Gas discharge regimes and transitions

Some of our present and recent efforts are listed in what follows:

 

Reduced order modelling of streamers and their characterization by macroscopic parameters 

Electric discharges in gases occur at various scales, and are of both academic and practical interest for several reasons including understanding natural phenomena such as lightning, and for use in industrial applications. Streamers, self-propagating ionization fronts, are a particularly challenging regime to study. They are difficult to study computationally due to the necessity of resolving disparate length and time scales, and existing methods for understanding single streamers are impractical for scaling up to model the hundreds to thousands of streamers present in a streamer corona. Conversely, methods for simulating the full streamer corona rely on simplified models of single streamers which abstract away much of the relevant physics. This disconnect highlights the need for a simplified model of individual streamers which captures the core dynamics but is scalable to ensembles of many mutually interacting streamers.

In this work, several such models are developed. First, a 1.5D model of a single streamer was created wherein particles are treated one dimensionally and electric fields two dimensionally (axisymmetric). This model incorporates developments in modelling streamer processes such as photoionization that were not available in the days when 1.5D models were first invesitgated. Next, a 1.5D model was created with the governing equations solved in the reference frame of the streamer. The existence of such a quasi-steady frame has previously been hypothesized; this work gives a thorough evaluation of the validity of a steady-state streamer model and finds it to be a reasonable approximation on the time scale of electron motion. Based on the success of the quasi-steady model, a further simplification is made wherein streamers are characterized by a small set of macroscopic parameters: tip electric field, velocity, radius and background electric field. A simple model is developed relating these various properties and an efficient graphical representation of their interdependencies is presented.

Related publications: Investigations of Positive Streamers as Quasi-Steady Structures Using Reduced Order Models

 

(L) Comparison of a simplified macroscopic parameter streamer model (contour plot) to 1.5D transient (solid black line) and 1.5D steady state (data markers) model results. The line representing the transient model traces the path the streamer follows in tip-field – velocity space as it propagates. (R) Comparison of at 1.5D transient streamer model (black) to 1.5D steady state streamer models (dashed lines) at an instant in time. Line colours correspond to the markers in the left figure, showing the where the steady-state streamer exists in tip-field – velocity space.

 

 

 


 

Glow corona in wind for electrically isolated electrodes

Experimental and theoretical works of corona discharge in wind have dealt with grounded electrode systems. These works report a current increase with wind speed, first explained by S. Chapman (1970), as the effect of the wind is to remove the shielding ions from the coronating electrode vicinity, strengthening the corona. In the context of atmospheric electricity, corona discharges can be initiated by thunderstorm electric fields at the tips of pointed objects such as towers or trees. These coronae produce space charge layers that influence the spatial distribution of the electric fields and as a result can influence the initiation and progression of leaders. In the presence of wind the ion clouds are dispersed and their shielding effect is reduced, which qualitatively explains the observed preference of lightning attachment to rotating blades of windmills rather than to static towers. There are a number of problems that are not directly addressed by the literature: those that include electrically isolated electrode configurations. Examples are corona discharge appearing at sharp extremities in aircraft, helicopter and isolated wind turbine blade tips when exposed to thunderstorm fields. Of particular interest to the APG is using corona discharge to artificially charge an aircraft in flight (see our work on Lightning to aircraft!). In this project we performed wind tunnel experiments in the MIT Wright Brothers Wind Tunnel (WBWT) and simulation of the effect of wind on a positive glow corona discharge in air from a wire-wing electrode system, that is electrically isolated from its environment. We showed that the classical scaling laws of corona discharge in wind do not apply for electrically isolated electrodes and that indeed corona can be used for charging. 

Chapman, S. (1970). Journal of Geophysical Research, 75 (12), 2165-2169.  

Related publications: Corona Discharge in Wind for Electrically Isolated Electrodes

 

 

Pulsed Nanosecond Dielectric Barrier Discharges at Atmospheric Pressure

Dielectric Barrier Discharge (DBD) is a self-sustained electrical discharge in electrode configurations that contain a dielectric material in the discharge path. DBD is a popular method in non-equilibrium plasma generation. It is favored because of its simple configuration, low power consumption and robustness. DBD can operate under atmospheric pressure and temperature, which makes it a good candidate in many surface processing and chemical processing applications. 

While DBD driven by AC voltage have been extensively studied, the Repetitive Pulsed Nanosecond Discharge (RPND) technique has recently attracted attention. RPND provides repetitive voltage pulses with a duration on the order of 10 nanoseconds at a frequency of 1-10 kHz. The high voltage pulses provide the high E/n breakdown condition, and the short duration time helps to maintain non-equilibrium and avoid thermalization of the discharge. 

In this project we study RPND plasma at atmospheric conditions and using a DBD configuration. Under AC voltage, DBD configurations present three distinct discharge modes: (i) self-organized patterned and (ii) uniform glow plasmas, proceeding in the Townsend breakdown regime; and (iii) microdischarges, proceeding in the streamer breakdown regime. When using RPND voltage waveforms, homogeneous, patterned uniform and microchannel appearance has also been recorded. Specifically, our interest is to characterize the different plasma regimes and transitions of RPND DBD plasma at atmospheric pressure conditions.