Spherical expansion of a plasma into an underdense plasma

Biased electrodes are common components of plasma sources and diagnostics. The plasma-electrode interaction is mediated by an intervening sheath structure that influences properties of the electrons and ions contacting the electrode surface, as well as how the electrode influences properties of the bulk plasma. A rich variety of sheath structures have been observed, including ion sheaths, electron sheaths, double sheaths, double layers, anode glow, and fireballs. These represent complex self-organized responses of the plasma that depend not only on the local influence of the electrode, but also on the global properties of the plasma and the other boundaries that it is in contact with. This review summarizes recent advances in understanding the conditions under which each type of sheath forms, what the basic stability criteria and steady-state properties of each are, and the ways in which each can influence plasma-boundary interactions and bulk plasma properties. These results may be of interest to a number of application areas where biased electrodes are used, including diagnostics, plasma modification of materials, plasma sources, electric propulsion, and the interaction of plasmas with objects in space.

Section: Fireball Onsetmentioning

confidence: 99%

Interaction of biased electrodes and plasmas: sheaths, double layers, and fireballs

Baalrud

Scheiner

Yee

et al. 2020

“…Anode spots have been studied in both unmagnetized plasmas [8,12,13,[28][29][30][31], and in magnetized plasmas [13,[32][33][34]. Their stability has been investigated in detail experimentally [8,29,35] and theoretically [9,12,[36][37][38].…”

Section: Introductionmentioning

confidence: 99%

Equilibrium states of anodic double layers

Baalrud

Longmier

Hershkowitz

2009

116

Equilibrium states of anodic double layers that form near a positively biased disk-shaped electrode immersed in a partially ionized plasma are studied experimentally using electrostatic probes. When the potential drop from the electrode to the plasma is less than a critical value, φ c , an anode glow is observed where a double layer potential drop is within a few millimeters of the electrode surface. For larger biases an anode spot forms where the double layer potential drop is centimeters from the electrode and the intervening region is a plasma more luminous than the bulk plasma. A theoretical model is developed which predicts that φ c ∝ 1/P + C, where P is the neutral pressure and C is a constant, and it predicts hysteresis in the current-voltage characteristic at the electrode; both effects are observed experimentally. The model also provides an estimate for the distance between the electrode and double layer potential drop that agrees with the measurements. Near small electrodes, anode spots are observed to be 'fireballs,' which are spherical in shape. Near larger electrodes 'firerods' are found instead, which have a cylindrical shape. It is shown that firerods are required by global current balance because they have a smaller effective electron collecting area than fireballs. Experiments also confirm that global nonambipolar flow (Baalrud S D et al 2007 Phys. Plasmas 14 042109) accompanies firerods. In this case all electrons are lost through the firerod to the electrode, while all positive ions are lost to the other plasma boundaries.

“…When a strong double layer, defined as one where the potential drop is considerably larger than IcT,fe, is present in a normal positive ion-electron plasma, the introduction of negative ions under the same operating conditions results in the formation of a second double layer [2]. Spherical double layers (anode spots) in a negative ion plasma have been made [3]. Both negative and positive ion beam-plasma interactions have been studied in negative ion plasmas in which there are virtually no electrons [4].…”

Section: Introductionmentioning

confidence: 99%

Observations on negative ion plasmas

Cooney

Aossey

Williams

et al. 1993

Experimental observations that may have general interest are presented concerning a low micrometre size dust panicles pressure plasma ("lo-' Torr) that consists of three components: positively charged ions, negatively charged ions and electrons ('negative ion plasmas'). These ObSeNatiOnS include Langmuir probe characteristics and sheaths that are germane to such a plasma. Properties of plasma waves in a negative ion plasma such as phase velocity, dispersion and soliton propagation are described.