Dispersion Relation of Dust Acoustic Waves in a dc Glow Discharge Plasma

Summary form only given. Measurements of the dispersion relation of dust acoustic (DA) waves in a dc glow discharge plasma will be presented. The glow discharge is formed on a 3 cm anode disk located in the center of a 60 cm diameter by 100 cm long grounded vacuum chamber. Relevant discharge and plasma parameters are: anode voltage 300 V, discharge current 15 mA, argon pressure 100-200 mtorr, electron temperature 1-3 eV, plasma density 10¹⁴-10¹⁵ m^-3. A magnetic field of 5 mT, applied parallel to the axis of the disk, magnetizes the electrons, allowing the formation of an axially elongated anodic plasma with a potential structure supporting both parallel and perpendicular (radial) electric fields. Aluminum silicate powder located on an electrically floating tray just below the anode is attracted into the anode plasma where it is trapped and levitated in the anode plasma potential structure. The trapped, charged dust is mainly in the size range of 0.5 -1 microns. DA waves with angular frequencies ap 300 s^-1 and wavenumbers ap 25 cm^-1 are spontaneously excited in the dusty plasma, and are detected using a CCD camera which images laser light scattered from the dust. Dispersion relation measurements are made by synchronizing the DA wave frequency to a desired value by applying a sinusoidal modulation to the discharge current. The resulting wavenumbers are measured from single frame video images of the DA wave fronts. Dispersion measurements covering a range of angular frequencies ap 100 s^-1 - 900 s^-1 will be presented. Earlier measurements covering a much smaller range of angular frequencies (50 s^-1 -225 s^-1) were reported. When the applied modulation frequency is above the frequency of the spontaneously excited wave, both the spontaneous and synchronized waves are present, and wave interference results in a beating of the two modes resulting in a "fine structure" in the vi- sible DA wave pattern.