Sheaths in laboratory and space plasmas

Abstract

The sheaths that occur at surfaces in laboratory and space plasmas are reviewed with an emphasis on numerical models that can be solved with modest computational resources. The surfaces in plasma may be the interior walls of confinement devices or inserted probes. Fluid and kinetic models are presented in some detail, and particle-in-cell models are discussed briefly. The numerical methods find the spatial profile of the potential, the particle densities near the surfaces and the current to the surfaces. Maxwellian electrons and cold ions are assumed at the outset and subsequently the models are expanded to encompass (1) multiple electron populations, (2) multiple ion species, (3) finite ion temperature, (4) surfaces that emit electrons such as heated cathodes or emissive probes and (5) surfaces that emit plasma as in the Q-machine. These complications may produce nonmonotonic sheaths in which the first derivative of the potential changes sign or double layers in which the second derivative changes sign. The effect of charge-exchange collisions on ion losses to the wall and on ion current to probes is discussed, but models with collisions of electron are omitted. Some recent advances are discussed, including experiments that measure the ion distribution function in sheaths using laser-induced fluorescence, experiments and numerical models on sheaths with multiple ion species and computational models of sheaths surrounding objects in flowing plasma.

Effect of total emitted electron velocity distribution function on the plasma sheath near a floating wall

“…[1][2][3][4] However, many laboratory and space plasma boundaries are emissive, and the electrons emitted via process of thermionic emission, photoelectron emission and secondary electron emission (SEE) influence sheath structure dramatically, and consequently the near-wall heat flux and bulk plasma. [5][6][7][8] Therefore, understanding the characteristics of sheath near emissive surfaces is crucial for many plasma related applications and phenomena, e.g. emissive probe, 9,10 plasma thruster, 11,12 fusion reactor, 13,14 charging of spacecraft, 15,16 and dust levitation near lunar surface.…”

Density Measurements in Low Pressure, Weakly Magnetized, RF Plasmas: Experimental Verification of the Sheath Expansion Effect

“…Additionally, the Debye length λ D derived from the density data on Figure 4, exhibits a value of about 0.15 mm in the high density region for both field modes, similar to the radius of the CP (r p = 0.125 mm). Since traditional theories on ion collection by a cylindrical probe [3,9] require the tip radius to be much smaller than the Debye length, the CP will not be used to measure the ion density in the present experimental setup and the related mechanism for ion density correction is beyond the scope of this study.…”

“…On the other hand, the modeling of ion collection by a cylindrical probe (e.g., [9,19]) requires the tip radius to be much smaller than the Debye length, which is violated under the present experimental condition. Hence the CP will not be used to obtain the ion density in this study and future PIC simulations, similar to the process reported in Sheridan [12], are need to correct the ion density measured by a cylindrical probe.…”

“…Previous experiments [9,14] have verified Sheridan's method (using particle-in-cell (PIC) simulations to obtain algebraic formulae for ion density correction [12]) in direct-current (DC) discharges by comparing the electron density and the corrected ion density measured through their respective saturation current by a single planar LP. Since the plasma breakdown voltage is lower for a radio-frequency (RF) power system compared to a DC system [15], most low pressure laboratory plasmas are sustained using a RF power supply according to Paschen's law.…”

“…This factor needs to be modified for high pressure plasmas where ion-neutral collisions play an important role in the presheath region [9]. A p = πr 2 p is the geometrical area of the collecting disc of radius r p ; e is the electron charge; u B = (eT e /m i ) 1/2 is the Bohm velocity where T e and m i are the electron temperature (in the unit of volts) and the ion mass, respectively.…”