Complex plasma experiment in cryogenic environment
C. Kojima, M. Kugue, T. Maezawa, M. Shindo, Y. Nakamura and O. Ishihara
Faculty of Engineering, Yokohama National University, Yokohama 240-8501, Japan
Abstract. Complex plasma in cryogenic environment is studied experimentally. Plasma is produced by electrical discharge in two methods, one in a helium vapor above liquid helium and the other in a glass tube with helium gas immersed in liquid nitrogen or in liquid helium. Micron sized acrylic particles are introduced into the plasma. A pressure of the helium vapor is ~300 Pa of neutrals at 1K, while the helium gas pressure in the glass tube can be controlled externally. In the helium vapor, a dust cloud is observed above a plate electrode placed just above liquid helium. In the glass tube, on the other hand, a structure formation is observed as well as a phase transition of dust particles from solid to liquid by changing temperatures of neutrals from 300 K to 77 K.
Keywords: complex plasma, liquid helium, structure
In a complex plasma, a Coulomb coupling constant is expressed as a ratio of Coulomb energy to thermal energy of dust particles, i.e., 220exp()4dDeZrrkTλπεΓ=− , where is the dust charge, deZris the mean inter-particle distance, BdkT is the thermal energy of dust particles and Dλ is the Debye length. Dust particles are strongly coupled when . The dust charge, the mean inter particle distance, the thermal energy of dust particles and the Debye length depend on electron temperature, ion temperature, temperature of neutrals, density of neutrals and density of dust particles. Most of the dusty plasma experiments in the laboratory have been carried out in the room temperature, where plasmas are characterized by electron and ion temperatures on the order of 0.1 ~ a few eV with a gas temperature on the order of 0.025eV. It is expected that the decrease of the gas temperature in the discharge makes the ion temperature low, resulting in the weakening of mutual repulsion among dust particles. Recently dust structures were observed in the striation of cryogenic dc discharges , while super-dense dust structures and a boundary-free worm-like structure were observed in the striations of dc discharges cooled in cryogenic temperature . 1Γ>>
Earlier, we produced a plasma in the liquid helium  as well as in a gas at a liquid helium temperature below 4.2 K by a pulse discharge (20kV with a pulse width 7μs) and the plasma was sustained for up to a few ms . We are now producing a plasma for an extended period of time by dc or rf discharge to sustain dust particles in the sheath in a cryogenic environment. We report dust structures observed in the sheath in a vapor of liquid helium and a phase transition of dust structures in the sheath in a helium gas cooled by surrounded liquid nitrogen.
An experimental apparatus #1 is shown in Figure 1(a), in which liquid helium is in a glass Dewar bottle of 10 cm in inner diameter with 100 cm in length. The temperature of liquid helium is cooled down to 1K by evaporative cooling. The pressure at the upper part of the Dewar bottle is ~300 Pa of neutrals at 1 K. The electrodes of 5 mm apart are placed about 20 cm above the surface of liquid helium, while a mesh and a plate of 5 cm in diameter are placed below the electrodes. The 10 kHz ac voltage (~1kV rms) is applied to the electrodes. The electrodes are DISCUSSION
Electron and ion fluxes to a dust particle are expressed by exp(/)eeIeφ∝ and i I . The charge of a dust particle is determined from the flux balance, II0ei . When neutral temperature and ion temperature are nearly equal to liquid nitrogen temperature, charge of the dust particle decreases as shown in Figure 4, thus a Coulomb coupling constant decreases. Thus the dust particles in cryogenic temperature are expected to move more rapidly than at room temperature.
FIGURE 4. Charge variation of a dust particle versus a ratio of an ion temperature to an electron
temperature. is a radius of a spherical dust. a
We have carried out complex plasma experiments in a cryogenic environment. In the helium vapor above the liquid helium, a dust cloud was observed in a sheath above a plate electrode placed just above liquid helium. Dust particles were observed to form a cluster in the sheath in helium plasma produced in a discharge tube surrounded by liquid nitrogen. The CCD camera observation shows that dust particles in the cluster at 77K move more rapidly than at 300K because of the decrease in the ion temperature. The size of the cluster at 77K was observed to become smaller in size as the pressure lowers.
This work is supported by Asian Office of Aerospace Research and Development and Japan Society for the Promotion of Science Grant-in-Aid for Scientific Research (B) under Grant No. (2) 16340179
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