Manfred A. Biondi

Mechanism of dc Electrical Breakdown between Extended Electrodes in Vacuum

A description is given of vacuum breakdown between extended copper electrodes in dc electric fields in terms of the relevant atomic‐collision processes. The theory is based on a model involving avalanche amplification of current in electrode vapor generated by the evaporation of an anode macroparticle during its transit to the cathode. Calculations are presented of the dynamics, heating, and evaporation of the macroparticle leading to the production of the vapor medium in the interelectrode gap. The inferred copper vapor density distribution accounts quantitatively for the absorption of resonance radiation measured just prior to current amplification in the gap. Calculations of electron avalanche multiplication in the vapor lead to predictions of breakdown conditions in agreement with our observations. The predicted size of the anode macroparticle which initiates breakdown is, on the average, of the order of 1 μ in diameter.

Vacuum Electrical Breakdown between Plane-Parallel Copper Electrodes

Measurements have been made of prebreakdown currents between thoroughly outgassed plane-parallel copper electrodes in ultrahigh vacuum, for electrode separations in the range 0.03 cm to 0.2 cm. These currents are found to be in good agreement with the Fowler-Nordheim theory of field emission from the pure metal surface. From combined measurements of prebreakdown current and breakdown voltage at different electrode separations it is deduced that the cathode microscopic field at breakdown is constant and of magnitude (6±1)×107 V/cm.Spectroscopic measurements of both resonance line absorption and line fluorescence have revealed that the density of neutral copper vapor present in the interelectrode space during the application of electric fields only fractionally (<1%) less than the breakdown field is considerably less than that necessary for volume ionization to occur. For the same experimental conditions, infrared radiation observations have revealed no hot spots on the anode surface.From the measurements i...

Measurement of the Electron Density in Ionized Gases by Microwave Techniques

Methods are described in which microwave techniques are used to measure the electron density during the period following a discharge. By measuring the detuning of a resonant cavity containing electrons, it is possible to measure electron densities between 1010 and 106 electrons/cc in gases whose pressure may be varied over a wide range. Design criteria are given to permit proper operation of the method under the conditions encountered in a given experiment.

The Effect of Electrode Temperature on Vacuum Electrical Breakdown between Plane‐Parallel Copper Electrodes

Combined measurements of current‐voltage, prebreakdown characteristics, and breakdown voltage have been made for plane‐parallel, copper electrodes in ultrahigh vacuum as a function of both cathode initial temperature and anode initial temperature. These measurements have been carried out for an electrode separation of 0.1 cm over the temperature range from 313° to 913°K. From the experimental data the temperatures of the hottest point of the cathode surface, (TC)s, and of the anode surface, (TA)s, at breakdown have been determined as a function of electrode initial temperature. Over the whole range of the investigation it is found that (TA)s is greater than (TC)s. Moreover, the calculated values of (TA)s at breakdown are essentially independent of anode initial temperature and have a mean value of (1100±150)°K. In contrast, the values of (TC)s at breakdown increase with increasing values of cathode initial temperature. The results suggest that a thermal instability of a point on the anode surface leads to...

Detection of Electrode Vapor Between Plane Parallel Copper Electrodes Prior to Current Amplification and Breakdown in Vacuum

Simultaneous, time‐resolved measurements of current growth and resonance line absorption have been made for the time intervals 0.2 and 0.8 μsec, respectively, preceding breakdown between plane‐parallel copper electrodes in vacuum. These measurements have been carried out for an electrode separation of 0.1 cm and residual pressures in the low 10−9‐Torr range. The experimental data show that neutral copper vapor is present in the interelectrode volume before the current increases sufficiently to produce breakdown. It is found that the vapor is generated during times of the order of some microseconds prior to breakdown, and is highly localized to the region of the subsequent spark channel. Further, the vapor density decreases from cathode to anode along the path of the subsequent spark channel. The results are consistent with a model for vacuum breakdown proposed recently in which the transient production of vapor immediately prior to breakdown occurs by the evaporation of an anode macroparticle during its t...

High‐Speed Nonrefrigerated Isolation Traps for Ultra High‐Vacuum Systems

A design is given for a nonrefrigerated isolation trap for use in ultra high-vacuum systems. The trap uses either artificial zeolite or activated alumina to retard migration of impurities and hold impurity molecules on contact. With zeolite the trap operated under ~3 x 10/sup -10/ mm Hg for 75 days; using alumina the trap remained effective for 100 days at < 1 x 10/sup -10/ mm Hg. (C.J.G.)

Atomic Collision Processes Occurring in Gas Discharges

Some atomic collision processes which influence the emission of radiation from gas discharge sources are discussed briefly The processes are divided into the following categories (1) Excitation of an atom by electron impact, (a) direct (from the ground state) and (b) indirect (from an excited state); (2) Collisions between excited atoms; (3) Transfer of excitation, and (4) Electron-ion recombination The relative importance of these reactions is discussed and possible consequences with regard to spectrochemical analyses are considered.

Microwave and optical techniques for gas discharge studies

SummarySome recently developed microwave and optical techniques for studying fundamental processes in gas discharges are described. Microwave apparatus which measures the change of resonant frequency andQ of a cavity containing a gas discharge is used to determine the concentration of electrons and their rate of collision with gas atoms in the discharge. Time sampling techniques are used to reduce the statistical fluctuations in data obtained in very low intensity optical emission studies. These techniques have been employed in direct recording spectrographic and interferometric equipments which require orders of magnitude less “exposure time” than conventional photographic recording methods. Time sampling has also been applied to optical absorption techniques used to study metastable atoms, with a hundred-fold increase in detection sensitivity over previous methods. The use of these methods is illustrated by a brief discussion of studies of electron-ion recombination, conversion of singlet metastable atoms to triplet metastables by collisions with slow electrons, and the formation and decay of excited mercury molecules.