Patent GB-838846-A (GB)
Title: Improved method of and apparatus for heating materials by electric discharges
Inventor: SMITH HUGH R
IPC: H01J37/305
Filed: 1956-11-22
Abstract: 838,846. Discharge apparatus. STAUFFER CHEMICAL CO. Nov. 22, 1956, No. 35682/56. Class 39(1). [Also in Groups XI and XXII] In heating materials in vacuo by electric discharges, especially in melting powders, sponges or granules e.g. metals such as Zr, Mo, Ti, Co and Ni, an electric field is directed from a thermionic cathode 15 upon the material 41 to be melted in a metal container 7 as anode all within a casing 5 which is continually evacuated by a pump 11 to a pressure of about 1 micron of Hg so as to ensure that the space charge in the electric field limiting the flow of electrons to the material is partially neutralised by the presence of positive ions. Gas such as argon or nitrogen is supplied under control of a valve 35 through a pipe 31 to the casing and a glow discharge is established, the current being a direct function of the gas pressure. The material 41 to be melted is supplied through a ceramic pipe 27. The thermionic cathode 15 is a rod or coil or convoluted wire of tungsten heated directly by passing current therethrough through leads 17 or indirectly by radiation, bombardment or conduction. The electrons are focussed on the anode receptacle 7 by the conformation of the cathode 15 and by a barrier 19 surrounding a major portion of the electric field and forming a converging electron lens to focus the electron beam on to the internal area of the receptacle. The metal barrier 19 is in the form of an inserted cup and is either connected by a wire 21 to the cathode to maintain it at the potential of the latter or is maintained at an adjustable potential of the latter or is maintained at an adjustable potential negative to the cathode to focus the electrons. The barrier may be adjusted by varying its geometry. When the electron beam is established, the copper container 7 and metal casing 5 are given a positive potential of about 200 volts from a D.C. generator 45, and inert gas is then admitted within the barrier and maintained at a pressure of ¢ - 1 Á of Hg, being regulated so that a glow discharge is established between the anode and cathode. The gas pressure within the barrier 19 is kept higher e.g. ten times higher than that in the casing 5 outside the barrier, so that the glow discharge does not spread to the wall of the casing and does not degenerate into an arc. The positive ions formed within the barrier bombard the cathode 15 so that current supply thereto is reduced or discontinued after a time. These ions also bombard the barrier 19, which is however water-cooled by a pipe 23. The anode comprises a metal stub or rod 39 inserted from below into the cup 7 so that molten metal collects on the rod 39, which is continuously lowered as additional metal 41 is supplied. The copper cup 7 is water-cooled by a pipe 9 so that the molten metal adjacent the cup is solidified and prevented from adhering to the cup. The rod 39 lowered through a vacuum seal 37 is cut off into lengths. In Fig. 2, the barrier 19 is open centrally at the top and has a small inward flange 53. Gas and material to be heated are fed through this opening and pass through a looped cathode 15 near the wall of the barrier 19, which need not be water-cooled and is supported by a bracket from the container 5. Owing to the top opening in the barrier, the difference of gas pressure within it as compared with that outside the barrier in the container 5 is much reduced, and a higher voltage is maintained between the cathode and anode to ensure the presence of sufficient gas ions to form a glow discharge. It is possible to operate the apparatus with a purely thermionic discharge by cutting off the gas entirely, the discharge being directed by inwardly sloping walls of the barrier on to the melt. By reducing the cooling of the copper container 7, the molten metal solidifies only adjacent the container and a discharge port is formed in the metal itself so that it can be withdrawn as a liquid for casing in other than ingot form. The metal may be melted continuously or in batches and may be evaporated.