Classified abstracta 3714-3722

work was restricted to only one initial pressure, and in this paper they extend the examined pressure range to obtain some general con- clusions on the pressure dependence of the spark behaviour. (Germany) H Hess et al, J Phys D: Appl Phys, 11(3), 1978,255-260.

12 3714. The axial electric field of a positive column in a transverse magnetic Beld. (GB) The axial electric field of a positive column with a transverse magnetic field of O-300 G is given for neon at a gas pressure of 0.3-10.0 torr with a discharge current of 40 mA. Measurements show that the axial electric field increases considerably with a transverse magnetic field in lower gas pressure. These measurements are compared with a theory which takes electron loss at the wall into account and found to be in good agreement. (Japan) Teruo Kaneda, J Phys D: Appl Phys, 11 (3), 1978,279-282.

12 3715. Relaxation process of the electron velocity distribution in neon. (GB) The relaxation process from an initial velocity distribution to the equilibrium distribution for electrons in neon is calculated by a finite difference method for the ratios of electric field to gas number density E/N between 56.6 and 566 Td (E/p0 = 20 and 200 V cm-l torr-’ at 0°C) without using the usual two-term spherical harmonics expansion of the velocity distribution. The pulsed Townsend (FT) condition, in which the evolution of all the electrons involved in an avalanche is observed as a function of time only, is assumed. The results suggest that the electron velocity distribution reaches through randomization the equilibrium distribution which has a structure with a minimu&near theorigin in the velocity space. At E/N = 566 Td, the shade of the eauilibrium distribution is found to be auite different from tie shape of‘ the distribution obtained by the two-term expan- sion. Despite this difference, the electron swarm parameters such as the ionization frequency and the drift velocity for the PT obtained by the two-term expansion are found to agree very well (within about 2%) with those obtained by the present finite difference method. At lower E/N, the agreement is better. The electron diffusion coefficient deduced by the two-term expansion is found to agree with an average of the diffusion coefficients parallel and perpendicular to the field deduced by the finite difference method. (Japan) K Kitamori et al, J Phys D: Appl Phys, 11 (3), 1978,283-292.

12 3716. Langmuir probe measurements in the negative glow of a sputtering dc discharge. (GB) Radial and axial profiles of the density n, and temperature T. of the slow electron group have been studied in the negative glow of an abnormal discharge in a dc diode sputtering system, by Langmuir probes. The variations of n, and T. have been also studied as a function of discharge current and discharge pressure. The contamina- tion of the probes has been examined. (France) G Lempt%&re et al, J Phys D: Appl Phys, 11 (3), 1978, 293-300 (in French).

12 3717. Helium singlet and triplet metastable number densities in hollow- cathode/metal-vapour lasers. (GB) Number densities of helium singlet and triplet metastable states in the hollow cathode discharge appropriate to metal vapour lasers have been measured; in particular their dependence on discharge current and pressure, cathode fall potential and cadmium partial pressure has been explored. Singlet and triplet densities behave differently with discharge parameters; whereas the former saturate with both current and pressure, the latter increase linearly with discharge current and follow the distinctive behaviour exhibited by the cathode fall potential as a function of pressure. Radial profiles of the metastable densities are also reported. A I McIntosh et al, J Phys D: Appf Phys, 11 (3), 1978, 301-311.

12 3718. Laser heterodyne measurement of electron densities in a hollow cathode discharge. (GB) A laser heterodyne system has been developed to measure electron densities in a hollow-cathode discharge as used in metal vapour lasers. The beat frequency between two helium-neon lasers, one of which contains the discharge under investigation within its cavity, is monitored in real time as the discharge is rapidly switched off. The beat frequency changes because of refractive index changes in the plasma. By comparing the beat frequency changes at two wavelengths

(0.6328 and I .15 pm). electron density and gas density changes have been determined. Electron densities have been measured in a cylindrical hollow cathode discharge (6 mm dia, IO cm long) in helium gas over a range of filling pressures (1-15 torr) and discharge currents (10-150 mA). Electron density is found to behave linearly with discharge current, and to follow the cathode fall potential with pressure. Results obtained by the heterodyne method are compared with those obtained by Stark broadening techniques. The high electron number densities (1013-10’4 cm-3) measured confirm the superiority of the hollow cathode over the positive column as a medium for Duffendack pumping. 1 K Bela1 and M H Dunn, J Phys D: Appl Phys, 11(3), 1978,313-323.

3719. Electron drift velocities in mercury, sodium and thallium vapo:: I. Experimental. (GB) A heat-pipe drift tube has been developed for the first time and has been used to measure electron drift velocities in mercury, sodium and thallium vapour. The drift velocity in mercury has been measured as a function of vapour pressure for 7 x lo-” c E/N c 1.4 x lo-l6 (units of V cm2) and shows good agreement with McCutchen. The drift velocities in sodium and thallium have been measured for the first time for 3 x 10-l’ < E/N < 5 x 1O-‘6 and 1.7 x 10-l’ -Z E/N < 3.9 x 10-16, respectively. The sodium velocities show a pressure dependency attributed to the presence of dimers. Y Nakamura and J Lucas, J Phys D: ApplPhys, 11(3), 1978,325-335.

12 3720. Electron drift velocity and momentum cross-section in mercury, sodium and thallium vapours: II. Theoretical. (GB) Theoretical values for the electron drift velocity have been calculated in mercurv. sodium and thallium for 7 x 1O-‘8 I E/N 5 5 x IO-l8 V cm2 usfig Boltzmann’s equation. The momentum’cross-section for electron-atom collisions has been obtained as a function of electron energy by using an iterative procedure which produces a good agreement between the calculated and experimental values of the drift velocity. Other electron swarm parameters have been calculated as a function of E/N: namely, electron mean energy, the ratio of radial diffusion coefficient to mobility and the fraction of swarm energy loss to inelastic collisions. Y Nakamura and J LUCBS, J Phys D: Appl Phys, 11(3), 1978,337-345.

I2 3721. The voltage, self-generated magnetic field and current distribution in a high-current vacuum arc. (GB) Measurements of the anode-cathode voltage’ of a high-current (>2 kA) vacuum arc are presented, attention being restricted to conditions which result in a smoothly varying voltage. It is found that under these conditions the voltage-current characteristics can be divided into three regions. namely linear, non-linear and irreproduc- ible. The current distribution within the arc is derived from magnetic probe measurements, attention being further restricted to repro- ducible and axisynunetric conditions. Annular current distributions are found and a simplified theory of the current conduction process is based on an idealized annular current distribution. This theory leads to an expression for the arc voltage which is in reasonable agreement with the voltage measurements in the linear region. J C Sherman et al, ./ Phys D: Appl Phys, 11(3), 1978,379-388.

II. Vacuum apparatus and auxiliaries

21. PUMPS AND PUMP FLUIDS 21

3722. Gas cooled rotary force pumps for high pressure differences. (France) At first the principle of the gas cooled roots pumps is explained. Then characteristics of pumping speed of one and two stage pumps are shown and it is referred to the calculation of the pumping speed as well as to the mathematical realization of the limits of possible temperatures, thus proving the safe control of operating temperatures at any pressure. Some applications of the pumps are mentioned. (Germany) H Lang, Vi& 32 (187). 1977,60-63 (in French).

375