Observation of plasma boundary induced Negative Differential Resistance (NDR) in a planar DC discharge system

Abstract

Observation of Negative Differential Resistance (NDR) in the discharge characteristics of a planar DC discharge system induced by specific combination of plasma boundaries is reported. In a previous work [Barnwal et al 2022 Phys. Plasmas 29 072102] it was shown by the authors that no NDRs are triggered in enclosures that have either a fully conducting boundary (viz., an ss vacuum chamber) or a fully insulating boundary (apart from the electrodes), comprising of glass tubes/mica sheets, etc. In the present work, it is shown that an NDR is triggered when the discharge produced within a small enclosure with an insulating boundary (IB), is allowed to leak out through a small aperture into a larger volume with a conducting boundary (CB). As the plasma density increases with increasing discharge current, the Debye length (λ De), which is initially larger than the gap width at low discharge currents, decreases to become of the order of gap width, triggering the NDR. Since the plasma boundary conditions before and after the NDR are widely different (insulating versus mixed) the NDR, which is unstable electrically, connects a low-current, high-voltage state before the NDR to a high-current, low-voltage state afterward. Further, issues related to changes in the anode sheath before and after the NDR are also explored. When the electrode system is nearly symmetric, the anode sheath prior to the onset of the NDR is an ion sheath that flips into an electron sheath after the NDR. When the electrode system is highly asymmetric, the anode sheath remains an electron sheath throughout (both before and after the NDR). Reasons for such behavior are discussed in the paper.