Y. Shimomura

Numerical calculations of helium ash enrichment and exhaust by a simple divertor

A simple poloidal divertor able to enrich and exhaust helium ash has been proposed for an INTOR-size reactor. Monte-Carlo simulations are carried out to investigate the motion of DT-fuel and He-ash particles after they have been re-injected from the divertor neutralizer plate. Helium-ash exhaust is shown to be feasible with a modest pumping speed of about 5 × 105 litre s−1 for most of the conceivable conditions of plasma particle containment.

HEAT FLUX TO THE MATERIAL SURFACES IN A TOKAMAK

Heat flux to the material surfaces in the scrape-off layer of a tokamak plasma is investigated experimentally. The time response of heat flux measurement has been improved to 0.2 ms by the recently developed thin-film thermometer. The result shows that the heat flux is calculated from a simple sheath model including secondary electron emission effects even if epithermal electrons are present.

Numerical Analysis of Magnetohydrodynamic Instabilities by the Finite Element Method

A finite‐element method has been applied to the analysis of magnetohydrodynamic instabilities in a cylindrical current‐carrying plasma embedded in a strong longitudinal magnetic field. To examine the applicability of the finite‐element method to the stability analysis, the growth rate of instabilities has been calculated numerically using an electronic computer. It has been shown that the numerical results, for which corresponding analytical results are available, are in good agreement with them. Several examples of the analysis of low‐m mode instabilities with various current distributions are demonstrated. It is concluded that the finite‐element method is a powerful tool for the study of magnetohydrodynamic instabilities in the cylindrical plasma as well as, possibly, plasmas with more complex geometries.

Carbon wall experiment in diva

Abstract The sputtering characteristics of various types of carbon limiter surfaces, e.g. pyrolitic graphite (PG), pulverized carbon film and carbon film produced by methane discharges, are tested. Arcing is only observed on the pulverized carbon surface in a stable discharge or the other surfaces in an unstable discharge. Ion sputtering is shown to be the dominant process of carbon efflux from the normal surfaces. Employing PG limiters and a carbon wall coated by rf-sputtering method, very low q plasmas with a good confinement characteristic are obtained. The coated carbon surface is pure and still not contaminated after 600 or more discharges.

Ion sputtering, evaporation and arcing in DIVA

Abstract An experimental study of the metal impurity origin in DIVA is described. Three processes for the release of the metal impurities, that is, ion sputtering, evaporation and arcing have been identified. Among these processes, ion sputtering is the dominant process in the quiet phase of the discharge, that is the phase characterized by no spikes in the loop voltage and no heat flux to a specific part of the first wall. Moreover it is demonstrated that a honeycomb structure can decrease the release of the metal impurity. This paper is being published separate [1].

Plasma behavior with a separatrix magnetic surface in JFT−2a tokamak

Results from the JFT−2a (DIVA) experiments made with a separatrix magnetic surface are described. The main conclusions are: (1) A separatrix magnetic surface is stably located inside the material surface. (2) A plasma enclosed in a separatrix magnetic surface is similar to that of a conventional tokamak as far as the magnetohydrodynamic properties are concerned. (3) Measured parameters of the main plasma column are consistent with those expected from a conventional tokamak. (4) The electron density and temperature in the scrape‐off layer are about ten times less than the values at the center of the main plasma column. (5) Heat and particle fluxes to the divertor region are axisymmetric and several times less than those of the total loss fluxes from the main plasma column. (6) Runaway electrons are well guided to the divertor region.

Effects of metal impurity re-cycling in the scrape-off plasma of a large tokamak

Re-cycling of self-sputtered impurities from a limiter or a divertor neutralizer plate is investigated numerically. An upper limit for the boundary plasma temperature is determined from the condition that wall material impurity ions do not increase. This temperature is shown to be very low in a conventional tokamak but not limited in a divertor tokamak with a molybdenum neutralizer plate.

Magnetohydrodynamic activity in the JFT-2 tokamak with high-power neutral-beam-injection heating

Neutral-beam power of up to 1.2 MW injected into the plasma has produced a volume-averaged β of up to 2.6% and a central beta β0 of up to 7%, due to the thermal components in the JFT-2 tokamak. In these beam-dominated discharges, the magnetohydrodynamic behaviour was studied. Four types of internal oscillations were observed: i) enhanced sawtooth oscillations with long repetition time and large sawtooth amplitude; ii) round sawtooth oscillations and/or reduced sawtooth oscillations with short repetition time and small sawtooth amplitude; iii) high-frequency oscillations without sawtooth oscillations, and iv) high-frequency oscillations with sawtooth oscillations. The measured beta values are compared with the critical ones as found from high-n ballooning-mode analysis, and the relationship between MHD behaviour and beta values is also investigated.

Divertor experiment for impurity control in DIVA

Abstract Divertor actions for controlling the impurity origin and penetration into the main plasma were investigated and demonstrated in DIVA.

Analysis of hydromagnetic plasma stability by the finite-element method

Though hydromagnetic plasma stability has been investigated analytically by many authors, analysis is not always applicable to complex plasma configurations. In such cases, numerical calculations can be made to obtain a condition for stability.