Patent GB-931980-A (GB)
IPC: H01J9/38
Abstract: 931,980. Discharge tubes. J. H. O. HARRIES. May 5, 1960 [May 14, 1959 (2); May 19, 1959; June 8, 1959], Nos. 16561/59, 16562/59, 16902/59 and 19480/59. Class 39 (1). A method of evacuating the envelope of an electric discharge tube comprises the steps of heating an adsorbent substance in an appendix connected to and communicating with the interior of the envelope in the presence of a flow of carrier gas, filling the envelope and appendix while still hot with a different gas, e.g. CO 2 , passed through during or just after the heating and then sealing an orifice in the appendix to close the space within the orifice and appendix, cooling the appendix to cause the adsorbent substance to adsorb gas and so reduce the pressure within the envelope, and finally sealing-off the connection between appendix and envelope to detach the appendix from the envelope. In manufacture of the electric discharge tube a vacuum gauge as in Specification 474,845, or U.S.A. Specification 2,197,079, which may have two plates forming a cathode and a ring anode in a magnetic field, at one end thereof and an appendix at the other end thereof holding activated charcoal are baked in an oven at 350‹ C. and carbon dioxide is passed through during or after baking, the external tubes at the ends of the vacuum gauge and appendix are sealed off and the appendix is immersed in solid CO 2  for adsorption of gas. The tube between discharge tube and appendix is now sealed off and the appendix detached. The getters of Ba-Al and Ti as described in Specification 931,979 are now flashed in succession. The appendix is lowered into a refrigerant in steps, for example the solid CO 2  referred to above, to permit adsorption of gas during processing of the electrodes and other parts, e.g. by a servomechanism actuated by the vacuum gauge and an amplifier, or manually. The processing may include the heating of the cathode to break down its carbonate coating to oxide, CO 2  gas being adsorbed by further immersing the appendix in the refrigerant by means of the servomechanism, and the degassing of getters. The refrigerant may also be liquid N 2 , solid CO 2  in trichlorethylene or in acetone or liquid He. Alternatively, cooling coils with circulating cooling fluid may be in contact with the adsorbent. To keep parts of the adsorbent effectively apart baffles of lowheat conductivity are provided in the appendix or the appendix is in separate compartments. To bring parts of the absorbent effectively closer together a perforated tube is inserted within the appendix. Automatic and repetitive exhaust cycles are effected by successive automatic connection of hot and cold appendices to successive envelopes by valves. Automatic processes may be used for filling and sealing the appendices. The envelopes may be vacuum or gas-filled. The Provisional Specifications refer to an ion pump connected to the discharge tube and state that envelope, appendix and connecting tube may be of metal. Getter wires or strips can be used and may be of Ba. The first and second Provisional Specifications state that processing includes degassing of electrodes and in the first Provisional Specification, breakdown and 'ageing of the oxide cathode and further baking of the envelope. The CO 2  gas may be replaced by nitrogen (see also the fourth Provisional Specification) and the charcoal adsorbent by silica gel. The second Provisional Specification states that there may be more than one appendix refrigerated successively. Successive refrigerations may be employed automatically and in sequence with processes successively applied to the electrodes or other parts. The cathode is coated with Ba Sr carbonates (see also the third and fourth Provisional Specifications). The third and fourth Provisional Specifications state that the baffles or different parts of the appendix may be of glass or ceramic or rings of ceramic or other baffling material are put between successive portions of activated charcoal or the appendix has constrictions. Each part of the adsorbent material may be connected to the envelope by separate tubes or orifices, e.g. a glass tube connected to the envelope and positioned along the centre of the tubular appendix and has orifices in it at appropriate places so gases may reach the various parts of the adsorbent from the envelope to be evacuated, successive cooling of different portions of the adsorbent being effected by automatic means in sequence with processes successively applied to electrodes or other parts. There may be a continuous or discontinuous increase in the volume cooled. A continuous increase of adsorbent being cooled can vary in accordance with the rate of evolution of gas caused by various stages of processing. The rate at which the adsorbent is cooled is made proportional to the pressure of gas inside the envelope by the output from the vacuum gauge. The fourth Provisional Specification states that means to refrigerate different portions of the adsorbent are provided during exhaust so that they become operative one after the other. The absorbent may be in the vacuum envelope itself. The second getter to be fired may be of Zr instead of Ti. The envelopes may be used as containers for storage of materials and devices sensitive to contamination.