摘要:

Collisionless heating occurs when a magnetoacoustic wave travels at right angles to a magnetic field in a plasma whose density is increasing in the direction of travel. Inclusion of pressure terms in a fluid description leads to an interaction of the wave with the density gradient which allows a net transfer of energy from the wave to the plasma. The mechanism is, however, reversible. Consequently a wave traveling out of the plasma is amplified, and the plasma loses energy to it. A method for obtaining a net transfer of energy to the plasma from a wave originating externally by varying the magnetic field strength is discussed along with the effect of the density gradient on other heating mechanisms.

ISSN:0031-9171    

DOI:10.1063/1.1691752

出版商:AIP    

年代:1968

数据来源: AIP

摘要:

A nonlinear process is considered where a tranverse wave with a frequency near the plasma frequency decays into a plasma oscillation and an ion acoustic wave. The inverse processes are investigated. Also, the interaction of four waves—one tranverse, one ion acoustic, and two plasma oscillations are investigated. The energy of the tranverse wave is partially converted to that of the ion acoustic wave in the presence of a plasma oscillation in both cases. A necessary condition for the process of four wave interaction is obtained. The phases of all interacting waves are assumed to be perfectly matched.

ISSN:0031-9171    

DOI:10.1063/1.1691753

出版商:AIP    

年代:1968

数据来源: AIP

摘要:

The radiative modes of electromagnetic wave oscillations in a cold magnetoplasma slab are investigated. The dispersion relations are examined analytically by resorting to various specific cases where simple solutions are obtainable. Numerical solutions are also obtained for the transverse magnetic mode of oscillations where the dispersion relation has a tractable form. For several other cases, the dispersion relation is reduced to forms that are identifiable to the transverse magnetic mode case. Numerous radiative modes that yield large radiative effects are shown to exist.

ISSN:0031-9171    

DOI:10.1063/1.1691754

出版商:AIP    

年代:1968

数据来源: AIP

摘要:

In the absence of spatial variations, the Vlasov‐Maxwell equations are solved exactly allowing for an initial plasma current to generate large‐amplitude, oscillatory, electric fields in the presence of a uniform external magnetic field. These results are extended to include the effects of weak spatial inhomogeneities for the case in which no external magnetic field is present. The analysis is carried out to first order in the small parameter, &lgr; ∼(Debye length/length scale of spatial variations), by means of a multiple‐time, multiple‐space, perturbation scheme. Due to the presence of weak spatial variations, a magnetic field is generated oscillating at the plasma frequency, whereas the zero‐order electric field is modified by oscillatory behavior at the second harmonic of the plasma frequency. The distribution function is obtained to order &lgr; in situations where the zero‐order electric field is initially curl free; under these circumstances no first‐order magnetic field is generated and the problem remains electrostatic to order &lgr;. In addition, a nonlinear differential equation is derived describing the long time(t ∼ 1/&lgr;)behavior of the zero‐order distribution function; the linear stability of this equation is discussed.

ISSN:0031-9171    

DOI:10.1063/1.1691755

出版商:AIP    

年代:1968

数据来源: AIP

摘要:

Earlier negative mass instability calculations with a mirror magnetic field have been extended to also include a B&thgr;field. The plasma is contained in a torus, of elliptical cross section, which is radially thin. From a new dispersion relation it is found that: (a) for a cold plasma the negative mass instability persists even for large B&thgr;, although the growth rate goes as B&thgr;−1; (b) for a finite velocity spread plasma, the addition of a B&thgr;field allows much higher densities, because the stability criterion becomes independent of density. These results appear to explain the beam disruption observed on the Stevens Institute plasma betatron surprisingly well.

ISSN:0031-9171    

DOI:10.1063/1.1691756

出版商:AIP    

年代:1968

数据来源: AIP

摘要:

Schlieren and Kerr cell photography is used to study the effect of reversed trapped field on a low‐energy theta pinch. At low plasma temperatures the rapid diffusion of magnetic field is observed to inhibit the growth of resistive instabilities of the tearing‐mode form, and this result is in accord with theoretical predictions.

ISSN:0031-9171    

DOI:10.1063/1.1691757

出版商:AIP    

年代:1968

数据来源: AIP

摘要:

An experimental investigation, which includes detailed spatial correlation measurements, reveals that the helium positive column is dominated up to many times the critical magnetic field by a finite amplitude helical oscillation. The equations describing this situation are solved by introducing into them experimental values for some of the variables. With this technique it is possible to avoid the use of unrealistic boundary conditions. The calculated values of axial wavelength, helical rotational frequency, and the magnitude of the radial flux of ions are in fair agreement with experimental values. The calculated growth rate is essentially zero up to four times the critical field, in agreement with the theory of Holter and Johnson, but increases steadily with further increase in magnetic field. A possible explanation of the latter region is that the previously stable helical oscillation turns into a helical relaxation oscillation. While the results of this investigation are contrary to the requirements of the turbulence theory of Kadomstev, there is a faint indication that the system may become turbulent at very high magnetic fields.

ISSN:0031-9171    

DOI:10.1063/1.1691758

出版商:AIP    

年代:1968

数据来源: AIP

摘要:

Calculations are presented which show the effect of resistivity in the expansion of a laser‐produced hot plasma against a magnetic field. Without resistivity, the expanding plasma has periodic behavior due to bouncing back from the field; but with resistivity, there is superimposed a slow diffusion (across the field) on the periodic oscillations. This is illustrated by considering a spherically symmetric plasma produced by a giant laser pulse (delivering power of the order of1010W) illuminating a particle of dimension10‐2cmand subsequently following the time development of its radius, skin‐depth, and temperature by integrating the nonlinear equations of motion and Ohm's law.

ISSN:0031-9171    

DOI:10.1063/1.1691759

出版商:AIP    

年代:1968

数据来源: AIP

摘要:

A high‐density helium plasma, accelerated from a conical pinch, is injected axially into a magnetic dipole field. Magnetic probe measurements show that, near the axis, a compression of the field is super‐imposed on the standard diamagnetic depression. The compression starts downstream and moves towards the injector. Simultaneously with the compression, an increase in the electron temperature and reflection of a small amount of plasma are seen. The amount of plasma transmitted through the dipole field is found to be nearly independent of the field strength.

ISSN:0031-9171    

DOI:10.1063/1.1691760

出版商:AIP    

年代:1968

数据来源: AIP

摘要:

The spectral transfer of turbulent energy at high wave‐numbers may be approximated byS/&egr; = (k5/3E/&agr; &egr;2/3)n. The approximations of Pao(n = 1)and of Kovasznay(n = 32)are members of this family.

ISSN:0031-9171    

DOI:10.1063/1.1691761

出版商:AIP    

年代:1968

数据来源: AIP