WT-3 Tokamak Research Articles

Soft X-Ray Computer Tomography of Double $m = 2$ Mode During Off-Axis Sawtooth Oscillation With Five Detector Arrays on the WT-3 Tokamak

An off-axis sawtooth oscillation is excited by electron cyclotron heating in a lower hybrid current drive plasma in the WT-3 tokamak. The internal structure of the plasma is investigated using computer tomography with five detector arrays after the application of singular value decomposition to the soft X-ray detector signals. It is found that the precursor oscillation of the off-axis sawtooth collapse is the m=2 mode. On the other hand, the double m=2 mode structure is observed as a post cursor of the collapse.

Soft X-ray computer tomography of sawtooth collapse associated with m=2 and m=3 modes with five detector arrays on the WT-3 Tokamak

Another kind of sawtooth collapse is observed in a lower hybrid current drive plasma on the WT-3 tokamak. The internal structure of the plasma is investigated using computer tomography with five soft X-ray detector arrays after the application of the singular value decomposition to the soft X-ray detector signals. It is found that the precursor and postcursor oscillations of this collapse are the m=3 mode and the m=2 mode, respectively.

Three-dimensional observation of an helical hot structure during a sawtooth crash in the WT-3 tokamak.

Sawtooth crashes in an Ohmically heated plasma in the WT-3 tokamak have been observed by using soft x-ray computer tomography at three different poloidal cross sections around the torus. Initially, collapsing proceeds slowly with keeping the helical structure of an m = 1/n = 1 hot core around the torus. It accelerates as the helical hot structure is strongly deformed and fades away in the manner that the hot core at the high field side becomes obscure and disappears, while that at the low field side is deformed into a thin crescent aligned along the inversion circle, which survives even at the completion of the crash.

Observation of sawtooth crashes by a multi-toroidally positioned soft x-ray computer tomography system in the WT-3 tokamak

Sawtooth crashes in an Ohmically heated plasma with a low safety factor of qa ≃ 2.7 in the WT-3 tokamak have been observed by using soft x-ray computer tomography (SXCT) systems attached at three different poloidal cross sections around the torus. With this multi-toroidally positioned (MTP) SXCT system we have examined how the core plasmas collapse at the three different toroidal locations in terms of the shift and the shape of hot cores in order to visualize the whole helical hot structure around the torus during the crash. A variety of crashes have been observed. In any case collapsing proceeds in two steps, called the first and second phases. In the first phase collapsing proceeds slowly keeping the helical hot structure of an m = 1/n = 1 mode and/or an m = 2/n = 2 mode around the torus. In the second phase, collapsing proceeds rapidly as the helical hot structure is strongly deformed, and reaches the end in such a manner that the hot core shifted to the low field side is deformed into a thin crescent shape aligned along the inversion circle while that shifted to the high field side fades away or is deformed into a broad, warm structure depending on the types of crashes. Thus, present MTP SXCT has revealed for the first time a remarkable deformation of the helical hot structure around the torus during the sawtooth crashes.

WT-3トカマクにおける電子サイクロトロン加熱によるm=2ティアリングモードの抑制

WT-3トカマクにおける電子サイクロトロン加熱によるm=2ティアリングモードの抑制

Investigation of coupling of magnetohydrodynamic modes by soft x-ray computer tomography on the WT-3 tokamak

The internal structure of the stationary m=1 and m=2 modes in an ohmic heating plasma and the double m=1 mode structure in a lower hybrid current drive plasma are investigated on the WT-3 tokamak [Maehara et al., Nucl. Fusion 38, 39 (1998)] using computer tomography after the application of the singular value decomposition to the soft x-ray signals. The results show that, in both cases, two coexisting modes have the same frequency and have a fixed mutual phase relation, indicating that two modes are coupled and rotate as one body in the toroidal direction. It is found that the mutual inductance of two loops of helical current filaments for producing magnetic islands always takes the maximum at the experimentally observed positions of two-mode structures. This result means not only that the electromagnetic coupling of two current loops is at the maximum, but also that the two loops are in the dynamically stable position.

