle dependence of spectral images. VUV spectra with spatial and time resolution have been obtained successfully in the GAMMA10 tandem mirror experiment.

Measurement of ICRF waves in the GAMMA10 tandem mirror using reflectometers

Microwave reflectometry has been applied to the central cell of the GAMMA 10 tandem mirror for measurements of fluctuations in the ion cyclotron range of frequency (ICRF). The instability with frequency ω<ωci is identified as the Alfven ion cyclotron wave driven by plasma β and ion pressure anisotropy. Internal magnetic fluctuations with δB/B∼10−4 as well as density fluctuations of the same level for this wave are determined by a combination of X‐ and O‐mode reflectometers. The fluctuations associated with the ICRF heating wave (ω=ωci) are also observed by three separated X/O mode reflectometers to study the physics of wave excitation and propagation. The radial profile of the fluctuation level is expected to give an estimation of the deposition profile of the heating power. The results will be applied in order to achieve efficient ion heating and to help the power balance study of central‐cell plasma.

Low frequency fluctuations measured by probes in the GAMMA10 tandem mirror

In the limited regions of operation of the GAMMA10 tandem mirror, a saturation or a reduction of the stored energy in the central cell is sometimes observed. By using electrostatic and magnetic probes, the low frequency (<1 MHz) fluctuations are studied in relation to plasma parameters. Dominant fluctuations are identified to be flute type and drift type modes. Magnetic fluctuations, of which frequency is much lower than the ion cyclotron frequency, are newly observed.