Dust particles in a plasma are charged either negatively or positively and are subject to various types of forces in a plasma, including a drag force by plasma particles and a force due to collective nature of a plasma. Dust particles are found in layers in ground experiments balanced dominantly by the gravitational force and the electric force, while dust particles in the gravitation-free environment form three dimensional structures subject mainly to forces of ion/neutral drag and the electric field. Dynamic motion of fine particles in a complex (dust) plasma is reviewed with the emphasis of collective nature of plasmas.
The theory of strongly coupled charged systems is briefly described. Emphasis is placed on thermodynamic properties and possible thermodynamic instability and its observation. When one looks at the subsystem composed of particles of one species and regards other particles in the charge-neutral system as background, the compressibility of the subsystem diverges at not-so-strong coupling and critical phenomena related to this thermodynamic instability are expected. In reality, however, the background gives larger contribution to the pressure and masks the critical behavior. In the case of dust particles in dusty plasmas, there is a possibility that the contribution of the background can be overcome and these phenomena can be directly observed for the first time. This is due partly to the large number of charges on macroscopic dust particles. At the same time, the large mass of dust particles are not free from the influence of gravity and the microgravity emvironment is necessary in investigation of critical phenomena in dusty plasmas.
Recent researches on the properties of dusty plasmas by experimental methods under microgravity as well as under gravity are reviewed. Crystal formation can be observed in dusty plasmas. The crystal was shown to have similar properties to a real crystal. In order to form three dimensional dusty plasma crystal under the condition of isotropic force balance, microgravity experiments were started. However fine particles were suspended around the peripheral of plasmas and formed voids in the center. New methods of plasma generation are being developed for the confinement of fine particles at the center. The directions of aligned fine particles under gravity were compared with those under microgravity and it was found that under gravity the mutual influence between positive ion flow and negative fine particles results in the vertical alignment of fine particles and the deformation of plasma.
Growth processes of particles in reactive plasmas have been studied mainly using SiH4 capacitively coupled HF discharges. In the SiH4 discharges, the particles grow to large ones through three phases: the initial growth phase including nucleation in the range of size below about 10 nm; the rapid growth phase in which particles grow due to coagulation between negatively and positively charged particles; the growth saturation phase in which almost all particles are charged negatively. The growth of particles (clusters) in the initial growth phase is characterized both by the time for neutral higher-order-silanes SinHx (n＜4) to grow up to a nucleation size n～ 4, and by that for them to be transported out of the bulk plasma by gas flow. The results of growth kinetics obtained for the SiH4 discharges until now have been applied to improving properties of devices such as amorphous silicon solar cells (growth suppression of clusters) , and also to fabricating new functional electronic devices by using not only SiH4 discharges but also and another material gas discharges (positive utilization of particles generated in plasmas by controlling their growth).
A uniform spray burner was developed to perform a fundamental study on partially-prevaporized spray combustion. A laminar stream of partially-prevaporized spray was generated by the condensation method using rapid expansion of a saturated ethanol vapor-air mixture. A coaxial nozzle burner of 8 mm in inner nozzle exit diameter was applied to the uniform spray burner. Microgravity experiments were performed for large droplet streams because large droplets fall down during the spray generation process at normal gravity. Ethanol was used as a liquid fuel. The mean droplet diameter was able to be varied within the range of 3-120 μm and was stable during an experiment under normal and microgravity conditions. Spray streams were laminar and fuel droplets were uniformly dispersed. The mean droplet diameter and the droplet diameter distribution were constant during spray stream generation. A fiat flame was realized successfully in a laminar spray stream with a stagnation plate. Spray flames of 0.9 in the total equivalence ratio are thicker and brighter than that of fuel vapor-air premixture with the same equivalence ratio.
For bridging between knowledge on droplet combustion and on spray combustion, the monodispersed suspendeddroplet cluster (MSDC) model with which both the droplet spacing and the initial droplet diameter were well controlled was developed, and combustion behavior of the MSDC models in a high-temperature air were examined under microgravity conditions. When the droplet spacing was in the intermediate range, each droplet was ignited to form an individual flame, which enveloped each droplet, and each flame merged into the group flame. The diameter of the burning sphere decreased at the beginning of combustion, and turned to increase afterward. The transition from the individual flame to the group flame occurred around the time when the burning sphere diameter reached its minimum. The ignition delay and the burning time increased with decreasing the droplet spacing, regardless of the droplet number and the model dimensions.
The thermo-acoustic streaming is a new type of convection that has been discovered experimentally under micro-gravity in our past work. The convection occurred when a droplet burns in a standing sound wave. When the droplet is placed between a node and an anti-node, the flame is flown in one direction. The flow is the ‘‘thermo-acoustic streaming’’. The detailed data on the flow field of the streaming does not exist sufficiently. The fields have no ways to be analyzed besides DNS so far. Since the DNS requires a long computational time, a new numerical model with time-averaged quantity is desired. To develop the time average model of the thermo-acoustic streaming, we have analyzed the time variation of temperature distributions on the streaming. It has been confirmed that a force based on the acoustic radiation force is the key to build the time average model. The force is taken into account to the Reynolds equation. The time average model has been named ‘‘ARF model’’. The DNS data are compared with the experimental data, and the ARF model data are compared with the DNS data. We have clarified that the ARF model can be developed using the acoustic radiation force with appropriate constant and allows low calculation cost.
Small-scale reusable sounding rocket system is under development to provide three-minutes microgravity condition to a 10-kg payload. The propulsion system is a hybrid type that uses solid fuel (plastics) and liquid oxygen as propellants and free from explosives, resulting in the dramatically reduced launch cost. To enhance the burning rate of the solid fuel and to augment the thrust, the rocket has employed a new fuel grain design. This new design, named CAMUI as an abbreviation of ‘‘Cascaded Multistage Impinging-jet’’ , allows mixing and combustion to occur around stagnation points on fuel surfaces. Successful launch experiments using a 50-kgf GAMUI engine have proved the feasibility of the basic idea of the system. Finally, a possible configuration of the microgravity test vehicle is presented.