Soft X-ray computer tomography of locked mode with five detector arrays on the WT-3 tokamak

In the WT-3 tokamak, mode locking of the m=2 mode is artificially induced by the application of lower hybrid current drive in an ohmic heating plasma. The time evolution of the plasma internal structure during the mode locking is investigated by soft X-ray computer tomography with five detector arrays. It is found that the m=1 mode does not rotate as well as the m=2 mode and that the m=1 and m=2 modes grow continuously during the mode locking.

Plasma Polarization Spectroscopy. Plasma Polarization Spectroscopy on Magnetically Confined Plasmas.

Plasma polarization spectroscopy experiments are reviewed. The OV and CV impurity emission lines from the WT-3 tokamak are found to be polarized. In the GAMMA 10 tandem mirror machine, the OV and low-ionization stage Fe and Cr lines are also found to be polarized. This polarization is ascribed to the anisotropic distribution of electron velocities. The PACR (Population-Alignment Collisional- Radiative) model is introduced, which interprets the experimental polarization data in terms of the anisotropic velocity distribution. The present status of development of the model is presented with the CV ion taken as an example.

Doppler-shifted cyclotron absorption of electron Bernstein waves via N( parallel)-upshift in a Tokamak plasma.

Extraordinary (X) waves are perpendicularly injected for electron Bernstein (B) wave heating into an Ohmically heated plasma from the inboard side in the WT-3 tokamak. Measurements show that absorption does not take place at the electron cyclotron resonance layer nor the upper hybrid resonance layer, but does happen midway between them. This is consistent with the ray tracing prediction, i.e., the poloidal field and poloidal inhomogeneity of toroidal field lead the B waves to have a large parallel refractive index N( parallel) (>1), and the B waves are damped away via the Doppler-shifted cyclotron resonance.

On-axis electron cyclotron current drive experiment on WT-3

The control of the current profile near the magnetic axis by electron cyclotron current drive (ECCD) is demonstrated experimentally on the WT-3 tokamak. Compared with the previous experiments, the driven current was increased by injecting the elliptically polarized microwave with the optimal polarization ellipticity. The effect of the improved absorption appears on the sawteeth behavior, soft X-ray emissivity and so on. The difference of the time evolution of the loop voltages between co-CD and counter-CD is also discussed.

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.

On-Axis Current Drive Experiment Using a Quasi-Optical Electron Cyclotron Heating/Current Drive System on the WT-3 Tokamak.

An 89 GHz quasi-optical Electron Cyclotron Heating (ECH)/Current Drive (ECCD) system has been installed on the WT-3 tokamak. In order to investigate the effect of localized ECH/ECCD on the MHD instabilities, this system is designed to focus the micro-wave beam into a small focal spot with the radius about 1cm in the plasma center. This radius is smaller than the inversion radius of the sawtooth oscillation observed in WT-3. Utilizing this feature, the experiment on control of sawtooth oscillation by on-axis current drive have been performed. Stabilization of sawtooth by counter-ECCD is observed when the power deposition is localized inside the inversion radius. This result may be due to the decrease in current density near the magnetic axis. For the optimal coupling of the micro-wave to obliquely propagating waves at the plasma surface, a linear-elliptical polarization converter has been installed recently. This polarization converter may raise the effective current drive efficiency by a factor of 1.5 for the injected micro-wave power.

Electron cyclotron current drive in a lower hybrid current drive plasma

Electron cyclotron (EC) current drive experiments were carried out in lower hybrid current drive (LHCD) plasmas on the WT-3 tokamak. Using oblique injection of EC waves, plasma current is ramped up in a frequency range ω/Ωe ≃ 1.2-1.4, where Ωe is the EC frequency at the plasma centre. This ramp-up is ascribed to selective EC heating of fast electrons in LHCD plasmas via the fundamental EC resonance at the upshifted frequency due to the Doppler effect. Fast electron momentum distributions are estimated from bremsstrahlung hard X ray spectra. These distributions indicate that the parallel momentum input to the fast electrons in the relativistic energy range from oblique EC waves is effective for efficient EC current drive. Current ramp-up is also observed in the second harmonic frequency range ω/ Ωe ≃ 1.9-2.2. In this case, bulk electrons are heated as well as fast electrons and no definite cause of current ramp-up is clarified.

