Real-time wall conditioning and recycling modification utilizing boron and boron nitride powder injections into the Large Helical Device

Abstract

Controlled particulate injections from the PPPL Impurity Powder Dropper(IPD) into the Large Helical Device (LHD) have demonstrated positive effects on the wall conditions in both an intra and inter-shot basis. Injections over a range of densities, input powers, pulse lengths, heating schemes, injection quantities and main ion species show conclusive evidence of improvement to plasma wall conditions. Successful injections are confirmed by both spectroscopic measurements as well as real-time visible camera signals. In 7s long plasmas the responses include a reduction in wall recycling as well as a reduction in native impurity content as observed over the course of several discharges. For plasmas longer than 40s, improvements to the recycling rate and increased impurity control are observed in real time as a consequence of the extended particulate injections. These experiments demonstrate the extended applicability of this solid particulate conditioning technique to the control and maintenance of the plasma wall conditions. In addition they are an important initial step in the development of the real time boronization technique as a supplement to standard conditioning scenarios.

Experimental study on boron distribution and transport at plasma-facing components during impurity powder dropping in the Large Helical Device

“…impurity gettering on the divertor plate. A long-term impurity reduction and effectiveness for hydrogen recycling during IPD experiments were already reported by Bortolon et al [4] and Lunsford et al [11]. Our study also confirms the real-time reduction of hydrogen recycling and of impurities by boron powder dropping, by validating the deposition region.…”

“…They confirmed reduced wall fueling, recycling and impurity content, as well as formation of a B-C layer on graphite samples exposed during boron injection. Lunsford et al [11] summarized temporal variations of emissions from boron and intrinsic impurities from the main plasma between shots in the LHD. They also investigated the building up of boron layers by inserting stainless steel samples at a relevant position on the first wall, and compared the boron coating by the IPD with a standard B 2 H 6 boronization.…”

First measurements of p11B fusion in a magnetically confined plasma

“…It conditions the walls of the vessel to decrease recycling and intrinsic impurity content (e.g., C, O, Fe). This leads to a decrease in turbulence and an improvement in global confinement 17 , 18 . It also acts as a supplemental electron source, increasing electron density and thus neutral beam (NB) deposition.…”

A reduced-turbulence regime in the Large Helical Device upon injection of low-Z materials powders

“…A first set of experiments demonstrated the successful injection of B and BN powder in the unique LHD magnetic geometry, and the response of the plasma to the powder injection was characterized [17]. Beneficial wall conditioning effects, such as reduction of recycling and of the intrinsic impurities (C, O, Fe) was observed in LHD both on a shot-to-shot basis and in real time [18]. Finally, during B powder injection experiments in plasmas of duration longer than 5 s, both the electron, ion temperature and stored energy have been observed to increase [3], while the input power and line-averaged electron density n e,av are kept constant.…”