Effect of a plasma turbulence on the properties of the sheath in a grazing magnetic field
- Lawrence Livermore National Lab., CA (United States)
The object of this study is the effect of plasma turbulence on the properties of the Debye sheath in the case where the magnetic field intersects the wall at a very shallow angle, {alpha}<<1. This situation is important for the divertor regions of tokamaks, as well as for some plasma processing and space applications. By the sheath we mean a region which includes both a quasineutral layer with a thickness of order of the ion gyro-radius where most of the density drop occurs, and a much thinner non-quasineutral layer immediately adjacent to the wall. Classical theory of the sheath in an oblique magnetic field is well developed (see, e.g., the most recent survey and references therein) but relatively little is known about possible effects of plasma turbulence. We analyze two types of the turbulent effects: (i) effects associated with the turbulence in the bulk of the plasma and, (ii) excitation of microturbulence in the sheath itself. When considering the effect of the {open_quotes}bulk{close_quotes} plasma turbulence, one should note that the wave structure of turbulent fluctuations is strongly modified in the vicinity of the wall; hence, one can not use the bulk plasma diffusion coefficient to describe the ion transport through the sheath. The {open_quotes}average{close_quotes} ion traverses the sheath during many ({approximately}1/{alpha}) gyroperiods. This sets a time-scale for the fluctuations which could cause an enhanced transport through the sheath: their frequency should exceed {alpha}{omega}{sub ci}. Slower fluctuations affect the ions only adiabatically, and cannot have a strong influence on the sheath structure (unless their amplitude is unreasonably high). We analyses the role of faster oscillations and find conditions under which they cause broadening of the sheath. The other group of turbulent effects is that caused by instabilities in the sheath itself.
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 489388
- Report Number(s):
- CONF-960354--
- Country of Publication:
- United States
- Language:
- English
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