K. Kawahata

Partial and major disruptions in the JIPP T-II tokamak

High-density disruptions and low-q disruptions have been investigated by soft-X-ray imaging on the JIPP T-II tokamak. Although an m = 3 / n = 2 mode is identified together with a 2/1 mode before partialdisruptions in high-density discharges, the resulting disturbance is limited and the central region is not affected. The growth of the 3/2 mode detected in low-q discharges leads to a rearrangement of the soft-X-ray profile by a 1/1 mode. The essential feature of major disruptions is the abrupt central thermal quench. Before major disruptions the predominant 2/1 mode is modulated by a central 1/1 component. The 1/1 mode is found to play an important role in the final stage of major disruptions, both in high-density discharges and in low-q discharges.

Observation of lower-hybrid-current drive in the JIPP T-II torus

Current driven by injecting lower hybrid waves has been observed in low-density plasmas in the JIPP T-II. It is confirmed that RF-driven current is generated by momentum transfer from lower hybrid waves to suprathermal electrons with an energy of 8–25 keV. The driving efficiencies in tokamak and stellarator configurations are 1 kA·kW−1 and 0.3 kA·kW−1, respectively, at a power level of 40 kW. Enhanced electron cyclotron emission due to pitch-angle scattering of RF-driven suprathermal electrons is observed, and spikes in loop voltage and X-ray bursts appear coincidentally. In a long RF-pulse, these rapid changes advance to relaxation oscillations. It is concluded that the pulsating changes originate in the instantaneous scattering of RF-driven suprathermal electrons by the unstable waves excited at an anomalous Doppler resonance.

MHD instabilities and their effects on plasma confinement in Large Helical Device plasmas

Characteristics of MHD instabilities and their impacts on plasma confinement are studied in current free plasmas of the Large Helical Device. Spontaneous L?H transition is often observed in high beta plasmas close to 2% at low toroidal fields (Bt ? 0.75?T). The stored energy starts to rise rapidly just after the transition accompanying the clear rise in the electron density but quickly saturates due to the growth of the m = 2/n = 3 mode (m and n: poloidal and toroidal mode numbers), the rational surface of which is located in the edge barrier region, and edge localized mode (ELM) like activities having fairly small amplitude but high repetition frequency. Even in low beta plasmas without L?H transitions, ELM-like activities are sometimes induced in high performance plasmas with a steep edge pressure gradient and transiently reduce the stored energy up to 10%. Energetic ion driven MHD modes such as Alfv?n eigenmodes (AEs) are studied in a very wide range of characteristic parameters (the averaged beta of energetic ions, ?b?, and the ratio of energetic ion velocity to the Alfv?n velocity, Vb?/VA), of which range includes all tokamak data. In addition to the observation of toroidicity induced AEs (TAEs), coherent magnetic fluctuations of helicity induced AEs (HAEs) have been detected for the first time in NBI heated plasmas. The transition of a core-localized TAE to a global AE (GAE) is also observed in a discharge with temporal evolution of the rotational transform profile, having a similarity to the phenomenon observed in a reversed shear tokamak. At low magnetic fields, bursting TAEs transiently induce a significant loss of energetic ions, up to 40% of injected beams, but on the other hand play an important role in triggering the formation of transport barriers in the core and edge regions.

Soft X-Ray Tomography of the m =2 Magnetic Island Structure in the JIPP T-II Tokamak

Soft X-ray Tomography has been developed in order to obtain a 2-dimensional view of a tokamak plasma cross-section. The image reconstruction of the m=2 magnetic island structure has been successfully demonstrated by the use of this technique. The higher Fourier components deform the m=2 magnetic islands. It is confirmed that the poloidal magnetic fluctuations called Mirnov oscillations are caused by the rotation of the magnetic islands.

Current density profile control by programming of gas puffing and plasma current waveform in the JIPP T-II tokamak

In the resistive-shell tokamak, JIPP T-II , a control of the current density profile has been attempted by programming both gas puffing and plasma current waveform. A stable high-density plasma has been obtained with the following parameters: the maximum line-average electron density is e = 8.5 × 1013cm−3, the minimum q(a)-value is 2.2, and the relative amplitude of the m/n = 2/l mode is suppressed to an extent less than 10−3. A derivation of the current density profile by solving the magnetic-diffusion equation on the basis of the experimental data shows that the current density profile favourable to the stability of low-m kink and tearing modes is realized by combining the effects of cooling through an increase in density and of heating by a current rise in the outer plasma region. The results of kink and tearing modes analysis agree well with the experimental observations. The criterion that the current density profile is successfully controlled by this method is derived as a function of the ratio of plasma current to electron density in the current-rise phase, i.e. 20 × 10−13 Ip/e 30 × 10−13 kA·cm3. The major disruption due to the density increase is completely suppressed by the method proposed in this paper. The major disruption due to a reduction of q(a) to less than 2.2 has, however, not yet been suppressed. In future, the current density profile should be maintained more precisely at its optimum shape by using a feedback-control technique and a control of the plasma boundary with titanium gettering, etc.

