Langmuir probe in collisionless and collisional plasma including dusty plasma

Abstract

Measurements of local plasma parameters in dusty plasma are crucial for understanding the physics issues related to such systems. The Langmuir probe, a small electrode immersed in the plasma, provides such measurements. However, designing of a Langmuir probe system in a dusty plasma environment demands special consideration. First, the probe has to be miniaturized enough so that its perturbation on the ambient dust structure is minimal. At the same time, the probe dimensions must be such that a well-defined theory exists for interpretation of its characteristics. The associated instrumentation must also support the measurement of current collected by the probe with high signal to noise ratio. The most important consideration, of course, comes from the fact that the probes are prone to dust contamination, as the dust particles tend to stick to the probe surface and alter the current collecting area in unpredictable ways. This article describes the design and operation of a Langmuir probe system that resolves these challenging issues in dusty plasma. In doing so, first, different theories that are used to interpret the probe characteristics in collisionless as well as in collisional regimes are discussed, with special emphasis on application. The critical issues associated with the current–voltage characteristics of Langmuir probe obtained in different operating regimes are discussed. Then, an algorithm for processing these characteristics efficiently in presence of ion-neutral collisions in the probe sheath is presented.

Origin of large-amplitude oscillations of dust particles in a plasma sheath

“…The potential difference between the biased probe and plasma produces a sheath around the probe, resulting in a current flow through the probe which carries information regarding the plasma environment. Our probe design makes use of "low pressure theory," which implicitly assumes that r P is much smaller than the characteristic Debye length λ D and the inverse plasma Knudsen numbers K −1 i,e = r P /λ in,en 1, where λ in,en are the ion-neutral and electron-neutral mean-free paths [30][31][32]. For the plasma system described above, the electron Debye length λ D ≈ 1-2 mm.…”

“…Given the linearity of the data, we assume that β = 1, and the resulting ion density at φ dc = −6 V is 2.24 × 10 13 m −3 ; at φ dc = −20 V, 4.28 × 10 13 m −3 ; and at φ dc = −40 V, 5.57 × 10 13 m −3 . We caution the reader that inferring the plasma density from the ion part of the probe characteristic (IPPC) may be subject to large errors (up to an order of magnitude) arising from the assumption made in orbital-motion-limited theory (OML) [31,37]. However, Chen et al [38] indicate that the extrapolation strategy provides estimates of n i that agree reasonably well with those obtained through other techniques at pressures of ∼0.3 Pa (near the regime we operate in).…”

Dust vortices in a direct current glow discharge plasma: a delicate balance between ion drag and Coulomb force

“…2015 c ; Bose et al. 2017). The electron temperature is determined from the electron retarding exponential region of the I–V characteristics, while the ion density is determined from the ion current.…”

“…Collision of ions with neutrals in the sheath surrounding the probe in the OML regime affect the ion collection by the probe in two ways (Schulz & Brown 1955; Bose et al. 2017). Orbital destruction of the ions in the sheath results in an increase in the ion current collected by the probe.…”

“…The method of determination of ion density from the ion current using the ‘modified TALBOT and CHOU model’ is well described by Bose et al. (2017).…”