On thermionic emission from plasma-facing components in tokamak-relevant conditions

Abstract

The first results of particle-in-cell simulations of the electrostatic sheath and magnetic pre-sheath of thermionically emitting planar tungsten surfaces in fusion plasmas are presented. Plasma conditions during edge localized modes (ELMs) and during inter-ELM periods have been considered for various inclinations of the magnetic field and for selected surface temperatures. All runs have been performed under two assumptions for the sheath potential drop; fixed or floating. The primary focus lies on the evaluation of the escaping thermionic current and the quantification of the suppression due to the combined effects of space-charge and Larmor gyration. When applicable, the results are compared with the predictions of analytical models. The heat balance in the presence of thermionic emission as well as the contribution of the escaping thermionic current to surface cooling are also investigated. Regimes are identified where emission needs to be considered in the energy budget.

Simulations of thermionic suppression during tungsten transient melting experiments

“…However, dedicated particle-in-cell (PIC) simulations can model the problem with sufficient accuracy. The 2D3V PIC code SPICE2 [11,12] was recently updated to include the generation of thermionic electrons [13]. In that study, scans were performed in the magnetic field inclination angle for selected surface temperatures and different wall configurations relevant to inter-and intra-ELM conditions of the JET divertor [13] aiming to gain insight into the complex interplay between the two suppression mechanisms.…”

“…The 2D3V PIC code SPICE2 [11,12] was recently updated to include the generation of thermionic electrons [13]. In that study, scans were performed in the magnetic field inclination angle for selected surface temperatures and different wall configurations relevant to inter-and intra-ELM conditions of the JET divertor [13] aiming to gain insight into the complex interplay between the two suppression mechanisms. In this contribution, plasma conditions are targeted that correspond to the dedicated AUG experiments and detailed scans in the surface temperatures for inclination angles relevant to the special lamellae are performed.…”

Magnetized electron emission from a small spherical dust grain in fusion related plasmas

“…The gyromotion can lead to prompt recollection of emitted electrons on the dust surface regardless of their energy. This effect has been studied on Plasma-Facing Components (PFCs), considered as planar surfaces oriented with a given angle to the magnetic field (using Particle-In-Cell modelling, 14 analytical models, [15][16][17] and in experimental measurements 18 ), but not in the case of a spherical emitting body, for which the ratio r d /k D might play an important role.…”

Numerical implementation of a cold-ion, Boltzmann-electron model for nonplanar plasma-surface interactions

“…34 However, the charge in a plasma sheath can mitigate these high field-enhancement factors. 35,36 The fieldenhancement factors of hemi-ellipsoidal protrusions are calculated in this subsection; these shapes are well-characterized for emission in a vacuum 30 and provide good representations of the elongated fuzz fibres and flattened bubbles which form on plasma-exposed surfaces. 34 Simulations are performed in axisymmetric coordinates by placing semi-ellipsoids of half-width 0:2k D and heights in the range ð0-1Þk D at the centre of the surface.…”