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

Abstract

Toward real-time wall conditioning, impurity powder dropping experiments with boron powder were performed in the 22nd experimental campaign of the Large Helical Device (LHD). To examine the deposition and desorption process of boron, we focus on boron hydride (BH) molecules which presumably populate near plasma-facing components. We performed spatially-resolved spectroscopic measurements of emission by boron ions and boron hydride molecules. From the measurement, we found that BH and B+ were concentrated on the divertor viewing chord, which suggest boron deposition in the divertor region. By comparing Hγ emissions with and without boron injection, neutral hydrogen shows uniform reduction in the SOL region, whereas less reduction of neutral hydrogen is confirmed in the divertor region. Although emissions from BH and B+ increased linearly, emissions by B0 and B4+ became constant after the middle of the discharge. Continuous reduction of carbon density in the core plasma was confirmed even after B0 and B4+ became constant. The results may show reduction of hydrogen recycling and facilitation of impurity gettering by boron in the divertor region and thus effective real-time wall conditioning.

Demonstration of aneutronic p-11B reaction in a magnetic confinement device

“…An impurity powder dropper was installed on the upper port, so-called 2.5-U of the LHD under the collaboration between the National Institute for Fusion Science and Princeton Plasma Physics Laboratory in order to realize the real time wall conditioning [55]. The continuous injection of impurity powder improved the plasma performance by suppressing the recycling of the plasma facing components [49,56] as well as turbulence inside the plasma [48,57]. The natural boron grains having the averaged diameter of submicrometers fall due to the gravity upon injection into the plasma.…”

“…The natural boron grains having the averaged diameter of submicrometers fall due to the gravity upon injection into the plasma. Once the boron grains reach the plasma, the boron is ionized and partly transported to the inner side of the plasma [56]. Note that the boron is natural boron composed of 80% of 11 B and 20% 10 B.…”

“…The central electron temperature T e0 measured by Thomson scattering diagnostics [71] was ∼2 keV, and line averaged electron density measured by far infrared interferometer [72] (n e_bar ) was <1 × 10 19 m −3 . We started the impurity powder dropper from ∼4.0 s. Here, the time trace of line-integrated BV line (I BV ) with fine time resolution was obtained using visible spectroscopy [56,73,74]. The boron profile was measured with CXRS by NB#4 modulation.…”

Calculation of electronic excitation cross sections and rate coefficients for boron monohydride (BH)

“…BH (or boron deuteride, BD) molecular bands have been confirmed in discharges after fresh boronization [5][6][7][8], suggesting a release of BH 2 from a:B-H layers upon exposures to thermal H 0 [9]. Kawate et al [10]. performed impurity powder dropping experiments [11] with boron powders in the Large Helical Device toward real-time wall conditioning.…”