K. Nagasaki

Stabilization of energetic-ion-driven MHD modes by ECCD in Heliotron J

Second harmonic electron cyclotron current drive (ECCD) has been applied in the stellarator/heliotron (S/H) device, Heliotron J, to stabilize magnetohydrodynamic (MHD) modes. The energetic particle mode (EPM) of 60–90xa0kHz frequency, one of the energetic-ion-driven MHD modes, is excited in a plasma heated by co- and counter-neutral beam injection and electron cyclotron heating (ECH). The EPM has been stabilized by counter-ECCD which decreases the rotational transform. Localized EC current driven by a few kA at the central region modifies the rotational transform profile, ι/2π, leading to the formation of a high magnetic shear at the radius where the mode is excited. An experiment scanning the EC-driven current shows that there is a threshold in magnetic shear and/or rotational transform to stabilize the EPM.

First measurement of time evolution of electron temperature profiles with Nd:YAG Thomson scattering system on Heliotron J.

A Nd:YAG Thomson scattering system has been developed for Heliotron J. The system consists of two 550 mJ 50 Hz lasers, large collection optics, and 25 radial channel (∼1 cm spatial resolution) interference polychromators. This measurement system achieves a S/N ratio of ∼50 for low-density plasma (ne ∼ 0.5 × 10(19) m(-3)). A time evolution of electron temperature profiles was measured with this system for a high-intensity gas-puff (HIGP) fueling neutral-beam-injection plasma. The peripheral temperature of the higher-density phase after HIGP recovers to the low-density pre-HIGP level, suggesting that improving particle transport in the HIGP plasma may be possible.

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

A fast video camera is verified to be a powerful tool 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.

Neoclassical parallel flow calculation in the presence of external parallel momentum sources in Heliotron J

A moment approach to calculate neoclassical transport in non-axisymmetric torus plasmas composed of multiple ion species is extended to include the external parallel momentum sources due to unbalanced tangential neutral beam injections (NBIs). The momentum sources that are included in the parallel momentum balance are calculated from the collision operators of background particles with fast ions. This method is applied for the clarification of the physical mechanism of the neoclassical parallel ion flows and the multi-ion species effect on them in Heliotron J NBI plasmas. It is found that parallel ion flow can be determined by the balance between the parallelviscosity and the external momentum source in the region where the external source is much larger than the thermodynamic force driven source in the collisional plasmas. This is because the friction between C6+ and D+ prevents a large difference between C6+ and D+flowvelocities in such plasmas. The C6+flowvelocities, which are measured by the charge exchange recombination spectroscopy system, are numerically evaluated with this method. It is shown that the experimentally measured C6+ impurity flowvelocities do not contradict clearly with the neoclassical estimations, and the dependence of parallelflowvelocities on the magnetic field ripples is consistent in both results.

Second harmonic ECRH breakdown experiments in the TJ-II stellarator

A summary of the second harmonic ECRH breakdown experiments performed in the TJ-II stellarator is given here. The goal of these experiments is to determine the dependence of second harmonic X-mode plasma breakdown efficiency on several relevant parameters as the EC power, wave polarization, beam injection angle, tokamak-like induced electric field, neutral gas pre-fill pressure and discharge creation time with respect to the ramp-up of the currents in the coils. For each different scenario, the breakdown properties are determined, as well as the initial pressure conditions and the level of initial radiation before the injection of the ECRH power, which plays an essential role in the breakdown process.

Interpretation of Plasma Fluctuation Data from Combination Measurement of a Perpendicular-View Camera and a Langmuir Probe in Heliotron J

Abstract In the medium-sized heliotron device Heliotron J, edge density fluctuation has been measured simultaneously with a high-speed video camera and a Langmuir probe. Poloidally propagating, parallel elongating filamentary structures with 20- to 30-kHz frequency and ~14-cm poloidal wavelength were observed by a camera. However, the radial position of this density mode is not well known with only camera data because the camera lens axis is perpendicular to the torus plane. To identify the span of this density mode, plasma-surface interaction (PSI) between the probe and the plasma has been analyzed. As the probe scanned into the plasma, enhanced brightness due to PSI was clearly observed in camera images. By comparing this enhanced brightness among different probe positions, the outmost margin of the 20- to 30-kHz mode observed by the camera has been identified to be within 10 mm outside from the last closed flux surface. This conclusion is supported by the spectrum of the probe data.

Effect of magnetic field configuration on parallel plasma flow during neutral beam injection in Heliotron J

The effect of magnetic field configurations on plasma flow velocity was investigated by measuring the parallel flow velocity (v∥) using a charge-exchange recombination spectroscopy during neutral beam injection in three different toroidal mirror configurations of Heliotron J: high, standard and reversed mirror configurations. The magnetic ripple strengths, γ, for these mirror configurations were γ = 0.073 m−1, 0.031 m−1 and 0.027 m−1, respectively, at the normalized averaged minor radius ρ = 0.07. The magnetic ripple strength is defined as γ = {〈(∂B/∂l)2/B2〉}1/2, where 〈...〉 is the flux surface averaged value and l is the length along the magnetic field line. At ρ = 0.07, the parallel flow velocity in the high mirror configuration (v∥ ~ 4 km s−1) was 2–3 times smaller than those in the standard and reversed mirror configurations (v∥ ~ 10–12 km s−1). An anticipated interpretation is that the difference in the neoclassical damping force contributes to the difference in v∥ among the three mirror configurations.