Robert L. Merlino

Laboratory observation of the dust‐acoustic wave mode

A laboratory observation of the dust‐acoustic instability is reported. The results are compared with available theories.

EXPERIMENTS ON ION-ACOUSTIC WAVES IN DUSTY PLASMAS

Abstract An experiment is described on ion-acoustic (IA) waves in dusty plasmas, which was performed in the dusty plasma device (DPD) of Xu et al. (Rev. Sci. Instrum. 63, 5266, 1992). It is found that, as expected, the presence of negatively charged dust grains increases the phase velocity of the waves and, at the same time, also reduces the strength of the collisionless (Landau) damping to which the waves are subjected.

Laboratory studies of waves and instabilities in dusty plasmas

Theoretical and experimental studies of low-frequency electrostatic waves in plasmas containing negatively charged dust grains are described. The presence of charged dust is shown to modify the properties of ion-acoustic waves and electrostatic ion-cyclotron waves through the quasineutrality condition even though the dust grains do not participate in the wave dynamics. If the dust dynamics is included in the analysis, new “dust modes” appear—dust acoustic and dust cyclotron modes. The results of laboratory experiments dealing with dust ion acoustic (DIA) waves and electrostatic dust ion cyclotron (EDIC) waves are shown. These modes are more easily excited in a plasma containing negatively charged dust. Finally, observations of dust acoustic (DA) waves are presented and measurements of the dispersion relation are compared with one obtained from fluid theory.

Dusty Plasmas in the Laboratory, Industry, and Space

Charged microparticles are an annoyance in the plasmas of fusion energy schemes and semiconductor manufacturing. But in laboratory plasmas and in space, they can be uniquely informative.

Dust acoustic waves in a direct current glow discharge

An experimental investigation of dust acoustic (DA) waves in a dc glow discharge plasma is described. The glow discharge is formed between a 3 cm anode disk and the grounded walls of a 60 cm diameter vacuum chamber which is filled with nitrogen gas at a pressure of about 100 mTorr. Dust located on a tray in the chamber is attracted into the plasma where it is trapped electrostatically. The dust acoustic waves were produced by applying a modulation signal (5–40 Hz) to the anode. The wavelength of the DA waves was measured from single frame video images of scattered light from the dust grains. The measured dispersion relation is compared with theoretical predictions.

Experimental study of shock formation in a dusty plasma

An experimental investigation of the effect of negatively charged dust on ion acoustic shock formation in a Q machine is described. Ion acoustic compressional pulses were observed to steepen as they traveled through a dusty plasma if the percentage of the negative charge in the plasma on the dust grains was ≳75%.

Understanding Langmuir probe current-voltage characteristics

I give several simple examples of model Langmuir probe current-voltage (I-V) characteristics that help students learn how to interpret real I-V characteristics obtained in a plasma. Students can also create their own Langmuir probe I-V characteristics using a program with the plasma density, plasma potential, electron temperature, ion temperature, and probe area as input parameters. Some examples of Langmuir probe I-V characteristics obtained in laboratory plasmas are presented and analyzed. A few comments are made advocating the inclusion of plasma experiments in the advanced undergraduate laboratory.

Charging of dust grains in a plasma with negative ions

The effect of negative ions on the charging of dust particles in a plasma is investigated experimentally. A plasma containing a very low percentage of electrons is formed in a single-ended Q machine when SF6 is admitted into the vacuum system. The relatively cold Q machine electrons (Te≈0.2eV) readily attach to SF6 molecules to form SF6− negative ions. Calculations of the dust charge indicate that for electrons, negative ions, and positive ions of comparable temperatures, the charge (or surface potential) of the dust can be positive if the positive ion mass is smaller than the negative ion mass and if ϵ, the ratio of the electron to positive ion density, is sufficiently small. The Q machine plasma is operated with K+ positive ions (mass 39amu) and SF6− negative ions (mass 146amu), and also utilizes a rotating cylinder to dispense dust into the plasma column. Analysis of the current-voltage characteristics of a Langmuir probe in the dusty plasma shows evidence for the reduction in the (magnitude) of the ne...

Charge neutralization of dust particles in a plasma with negative ions

Charging of dust grains in a plasma with negative ions is studied experimentally. When the relatively mobile electrons are attached to heavy negative ions, their tendency to charge the grains negatively is reduced. In a plasma in which a substantial fraction of the electrons are eliminated (positive ion/negative ion plasma), the grain charge can be reduced in magnitude nearly to zero (“decharging” or charge neutralization). If the positive ions are lighter than the negative ions, dust grains having a small net positive charge can be produced.

Current-driven dust-acoustic instability in a collisional plasma

Abstract The excitation of the dust-acoustic instability in a collisional dusty plasma is investigated. For conditions similar to those of recent laboratory experiments with neutral gas pressures of ∼ 100mTorr, it is found that zero-order electric fields E0 ≳ 10 Vm−1 are sufficient for the growth of perturbations with centimeter wavelengths. Much larger wavelengths generally require larger values of E0. Free electrons in the dusty plasma have a stabilizing effect, which can be very pronounced at the longest wavelengths.