Theory of probe measurements at the divertor plate
- Lawrence Livermore National Lab., CA (United States)
Probe measurements represent a technically simple and inexpensive approach to the characterization of plasma parameters in the divertor region. On the other hand, the interpretation of the probe signals is sometimes not straightforward, with discrepancies between the results of probe measurements and Thomson scattering measurements often arising. The difficulty of interpretation of probe measurements stems predominantly from the unknown influence of a strong magnetic field on the probe current-voltage characteristics (CVC). There have been many studies of this issue, among the most recent ones papers. In our paper, we present analysis of the physics issues which determine the performance of so called {open_quotes}flush mounted probes{close_quotes}. We note that, in case of an infinitely strong magnetic field, the flux-tube whose footprint coincides with the probe surface, is completely isolated from the rest of the plasma, and the probe CVC becomes more similar to the CVC of a double probe. As a next step in the analysis, we consider classical cross-field transport processes and conclude that, for the flux tubes with dimensions of a few ion gyroradii (as is the case for the probes of the type used in DMD) the cross-field currents are dominated by the ion viscosity. We derive the probe CVC for this case and find that it has a remarkable similarity with the aforementioned characteristics of the double probe. We consider possible effects of plasma turbulence on the cross field transport through a thin flux tube leaning on the probe. We conclude that the character of this effect strongly depends on the spatial and temporal scale of the plasma fluctuations: the influence of fast short-wavelength fluctuations can be described in terms of enhanced diffusion across the flux tube, while the influence of the slow ones can not. As the SOL turbulence is usually slow, we consider in more detail the effect of slow fluctuations.
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 489444
- Report Number(s):
- CONF-960354-; TRN: 97:011591
- Resource Relation:
- Conference: International Sherwood fusion theory conference, Philadelphia, PA (United States), 18-20 Mar 1996; Other Information: PBD: 1996; Related Information: Is Part Of 1996 international Sherwood fusion theory conference; PB: 244 p.
- Country of Publication:
- United States
- Language:
- English
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