Spatio-Temporal Plasma Afterglow Induces Additional Neutral Drag Force on Microparticles

Abstract

An emerging topic in complex plasma physics is the interaction between dust particles and afterglow plasmas. Control of plasma-particle interactions and specifically of the particle trajectories is especially relevant for plasma based contamination control applications. In systems where this contamination control is relevant, emerging or applied plasmas can be of highly transient nature, due to which contaminating particles interact with a combination of a spatial and a temporal afterglow plasma. Until now this type of plasmas and the possible interaction with embedded microparticles has remained far from fully explored in literature. In this work we visually record falling microparticles in a spatio-temporal afterglow of a low pressure inductively coupled plasma and observe a sudden and temporary reversal in their vertical velocity. Numerical simulations confirm that this effect is due to the cooling of the heated background gas in the former active plasma region, which creates a pressure wave and causes microparticles in the spatial afterglow to experience an additional neutral drag force in direction of the plasma bulk. Besides being an interesting principle phenomenon, the presence of this effect could have added value for developing plasma-driven particle contamination control applications. Moreover, for a well defined vacuum vessel geometry and plasma heating volume, this enables the use of microparticles in the spatio-temporal afterglow as probe for the neutral gas temperature in plasma.

The role of neutral drag force in pulsed plasma enabled particle contamination control

“…Figure 3: Vertical velocity (z-direction) of the particles from just before to just after the plasma power was terminated. Data adapted from [19].…”

“…In absence of otherplasma-induced and electric field relatedforces (verified, see [19]), the vertical velocity of the particles through the gas in the glass tube, i.e. the settling velocity with which the particles travels in a laminar flow (as is the case in these experiments), is determined by two competing forces; the gravitational force and the neutral drag force.…”

“…a spatio-temporal afterglow plasma, which is extremly complex and requires further research efforts to explore the relevant physical mechanisms and scaling laws. Our previous works reported already on residual charge measurements of microparticles in spatio-temporal afterglow plasmas [8], [19].…”

Temporal afterglow between two pulses of repetitively pulsed argon-acetylene plasma: measuring electron and negatively charged species densities

“…plasmas extinguishing over time) [22], spatial afterglow plasmas [23] (i.e. plasma residues spatially away from the active (powered) plasma region) or combination of the two [24]. The focus of the current work is on the temporal afterglow of a particle forming plasma.…”

Particle charging during pulsed EUV exposures with afterglow effect

“…Several other experiments have also been performed to understand the afterglow physics on particle charging, such as the effect of an external electric field on microparticle charge distribution [22], spontaneous dust pulse formation under microgravity [23], impact of the initial particle cloud configuration/shape on cloud dynamics (simple Coulomb expansion to more complex behaviors, such as Coulomb fission) [24,25]. In recent times, the dust response to the afterglow (both temporal and spatial) plasma has gained attention [26][27][28][29][30][31][32][33][34][35][36]. In the case of 20 µs pulsed EUV environment in lithography machines, the afterglow physics with nonlocal thermodynamic equilibrium conditions is fundamentally different from those mentioned above.…”