Recycling behaviour during long pulse discharges after ICRF boronization in the HT-7 tokamak

Abstract

The evolution of recycling behaviour has been investigated during long pulse discharges in the HT-7 tokamak after ICRF boronization (C2B10H12) using the H/(H+D) ratio and the edge recycling coefficient R. After boronization, impurity reduction is observed, attributed to the fresh boron film, but the recycling coefficient can exceed unity due to a large amount of hydrogen absorbed in the film, leading to an uncontrollable density rise and discharge termination. When the H/(H+D) ratio was reduced to less than 25%, the electron density was easily controlled. The longest discharge, up to 240 s with central electron temperature Te(0) of about 1.0 keV and central electron density ne(0) of 0.8 × 1019 m−3, was achieved following boronization. After many discharges the effectiveness of boron film was weakened, and the density rise was correlated with an increase in both carbon and oxygen radiation which limited the duration of long pulse discharges.

Cross section of the impact single ionization of B2+by H+

“…Enhanced energy confinement in current tokamak experiments has been achieved by coating heavy metal facing components with boron. [20] Besides, boronization is a widely used strategy in carbon-based plasma facing components to improve their erosion, hydrogen retention, and some thermomechanical properties. [21] Thus, a collision process involving boron atoms (ions) has attracted increasing attention.…”

“…[22,23] Hydrogen isotopes are magnetic confinement and inertial controlled fusion (ICF) fuel. [20] In the ICF experiments, the heated high-energy protons easily interact with wall materials, so the following collision process:…”

Analysis of D α (H α ) spectrum emitted in front of the limiter in HT-7

“…For the HT-7 tokamak of a limiter configuration, recycling particles contribute more than 90% of the total fuelling to sustain particle balance during the plateau of a discharge. [5] The limiter plays an important role in edge recycling, so the study of recycling from the limiter is the focus of this paper. To investigate the recycling mechanism in detail, the D α (H α ) line shape was simulated by using DEGAS 2.…”

Preliminary observations of hot spot for HT-7 long pulse discharges

“…For long pulse discharges, there are different factors limiting the plasma duration. It is observed for many times that towards the end of the discharges, carbon influx is related to the uncontrollable density rise [7]. Under this condition, the coupling of the 2.45 GHz lower hybrid wave (LHW) with the plasma becomes worse because the reflected wave power is observed to be higher with the same input power.…”