Joe Khachan

Measurements of ion energy distributions by Doppler shift spectroscopy in an inertial-electrostatic confinement device

Doppler shift spectroscopy was carried out on the discharge in a spherically symmetric inertial-electrostatic confinement system. This enabled the ion energy distributions, types, and densities of ionic species to be determined. A weakly ionized hydrogen radio-frequency discharge was used as the ion source for two spherical and concentric electrostatic grids. The inner and outer grids were the cathode and anode, respectively. It was found that the ion energy distribution consisted of a non-Maxwellian directional component, as well as a spatially isotropic Maxwellian distribution. The directional component consisted of three broadened energy peaks belonging to H3+ (20%), H2+ (60%), and H+ (20%). These ions had energies approximately 20% of the cathode potential. The temperature (in electronvolts) of the Maxwellian distribution was approximately 15% of the cathode potential.

Diverging ion motion in an inertial electrostatic confinement discharge

An inertial electrostatic confinement device operating in the gaseous discharge pressure regime (units to tens of mTorr) is shown to consist of a substantial flux of neutrals diverging from the cathode center. Using Doppler shift spectroscopy, it is shown that directional ion beams, originating from the center, increase in energy as they move away from the center. Moreover, through charge exchange, these ions become energetic neutrals and travel out of the cathode to the anode. Although naturally there are converging ions, it is shown that this is a lesser component of the energetic particle beams in this pressure range.

Spatial distribution of ion energies in an inertial electrostatic confinement device

The spatial distribution of ion energies was measured in an inertial electrostatic confinement device, which can be used as a small nuclear fusion source for the generation of neutrons. Using Doppler shift spectroscopy and a single ended Langmuir probe it was found that a virtual anode was established at the center of the electrostatic potential well resulting in much reduced ion energies at this point. Moreover, there was direct correlation between the ion energies and the spatial plasma potential showing the validity of the probe measurements. It was shown that the ion energy is a maximum at the point where the potential is the deepest, which occurs about halfway between the center and the cathode radius.

A simple electric thruster based on ion charge exchange

An electric thruster is presented that makes use of the properties of an asymmetric hollow cathode glow discharge that ejects a collimated plume of high velocity neutral atoms. Ions are accelerated electrostatically outwards from within the asymmetric hollow cathode and undergo charge exchange with the background gas resulting in energetic neutrals being ejected, thus producing thrust. This thruster is entirely self-contained allowing thrust generation and beam neutralization within the discharge. Doppler spectroscopy was used to determine the speed of atomic hydrogen in the plume and was found to produce a specific impulse greater than 3 × 104 s for applied voltages and powers of the order of 5 kV and 100 W, respectively. An estimate of the thrust of 1 mN for a power input in the order of 1 kW was obtained from previously measured ion densities in similar discharges. This work builds on recent work associated with the ejection of high velocity neutrals from hollow cathode discharges. The simple but effective design of the thruster has the potential for large thrust densities as well as successful long term operation.

Low beta confinement in a Polywell modelled with conventional point cusp theories

The magnetic field structure in a Polywell device is studied to understand both the physics underlying the electron confinement properties and its estimated performance compared to other cusped devices. Analytical expressions are presented for the magnetic field in addition to expressions for the point and line cusps as a function of device parameters. It is found that at small coil spacings, it is possible for the point cusp losses to dominate over the line cusp losses, leading to longer overall electron confinement. The types of single particle trajectories that can occur are analysed in the context of the magnetic field structure which results in the ability to define two general classes of trajectories, separated by a critical flux surface. Finally, an expression for the single particle confinement time is proposed and subsequently compared with simulation.

The dependence of the virtual cathode in a Polywell™ on the coil current and background gas pressure

Floating potential measurements have been carried out on a Polywell™ inertial electrostatic confinement device that uses magnetic cusps to trap electrons and establish a virtual cathode. In particular, the dependence of the floating potential on the coil current and background gas pressure was studied. The magnetic field coils were driven by a pulsed current supply and it was found that the virtual cathode could only be established within a narrow range of currents. In addition, it was shown that the magnitude of the floating potential increased with decreasing background gas pressure. It is conjectured that the depth of the virtual cathode and its lifetime are dependent on the magnitude of the injected electron current.

Quenching of excited Ar I and H by H2 in a gas discharge

The cross sections for quenching by H2 of the 4p´[1/2]1 and 4p´[1/2]0 states of Ar I, corresponding to emission lines at 696.5 and 750.4 nm, are obtained in a gas discharge using emission spectroscopy in conjunction with a laser-induced fluorescence measurement of the atomic hydrogen density, and found to be (1.3±0.3) × 10-19 and (1.5±0.3) × 10-19 m2, respectively. The cross sections for the quenching of H(n = 4) and H(n = 5) by H2 are obtained using emission spectroscopy and the known quenching cross section for H(n = 3) and are found to be 3.1(±0.1)(-0.9) × 10-19 and 8.3(±0.7)(-2.5) × 10-19 m2, respectively, where the first uncertainty is the random error and the second uncertainty is a possible systematic error.

Effect of repetition rate of a pulsed microwave diamond forming plasma on the density of C2

The relative density of C2 in a pulsed microwave discharge, used for diamond deposition, was measured by optical emission spectroscopy. It was found that the average density of C2 increases with increasing plasma repetition rate (200 Hz–8 kHz) and approaches a limit at high frequencies. These results are explained with a simple plasma chemical kinetics simulation of the pulsed discharge.

Student evaluation of research projects in a first-year physics laboratory

We describe the evaluation by students of a scheme of open-ended, research-based group project work which has become a standard component of first-year physics courses at the University of Sydney and is now in its 19th year of operation. Data were gathered from two sources: direct observations of the classes and a written survey. A summary of the classroom observations and the results from a detailed analysis of the survey responses are presented. The feedback from the cohort of approximately 800 students is largely positive but we identify a few discrepancies between stated course goals and the results from the survey.

Production and loss of H atoms in a microwave discharge in

Two-photon laser-induced fluorescence (LIF) is used to study the production and loss of H atoms in a pulsed microwave discharge in over the pressure range 1-50 Torr. Absolute measurements of the H atom density are made at the end of the pulse. These measurements were calibrated using a new technique based on the decay rate of the LIF signal. The temporal variation of emission during pulsing of the discharge is used to estimate the rate of dissociation of , which compares well with the predictions of a one-dimensional model for the electron energy distribution function. This measurement also gives the wall recombination probability for H atoms, which is compared with that obtained by LIF measurement of the decay of the H atom density in the pulse afterglow.