Development of Plasma Electron Guns to Produce Narrow Focusing Beams Under the Higher Operation Pressure

Summary form only given. One of the main advantages of the plasma cathode electron guns is its ability to produce electron beam under the higher operation pressure up to fore-pump pressure range. Present work is devoted to plasma electron guns that were designed especially to generate narrow focusing electron beams under the high pressure. To produce narrow focusing beam two contradictory requirements should be satisfied. From the one hand the plasma emission area must be as small as possible to realize beam focusing and to prevent plasma penetration from discharge gap to accelerating one. From the other hand to reach necessary total beam current the emission area has to be increased. One of the reasonable ways to solve the problem is to enhance plasma density just near plasma surface in emission area. Two different versions of the plasma guns were investigated. First one has the following electrode configuration: hollow cathode, flat anode with emission hole and accelerating electrode with special configuration. Recently this electrode configuration was already introduced for generation of cylindrical electron beam. Experiments showed strong beam current dependence on discharge gap geometry such as hollow cathode diameter and distance between cathode and anode. Under the 50 mtorr gas pressure electron beam current of 0.1 A DC with energy up to 12 keV were reached. Under the lower pressure of 30 mtorr it was possible to increase high voltage up to 17 kV. With magnet focusing system applied the beam diameter, measured by rotating probe at 20 cm distance far from extractor, was as small as 1 mm. Another version was based on reflected (PIG) discharge with hollow cathode. This gun was used in pulsed mode of operation (100 mus, 2-500 pps). With acceleration voltage of 50 kV it produces 150 mA of the beam current, with the beam diameter of smaller than 1 mm under 0.1-1 torr gas pressure in work chamber (after two gas-dynamic windows). Main application fields for such pla- ma electron guns are surface treatments including electron beam heating, welding, cladding, evaporations as well as plasma chemistry