N. Yamaguchi

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.

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.

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.

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.

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.

X‐ray detection characteristics of microchannel plates using synchrotron radiation in the energy range from 0.06 to 0.6 keV

The characteristics of microchannel plates (MCPs) for x‐ray detection have been investigated with continuous x‐ray energy variation using synchrotron radiation in the XUV and soft x‐ray region (from 60 to 600 eV). The current response data have shown x‐ray absorption fine structure (EXAFS or XANES) near the oxygen K absorption edge as well as a jump structure near the silicon L edge; these structures are explained by the surface composition of channel walls (SiO2). The data show that the surface layer of a microchannel is essential for x‐ray detection by MCPs in this energy range. The precise current response curve has been completed for the wide x‐ray energy range from 0.06 to 82 keV combined with the previously reported data [Kondoh et al., Rev. Sci. Instrum. 59, 252 (1988); Cho et al., ibid. 59, 2453 (1988); Yamaguchi et al., ibid. 60, 368, 2307 (1989)].

Current response characteristics of microchannel plates x‐ray detector using synchrotron radiation (0.6–2 keV and 5–20 keV)

The characteristics of microchannel plates (MCPs) for detection of x rays have been investigated using synchrotron radiation in the energy ranges from 0.6 to 2.0 keV and from 5 to 20 keV. Microchannel plates are operated under the condition of an unsaturated pulse‐height distribution mode. The current response curve of MCPs is measured continuously with x‐ray energy variation for the first time. The experimental result of some discontinuous jumps in the response is obtained at the energies corresponding to the absorption edge of the MCP materials. In the low‐energy range (hν 5 keV), a weak dependence on θ is observed, and is attributed mainly to the penetration of x rays through multiple channels.

Multichordal spectroscopy on JT‐60

In order to study impurity behavior on the JT‐60 plasma, a spectroscopic diagnostic system, which is capable of measuring many spectra of impurity ions and their spatial distributions simultaneously, has been developed. This system consists of interchangeable unit‐type spectrographs which have different lines of sight. Four types of grazing incidence spectrographs are prepared as unit‐type spectrographs. These spectrographs cover the nominal wavelength regions 0.5–5, 0.5–50, 2.0–50, and 50–122 nm, respectively, and have holographic gratings with flat focal fields. Image‐intensified 1024‐ or 512‐channel photodiode arrays are used as detectors for all spectrographs. Four unit‐type spectrographs are used for observing the main plasma, and two unit‐type spectrographs are used for observing the plasma near the null point of the diverted discharge. One of these spectrographs has been calibrated absolutely in the wavelength region 27.5–120 nm using synchrotron radiation from the Photon Factory (National Laborato...

Development of Atmospheric Plasma Jet With Long Flame

In this paper, an atmospheric plasma jet with long flame has been developed. The plasma jet is covered with a cylindrical glass container, where the inner volume is purged by the working gas. The flame of N2 plasma jet has been elongated to about 200 mm, while the conventional plasma jet has a flame of only 10-20-mm length. It has been confirmed that the long-flame plasma jet can modify polymer film with a work distance of over 200 mm. This plasma-jet device could be applicable for treating and cleaning objects having deep channels or large structures and could be used in the wider application fields.