N. Noda

Behavior of plasma-sprayed tungsten coatings on CFC and graphite under high heat load

Tungsten coatings of 0.5 and 1 mm thickness were successfully deposited by the vacuum plasma spraying technique on carbon/carbon fiber composite (CFC), CX-2002U, and isotropic fine grained graphite, IG-430U. High heat flux experiments by irradiation of electron beam with uniform profile were performed on the coated samples in order to prove the suitability and load limit of such coating materials. Heat load properties, gases emission, surface modification and structure changes of cross-section of the samples were investigated. Cracks on the surface and exfoliation between the joint interface of the samples were not formed below the melting point. These results indicated that the thermal and adhesion properties between the substrate and coatings were good under high heat flux. Microstructure of the joint interface of the sample was changed in the case of a peak temperature at about 2800°C. Many cracks and traces of melted tungsten flow were observed on the surface after melting and solidification. Large cavities were also formed inside the resolidified tungsten layer.

Overview of confinement and MHD stability in the Large Helical Device

The Large Helical Device is a heliotron device with L = 2 and M = 10 continuous helical coils with a major radius of 3.5–4.1 m, a minor radius of 0.6 m and a toroidal field of 0.5–3 T, which is a candidate among toroidal magnetic confinement systems for a steady state thermonuclear fusion reactor. There has been significant progress in extending the plasma operational regime in various plasma parameters by neutral beam injection with a power of 13 MW and electron cyclotron heating (ECH) with a power of 2 MW. The electron and ion temperatures have reached up to 10 keV in the collisionless regime, and the maximum electron density, the volume averaged beta value and stored energy are 2.4 × 1020 m−3, 4.1% and 1.3 MJ, respectively. In the last two years, intensive studies of the magnetohydrodynamics stability providing access to the high beta regime and of healing of the magnetic island in comparison with the neoclassical tearing mode in tokamaks have been conducted. Local island divertor experiments have also been performed to control the edge plasma aimed at confinement improvement. As for transport study, transient transport analysis was executed for a plasma with an internal transport barrier and a magnetic island. The high ion temperature plasma was obtained by adding impurities to the plasma to keep the power deposition to the ions reasonably high even at a very low density. By injecting 72 kW of ECH power, the plasma was sustained for 756 s without serious problems of impurities or recycling.

Steady-state operation and high energy particle production of MeV energy in the Large Helical Device

Achieving steady-state plasma operation at high plasma temperatures is one of the important goals of worldwide magnetic fusion research. High temperatures of approximately 1?2?keV, and steady-state plasma sustainment operations have been reported. Recently the steady-state operation regime was greatly extended in the Large Helical Device (LHD). A high-temperature plasma was created and maintained for 54?min with 1.6?GJ in the 2005FY experimental programme. The three-dimensional heat-deposition profile of the LHD helical divertor was modified, and during long-pulse discharges it effectively dispersed the heat load using a magnetic axis swing technique developed at the LHD. A sweep of only 3?cm in the major radius of the magnetic axis position (less than 1% of the major radius of the LHD) was enough to disperse the divertor heat load. The steady-state plasma was heated and sustained mainly by hydrogen minority ion heating using ion cyclotron range of frequencies and partially by electron cyclotron of fundamental resonance frequency. By accumulating the small flux of charge-exchanged neutral particles during the long-pulse operation, a high energy ion tail which extended up to 1.6?MeV was observed. This is the first experimental evidence of high energetic ion confinement of MeV range in helical devices. The long-pulse operations lasted until a sudden increase in radiation loss occurred, presumably because of metal wall flakes dropping into the plasma. The sustained line-averaged electron density and temperature were approximately 0.8 ? 1019?m?3 and 2?keV, respectively, at a 1.3?GJ discharge (#53776) and 0.4 ? 1019?m?3 and 1?keV at a 1.6?GJ discharge (#66053). The average input power was 680?kW and 490?kW, and the plasma duration was 32?min and 54?min, respectively. These successful long operations show that the heliotron configuration has a high potential as a steady-state fusion reactor.

High heat load properties of tungsten coated carbon materials

Tungsten coatings of 0.5 and 1.0 mm thickness were successfully deposited by the vacuum plasma spraying technique (VPS) on carbon/carbon fiber composite, CX-2002U, and isotropic fine grained graphite, IG-430U. High heat flux experiments were performed on the coated and non-coated samples in order to prove the suitability and load limit of such coating materials. The electron beam irradiation experiments showed that there was little difference in temperature increases among CX-2002U and the coated materials below surface temperature of 2200°C. These results indicated that thermal and adhesion properties between the substrate and coatings were good under high heat flux. A few cracks with a width of 15 μm were formed from the surface to the bottom side of the all coated samples, but plastic deformation and microcracks due to grain growth by recrycrallzation were not observed below a surface temperature of about 2200°C. The cracks are expected to be formed due to local thermal stress produced by spot-like beams.

