Residual dust charges in an afterglow plasma
L. Couëdel¤, M. Mikikian¤, L. Boufendi¤ and A. A. Samarian
¤GREMI (Groupe de Recherches sur l’Énergétique des Milieux Ionisés), CNRS/Université d’Orléans, 14 rue
d’Issoudun, 45067 Orléans Cedex 2, FRANCE
School of Physics A28, University of Sydney, NSW 2006, AUSTRALIA
Abstract. An on-ground measurement of dust particle residual charges in the afterglow of a dusty plasma was performed in a
rf discharge. An upward thermophoretic force was used to balance the gravitational force. It was found that positively-charged,
negatively-charged and neutral dust particles coexisted for more than one minute after the discharge was switched off. The
mean residual charge for 200 nm radius particles was measured. The dust particle mean charge is about ¡5e at pressure of
1:2 mbar and about ¡3e at pressure of 0:4 mbar.
Keywords: Discharging, dust, plasma, residual charges, afterglow
Dusty or complex plasmas are partially ionized gas composed of neutral species, ions, electrons and charged dust
particles. In laboratory experiments, these particles can be either injected or grown directly in the plasma. Injected
dust particles are usually micron-size particles. And with their small mass, they can be conned near the electrode
where the electric force counterbalance with gravity. Microgravity condition is necessary to study dust clouds of
micrometer size particles lling the whole plasma chamber . In laboratory, dense clouds of submicron particles
light enough to completely ll the gap between the electrodes can be obtained using reactive gases such as silane
 or using a target sputtered with ions from plasma [3, 4, 5, 6]. Dust particle charge is a key parameter in complex
plasma. It determines the interaction between a dust particle with electrons, ions, its neighboring dust particles, and
electric eld. The determination of the dust particle charge is so one of the basic problems in any complex plasma
experiments. The knowledge of dust charge will allow us to understand the basic properties of dusty plasma, particle
dynamics in dust clouds, and methods to manipulate the particles.
In this paper, we report the rst on-ground experiment on the residual charges of dust particles after decay of a dusty
plasma. The experiment was performed in the PKE-Nefedov reactor where the dust particles were physically grown
in discharge chamber. It was found coexistence of positively and negatively charged dust as well as and non-charged
dust for more than one minute after the discharge was switched off. The residual charges for 200 nm radius particles
have been measured for two different pressures. It was revealed that dusts kept the residual charges only when the
discharge was abruptly switched off. In the case when the discharge power is decreased slowly until the plasma
disappeared, there was no residual charge on dust particles.
EXPERIMENTAL PROCEDURE AND RESULTS
The work presented here is performed in the PKE-Nefedov (Plasma Kristall Experiment) chamber designed for
microgravity experiments . It is a rf discharge operating in push-pull excitation mode. It consists of 4 cm diameter
parallel electrodes separated by 3 cm. The injected power varies in the range 0¡4 W. Dust particles are grown in an
argon plasma (0:2¡2 mbar) from a sputtered polymer layer deposited on the electrodes and coming from previously
injected dust particles (3:4 mm, melamine formaldehyde). A detailed description of this experiment and previous
results are presented in Ref [1, 4, 3]. For the study concerning residual charges, the top electrode was cooled. An
upward thermophoretic force was applied to dust particles in order to counterbalance gravity  when a plasma is off.
In order to study particle charges, a sinusoidal voltage produced by a function generator with amplitude §30 V and
frequency of 1 Hz was applied to the bottom electrode. Induced low frequency sinusoidal electric eld E(r; t) generated
dust oscillations if they kept a residual electric charge. A thin laser sheet perpendicular to the electrodes illuminates
dust particles and the scattered light is recorded at 90± with standard charge coupled device (CCD) cameras with 25
images per second. Video signals were transferred to a computer via a frame-grabber card with 8 bit gray scale and
560£700 pixel resolution. In order to avoid edge effect, a eld of view over 8:53£5:50 mm2 restrained to the center
of the chamber is used for residual charge measurement. By superposition of video frames particles trajectories have
been obtained. The coordinates of the particles were measured in each third frame. The amplitude of the oscillations
was gured out from the measured particle positions. Absolute values for the oscillation amplitude were obtained by
scaling the picture pixels to the known size of the eld of view.
From the measurement of oscillation amplitude, the residual charge on a dust particle can be obtained. As the gravity
is compensated by the thermophoretic force.
Residual dust particle charges have been measured in the late afterglow of a dusty plasma. Positive, negative and
non-charged dust particles have been detected. Mean residual charge for 200 nm radius particles was measured. The
particle charge is about 5e at pressure of 1:2 mbar and about 3e at pressure of 0:4 mbar. A model for the dusty plasma
decay was exploited to explain the experimental data. According this model the dust plasma decay occurring in four
stages: temperature relaxation stage, density decay stage, dust charge volume stage, and frozen stage (ice age IV). The
main decreasing of the dust charge happens during the rst stage due to cooling of the electron gas. The nal residual
charge established during the third stage when the density of ions exceeds the density of electrons and the plasma
density is still high enough to vary the charge. Measured values of the dust residual charges are in a good agreement
with values predicted by the model. However the residual charge dependence on discharge condition and detection of
positively charged particle show that more detailed model taking into account various phenomena (electron re-heating,
electron release, afterglow chemistry) in decaying plasma have to be developed for better understanding of dust plasma
The PKE-Nefedov chamber has been made available by the Max-Planck-Institute for Extraterrestrial Physics, Germany,
under the funding of DLR/BMBF under Grant No.50WM9852.
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