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

Abstract

Experimental data, together with interpretive modeling tools, are examined to study trends in the tungsten (W) source in the all-W environment of the WEST tokamak, with significant focus away from the divertor. In particular, a poloidal limiter protecting an Ion Cyclotron Resonance Heating (ICRH) antenna is used as proxy for main chamber sourcing. The key study is carried out by stepping up Lower Hybrid Current Drive (LHCD) power, as the only auxiliary power source. The dependence of the W source from both the divertor and this outboard limiter on the total power crossing the separatrix, PSEP, show that although the main chamber source is substantially lower than the divertor source, it scales similarly with PSEP. Intepretive modeling of the limiter source is carried out with a Particle-in-Cell (PIC) sheath model coupled to a surface sputtering model. Oxygen is used as a proxy for all light impurity species allowing for characterization of the critical W erosion regions. To get a good quantitative match to the data, it is necessary to assume that the oxygen arrives at the surface mostly at high ionization stages (4+ and above). A separative simulation with SOLEDGE-EIRENE, constrained to measured upstream scrape-off-layer plasma profiles, gives oxygen fractional abundances that are compatible with the PIC simulation result. This is understood to arise from transport processes that dominate over recombination. Substituting the LHCD by ICRH, in an equivalent experiment, the local W source exhibits a x3 enhancement. This can be matched by the simulation, by assuming local RF electric field rectification, based on ~100eV peak-to-peak, near-antennna electric field. This work has highlighted the particular importance of understanding the ion charge state balance of light impurities as these are most likely the dominant sputtering species in fusion devices with high-Z walls

Tungsten accumulation during ion cyclotron resonance heating operation on WEST

“…The inferred tungsten concentration is also similar for all the pulses at 0.5 MA, and it increases with the ICRH power. This increased plasma contamination is qualitatively consistent with the increase of the power entering the scrape-off layer region, and associated with increased tungsten sources [5,14,15]. The ICRH power influences the core electron temperature only up to 1 MW, and has little effect above.…”

Developing high performance RF heating scenarios on the WEST tokamak

“…The photons flux of the neutral W line (λ = 401 nm) is converted in W atoms flux with the aid of the inverse of the photon efficiency S/XB [38]. The poloidal profile of the W emission of the WEST antenna protection limiter (APL) was successfully simulated with the coupled codes hPIC and RustBCA when these S/XB factors are used [39].…”

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

“…This section will focus on VS measurements at the lower outer divertor, due to the LSN configuration, and at the ICRH antenna limiter, which was radially positioned 5 mm in front of the LHCD antennas and acted as the outboard limiting surface. 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].…”