Classical Plasma Applications

Plasma electronics

Vacuum tubes containing ionized gases and capable of carrying high currents are one of the oldest applications of plasmas, going back to the researches of Irving Langmuir and Levi Tonks in the 1920's. The modern plasma electronics industry uses mercury rectifiers, hydrogen thyratrons, ignitrons and arc switches for electricity transmission and control. Fluorescent lamps, intense microwave plasma light sources and plasma flat panel displays are still other applications of gas discharges.

Classical DC (direct current) electrical discharge tube. In the glow discharge regime (1014 to 5×1018 charged particles/m3, 104 to 5×104 K) the plasma is luminous due to excitation collisions by accelerated electrons. At high current densities the high heat load on the cathode triggers electron emission and the discharge changes to the highly luminous arc regime (1020 to 1025 charged particles/m3, 2×103 to 105 K).

Magnetohydrodynamic (MHD) generator

Plasma moving with velocity v perpendicular to a magnetic field B generates an electromotive force perpendicular to both the direction of plasma flow and the magnetic field. This dynamo effect can drive a current in an external circuit connected to electrodes in the plasma, producing electric power without the inefficiency of a thermal cycle.

According to the Lorentz force the positive ions move upward in the figure and the electrons move downward, charging the electrodes with a potential difference.

Historical comment - The principles of MHD have been known since the studies of Michael Faraday in the 1830s. However, the first attempts to construct a large MHD generator, made in 1938, were unsuccessful due to poor knowledge of the plasma properties. By 1959 the understanding and the technology progressed to the point that 10 kW of electric power was generated in an MHD device. A large central station power MHD generator, followed by a conventional steam generator, has the potential for using fuels more effectively, due to higher cycle efficiencies, with reduced heat loss to the environment. Unfortunately, extensive research programs were discontinued by 1970 in view of the large investment needed to solve the remaining technical problems.

Magnetoplasmadynamic (MPD) thruster

The inverse of the dynamo effect can be used to accelerate plasma for electric propulsion of space vehicles. In the motor effect electric energy is converted in mechanical energy by the action of perpendicular electric and magnetic fields.

Current density j is driven through the plasma by applying a voltage to the electrodes. The plasma is ejected with large exhaust velocity by the j × B force. The reaction force can be used to accelerate a craft in deep space.

Historical comment - Robert Hutchings Goddard informally expressed, in 1906, some of the concepts of electric propulsion, and Hermann Oberth devoted one chapter of his book "Wege zur Raumschiffahrt" (1929) to the subject. Some conceptual progress on electric propulsion was made from the mid 40's onward, when light-weight space electric power plants, based on nuclear fission and solar panels, became foreseeable. However, only after 1957 actual small scale experiments were conducted in government laboratories and many independent companies mainly in the USA. At that time it was realized that electric propulsion is not limited to the electrostatic or ion thrusters envisioned in the earlier years, but can be extended to electrothermal and electromagnetic systems. The first space tests of an electric thruster, involving an electrostatic ion engine, were made in mid 1964. These ion engines are now used for the control of satellites and primary propulsion of deep space probes. On the other hand, magnetoplasmadynamic thrusters provide a combination of high exhaust velocities with high mass flow, but up to this day have only been tested in the laboratory, due essentialy to the high powers involved and remaining technical problems. The early history of electric propulsion can be found in the book "Ion Propulsion for Space Flight", by Ernst Stuhlinger (1964).