K. Yatsu

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.

Plasma confinement in the GAMMA 10 tandem mirror

The central cell density and the diamagnetic signal were doubled as a result of plug potential formation by ECRH in hot ion mode experiments on the GAMMA 10 tandem mirror. In order to obtain these remarkable results, the axisymmetrized heating patterns of ECRH and ICRF heating were optimized. Furthermore, conducting plates were installed adjacent to the surface of the plasma along the flat shaped magnetic flux tube located in the anchor transition regions; the plates may contribute to the reduction of some irregular electric fields produced possibly with ECRH in these thin flux tube regions. The conducting plates contributed to reducing the radial loss rate to less than 3% of the total particle losses, along with improvements in the reproducibility of the experiments and the controllability of the potential confinement. The increases in central cell density and diamagnetism in association with the increase in plug potentials scaled well with increasing ECRH power. A plug potential of 0.6 kV and a density increase of 100% were achieved using an ECRH power of 140 kW injected into both plug regions. The plasma confinement was improved by an order of magnitude over a simple mirror confinement owing to the tandem mirror potential formation.

A DIFFERENTIAL-SPECTRUM ION-ENERGY ANALYZER WITH ELECTROSTATIC SLANTED GRIDS

A novel end‐loss ion‐energy spectrometer is designed for plasma‐ion diagnostics in open‐ended plasma‐confinement devices. This analyzer significantly upgrades a previous slanted grid end‐loss analyzer (SELA) to essentially eliminate secondary‐electron current, and to provide a differential‐spectrum mode of operation, in addition to the usual integral‐spectrum operation of gridded ion‐energy analyzers. The upgraded SELA does not perturb the ambient magnetic field due to its electrostatic operation. Either the differential or integral spectra are obtained by a time sweep of grid voltages, collecting the ion current on a single‐channel plate. Because the angular alignment of the SELA is not critical, it can be used as a spatially scannable diagnostic of ion‐energy distributions and plasma potentials. It is characterized using computer simulations of ion trajectories, monoenergetic ion beams, and end‐loss plasma from the world’s largest tandem mirror—GAMMA 10.

New findings of X-ray energy responses of silicon surface barrier detectors and their generalized theoretical extension to X-ray responses of position sensitive detectors

Abstract X-ray energy responses of silicon surface barrier (SSB) semiconductor detectors are found to be explained neither by the commonly believed conventional model using the depletion layer thickness of an SSB detector nor by a recently proposed model using its wafer thickness as the X-ray sensitive layer. Our new theory using both the depletion layer sensitivity and the X-ray response due to a three-dimensional charge diffusion effect in the field-free substrate region of a SSB detector can well fit the response data. This theory is newly extended to analyse each channel response of a multichannel semiconductor X-ray detector fabricated on one silicon wafer; these detectors are widely utilized as position sensitive X-ray detectors for nuclear fusion oriented plasma research as well as for high energy elementary particle studies. A numerical simulation of multichannel detector outputs using our three dimensional diffusion theory is carried out; generalised theoretical formulae are also presented.

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.

Detection characteristics of an ultralow‐energy measurable pure‐germanium detector in the hundreds‐eV photon‐energy region

In the energy range from 1 keV down to a few hundred eV, a newly developed ‘‘ultralow‐energy measurable’’ pure‐Ge detector for a pulse‐height analysis and a current‐mode observation has been characterized using synchrotron radiation monochromatized by a grasshopper monochromator at the Photon Factory of the National Laboratory for High Energy Physics (KEK). X‐ray measurements in this low‐energy region were previously tried out using several types of ‘‘windowless’’ Si(Li) detectors. These detectors, however, had trouble, including temporal variations in the quantum efficiencies because of their detector‐surface deteriorations due to various impurities in vacuum chambers. Our pure‐Ge detector has a 4000‐A thick polymer window metalized by a 1400‐A thick Al supported by a 100‐μm apart Si‐ribbed structure. However, for this liquid‐nitrogen‐cooled detector with the special window, its actual energy‐response data are not available at this time. Therefore, the investigations of its characteristics are reported p...

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.

Temporal behaviour of the potential confined electrons in the central cell and in the plug region during a period with thermal barriers

An increase with time of the central-cell electron temperature during a period with thermal barrier potentials has been observed. The increase is explained by an improvement of the electron energy confinement due to the presence of thermal barriers. Different evolutions of the electron energy in the plug region have been observed; these are closely related with the variation in time of the confining potential of the plug electrons. The observed behaviour of the potential confined electrons provides the first data set for the evolution of the effect of thermal isolation caused by the formation of a thermal barrier in the kilovolt range.

Objectives and design of the JT-60 superconducting tokamak

A fully superconducting tokamak named JT-60SC is designed for the modification programme of JT-60 to enhance economical and environmental attractiveness in tokamak fusion reactors. JT-60SC aims at realizing high-β steady-state operation in the use of low radio-activation ferritic steel in a low ν* and ρ* regime relevant to the reactor plasmas. Objectives, research issues, plasma control schemes and a conceptual design for JT-60SC are presented.

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.