M. Hosokawa

Compact Helical System physics and engineering design

This paper reports on the Compact Helical System designed for research on transport in a low-aspect-ratio helical system. The machine parameters were chosen on the basis of a physics optimization study. Considerable effort was devoted to reducing error fields from current feeds and crossovers. The final machine parameters are as follows: major radius of 1 m; minor radius of the helical field coil of 0.313 m; plasma aspect ratio A{sub p} = 5; pole number and toroidal period number of the helical field coil of l = 2 and m = 8, respectively; and helical pitch modulation of {alpha}{sup *} = 0.3.

Shafranov shift in the low aspect ratio heliotron/torsatron Compact Helical System

The MHD equilibrium properties of neutral beam heated plasmas have been experimentally investigated in the Compact Helical System (CHS)-a low aspect ratio (Ap ~ 5) heliotron/torsatron. This configuration is characterized by a strong breaking of helical symmetry. The radial profiles measured by various diagnostics have shown a significant Shafranov shift due to the plasma pressure. The deviation of the magnetic axis from is vacuum position has become as large as 50% of the minor radius. When the three-dimensional equilibrium code VMEC is used to reconstruct the equilibrium from the experimental data, the result is in good agreement with the experimentally observed Shafranov shift as well as with the diamagnetic pressure in plasmas with β ≤ 1.2% and β0 ≤ 3.3%. This beta values corresponds to half of the conventional equilibrium β limit defined by the Shafranov shift reaching a value of half of the minor radius. Although tangential neutral beam injection causes pressure anisotropies, p||/p⊥ ≤ 3, the description of the equilibrium assuming isotropic pressure is consistent with the experiment

Space and time‐resolved measurements of plasma density by a lithium neutral beam probe in NBT‐1M

A lithium neutral beam probe has been improved for space and time‐resolved measurements of plasma density in NBT‐1M. A lithium neutral beam (4 keV, 10–30 μA) is injected into the plasma and photon flux emitted from the injected lithium atoms by electron impact excitation is detected. This cross section is not sensitive to the electron temperature in a wide range (10 eV 1 keV), where the attenuation through charge‐exchange process becomes dominant. This method is not influenced by the magnetic field and can be applied to plasmas in any magnetic field configuration.

Generation of energetic electrons by electron cyclotron heating in a magnetic mirror field

Electron cyclotron heating that generates hot-electron plasma in a magnetic mirror trap by microwaves is studied experimentally. The evolution of the energy distribution functions for the high-energy electrons is observed in steps of milliseconds during 200 ms of the heating period from the initial stage at the microwave power input until the stationary final state. According to the proposed statistical model for the cyclotron heating, heating rates are estimated to be 10 MeV/s typically, in the three characteristic cases of mirror field configuration with heating microwave power as a parameter. Some problems associated with stochastic cyclotron heating are discussed in the light of the experiments.

Pre-ionization and heating of stellarator plasma at electron cyclotron frequency in JIPP T-II

Experimental studies of electron cyclotron pre-ionization and heating have been carried out in the JIPP T-II torus by injecting a power of 36 kW at the frequency of 35.5 GHz. Pre-ionization effectively decreases the loop voltage at the initial stage and eliminates strong spikes in the signals of electron density and of light and hard-X-ray emission which is due to runaway electrons in the initial breakdown phase of the Joule heating. From the measurement of the central electron temperature only, it is seen that electron cyclotron heating of a stellarator plasma with ordinary-mode radiation shows a heating efficiency of 1.6 eVkW−1 and, from power balance considerations, the absorption rate of microwave power is estimated to be around 50%.

Transient local magnetic field measurement in a bumpy torus by rapid‐frequency‐scan laser spectroscopy

A new technique for local magnetic field measurement in a bumpy torus NBT‐1M (which is a magnetic plasma confinement device) has been developed using a rapid‐frequency‐scan (RAFS) laser system combined with sodium atomic beam probing. The RAFS laser beam excites the sodium D1 transition in the plasma cavity, and the Zeeman pattern is determined with laser fluorescence spectroscopy in a single laser shot. From the Zeeman splitting, the transient magnetic field distribution in the plasma, produced by electron cyclotron resonance heating (ECH), is measured with an accuracy of ±3.2%, a spatial resolution of 5 mm, and a temporal resolution of 5 μs.

Results from the CHS device

This paper reviews recent experimental results from compact helical system (CHS), a heliotron/torsatron type device with a low aspect ratio of 5. Transport phenomena are discussed through analyses of MHD equilibrium, thermal/particle diffusivity and viscosity. It is seen that transport is dominated by anomalous processes which are thought to be due to electrostatic fluctuations. On the basis of these results, optimization of confinement by shifting the vacuum magnetic axis is briefly discussed.

HEATING SYSTEMS OF LARGE SUPERCONDUCTING HELICAL DEVICE

The heating systems for the new large helical device which has been designed as a joint effort among Japanese universities are reported. The conceptional design of the heating devices and basic hardware structures are discussed. ECH heating system (112 GH z ) with maximum power of around 10MW is proposed to produce and heat the net current-less plasma. For the further heatings, NBI (20 MW, 100 kV, H 0 ) and ICRF (9 MW, 30–90 MH z ) heating systems have been considered. High energy particle loss, which is a fairly serious problem in helical configurations (stellarator/heliotron), have been analyzed numerically. Monte-Carlo calculation shows that, for tangential injection, the beam energy over 100 kV is necessary to obtain the thermalization efficiency of above 60% inside the half radius. On the fast-wave (ICRF) heating, theoretical analysis of the wave field and velocity distribution function, which includes the loss cone effect, has been developed. In the case of 3 He minority heating, the orbit loss problem becomes small than in the case of proton minority heating. From these theoretical analysis and hardware considerations, the required thermalized power of around 20 MW can be obtained by ECH, NBI and ICRF heatings.

Edge plasma control by rf electric field in the compact helical system (CHS)

The reduction of the plasma heat flux on the material limiter and the stabilization of the fluctuations at the plasma boundary by the use of an rf ponderomotive potential barrier on the limiter (rf limiter) have been demonstrated in the compact helical system (CHS) for the ECH heated plasma. The rf current on a graphite limiter surface along magnetic field lines produces a ponderomotive potential barrier, which is large at the limiter edges. The high heat load on the limiter edges, which is measured using an infrared TV camera, has been shown to be reduced by an rf power of 1.4 kW at a frequency of 10 MHz ( ω ci ω ω ce ). Another remarkable effect of the ponderomotive potential barrier on the limiter that has been observed is the suppression of density fluctuations (

Intensity measurements of extreme ultraviolet radiation from helium plasma by photoelectron spectroscopy

Abstract Extreme ultraviolet radiation emitted from He plasma has been studied by means of photoelectron energy analyzer. Relative and absolute intensities of Lyman series of ionized helium have been measured. From these spectral intensities, the electron temperature of the plasma has been estimated at several tens of eV.