Electron characterization in weakly ionized collisional plasmas: from principles to techniques

Abstract

Weakly ionized plasmas at or near 1 atm pressure, or atmospheric-pressure plasmas, have received increasing attention due to their scientific significance and potential for use in a variety of applications, particularly for medicine, agriculture, and food. However, there is a large imbalance between scientific research on plasma physics and applications, which is partly due to the considerable differences in the characteristics of these plasmas compared with those of low-pressure plasmas. This discrepancy is particularly related to the difficulty in performing plasma diagnostics for highly collisional plasmas. Information on electrons (such as the electron density and temperature) is essential since electrons play a dominant role in the generation of active species related to the physical and chemical processes inside the plasma. So far, limited diagnostics have been available for electrons such as Thomson scattering and optical emission diagnostics based on equilibrium models. Here, we review the available diagnostic methods along with their merits and limitations for characterizing electrons in weakly ionized collisional plasmas. Particular attention is paid to continuum radiation-based spectroscopy, which facilitates multidimensional imaging of electron density and temperature. The future impact of these plasmas on relevant fields (i.e. laboratory and industrial plasmas and their applications) is also addressed.

Selective modulation of plasma parameters in an atmospheric dielectric barrier discharge driven by sawtooth-type tailored voltage waveforms

“…In this figure, we can see that the discharge operates primarily in a-mode, where the primary electron heating phenomenon is largely dominated by the expansion and collapse of the sheaths. 47 At a low frequency (1 MHz), the strongest heating occurs within the plasma bulk, coinciding with typical features obtained at atmospheric pressure. Different heating processes in each half cycle give rise to the occurrence of an asymmetric electron heating phenomenon when the system is driven by such tailored voltage waveforms.…”

Determination of electron properties of a helium atmospheric pressure plasma jet with a grounded metallic target

“…In atmospheric pressure plasma, continuum radiation is produced via deceleration of a free electron due to the induced dipole field of a neutral atom, which is called free-free electron-neutral atom bremsstrahlung or neutral bremsstrahlung; time-dependent dipole moment of the neutral atom; property of the potential profile of diatomic molecules; ion-atom recombination; electron-ion recombination; and radiative charge exchange, among others. In He atmospheric pressure plasmas, neutral bremsstrahlung is known to be the dominant continuum radiation mechanism, [42] which is described in detail in References [41,[43][44][45]. The emissivity of neutral bremsstrahlung ϵ ea is given by [41,[43][44][45] the equation given below: , and E are the fine structure constant, the Planck constant, the electron mass, the speed of light, the electron density, the atom density, the electron temperature, the wavelength, the Boltzmann constant, the emitted photon energy, the momentum transfer cross-section for electron-atom, [46] and the electron energy, respectively.…”

Investigation of near cut‐off properties of electromagnetic wave propagation in homogeneous, collisional plasma slab

“…[28] Recently, some experimental methods were developed to predict plasma properties with the aid of microwave propagation in low-temperature plasma. [29][30][31] In many diagnostic facilities such as microwave reflectometers, substantial irregularities in the reflected signal are observed, mainly due to density fluctuations or scattering of the EM wave from the reflection (cut-off) layer of the plasma. Such fluctuations of the received (reflected) signal, in the vicinity of the cut-off layer, can cause significant errors in phase detection, especially in homodyne microwave systems.…”