A talk on travelling double layer in unipolar vacuum arcs by Dr André Anders

Last month,July 22, I attended Dr André Anders' conference on the physics of vacuum arcs[1] organised by the American institute of physics. VA are fascinating because of the complex phenomenon that leads to the ignition of plasma in vacuum. 

Starting as an initial field emission the current through metallic imperfections increases with voltage leading to rapid resistive heating which causes evaporation of tiny sections of metal in the vicinity. The high energy electrons ionise these vapors leading to formation of plasma between electrodes and establishment of a vacuum arc. 

The number density for these evaporated sections is extremely high causing a massive pressure spike in regions close to the negative electrode and it's this pressure spike that drives the ions towards the positive electrode overpowering the electric force between them. 

This high pressure phenomenon itself is quite fascinating as the temp in these tiny vapor spots can reach 20000K. This leads to rapid isochoric heating of the vapor which creates pressures of upto 10Gpa[2]. Such high pressures are only possible because the energy is dumped so rapidly that atoms don't have a chance to escape leading to an inertial confinement like situation. 

But the topic that Dr Anders covered was even more fascinating. That is of unipolar arcs,in which an arc is established on a single floating electrode. In this case a background plasma charges the conductive surface and as the charge density increases it leads to the formation of a cathode spot that becomes so hot that it vaporises parts of metals on the surface leading to emission of vapor which ionises to form plasma. 

But here's the question. If there is only one electrode the plasma should extinguish quickly due to charge buildup. Yet it does not. There seems to be a return current flowing through this circuit but how does it happen and where does it terminate? 

Dr Anders’ proposal of a travelling double layer that forces the electron back on to the surface making it behave as a virtual anode was not simply astonishing but it also seems to explain the steady wireless current loop that forms between the metal electrode and the plasma.  

While vacuum arcs themselves are useful in applications like coating, so far unipolar arcs have been detrimental. This research could help us understand it better and maybe even find an application for it. 

The physics of plasmas continue to provide a great platform for plasma researchers to interact with each other and push the boundaries of plasma science which has nearly limitless applications. I look forward to more such events in the future. 

REFERENCES 

1  Mechanism and current balance of unipolar arcs and implications for vacuum arcs

https://www.researchgate.net/publication/407143303_Mechanism_and_current_balance_of_unipolar_arcs_and_implications_for_vacuum_arcs

2  The vacuum arc spot-a high pressure phenomenon

https://iopscience.iop.org/article/10.1088/0022-3727/24/1/007