  It is widely assumed that thermionic current flow through a plasma is limited
by a "space-charge limited" (SCL) cathode sheath that consumes the hot
cathode's negative bias and accelerates upstream ions into the cathode. Here,
we formulate a fundamentally different current-limited mode. In the "inverse"
mode, the potentials of both electrodes are above the plasma potential, so that
the plasma ions are confined. The bias is consumed by the anode sheath. There
is no potential gradient in the neutral plasma region from resistivity or
presheath. The inverse cathode sheath pulls some thermoelectrons back to the
cathode, thereby limiting the circuit current. Thermoelectrons entering the
zero-field plasma region that undergo collisions may also be sent back to the
cathode, further attenuating the circuit current. In planar geometry, the
plasma density is shown to vary linearly across the electrode gap. A continuum
kinetic planar plasma diode simulation model is set up to compare the
properties of current modes with classical, conventional SCL, and inverse
cathode sheaths. SCL modes can exist only if charge-exchange collisions are
turned off in the potential well of the virtual cathode to prevent ion
trapping. With the collisions, the current-limited equilibrium must be inverse.
Inverse operating modes should therefore be present or possible in many plasma
devices that rely on hot cathodes. Evidence from past experiments is discussed.
The inverse mode may offer opportunities to minimize sputtering and power
consumption that were not previously explored due to the common assumption of
SCL sheaths.
