T. Baba

Recent Heliotron E physics study activities and engineering developments

Recent studies of transport, magnetohydrodynamic stability, and divertor action on Heliotron E are summarized. A pellet injector and a new diagnostic system are developed. Moreover, the Heliotron groups is conducting research and development on heating and other new systems for the Large Helical Device.

Studies of currentless, high-beta plasma in the Heliotron E device

A currentless plasma with a volume-averaged beta value of 2% has been produced with neutral beam heating. Target plasmas were created by second harmonic resonance heating with electron cyclotron waves (150–350 kW and 53.2 GHz) at a magnetic field strength of 0.94 T. Neutral beam injection (23–30 keV and 1.3−2.6 MW) was used to heat the plasma further. MHD stable and unstable high-beta plasmas were observed. The Q-mode plasmas were produced with the help of intense neutral gas puffing. Properties of the MHD activity and confinement of high-beta plasmas are discussed and compared with theoretical studies.

Pellet injection experiment on NBI current-free plasmas in Heliotron E

The first pellet injection experiments on NBI-heated current-free plasmas in Heliotron E have been carried out. The pellet and plasma species are hydrogen. The increment of line-averaged density is up to 8.2 × 1019m−3, which is consistent with particle numbers of 1020 contained in a pellet used in the experiment. The behaviour of the plasma is studied in two cases of magnetic field strength, B = 1.9 T and B = 0.94 T. The density after pellet injection decays slowly (50–150 ms) in most cases, but in some cases with B = 0.94 T it decays rapidly (< 10 ms). This may be related to power balance.

Optimum confinement of ECH plasmas in Heliotron E

The optimum confinement of plasmas produced by electron cyclotron heating (ECH) in Heliotron E and some other significant transport characteristics of ECH plasmas have been studied experimentally by changing the magnetic field configuration. It is found that the plasma confinement can be improved by minor changes of the magnetic field configuration. The main results are as follows: (1) Plasmas with a minor radius reduced (by means of a limiter) to two thirds of the value in the original (non-limited) configuration have optimum confinement properties when the magnetic axis is shifted inward by 20 mm. (2) The electron pressure in an optimized plasma configuration (confirmed by neutral beam injection) is higher than that in a standard ECH plasma. (3) The insertion of a carbon limiter causes the plasma line density to increase, with no significant degradation of the core temperature or density. (4) Magnetic field perturbations hardly affect the temperature and density profiles, unless the magnetic axis is shifted outward by a large (50 mm) distance. (5) Transport analysis indicates that significant improvement in the confinement properties can be achieved by slightly manipulating the magnetic field configuration.

Effects of NBI power modulation on pellet injection into a current free plasma in Heliotron E

Hydrogen pellets are injected into the current free neutral beam injection (NBI) heated plasma in Heliotron E to effectively increase the plasma density and the internal energy. The chord averaged density of the target plasma is about 2 × 1019 m−3, and the density increase by pellet injection is (4 – 6) × 10l9 m−3. During this process the plasma remains stable. Under the present operational conditions, the pellet reaches the central axis: the penetration depth ranges from 27 cm to 35 cm (the magnetic axis corresponds to 30 cm). To optimize the plasma parameters obtainable by pellet injection, the NBI heating power is modulated. It is found that the mode of operation in which pellets are injected at relatively low NBI power followed by a higher-NBI-power phase is preferable to that in which pellets are injected during a constant high power phase. The main reason for this difference is the penetration depth of the pellet in the plasma which determines the deposition profile of particles in the plasma.

Ablation model including the particle energy distribution function and pellet ablation by hot ions in Heliotron E

The authors present a neutral gas shielding model including the distribution function of particles carrying the heat for ablation. In an ohmically heated tokamak, high energy electrons in the tail part of the Maxwellian with E approximately=6-8 Te contribute significantly to the ablation process at the pellet surface. A simple scaling for the pellet ablation rate is given which takes into account the Maxwellian electron energy distribution. The pellet penetration depth obtained with the model presented agrees well with the multigroup approximation for an energy distribution during ablation by Houlberg et al. (1988). Pellet ablation dominated by hot ions was obtained in a special NBI heated Heliotron E plasma with an electron temperature much lower than the ion temperature

Study on edge plasma physics and particle control in the Heliotron-E device

The edge plasma physics and the particle control under the intrinsic magnetic limiter configuration of a helical system have been studied with the Heliotron-E device, where currentless plasmas of Te ≤ 1–2 keV, Ti ≤ 1 keV and ne ≤ 2 × 1020/m3 are produced by a combination of ECRH, NBI and/or ICRH. It is indicated that the separatrix region of the heliotron device is able to act as a divertor magnetic field. According to calculations of the magnetic field line in the edge region, the separatrix region has some different characteristics from the scrape-off layer in tokamak devices; the existence of a fine structure in the separatrix region and asymmetry of the region in toroidal and poloidal directions are observed. A localized pattern of the heat load on the first wall is experimentally observed. This agrees with the heat-load profile expected from the magnetic configuration and the distribution of the plasma in the edge region. A carbonization of the first wall is successfully applied to the Heliotron E device for reduction of metallic impurity contents. The heat load at the divertor trace decreased and that on the other part of the first wall increased in the high recycling conditions after the carbonization.

Effects of carbon wall on the behavior of Heliotron-E plasmas

Abstract Carbonization was successfully applied to Heliotron-E. Iron-inpurity radiations were strongly reduced with the carbonized wall. Main impacts of the metal reduction on plasma behaviors are sustainment of stored energy during high power, long pulse heating by NBI, achievement of a quasi-steady discharge with a low helical field and high beta, and highest electron density with pellet injection in a quasi-steady state. Hydrogen recycling was very high with the carbonized wall and low density operation was impossible. Helium glow discharge was found to be effective to control the hydrogen recycling with a carbon-tiled wall.

PELLET INJECTION EXPERIMENTS ON HELIOTRON E AND DEVELOPMENTS OF HIGH SPEED PELLET INJECTOR

Deuterium/hydrogen pellet injection up to 6 pellets in one discharge (in the pellet velocity range up to 1.4 km/s) is used for investigating transport and MHD properties of currentless Heliotron E plasmas as well as for studying pellet ablation mechanisms in NBI and ECH plasmas. In addition to pellet injection experiments, a two stage pneumatic pellet injector for refuelling plasmas was constructed and tested in order to increase pellet velocity to the range of 3 km/s.

Development of hydrogen pellet acceleration system using a two-stage light gas gun

Hydrogen pellets without sabots were accelerated to high speeds using a two-stage light gas gun for Heliotron-E. The primary application of this technology is plasma fueling of fusion devices. Conventional pellet injectors have limited pellet speeds in the range of 1-2 km/s. Higher velocities are desirable for more flexible density profile control and for deep penetration into a high-temperature plasma. By developing a new fast valve with high conductance to accelerate the piston and other components, a 2-mm-diam hydrogen pellet with a velocity of 3.2 km/s has been successfully accelerated without a sabot. The pipe gun technique for freezing hydrogen operation is simulated with the code MYKE. Development of new pistons instead of the plastic piston for the repetitive two-stage pellet injector is being carried out using metal and ceramic, as the surface of the plastic piston tends to wear out through repetitive operation and carbon powder produced from the plastic piston may cause trouble with the fusion device.<<ETX>>