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The abnormal glow discharge in mercury vapour and xenon
Classified abstracts 51 O-51 9 18. GASEOUS
ELECTRONICS
18 510. Effects of clumps and ion bombardment on electrical breakdown in vacuum. (GB) The formation of emitting sites on highly polished stainless steel and titanium electrodes in vacuum is investigated. It is shown that during the first application of an electric field between broad-area electrodes clumps damage the cathode surface. These clumps originate both from the cathode and the anode. If a spark-conditioned cathode with a field enhancement factor /3 = 40 is placed opposite a new anode, an electric field of 6.5 x lo6 V m-l is enough to damage the cathode, resulting in a field enhancement factor of 100. It is demonstrated that conditioning of electrodes by sparks results from the detachment of more or less loosely bound particles from the electrodes. For small electrode separations (d < 1 mm) clumps initiate a breakdown if the microscopic field strength fl V/d after the impact on the cathode is higher than a critical field strength F,,, which equals -7 x lo9 V m-l. The field enhancement factor fl can be decreased by ion bombardment, but the breakdown voltage is the same as after spark conditioning. It is shown that after conditioning by ion bombardment clumps are responsible for the onset of a spark. (Netherlands) G P Beukema, JPhys D: ApplPhys, 7 (12), 1974,1740-1755. 18 511. The positive column of low-pressure Hg/Ne and Hg/Ne/Ar discharges. (GB) The electron density at the axis, the electron temperature and the electric field strength of the positive column of low-pressure Hg/Ne discharges have been measured using electrostatic probes. The measurements have been carried out for discharge currents between 0.1 and 0.8 A, noble gas pressures between 0.3 and 10 torr, tube wall temperatures in the range of 13-70°C and an inner tube radius of 18 mm. The addition of argon to neon as a filling gas has also been studied. By computing the volume and wall losses from the measured data, the authors have worked out the effect of the discharge conditions on the radiation efficiency of the column. These data have heen compared with the results of a theoretical model of the positive column, showing a reasonable agreement. From this comparison it has been found that elastic collisions of electrons and of ions with neutral mercury atoms have a decisive influence on column properties at mercury pressures above - lo-* torr. Furthermore a higher radiation efficiency of the column has been found in Hg/Ne discharges compared to Hg/Ar discharges for 3 torr noble gas pressure, wall temperature 42°C and discharge current 0.4 A. (Netherlands) T G Verbeek and P C Drop, J Phys D: Appl Phys, 7 (12), 1974, 1677-1683.
18 513. Microscopic evolution of the ionizing collision frequency in Townsend avalanches. (GB) Information obtained from the analysis of the spectra of the Townsend avalanche amplification provides the basis for the study of average microscopic behaviour of the electrons. The electrical field E is unform, and the avalanches are triggered off at the cathode by a single primary electron. Experimental conditions are such that no secondary effects or space charge can take place. The results of these experiments (methane, 50 < E/p < 300 V cm-’ torr-‘) prove that the multiplication process is a non-markovian one, and also that it rapidly attains asymptotic behaviour. When interpreted in the light of the age-dependent branching processes, the results enable one to determine some distribution functions: the distribution g(0) of the time intervals 0 between two ionizing collisions, the multiplication coefficient a(r) characterizing the ionizing capacity of the electron as a function of its ‘age’ 7. U(T)describes the phenomenon more precisely than does the first Townsend coefficient. The application of this method when E/p and pd are constant (p represents gas pressure and d gap distance) shows that these distributions comply with the similarity principle. (France) G Vidal et al, J Phys D: Appl Phys, 7 (12), 1974, 16841698. 18 514. Unstable ionization waves in helium. (GB) A discussion is given of some dispersion curves of ionization waves in the helium positive column. A T Jackson, JPhys D: Appl Phys, 7 (12), 1974,1699-1702.
