T. Maekawa

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.

A survey of mode-conversion transparency windows between external electromagnetic waves and electron Bernstein waves for various plasma slab boundaries

For the plasma slab boundary with monotonically increasing density profile along the x axis and the magnetic field along the z axis, both Nz and Ny components of the refractive index are parallel to the plasma slab and are conserved in the mode-conversion process between the vacuum transverse electromagnetic (TEM) waves and the electron Bernstein (B) waves. Information of Nz and Ny is sufficient to identify the waves uniquely both for TEM waves and B waves coupled by mode conversion. Furthermore, the wave differential equation which governs the mode-conversion process can be written in the normalized form with a few numbers of the normalized parameters and variables for the linear density profile. Thus, the mode-conversion transparency window, which is presented as a contour plot of the mode-conversion rate versus the Nz–Ny plane, can be categorized for the pair of parameters of the density scale length normalized to the wavelength in vacuum Ln/λ0 and the frequency to the cyclotron frequency ω/Ω.A survey of the transparency windows for various parameter ranges of Ln/λ0 and ω/Ω is presented. The windows are categorized into four types. The frosted type at the steepest density gradient region has a broad transparency profile but even the peak is not completely transparent. The perpendicular-X type at the next steep density gradient region also has a broad transparency profile with a completely transparent peak by the perpendicularly propagating extraordinary waves. The OXB type at the gentle density gradient region has a pair of completely transparent sharp peaks by the obliquely propagating ordinary waves at the optimal propagation angles with Nz = ±N∥opt and Ny = 0. The fourth is the g1 type in the intermediate density gradient region between the above two cases, which has two completely transparent peaks in the window. Finally, a simulation to examine the applicability of the survey to experiments is made using a test density profile, which elucidates key points for the application of the survey.

Polarizers with non-rectangular grooves for high power millimeter waves

Abstract Polarizers with non-rectangular grooves are studied in high power millimeter wave transmission lines for electron cyclotron heating (ECH) and electron cyclotron current drive (ECCD) of fusion plasmas. The groove shape is important for determining the polarization parameters and avoiding arc breakdown in the system. A low-power measurement has been carried out for several polarizers with different groove depths. The polarization characteristics experimentally measured are in good agreement with numerical results in which the actual groove shape is taken into account. The polarizers are designed and applied to different frequencies of ECH/ECCD systems. Favorable results have been obtained in high-power transmission up to 500 kW, 0.2 s.

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

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.

Non-Inductive Start up of QUEST Plasma by RF Power

Both start-up and sustainment of plasma were successfully achieved by fully non-inductive current drive using microwave with a frequency of 8.2 GHz. Plasmas current of 15 kA was implemented for 1 s. Magnetic surface reconstruction exhibited a plasma shape with an aspect ratio of below 1.5. The plasma current was dependent significantly on the launched microwave power and vertical magnetic field, while not affected by the mode of launched wave and the toroidal refractive index. Hard X-ray (HXR) emitted from energetic electrons accelerated by the microwave was observed, and the discharge with a plasma current over 4 kA followed the same trend as the number of photons of 10 keV to 12 keV. This suggests that the plasma current may be driven by energetic electrons. Based on the experimental conditions, alternative explanations of how the plasma current could be driven are discussed.

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.