Y. Terumichi

Formation of spherical tokamak equilibria by ECH in the LATE device

The main objective of the Low Aspect Ratio Torus Experiment (LATE) device is to demonstrate the formation of spherical torus (ST) plasmas by electron cyclotron heating (ECH) alone without a centre solenoid and establish its physical bases. By injecting a 2.45 GHz microwave pulse for 4 s, a plasma current of 1.2 kA is spontaneously initiated by P = 5 kW under a weak steady vertical field of Bv = 12 G and then ramped up slowly with a slow ramp-up of Bv for the equilibrium of the plasma loop and finally reaches 6.3 kA by P = 30 kW at Bv = 70 G. This current amounts to 10% of the total coil currents of 60 kA flowing through the centre post for the toroidal field. Magnetic measurements show that an ST equilibrium, having the last closed flux surface with an aspect ratio of R0/a 20.4 cm/14.5 cm 1.4, an elongation of κ 1.5 and qedge 37, has been produced and maintained for 0.5 s at the final stage of discharge. Spontaneous formation of ST equilibria under steady Bv fields, where plasma current increases rapidly in the time scale of a few milliseconds, is also effective and a plasma current of 6.8 kA is spontaneously generated and maintained at Bv = 85 G by a 5 GHz microwave pulse (130 kW, 60 ms). In both cases, the plasma centre locates near the second or third harmonic EC resonance layer and the line averaged electron density significantly exceeds the plasma cutoff density, suggesting that the harmonic EC heating by the mode-converted electron Bernstein waves supports the plasma.

Numerical Studies of Wave Trajectories and Electron Cyclotron Heating in Toroidal Plasmas

Wave trajectories and cyclotron damping of the electromagnetic waves (O- and X-modes) and the electron Bernstein wave (B-mode) propagating in three dimensional toroidal plasmas are investigated, assuming the geometrical optics. Electron cyclotron heating (ECH) by injection of O- and X-mode is effective in low density (\(\omega_{\text{pe}}{ }\varOmega_{\text{e}}{\approx}\omega\)), where B-mode is linearly converted from X-mode or O-mode (the latter being converted to X- and then to B-mode). Because of strong damping, B-mode is also useful for low T e plasmas. Propagation characteristics and mode-conversion in these high density plasmas are studied in detail.

Lower hybrid wave driven current and associated instabilities in the WT-2 tokamak

Abstract During rf injection near the lower hybrid frequency, the loop voltage decreases and reverses in a low-density discharge in the WT-2 tokamak, implying that an rf-driven current is produced. The associated ion-cyclotron instability and relaxation oscillation, where voltage spikes, step-like increases in electron-cyclotron emission and X-ray bursts appear, were observed.

Microwave preionization of the tokamak discharge at the electron cyclotron resonance

Abstract When ohmic heating power is applied during the microwave discharge at the electron cyclotron resonance, the plasma current and density build-up become fast and the loop voltage required to start the tokamak discharge decreases. The micro-wave burst observed in the initial stage is reduced and the burst of hard X-rays, occurring at the quasi-steady stage of discharge in the case of no microwave injection, disappears, suggesting that the production of run-away electrons is suppressed strongly.

Suppression of a pressure driven m=1 mode in a lower hybrid current drive plasma by electron cyclotron heating in the WT-3 tokamak

A pressure driven m=1/n=1 mode is excited by lower hybrid current drive in the WT-3 tokamak [T. Maehara et al., Nucl. Fusion 38, 39 (1998)]. The excitation of the mode is accompanied with the decrease of the magnetic shear and with the peaking of the soft x-ray emissivity profile inside the q=1 surface. The crescent-shaped mode structure appeared on the contour map of the soft x-ray emissivity is consistent with that of the quasi-interchange mode. The m=1 mode can be suppressed by electron cyclotron heating near the q=1 surface. The range of the location of the electron cyclotron resonance layer effective for the complete suppression is much wider and the time scale for the suppression is much faster than those in the case of the suppression of the tearing mode in the ohmic heating plasma.

