T. Fujita

Recent results of ultra low q experiments in TORIUT-6 and REPUTE-1

In REPUTE-1, the radiation barrier has been overcome and ion and electron temperatures of 0.6 keV (Ti) and 0.3 keV (Te) have been achieved at an electron density of (0.2?0.7) ? 1020 m?3 and toroidal beta values of 12-18%. A resistivity anomaly due to m = l kink activity has been observed which may be connected with the peaked temperature profile and the rather broad current density profile. The plasma resistance is lower than that of reversed field pinch discharges in REPUTE-1 and decreases with increasing plasma current. In TORIUT-6, the effects of current ramp-up and carbonization of the first wall have been studied. A hollow current density profile, typical of ultra low q equilibria, is preserved which may be due to the skin current effect. The difference between current ramp-up discharges and standard discharges is the extended duration of the quiescent phase and the preservation of the position of the pitch minimum away from the axis in the ramp-up discharges. After carbonization, the duration of the quiescent phase increases and the plasma resistivity is reduced by 50%.

Absorption of fast waves excited by a phased four-loop antenna array in the JFT-2M tokamak

The absorption characteristics of fast waves excited by a phased four-loop antenna array have been investigated experimentally in JFT-2M. The frequency of the fast waves is 200 MHz, which corresponds to approximately the tenth harmonic of the ion cyclotron frequency of hydrogen. The fast wave power is absorbed mainly by the bulk thermal electrons. It is shown that the absorption efficiency of the excited fast waves is improved with increasing density and temperature, and with decreasing phase velocity of the fast wave. The results are consistent with the theoretical predictions obtained from ray-tracing calculations. The power deposition profile is obtained through synchronous detection of the electron cyclotron emission modulated by a periodic heat source. In this modulation experiment with a limiter plasma on JFT-2M the electron thermal diffusivity is 2-3 m2s−1 and the convection velocity is 20-40 ms−1 at e = 2 × 1019 m−3 and Ip = 230 kA. The resultant power deposition profile has a peak at the plasma centre and agrees well with that calculated with the ray-tracing code. The absorption efficiency calculated from the power deposition profile is 0.3-0.4 at e = 2 × 1019 m−3 and Te0 = 0.7 keV, which agrees roughly with that estimated from the initial rise of the plasma stored energy. The electron heating efficiency estimated from the absorption efficiency is (4–5) × 1019 eVm−3kW−1 and the incremental confinement time is 8-10 ms, which is slightly longer than that in L-mode plasmas heated by neutral beam injection and/or ion cyclotron heating in JFT-2M.

Coupling of fast waves launched into the JFT-2M tokamak by a phased four-loop antenna array

The coupling characteristics of the fast wave excited by a phased four-loop antenna array are described. Fast waves in the lower hybrid frequency range have the potential of generating plasma current in hot and dense plasmas. Fast waves are excited at the plasma edge by an oscillating magnetic field parallel to the toroidal direction. The parallel wavenumber of the excited fast waves is determined by the relative phase of the RF current on each antenna. The loading resistance of the antenna increases with density, but at densities above 2 × 1019 m−3 it starts to decrease with density, because of the steepening of the density gradient. The loading resistance is strongly dependent on the antenna phasing. The maximum loading due to surface wave excitation is obtained at Δ = 0 and the minimum at Δ = π. Substantial absorption of the excited fast waves is observed at the plasma centre when the antenna phasing is Δ = π. The absorption efficiency rises with decreasing phase velocity of the excited waves. The experimental results obtained in the coupling experiment are consistent with theoretical predictions.

Mössbauer Study of FeCl2·4H2O in the Temperature Range 4.2° to 0.025°K

The Mossbauer spectrum of 57Fe in FeCl2·4H2O has been examined in the temperature range 4.2° to 0.025°K. The temperature dependence of the internal magnetic field indicates that the spin ordering starts at 1.3°K. The spectrum below the Neel temperature is well‐explained with the parameters H = 266 kOe, e2qQ/2 = 3.10 mm/sec, η= −0.2 and ∠Hq = 25°, where H, e2qQ/2, and η have the conventional meanings and ∠Hq is the angle between the internal magnetic field and the electric field gradient at the iron nucleus. The spectrum for a single crystal with gamma rays being parallel to the monoclinic crystal axis b determines ∠Hb = 15°. Thus, below the Neel temperature the directions of the internal magnetic field at the two sites of iron nuclei in this material make an angle of 30° with each other.

Magnetic field fluctuations in ripple reduction experiments on the REPUTE-1 RFP

The loop voltage of REPUTE-1 RFP is decreased by reducing the toroidal field ripple. The low frequency part of the toroidal field fluctuations, which is mainly composed of the m = 1 mode and has good coherence in space and time, is found to be reduced, especially outside the reversal surface, when a trimming field is applied. High frequency fluctuations, which have little coherence, are not influenced effectively.

3D code for calculation of iron‐core field in fusion devices

Three‐dimensional codes for calculations of a stray magnetic field of iron core of a toroidal plasma confinement system have been developed. The calculated results agree well with the measured ones. It is found that up‐and‐down asymmetry of the iron‐core gap deforms the stray field asymmetrically when the primary coil current is not equal to the plasma current and that anisotropy of permeability of the iron core does not have a large influence on the field.

High-Current Runaway Electron Beam in a Tokamak Plasma

An equilibrium of toroidal plasma with a large electron-beam current has been realized using the runaway effect in a tokamak. Reproducible runaway-mode discharges are obtained with pure hydrogen gas by the help of intense titanium flashing which results in a low electron density. The beam current is estimated to be more than a half of the total toroidal current. The equilibrium of this discharge is maintained by a strong vertical field because the beam pressure gives rise to an additional increase in Shafranov Λ. The beam pressure is estimated to be more than 80% of the total pressure. The kinetic energy and the spatial distributions of beam electrons are studied by seeing X-ray emission from a tungsten wire inserted into the plasma. The increase of Shafranov Λ due to beam pressure is enhanced by puffing gas into the discharge.

Bolometer measurements of ultra-low-q and reversed field pinch plasmas in REPUTE-1

The energy loss by radiation is estimated in REPUTE-1 by bolometric measurements. Burning out of the radiation barrier is observed in both ultra-low-q (ULQ) and reversed field pinch (RFP) discharges. The conditions for burning out of the radiation barrier are studied for various operation parameters. A reduction of the radiation loss is observed after wall conditioning by carbonization.

Observation of m ≥ 2 modes in plasmas with 1/2 ≤ qa < 1

Internal modes with m = 1 and m ≥ 2, which localize around a pitch minimum, have been studied for ultra-low-q plasmas with 1/2 ≤ qa < 1 in REPUTE-1, where m is the poloidal mode number and qa is the safety factor at the plasma surface. One-dimensional stability analysis has shown that, for finite beta, m ≥ 2 modes have a relatively large growth rate compared with those of the m = 1 modes. The internal structure of the magnetic fluctuations with m = 1 and m ≥ 2 is found to be consistent with the numerically calculated eigenfunctions.

Toroidal Equilibrium of Plasma with Concentrated Relativistic Electron Beam

A simplified model has been given for toroidal equilibrium of a tokamak-type plasma with high-current concentrated electron beam. The plasma has a thermal pressure, and the electron beam has effective inertial pressure. Strong deformations of tokamak equilibria have been simulated by numerical calculations. Toroidal equilibria with relatively large vertical field are obtained when we consider high-energy intense electron beam. The beam orbit, which is shifted outward from the magnetic axis of the plasma, is closed by the sum of the externally applied relatively large vertical field and the poloidal magnetic field of the plasma.