Characteristics of Cleaning and Carbonizing Plasmas in TEXTOR

Abstract

Plasma parameters for discharge cleaning and carbonization in TEXTOR (Torus Experiment on Technology Oriented Research in the Kernforschungsanlage Julich) have been measured with Langmuir probes. In the rf-assisted glow (RG) discharge of hydrogen, at 1.6×10-1 Pa, the plasma density is n e = 3.0-4.0×108 cm-3, the electron temperature T e = 10-12 eV, and the space potential V s≦265 V. In the electron cyclotron resonance (ECR) discharge, at 1.1 × 10-2 Pa, n e=2.0-2.7×1010 cm-3, T e=3.5-5.2 eV, and V s=20 V in the resonance region. The width of this region is about 20 cm. In the carbonization by the RG discharge with an admixture of methane into deuterium at a volume ratio of 33%, at 1.7×10-1 Pa, n e = 0.9-1.4×109 cm-3, T e = 2.7-3.3 eV and V s≦280 V. The flux of ions and neutral atoms are estimated from measured parameters. In RG(H2), both H+ 2 and H are effective for cleaning. In ECR, an appreciable flux of H is obtained, and the H+ 2 flux, though reduced by the radial magnetic field, has an effect of cleaning. In RG(CH4/D2), the positive ion flux of the order of 6×1013 cm-2s-1 is responsible for the carbonization.

Wall conditioning in fusion devices with superconducting coils

“…The main species striking the wall are single ionised helium ions and H2 + -ions stemming from direct ionization of wall released H2 [18]. Their flux is proportional to the gas pressure ( and ), the ionisation rate ( and ) and the electron density ( ).…”

Isotope exchange experiments on TEXTOR and TORE SUPRA using Ion Cyclotron Wall Conditioning and Glow Discharge Conditioning

“…eV [2]. The significantly lower density in GD plasmas directly leads to a much lower reionization probability .…”

“…The results of ICWC are compared with isotope exchange experiments using conventional glow discharges on the same machines with typically the same wall pre-treatment conditions. The physics of GDC is well documented [1,2]. The flux of particles to the wall consists mainly of ions /s = /sm 2 (TEXTOR) with ion energies of typically eV, obtained in the cathode fall in front of the walls.…”

“…Also an additional contribution of neutrals is not excluded [2]. This paper concentrates on a comparison of measured and deduced quantities in both wall conditioning procedures, without regarding in more detail the dominant Plasma Wall Interaction (PWI) processes at the wall itself.…”