J. Kohagura

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.

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.

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...

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.

Active control of internal transport barrier formation due to off-axis electron-cyclotron heating in GAMMA 10 experiments

The controlled formation of an internal transport barrier (ITB) is observed in GAMMA 10 [T. Cho et al., Nucl. Fusion 45, 1650 (2005)]. The barrier is localized within a layer of a strongly sheared Er×B plasma rotation (5.5<rc⩽10cm). This high-vorticity layer is formed and maintained by off-axis electron-cyclotron heating, which generates a cylindrical layer (4<rc<7cm) with a high-energy electron population that modifies the initial Gaussian radial potential profile into a nonmonotonic one with a hump structure. The local gradients of Ti and Te are appreciably enhanced in the ITB layer, similarly to those of the ITB in tokamaks and stellarators. Reductions in the effective ion and electron thermal diffusivities are obtained in the barrier layer. A reduction of the observed low-frequency turbulence in the ITB layer and a partial decoupling of the turbulent structures localized on either side of the layer are demonstrated by two-dimensional x-ray diagnostics.

Potential confinement experiments with long sustainment and high density on GAMMA 10

The improvement of potential confinement reported at the last IAEA Fusion Energy Conference in 1998 was attained by axisymmetrization of the heating pattern of ECRH. It was experimentally shown that the axisymmetrization of ECRH actually produced an axisymmetric potential profile. GAMMA 10 experiments on potential confinement have advanced in the direction of longer sustainment and higher density. Experiments to achieve long sustainment of potential confinement were carried out in order to study problems of steady state operation of a tandem mirror reactor. A confining potential was sustained for 150 ms by sequentially injecting two ECRH pulses into the plug region. It had previously been difficult to increase the central cell density to higher than about 2.5 × 1012cm-3 with or without potential confinement, owing to some density limiting mechanism. In order to overcome this problem, a new, higher frequency ICRF system (RF3: 36-76 MHz) has been installed. A higher density plasma has been produced with RF3. In addition to RF3, NBI in the anchor cells was recently made more effective through the reduction of neutral gas from the beam injectors. Potential confinement experiments have advanced to higher central cell densities of up to 4 × 1012 cm-3 with RF3 and NBI. A 35% density increase due to the potential confinement was obtained in the high density experiments.

EFFECTS OF NEUTRONS ON SEMICONDUCTOR X-RAY DETECTORS INCLUDING N-TYPE JOINT EUROPEAN TORUS AND P-TYPE GAMMA 10 TOMOGRAPHY DETECTORS

Characterization experiments have been carried out so as to investigate the effects of fusion-produced neutrons on the x-ray-energy responses of semiconductor detectors for x-ray tomography in the Joint European Torus (JET) tokamak (n-type silicon) and the GAMMA 10 tandem mirror (p-type silicon). Neutron effects on the x-ray-energy responses of these detectors are studied using synchrotron radiation from a 2.5 GeV positron storage ring at the Photon Factory. Changes in the material properties of the detectors have been investigated using an impedance analyzer to estimate neutron effects on x-ray-sensitive depletion thicknesses. A cyclotron accelerator is employed for well-calibrated neutron irradiation onto these plasma x-ray detectors; a fluence of 2–5×1013 neutrons/cm2 is utilized for simulating the effects of fusion-produced neutrons in JET. Modifications of the x-ray responses after neutron exposure due to fusion plasma shots in JET as well as cyclotron-produced neutron irradiations are found to have ...

A diagnostic method for both plasma ion and electron temperatures under simultaneous incidence of charge-exchange particles and x rays into a semiconductor detector array

An idea for using semiconductor detectors to simultaneously observe both plasma ion Ti and electron Te temperatures is proposed. The idea is also experimentally verified in tandem-mirror plasma shots. This method is developed on the basis of an alternative “positive” use of a semiconductor “dead layer” as an energy-analysis filter. Filtering dependence of charge-exchange neutral particles from plasmas on the thickness of a thin (on the order of nm thick) SiO2 layer is employed for analyzing Ti in the range from hundreds to thousands of eV. Even under the conditions of simultaneous incidence of such particles and x rays into semiconductor detectors, the different dependence on their penetration lengths and deposition depths in semiconductor materials makes it possible to distinguish particles (for Ti) from x rays (for Te). In this letter, proof-of-principle plasma experiments for the proposed idea are carried out to verify the availability of this concept of distinguishing and identifying each value of Ti ...

Newly developed matrix-type semiconductor detector for temporally and spatially resolved x-ray analyses ranging down to a few tens eV using a single plasma shot

For the purpose of the measurements of temporally and spatially resolved electron temperatures (Te) during a single plasma shot alone, we propose and fabricate a new matrix-type semiconductor x-ray detector. This detector is fabricated using the precise formation of thin dead layers (SiO2) with six different thicknesses (from 10 to 5000 A) aligned in line on its surface compactly. Each “row” has seven channels for the measurements of plasma x-ray radial profiles so as to make x-ray tomographic reconstructions; namely, the compact-sized matrix detector having six rows and seven columns with a 5×5 mm2 active area for each matrix unit. These various SiO2 layers are proposed to be utilized as ultrathin “x-ray absorption filters” with different thicknesses, which are never obtained as “self-supporting material absorbers” because of their ultrathin properties. This novel idea enables us to analyze x-ray tomography data including in the Te region down to a few tens eV. The simultaneous comparison of each tomogra...

Electron density fluctuation measurements using a multichannel microwave interferometer in GAMMA 10

Measurement of fluctuation in plasma is important for studying the improvement in plasma confinement by the formation of the plasma confinement potential. The density fluctuation is observed by microwaves by methods such as interferometry, reflectometry and Fraunhofer diffraction method. We have constructed a new multichannel microwave interferometer to measure the plasma density and fluctuation radial profiles in a single plasma shot. We successfully measured the time-dependent density and line-integrated density fluctuation radial profiles in a single plasma shot using the multichannel microwave interferometer. Thus, we have developed a useful tool for studying the improvement in plasma confinement by the formation of plasma confinement potential.