Rare Gas Afterglow Plasmas Research Articles

Cyclotron emission and thermalisation of electrons in rare gas afterglow plasmas

An emission component is observed in the ECR spectrum of free electrons produced in rare gas afterglow plasmas and the rate of thermalisation of the electrons is obtained from the variation of the emission intensity with time.

Stationary states for electron oscillators in a rare-gas afterglow plasma

An explanation is advanced for the presence of populations of low energy oscillating paired electrons in rare-gas afterglow plasmas arranged in a system of coaxial shells, and enclosing a core plasma of free electrons. The common axis of the system is that of the long glass tube containing the plasma. The explanation is based on Fermi's analysis of the collisional interaction of very low energy electrons with gas atoms or molecules. A model afterglow is evolved based on the coaxial shell hypothesis, and is used to interpret two sets of afterglow phenomena, namely the plasma shell resonances obtained when energy is radiated by electrons making transitions from inner to outer shells, and the multiple values for accelerated decay times observed for afterglows of this type.

Observations of electron temperature relaxation rates in rare gas afterglow plasmas

The single Langmuir probe technique has been used to determine the relaxation rates of the electron temperature, Te, in repetitively produced, time-resolved afterglow plasmas. Comprehensive data is presented for krypton plasmas together with preliminary data for neon and argon plasmas. The electron temperature cooling curves for a given gas pressure, p0, have been interpreted in terms of a characteristic time, tau en, for electron cooling via electron-neutral collisions. Spatial measurements of Ie indicated that no significant Ie gradients exist in the plasmas during the periods over which cooling curves were obtained; it is also argued that there was no significant elevation of the gas temperature above the wall temperature during the afterglow. In all three gases good agreement is obtained with previously published data.

Measurements of magnetized rare gas afterglow plasmas in the frequency range 200-1500 MHz

Measurements of magnetized rare gas afterglow plasmas in the frequency range 200-1500 MHz

Measurements of magnetized rare gas afterglow plasmas in the frequency range 200-1500 MHz

The change in the signal absorptions by rare gas afterglow plasmas as a DC magnetic field is varied in the range 0-400 G is studied in the frequency range 200-1500 MHz. The measurements are made using large aluminium cavities excited in the TM0m0 family of modes with a uniform DC magnetic field, B, applied along the direction of the alternating electric field. Signal absorptions are observed to occur, B=0, at several successive times during the afterglow for signal frequency, fM, when the pulsed plasma is excited by means of an 8 ms pulse of high frequency energy at 54 MHz. The absorptions get smaller as the magnetic field is increased slowly in magnitude, and disappear, in turn, for certain critical values of field, Bcrit. Plots of fM against Bcrit fall on straight lines having almost integrally related slopes, the lowest slope having a value close to 2.80*106 HzG-1 in magnitude. The phenomena are believed to be explicable in terms of the oscillations of a low energy gas of electron pairs which break up at critical values of B. The plasma oscillations of a free electron gas are also briefly discussed.

Three body conversion reactions in pure rare gases

Measurements of the conversion reaction coefficients, beta , for the three body conversion of krypton and xenon atomic to molecular ions in pure rare gas afterglow plasmas have been made. The values for beta at 300 K are (342+or-15) Torr-2s-1 in krypton, and (447+or-22) Torr-2s-1 in xenon. Preliminary investigations in the case of krypton indicate that beta is essentially the same at gas temperatures of 180 K and 300 K, but that there is a marked decrease in beta as the gas temperature is increased to 510 K. These results, coupled with the previous measurements, in this laboratory, of beta in helium, neon and argon have allowed an assessment to be made of the validity of several theoretical models. It is shown that good agreement exists between the experimental values and those calculated using the theoretical approach of Mahan (1965).

Investigations of the afterglows of pulsed rare gas discharges in the frequency range 200-1500 MHz

An investigation is made into the dielectric properties of rare gas afterglow plasmas under varying conditions of pulsed discharge excitation in the pressure range 0·1-5 torr. Measurements are carried out in the frequency range 200-1500 MHz using a circular cylindrical cavity energized in the TM0m0 family of modes, the signal frequency increasing as m is increased. A study is made of the way in which the change in the cavity resonant radian signal frequency at a given time in the afterglow, (Δω)t, varies with m. Two kinds of afterglow are distinguished from experiment: those for which (Δω)t increases with increase in m, for example the afterglows following pulsed thermionic arc discharges, and those for which the sense of change of (Δω)t with m is reversed, for example afterglows following long-pulse (similar 11 ms), relatively low-power, radio-frequency discharges. At low pressures, and for a specific signal frequency, the second kind of afterglow causes the cavity to resonate at two different times during the decay; this does not happen with the first kind. Associated with the second kind are greatly increased afterglow decay rates which are dependent on the current pulse width δ. A change-over from the first to the second kind of afterglow is observed in xenon when the radio-frequency current pulse width is increased in the range 10 μs to 2 ms at fixed input power, or when the input power is increased in the range from about 200 w peak to about 1 kw peak at fixed δ. The phenomena are interpreted in terms of afterglows populated mainly by a free-electron gas (first kind), or mainly by a quasi-bound electron gas (second kind), or by a combined population of both gases (change-over regime). Measurements enable the plasma frequency and collision frequency for momentum transfer with neutral atoms to be determined for both kinds of afterglow and, in addition, determinations made of the internal frequency of oscillation of the quasi-bound electrons, yielding values for this parameter in the range 6 × 108-8 × 109 rad s-1.

Reply to comments by B. A. Anicin

The suggestion that anomalous results obtained for the low-frequency dielectric constant of rare-gas afterglow plasmas can be explained in terms of the longitudinal oscillations of a warm-plasma slab is discussed. Experimental results on mercury vapour afterglows are presented; it is shown that the nature and constitution of the conducting electron gas depends on the method of excitation of the preceding pulsed discharge.