18 515. The abnormal glow discharge in mercury vapour and xenon. (GB) Measurements have heen made of the current-voltage characteristics of abnormal glow discharges in mercury vapour and xenon, both with a mercury cathode, over a pressure range from 0.5 to 15 mm Hg. Measurements have also been made of the cathode fall distance as a function of the voltage, but at low voltages only. A theoretical description is presented and is in good agreement with experiment. The main features of the theory are: at high current densities, the gas density increases towards the cathode as a result of momentumexchanging collisions between the positive ions and the gas molecules; the dominant secondary emission process is photoelectric emission by photons from the negative glow, except near the glow-arc transition where emission by excited atom bombardment makes a small but significant contribution; and only a small fraction of the photons produced in the negative glow reach the cathode, because of frequent reabsorption and subsequent ionization. A J T Holmes and J R Cozens, J Phys D: Appl Phys, 7 (12), 1974, 1723-1739. 18 516. Study of the characteristics of the ion beam extracted from an rf discharge in hydrogen under conditions of resonance. (USA) The characteristics of an ion beam issued from a hydrogen plasma have been studied. This plasma is obtained by means of a highfreauency discharge set under conditions of resonance. This resonance is obtained by inductively exciting the discharge in a transverse static magnetic field (perpendicular to the axis of the vessel). The resonance, of which we give the characteristics, permits a considerable increase in the ionization degree, and is therefore of interest as regards to ion sources. The beam of particles extracted from this discharge in resonance is studied by means of a magnetic analyser. The energy distributions of the various ions consist of two parts. A comparative analysis of the characteristics of the beam, with and without resonance, shows that this resonance improves the properties of the system. In particular, for the H1+ ion, the energy dispersion is reduced to 25 eV and its proportion in the beam reaches 85%. This resonance proves to be a convenient and very simple means to obtain an HI + ion beam. (France) B Grollean, J Appl Phys, 45 (8), 1974, 3385-3393. 18 517. Effect of electron deexcitation and self-absorption on the intensity of the Ha 2537-A radiation from Ha + Ar discharges - (ac fluorescent lamps). (USA) The intensity of the Hg 2573-A radiation from Hg + Ar discharges was measured as an independent function of mercury pressure (0.2-50 mtorr), ac current (5&2100 mA) and tube radius (0.79 cm and 1.27 cm) at a constant Ar pressure of - 4 torr. For various constant mercury pressures, the Hg 2537-A intensity initially rises linearly with increasing current, but then tends to bend over and approach an asymptotic limit. The nonlinear, asymptotic behaviour is due to electron deexcitation of the Hg 63P, state at the higher currents in the presence of Hg 2537-A self-absorption. The Hg 2537-A intensity was also measured as a function of mercury pressure at various constant currents. The intensity rises to a peak (which defines an optimum Hg pressure) and then decreases with further increase in mercury pressure due to the combination of self-absorption and electron deexcitation. for high ac currents, the optimum Hg pressure is independent of current but varies inversely with the tube diameter. All this behaviour is relevant to the problem of obtaining high efficiency from fluorescent lamps at high powers. T J Hammond and C F Gallo, Appl Opt, 13 (9), 1974,2164-2170. 18 518. Electron impact ionization of inert gases. (Canada)
Electron impact total ionization cross sections of neon, argon, krypton, and xenon have been calculated for the electron impact energy varying from threshold to 20 keV. The method of Khare and Padalia, modified to include approximately the effect of multiionization, has been employed. The results are in fair agreement with the experimental data. S P Ware et al, Gun J Phys, 52 (18), 1974, 1755-1758. 18 519. Determination of diffusion velocities in argon and helium plasmas. (USA) Some results of the investigation of plasma diffusion velocity in collisionless argon plasma and collision-dominated helium plasma in 175
ELECTRONICS
18 510. Effects of clumps and ion bombardment on electrical breakdown in vacuum. (GB) The formation of emitting sites on highly polished stainless steel and titanium electrodes in vacuum is investigated. It is shown that during the first application of an electric field between broad-area electrodes clumps damage the cathode surface. These clumps originate both from the cathode and the anode. If a spark-conditioned cathode with a field enhancement factor /3 = 40 is placed opposite a new anode, an electric field of 6.5 x lo6 V m-l is enough to damage the cathode, resulting in a field enhancement factor of 100. It is demonstrated that conditioning of electrodes by sparks results from the detachment of more or less loosely bound particles from the electrodes. For small electrode separations (d < 1 mm) clumps initiate a breakdown if the microscopic field strength fl V/d after the impact on the cathode is higher than a critical field strength F,,, which equals -7 x lo9 V m-l. The field enhancement factor fl can be decreased by ion bombardment, but the breakdown voltage is the same as after spark conditioning. It is shown that after conditioning by ion bombardment clumps are responsible for the onset of a spark. (Netherlands) G P Beukema, JPhys D: ApplPhys, 7 (12), 1974,1740-1755. 18 511. The positive column of low-pressure Hg/Ne and Hg/Ne/Ar discharges. (GB) The electron density at the axis, the electron temperature and the electric field strength of the positive column of low-pressure Hg/Ne discharges have been measured using electrostatic probes. The measurements have been carried out for discharge currents between 0.1 and 0.8 A, noble gas pressures between 0.3 and 10 torr, tube wall temperatures in the range of 13-70°C and an inner tube radius of 18 mm. The addition of argon to neon as a filling gas has also been studied. By computing the volume and wall losses from the measured data, the authors have worked out the effect of the discharge conditions on the radiation efficiency of the column. These data have heen compared with the results of a theoretical model of the positive column, showing a reasonable agreement. From this comparison it has been found that elastic collisions of electrons and of ions with neutral mercury atoms have a decisive influence on column properties at mercury pressures above - lo-* torr. Furthermore a higher radiation efficiency of the column has been found in Hg/Ne discharges compared to Hg/Ar discharges for 3 torr noble gas pressure, wall temperature 42°C and discharge current 0.4 A. (Netherlands) T G Verbeek and P C Drop, J Phys D: Appl Phys, 7 (12), 1974, 1677-1683.
