M. Hirata

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.

Charge-exchange neutral particle measurements on electron cyclotron resonance heating experiments in hot-ion-mode plasmas of the GAMMA 10 tandem mirror

Charge-exchange (C–X) neutral particle measurements have been carried out in hot-ion-mode plasmas of the GAMMA 10 tandem mirror. In the present experiment, a microwave power of 40 kW in 28 GHz is injected toward a second harmonic ECR layer located in the vicinity of the ICR layer at the central region and the radial profiles of ion temperatures determined from the energy spectrum of the C–X neutrals by using a neutral particle energy analyzer (NPA) are investigated from the viewpoint of ion energy balance. At the onset of the ECRH pulse, a remarkable increase of C–X neutral flux with high energy (few keV to few tens keV) is observed with NPA and the resultant ion temperature on the plasma axis is found to increase from 2.5 to 5.0 keV at the electron line density of 3×1013u2009cm−2. Based on the measured plasma parameters, radial profiles of ion-energy losses due to classical processes are evaluated and ECRH in the central region is confirmed to reduce the energy loss due to electron drag significantly in the ...

A theory on the x‐ray sensitivity of a silicon surface‐barrier detector including a thermal charge‐diffusion effect

An analytical method based on a new theoretical model for the x‐ray energy responses of silicon surface‐barrier (SSB) detectors has been proposed. This method may address a recent confusing issue in the x‐ray detection characteristics of SSB semiconductor detectors; that is, the x‐ray responses of SSB detectors as well as p‐i‐n diodes used in underbiased operations were recently found to be contrary to the commonly held belief that the x‐ray sensitivity of an SSB detector is determined by the thickness of the depletion layer. The model presented includes a signal contribution from thermally diffusing charge that is created in the field‐free substrate region within a diffusion length from the depletion layer along with a signal contribution from charge created in the depletion layer. This model predicts a large signal contribution from the charge‐diffusion effect on the SSB responses to high‐energy x rays. Formulas and calculated results supporting SSB calibration data have been represented. These analytic...

Coupling of ICRF waves and axial transport of high-energy ions owing to spontaneously excited waves in the GAMMA 10 tandem mirror

Plasmas with high ion temperature of several kiloelectronvolts and a strong temperature anisotropy of greater than 10 were produced by ion cyclotron range of frequency (ICRF) heating in the GAMMA 10 tandem mirror. In such high-performance plasmas with strong anisotropy, high-frequency fluctuations, so-called Alfv?n-ion-cyclotron (AIC) waves, are excited spontaneously. These AIC waves have several discrete peaks in the frequency spectrum. Coupling of the ICRF heating waves and the excited AIC waves was clearly observed in the density fluctuations measured with a newly developed reflectometer. Parametric decay from the heating ICRF waves to the AIC waves and low-frequency waves was also indicated. Alfv?n waves with difference frequencies between the discrete peaks of the AIC waves were detected in a signal that measured the number of axially transported high-energy ions (over 6?keV) at the machine end, indicating pitch-angle scattering caused by the low-frequency waves. Energy transport along the magnetic field line is an important consideration when ICRF power is injected in the perpendicular direction to a magnetic field line. The importance of the spontaneously excited AIC waves for axial confinement of a tandem mirror through wave?wave couplings was demonstrated.

Extension Of operational regimes with ICRF heating on gamma 10/PDX

Abstract Recent ICRF heating experiments performed in GAMMA 10/PDX are reported. Owing to antenna-phasing technique and four ICRF antennas installed in the mirror cells other than the central cell, the operational regime of GAMMA 10/PDX is significantly extended. Several ICRF heating schemes to increase the parameters (plasma density in the central cell, particle flux flowing to the machine end and its ion temperature parallel to the field lines) are demonstrated. These results present positive progress in the ongoing divertor simulation experiments on GAMMA 10/PDX.

Wave excitation by nonlinear coupling among shear Alfvén waves in a mirror-confined plasma

A shear Alfven wave at slightly below the ion-cyclotron frequency overcomes the ion-cyclotron damping and grows because of the strong anisotropy of the ion temperature in the magnetic mirror configuration, and is called the Alfven ion-cyclotron (AIC) wave. Density fluctuations caused by the AIC waves and the ion-cyclotron range of frequencies (ICRF) waves used for ion heating have been detected using a reflectometer in a wide radial region of the GAMMA 10 tandem mirror plasma. Various wave-wave couplings are clearly observed in the density fluctuations in the interior of the plasma, but these couplings are not so clear in the magnetic fluctuations at the plasma edge when measured using a pick-up coil. A radial dependence of the nonlinearity is found, particularly in waves with the difference frequencies of the AIC waves; bispectral analysis shows that such wave-wave coupling is significant near the core, but is not so evident at the periphery. In contrast, nonlinear coupling with the low-frequency backgro...

X-ray detector characterization and applications for plasma research

Abstract Using synchrotron radiation, the detection characteristics of microchannel plates, silicon surface barrier detectors and gold photocathodes are investigated; in particular, the detailed structures of the detection efficiency including an extended X-ray absorption fine structure (EXAFS), an X-ray absorption near-edge structure (XANES) and a chemical shift is reported along the with discussion on the physical mechanisms. Also, the applications of these detectors to X-ray tomography diagnostics for the observation of the cross-sections of fusion-oriented tandem-mirror plasmas are described. These data give information on the spatial distribution of the hot electrons which play an important role in the formation of thermal-barrier potentials φb. Also, the tomographic reconstructed X-ray data in the central cell show the effect of φb formation on the improvement in the central-cell electron confinement for the first time.

Recent ICRF Heating Experiments for a Divertor Simulation Study on GAMMA 10

In GAMMA 10, a divertor simulation study has been started with open magnetic field configuration in the end region. High heat and particle fluxes are required along the magnetic field line to the end region. Plasmas with high ion-temperature of several keV and strong temperature anisotropy of more than 10 have been produced by using ion-cyclotron range of frequency (ICRF) heating in the central cell. Direct anchor heating experiments with new anchor antennas have been performed and the enhancement of the MHD stabilization has been observed. High energy ions whose energy is more than 50 keV have been observed in the end-loss ions. The axial transport of high-energy ions due to loss processes other than the classical Coulomb scattering has been discussed. Alfvén-ion-cyclotron (AIC) waves are spontaneously excited owing to such the strong temperature anisotropy and considerable energy transport along the magnetic field line due to the AIC waves is expected. In this review, recent ICRF heating experiments for the divertor simulation study are described.