Speaker
Description
In the Large Helical Device (LHD), boron powder injection experiments have been conducted using an impurity powder dropper (IPD), developed by PPPL, for real-time wall conditioning. A pronounced effect of boron powder injection has been observed on the line emission intensity of iron, the main constituent element of the stainless steel type 316L first wall material in LHD. The emission intensity of highly charged iron ions (Fe XXIV) radiating in the main plasma is significantly reduced by boron injection, in some cases to less than one-tenth of its original value. In contrast, the emission intensity of low-charge iron ions (Fe VIII, Fe IX) radiating in the scrape-off layer (SOL) shows little or no change, suggesting that the generation of iron impurities remains largely unaffected. These observations indicate that boron powder injection alters the transport of the first wall material into the main plasma. Such effects on iron transport have also been observed with the injection of carbon powder and lithium granules.
In fusion devices with metallic walls, it is desirable to minimize the penetration of wall materials into the main plasma as impurities. Understanding the physical mechanisms by which impurity powder injection reduces impurity contamination may contribute to impurity control in future fusion reactors. In LHD, the divertor plates are made of carbon, while the first wall material is stainless steel. With regard to the transport of iron originating from the first wall, previous studies have examined the effects of radial electric field formation in the peripheral region of the main plasma, the balance between frictional forces due to plasma flows toward the divertor in the scrape-off layer (SOL), and thermal force arising from temperature gradients, as well as turbulent transport. In this presentation, results obtained by varying the background plasma conditions to examine the effect of impurity powder injection on iron transport will be shown, and the mechanisms by which impurity powder injection suppresses the penetration of iron impurities into the main plasma will be discussed.