摘要:

The linear theory of a free electron laser based on a sheet beam propagating in a narrow gap is developed. The three‐dimensional structure of the radiation field is determined for an infinitely thin sheet beam. Included are the effects of diffraction and space charge. The possibility of diffractive suppression of unwanted modes is discussed.

ISSN:0031-9171    

DOI:10.1063/1.866903

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The electron dynamics in a free electron laser (FEL) is studied in the case when the radiation field contains many modes. This situation arises when unstable modes (sidebands) are excited during operation. It is observed that when the strength of these sidebands exceeds certain levels the electron motion becomes chaotic. This may lead to extensive particle detrapping and loss of amplification for the FEL signal. The threshold for the onset of stochastic electron motion is computed. The evolution of the trapped electron distribution exhibits a diffusive behavior. The rate of particle detrapping is parametrized by the diffusion coefficient@FDin action space. Thee‐folding length for the number of trapped electrons is parametrized byJ2s/@FD, whereJsis the action at the separatrix. It is found that the diffusion rates are related to the type of the sideband spectrum. The diffusion coefficient is always proportional to the ratio of the sideband power in all frequencies to the power of the carrier signal. The coefficient of the proportionality, however, scales differently on the FEL parameters for each of the three spectral categories: a narrow, a broad discrete, and a broad continuous spectrum. The diffusion coefficient is computed analytically for the last two cases and is in good agreement with numerical results. The narrow spectrum yields the highest and the broad continuous the lowest diffusion rates under constant sideband power. It is also found that, in all cases, the diffusion length, measured in wiggler periods, is independent of the electron energy &ggr;.

ISSN:0031-9171    

DOI:10.1063/1.866904

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A model for the hose instabilities of rotating electron beams is developed using the dynamics of single particles as a guide. Three unstable modes are found, all of which are helical in character. This model, which uses the spread mass formalism to incorporate phase mixing, is shown to give the correct high and low frequency limits obtained from a Vlasov treatment. More generally, the predicted growth rates agree well with simulation results.

ISSN:0031-9171    

DOI:10.1063/1.866905

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The equilibrium and stability of three‐dimensional (3‐D) ionospheric plasma clouds is investigated. The equilibrium density profile is assumed to be a 3‐D water bag with a circular cross section transverse toB, and an ellipsoidal cross section parallel toB. The equilibrium potential consists of two parts: a polarization potential driven by a uniform neutral wind, and an ambipolar potential driven by parallel electron pressure. The latter potential is shown to cause sheared azimuthal plasma flows perpendicular toB. A stability analysis of this equilibrium is carried out. It is found that theE×Binstability, which causes the structuring of ionospheric plasma clouds, can be stabilized by the shear in the azimuthal flow. A marginal stability criterion is derived. On the basis of this stability criteria, striations of barium clouds in the ionosphericFregion should be stable to further bifurcation, as is observed

ISSN:0031-9171    

DOI:10.1063/1.866906

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A description of the generation and evolution of ionospheric oxygen ion conic distributions by electromagnetic ion cyclotron resonance heating is formulated in terms of a path integral. All of the relevant physics is contained in this path integral, which may be used to calculate measurable properties of the conic distribution. Although the presentation is applied to this specific ionospheric context, the treatment may be generalized to treat other diffusion problems of interest.

ISSN:0031-9171    

DOI:10.1063/1.866907

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

In this work the experimental verification of the appearance of square patterns in Rayleigh–Be´nard systems is described. Convective square cells are observed above the threshold of convection in a layer of water sandwiched between two Plexiglas plates. These experimental results agree with earlier theoretical predictions [J. Fluid Mech.139, 461 (1984);152, 113 (1985);178, 491 (1987)], and provide, as far as can be determined, the very first direct observation of the stability of square patterns due to a hybrid thermal boundary condition on the top and bottom of the convecting fluid layer.

ISSN:0031-9171    

DOI:10.1063/1.866908

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The effect of an electric field on the long wave motion of the surface of a conducting fluid layer is considered. A Korteweg–de Vries (KdV)‐type equation with coefficients depending on the applied field is derived. The speed of the solitary wave on the fluid layer is seen to be reduced by the electric field. It is found that there are two critical values of the applied voltage that lead to (i) breaking up of the solitary waves and (ii) bifurcation of solutions of the governing equations. For a given value of the imposed potential, solitary waves of elevation and depression are possible depending on the value of surface tension.

ISSN:0031-9171    

DOI:10.1063/1.866909

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The sideband instability of a large‐amplitude electromagnetic wave that traps electrons near the cyclotron resonance in a magnetized plasma is analyzed. The wave‐trapped particles produce a weak instability. The growth rate and the criterion for the instability to exceed the threshold determined by collisional damping of the sidebands are given. This instability will have little consequence for the parameters expected in the microwave tokamak experiment at Lawrence Livermore National Laboratory [Proceedingsofthe14thAnnualConferenceonPlasmaPhysics, St. Andrews, Scotland, 1987 (Pergamon, London, 1988), Paper No. 25].

ISSN:0031-9171    

DOI:10.1063/1.866910

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Pressure anisotropy cannot stabilize the kinks with short axial wavelengths that, according to ideal magnetohydrodynamics, are unstable at an elliptic magnetic stagnation line. This result is derived by applying the guiding center theory to the simple but generic case of circularly cylindrical Z‐pinch equilibria with anisotropic Maxwellian distribution functions of ions and electrons.

ISSN:0031-9171    

DOI:10.1063/1.866911

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A new analytic, self‐similar solution of the fluid equations with losses in a stabilized Z pinch is presented. A scaling is suggested for the net energy loss with plasma density and temperature typical for a Z pinch immersed in an external multipole magnetic field. The solution of the strongly nonlinear system of fluid equations is obtained by self‐similar methods. Strongly aharmonic high frequency oscillations of the plasma parameters are found. It is emphasized that a static Z pinch cannot be stabilized by a stationary field of a magnetic multipole. Therefore the potentiality of these oscillations to affect the stability of Z pinches embedded in a magnetic multipole is investigated. The effect of the dynamic stabilization is considered by taking estimates.

ISSN:0031-9171    

DOI:10.1063/1.866912

出版商:AIP    

年代:1988

数据来源: AIP