Kiyoshi Hayase

Power balance in a reversed-field-pinch plasma with partial poloidal current drive by neutral beams

Zero-dimensional power balance is analyzed, and an operation boundary is deduced in a `beam-assisted reversed-field pinch`; the latter utilizes partial poloidal current drive by neutral beams so that transport losses arising from magnetohydrodynamics (MHD)-dynamo, i.e., tearing mode instability are reduced. Changes of power flow and heat conductivity due to a beam driven current are treated by considering an MHD-dynamo-based power balance model that assumes linear dependence of magnetic fluctuation level on the externally driven current. It is shown that a ratio of a beam driven current to a dynamo current must not exceed nearly 40% regarding a beta-limit in the next generation of plasma experiments (minor radius/major radius = 0.6 m/1.8 m, plasma current = 1 MA, poloidal beta = 0. 1). At that point, the energy confinement time is predicted to increase by a multiple or so of that estimated from the MHD dynamo model without a current drive. 41 refs., 12 figs., 2 tabs.

Poloidal-Open Divertor Experiments on TPE-2M Reversed Field Pinch

Reversed field pinch configuration with a poloidal-open divertor has been realized quasi-continuously on TPE-2M device with close-fitting shell. Significant increase of liner temperature near the separatrix has been observed, indicating strong heat transport through the separatrix, consistent with calculated last closed flux surface from measured magnetic fields. Spectroscopic measurement has shown that impurities from the limiters and the first wall rapidly increase right after divertor configuration is realized, which are supposed to be due to a short connection length between the separatrix and the wall. Characteristics of resistive kink modes have been found to be similar to those without divertor. Apppearance of local magnetic fluctuations near separatrix, however, have been observed by the onset of divertor fields, but not to grow to the disruption.

Reduction of Plasma Resistance by Toroidal Divertor in TPE-2M Reversed-Field Pinch.

A reversed-field pinch configuration with a four-separatrix toroidal divertor has been shown to be sustained without deterioration of MHD instabilities. By applying toroidal divertor fields, the plasma current increases by about 30% of that of discharge without a divertor, resulting in 40% reduction of plasma resistance, for the same charging voltage of an ohmic circuit in low-plasma-current operation (40 kA-55 kA). Decrease of impurity lines of carbon and oxygen is also observed.

A magnetic limiter study of a reversed field pinch plasma in TPE-2M

Abstract A fully axisymmetric poloidal magnetic limiter/divertor (PD) experiment with a close fitting shell was carried out in the newly constructed reversed field pinch (RFP) device TPE-2M. The interaction of the diverted SOL plasma with the wall and limiter tiles is observed by a fast spectroscopic CCD camera. The strong interaction of SOL plasma with the wall is inevitable in this open divertor configuration. A preliminary experiment of toroidal divertor (TD) is also carried out. MHD phenomena in both types of divertors are similar to those of standard discharges (without divertor). The results from these observations indicate that magnetic divertors can usefully be adopted to control the plasma-wall interaction in an RFP, if the back flow of wall impurity is effectively avoided by a separated divertor room.

Equilibrium Configuration of a High-Beta Screw Pinch

The magnetohydrodynamic equilibrium of a moderately elongated screw pinch is considered. From comparisons of experimental and calculated data on the elongation factor and radial shift of the plasma column, the configuration is described by a two-region flat-current tokamak model. The inner high-beta plasma is surrounded by a force-free current region.

High-Beta Plasma Production and Confinement in Screw Pinch TPE-2

A high-beta plasma of βt~0.1 and eβp>2 is produced and its confinement properties are studied in a screw pinch of moderately elongated cross section. The initially constricted high-beta plasma column is surrounded by a force-free current layer and expands radially. The energy confinement time is much less than that from the empirical scaling of an ohmically heated tokamak, but it gradually improves as βp decreases. The fast plasma-energy decay and the fast boundary diffusion are considered to be caused by the ballooning mode.