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Characteristics of a 600 W convective flow CO laser
Classified abstracts 3615-3622 tirst detectable light (second positive group of the N1 molecule) is emitted with sufficient intensity just after the critical amplification. This type of radiation continues till the start of the heating of the plasma in the pmdischarge channel; its intensity grows by a factor IO’ during this time. It allows one to determine the electron density, which grows from 10” cm-’ up to 1Or6 cm-‘, and to calculate the current in the predischarge channel. In this stage the electron temperature remains 24 eV. At about 170 ns after the start of the avalanche the dissociation of the molecules and the ionization of the atoms takes place near x,,~, in times of a few ns. This causes a rapid increase of the current and the emission of a strong continuum intensity as well as the emission of atom and ion lines. Now the predischarge channel transforms into a plasma channel at a current of about 10A. This rapid growth of the current in the thin channel produces a high temperature and a high pressure and as a further consequence an expansion of the plasma channel. Applying plasma diagnostic,method in the spectral range of 300&8600 A the temperatures and the electron densities as function of time and radius of the channel can be derived in the plasma channel. At early times of the temporal development the temperature in the axis of the channel reaches values between 5 x 1O4-6 x lo4 K. It decreases after 1.5 e to values of about 3 x lo4 K because of the radial expansion of the fully ionized plasma channel and the lack of further energy supply. Electron densities up to lOi cm-’ are registered at the boundary of the plasma channel. The agreement of the temperatures derived from different lines indicates the existence of local thermodynamic equilibrium (LTE) which is established under the given experimental conditions in nitrogen 200 ns after about IO2 primary electrons have been released from the cathode. Similar measurements have been performed to determine the radial temperature distribution in the hot stage of an argon plasma. (Germany) P Stritxke et al, J Phys D: Appl Phys, 10 (I 6), 1977, 2285-2300.
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3615. Atomic physics algorithms for plasmas undergoing transient ionization and recombination. (GB) Algorithms are presented for solving the reduced rate equations describing the population densities of different ion species in onizing and recombining plasmas. Computationally, these models are efficient and can serve as an extension to numerical MHD models of plasmas. Their particular relevance to studies of plasmas produced by short laser pulses is illustrated by results from calculations on carbon plasmas. Comparisons are made between calculations using either a steady-state model for the ion population densities or the timedependent models presented. J Magill, J Phys D: Appl Phys, 10 (I 6), 1977,2257-2268. I2 3616. Negative glow and positive column discharges for helium/metalvapour lasers-a comparison. (GB) The comparative abilities of the negative glow and positive column regions of one and the same helium/zinc discharge to tolerate an increasing density of the metal vapour have been investigated. Spatially resolved profiles of sidelight emission from particular upper laser levels of Zn II were recorded over a wide range of discharge conditions. Evidence that the helium ion production rate can be maintained in the negative glow at pressures of zinc vapour as high as 0.054.5 torr is the undiminished intensity of Zn II levels known to be populated by heavy-body collision processes. The same transitions, excited in the positive column, show a rapid decline in their intensity at zinc vapour pressures above 0.005 torr. The difference in behaviour is ascribed to the existence, independent of metal vapour density, of a significant tail of high energy electrons in the negative glow electron energy distribution. The factors which determine the optimum partial pressure in the two cases are compared. P Gill and C E Webb, J Phys D: Appl Phys, 10 (I 6). 1977,2235-2244. I2 3617. Energy dissipation in the cathode of a vacuum arc. (GB) Measurements have been performed to determine the heat flow to the cathode during arcing in vacuum. It is shown that for a range of metals the heat flow is proportional to both current and arcing time. Depending on the cathode metal the losses vary from 25 to 34% of the total arc input. A relation between cathode heat loss and arc voltage is established. The experimental results are compared with theoretical calculations as found in literature. (The Netherlands) J E Daalder, J Phys D: Appl Phys, 10 (16). I977,2225-2234.
