Yousuke Nakashima

A new end‐loss‐ion energy analyzer with obliquely placed multigrids for open‐ended plasma diagnostics

A new type of end‐loss‐ion energy analyzer (ELA; a multigridded Faraday cup) is applied to measure plasma ion temperatures Tip and plasma potentials ΦP in the GAMMA 10 tandem‐mirror device. As compared with conventional‐type ELA, this new ELA has obliquely placed ion‐ and secondary‐electron‐repeller grids and a collector plate with respect to external tandem‐mirror magnetic fields. One of the most useful capabilities of this new ELA for precise ion measurements is to greatly reduce the current of high‐energy electrons onto the collector plate. Also, this compact‐sized ELA is scannable in the plasma to obtain radial profiles of Tip and ΦP without providing any perturbations of the tandem‐mirror magnetic fields because it neither adds its own magnetic field nor needs to shield against the ambient magnetic field. Ion trajectories in the new ELA are numerically calculated so as to make a further understanding of its physics principles and to optimize its design. The energy‐calibration experiments for the new ...

Recent results of divertor simulation experiments using D-Module in the GAMMA 10/PDX tandem mirror

Abstract This paper describes the recent results of divertor simulation research toward the realization of the detached plasma using the end-mirror of a large tandem mirror device. The additional ion cyclotron range of frequency heating in the anchor-cells for higher particle flux generation significantly increases the density, which attained the highest particle flux up to 1.76 × 1023 particles/·m2 at the end-mirror exit. Massive gas injection (H2 and noble gases) into the divertor simulation experimental module (D-module) was performed, and a remarkable reduction of the electron temperature on the target plate was successfully achieved associated with the strong reduction of particle and heat fluxes in D-module. Two-dimensional images of Hα emission in D-module observed with a high-speed camera showed strong emission in the upstream region and significant reduction near the target plate. These results clarified the effect of radiation cooling and formation of detached plasma due to gas injection. It is also found that Xe gas is much more effective in achieving detached plasma than Ar gas. Numerical simulation studies also have been performed toward the understanding of the cooling mechanism of divertor plasma. The above results will contribute to establishment of detached plasma control and clarification of the radiation cooling mechanism toward the development of future divertor systems.

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 3×1013 cm−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 ...

Observation of AIC-mode in ICRF-heated plasmas in the central cell of GAMMA 10 tandem mirror

Alfven ion-cyclotron modes are generated in an ICRF heated plasma in the central cell of the GAMMA 10 tandem mirror. Identification of the mode, wave characteristics and its effects for the plasma parameters are described. General results of the GAMMA 10 experiments are also reviewed including the experimental scaling of the potential confinement.

Behavior of high energy ions in the GAMMA 10 tandem mirror

In the GAMMA 10 tandem mirror, ion cyclotron range of frequency heating has been used effectively in the central cell. The ion temperature becomes several keV and an ion beta value is a few %. Semiconductor detectors are used for measurement of high energy protons in both parallel and perpendicular directions to the magnetic field line. Observed pitch angle distribution peaks near the cyclotron resonance layer and estimated pressure profile in the axial direction is consistent with the profile from a diamagnetic loop array. Strong temperature anisotropy can drive an Alfven ion cyclotron (AIC) mode unstable. In a typical discharge, the temporal evolution of the endloss high energy ions has strong correlation with that of the AIC-mode amplitude. The enhancement of the loss of high energy ions due to the AIC mode is suggested.

Observation of Ion Confining Potential Enhancement due to Thermal Barrier Potential Formation and Its Scaling Law in the Tandem Mirror GAMMA 10

In the tandem mirror GAMMA 10 , (i) the enhancement of the ion confining potential, φ c , only during the period of the thermal barrier potential φ b -formation, has been observed first by using not only end-loss-analysers ( ELA s) of GAMMA 10 but an end-loss-ion-spectrometer ( ELIS ) installed from TMX-U . This results in strong end-loss-ion plugging with increased central cell density. (ii) The first experimental observation of the φ c vs φ b -scaling law is obtained, where φ c increases with φ b . This scaling law is consistently interpreted by Cohens theories of tlte weak- ECH and the strong- ECU in the plug region, (iii) Good agreement of the plug potential measuredwith the ELA s and the ELIS is achieved.

Relaxation Oscillation in an Ohmic-Heated Heliotron E Plasma with a Zero Rotational Transform Surface

Nonlinear evolution of tearing instability with a poloidal mode number m =1 and a toroidal mode number n =0 is studied in a straight helical system with an \(\imath{=}0\) surface, where \(\imath\) denotes a rotational transform. On the basis of a low beta resistive MHD model derived by using the stellarator expansion, it is shown that the m =1/ n =0 magnetic island grows to cover the whole central region; then, plasma current is readjusted and it almost recovers the initial profile. The large magnetic island causes outflow of ions, since the level of poloidal field fluctuations reaches about 10%. This whole process seems to repeat and may explain relaxation oscillations of ion temperature detected by neutral particle analyzer in the Heliotron E experiment.

Investigation of Edge Plasmas in the Anchor Cell Region of GAMMA 10

The first results of Langmuir probe measurements at the outer transition region of the anchor cell of GAMMA10 are given. A probe current asymmetry in vertical direction is found in this region. It is also found that the asymmetry of probe current increases in outward direction and the direction of the asymmetry is independent on movable limiter position. A relation of this plasma asymmetry with the main magnetic field configuration is investigated. Plasma flow through the non-axisymmetric magnetic field configuration region is thought to be the source of plasma asymmetry in this region, i.e., ∇ B and curvature drifts are responsible for the asymmetry. Possibility of cold plasma formation in the anchor cell region is obtained during plug electron cyclotron resonance heating (ECRH) and can be explained with the desorption of particles due to the collision of the drifted out particles with the wall.

Development and characterization of semiconductor ion detectors for plasma diagnostics in the range over 0.3 keV

For the purpose of plasma-ion-energy analyses in a wide-energy range from a few hundred eV to hundreds of keV, upgraded semiconductor detectors are newly fabricated and characterized using a test-ion-beam line from 0.3 to 12 keV. In particular, the detectable lowest-ion energy is drastically improved at least down to 0.3 keV; this energy is one to two orders-of-magnitude better than those for commercially available Si-surface-barrier diodes employed for previous plasma-ion diagnostics. A signal-to-noise ratio of two to three orders-of-magnitude better than that for usual metal-collector detectors is demonstrated for the compact-sized semiconductor along with the availability of the use under conditions of a good vacuum and a strong-magnetic field. Such characteristics are achieved due to the improving methods of the optimization of the thicknesses of a Si dead layer and a SiO2 layer, as well as the nitrogen-doping technique near the depletion layer along with minimizing impurity concentrations in Si. Such...

Gain Curves of a Microchannel Plate Ion Detector for Measurement of End-Loss Ions in the Tandem Mirror

A microchannel plate (MCP) is used as an ion detector in the end-loss energy component analyzer, which has been developed in order to measure velocity distribution functions of end-loss ions in the tandem mirror GAMMA10. The gain of the MCP depends on variable parameters of the incident ion energy, the incident ion current density, and the MCP bias voltage. The gain characteristics was obtained using a hydrogen beam test stand, and the gain curves were described as a function of the parameters for the purpose of the quantitative evaluation of end-loss ions.