Resonant helical divertor experiments in ohmic and auxiliary heated JIPP T-IIU plasmas

Abstract A series of initial resonant helical divertor (RHD) experiments have been carried out in ohmically and auxiliary heated JIPP T-IIU plasmas. Disruptive and MHD instabilities make the interpretation of the RHD results difficult but an apparent increase in the energy confinement time is observed when the helical magnetic perturbation is applied. This may be due to the suppression of MHD activity or to a reduction in the edge convective heat losses. Magnetic island effects have been observed on the floating potential of a Langmuir probe array and energy scrape-off layer widths have been measured with and without helical perturbations during ICRF operation. Basic pump limiter data is presented including ion temperatures and C4+ impurity profiles. Energy confinement times are reported in ohmically and NBI heated discharges.

Experimental studies of energetic-ion-driven MHD instabilities in Large Helical Device plasmas

Conditions for the excitation of Alfven eigenmodes (AEs) by energetic ions are investigated in neutral-beam-injection (NBI) heated plasmas of the Large Helical Device (LHD). This study is carried out in a wide parameter range of the beta values of the energetic ion components and the ratio of the energetic ion velocity to the Alfven velocity (up to with the assumption of classical slowing down and ). These ranges of parameters cover those predicted for the International Thermonuclear Experimental Reactor (ITER). During this experimental campaign of LHD, toroidicity-induced AEs (TAEs) with n = 1–5 (n being the toroidal mode number), global AEs (GAEs) with n = 0 and 1, and energetic particle modes (EPMs) were observed. The effect of the magnetic configuration on the TAE spectrum was also investigated. In magnetic configurations with relatively high magnetic shear, only TAEs with n = 1 and 2 were observed. On the other hand, TAEs with n up to 5 were observed in magnetic configurations with low magnetic shear. For two typical shots obtained in magnetic configurations characterized by different values of the magnetic shear, eigenfunctions of TAEs were calculated by using a global mode analysis code CAS3D3. The calculated results indicate that the eigenfunctions tend to be localized around the relevant TAE gaps. When the gap is located in the plasma core region (normalized minor radius ρ ≤ 0.4), the TAE tends to become a core-localized type. When the gap is in the outer region (typically 0.5 ≤ ρ ≤ 0.9) of the plasma, the TAE tends to (a) either become a global type having a radially extended structure if the magnetic shear is very weak in the core region inside the gap, (b) or become a gap localized type in the case of finite central magnetic shear. Transition of the eigenmode from the core-localized type with m ~2/n = 1 TAEs (m being the poloidal mode number) to the n = 1 GAEs (or cylindrical AEs) has been observed when the rotational transform at the core ι (0)/2π exceeds the specific value of ι(0)/2π = 0.4.

Soft X-ray tomography in the JIPP T-II stellarator

Tomography based on poloidal harmonic expansion is modified to be applied to an elliptical source in order to investigate the internal mode structures due to tearing modes in the l = 2 stellarator configuration. This method was applied to the soft X-ray signals of the JIPP T-II stellarator and the image of the m = 2 magnetic island of the plasma was obtained.

ECR discharge cleaning experiment in the JIPP T-II

The first experiment of electron cyclotron resonance discharge cleaning (ECR-DC) on a tokamak device was carried out by using the JFT-2 (JAERI) [I]. In the experiment, the cleaning effect was estimated from comparison of QMA (quadrupole mass analyzer) signals of H,O, CH, and CO with those of Taylor-DC, but the influence of ECR-DC on the tokamak discharge plasma was not investigated. In the present work, we focus our attention on the decrease of light impurities (especially oxygen) in tokamak discharge plasma by virtue of ECRDC. To characterize the tokamak plasma property, the mean plasma resistivity, the intensity of an 0 II line and the mean electron density without gas puffing were observed. These quantities are expected to decrease with the decrease of impurities.

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%.