VOLUME 83, NUMBER 8 PHYSICAL REVIEW LETTERS 23 AUGUST 1999 Condensed Plasmas under Microgravity G. E. Morfill, 1 H. M. Thomas, 1 U. Konopka, 1 H. Rothermel, 1 M. Zuzic, 1 A. Ivlev, 1 and J. Goree 2 1 Max-Planck-Institut f ür extraterrestrische Physik, 85740 Garching, Germany 2 University of Iowa, Department of Physics and Astronomy, Iowa City, Iowa 52242 (Received 28 August 1998; revised manuscript received 28 May 1999) Experiments under microgravity conditions were carried out to study “condensed” (liquid and crystalline) states of a colloidal plasma (ions, electrons, and charged microspheres). Systems with 10 6 microspheres were produced. The observed systems represent new forms of matter — quasineutral, self-organized plasmas — the properties of which are largely unexplored. In contrast to laboratory measurements, the systems under microgravity are clearly three dimensional (as expected); they exhibit stable vortex flows, sometimes adjacent to crystalline regions, and a central “void,” free of microspheres. PACS numbers: 52.25.Vy, 52.90. + z, 81.10.Mx, 82.70.Dd “Colloidal plasmas” may undergo phase transitions and condense to form “liquid plasmas” and “plasma crystals” [1 – 9]. These plasma states are not normal condensed mat- ter states — the individual plasma components (electrons, ions, microspheres) coexist in an ordered (or organized) structural form, which arises as a consequence of the strong Coulomb coupling between them — they therefore represent new states of matter. Research into the proper- ties of these new plasma states, e.g., the thermodynamics, microscopic, and collective processes, benefits from the fact that one plasma component, the microspheres, may be visualized and analyzed at the kinetic level. The charged microspheres are crucial; without them the “plasma condensation” would not occur [10]. However, there exists a major constraint — gravity. The external grav- itational pressure restricts laboratory investigations to a limited range of state variables. Consequently, it was recognized fairly early that microgravity measurements were needed to complement laboratory research [1,11,12]. Experiments in space have now been performed in a radio-frequency (rf ) discharge colloidal plasma, and the first results are presented together with a discussion of the salient features observed. In order to appreciate the scope of microgravity research for this fundamental physics field, we summarize some of the relevant issues: On Earth, the microspheres have to be electrostatically supported against gravity. The electric fields required are E  10 Vcm for particles of mass m  10 210 g and charge Q  10 4 e. Such strong fields occur in plasma boundary layers, e.g., the sheath above the electrodes, and are characterized by supersonic ion flows and nonequilibrium conditions. Over a lattice separation, D, the (unavoidable) electric bulk force in this sheath varies by an amount DF  Q dE dz D, which is comparable to the interparticle forces Q 2 D 2 . This and the ion drag have a profound influence on liquid flow properties and crystal structures. Gravity also causes “crystal crushing” as well as shear forces and therefore prevents the establishment of large 3D crystals or liquids (see [11]). The internal pressure generated by n L lattice planes is P g  QD dE dz n L 2 1. It is clear that the existence of such large body forces makes it difficult, if not impossible, to investigate experimentally whether “plasma crystals” have self-confining bind- ing forces [13–15] and whether “plasma fluids” have properties such as surface tension. Finally, the de- tailed determination of system properties — in particular the thermodynamics, phase transitions, and flow dy- namics — requires investigations over a large range of “parameter space” (e.g., the “coupling parameter” G  Q 2 4p´ 0 DkT  Coulomb energy / thermal energy and the “structure parameter” k  Dl D  mean par- ticle separation/Debye length of the plasma) in order to understand the equation of state of condensed plasmas [16]. While in principle the charge, Q, and hence G, can be varied quite easily, gravity constrains this in practice through the levitation requirement mg  QE. The same is true for k, which is governed by the external pressure acting on the system—and which is again constrained by the action of gravity. In the absence of gravity, microspheres can in principle be embedded in the main plasma, where the major bulk forces — electric fields, QE, thermophoresis, F th , density gradients F gr , ion drag, F i , and neutral drag, F n —are much smaller. F th and F i are directed outwards, QE and F gr into the main plasma, and F n is a friction force slowing the particles down. In the main plasma, the ion drag force is due to subsonic drifts, v i , governed by collisions with neutrals at the rate v in . Throughout the subsonic ion drift regime of the main plasma we get the interesting result that the ratio of the bulk forces for a particle at rest is jFQEj  8.06 3 10 212 n i P mb T 300 a m f e  j , (1) i.e., it depends only on neutral gas pressure (P mb in millibar), ion temperature (in units of 300 K), ion density cm 23 , and particle size a m (in microns). f e is the cross section enhancement due to ion-microsphere Coulomb collisions [17]. Hence for particles in the micron size range, it should be possible under microgravity conditions 1598 0031-9007 99  83(8)  1598(4)$15.00 © 1999 The American Physical Society