Potential Distribution Between a Hot Cathode and an Anode when the Current is Limited*

Space-charge limited (SCL) cathode sheaths have been predicted to limit the thermionic current flow into the plasma in many devices that rely on hot cathodes. But recently, we showed that SCL sheaths are unstable and unlikely to exist because charge-exchange ion trapping in the virtual cathode forces a transition to a current mode with an inverse cathode sheath.  The unique physics of the inverse mode has now been modeled for the first time [1]. Moving outward from the cathode, the potential first decreases, then flattens across the quasineutral plasma region, and then rises across the anode sheath to reach the bias value. All plasma ions are confined between the electrode sheaths.  Inverse modes have current flow, electrode erosion, and power dissipation properties that strongly differ from conventionally theorized modes with classical or SCL cathode sheaths. Experimental evidence of inverse modes in devices will be discussed. Recent work [2] predicts that inverse sheath formation leads to ion-ion plasma formation in negative ion sources.

[1]Campanell, PRE 97, 043207 (2018).  [2]Zhang et al., PSST 27, 06LT01 (2018).