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3618. Theory of electrostatic stagnation point probes in a low-dens~~ continuum plasma: II. The effects of the Knudsen number. (GB) The continuum stagnation point electrostatic probe theory by Kodera and Chang has been extended for a finite-charged-particle mean free path under continuum neutral flow conditions, i.e. those in which the gas particle mean free path < sphere radius and charged particle mean free path 5 sphere radius < Debye length. The numerical results show that: (1) an effective Knudsen number (for charged particle) smaller than 10m2 is required for continuum conditions; (2) the influence of effective Knudsen number increases with diffusion Reynolds number; (3) the influence of effective Knudsen number increases with retarding surface potentials and decreases with increasing attracting surface potential; (4) the usual retarding potential method for temperature determination will lead to an increasing overestimate of 7’. as the-effective Knudsen number decreases; (5) the effect of effective Knudsen number is to smooth the ‘knee’ of the probe characteristic and to render more imprecise the determination of space potential. (France) Jen-Shih Chang and K Kodera, J Phys D: Appl Phys, 10 (l6), 1977. 2207-22 12. 12 3619. Poisoning of LaB6 cathodes. (GB) This paper describes an investigation of the thermionic emission decay due to chemical interaction between lanthanum hexaboride and the residual gases in a vacuum system. The results of the thermochemical analysis are in good agreement with the experimental data on poisoning. It is shown that oxygen poisoning is possible only at a low cathode temperature (5 1300°C) and relatively high gas pressure, but the LaBe cathode is much more sensitive to carbon poisoning. The carbon poisoning takes place at cathode temperatures up to 1500°C and at low gas pressures (lo-’ torr) if the residual atmosphere contains heavy hydrocarbons or vapours of the organic solvents. It is found that carbon poisoning may be offset by increase of the partial pressure of carbon dioxide. (USSR) A A Avdienko and M D Malev, 27 (IO/l I), 1977,583-588. 3620. A novel electrode geometry for high-current vacuum arcs. (G;;) A new electrode geometry, the stirrup, utilizes its own magnetic field to maintain a diffuse high-current vacuum arc at the centre of the electrode system. This arc is reproducible and stable, and the voltage is hash-free. High-speed photographs of a high-current vacuum arc are shown, together with arc voltage and current oscillograms and characteristics. A modification of the electrode is described giving hash-free arc voltages for peak currents up to at least 23 kA peak. Results for arcs between disc electrodes under similar conditions are also shown for comparison. MRBarraultetal,JPhys D: ApplPhys, 10(16), 1977,L213-218. 3621. Spark channel formation in helium-neon mixtures. (GB) An image converter is used to study the development of breakdown in a uniform field produced between two 5 cm plane electrodes. Discharges are recorded in helium (25x)-neon (75%) mixtures for pressures of several hundred torr and for large electric-field-topressure ratio values (25-40 V cm-’ torr- I). The discharge is simultaneously photographed with streak and still cameras. The formation and propagation of the ionization fronts are seen to move towards the cathode and the anode. Special attention is paid to the secondary avalanches that originate from the cathode and move towards the cathode ionizing wave. The densitometry of recordings gives rise to the characterization of the isodensity lines through which we obtain a detailed analysis of the spatio-temporal growth of light emission from the discharge. The formation of a strong anodic spot localized in time is demonstrated as the ionizing front reaches the anode. (France) M Bayle et al,JPhys D: ApplPhys. 10 (16) 1977, 2181-2187. I2 3622. Characteristics of a 600 W convective flow CO laser. (USA) Operating characteristics of a moderate power, convective flow, longitudinal discharge CO laser are presented. Incorporation of a novel electrode geometry has resulted in a highly stable and reliable device, useful as a laboratory tool. D G Lim et al, Rev scient Instrum, 48 (I I), 1977.1430-1433. 253
.IL.
3615. Atomic physics algorithms for plasmas undergoing transient ionization and recombination. (GB) Algorithms are presented for solving the reduced rate equations describing the population densities of different ion species in onizing and recombining plasmas. Computationally, these models are efficient and can serve as an extension to numerical MHD models of plasmas. Their particular relevance to studies of plasmas produced by short laser pulses is illustrated by results from calculations on carbon plasmas. Comparisons are made between calculations using either a steady-state model for the ion population densities or the timedependent models presented. J Magill, J Phys D: Appl Phys, 10 (I 6), 1977,2257-2268. I2 3616. Negative glow and positive column discharges for helium/metalvapour lasers-a comparison. (GB) The comparative abilities of the negative glow and positive column regions of one and the same helium/zinc discharge to tolerate an increasing density of the metal vapour have been investigated. Spatially resolved profiles of sidelight emission from particular upper laser levels of Zn II were recorded over a wide range of discharge conditions. Evidence that the helium ion production rate can be maintained in the negative glow at pressures of zinc vapour as high as 0.054.5 torr is the undiminished intensity of Zn II levels known to be populated by heavy-body collision processes. The same transitions, excited in the positive column, show a rapid decline in their intensity at zinc vapour pressures above 0.005 torr. The difference in behaviour is ascribed to the existence, independent of metal vapour density, of a significant tail of high energy electrons in the negative glow electron energy distribution. The factors which determine the optimum partial pressure in the two cases are compared. P Gill and C E Webb, J Phys D: Appl Phys, 10 (I 6). 1977,2235-2244. I2 3617. Energy dissipation in the cathode of a vacuum arc. (GB) Measurements have been performed to determine the heat flow to the cathode during arcing in vacuum. It is shown that for a range of metals the heat flow is proportional to both current and arcing time. Depending on the cathode metal the losses vary from 25 to 34% of the total arc input. A relation between cathode heat loss and arc voltage is established. The experimental results are compared with theoretical calculations as found in literature. (The Netherlands) J E Daalder, J Phys D: Appl Phys, 10 (16). I977,2225-2234.
