[Ion sources for surface treatments of materials]. Final report on the Contract 0248U0016-35 between the Los Alamos National Laboratory and the Institute of Electrophysics Page: 4 of 8
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breakdown voltage, the discharge operating voltage, and the working pressure of the gas
decrease. The glow discharge initiation time in the chamber having the characteristic dimension
of 0.1 to 0.5 m is a few tens of microseconds. It decreases with increasing magnetic field, gas
pressure, and voltage applied to the gap.
The study of the discharge initiation dynamics revealed a new effect: the pulse repetition
rate strongly affected the discharge delay time. The analysis showed that the observed effect
was not reduced to the decrease in the statistical delay time of the discharge initiation (which
was detected earlier by many researchers) caused by the growth of the rate of appearance of
primary electrons, with increasing pulse repetition rate f. In our experiments, when f was
increased, the statistical spread of the delay time dropped almost to zero. If f was increased
further, the discharge formation time was reduced, too. According to our analysis, the
observed effect can be explained as follows: When the discharge is produced at a large hollow
cathode, the ion current density at the cathode proves to be insufficient for conditioning of the
cathode surface. Under incomplete vacuum thin non-conductive films are formed and
preserved for a long time on the surface of the glow-discharge cathode. These films affect the
secondary ion-electron emission and cause the post-discharge electron emission of the cathode.
The post-discharge emission of electrons is due to the surface charge of the thin films and is
characterized by an abrupt drop of the current with time. Therefore, when the pulse repetition
rate is increased and the pause between pulses is shortened, the generation rate of primary
electrons grows. If the frequency f is high enough, the generation rate of primary electrons
grows so large that it provides the multi-electron mechanism of the discharge initiation and the
corresponding decrease in the discharge formation time.
This new effect appeared and was detected thanks to specific experimental conditions.
The large surface area of the cathode provided the proportional increase in the generation rate
of primary electrons and the corresponding decrease in the statistical spread of the discharge
delay time. The increase in the pulse repetition rate also caused growth of the rate -of
appearance of primary electrons and the decrease in the time spread. Increasing the surface area
by -1000 times and the frequency by -100 times as compared to the values usually used in
such experiments, we managed to minimize the contribution from the static delay time almost
to zero. It was also possible to determine the discharge formation time at a low current density
of the post-discharge emission and analyze how the discharge development depends on the
cathode processes. The electron past-emission intensity of the cathode was found to be very
high after the discharge quenching and drop abruptly with time.
The detected effect accounts for the large difference between the calculated and
experimental discharge initiation time. This effect ensures reliable and efficient initiation of
discharge and provides conditions for generation of ion beams at a high pulse repetition
frequency even using a large-size cathodic chamber, inspite of the large values of the calculated
discharge formation time (-0.1-1 ms).
Task 2.2. The study of the conditions and the regimes of the discharge operation.
Application of a magnetic field altered operating conditions of the hollow-cathode
discharge. Depending on the magnetic induction value, three main regimes of the discharge
operation were observed in our experiments with the 150-mm in diameter discharge system.
1) Magnetic field induction of 0-2 mT
The discharge operating voltage decreased with growing field as a result of the increase
in the energy relaxation of fast electrons in plasma. Moreover, the magnetic field altered the
conditions of generation and transport of charged particles. Due to the ionization frequency
increase at the discharge periphery, the radial profile of plasma becames more homogeneous.
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[Ion sources for surface treatments of materials]. Final report on the Contract 0248U0016-35 between the Los Alamos National Laboratory and the Institute of Electrophysics, report, December 1, 1998; New Mexico. (digital.library.unt.edu/ark:/67531/metadc678518/m1/4/: accessed September 22, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.