Electrodeless Discharge

one of the kinds of AC electrical discharges in which energy is supplied without a contact and the circuit of the discharge current is completed by means of a displacement current or induction current. The chief role is played by the volume ionization of a gas; the processes on surfaces that border on the discharge region are of secondary importance. There are two types of electrodeless discharge: the E-discharge (linear electrode-less discharge) and the H-discharge (ring electrodeless discharge).

Ionization of the gas occurs in the E-discharge as a result of collisions between gas molecules and electrons accelerated by an alternating electrical field E. The discharge current in this type is completed via the displacement current (capacitative current) that flows through the capacitance between the external metallic electrodes and the ionized gas within the discharge vessel. It is possible to obtain an E-discharge by placing a tube containing a rarefied gas between the plates of a condenser in an oscillatory circuit in which electromagnetic oscillations are occurring (Figure 1, a).

Figure 1. Circuits for obtaining an electrodeless discharge: (a) linear, (b) ring discharge. DT is a discharge tube containing a rarefied gas, C is the capacitor of an oscillatory circuit, L is the inductance coil, and G is an electromagnetic oscillation generator.

An H-discharge develops, for example, in a tube containing a rarefied gas that is placed within the coil of an oscillatory circuit if the electromotive force induced by the magnetic field H exceeds the voltage required to start a discharge (Figure 1, b). Because of its construction (discs or rings that are coaxial with the coil), this type of discharge is also called a ring discharge. The ionization of the gas in a ring discharge is caused by a rotational electrical field, and the discharge current is an induction current. The ring discharge constitutes a closed circuit for a current that depends only on the internal resistance of the discharge itself. Owing to this, it is possible to achieve a high degree of ionization and a high temperature of the ionized gas (plasma). This characteristic, and also the excellent thermal insulation of the discharge’s plasma column from the walls of the vessel (because of reduced transverse diffusion of the charged particles to the vessel’s walls), has led to attempts to utilize a ring discharge in a magnetic field for obtaining controlled thermonuclear reactions. In a toroidal discharge chamber filled with deuterium, with the application of a magnetic field from several thousands to several tens of thousands of oersteds in intensity and directed along the axis of the toroid and with a sufficiently high current strength, it is possible to achieve practically complete ionization of the high-temperature plasma. In equipment of this kind the temperature of the plasma ions reaches several tens of millions of degrees.

The capability of obtaining an electrical discharge without introducing metallic parts into the discharge vessel is also of use as an ion source in accelerators, in the spectral analysis of gaseous mixtures, for carrying out chemical reactions in the discharge, and so on.