Microwave plasma generation by end-on and side-on irradiation in the pressure range 0.1–10 mTorr of hydrogen

Abstract

This paper describes investigations of hydrogen plasmas produced by mean of linearly polarized microwaves (f=2.45 GHz,P ₀=2.2–5.0 kW). A superimposed uniform and stationary magnetic field$\vec B_z$ is tuned in most of the experiments to the electron cyclotron resonance. The RF is coupled to the plasma by different antennae. A conical horn is used for end-on irradiation (wave vector$\vec k\parallel \vec B_z$). With regard to toroidal discharge geometries the side-on irradiation$\vec k \bot \vec B_z$ is of particular interest. For this purpose two newT-shaped antennae were developed.

In the pressure range investigated the plasma produced by the microwaves fills up the whole discharge vessel (l=200 cm,V≈10⁴cm³). The electron densities are in the range between 2.5 · 10¹⁰ and 1.5 · 10¹¹ cm⁻³ and the electron temperatures between 1.2 and 5.0 eV.

The plasma produced by side-on RF irradiation$(\vec k \bot \vec B_z ,\vec E\parallel \vec B_z )$ is used as a pre-ionization plasma for aZ-pinch discharge. The degree of ionization obtained in this way is ≳5%.

The optimum conditions for this type of preionization are evaluated from density measurements at higher cyclotron harmonics.