Lunar and Planetary Science XXXI 1098.pdf

PLASMA CONDENSATION AND THE ONE COMPONENT PLASMA MODEL. J. A. Vasut and T. W. Hyde, Center for Astrophysics, Space Physics and Engineering Research, Baylor University, Waco, TX 76798- 7316, USA, phone: 254-710-2511 (email: [email protected] & [email protected]).

Introduction: Dusty plasmas are of current interest fied to include electrostatic forces by Lorin Mat- in a number of research areas. The discovery that under thews[6]. The box_tree code models a colloidal certain conditions a dusty plasma can organize itself plasma system by first dividing it into self-similar into an ordered crystal has proven to be important in patches, where the box size is much greater than the areas as diverse as semiconductor manufacturing and radial mean excursions of the constituent dust grains. nanotechnology. This ‘plasma condensation’ was first Boundary conditions are met using twenty-six ghost suggested in a short paper by Ikezi[1] in 1986 and dis- boxes. A tree code is incorporated into the box_tree covered experimentally in 1994 by a team working at routine to allow it to deal with gravitational and elec- the Max Plank Institute under Morfill[2]. Since then it trostatic interactions between the particles. A full has been observed by several teams around the world. treatment of rigid body dynamics, including rotation, Ordered crystal formation is not generally observed is therefore possible allowing for both cluster trajecto- in simple ion-electron plasmas. For plasma condensa- ries and the orientation of fractal aggregates to be exam- tion to occur, colloidal plasmas (plasmas enriched with ined. Additionally, the code provides a method for large numbers of microscopic dust particles) have to be including timers and system events allowing for the employed. It has long been known[3] that particles generation of statistics, images, and other data collec- with like charges could organize themselves into a tion mechanisms. solid, crystalline structure if the ratio of their average Coulombic energy divided by their average kinetic Simulation Results: The modified box_tree energy (G) was high enough. In the late 1970’s and code mentioned above is used to calculate G values early 1980’s it was shown that for a one component under varying plasma and dust grain densities. Vari- plasma (OCP), G must be around 170[4] for this to ous interparticle scale body forces are also examined to occur thus preventing such crystallization in most determine their interaction with G under these condi- cases. At around the same time, Ikezi predicted that tions. The results of these simulations are compared dust particles immersed in a plasma should be able to with theoretical calculations in the literature where the acquire a large enough charge to easily surpass the 170 dusty plasma has been assumed to be a one component level. This would allow them to form a solid, even plasma. when the Coulomb potential was replaced by the Yu- kawa potential to take into account plasma shielding. References: Recently, such plasma condensation has been observed [1] H. Ikezi (1986) Phys Fluids, 29, 1764-1766. leading to both liquid and crystalline system states. [2] H. Thomas and G. Morfill, (1994), Phys Rev Let- Both states retain the individual plasma components as ters, 73, 652-655. [3] E. Winger (1939) Trans. Fara- charged particles with structuring goverened by the day Soc. 34, 678. [4] S. Ichimaru (1982) Rev. Modern larger scale body forces. Interest in the above has risen Phys, 54, 1017-1059. [5] D. Richardson (1994), Mon. dramatically since 1994 due to possible applications in Not. R. Astron. Soc, 269, 493-511. [6] L. Matthews, atomic and molecular systems research and plasma PhD dissertation, Baylor University, 1998. processing applications. At least as important is the emergence of the new field of dry colloidal physics along with its possible ramifications.

Computer Model: Most observed Coulomb crys- tals have values for G in the tens of thousands. Such systems form strongly ordered crystals. However, in order to accurately study colloidal plasma ‘phase’ tran- sitions, a better understanding of the manner in which G is determined by the dusty plasma system is re- quired. In particular, a much better understanding of the manner in which G is affected by the fact that such a system is not well described by the one component plasma theory is needed. To accurately model such systems, it is necessary to keep track of a large number of particles. In this case, this is accomplished using a Barnes-Hut tree code known as “box_tree” devel- oped by Derek Richardson[5] and subsequently modi-