Section 1 Coulomb Crystals

**3**

**Chapter 1**

**Abstract**

**1. Introduction**

Plasmas

*Yasuaki Hayashi*

outward from the center forming a void.

component plasma, strongly coupled plasma

Observation and Analyses of

Coulomb Crystals in Fine Particle

Observations of crystal-like ordering of fine particles in plasmas were first reported in 1994, when we succeeded to observe it by growing carbon fine particles in a methane plasma. Video cameras and Mie-scattering ellipsometry were applied for the analyses of fine particles and their crystal ordering. 3D and 2D crystal structures were observed for smaller and larger particles, respectively. The former structures were fcc, fco, and bct, but bcc structure was not observed. The result is due to the fact that the rearrangement from fcc to fco or bct occurs with both constant particle density in horizontal planes and constant interplane vertical distance. Behaviors of fine particles under microgravity were observed and analyzed using ready-made and injected fine particles. Its experimental result showed that the resultant force composed of electrostatic and ion drag forces pushes fine particles

**Keywords:** fine particle plasma, fine particle, plasma, Coulomb crystal, ellipsometry,

Observations of crystal-like ordering of fine particles in plasmas were first reported in 1994 [1–4]. They were performed by growing carbon particles in a methane plasma [1], silica particles growing in plasma [2], and ready-made polymer particles in argon plasmas [3, 4]. The possibility of observation of crystal-like ordering of fine particles, which was called Coulomb solid or Coulomb crystal, in a low-pressure plasma was predicted through calculation by Ikezi [5]. It was similar to those in the solution of colloids generally negatively single-charged, while micron-sized fine particles get thousands of electrons or more on their surface in a plasma. They can form solid state easily because

> /4*π ε*<sup>0</sup> *a* \_\_\_\_\_\_\_\_\_\_\_ *kBT*

where *e*, *Q*, *a*, and *T* are the unit charge, the number of electron charge per one particle, the Wigner-Seitz radius, and absolute temperature, respectively. The numerator and denominator of Eq. (1) are the average Coulomb energy and the kinetic energy of a fine particle, respectively. In the case that *a* ≳ *λD*, where *λD* is

, (1)

Mie scattering, Mie-scattering ellipsometry, dusty plasma, microgravity, one-

of larger value of the Coulomb-coupling parameter, Γ, as follows:

*Γ* = (*eQ*)<sup>2</sup>

## **Chapter 1**
