T. Cho

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.

Observation of hot electrons produced by second harmonic electron cyclotron heating in the axisymmetric tandem mirror GAMMA 10

Microwave power, PECH ≤ 140 kW, has been injected at 28 GHz into the axisymmetric plug/barrier cell in the axisymmetrized tandem mirror GAMMA 10. As observed by soft X-ray measurements, the microwaves generate a hot (50-60 keV) electron population, radially peaked on the magnetic axis, which results in the formation of a thermal barrier. The production mechanism of these hot electrons is found to be second harmonic electron cyclotron heating (ECH), corrected for the effects of the relativistic mass variation and the Doppler shift. This mechanism also explains the first experimental observation of a saturation of the single-component hot electron temperature Teh as being caused by the finite width of the incident microwave lobe. The dependence of the plasma parameters on the filling gas pressure, the plasma density and the ECH power is studied. It is found that the heating process can be interpreted as a competition between electron acceleration by the incident wave, electron deceleration by collisions, and the mirror trapping efficiency of the source electrons for hot electrons. The axial profile of the soft X-rays is investigated in relation to the mechanism of the second harmonic ECH. The heating process is discussed in terms of the electron pitch angle and the magnetic field intensity.

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.

X-ray detection characteristics of gold photocathodes and microchannel plates using synchrotron radiation (10 eV–82.5 keV)

Abstract X-ray detection characteristics of gold photocathodes, and microchannel plates (MCP) have been investigated using synchrotron radiation in the energy range from 12 eV to 82.5 keV. The quantum efficiency of gold is compared with the published data which were reported at some discrete energies, and we add several new data points in the 12–35.9 eV. The detection response of MCP has been represented for photons from the VUV regime of 12 eV to the hard X-ray region of 82.5 keV along with its incident angle dependence. This MCP response has demonstrated the following remarkable sensitivity in this wide energy range: The energy response of MCP remains within about one order of magnitude through this broad range. This comes from the contribution of the combination of various ingredients of MCP; namely, C, O, Si, Ba and Pb. For the energy response, these ingredients, in turn, make some jumps and humps at their edge energies. Another useful feature of the MCP response we obtained is a rapid decrease in the MCP response for the energies less than about ten eV; this is desirable for avoiding the effect of visible light on the X-ray data. Also, the MCP response is compared with the detection efficiency of a silicon surface barrier detector for the application to plasma X-ray diagnostics.

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.

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...

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.

High-density plasma production with potential confinement in the GAMMA 10 tandem mirror

The improvement of potential confinement was attained in the GAMMA 10 tandem mirror [Phys. Rev. Lett. 55, 939 (1985); Proceedings of the 13th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Washington, 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. 2, p. 539] by axisymmetrization of heating systems for the plasma production, heating, and potential formation. A significant increase of the density and diamagnetism by the potential confinement was observed. In the previous experiment, it was difficult to increase the central cell density higher than 2.7×1018 m−3. One of the possible mechanisms is the density clamping due to the eigenmode formation of the ion–cyclotron-range of frequency (ICRF) waves in the axial direction. With high harmonic ICRF waves (RF3), the experiments to overcome this problem have been performed. In preliminary experiments with RF3 and NBI the maximum density of 4×1018 m−3 was attained.

Production of hot electrons for axisymmetric thermal barrier formation in a tandem mirror

Hot electrons have been produced by second harmonic electron‐cyclotron resonance heating in axisymmetric end mirrors of the tandem mirror GAMMA10 [Phys. Rev. Lett. 55, 939 (1985)] with an on‐axis density fraction reaching 80% and temperature of 25–50 keV, satisfying theoretically required conditions for the formation of thermal barriers. The successful control of the electron temperature may be attributed to the relativistic detuning of the second harmonic resonance for localized microwave power absorption. The time evolutions of relevant parameters are studied with extensive diagnostics.