Plasma Confinement Characteristics in Heliotron J——Spontaneous Change of Plasma Confinement State

Spontaneous transition of the plasma confinement mode was observed in thehelical-axis heliotron device 'Heliotron J' for three different plasma heating schemes, i.e.ECH-only, NBI-only and the combination of ECH and NBI. The transition seems to occur above a certaincritical density. In addition to the confinement transition, a spontaneous shift of the hittingposition of the divertor plasma flux on the wall was observed. This shift could be related with thechange of the edge field topology caused by non-inductive toroidal currents.

Observation of Edge Plasma Fluctuations with a Fast Camera in Heliotron J

A fast video camera is verified to be a powerful t0ol for observation of filaments/blobs near the last closed flux surface （LCFS）. In order to extract the fluctuation component from the raw data of the fast camera, a pre-processing technique, sliding time window averaging subtraction （STWAS） has been developed to remove the background of slowly varying emission from the bulk plasma. By using this pre-processing technique, the fast camera data are analyzed. A method to identify the filaments in the pre-processed image is also discussed.

Recent Progress in Plasma Control Studies on the Improvement of Plasma Performance in Heliotron J

Recent progress in plasma control studies on the improvement of plasma performance in Heliotron J is reviewed. The supersonic molecular beam injection （SMBI） fueling is successfully applied to Heliotron J plasma. A supersonic H2-beam is effectively injected to increase fueling efficiency and generate a peaked density profile. Local fueling with a short-pulsed SMBI can increase the core plasma density and avoid the degradation arising from edge cooling. Second harmonic electron cyclotron current drive （ECCD） experiments were conducted by launching a focused Gaussian beam with a parallel refractive index of -0.05 ≤ Nil 〈 0.6. Results show that the electron cyclotron （EC） driven current is determined not only by Nil but also by local magnetic field （B） structure where the EC power is deposited. Detailed analysis of the observed NI and B dependences is in progress with a ray-tracing simulation using the TRAVIS code. Fast ion velocity distribution was investigated using fast protons generated by ion cyclotron resonant frequency （ICRF） minority heating. For the standard configuration in Heliotron J, charge ex- change neutral particle analysis （CX-NPA） measurements show higher effective temperature of fast minority protons in the on-axis resonance case compared to that in the HFS （high field side） off-axis resonance case. However, the increase in bulk ion temperature in the HFS resonance case is larger than that in the on-axis resonance.

Assessment of Flow Drive by Use of Ion Bernstein Wave on Heliotron J and EAST Devices

The possibility of driving poloidal flows by use of ion Bernstein wave is assessed for Heliotron J and EAST devices by means of ray tracing analysis. Sheared poloidal flow is expected to suppress plasma turbulences due to the decorrelation of the waves. In Heliotron J and EAST plasma, the rays of Ion Bernstein Wave travel into the central region with oscillations along the magnetic lines of force and their power is absorbed by ions at the cyclotron resonance layers. The momentum input has been estimated by calculating the momentum change of rays and the poloidal flow has been estimated using neoclassical viscosities. The wave momentum changes its sign as it propagates inward, depositing sheared momentum to the plasma, and therefore causes sheared poloidal flows.

Soft x-ray tomographic reconstruction of Heliotron J plasma for the study of magnetohydrodynamic equilibrium and stability

The preparation for an experimental soft x-ray tomography study on the Heliotron J (H-J) machine is carried out,with the objectives of evaluating the capability of the current soft x-ray tomographic system in terms of the identification of different mode structures and their poloidal rotation,and the axis shift with different plasma and machine parameters,and fixing the physics goals for the experimental study.These preparations were carried out via a simulated soft x-ray data set arising from different plasma conditions,such as magnetic islands,low beta and high beta.Soft x-ray tomography (SXT) is performed by the discrete pixel method including singular value decomposition and Phillips-Tikhonov regularization,to obtain clear and smooth images.The H-J soft x-ray tomography results from simulated soft x-rays for the equilibrium H-J plasma sensed the magnetic axis shift clearly and an estimate was also achieved.Successful reconstruction for mode structure m =1/n =1 was obtained along with the realization of the poloidal rotation of the structure.The reconstruction for the m =2/n =1 mode was not very clear for the current soft x-ray diagnostic design.Effective mode identification was not possible due to the lack of measurements.The SXT from the current soft x-ray diagnostic on H-J,the magnetic axis shift can be estimated and the m =1/n =1 mode can be studied.Study of higher poloidal modes is difficult with the current design.

Effects of Sheared Flow on Microinstabilities and Transport in Plasmas

Theoretical and experimental studies associated with electric field effectson the stability and transport are briefly surveyed. The effects of radial electric field on thesuppression and/or enhancement of various microinstabilities such as drift waves, flute mode andtemperature gradient modes are discussed. The suppression of flow shear on the electron temperaturegradient mode in plasmas with slightly hollow density profiles is investigated by solving thegyrokinetic integral eigenvalue equation. Comparison between theoretical predictions andexperimental observations based on the HIBP measurements with high temporal and spatial resolutionsis made in bumpy tori and heliotron (CHS) devices.

New tomographic reconstruction technique based on Laplacian eigenfunction

This letter proposes a new tomographic reconstruction procedure based on the Laplacian eigenfunction(LEF) patterns, which are independent of the plasma cross-section and do not require the flux surface information. The process is benchmarked for the experimental data of Heliotron J plasma and the results are compared with the least-squares approximation by a Phillips–Tikhonov(PT)-type regularization, which is widely used as the standard technique for tomographic reconstruction. The reconstruction based on the LEF is found to be capable of determining the magnetic axis at different time locations efficiently in comparison with the PT-type regularization.