Model of an inductively coupled negative ion source: II. Application to an ITER type source

Abstract

The injection of energetic neutral deuterium atoms will be one of the major heating methods of the ITER plasma. The 1 MeV, 16.5 MW neutral atom beam will be obtained by acceleration and collisional neutralization of negative ions extracted from an inductively coupled low-temperature plasma source. This negative ion source is composed of driver volumes where the RF (radio-frequency) power is inductively coupled to the plasma electrons, an expansion chamber including a magnetic filter, and the extraction grids. In this paper we present the first results of a 2D fluid model of a single-driver prototype of the source, for an H 2 plasma under realistic ITER-relevant conditions. We discuss the general plasma properties: plasma density, electron and neutral particle temperatures, ion composition (H + , H + 2 , H + 3 ), the dissociation degree of H 2 and the effect of the magnetic filter, in a large range of input powers (10-80 kW) and source pressures (0.2-0.8 Pa). Negative ions are not described self-consistently in this first approach. The results show a decrease in the gas density when the plasma is turned on, due to gas heating and to the neutral gas depletion induced by ionization. The low gas density leads to a high electron temperature in the driver, and to the saturation of the plasma density growth with power for pressures below 0.3-0.4 Pa. The H 2 temperature is in the 0.1 eV range while the H temperature is much higher (up to 1 eV) because hydrogen atoms are generated at high energies by the dissociation of H 2 or ion recombination at the wall surface. The simulation results are globally consistent with recent experiments on the negative ion source developed at IPP Garching. Because of the large Hall parameter in the magnetic filter, electron transport across the filter is complex and the ability of a 2D fluid model to grasp this complexity is discussed.

Effect of fast positive ions incident on caesiated plasma grid of negative ion source

“…1). The observation of high plasma potential in the driver of the IPP RF source is confirmed by the simulation of Boeuf et al 3 The experimental information about the profile of the plasma potential variation between the plasma grid and the driver is limited to an off-axis measurement (in front of the bias plate) presented by Christ-Koch and Fantz, 4 which shows that the plasma potential is constant at a value of 20 V on a distance from the plasma grid from 5 cm to 21 cm. It can be concluded that the plasma potential goes up on a short distance beyond 21 cm to the value of 50 V, corresponding to the measured in Ref.…”

Negative ion extraction from hydrogen plasma bulk

“…For example, substituting electrons by negative ions reduces the problem of differential surface charging in etching application 1 and the damage induced by energetic electrons and ions accelerated by a large sheath in surface treatment of polymers. 2,3 Negative ions are also used for neutral beam heating system of controlled thermonuclear fusion reactors [4][5][6] and for space propulsion. 7 The negative ion to electron density ratio (electronegativity a n À =n e ) is one of the most important parameter characterizing electronegative plasmas.…”

Can we estimate plasma density in ICP driver through electrical parameters in RF circuit?

“…As a consequence, studies related to those power absorption physics are not present in this manuscript. The RF driver density value can be considered as initial boundary condition to study the plasma transport inside the source volume through either Monte-Carlo technique [36] or fluid model [37]. The output of plasma transport [36 or 37] may establish the link between driver plasma density to the extraction region plasma density; i.e.…”