It is widely assumed that thermionic current flow through a plasma is limitedby a "space-charge limited" (SCL) cathode sheath that consumes the hotcathode's negative bias and accelerates upstream ions into the cathode. Here,we formulate a fundamentally different current-limited mode. In the "inverse"mode, the sheath potentials at both electrodes are positive, trapping the ionsin the plasma. The bias is consumed by the anode sheath. There is no potentialgradient in the neutral plasma region from resistivity or presheath. Theinverse cathode sheath potential phi_inv pulls some thermoelectrons back to thecathode, attenuating the current by a factor exp(qe*phi_inv/Temit).Thermoelectrons entering the zero-field region that undergo collisions may alsobe sent back to the cathode. In planar geometry, the plasma density linearlydecreases across the gap and the current attenuation factor from collisions isemfp/(Lp+emfp) in terms of the mean free path and plasma length. A continuumkinetic planar plasma diode simulation model is set up to compare theproperties of current modes with classical, conventional SCL, and inversecathode sheaths. SCL modes can exist only if charge-exchange collisions areturned off in the virtual cathode region to prevent ion trapping. With thecollisions, the current-limited equilibrium is inverse. Inverse operating modesshould therefore be present or possible in many plasma devices that rely on hotcathodes. Evidence from past experiments is discussed. The inverse mode mayoffer opportunities to minimize sputtering and power consumption that were notpreviously explored due to the common assumption of SCL sheaths.
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