B2.5-Eunomia simulations of Magnum-PSI detachment experiments: II. Collisional processes and their relevance

Abstract

Detachment is achieved in Magnum-PSI by increasing the neutral background pressure in the target chamber using gas puffing. The plasma is studied using the B2.5 multi fluid plasma code B2.5 coupled with Eunomia, a Monte Carlo solver for neutral species. This study focuses on the effect of increasing neutral background pressure to the plasma volumetric loss of particle, momentum and energy. The plasma particle and energy loss almost linearly scale with the increase of neutral background pressure, while the momentum loss does not scale as strongly. Plasma recombination processes include molecular activated recombination (MAR), dissociative attachment, and atomic recombination. Atomic recombination, which includes radiative and three-body recombination, is the most relevant plasma process in reducing the particle flux and, consequently, the heat flux to the target. The low temperature where atomic recombination becomes dominant is achieved by plasma cooling via elastic H+-H2 collisions. The transport of vibrationally excited H2 molecules out of the plasma serves as an additional electron cooling channel with relatively small contribution. Additionally, the transport of highly vibrational H2 has a significant impact in reducing the effective MAR and dissociative attachment collision rates and should be considered properly. The relevancy of MAR and atomic recombination occupy separate electron temperature regimes, respectively, at Te = 1.5 eV and Te = 0.3 eV, with dissociative attachment being relevant in the intermediary. Plasma cooling via elastic H+-H2 collisions is effective at Te ≤ 1 eV.

Comparison between SOLPS-ITER and B2.5-Eunomia for simulating Magnum-PSI

“…In Eunomia more energy is extracted from the plasma ions, particularly due to MAR and p + H 2 elastic collisions. Moreover, a double counting of p + H interactions in Eunomia was identified, whose impact in coupled runs will be addressed in a erratum for [7,16]. In Magnum-PSI simulations, Ohmic heating in the plasma beam is directly controlled by the electric potential at the source, which is unknown and it is manually adjusted to match experimental data at the TS target position [7].…”

“…An erratum is being prepared to address the impact of this issue in previous published coupled runs, as those in Ref. [7,16]. Figure 5 presents the radial profiles of atomic Hydrogen at z = −1.2 m, a midpoint in the source chamber for three cases: Eirene (using only CX), Eunomia using CX and EL and Eunomia using only CX.…”

“…Molecular Assisted Recombination (MAR) is of huge importance [16] to understand molecule-plasma interaction in Magnum-PSI. This occurs when a proton exchanges charge with a H 2 molecule and then a free electron dissociates the charged molecule, resulting in three H atoms.…”

“…Thus, it is important to study the effect of this erroneous implementation and also to check if both codes produce similar results when only backwards scattering is accounted for. An erratum is being prepared to address the impact of this issue in previously published coupled runs, such as those in [7,16]. Figure 5 presents the radial profiles of atomic Hydrogen at z = −1.2 m, a midpoint in the source chamber for three cases: Eirene (using only CX), Eunomia using CX and EL and Eunomia using only CX.…”

“…This will ease the analysis of Magnum-PSI experiments aimed to recreate ITER divertor like conditions and will allow to extract relevant information of plasma-surface interaction in fusion devices. However, first, a comparison between the two neutral codes needs to be carried out, to ensure that the implementation of plasmaneutral interactions in SOLPS-ITER is capable to reproduce the B2.5-Eunomia cases that have been previously compared with experiments [7,16]. Good agreement between simulations and experiments was found for the high and low-density cases and for a wide range of pressures, although discrepancies between the two still appeared, particularly in the plasma density.…”