Optimum Gasdynamic Conditions of Use of a Railgun Accelerator for Spraying of Various Materials on Target Surfaces

Abstract

A model of plasma outfl ow from the channel of a railgun accelerator to an inert-gas atmosphere has been developed with the aim of optimizing the process of deposition of the substance from the plasma phase on the target. It has been established that the interaction between the plasma jet formed from the plug material at exit from the railotron and the gas fl ow preceding the plug must be minimized. It has been shown that the conditions necessary are suffi ciently universal and are reduced to correct choice of the distance at which the target must be installed in relation to the exit section of the railotron channel.Keywords: railotron, plasma spraying, plasma plug, shock waves in the plasma.Introduction. Some of today′s technologies of spraying can successfully be implemented using railgun accelerators (railotrons) [1, 2], making it possible to combine the high (much higher than the velocity of sound) rate of spraying with the high density of atoms (or ions) incident on the target. For a variety of reasons, such combination of parameters is quite diffi cult to realize in alternative technologies.In a railotron, current fl owing through a short-circuited bridge is conveyed to the ends of the electrodes (parallel rails) [3,4]. The bridge can be dielectric or metallic. In the fi rst case the dielectric is accelerated by the pressure of the plasma which is generated on its rear by the discharge current. In this case we speak of the acceleration by a plasma plug effi cient for acceleration of macroscopic bodies weighing as little as a few milligrams. On the other hand, a free plasma plug without a striker represents the source of dense and hot (~2 eV) low-temperature plasma with required composition, concentration, and velocity.The advantages of a railotron can be illustrated with a number of concrete physicotechnological problems. Thus, when a railotron is used for spraying of fullerene fi lms, their vapor is deposited using a supersonic molecular beam of inert gas enriched with C 60 molecules. The kinetic energy of adsorbed molecules may be as high as 10-20 eV [5]. As applied to a fullerene molecule, this corresponds to a velocity of (1.5-2) km/s. The degree of ordering of the fi lm turns out to be much higher than in the traditional deposition of fullerenes in vacuum, which manifests itself as the higher-than-average resistance to laser treatment and stability to the desorption temperature. Experiments conducted in [6] have shown that in such fi lms there form polymeric structures analogous to the structures formed in 3D fullerene samples at high pressure and temperature. From this viewpoint, the opportunities provided by the railotron method in producing thin fullerene fi lms are obvious. Even when a macroscopic body is accelerated in an air atmosphere in the railotron, this body can pick up a velocity of to 4-5 km/s. It is possible to attain a twice as large rate of acceleration of such a body in a helium atmosphere in the case of a free plasma plug [4,7]. Therefore, it should be expected that the quality of a p...