Patent GB-1373402-A (GB)
Title: Method and apparatus for producing a controlled
IPC: H01J17/00
Filed: 1971-09-16
Abstract: 1373402 Electron guns; lasers AVCO CORP 16 Sept 1971 [17 Sept 1970] 40171/71 Headings H1D and H1C A method of producing a spatially uniform controlled discharge (e.g. for lasers, MHD generators or accelerators, or ozone production) comprises (a) providing a gaseous working medium at such a pressure in a working region in a cavity having imperforate walls, as defined, that upon production of secondary electrons in the medium, the ambipolar and thermal diffusion rates are incapable of damping local increases in secondary electron density, (b) generating ionizing radiation (e.g. electrons, X-rays, u.v., a particles, light &c.) externally of the cavity, (c) and introducing that radiation into the cavity through one wall to produce a substantially spatially uniform density of secondary electrons by ionization, the one wall being impervious to gases, as defined, and pervious to the ionizing radiation and (d) providing a sustainer field for providing substantially uniformly a predetermined electron temperature effective to increase the average energy of the secondary electrons without increasing the predetermined electron density by self regenerative ionization, the electron temperature being such that the controlled discharge is produced substantially uniformly throughout the working region at a predetermined level, and effecting population inversion in the laser embodiment. "Imperforate walls" or "walls impervious to gases" includes an arrangement wherein a wall has one or more holes for admitting electrons but where gas diffusion is minimized. The external ionization extends over the cross-section of the working region and may include a scanned narrow electron beam as well as a broad area beam. Fig. 1, and Figs. 2, 3 (not shown) indicate a laser source and Fig. 4 (not shown) the enlarged electron source from which electrons are injected into the gas medium. In Fig. 1, gas flows through cavity 10 between electrodes 50, 52 and electron temperature control may be achieved by varying the voltage of the injected electrons from gun 25, which may also be formed by plasma or field emission, photoemission, u.v., electron or ion bombardment or photo-electric emission. Arcuate members 16, 17 provide smooth laminar flow and also included are mirror assemblies 21, 22, anode 52 (e.g. Au), stainless steel gun enclosure 26, spaced thermionic filaments 27 supported by electrically conductive stand-offs 29 in insulator plate 28 (also Fig. 4, not shown), supports and pulse circuit connectors 33, 34, reticulated screen 35, higher potential stainless plate 45, conductive or insulative diaphragm 47 (Al, Be, Ti or C) permitting pressure differentials and cooled by gas flow or conduction and insulated wire mesh cathode 50 preventing spurious arc damage to diaphragm 47. The beam may be defocused to cover a broad area or may be a beam rapidly scanned and may be from a plurality of electron beam welding type guns. The sustainer field may be D.C., r.f., and electrodes capacitative or inductive. Pressures of less than 1 Á and up to 1 atmos. or more are mentioned and an N 2 , CO 2 , He mix (3 : 2 : 1) or combinations of CO, H 2 O, SO 2 , HCN, NO, O 2 , H 2 , Ar, NO 2 , N 2 O, HF. Diaphragm 47 may be replaced by small holes in a plurality of plates defining differentially pumped chambers, plate voltages ensuring optimum focusing. Laser gas flow may be up to Mach 1. Beam uniformity may be tested by a fluorescent sodium salicylate coated plastic wall. Mirror details are given, and a 10À6 Á filter and Ge infra-red detector included. The electron beam pulse length may be varied to create a CW or pulsed laser. For ozone production, the working medium may be air of pure oxygen in apparatus as in Figs. 1 to 3, but without mirrors. MHD generation using the gas is also discussed, with a beam of electrons injected at a suitable angle to the B field direction. The apparatus may constitute the means for increasing or providing the required electrical conductivity. A jet of He may cool the diaphragm or foil. Discharge mechanisms are discussed in detail and many quantitative parameters disclosed.