Ion-acoustic model of instability of strong shock waves

Yu.L.Serov

Ioffe Physical-Technical Institute of RAS

St.-Petersburg, Russia

yuserov@mail.ioffe.ru

Till now the instability of strong ionizing shock waves was one of not solving problems of gas dynamics. For the last 40 years at the movement of high-speed bodies and strong shock waves many contributors registered a lot of plasma effects, which it was not possible to explain. There were oscillations of intensity of radiation, turbulization of stream. Additional acceleration of a shock, doubling and even trebling of a shock front, nonmonotone distribution of density of the charged and neutral particles were observed. At some Mach- numbers there wasљ a destruction ofљ a shock front. The similar effects exist in the artificially created discharge plasma at considerably smaller speeds of the bodies in a practically important range 700 ¸ 3500 Н/s. At supersonic movement of the bodies in discharge plasma strong nonthermal effects are observed. There are modulation of radiation, significant increase of a standoff distance of a bow shock wave. Its weakening and number of other effectsљ also are observed. These effects can be used in applied gas dynamics forљ development of plasma technologies for improvement of the aerodynamic characteristics of aircrafts of a new generation.

The anomalies at the movement of bodies and shock waves in plasma in some conditions (resonant ranges) arise owing to formation near to shock front or in a zone of relaxation of ion- acoustic soliton bunch, in which the coefficient of ionization sharply increases. In result in domain of such condensation there is a strong interaction of the charged and neutral components leading to occurrence of complex plasma structure. Therefore it is impossible to explain observable effects from the point of view of classical representations ofљ plasma as heated up conducting neutral gas. The range of speeds, in which an ion - acoustic soliton bunch can exist, is rather narrow. It will be determined by "competitiveness" of nonlinearity, dispersion, dissipation, by electron heating and ionization of atoms and molecules. Additional soliton bunches will be formed at perturbations of the function of electron distributionљ on energy. A condition ofљ soliton existenceљ and appearance of anomalies at the movement of bodies and shock waves in plasma is the execution of an inequalityљљљ 1< н £ 1,6, where н љ is ion Mach- number. If the structure of atomic terms is known, the equations of reactions of ionization in plasma, formed by a shock wave, allow to determine the energies of additional electron groups, perturbing the function of distribution of electrons on energy. This allowљ to determine the high-speed ranges of instability of the shock waves or the ranges of abnormal flow ofљ supersonic bodies. Also this allow to predict the characteristic values of speeds, when it is possible to expect the sharp change of shock structureљ or strong instability of a flow in a zone of relaxation. In report the main speed ranges are represented, in which there are the ion - acoustic bunches at the movement of a shock in carbonic gas and xenon. The reactions ofљ dissociation, pair collisions of exited molecules and atoms, ion structure, electron heating were taken into account. These ranges in carbonic gas is following: 1,6-2,6; 2-3,2; 2,7-4,3; 3,4-5,5; 4,4-7,1; 5,8-9,3 km/s. Good correspondence of known results of experiments and of calculations that both for carbonic gas, and for xenon is observed. The resultsљ of ballistic researches of abnormal flow of supersonic bodies by discharge plasma of xenon are represented. The abnormal flow of a spherical body with considerable increase of a standoffљ distance of a bow shock wave is revealed. The boundary of an abnormal flow coincides with maximum phase speed of ion - acoustic soliton bunch. Thus, the behaviour of a shock and of the flow of supersonic bodies in plasma has resonant character and can be determinedљ by ion structure of plasma and by the electron function of distributionљ on energies. The considered effects can have essential significance for understanding of plasma processes accompanying the descent of a space vehicle in an atmosphere.

So:

ћ        the classical aerodynamics cannot describe the movement of a shock wave and flow of bodies in plasma when 1< M £ 1,6;

ћ        there is an ion-acoustic shock wave in plasma when н > 1;

ћ        the ion Mach - number is the important parameter describing the movement of a shock wave and a flow of bodies by plasma at high speeds of movement;

ћ        there are the ranges of instability, connected with nonlinear character of interaction of a shock wave with plasma;

ћ        the parameters of ranges of instability and their number are determined by form of the function of distribution of electrons on energy and by ionic structure of plasma;

the structure of a shock wave is determined by a stage of development of ion-acoustic soliton bunch at variation of ion Mach - number in a range 1 < M 1,6.