RF wave coupling, plasma heating and characterization of induced plasma-material interactions in WEST L-mode discharges

Abstract

Plasma heating in the full Tungsten (W) Environment in Steady-state Tokamak (WEST) relies on electromagnetic waves in both Lower Hybrid (LH) and Ion Cyclotron Range of Frequencies (ICRF). The present study focuses mostly on the optimization of discharges heated with ICRF, by reporting different methods to first optimize wave coupling, optimize their absorption and reduce the impurity production. It is shown that ICRF coupling can be optimized either by moving the plasma closer to antennas, increasing the plasma density, wave frequency and LH power. We show that the absorption efficiency correlates with the hydrogen concentration with the existence of an optimum between 7 and 10% as expected for a minority heating scenario.Absolutely calibrated visible spectroscopy sightlines were used to monitor ion fluxes in different locations as part of an effort to quantitively estimate the contribution of different impurity sources to the core contamination by tungsten. It is typically found that in discharges with high total RF-power (above 5MW of LH and 3MW of ICRF), divertor and antenna limiter sources can reach similar order of magnitude during the ICRF phase.

Initial results from boron powder injection experiments in WEST lower single null L-mode plasmas

“…The ICRH antenna, a potential source for gross W erosion [25], was placed here for comparison with powder-free W sourcing experiments that positioned the antenna in the same location [26]. No ICRH power was used during the powder injection experiments to keep consistency with the W sourcing experiments and to better isolate the effects of powder injection, as ICRH is known to significantly increase impurity sputtering in WEST [27].…”

Characterizing W sources in the all-W wall, all-RF WEST tokamak environment * , **

“…Figure 12 shows processing of shots from both the LH-only and ICRH-only sets, using the heuristic approach taken in [31,32] whereby the W I radiance values (Fig. 12b) are divided by the intensity a spectrally-near, deuterium Balmer-series line (Dε, in this case) providing an effective yield (Fig.…”

The geometry of the ICRF-induced wave–SOL interaction. A multi-machine experimental review in view of the ITER operation

“…More energetic ions are also more efficient for sputtering wall elements. RF-specific impurity production during ICRH has been studied mainly in metallic machines, using as a proxy the line emission in the visible range for tungsten (W) [5,21,27,[39][40][41][42][43][44][45][46], molybdenum (Mo) [10,12] and beryllium (Be) [47,48]. In the absence of systematic measurements of V DC , ASDEX Upgrade and WEST publications used the ratio of W over D line brightness as a proxy for the effective W sputtering yield in order to assess the magnitude of rectified DC potentials.…”

“…Visible spectroscopy has generally been used in conjunction with VUV spectroscopic line emission from the plasma core in order to assess the central contamination for W [5, 21, 27, 39-46, 50, 51, 53], Mo [10,12,27,45,50], but also nickel in JET [53][54][55], titanium or iron in EAST [45,50], iron and copper in Tore Supra [56] and silver in WEST [27,46]. Contamination of the plasma core has been studied extensively, as large radiation from the plasma centre can hinder the fusion performance or even the stability.…”