Particle motion in a sheath in a strongly tilted magnetic field
- Lawrence Livermore National Lab., CA (United States)
The ions approaching the wall along the strongly tilted magnetic field, get gradually {open_quotes}scraped-off{close_quote} by the wall, so that the ion density begins to drop at a distance of the order of the ion gyro-radius, {rho}{sub i}, from the wall. The quasineutrality constraint then leads to formation of the ambipolar potential that repels electrons and attracts the ions. This quasineutral region that can be called the {open_quotes}ion sub-sheath{close_quotes} is followed by a thin non-quasineutral layer adjacent to the wall which we call the {open_quotes}electron sub-sheath{close_quotes}. Particle motion in the ion sub-sheath has been studied in some detail both numerically and analytically. In our paper we point out that, if the angle a between the magnetic field and the wall is small, the ion motion in the ion sub-sheath can be described in terms of an adiabatic invariant, despite the fact that electrostatic potential varies at a scale {approximately} {rho}{sub i}. The presence of an adiabatic invariant makes it possible to carry out a general classification of the ion trajectories. We find that ions with high energies approach the electron sub-sheath at shallow angles, less than {alpha}{sup {1/2}}, while ions with low energies {open_quotes}dive{close_quotes} towards the wall from the middle of the ion sub-sheath. We present simple expressions for the ion distribution function at the wall for high and low electron temperatures. We consider the motion of heavy impurity ions in different charge states and present the classification of the ion trajectories for different values of the parameters a and Z/M. Our assessment of the ion motion provides a basis for evaluating the sputtering rate and for analyzing the sheath stability. In our present paper, we consider electron motion within the electron sub-sheath under conditions where the electron Debye radius is much greater than electron gyroradius.
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 489488
- Report Number(s):
- CONF-960354--
- Country of Publication:
- United States
- Language:
- English
Similar Records
Effect of a plasma turbulence on the properties of the sheath in a grazing magnetic field
Sheath over a rough surface in a tilted magnetic field