Ultra-long pulse operation (> 3 min) was achieved on the superconducting high field tokamak TRIAM-1M. In this operation, the plasma current was maintained with a relatively peaked current distribution by the 2.45 GHz radiofrequency power (PRF ≤ 35 kW) alone. A stationary plasma with a driven current of up to 35 kA and a line averaged electron density of up to 3 × 1012 cm−3 was produced by precise plasma position and gas feed control. The extremely long discharge showed the interesting characteristics that the high temperatures of about 1 keV for the electrons and about 0.5 keV for the ions were kept almost constant during steady state current drive and that there was no impurity accumulation which could have a fatally adverse effect on steady state tokamak operation.

The hard X-ray emission from the TRIAM-1M tokamak plasma during steady state lower hybrid current drive with a discharge duration of a few minutes was measured with sodium iodide scintillation spectrometers. The radial profiles of the X-ray emission were also measured and indicate that, in the low density regime (ne = (1−3)×1012 cm−3), the current carrying high energy electrons are mainly in the inner region of the plasma column and their radial profile remains unchanged during current drive. On the other hand, high density discharges (ne = (3−6) × 1012 cm−3) are always accompanied by an abrupt drop of the plasma current, and the X-ray emission profile changes from peaked to broad. This change can be attributed to the conditions of wave accessibility. As the electron density increases, the accessibility of the plasma to lower hybrid waves with low values of the parallel wave number n1 is significantly reduced and high energy electrons resonating with the waves are produced at the plasma periphery. Interaction of these electrons with the limiters causes an increase of the electron density in this region; waves with low n1 then become completely excluded from the inner part of the plasma column. This interpretation is supported by measurements of the density profile and impurity radiation, and has been confirmed in an investigation of discharges with additional gas puffing.

In order to evaluate the electron temperature and the effective charge number (Z eff) of the ultralong discharge by a lower hybrid current drive in the TRIAM-1M tokamak, the soft X-ray energy spectrum is measured by the pulse height analysis system. The accuracy of the electron temperatures determined by the soft X-ray spectrum measurements is confirmed by comparing with the temperature obtained by the Thomson scattering system for OH plasmas. We obtained a bulk electron temperature of about 1.5 keV and a Z eff value of about 2 during the long-term discharge. It was found that the impurities observed in the spectrum were K α-lines of Fe, Cr and Ni originating from the stainless-steel limiter and they did not increase during the long-term discharge. Thus we obtained clean plasmas with relatively high temperature by a lower hybrid current drive.

A method of estimating the plasma elongation and position using the magnetic field picked up by magnetic coils is described. The method is based on the approximate solution for the Grad-Shafranov equation including the effect of the eddy current on the resistive shell. When this method was applied to noncircular plasmas on TRIAM-1M, the analytical results were found to be in good agreement with the plasma shape observed by a TV camera and with the outermost magnetic surface obtained through the calculation of an MHD equilibrium code. This approach is suitable for the real-time analysis and feedback control of plasma shape in a noncircular tokamak.