0D model of magnetized hydrogen–helium wall conditioning plasmas

Abstract

Abstract. In this paper the 0D description of magnetized toroidal hydrogen-helium RF discharges is presented. The model has been developed to obtain insight on ICRF plasma parameters, particle fluxes to the walls and the main collisional processes, which is especially relevant for the comprehension of RF wall conditioning discharges. The 0D plasma description is based on the energy and particle balance equations for 9 principal species: H, H + , H 2 , H + 2 , H + 3 , He, He + , He 2+ and e − . It takes into account (1) elementary atomic and molecular collision processes, such as excitation/radiation, ionization, dissociation, recombination, charge exchange, etc... and elastic collisions, (2) particle losses due to the finite dimensions of the plasma volume and confinement properties of the magnetic configuration, and particle recycling, (3) active pumping and gas injection, (4) RF heating of electrons (and protons) and (5) a qualitative description of plasma impurities. The model reproduces experimental plasma density dependencies on discharge pressure and coupled RF power, both for hydrogen RF discharges (n e ≈ 1 − 5 · 10 10 cm −3 ) as for helium discharges (n e ≈ 1 − 5 · 10 11 cm −3 ). The modeled wall fluxes of hydrogen discharges are in the range of what is estimated experimentally: ∼ 10 19 − 10 20 /m 2 s for H-atoms, and ∼ 10 17 − 10 18 /m 2 s for H + -ions. It is found that experimentally evidenced impurity concentrations have an important impact on the plasma parameters, and that wall desorbed particles contribute largely to the total wall flux.

Isotope exchange by Ion Cyclotron Wall Conditioning on JET

“…The correlation is calculated as the Pearson coefficient and optimized via regression analysis of the logtransformed data. The density scaling for the same pressure and power range is confirmed by 0D ICRF plasma simulations [7] (correlation coefficient R>0.99) using fixed coupled power fraction to electrons and atomic ions of resp. 0.9 and 0.1, based on JET ICWC RF coupling studies [1], and using fixed charged particle confinement time of 5ms.…”

“…Experimental data are complemented by 0D plasma modelling (TOMATOR code [7]) solving energy and particle balance equations for hydrogen atomic and molecular plasma species, taking into account (1) elementary atomic and molecular collision processes, (2) RF heating of electrons and protons, (3) particle and energy confinement, as well as (4) wall flux recycling, active pumping and gas injection. and are fitted with good correlation, R=0.79, using power n=-1/3.…”

Wall conditioning for ITER: Current experimental and modeling activities

“…ICWC has been the subject of considerable study on current tokamaks [3,4,[26][27][28][29][30][31]. The principle of ICWC discharge production, in the presence of the toroidal magnetic field, has been described elsewhere (see e.g.…”

“…Operation in monopole phasing [4,29] and/or mode conversion scenarios [3,29] are proven methods to improve discharge homogeneity and optimize wall coverage [4]. The main flux to the wall is that of isotropic neutral hydrogenic species either desorbed from the wall surfaces [27] or created by dissociation of molecular Hydrogen by electron collisions, these having Franck-Condon energies of a few eV. In addition to ion fluxes parallel to the magnetic field lines, fast neutrals are produced in ICWC by charge exchange between protons or deuterons accelerated in the ICR layer and the background neutral gas, with temperatures above 1 keV and energies up to 50 keV, making them particularly attractive for T-removal [31].…”

A crude model to study radio frequency induced density modification close to launchers

“…Incorporating a large set of relevant processes and their respective reaction rates, Wauters successfully developed a model for the birth of plasma under the influence of RF waves, which enabled him to get a grip on the dynamics of ion cyclotron wall conditioning and RF assisted plasma startup. 30,31 The so far described sources scale with the density. Aside from this kind of sources, the density may change due to the processes that are independent of the local density.…”