In-situ measurement of dust charge density in nanodusty plasma

A dust grain immersed in a low-pressure gas discharge obtains a permanent negative surface charge due to the high mobility of electrons compared to that of ions. This charge essentially governs all fundamental processes in dusty and complex plasmas involving dust grains, neutrals, (an)ions and electrons and-consequently-virtually all industrial applications of these types of plasmas are affected and steered by it. In this work, we have measured the surface charge by application of laser-induced electron detachment from nanosized dust grains in concert with microwave cavity resonance spectroscopy and laser light extinction. The main result is that the electron release is governed by photodetachment rather than by thermionic emission, and that recharging of the dust grains occurs on timescales that are well in agreement with the orbital-motion-limited (OML) theory. The total surface charge density residing on the dust grains inside the laser volume follows from the saturation of the photodetachment signal, which was used in combination with dust density values derived from extinction measurements to estimate the mean dust charge. The negative dust charge on the 140 nm (average) diameter dust grains in this work is obtained to be in the range of 273 - 2519 elementary charges, of which the lower bound matches well with analytical predictions using the OML theory

Resonant microwaves probing acoustic waves from an RF plasma jet

Microwave cavity resonance spectroscopy is introduced and demonstrated as a new approach to investigate the generation of acoustic waves by a pulsed radio-frequency driven atmospheric-pressure plasma jet. Thanks to recent advancements in the diagnostic method, the lower detection limit for pressure changes in air is ∼0.3 Pa. Good agreement with conventional pressure transducer measurements with respect to the temporal evolution, the pressure amplitude and the spectral response is found. Fourier analysis revealed that the acoustic waves induced by the plasma can most likely be attributed to standing waves in the discharge geometry. Additionally, the plasma-induced acoustic waves of a few (tens of) Pa are proposed as an active mechanism in plasma medicine

The underexposed effect of elastic electron collisions in dusty plasmas

Dusty plasmas comprise a complex mixture of neutrals, electrons, ions and dust grains, which are found throughout the universe and in many technologies. The complexity resides in the chemical and charging processes involving dust grains and plasma species, both of which impact the collective plasma behavior. For decades, the orbital-motion-limited theory is used to describe the plasma charging of dust grains, in which the electron current is considered collisionless. Here we show that the electron (momentum transfer) collision frequency exceeds the electron plasma frequency in a powder-forming plasma. This indicates that the electron current is no longer collisionless, and the orbital-motion-limited theory may need corrections to account for elastic electron collisions. This implication is especially relevant for higher gas pressure, lower plasma density, and larger dust grain size and density

Resonant microwaves probing acoustic waves from an RF plasma jet

\u3cp\u3eMicrowave cavity resonance spectroscopy is introduced and demonstrated as a new approach to investigate the generation of acoustic waves by a pulsed radio-frequency driven atmospheric-pressure plasma jet. Thanks to recent advancements in the diagnostic method, the lower detection limit for pressure changes in air is ∼0.3 Pa. Good agreement with conventional pressure transducer measurements with respect to the temporal evolution, the pressure amplitude and the spectral response is found. Fourier analysis revealed that the acoustic waves induced by the plasma can most likely be attributed to standing waves in the discharge geometry. Additionally, the plasma-induced acoustic waves of a few (tens of) Pa are proposed as an active mechanism in plasma medicine.\u3c/p\u3

Resonant microwaves probing the spatial afterglow of an RF plasma jet

\u3cp\u3eThe electron density and effective electron collision frequency in the spatial afterglow of a pulsed radio frequency driven atmospheric-pressure plasma jet are obtained by using microwave cavity resonance spectroscopy in a temporal manner with an ∼1 μs resolution. During the plasma on phase, values of 1.7 ± 0.3 × 10 18 m\u3csup\u3e-3\u3c/sup\u3e for the electron density and 0.12 ± 0.01 THz for the electron collision frequency were found. These values and standard deviations represent the collective measurement set with repetition rates ranging from 125 to 8000 Hz. The spread in the plasma parameters during this phase within one repetition frequency is smaller than 3%. It is observed that remnant species, e.g., metastables, of previous discharges influence the decay of the plasma. The work reported is enabled by recent developments in the applied diagnostic with respect to the resolution in the plasmas' permittivity. Moreover, a multiplying probe is used for the electrical characterization of the plasma and the presence of the cavity did not influence the plasma impedance. This strongly suggests that the cavity did not affect the discharge.\u3c/p\u3

Probing Collisional Plasmas with MCRS: Opportunities and Challenges

Since the 1940s, Microwave Cavity Resonance Spectroscopy (MCRS) has been used to investigate a variety of solids, gases, and low-pressure plasmas. Recently, the working terrain of the diagnostic method has been expanded with atmospheric-pressure plasmas. This review discusses the advancements that were required for this transition and implications of studying highly collisional, with respect to the probing frequencies, plasmas. These developments and implications call for a redefinition of the limitations of MCRS, which also impact studies of low-pressure plasmas using the diagnostic method. Moreover, a large collection of recommendations concerning the approach and its potential for future studies is presented

Influence of a magnetic field on an extreme ultraviolet photon-induced plasma afterglow

Understanding extreme ultraviolet (EUV) photon-induced plasma dynamics is key to increasing the lifetime of the new generation of lithography machines. The plasma decay times were determined by means of a non-destructive microwave method, microwave cavity resonance spectroscopy, for unmagnetized and magnetized EUV photon-induced plasma afterglows with the argon pressure ranging from 0.002 to 10 Pa. As a result of an external magnet with a magnetic field strength of (57 ± 1) mT, the plasma decay times were extended by two orders of magnitude. Good agreement was found between these measured plasma decay times and four diffusion models, i.e. the ion acoustic, ambipolar, classical-collision, and Bohm's diffusion model