Formation of voidless dusty plasma using coaxially-segmented RF discharge under microgravity condition

Paper number

IAC-05-A2.3.02

Author

Dr. Satoru Iizuka, Tohoku University, Japan

Coauthor

Mr. Wataru Suzukawa, Tohoku University, Japan

Year

2005

Abstract

Dusty plasma has been known to form Coulomb crystals in strongly coupled state since dust particles in plasmas are charged up strongly to provide the condition that the Coulomb energy of dusts well surpass their thermal energy. On the earth, however, the mass of dust cannot be neglected, so the dusts are forced to move downward into plasma sheath with strong electric field, where upward electric force can be balanced with the gravity. Therefore, the crystal structure is strongly modified by the electric field. In order to reduce such gravity effect for ideal Coulomb crystal formation the experiments under microgravity condition had been proposed. However, it became a problem that a void was included in the dust cloud. The void was formed by ion drag forces, which push the dusts from plasma center to plasma periphery, depending on the plasma density. Therefore, it has been a subject how background plasma density can be reduced without switching off the plasma discharge.
    In this paper we propose a technique for the formation of dust cloud without a void inside in the microgravity experiment. Here, we introduce a coaxially segmented radio-frequency (RF) discharge, where the plasma density in the center region can be reduced sufficiently to suppress the ion drag force. Two parallel RF electrodes of 10 cm in diameter are placed with spacing of 5 cm. On each RF electrode dielectric cylinders of 3 cm in diameter and 2 cm in height are attached at the center. Therefore, in the center region of the RF electrode the dielectric cylinders are facing with a spacing of 1 cm, while in the outer region the metal RF electrodes are facing with a spacing of 5 cm. When we increase RF power, the discharge starts first in the outer region. Therefore, the plasma density in the center region can be very weak. However, with increasing the RF power the discharge in the center region also starts, therefore, the plasma density in the center region gradually increases. We can more precisely control the plasma density in the center region.
    The microgravity experiment has been carried out by a parabolic flight of aircraft. After reaching the microgravity dust particles of 10 micron meters are injected between the RF electrodes. We have observed for the first time a generation of disc-shape dusty cloud in the center region of RF electrode, where no void is contained inside the dust cloud in case eof low RF power discharge. On the other hand, in case of high RF power, the dusts are shifted toward both dielectric cylinders to form a void in the center region.
    In conclusion, we have demonstrated the formation dust cloud without void inside by introducing a segmented RF electrode in the microgravity experiments.

Abstract document

IAC-05-A2.3.02.pdf

Manuscript document

IAC-05-A2.3.02.pdf (🔒 authorized access only).

To get the manuscript, please contact IAF Secretariat.