A. Funahashi

Confinement and fueling studies during additional heating phase in the JFT-2M tokamak

Increments of peripheral hydrogen/deuterium neutral gas pressures (PH2/PD2) during the additional heating phase (neutral beam and RF heatings) have been observed in a D2 gas-puff fueled JFT-2M tokamak with H2-absorbed graphite limiters/divertor plates. In the beam heating phase, a large increment of PH2 raises the plasma density 2 times or more without the degrading energy confinement time. The D2 gas-puff valve is closed in this phase. This improvement is interpreted as being due to “wall fueling”, i.e., fueling by desorbed hydrogen from the graphite wall by energetic particles. The first observation of the H-mode in the INTOR-type stubby open divertor with a short divertor channel (1–8 cm) at the high density regime (4–7 × 1013 cm−3) enhanced by the wall fueling is presented. The improved energy confinement time is comparable to or higher than that of ohmically heated discharges.

Electron cyclotron resonant heating with an ordinary-mode antenna in the JFT-2 tokamak

Radio-frequency power of up to 110 kW for 14 ms at 28 GHz is launched in a horizontal polarization from the outside midplane at an angle nearly perpendicular to the 10-kG toroidal field of the JFT-2 tokamak. With initial Ohmic power comparable to the RF power, the central electron temperature increases from 600 to 1000 eV in 10 ms. Details of the electron heating are reported.

Transition from the L-mode to the H-mode by electron cyclotron heating of a tokamak edge plasma

Transitions of L-mode plasmas to the H-mode have been induced by an electron cyclotron heating (ECH) pulse. The transitions occur when ECH is applied to plasmas preheated either by a neutral beam or by waves in the ion cyclotron range of frequency with power levels well below their own threshold power for the H-mode transition. The position of the electron cyclotron resonance layer has been scanned and it has been shown that edge heating rather than central heating is effective in inducing the transition to the H-mode.

Electron cyclotron heating experiments on the JFT‐2 tokamak using an inside launch antenna

An electron cyclotron heating experiment is described in which 28 GHz microwave power is launched from the high field side (inside) of a tokamak discharge from a steerable phased array antenna exciting the extraordinary mode. The central temperature was doubled (from 600 eV to 1200 eV) with a power at the antenna approximately two‐thirds the Ohmic input. An oblique launch of the extraordinary mode was found to heat more efficiently and to a higher density than either the perpendicularly launched extraordinary mode or the ordinary mode launched from the outside.

Improvement of plasma parameters by titanium gettering in the JFT-2 tokamak

Abstract Oxygen impurity has been reduced to about 1–2% of the electron density by titanium gettering onto the torus wall (~ 1 2 ) and the limiter. Radiation loss and effective ionic charge were decreased by a factor of ~2. As a result of reduced impurity influx, broader electron temperature profiles have been obtained. The energy confinement time and the scaling factor of the maximum electron density ( n e R/B t ) were improved by a factor of 1.6 with the titanium gettering. Limits on the density increase were investigated in connection with the radiation power. Mechanism limiting the density maximum is discussed.

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.

Characteristics of the edge localized mode in the JFT-2M H-mode

The characteristics of the edge localized model (ELM), also referred to as edge relaxation phenomenon, were investigated in H-mode plasmas of JFT-2M. It was found that the ELM is mainly a density fluctuation phenomenon at the edge and that the electron temperature at the edge, except near the separatrix, is not very strongly perturbed. Some experimental conditions controlling the ELM intensity are plasma density, plasma ion species, heating power and plasma current ramping. ELMs found in low density deuterium discharges are suppressed by raising the density. ELMs are more strongly pronounced in hydrogen plasmas than in deuterium plasmas. ELMs seen in hydrogen plasmas or in near marginal H-mode conditions are suppressed by increasing the heating power. ELMs are found to be suppressed by plasma current ramp-down whereas they are enhanced by current ramp-up. The MHD aspects of the ELM were investigated. Although no consistent MHD features of the ELMs were found, they sometimes trigger bursts of the m = 2 mode. The enhanced broadband magnetic fluctuation suggests a temporary return to the L-mode during the ELM event.

Spectroscopic study of impurities in the JFT-2 tokamak

Impurity contamination and radiation power have been studied with a vacuum UV monochromator (10–1300 A) on JFT-2 for typical 160-kA and 18-kG hydrogen discharges. Oxygen has been found to be a dominant impurity element (5–7% of the electron density) and to be radially distributed according to the ionization sequence. The ground level population densities of the O6+ and O7+ ions in the central hot region have been determined, and that of fully ionized oxygen has been also estimated, combined with enhanced bremsstrahlung radiation measurement. The radial profile of the effective ionic charge Zeff has been determined from that of the impurity ions. The Zeff value has been found to be consistent with the value derived from the electrical conductivity. The total power of line radiation from the first resonance transitions of the oxygen ions amounts to 100–50 kW (34–17% of Joule heating power).

Electron heat conduction coefficient at internal disruption in low-q DIVA plasmas

Electron temperature and density profiles are measured by Thomson scattering of ruby laser light in a low-q DIVA plasma; the profiles are strongly affected by internal disruption. The effective electron conduction coefficients χep at the moment of internal disruption are calculated on the basis of the diffusion equation. – The average value of χep outside the q = 1 singular surface rD, 7.2 × 104cm2s−1 is much larger than that expected for the base discharge, which implies that the internal disruption substantially enhances the thermal transport throughout the region rD<r≤a. The χep(r) profile inside the q = 1 surface is approximated by the equation χep(r)= 1.4 × 105 × (1–(r/11)1.5)cm2s−1 at 0≤r≤rD.

Plasma surface interaction during rf heating on JFT-2 tokamak

Abstract Plasma-surface interactions during RF heating on JFT-2 tokamak were investigated. Light impurities (O, C) and also metal impurities (Mo, Fe, Ti) increased by about twice for 220 kW RF power, however, because impurities have been reduced to an allowable level by Ti gettering onto the wall and the limiter, it was not so deleterious on plasma confinement. The increase of metal impurities can be explained by the light impurity sputtering and the effect of high energy ions produced by RF heating seems to be minor for metal release. On condition of clean surface produced by Ti gettering, decrease of electron density was observed during RF heating. This phenomenon is discussed in connection with the behaviour of high energy ion.