DC discharge experiments at low and medium pressures: from swarm to abnormal glow
The maturity reached by the non-equilibrium plasma community has opened new levels of sophistication in the models that have made it necessary to include more of the relevant physics which requires more experimental results.
From one side, the physics of swarms has always provided the foundation for non-equilibrium plasma modelling. Energy dependent cross sections for electron excitation of ionic levels of rare gases may be of interest for modeling and diagnostics of a great number of different discharges and gas discharge devices. At the same time swarm physics related theory has continued to develop new methods and concepts calling for new experimental data. In our swarm experiment, steady state Townsend (SST) discharge operating in self-sustained regimes is used to obtain the excitation and ionization coefficients for a wide range of gases. After the pressure is set up to a certain value, stable low-current discharge is ignited between plane parallel electrodes. Spectrally resolved emission is acquired from side-on direction as the optical system moves and scans the space of the electrode gap. The coefficients are determined from the measurements of the complete optical signal after correction for detector quantum efficiency. By changing the pressure, data can obtained for a wide range of reduced electric field values (E/N). In case of moderate E/N values, for many gases investigated, electrons are in equilibrium and the profile obtained from the electrode gap peeks close to the anode. However, at very high E/N values, where electrons may not be in equilibrium with the field and heavy particles may contribute to the excitation, peek on the spatial profile is also present at the cathode side.
Besides, of practical importance for modeling and optimizing kinetics processes in gas discharge devices, such as plasma displays, light sources, excimer lasers, ion lasers, ion thrusters, particle detectors, in microwave afterglows etc. is understanding of the kinetics of particles in gases. Apart low-current conditions, where swarm conditions are valid, many applications operate in space charge-dominated regimes, glow and abnormal glow, at higher currents. Particle kinetics of these regimes is governed with field gradients i.e. cathode fall in the electrode gap. Additionally, the existence of space charges allows formation of oscillations in current ranges preceding the normal glow regime. Oscillation regimes prove to be extremely useful for some applications so, beside the steady-state operation, these investigations attract a lot of interest. Our pulsed DC discharge experiment covers a wide range of discharge currents (from a few microA to several tens of mA) and enables ignition of different regimes of operation: low-current Townsend discharge, transient regime with oscillations, normal and abnormal glow. In this way the experiment is utilized in studies of electrical and emission properties of low-pressure DC discharge operating in different regimes. These studies include voltage and current measurements synchronized with discharge side-on and end-on imaging by very fast ICCD camera. Thus, spatial profiles of particular regime can be acquired together with precise information on current and voltage. Moreover, pulsing feature of the experiment allows discharge to reach and operate in steady-state or oscillating high-current regime for shot time interval, avoiding cathode surface conditioning and gas heating and thus providing stable and reproducible results of Volt-Ampere characteristics and spatial profiles of different regimes low-pressure discharges. In case of low-light imaging, the pulse can be applied several times, with appropriate delays, and the discharge emission from all pulses can be accumulated, forming a visible profile. Precise control of the camera gating in time associated with imposing the pulse, also permits following of development of discharge regimes: from ignition, i.e. low-current limit, to the steady-state current or regime of free oscillations. In this way spectrally integrated recordings of development of different regimes in space and time supported by time-dependent monitoring of discharge voltage and current can be obtained. All these measurements can be performed on discharge vessels of different characteristic dimensions and geometries: from cm-size to micrometer-size discharge gaps, i.e. from standard to micro-discharges, and for parallel plate electrodes or some complex geometry electrodes. Several discharge chambers exist: parallel-plate chamber where electrode gap can be adjusted in cm range, small chamber with micrometer-size electrode gap with electrodes available in plane or complex shape and standard size chamber with a hollow cathode.
Measurements from these complementary experiments, swarm and pulsed DC low-pressure discharge, for a particular gas can create database with complete set of data required for successful modeling of low-pressure non-equilibrium discharges and their applications.