36 a comparator unit 37 senses the amount of grid current being drawn by grid 18 compares it to a reference amount and regulates cathode supply 23 in a manner to stabilize the plasma generated by the gun for example after the plasma is fully generated across the entire length of electrodes 22 excessive grid currents would otherwise occur unless the current provided by cathode supply unit 23 between lines 24 and 26 was considerably reduced in accordance with well known vacuum tube theory grid 18 is maintained at substantially the same potential as the cathode in order to control and regulate the flow of electrons to the anode while the electrodes may be of any number of which three is a lower limit typically one would have a larger number i e from 20 to 100 or more but the number may be less or more depending upon the application the thickness of the individual electrodes is not particularly critical although it is not expected that a thickness less than 1 1000 of an inch would be practical as the thickness increases the useful lifetime of operation of the electrode also increases until the thickness reaches a point at which erosion of the material from vaporization of the structure of the electrode causes preferential and undesired emission from the projections formed from the material remaining after erosion thus the lifetime cannot be extended beyond a practical limit directly related to about 2 3 millimeters thickness accordingly a range of thicknesses of the electrodes of the order of 1 1000 of an inch to 2 millimeters typical electrode materials are molybdenum tungsten.