Initiation of plasma current with the assistance of electron cyclotron waves in the WT-3 tokamak

In the WT-3 tokamak, the toroidal plasma current is started and ramped up to 6.3 kA by the electron cyclotron (EC) wave alone, without ohmic heating (OH) power. After generation of the plasma current by the EC wave, the OH current can be started up with a very low ohmically induced electric field, i.e. <0.32 V/m, which is extremely small compared with that (2.5 V/m) in an OH discharge with the assistance of EC resonance preionization. The low value of the starting loop voltage is in accordance with the requirements of the ITER design. It is demonstrated that the loop voltage and the flux of the OH transformer can be reduced considerably when the EC power is used for preionization and startup of the toroidal current before initiation of an OH discharge in tokamaks

Pre-ionization and heating of stellarator plasma at electron cyclotron frequency in JIPP T-II

Experimental studies of electron cyclotron pre-ionization and heating have been carried out in the JIPP T-II torus by injecting a power of 36 kW at the frequency of 35.5 GHz. Pre-ionization effectively decreases the loop voltage at the initial stage and eliminates strong spikes in the signals of electron density and of light and hard-X-ray emission which is due to runaway electrons in the initial breakdown phase of the Joule heating. From the measurement of the central electron temperature only, it is seen that electron cyclotron heating of a stellarator plasma with ordinary-mode radiation shows a heating efficiency of 1.6 eVkW−1 and, from power balance considerations, the absorption rate of microwave power is estimated to be around 50%.

Numerical Study of Propagation and Damping of Lower Hybrid Wave in Tokamak Plasmas

Incident lower hybrid waves propagate toward the center of the plasma in a spiral form in the poloidal and the toroidal sections and finally are absorbed by the ion and/or the electron Landau damping within the limited spatial region in accordance with the refractive index parallel to the magnetic field, N // , which is varied considerably along the trajectory, because of the toroidicity and the rotational transform. We propose the scaling law of the wave trajectories on plasma parameters, which shows that the control of N // or time applied frequency is necessary during the lower hybrid heating. These wave trajectories and damping based on the unmagnetized ion model are compared with those based on the magnetized ion model.

ANOMALOUS MICROWAVE RADIATION AT CYCLOTRON RESONANCE IN PARTIALLY IONIZED PLASMAS

An anomalous microwave radiation from a d c discharge plasma in a magnetic field was observed. The intensity was extremely large compared with that of black body (about 1×10 4 °K) and amounted to the power radiated from the plasma in the thermal equilibrium equivalent to 5.5×10 6 °K in the case of Kr and 1.5×10 6 °K in the case of Xe. This radiation occurred very sharply at cyclotron resonance field, the width being by one order smaller than that of normal cyclotron radiation. The radiation was observed in Xe, Kr and Ar plasmas in the pressure range of 1×10 -1 ~5 mmHg, but not in Ne and He plasmas at any pressure. Further, the radiation was observed when the whole tube was placed in a longitudinal magnetic field, but not when only a part of the positive column was immersed in the field. Finally, it is discussed whether this anomalous radiation can be interpreted as a phenomenon of the negative radiation temperature at cyclotron resonance in partially ionized plasmas.

Non-inductive current drive using second harmonic electron cyclotron waves on the WT-3 tokamak

A plasma current of up to 70 kA has been sustained in WT-3 discharges by second harmonic (2Ωe) electron cyclotron (EC) waves alone, with zero loop voltage, after shutting off the Ohmic heating power. Further, in the case of high power EC wave injection, ramp-up discharges have been obtained. Pulse height analysis of hard X-rays in the line of sight at various angles to the toroidal field shows that the velocity distribution function of the high energy tail electrons is asymmetric in the toroidal direction. The 2Ωe EC wave is mainly absorbed by the tail electrons, and a 2Ωe EC driven current is generated by enhancing the asymmetry of the distribution. The figure of merit of 2Ωe EC current drive (ECCD) is ηEC(2) = (3.2-6.4) × 10−2 (1019 A/Wm2), which is one order of magnitude smaller than that of lower hybrid current drive in WT-3. This low value of ηEC(2) can be attributed to low confinement of the current carrying, high energy tail electrons produced by 2Ωe ECCD.