Material probe analysis for plasma facing surface in the Large Helical Device

Material probes have been installed at the inner walls along the poloidal direction in large helical device (LHD) from the first experimental campaign. After each campaign, the impurity deposition and the gas retention have been examined to study the plasma surface interaction and the degree of wall cleaning. In the 2nd campaign, the entire wall was thoroughly cleaned by glow discharge conditioning and the number of main discharge shots increased. For the 3rd and 4th campaigns, graphite tiles were installed over the entire divertor strike region, and then the wall condition was significantly changed compared with the case of a stainless steel (SS) wall. It was seen that graphite tiles in the divertor were eroded mainly during main discharges, and the SS first wall mainly during glow discharges. During main discharges the eroded carbon was deposited on the entire wall. A reduction of metal impurities in the plasma was observed, which corresponds to the carbonized wall. The deposition thickness was great at the wall far from the plasma. Since the entire wall was carbonized, the amount of discharge gases retained such as H and He became large. In particular, helium retention was large at a position close to the anodes used for helium glow discharge cleanings. One characteristic of the LHD wall is a large retention of helium since the wall temperature is limited to below 368 K. In order to reduce the recycling of the discharge gas, wall heating is necessary.

Changes of composition and microstructure of joint interface of tungsten coated carbon by high heat flux

Abstract Tungsten coatings of 0.5 and 1 mm thickness were successfully deposited by the vacuum plasma spraying (VPS) technique on carbon/carbon fiber composite (CFC), CX-2002U and isotropic fine grained graphite, IG-430U. High heat flux experiments by irradiation of electron beam with uniform profile were performed on the coated samples in order to prove the suitability and load limit of such coating materials. The cross-sectional composition and structure of the interface of VPS–W and carbon material samples were investigated. Compositional analyses showed that the Re/W multi-layer acts as diffusion barrier for carbon and suppresses tungsten carbide formation in the VPS–W layer at high temperature about 1300°C. Microstructure of the joint interface of the sample changed in the case of a peak temperature of about 2800°C. The multi-layer structure completely disappeared and compositional distribution was almost uniform in the interface of the sample after melting and resolidification. The diffusion barrier for carbon is not expected to act in this stage.

Boronization in future devices – protecting layer against tritium and energetic neutrals

A thin boron film is attractive as a deuterium/tritium free wall, and as a protecting layer against impact of energetic charge-exchange neutrals in future fusion devices with long pulse operation. New experimental evidence is given for desorption of hydrogen isotopes from these films at relatively low temperature. Most hydrogen atoms in a boron-coated layer are re-emitted to the plasma side below 400°C without penetration into the substrate of stainless steel. The maintainability of a thin boron layer during a long pulse operation may be a problem. Boron atoms are hardly removed by pumping because their hydrides are easily disintegrated and redeposited. Gross migration of boron atoms inside the vessel is a concern. A condition required for avoiding the migration is discussed.

Microscopic modification of wall surface by glow discharge cleaning and its impact on vacuum properties of LHD

Glow discharge cleaning (GDC) is a widely used technique for wall conditioning in fusion experimental devices. Though the cleaning effects of GDC are essentially related to the microscopic modification of the wall surface, there are few reports about it. In the present study, specimens of wall materials were exposed to GDC plasma of helium, hydrogen and neon in the Large Helical Device (LHD) by using the retractable material probe transfer system and irradiation damage was examined by transmission electron microscopy to understand the underlying microscopic mechanism of GDC. In the case of Ne-GDC, the specimen surface was covered with a thick deposited layer of Fe and Cr but no radiation induced defects were observed. Due to the high sputtering efficiency and very shallow penetration, it is likely that neon atoms effectively sputter the surface contamination without leaving serious damage or remaining in the sub-surface region. After the Ne-GDC phase, retained Ne can be successfully removed by the following short hydrogen GDC. It was shown that a two-step GDC with Ne and H is very effective to clean the metallic surface of the LHD.

High heat flux test of actively cooled tungsten-coated carbon divertor mock-ups

Abstract CX-2002U and IG-43OU coated by VPS-W were developed to be as a light high-Z plasma-facing material. After brazing them on OFHC blocks using a Ti foil, their thermal response and thermal fatigue properties were examined with active cooling. No cracks and no exfoliation occurred on the W surface and the braze interface even after 160 cycles of heat load for 20 s at 10 MW/m2. This result indicates that the Ti-brazing is a possible alternative to Ag-brazing for joining carbon to Cu. Heat load resistance of the VPS-W coated CX-2002U/OFHC was much better than the VPS-W coated IG-430U/OFHC due to the excellent thermal conductivity of CX-2002U. VPS-W coated CFC/OFHC is a potential candidate for a high heat resistance armor material on plasma facing components.

Analysis for surface probes of third experimental campaign in the large helical device

For the third experimental campaign of LHD, graphite tiles were installed in entire region of divertor strike region. Several material probes of SS 316L and graphite were placed along the poloidal direction at #7 toroidal sector. These probes were extracted after the campaign, and impurity deposition on the probe and retention of discharge and impurity gases were analyzed by AES and TDS, respectively. Major impurity species deposited on the probe were C, Fe and O. The iron deposition rate per one main discharge was approximately a half of that in the second campaign. The carbon deposition was significantly large, and the carbon concentration was as high as 90 at.%. These results correspond to the reduction of metal impurity level in the plasma. The retention of discharge gas and impurities doubled compared to the case of the second campaign, due to the carbonized wall. Significant retention of helium was observed, which is one of the characteristics of the LHD wall. The retention of helium is due to charge exchanged He during He main discharge shots and implantation of He ion during He glow discharges.