Alternative Model of Space-Charge Limited Thermionic Current Flow Through a Plasma

TL;DR

This paper introduces an alternative inverse current mode in thermionic plasma devices, challenging the traditional space-charge limited model, with implications for reducing sputtering and power use.

Contribution

It formulates a new inverse current-limited mode where the plasma potential is above electrode potentials, contrasting with the classical SCL model, and demonstrates its relevance through simulations.

Findings

Inverse mode limits current by anode sheath bias.

SCL modes require collisionless conditions in virtual cathode.

Inverse mode may be common in plasma devices with hot cathodes.

Abstract

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.