particles: horizontal in a d.c. discharge, and vertical in an r.f. discharge. Theoretical investigations into the charging of horizontally and vertically oriented particles, their equilibrium positions (levitation height and angle), and the energy of electrostatic interaction between cylindrical particles depending on their orientation can be found in (Ivlev et al. 2003a).

11.5.4Dusty plasma at cryogenic temperatures

The coupling in the dust component increases as the temperature of a gas discharge decreases. This is connected both with a direct fall in dust particle kinetic energy and a diminution in the screening length due to a decrease in the ion temperature. The first experiments with dusty plasmas in a cryogenic gas discharge under a liquid–nitrogen temperature of 77 K were performed by Fortov et al. (2002). Both glow d.c. discharge and capacitively coupled r.f. discharge were used. Ordered dust structures in the glow cryogenic discharge were similar to the structures observed at room temperatures. However, a much stronger influence of the thermophoretic force on the dynamics and stability of dust structures was observed. Very extensive (about 30 cm) ordered structures consisting of long chains and occupying practically the whole volume of the positive column of a d.c. gas discharge were observed for the first time in a d.c. discharge as well.

In experiments with an r.f. discharge it was found that at cryogenic temperatures the density of dust particles in the main volume of the ordered structures can increase considerably, while at the periphery it is similar to that in common r.f. discharges. In the lower part of dust structures, the propagation of travelling density waves was observed. The dust–acoustic wave velocity in cryogenic conditions was several times larger than in normal conditions. With decreasing pressure, instabilities led to structure separation into transverse layers with clear boundaries. Fortov et al. (2002) explain the formation of much more dense structures in cryogenic plasmas mainly by the decrease of the Debye radius, which leads to an exponential decrease in the interparticle interaction energy and allows the particles to be closer to each other.

11.5.5Possible applications of dusty plasmas

Dusty plasmas have already been applied in industry for many decades, for example, in technologies such as precipitation of aerosol particles in combustion products of electric power stations, plasma spraying, and electrostatic painting, as well as in a number of other areas. In the beginning of the 1990s it was understood that a large part of the contamination on the surface of silicon wafers during manufacturing of semiconductor elements for electronics not come from insu cient cleaning of the production area of dust, but was an unavoidable consequence of plasma etching and deposition technologies. In widespread capacitively coupled r.f. discharge reactors, all particles are charged negatively and levitate close to one of the electrodes. After switching o the discharge they deposit on the surface of the silicon wafer. Submicron-sized particles deposited on the surface of processable wafers can reduce the working surface, cause dislocations and

voids, and reduce adhesion of thin films. Great e orts focused on reduction of the number of undesirable dust particles in industrial plasma reactors have given positive results (Bouchoule 1999; Kersten et al. 2001; Selwyn et al. 1989), and this problem can be considered as practically solved.

For the power supply of space vehicles, automatic weather stations, antisubmarine buoys, etc., compact autonomous power–supply sources with a power about 1–10 kW and with working resources of several years are necessary. At present, solar power photoelectric converters, thermoelectric sources with thermoemissive elements on 90Sr, 238Pu, or 210Po, and thermoemissive converters (TEC), where a nuclear reactor fueled by 235U is used as the heat source, are widely applied. All these sources have disadvantages, in particular, very low efficiency. Moreover, a nuclear reactor is very complicated to produce.

Recently, Baranov et al. (2000) suggested the conversion of nuclear energy into electric energy by the photovoltaic e ect in wide bandgap semiconductors based on diamond films obtained by precipitation of the carbon from the gas phase (CVD diamond) and boron nitride. Creation of such sources has become possible due to the investigation of diamond-film synthesis, resulting in the production of semiconducting structures, and the investigation of the physics of low–temperature dusty plasmas.

The principle of the action of the sources, which convert the energy of radioactive isotopes into electricity by the photovoltaic e ect, is the following. Under the influence of ionizing radiation a specially selected gaseous mixture is excited and radiates in the UV range. The ultraviolet radiation due to the photovoltaic e ect induces the electromotive force in the wide bandgap semiconductor. For this purpose, semiconductors based on diamond structures are the most convenient because they have high radiation resistance and high conversion e ciency (up to 70%). As a radioactive isotope it is possible to utilize β–active isotopes having a comparatively high half–life period (10–30 years), for example, 90Sr or similar solid isotopes, e.g., α–active 238Pu.

To use solid isotopes in the photovoltaic converters, it is necessary to have the isotope surface area as large as possible. A homogeneous mixture of gas and isotope dust, which has a maximum possible surface-to-volume ratio, is the most attractive. Excitation of the gas mixture is accomplished by β– or α–radiation from the radioactive dust. Estimates show that at a dust size of 1–20 m and dust number density in the gas of 105−109 cm−3 it is possible to obtain a power density of 1 W dm−3. The gas pressure has to be on the order of 1–10 atm for e ective energy conversion of β– or α–radiation into UV radiation.

The main physical problem arising during the creation of such a battery consists in the necessity of having the homogeneous gas–dust medium at pressures of several atmospheres. Results received recently in investigations of dusty plasmas and their condensation and crystallization (Pal’ et al. 2001a,b) demonstrate such a possibility. Self–consistent processes in such plasmas result in the establishment of an ordered stationary state which is necessary for the transport of the radiation from the volume of the exited gas to photo–converters.