Motomi Iida

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

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.

Computer tomography of M=1 mode during sawtooth oscillation with three soft X-ray detector arrays on the WT-3 tokamak

Soft X-rays (SXRs) emitted from plasma in the WT-3 tokamak were measured by three SXR detector arrays, viewing the plasma from three different angles. From these experimental data, the authors reconstructed the two dimensional (2-D) contour of constant SXR emissivity as a function of time, using the technique of computer tomography (CT). Computer simulation results show that data from three or more sets of detector arrays are required to reconstruct the 2-D emissivity structure with sufficient accuracy to distinguish Kadomtsevs model from the quasi-interchange (Wessons) model of sawtooth crash. Further, it is shown that if the noise amplitude is more than 1.5%, reconstruction of the real 2-D SXR emissivity structure is difficult.

Study of the fast electron distribution function in lower hybrid and electron cyclotron current driven plasmas in the WT-3 tokamak

The distribution function f() of fast electrons produced by lower hybrid current drive (LHCD) is investigated in the WT-3 tokamak, using a combination of measurements of the hard X-ray (HXR) angular distribution with respect to the toroidal magnetic field and observations of the HXR radial profile. The data obtained indicate the formation of a plateau-like region in f() which corresponds to a region of resonant interaction between the lower hybrid (LH) wave and the electrons. The energy of the fast electrons in the peripheral plasma region is observed to be higher than that in the central plasma region under operational conditions with a high plasma current (Ip ≥ 80 kA). At low current (Ip 50 kA), however, the energy of fast electrons is constant along the plasma radius. In the current ramp-up phase, fast electrons are generated in the directions normal to and opposite to the LH wave propagation. The latter case is ascribed to a negatively biased toroidal electric field induced by the current ramp-up. To study the characteristic change of f() for various current drive mechanisms, HXR measurements are performed in electron cyclotron current driven (ECCD) plasma and in Ohmic heating (OH) plasma. In ECCD plasma, the perpendicular energy of fast electrons increases, which indicates that fast electrons are accelerated perpendicularly by electron cyclotron heating. In both LHCD and ECCD plasmas, fast electrons flow in the direction opposite to the wave propagation, while no such fast electrons are formed in OH plasma.

Production of high poloidal beta plasma by bidirectional lower hybrid current drive in the WT-3 tokamak

Lower hybrid waves propagating both parallel and anti-parallel to the toroidal field (bidirectional waves) produce suprathermal electron tails in both directions. The contribution of these tails is additive for the pressure but subtractive for the plasma current Ip. Therefore, the poloidal beta due to tail electrons, βptai1, becomes much larger than that in the usual case of unidirectional current drive, where βptai1 exceeds unity only when Ip is smaller than the Alfven critical current IA. In the WT-3 tokamak, lower hybrid current drive plasmas produced by bidirectional and unidirectional injection have been compared in terms of βp. The experimental results show that a high βp plasma is generated with Ip larger than IA in the bidirectional case.

Enhancement of efficiency for lower hybrid current drive by electron cyclotron heating in the WT-2 tokamak

Injection of electron cyclotron (EC) waves into the lower hybrid (LH) current-sustained plasma in WT-2 increases the efficiency and the ramp-up rate of the LH-driven current when the high-energy electrons that couple to the LH waves are EC-heated. On the contrary, the efficiency decreases and the current ramp-up rate becomes negative when the bulk electrons are EC-heated. This selective EC-heating of bulk electrons or high-energy electrons is achieved by changing the radial position of the EC resonance layer in the plasma column.

Investigation of high energy electrons in lower hybrid current drive plasma with electron cyclotron emission measurement in the WT-3 tokamak

Electron cyclotron emission (ECE) from lower hybrid current drive plasma in the WT-3 tokamak has been measured along a vertical chord where the magnitude of the toroidal field is constant. From the frequency of the peak of the down-shifted second harmonic X-mode emission, the perpendicular temperature is evaluated to be about 50, 40 and 25 keV for plasmas with a current of 65, 50 and 24 kA, respectively. Also, the line integrated tail electron density is evaluated from the peak intensity for these plasmas and found to be 1.7, 0.9 and 0.5 × 1012 cm−2, respectively. It is found that the perpendicular temperature is an increasing function of the component of the momentum parallel to the plasma current, p1 , and that the gradient of the perpendicular temperature along p1 increases as the plasma current decreases.

Sawtooth stabilization by localized electron cyclotron heating in the WT‐3 tokamak

The effect on sawtooth oscillations (STO) by localized electron‐cyclotron‐resonance heating (ECH) on the WT‐3 tokamak [Plasma Physics and Controlled Nuclear Fusion Research, 1988 (International Atomic Energy Agency, Vienna, 1989), Vol. 1, p. 563] is studied. STO are strongly modified or stabilized by ECH near the q=1 surface, where q refers to the safety factor. The effect of ECH is much stronger when it is applied on the high‐field side as compared to the low‐field side. Further, even when ECH is applied outside the q=1 surface, the amplitude of STO decreases and STO stabilizes. In the very high qL discharge, the excitation of STO can be obtained by applying ECH.

Heat flux of fast electrons to the limiter in lower hybrid current drive plasma on WT-3

The heat flux of fast electrons to the local limiter in LHCD plasmas in WT-3 has been investigated by thermal measurement of the limiter. The amount of the heat flux (PFE) is found to be about on third of the net radiofrequency power (Prf) injected into the plasma for various discharge conditions. The results combined with other measurements show that the confinement of fast electrons deteriorates as Prf increases. This direct loss of fast electrons is one of the causes of the degradation of the current drive efficiency. Heat transport of the bulk electrons is also found to increase as Prf increases. Experimental results indicate that a significant part of the remaining RF power (2Prf/3) flows to the bulk electrons. The slowing down power of fast electrons in the energy range above several tens of keV is estimated to be quite small compared with 2Prf/3, suggesting that a significant part of the remaining power flows to the bulk electrons via other channels. A plausible channel is the absorption of RF power via lower energy electrons by an upshift of the parallel refractive index of the injected lower hybrid waves. This seems to be another cause of the degradation of the current drive efficiency

Particle confinement of lower hybrid current driven plasmas and electron cyclotron heated plasmas in the WT-3 tokamak

Confinement properties of lower hybrid current driven (LHCD) plasmas and electron cyclotron heated (ECH) plasmas in the WT-3 tokamak are studied. The outward particle and energy fluxes are estimated from parameters of the scrape-off layer (SOL) plasma. During the LHCD period the density and temperature of the SOL plasma decrease drastically, while the density of the core plasma increases, suggesting a decrease of the outward flux. These results show that particle confinement is improved by LHCD. On the contrary, during ECH the density of the SOL plasma increases and the density of the core plasma decreases, suggesting a deterioration of particle confinement. Improved confinement is not obtained near the density limit of LHCD; the improvement is not related with MHD activities during LHCD.