摘要:

A highly ionized plasma in helium at a pressure of 10 atm was produced by a giant laser pulse. The properties of the plasma were studied by a dc helium‐neon gas laser. Measurements of the variation of the shape and size of the plasma, its electron density and temperature, and its absorption coefficient at 6328 A˚ were made during the afterglow between 100 and 600 nsec. The shock wave emitted by the plasma was also studied.

ISSN:0031-9171    

DOI:10.1063/1.1692074

出版商:AIP    

年代:1968

数据来源: AIP

摘要:

Experimental observation of a new nonlinear plasma phenomenon, the plasma wave echo, is reported. A recent theory predicts that if a longitudinal electron plasma wave is excited at one position in a collisionless plasma and Landau damps away, and a second wave is excited at another position and also damps away, then a third wave (i.e., the echo) will spontaneously appear at a third position. The existence of various echoes, associated with various orders of the perturbation theory, is demonstrated experimentally. The echoes appear at the predicted position in the plasma. The frequency of the echo wave and the dependence of its amplitude on the amplitude of the initial wave are also in agreement with the theory. Experimental data on the saturation of echo amplitude with increasing initial wave amplitude and increasing distance are given. Data are also presented on another new, but related, phenomenon “the sheath echo.”

ISSN:0031-9171    

DOI:10.1063/1.1692075

出版商:AIP    

年代:1968

数据来源: AIP

摘要:

The linearized Vlasov and Poisson equations governing electron plasma oscillations are solved numerically for the case of a Maxwellian electron gas confined by a linear potential. An iterative method of solving the wave equation is developed using a local Fourier transform to demonstrate convergence of the method. The numerical solution verifies and extends the results obtained analytically with the WKB theory by Berk, Horton, Rosenbluth, and Sudan. The reflection coefficient obtained from the numerical solution is compared with the first‐order WKB reflection coefficient. The numerical calculation demonstrates that the higher‐order reflection processes are smaller than the first‐order process.

ISSN:0031-9171    

DOI:10.1063/1.1692076

出版商:AIP    

年代:1968

数据来源: AIP

摘要:

A procedure for systematically calculating a wide class of exact, nonlinear wave solutions of the Vlasov equation that are stationary in the wave frame is given. The essential feature of the method is the expansion of the current density in an infinite series of the vector potential or the expansion of the charge density in an infinite series in the scalar potential. The potentials obey a differential equation of the form of the equation of motion for a point particle in a conservative potential. This nonlinear differential equation has numerous analytic solutions depending on the choice of physical parameters.

ISSN:0031-9171    

DOI:10.1063/1.1692077

出版商:AIP    

年代:1968

数据来源: AIP

摘要:

The Landau decrement associated with sinusoidal electron‐plasma wave propagation undergoes decaying oscillations in time. The weakening of the oscillations between growing and damped waves is due to phase mixing of resonance particle motions. The rate of phase mixing is increased by the effects of collisions between resonance electrons and the bulk of the plasma particles. The phase mixing problem has been studied, and it is found that the amplitude of the oscillations in the Landau decrement decreases inversely with the square root of the time for early times, and inversely with the seven‐halves power for late times.

ISSN:0031-9171    

DOI:10.1063/1.1692078

出版商:AIP    

年代:1968

数据来源: AIP

摘要:

The general dispersion formulas of plasma waves propagating perpendicular to an applied uniform magnetic fieldB0in an infinite plasma are derived. Stability of one of the wave modes, a linearly polarized electromagnetic wave whose electric field is directed alongB0, is analyzed under the assumption that the wavelength is much larger than the cyclotron radii of electrons. The analysis shows that the waves are stable for a low‐density plasma. However, if the plasma density becomes sufficiently high such that the plasma frequency becomes higher than the electron cyclotron frequency, then an instability may develop. The stability criterion is compared with and is analogous to the criterion for the firehose instability. the allowable frequency range for this mode is analyzed, and it indicates that there is no longer a low‐frequency cutoff in the rigorous sense for a hot plasma.

ISSN:0031-9171    

DOI:10.1063/1.1692079

出版商:AIP    

年代:1968

数据来源: AIP

摘要:

Experiments are described on ion‐acoustic wave propagation and damping in alkali plasmas of various degrees of ionization. An increase of the ratioTe/Tifrom 1 to approximately 3‐4, caused by ion‐atom collisions, results in a decrease of the (Landau) damping of the waves. At high gas pressure and/or low wave frequency a “fluid” picture adequately describes the experimental results.

ISSN:0031-9171    

DOI:10.1063/1.1692080

出版商:AIP    

年代:1968

数据来源: AIP

摘要:

A set of equations are derived for the amplitudes of the oscillating modes which arise in a slightly unstable weakly ionized plasma. The plasma is initially in a time‐independent but spatially dependent and linearly stable equilibrium. It becomes unstable upon a small fractional increase &Dgr; of some external parameter. The amplitudes of the final steadily oscillating states, the frequency shifts of these states, and the transport rates are determined in terms of &Dgr;.

ISSN:0031-9171    

DOI:10.1063/1.1692081

出版商:AIP    

年代:1968

数据来源: AIP

摘要:

A weakly ionized plasma in a nonuniform magnetic field exhibits an instability analogous to the well‐known flute instability of a fully ionized plasma. The instability sets in at a critical magnetic field. When this external field is slightly increased above the critical value by the fractional amount &Dgr;, to begin with only one mode goes unstable and it grows in amplitude to a finite limiting value. Using a general theory of the nonlinear behavior derived in an accompanying paper, we evaluate the mode amplitude and frequency shift as functions of &Dgr;, as well as the corresponding enhanced particle transport. The results are first obtained in the quasilinear approximation, but it is then shown that the mode‐coupling contribution is negligible and hence that the result is generally valid for small &Dgr;.

ISSN:0031-9171    

DOI:10.1063/1.1692082

出版商:AIP    

年代:1968

数据来源: AIP

摘要:

The stability of plasma in a magnetic field against collision‐free tearing of current layers is considered in plane geometry. While previous studies were confined to specific equilibria without charge separation, the present (variational) approach allows for a large class of distribution functions and for an equilibrium electrostatic field, thus also applying to strongly inhomogeneous equilibria. A simple criterion is obtained, which is necessary and sufficient for stability against perturbations that do not vary along the direction of the equilibrium current (tearing mode). Strictly one‐dimensional perturbations requiring a somewhat modified procedure are discussed separately.

ISSN:0031-9171    

DOI:10.1063/1.1692083

出版商:AIP    

年代:1968

数据来源: AIP