A discussion on electron sticking coefficient of dielectrics and some potential applications of dusty plasmas

It was a great learning experience for me to hear from researchers Franko Greiner and Franz Xaver Bronold of Kiel University and University of Greifswald respectively about the new model they have developed to measure the sticking coefficient of low energy electrons. 

When an insulating material is placed in a stream of electrons it acquires surface charge. For low energy electrons this charge is negative as they lack energy to cause electron emission from the dielectric surface. In this presentation researchers  Franko and Franz demonstrated a highly creative method of measuring the number of electrons that remain trapped on the dielectric surface. 

Their method was to suspend dust particles inside the plasma sheath causing them to levitate as they acquire charge. On these charged particles when an oscillating electric field was applied it resulted in the oscillation of the particle itself. The resulting oscillation of the particle was captured by high speed cameras. Through the oscillation frequency of the particle its charge to mass ratio was extracted and since the mass of the single dust particle was known its charge was calculated. 

This method of determining charge on the dielectric gives exceptionally clean results because the measurement is done without any direct contact with the particle — that is its non-invasiveness. What's more appealing though is that it provides a method to directly probe individual dust particles floating within a plasma. 

Dusty plasmas have become a highly researched topic in plasma nano-synthesis  as well as in fundamental sciences to model behaviour of solid particles in charged fluids and this technique could help further our understanding. 

That is not to say that more traditional methods of measuring time averaged current behind the dielectric wall to infer sticking coefficients have become any less useful. They are still good enough especially where a low tech measurement may be desired or in situ measurement may be irrelevant. The technique presented uses highly sophisticated experimental equipment to provide accuracy. Other techniques still remain relevant where such accuracy may not be needed. 

However the real prize remains novel application of dusty plasmas. The theory that is built for insulating particles could be applied to unique dusty plasma devices that land somewhere between vacuum tubes , dielectric barrier discharge and arc plasmas when those insulating particles are substituted with conductive particles. In this presentation the speakers used gold coated particles as a ‘perfect absorbing material’ but other conductive particles could provide similar behaviour—at the very least offer a recombination and charge quenching mechanism. 

Because these conductive dusty particles provide recombination centers that quench charges inside a plasma , they could allow the use of a dbd to create continuous non fluctuating plasma even as electrodes remain confined behind the dielectric. 

This is significant because it protects the electrodes and enables the use of air as a plasma, potentially unlocking creation of plasma based rectifiers and diodes using conductive dust particles instead of toxic and difficult to manage mercury or other metals like Na or Cs. Of course conductivity could still be tuned using a triode-like setup. 

By using conductive dust particles within the dbd we can overcome the limitations of vacuum tubes— namely low pressure requirement and electrode degradation while gaining tunable conductivity and continuous operation. This could have significant applications in electronics and communication systems. 

I thank the American institute of Physics for continuing to organise such thought provoking discussions on plasma science. I look forward to more such meetings in the future. 

REFERENCES 

Evidence of Different Charging Behavior of Conductive and Dielectric Materials in Low-Temperature Plasmas and a New Diagnostic for Low-Energy Electron Absorption

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.133.185301