.”
3618. Theory of electrostatic stagnation point probes in a low-dens~~ continuum plasma: II. The effects of the Knudsen number. (GB) The continuum stagnation point electrostatic probe theory by Kodera and Chang has been extended for a finite-charged-particle mean free path under continuum neutral flow conditions, i.e. those in which the gas particle mean free path < sphere radius and charged particle mean free path 5 sphere radius < Debye length. The numerical results show that: (1) an effective Knudsen number (for charged particle) smaller than 10m2 is required for continuum conditions; (2) the influence of effective Knudsen number increases with diffusion Reynolds number; (3) the influence of effective Knudsen number increases with retarding surface potentials and decreases with increasing attracting surface potential; (4) the usual retarding potential method for temperature determination will lead to an increasing overestimate of 7’. as the-effective Knudsen number decreases; (5) the effect of effective Knudsen number is to smooth the ‘knee’ of the probe characteristic and to render more imprecise the determination of space potential. (France) Jen-Shih Chang and K Kodera, J Phys D: Appl Phys, 10 (l6), 1977. 2207-22 12. 12 3619. Poisoning of LaB6 cathodes. (GB) This paper describes an investigation of the thermionic emission decay due to chemical interaction between lanthanum hexaboride and the residual gases in a vacuum system. The results of the thermochemical analysis are in good agreement with the experimental data on poisoning. It is shown that oxygen poisoning is possible only at a low cathode temperature (5 1300°C) and relatively high gas pressure, but the LaBe cathode is much more sensitive to carbon poisoning. The carbon poisoning takes place at cathode temperatures up to 1500°C and at low gas pressures (lo-’ torr) if the residual atmosphere contains heavy hydrocarbons or vapours of the organic solvents. It is found that carbon poisoning may be offset by increase of the partial pressure of carbon dioxide. (USSR) A A Avdienko and M D Malev, 27 (IO/l I), 1977,583-588. 3620. A novel electrode geometry for high-current vacuum arcs. (G;;) A new electrode geometry, the stirrup, utilizes its own magnetic field to maintain a diffuse high-current vacuum arc at the centre of the electrode system. This arc is reproducible and stable, and the voltage is hash-free. High-speed photographs of a high-current vacuum arc are shown, together with arc voltage and current oscillograms and characteristics. A modification of the electrode is described giving hash-free arc voltages for peak currents up to at least 23 kA peak. Results for arcs between disc electrodes under similar conditions are also shown for comparison. MRBarraultetal,JPhys D: ApplPhys, 10(16), 1977,L213-218. 3621. Spark channel formation in helium-neon mixtures. (GB) An image converter is used to study the development of breakdown in a uniform field produced between two 5 cm plane electrodes. Discharges are recorded in helium (25x)-neon (75%) mixtures for pressures of several hundred torr and for large electric-field-topressure ratio values (25-40 V cm-’ torr- I). The discharge is simultaneously photographed with streak and still cameras. The formation and propagation of the ionization fronts are seen to move towards the cathode and the anode. Special attention is paid to the secondary avalanches that originate from the cathode and move towards the cathode ionizing wave. The densitometry of recordings gives rise to the characterization of the isodensity lines through which we obtain a detailed analysis of the spatio-temporal growth of light emission from the discharge. The formation of a strong anodic spot localized in time is demonstrated as the ionizing front reaches the anode. (France) M Bayle et al,JPhys D: ApplPhys. 10 (16) 1977, 2181-2187. I2 3622. Characteristics of a 600 W convective flow CO laser. (USA) Operating characteristics of a moderate power, convective flow, longitudinal discharge CO laser are presented. Incorporation of a novel electrode geometry has resulted in a highly stable and reliable device, useful as a laboratory tool. D G Lim et al, Rev scient Instrum, 48 (I I), 1977.1430-1433. 253