Plasma discharge fuelling by trimethylboron-a new wall conditioning technique

Abstract

A new in situ wall conditioning method by reactive gas fuelling has been investigated in TEXTOR; this method is applicable during plasma discharges of the operating machine. For these experiments, trimethylboron, B(CH3)3, was fed into the torus instead of hydrogen. While the hydrogen of the trimethylboron molecules fuels the plasma to sustain the discharge, boron and carbon are screened at the plasma edge and redeposited at the wall. The impurity concentration in the plasma is negligible, as it is after the usual boronization by radiofrequency assisted glow discharge. The plasma performance was improved shot by shot. This new technique may open a way to direct local control of erosion and redeposition processes at wall elements subject to high heat loads

Impact of the Wall Conditioning Program on Plasma Performance in NSTX

“…During the fueling sequence, it was found that edge O and C luminosities were comparable to before TMB fueling, within the limited statistics, due to the initially clean conditions (Fig.3). Core oxygen was reduced, and as was found previously in larger aspect ratio tokamaks [10,11]. Edge fueling with the TMB mixture did not increase core B V and C VI for the close-wall NSTX geometry (Fig.3).…”

“…It was found that the fraction of radiated power to total power during NBI fiducial discharges before and after the fueling sequence remained about the same at about 0.11. This is suggestive of the possible screening of the B and C components of TMB molecules discussed previously [10]. In addition, it was found that although the oxygen and carbon impurity levels decreased initially, and that the plasma performance continued to improve during the fueling sequences, these O and C luminosities remained relatively high.…”

“…Previously, it was found that boron gases injected into the plasma edge of large aspectratio tokamaks re-boronizes plasma-wetted surfaces and improved plasma performance [10,11].…”

Wall Conditioning on TEXTOR

“…Trimethylboron 41 and di-silane 42 have been injected into the SOL during tokamak discharges. Boron and silicon are redeposited on the walls while the hydrogenic atoms fuel the plasma with the same efficiency as deuterium gas, whereas the impurities are well shielded by the SOL from penetrating into hot plasma zones.…”

Boronization on NSTX using deuterated trimethylboron

“…Plasma fueling with TMB had been tested previously on TEXTOR [6] and TdeV [7] and found to transport boron in the edge plasma to plasma wetted surfaces.…”