Active particle control experiments and critical particle flux discriminating between the wall pumping and fuelling in the compact plasma wall interaction device CPD spherical tokamak

Abstract

Two approaches associated with wall recycling have been performed in a small spherical tokamak device CPD (compact plasma wall interaction experimental device), that is, (1) demonstration of active particle recycling control, namely, ‘active wall pumping’ using a rotating poloidal limiter whose surface is continuously gettered by lithium and (2) a basic study of the key parameters which discriminates between ‘wall pumping and fuelling’. For the former, active control of ‘wall pumping’ has been demonstrated during 50 kW RF current drive discharges whose pulse length is typically ∼300 ms. Although the rotating limiter is located at the outer board, as soon as the rotating drum is gettered with lithium, hydrogen recycling measured with Hα spectroscopy decreases by about a factor of 3 not only near the limiter but also in the centre stack region. Also, the oxygen impurity level measured with O II spectroscopy is reduced by about a factor of 3. As a consequence of the reduced recycling and impurity level, RF driven current has nearly doubled at the same vertical magnetic field. For the latter, global plasma wall interaction with plasma facing components in the vessel is studied in a simple torus produced by electron cyclotron waves with I p < 1 kA. A static gas balance (pressure measurement) without external pumping systems has been performed to investigate the role of particle flux on a transition of ‘wall fuelling’ to ‘wall pumping’. It is found that a critical particle flux exists to discriminate between them. Beyond the critical value, a large fraction (∼80%) of pressure drop (‘wall pumping’) is found, suggesting that almost all injected particles are retained in the wall. Below it, a significant pressure rise (‘wall fuelling’) is found, which indicates that particles are fuelled from the wall during/just after the discharge. Shot history effects (integrated particle recycling behaviour from the plasma facing surfaces) are seen on that the critical particle flux is reducing from 0.8 × 10−4 to ∼0.1 × 10−4 Torr during the experimental campaign (∼3000 shots). In the wall pumping pressure range the wall pumping fraction is reduced with increasing surface temperature up to 150 °C.

Analysis of the footprint traces on the first walls of the compact plasma wall interaction device (CPD) using surface analysis and electron orbit calculations

“…Both height and width of the vacuum chamber are ∼1.2 m. Although there are four TF coils, since the return coils are located far from the plasma area, the ripple in the toroidal magnetic field (B T ) is within a tolerable limit (∼8% at R = 0.5 m). The maximum B T is 0.24 T at R = 0.3 m. A more detail description of CPD is given in [13]. In the initial phase of the experimental campaign (2006), the plasma was produced by ohmic as well as RF discharges.…”

“…The CS cover made of stainless steel sprayed with tungsten acts as the inboard limiter at R = 0.13 m and is extended up to ±150 mm from the mid-plane in the vertical direction. The RL setup is mounted horizontally at R ∼ 0.43 m on the torus equator plane [13]. The stiffeners made of stainless steel are attached on the walls to provide the mechanical strength.…”

“…It has been known from the spherical tokamak LATE [12] that in the non-inductive current startup scenario using the electron cyclotron waves (ECWs), the energetic electrons play an important role in the region of the open magnetic field. In compact plasma wall interaction device (CPD), the relatively symmetric footprint traces after the non-inductive current startup experiments have been observed [13] and the loss of energetic electrons is considered to contribute to these traces instead of impurity ions. Furthermore, in ITER, an ECR system having an output power of 3-10 MW would be used to initiate a plasma [14].…”

22nd IAEA Fusion Energy Conference: summary of sessions EX/D, EX/S and EX/W

“…Liquid lithium is investigated to clarify its potential as a first wall material for a fusion reactor. Two kinds of lithium plasma facing walls are studied in several devices [53][54][55][56][57]. One is an experiment with a lithium coated first wall by using lithium evaporation [53][54][55][56].…”

“…Two kinds of lithium plasma facing walls are studied in several devices [53][54][55][56][57]. One is an experiment with a lithium coated first wall by using lithium evaporation [53][54][55][56]. Figure 32 shows energy confinement time versus line density for B and Li coated wall discharges in TJ-II.…”