Canada

`ä~ìÇÉ=_çìÅÜÉê=EfkopF boucher@emt.inrs.ca

Collaboration between FZ Jülich and the University of Toronto relating to the EIRENE kinetic neutral code

S. Lisgo and D. Reiter

The purpose of this study is to model the level of Lyman photon trapping (and photon transport) in divertor plasmas as close to the conditions of ITER as possible, thereby providing the best test con-ditions for models. Photon trapping appears to play a role in determining the plasma concon-ditions and pressure in divertors. The level of divertor Lyman alpha trapping is proportional to n0L (n0 the neu-tral density and L the divertor dimension). The C-Mod divertor, where the neuneu-tral density is high, thus provides the best source of data.

Objectives

¾ Development of credible plasma solutions for partially detached (high-density) divertor plasmas on C-Mod and JET, and modelling of the associated neutral and photon dynamics, to assess the impact of photon trapping on divertor performance (neutral pressures and pumping, onset of detachment, parasitic plasma formation due to photo-ionisation).

¾ Determine if the current application of Lyman series trapping in codes is correct and appli-cable to ITER. This will require local comparison between the model and the observed Lyman-β trapping on C-Mod, as inferred from Lyman-β / Balmer-α emissivity data taken along the same line of sight (collection of corresponding JET data pending). The handling of volume recombinations and radiation transport in the codes will be an area of close scrutiny as well.

Divertor Plasma Solution

Interpretive modelling analysis with the OSM-EIRENE code has been conducted for three C-Mod discharges, with continuing refinement of the calculated plasma solution (which is based on divertor experimental data), neutral model and photon transport. The discharges cover a large range in diver-tor neutral pressures, with values of 25, 75 and 150 mTorr (3.3, 9.9 and 19.7 Pa) respectively.

The plasma solutions formulated thus far are based on ‘plasma reconstruction’, which strives to give the best overall agreement with the available experimental data for the divertor. For C-Mod, the diagnostic data set consists of line-of-sight Dα, target Langmuir probes, and for the detached plasma regions, volume measurements of ne from Stark broadening and Te from fits of the high-n Balmer line emissions to the Saha equation. Figures 1 and 2 are for the 25 mTorr case, and illustrate the level of agreement that has been achieved.

Divertor Neutral Solution

A by-product of the having high neutral densities leading to Lyman series absorption is that the high neutral densities will also provide tests of n-n and D⁺–D2 collision physics for mean free paths small compared to the relevant plasma fan or divertor dimension (λnn ~ 1.3-7.8 mm, λp-D2 ~ 1-8 mm for the range in cases under study). As seen in Figure 3 the above neutral collision processes play an important role in neutral transport and determining the pressure in the divertor. (A representation of molecular activated recombination is included, but this appears to be a second order effect at pre-sent).

Fig. 1: Comparison between model (solid lines) and probe data (solid squares). (a) Inner target, also with Te

from spectroscopic data (open squares), (b) ion flux to inner target, (c) outer target Te, (d) outer target ion flux, (e) pressure profiles (dashed line – inner target, solid line – outer target, dotted line –

upstream from reciprocating probe).

Fig. 2: Comparison between model (solid lines) and spectroscopic data. (a) temperatures in the volume recombination zone from the distribution of high-n Balmer emission lines, (b) densities in the volume

recombination zone from Stark broadening, (c) Dα from line-of-sight diode arrays (dashed line).

Fig. 3: The effect of turning off individual collisional processes in EIRENE, one at a time, for the three C-Mod cases modelled (OSM, static plasma solution) thus far, to illustrate the controlling physics.

Numerical simulations

M. Shoucri, H. Gerhauser and K.H. Finken

Two papers are under preparation in collaboration with H Gerhauser and K.H. Finken, to study the problem of the oscillation in the sheath and plasma-wall transition, at grazing incidence of the mag-netic field, when the ions gyrating across the magmag-netic field have large gyro-radii, and the electrons, frozen by the magnetic field lines, cannot move across the magnetic field to catch the gyrating ions.

Increasing the angle of incidence of the magnetic field, or decreasing the ions gyro-radius, result in a more standard behaviour of the sheath. This transition between the oscillations and the standard behaviour of the sheath will be studied.

Collaboration between FZ Jülich, University of Düsseldorf and University of Alberta, Canada

R.D. Sydora, A. Rogister and K.H. Spatschek

This collaboration concerns development of theoretical and computational approaches in fluctua-tion-driven anomalous transport. We have been working on two projects of experimental relevance to the TEXTOR, ALCATOR C-Mod and JET programmes. This collaboration on the development of theoretical and computational approaches in fluctuation-driven anomalous transport is ongoing.

First, we have been investigating the relation between onset of parallel velocity shear instability – or Kelvin-Helmholtz (KH) – and transition from ELM (edge localized modes)-free to EDA (Enhanced D-alpha) behaviour in ALCATOR C-Mod high confinement H-mode plasmas. The mode number of the quasi-coherent mode which appears in the EDA phase is close to theoretically predicted

maxi-mally growing KH mode driven by parallel velocity flow shear. We are investigating the linear in-stability of the KH mode in the presence of a temperature gradient, the relation of this mode to the ion temperature gradient (ITG) instability and the nonlinear relaxation and associated anomalous particle, momentum and energy fluxes. This work also has relevance to the Type II ELMS observed in JET and the so-called Washboard (WB) mode.

The second project involves the radial plasma transport across a magnetic island in tokamak plas-mas. The effect of poloidally mode coupled ballooning-type electrostatic drift waves on a magnetic island is investigated theoretically and by gyrokinetic simulations. The transport can be significant, even when the flux surface is not destroyed, because the electron motion is highly stochastic due to resonance effects between the periodic particle motions in the magnetic island and the ExB drift due to the electrostatic drift waves. We are investigating the physical mechanism of the diffusion proc-ess and scaling with plasma parameters.

D. PARTNERS OF THE IEATEXTORIMPLEMENTING AGREEMENT