Collisional-radiative model for helium and its application to a tokamak plasma

We have reviewed the cross-section data of electron impact excitation for transitions starting from the ground and metastable states, and for those between different − l levels within the same n for neutral helium. We have taken into account the effect of the helium singlet and triplet wave function mixing caused mainly by the spin-orbit interaction. The dependence of the degree of the mixing on the magnetic field strength was also calculated. We have incorporated these data in our collisional-radiative model code and analysed the population of the 3 3 P and 3 3 D levels measured in the WT-3 tokamak plasma. The ground and 2 3 S metastable state populations thus determined are compared with those given by the neutral transport code.

Polarization of impurity emission lines from a tokamak plasma.

The visible-uv spectrometer was equipped with a double refracting calcite plate for the purpose of resolving a spectral line into the two linearly polarized components and observing these components simultaneously. Relative intensities of these components have been measured for heliumlike carbon and berylliumlike oxygen impurity lines from the WT-3 tokamak plasma at Kyoto University. In the joule heating mode, for transition from the joule heating to the lower-hybrid current drive mode, and for application of the electron cyclotron heating, changes in the relative intensities were found, which indicated that these lines were polarized. Implications of the polarization of the emission lines from plasma are discussed.

Fast electron momentum distributions in lower hybrid and electron cyclotron current drive plasmas

We investigate fast electron momentum distributions in lower hybrid (LH) and electron cyclotron (EC) current drive (CD) plasmas in the WT-3 tokamak by a new method for the reconstruction of momentum distribution from bremsstrahlung hard X-ray spectra measured at various viewing angles to the toroidal field. The results show that fast electrons in an LHCD plasma are further accelerated by an additional ECCD beyond the energy cut determined by the LH wave accessibility condition.

Transport Coefficient and Heat Pulse Propagation

The problem of deducing {Chi}{sub e} from heat pulse propagation measurements is addressed. It is indicated that diffusive models can not explain the experimental observations on WT-3 tokamak. The equation taking account of the convective term gives a good fit to experimental results. It may indicate that for the sawtooth free plasma, there exists a convection of heat pulse. 9 refs., 4 figs., 1 tab.

Electron cyclotron heating combined with lower hybrid current drive on WT-3

Lower hybrid current drive (LHCD) is applied to improve the confinement in the WT-3 tokamak. With increasing the lower hybrid (LH) power, a sharp decrease in electron thermal diffusivity χ e is observed at a certain LF power. The χ e is directly measured by applying the simple time-to-peak method to propagation of a heat pulse induced by a short electron cyclotron heating (ECH) pulse. A high electron temperature is obtained by applying ECH to the LHCD plasma with improved χ e. The efficiency of ECH in the LHCD plasma with improved χ e becomes three times that in ohmic heating plasmas.

Control of magnetohydrodynamic activities by electron cyclotron heating in WT-3

Abstract Control of the m = 2, n = 1, magnetohydrodynamic mode in an ohmically heated plasma by localized electron cyclotron heating is studied systematically in the WT-3 tokamak. It is found that both m = 2 and m = 2 modes can be simultaneously suppressed by electron cyclotron heating near the q = 1 surface as well as the q = 2 surface, where q is the safety factor.

Heat Flow during Sawtooth Collapse in Tokamak Plasmas

Heat flow during sawtooth collapse was studied on the WT-3 tokamak by using temporal evolution of soft X-ray intensity profile in the poloidal cross section in a lower hybrid current driven plasma as well as an electron cyclotron heated plasma. Two phases in sawtooth collapses were observed. In the first phase, the hottest spot that is the peak of the soft X-ray distribution approaches the inversion surface and heat flows out through a narrow gate on the inversion surface. In the second phase, the hottest spot stays on the inversion surface, and heat flows out through the whole inversion surface. This suggests that magnetic reconnection as predicted by Kadomtsev's model occurs in the first phase, but in the second phase, a different mechanism dominates heat flow.