18 513. Microscopic evolution of the ionizing collision frequency in Townsend avalanches. (GB) Information obtained from the analysis of the spectra of the Townsend avalanche amplification provides the basis for the study of average microscopic behaviour of the electrons. The electrical field E is unform, and the avalanches are triggered off at the cathode by a single primary electron. Experimental conditions are such that no secondary effects or space charge can take place. The results of these experiments (methane, 50 < E/p < 300 V cm-’ torr-‘) prove that the multiplication process is a non-markovian one, and also that it rapidly attains asymptotic behaviour. When interpreted in the light of the age-dependent branching processes, the results enable one to determine some distribution functions: the distribution g(0) of the time intervals 0 between two ionizing collisions, the multiplication coefficient a(r) characterizing the ionizing capacity of the electron as a function of its ‘age’ 7. U(T)describes the phenomenon more precisely than does the first Townsend coefficient. The application of this method when E/p and pd are constant (p represents gas pressure and d gap distance) shows that these distributions comply with the similarity principle. (France) G Vidal et al, J Phys D: Appl Phys, 7 (12), 1974, 16841698. 18 514. Unstable ionization waves in helium. (GB) A discussion is given of some dispersion curves of ionization waves in the helium positive column. A T Jackson, JPhys D: Appl Phys, 7 (12), 1974,1699-1702.
18 515. The abnormal glow discharge in mercury vapour and xenon. (GB) Measurements have heen made of the current-voltage characteristics of abnormal glow discharges in mercury vapour and xenon, both with a mercury cathode, over a pressure range from 0.5 to 15 mm Hg. Measurements have also been made of the cathode fall distance as a function of the voltage, but at low voltages only. A theoretical description is presented and is in good agreement with experiment. The main features of the theory are: at high current densities, the gas density increases towards the cathode as a result of momentumexchanging collisions between the positive ions and the gas molecules; the dominant secondary emission process is photoelectric emission by photons from the negative glow, except near the glow-arc transition where emission by excited atom bombardment makes a small but significant contribution; and only a small fraction of the photons produced in the negative glow reach the cathode, because of frequent reabsorption and subsequent ionization. A J T Holmes and J R Cozens, J Phys D: Appl Phys, 7 (12), 1974, 1723-1739. 18 516. Study of the characteristics of the ion beam extracted from an rf discharge in hydrogen under conditions of resonance. (USA) The characteristics of an ion beam issued from a hydrogen plasma have been studied. This plasma is obtained by means of a highfreauency discharge set under conditions of resonance. This resonance is obtained by inductively exciting the discharge in a transverse static magnetic field (perpendicular to the axis of the vessel). The resonance, of which we give the characteristics, permits a considerable increase in the ionization degree, and is therefore of interest as regards to ion sources. The beam of particles extracted from this discharge in resonance is studied by means of a magnetic analyser. The energy distributions of the various ions consist of two parts. A comparative analysis of the characteristics of the beam, with and without resonance, shows that this resonance improves the properties of the system. In particular, for the H1+ ion, the energy dispersion is reduced to 25 eV and its proportion in the beam reaches 85%. This resonance proves to be a convenient and very simple means to obtain an HI + ion beam. (France) B Grollean, J Appl Phys, 45 (8), 1974, 3385-3393. 18 517. Effect of electron deexcitation and self-absorption on the intensity of the Ha 2537-A radiation from Ha + Ar discharges - (ac fluorescent lamps). (USA) The intensity of the Hg 2573-A radiation from Hg + Ar discharges was measured as an independent function of mercury pressure (0.2-50 mtorr), ac current (5&2100 mA) and tube radius (0.79 cm and 1.27 cm) at a constant Ar pressure of - 4 torr. For various constant mercury pressures, the Hg 2537-A intensity initially rises linearly with increasing current, but then tends to bend over and approach an asymptotic limit. The nonlinear, asymptotic behaviour is due to electron deexcitation of the Hg 63P, state at the higher currents in the presence of Hg 2537-A self-absorption. The Hg 2537-A intensity was also measured as a function of mercury pressure at various constant currents. The intensity rises to a peak (which defines an optimum Hg pressure) and then decreases with further increase in mercury pressure due to the combination of self-absorption and electron deexcitation. for high ac currents, the optimum Hg pressure is independent of current but varies inversely with the tube diameter. All this behaviour is relevant to the problem of obtaining high efficiency from fluorescent lamps at high powers. T J Hammond and C F Gallo, Appl Opt, 13 (9), 1974,2164-2170. 18 518. Electron impact ionization of inert gases. (Canada)
Electron impact total ionization cross sections of neon, argon, krypton, and xenon have been calculated for the electron impact energy varying from threshold to 20 keV. The method of Khare and Padalia, modified to include approximately the effect of multiionization, has been employed. The results are in fair agreement with the experimental data. S P Ware et al, Gun J Phys, 52 (18), 1974, 1755-1758. 18 519. Determination of diffusion velocities in argon and helium plasmas. (USA) Some results of the investigation of plasma diffusion velocity in collisionless argon plasma and collision-dominated helium plasma in 175