科研之友
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摘要
The intensity of the maintenance electric field of a given discharge is one of its internal parameters. Under ambipolar diffusion conditions, it is almost exclusively set by particle losses, which are related to the dimensions of the discharge vessel and to the gas pressure, and ultimately are determined by the electron energy distribution function. For instance, raising the density of microwave power absorbed in a discharge tube essentially increases the electron density without much increasing the amplitude of the maintenance E-field. To raise the intensity of this E-field in such a case, one needs to reduce the volume into which electromagnetic power is absorbed relative to the diffusion volume, i.e. the volume within which electrons transfer their power through collisions with heavy particles. To show this point, we consider a power balance based on the power lost per electron through collisions with heavy particles, theta(L), to the power absorbed (over a period of the microwave field) per electron in the discharge, theta(A). The power theta(A), which depends on E-0(2), the square of the amplitude (intensity) of the maintenance electric field, adjusts to compensate for the power lost theta(L). The analysis presented is achieved for a particular microwave discharge configuration that is known to provide an intense E-0-field, which means x >= lambda(De), where x is the oscillation amplitude of electrons in the E-0-field and lambda(De) the electron Debye length. Such a condition allows one to observe periodic parametric instabilities at, or close to, the electron-plasma frequency f(pe) and at their corresponding ion-plasma frequency f(pi), these oscillations being caused by the simultaneous propagation of an electron-plasma wave and an ion-plasma wave in the discharge as a result of an applied 'pump' power, which also sustains the discharge. A 2D hydrodynamic calculation of the specific plasma discharge system is performed, which yields the value of the x/lambda(De) ratio in particular through a global energy balance equation. This equation requires that the volume over which power is absorbed in the discharge multiplied by theta(A) must be equal to the discharge diffusion volume multiplied by theta(L), to which must be added the power lost in the ambipolar field Ea and in the plasma sheath. By comparison, in electromagnetic surface-wave discharges these two volumes are equal. The present considerations could possibly be extended to obtain a better insight into dc, RF and microwave micro-discharges when their discharge volume appears to be smaller than the volume over which particles are lost. Such a situation could explain their unusually high level of absorbed power density, as a result of which highly excited and ionized plasmas are generated.
- 出版日期2015-11-18
