T. Sasajima

Performance of B4C-converted carbon fiber composites in high power neutral beam heated divertor discharges in JT-60U

B 4 C conversion-coated carbon fiber composites (B 4 C-converted CFC) have been installed on the outboard strike point of the divertor plate in JT-60U to form a complete toroidal ring. The plasma-surface interactions of the B 4 C-converted CFC have been investigated in high power divertor operation with neutral beam (NB) heating power of - 30 MW for 2 s. It is found that the carbon impurity is reduced without a significant increase of the boron impurity using a 100 μm thick B 4 C layer with a B /C ratio of 2.7. The oxygen impurity is reduced significantly by the evaporation of a 300 μm thick B 4 C layer with a B /C ratio of 3.8. A reduction in the detrimental effects on plasma-surface interactions with carbon-based materials has been demonstrated for more than 1000 shots by the use of surface-boronized carbon tiles in JT-60U

Investigation of plasma facing components in JT-60U operation

Abstract The mechanical fracture of three carbon fiber composite (CFC) first wall tiles was observed. This damage was probably caused by the electromagnetic force due to halo current during disruption. The required current to break the CFC tile is estimated to be 25 kA. The broken tile was rotated poloidally around the plasma with a speed of about 10 m/s during the following discharge. A possible driving force of this rotation might be the electromagnetic force due to the scrape-off layer (SOL) current. The required current to rotate the piece of the broken tile is 1 kA. These results indicate that electromagnetic interaction between SOL plasma and the plasma facing component is important in the research on the plasma wall interactions in fusion devices.

Development of a compact W-shaped pumped divertor in JT-60U

In JT-60U, the modification to a W-shaped pumped divertor will be completed in May 1997, aiming to realize sufficient reduction in heat flux to the targets and good H-mode confinement simultaneously. W-shaped geometry is optimized not only for forming radiative divertor plasmas and reducing the back flow of neutral particles but also for allowing various experimental configurations. Toroidally and poloidally segmented divertor plates, dome and baffles are arranged in a W-shaped poloidal configuration. The pumping speed can be changed during a shot by variable shutter valves in the three pumping ports under the outer baffle. The net throughput is enough for particle control in the steady radiative operations with high power NBI heating. Carbon fiber composite (CFC) tiles are used for the divertor targets and the divertor throat where large heat flux is expected. Gaps between two adjacent segments are carefully sealed to suppress the leak of neutral gas from the exhaust duct below the divertor and baffles. The strength of the whole structure is confirmed by an electromagnetic force analysis and structural analysis carried out for disruptions of 3 MA discharges with a halo current.

Design of a compact W-shaped pumped divertor in JT-60U

In JT-60U, a compact W-shaped pumped divertor, which is optimized for realizing a radiative divertor with high confinement performance in various experiments, will be implemented in February-May, 1997. Pumping and fueling systems are arranged to provide a sufficient capability of particle control. Segmented baffle structure with insulated flexible gas seal is adopted to satisfy experimental and structural requirements.

Material behavior of JT-60U plasma facing components and installation of B/sub 4/C-converted CFC/graphite tiles

Wall damages of the JT-60U plasma facing components in the recent operation are more serious than in the previous one. Erosion of carbon fiber composite (CFC) divertor tiles due to field ripple of the toroidal coil is observed. Several bolt-loosening divertor tiles are eroded or disconnected. Mechanical fracture of CFC first wall tiles is found. The first application of B/sub 4/C-converted CFC tiles (surface-boronized ones using conversion method) is also shown. The B/sub 4/C-converted tiles have been installed to the outboard striking zone of the divertor plate and in use satisfactorily in the high power heating divertor operation.

Effects of plasma behavior on in-vessel components in JT-60 operation

Damage of in-vessel component tiles such as divertor and first wall tiles have been investigated from the view point of the effects of plasma behavior under the JT-60 operation.

JT-60U W-shaped divertor with in-out divertor pumping slots

In JT-60U, an outer divertor pumping slot was added to the W-shaped divertor with an inner pumping slot during the maintenance period, November to December 1998, in order to improve plasma performance especially in a detached state with no or inverse in-out particle flux asymmetry. This modification was already planned at the beginning of W-shaped divertor design activity. Hence, the previous W-shaped divertor had adaptable structure to the in-out pumping slots. The modified components, especially armor tiles, may be exposed to high heat load and high electromagnetic load in an extreme plasma configuration. Thermal analyses showed that peak surface temperatures exceeded 2000/spl deg/C. The maximum induced thermal stress, however, was below an acceptable level. Furthermore, each armor tile size and position of the tile attachment were optimized to avoid fracture due to halo current. Therefore, high power operations are possible without tile fracture in an extreme condition. With the modified W-shaped divertor, it was observed that the efficiency of He pumping was improved, and that the L-H transition threshold power was reduced. The integrated plasma performance has also been improved. These results indicate efficient divertor pumping of the modified W-shaped divertor.