Impact of carbon and tungsten as divertor materials on the scrape-off layer conditions in JET

Abstract

The impact of carbon and beryllium/tungsten as plasma-facing components on plasma radiation, divertor power and particle fluxes, and plasma and neutral conditions in the divertors has been assessed in JET both experimentally and by simulations for plasmas in low confinement mode. In high-recycling conditions the studies show a 30% reduction in total radiation in the scrape-off layer when replacing carbon with beryllium in the main chamber and tungsten in the divertor. Correspondingly, at the low field side divertor plate a twofold increase in power conducted to the plate and a twofold increase in electron temperature at the strike point were measured. In low-recycling conditions the SOL was found to be nearly identical for both materials configurations. These observations are in qualitative agreement with predictions from the fluid edge code package EDGE2D/EIRENE. The rollover of the ion currents to both plates was measured to occur at 30% higher upstream densities and radiated power fraction in the Be/W configuration. Past rollover, it was possible to reduce the ion currents to the low field side targets by a factor of 2 and to operate in stable, detached conditions in the JET-ILW configuration; in the JET-C configuration the reduction was limited to 50%. Plasmas with low and high triangularity (and thus magnetic separation to the top of the device), and horizontal and vertical target configurations were investigated and compared to EDGE2D/EIRENE predictions.

Improved EDGE2D-EIRENE simulations of JET ITER-like wall L-mode discharges utilising poloidal VUV/visible spectral emission profiles

“…The simulations of the JET-ILW, L-mode, NBI-heated discharges of Groth et al [1] have been used to determine P rad contributions (table 1). They apply to a density scan series of 2.5MA / …”

“…Both fuel and edge impurities affect the power balance, thus determining the power reaching the divertor-target plates, which is limited by the mechanical and thermal properties of the plates. A recent comparison [1] of Lmode discharges during the present JET-ILW campaign and previous JET-C campaigns, in which the plasma-facing surfaces were C based materials (Carbon-Fibre Composite), has consistently shown a shortfall in the radiated power in the Scrape-Off Layer (SOL) and divertor calculated from EDGE2D-EIRENE simulations [2] below that measured by bolometry. In order to understand this discrepancy, the contributions to the divertor radiated power (P rad ) as predicted by 3 the simulations have been quantified and the results compared with measurements from bolometry and an upgraded poloidally scanning VUV/visible spectrometer.…”

First scenario development with the JET new ITER-like wall

“…The experiments to qualify the efficiency of the massive gas injection (MGI) on disruption have shown that the MGI increases the radiation and thus reduces the heat loads and forces to the level of those observed in the JET-C [20]. Given these results, the MGI has been used routinely in closed loop above a plasma current of 2.5MA.…”

Kinetic corrections from analytic non-Maxwellian distribution functions in magnetized plasmas

“…The divertor plasma is a crucial component for the fusion energy because it is the main material component in direct contact with the plasma and can highly impact the cost efficiency due to the increase of required maintenance. The difference of transport and plasma profiles between experimental measurements and edge simulations seems to be explained by the presence of NMDFs [24][25][26]. The main issue of these edge simulations is the description of the radiation which is very sensitive to kinetic effects.…”

“…[22]), or the description of the transport. Indeed, some results suggest that fluid models can be similar to kinetic codes such as the observation of asymmetric heat flux inside an island by using a fluid model with some finite Larmor radius terms [65], using profiles of transport coefficients [24][25][26] to reduce the radiation shortfall or using nonlocal fluid closures [66,67]. A clarification of these examples will be detailed later.…”