ISSN:0899-8221    

DOI:10.1063/1.3480473

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Free‐electron laser (FEL) theory and experiments are reviewed. The physical mechanism responsible for the generation of coherent radiation in the FEL is described and the fundamental role of the ponderomotive wave in bunching and trapping the beam is emphasized. The relationship of the FEL interaction to the beam–plasma interaction is pointed out. Various FEL operating regimes are discussed. These include the high‐gain Compton and Raman regimes, both with and without an axial guiding magnetic field. The linear and nonlinear regimes are examined in detail, with particular emphasis on techniques for achieving efficiency enhancement. The quality of the electron beam used to drive FEL’s is a critical factor in determining their gain and efficiency. The subject of electron beam quality, for different accelerators, is discussed. Key proof‐of‐principle experiments for FELs in an axial guiding magnetic field, as well as those driven by induction linacs, rf linacs, electrostatic accelerators, and storage rings, are reviewed. Finally, the requirements on wigglers and resonators are discussed.

ISSN:0899-8221    

DOI:10.1063/1.859102

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Exact and almost exact solutions to the Vlasov–Maxwell systems describing a variety of plasma configurations with density, temperature, and current gradients are presented. Possible consequences of these solutions are also discussed.

ISSN:0899-8221    

DOI:10.1063/1.859103

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Wave energy flow conservation is demonstrated for Hermitian differential operators that arise in the Vlasov–Maxwell theory for propagation perpendicular to a magnetic field. The energy flow can be related to the bilinear concomitant, for a solution and its complex conjugate, by using the Lagrange identity of the operator. This bilinear form obeys a conservation law and is shown to describe the usual Wentzel–Kramers–Brillouin (WKB) energy flow for asymptotically homogeneous regions. The additivity and lack of uniqueness of the energy flow expression is discussed for a general superposition of waves with real and complex wave‐numbers. Furthermore, a global energy conservation theorem is demonstrated for an inhomogeneity in one dimension and generalized reflection and transmission coefficients are thereby obtained.

ISSN:0899-8221    

DOI:10.1063/1.859104

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

A self‐adjoint, gauge invariant stability functional &dgr;WTGfor determination of a sufficient condition of tearing mode stability is obtained for a plasma with a uniform pressure. The plasma is stable if there is no free energy available to support energy flux into the singular layer. The &Dgr;’concept, valid in simple geometries, is generalized to reflect variation of equilibrium quantities on the flux surface. The relationship of &dgr;WTGto the ideal stability functional and other possible forms of tearing mode stability functionals is clarified through gauge transformations.

ISSN:0899-8221    

DOI:10.1063/1.859105

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Plasma flow to an obstacle is examined using the two‐fluid equations. In this model the obstacle is assumed to be a two‐dimensional strip that extends to infinity in theydirection (slab geometry). An obstacle inserted into a magnetized plasma will cast a ‘‘shadow’’ along the magnetic field lines. The natural collection length of such an obstacle is a measure of the length of its shadow. This study shows that in a typical fusion tokamak, wherecs/&OHgr;cid≪1 (&OHgr;ci,csare the ion cyclotron frequency and the ion acoustic speed, respectively,dis the half‐width of the strip), the particle collection length of an obstacle can be approximated asL∥=0.23csd2/D⊥, ifD⊥/csd≪1; or 0.30d, ifD⊥/csd≫1. For the cases examined in this study, the inclusion of the electron–ion collisional drag parallel toB0changes the solution only by an insignificant amount.

ISSN:0899-8221    

DOI:10.1063/1.859106

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Spatially localized unstable waves are observed around a dipole magnet in a low‐density plasma. The observed wave frequency (&bartil;150–200 kHz) with a high fluctuation level (&bartil;20%) is much higher than an ion‐cyclotron frequency. Correlation measurements confirm azimuthal propagation of the unstable wave on the axis of the dipole magnet. The calculation using a fluid model is found to account qualitatively for the results, and predicts that the waves are excited by two stream instabilities ascribed to an electron drift caused by the density and the potential gradients perpendicular to the magnetic field.

ISSN:0899-8221    

DOI:10.1063/1.859107

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The bi‐Maxwellian model is employed to study the time evolution of anisotropic distributions resulting from Coulomb collisions. The rate of change of temperatures caused by like‐particle collisions is described by a single ordinary differential equation for &Egr;, where &Egr;=T⊥/T∥. It exhibits a logarithmic singularity in the limit &Egr;→0, expressing the velocity space geometry and nonlinearity in the collision operator. The temporal relaxation is compared with a numerical solution of the nonlinear Fokker–Planck equation and is found to be insensitive to changes in the collision operator in the case of &Egr;>1. For &Egr;<1, however, the relaxation toward thermal equilibrium turns out to be dependent on the chosen model. The influence of unlike‐particle collisions is also investigated.

ISSN:0899-8221    

DOI:10.1063/1.859108

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

A dissipative Benjamin–Ono equation is introduced to study turbulence of a magnetic two‐fluid system. An exact nonlinear mode truncation method is applied in which a finite number of poles are used to represent completely the asymptotic behavior of the system. The pole dynamics exhibit a wide range of nonlinear phenomena, including periodic and chaotic orbits. Statistical properties of the resultant turbulent flows such as correlation functions and energy spectra are also studied. The results show that the system can indeed reach the strong turbulent state.

ISSN:0899-8221    

DOI:10.1063/1.859109

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The results of a study of the role of multiple‐helicity nonlinear interaction of tearing modes and the dynamics of magnetic relaxation and dynamo activity in a high‐temperature, current‐carrying plasma are described. A set of fluid equations for tearing modes in the semicollisional regime is derived, and a previous resistive magnetohydrodynamic study of tearing mode turbulence is thus extended to high‐temperature regimes. Because of the direct connection between fluctuation evolution and configuration evolution, a generalized nonlinear theory of the turbulent dynamo and magnetic relaxation is proposed, and a two‐point ⟨(∇⊥p˜)&psgr;˜⟩‐correlation evolution is determined by calculating the relaxation time &tgr;cl. This calculated relaxation time is shown to serve as the phase shift betweenv˜rand &psgr;˜ and hence to control magnetic energy relaxation and dynamo processes. Careful study of the two separated regions of kink‐tearing modes (i.e., the resonant region and the exterior region) reveals the direct relationship between the equilibrium magnetic energy relaxation and average magnetic flux evolution. Thus a theoretical interpretation of the observed correlation between magnetic fluctuations and maintenance of field reversal is provided. Finally, the correlation lifetime of the flux transport term is evaluated from the renormalized two‐point theory, and the saturated magnetic fluctuation level is estimated. The result shows that the saturation level is independent of or very weakly dependent on the Lundquist numberS. The implications of this result for anomalous thermal transport and dynamo activity in high‐temperature reversed field pinch experiments are discussed.

ISSN:0899-8221    

DOI:10.1063/1.859056

出版商:AIP    

年代:1989

数据来源: AIP