M. Inutake

Observation of scaling laws of ion confining potential versus thermal barrier depth and of axial particle confinement time in the tandem mirror GAMMA 10

In the thermal barrier tandem mirror GAMMA 10, the scaling law governing the enhancement of the ion confining potential, c, resulting from thermal barrier formation, is obtained experimentally, and is consistently interpreted in terms of the weak and strong ECH theories set up by Cohen and co-workers. The scaling law on the axial particle confinement time, τp||, related to this c formation, is also demonstrated in detail; it is in good agreement with the Pastukhov theory as modified by Cohen and co-workers. This scaling is verified at any radial position in the core plasma region and at any time through the various stages of a discharge; this indicates a scaling with drastic improvement of τp||, due to the potential formation in the tandem mirror plasma.

Anchor plasma buildup by using central cell ICRF antennas in the tandem mirror GAMMA 10

The paper presents a study of anchor cell plasma buildup in the tandem mirror GAMMA 10 with the use of central cell ICRF antennas. For plasma buildup, it is demonstrated that an ion cyclotron resonance layer exists within the closed mod-B surface in the minimum-B anchor cell. The experimental plasma buildup rate in the anchor cell is compared with a one-dimensional Fokker-Planck simulation. A rapid increase in the anchor plasma density is observed and an associated reduction of the end-loss current when radiofrequency is applied.

Investigation of recycling in the GAMMA 10 tandem mirror

This paper describes the detailed behavior of particle recycling on the stainless-steel wall of the GAMMA 10 tandem mirror device. A comparison is made with a model calculation. The ion temperature dependence of the recycling coefficients has been deduced from experimental data in a wall conditioning period. It was found that the recycling coefficient increased with the ion temperature and that the recycling due to particles desorbed from the wall prevailed over that due to wall-reflection. A simple recycling model including both atomic and molecular processes was applied for the ICRF-heated plasma. The result of the model calculation is consistent with the above ion-temperature dependence. The DEGAS neutral transport code has been applied to the GAMMA 10 configuration and the results of the DEGAS code show a good agreement with results obtained from Hα-emission, which predicted a significant influence due to neutral molecules near the plasma boundary.

Neoclassical resonant plateau transport calculation in an effectively axisymmetrized tandem mirror with finite endplate resistance

Calculations are made for neoclassical resonant plateau transport in the geometry of the effectively axisymmetrized tandem mirror GAMMA 10 magnetic field, which has minimum B inboard anchors inside the axisymmetric plug/barrier mirror cells. Azimuthal drifts in the local, non-axisymmetric regions are included. The radial potential profile is determined by solving the charge neutrality equation self-consistently. A finite resistance connecting the end plate to the machine ground provides appropriate boundary conditions on the radial electrostatic potential distribution so that it can be determined uniquely. The calculation is consistent with experimental results of GAMMA 10.

Measurement of Alfvén ion-cyclotron wave using both X- and O-mode reflectometers

Alfven ion‐cyclotron wave excited in the central‐cell plasma of the GAMMA 10 tandem mirror is investigated by a reflectometer system. The Fourier amplitude of the instability strongly depends on the anisotropy of the ion velocity distribution function and the plasma β value. The ratio 0.2–0.3 between O‐mode and X‐mode reflectometer signals suggests that a magnetic fluctuation level similar to the density fluctuation level is excited in the plasma due to the instability.

Temperature Anisotropy Measurement Using Diamagnetic Loop Array

The anisotropy of the ion distribution function in the velocity space is observed in the ion cyclotron range of frequency heating experiments on the tandem mirror GAMMA 10. The anisotropy, defined as T⊥/T// (ratio of ion temperatures perpendicular and parallel to the magnetic field line), is evaluated quantitatively using three diamagnetic loops aligned in the direction of the magnetic field line. Depending on the anisotropy and the plasma pressure, fluctuations below the ion cyclotron frequency are excited. In terms of the quantitative estimation of the anisotropy, it is established clear that the parameter region in which the fluctuations are observed agrees well with a theoretically predicted region of the convectively unstable Alfven ion cyclotron mode.

Observation of spatial Alfven resonance

Abstract By exciting axisymmetric Alfen waves with Stix coils, the spatial properties of shear Alfven resonance are investigated. The maximum azimuthal magnetic field of the excited waves is located at this resonance layer, which is a function of both frequency and wavelength. The polarization also changes continuously through the resonance layer.

Scaling study of potential in the end region of a tandem mirror based on end‐loss electron measurement

An empirical scaling for the potential difference between the plug potential ΦP and the floating potential ΦEP of an endplate is obtained in terms of end‐loss electron temperatures. Results from the GAMMA 10 tandem mirror [Phys. Rev. Lett. 55, 939 (1985)] indicate that ΔΦ=ΦP−ΦEP scales much better with an effective temperature Teff≡(1−β)TeL+βTeH than with a single temperature TeL or TeH, where TeL and TeH are temperatures of the bulk and the high‐energy tail components of the end‐loss electrons and β denotes the flux fraction of the TeH component. Moreover, the observed scaling is consistent with a recently developed theoretical model [K. Kurihara et al., J. Phys. Soc. Jpn. 61, 3153 (1992)].

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.

Non-Axisymmetric Alfvén Wave Excited by a Helical Coupler in an Inhomogeneous Plasma

Non-axisymmetric Alfven waves are excited with a helical coupler in a finite beta and a cylindrical inhomogeneous plasma surrounded by a conducting wall. The helical coupler, which consists of two symmetrical helical windings, exhibits a strong propagation directionality; slow waves of the m=-1 mode and fast waves of the m=+1 mode can be launched simultaneously from the coupler, but propagate in opposite directions along the static magnetic field. Dispersion relations including the attenuation length for both modes are compared with a magnetohydrodynamic theory given by Woods on the assumption of appropriate boundary conditions. As a nonlinear phenomenon, subharmonic slow Alfven waves of the m=-1 mode have been observed when the pump frequency is close to or above the ion cyclotron frequency.