摘要:

A one‐dimensional transport model is developed to simulate the confinement of plasma and magnetic flux in a field‐reversed configuration. Given the resistivity, the confinement times can be calculated. Approximate expressions are found which yield the magnitude and gross profile of the resistivity if the confinement times are known. These results are applied to experimental data from experiments, primarily TRX‐1, to uncover trends in the transport properties. Several important conclusions emerge. The transport depends profoundly, and inexplicably, on the plasma formation mode. The inferred transport differs in several ways from the predictions of local lower‐hybrid‐drift turbulence theory. Finally, the gross resistivity exhibits an unusual trend withxs(separatrix radiusrsdivided by the conducting wall radiusrc), and is peaked near the magnetic axis for certain predictable conditions.

ISSN:0031-9171    

DOI:10.1063/1.865059

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Five different codes have been used to simulate the identical problem in reversed‐field pinch (RFP) dynamics using the resistive magnetohydrodynamic (MHD) model in three dimensions with the same set of initial and boundary conditions. Three codes are compressible, while two are incompressible. The assumption of incompressibility was made in the spirit of reducing the model so that, for example, the codes would run faster while retaining the relevant physics. The results show that the three compressible codes agree quite well with each other and the two incompressible codes agree with each other also, but the compressible and incompressible models show qualitatively different behavior. Most importantly, for a certain set of boundary and initial conditions, the compressible codes predict field reversal maintenance while the incompressible codes do not. Thus compressibility is an important feature of RFP physics. This is in contrast to tokamak physics where the strong toroidal field enforces incompressibility at low poloidal beta.

ISSN:0031-9171    

DOI:10.1063/1.865060

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The equations of resistive magnetohydrodynamics (MHD) are recast in a form that is useful for studying the evolution of those toroidal systems where the fast magnetosonic wave plays no important role. The equations are exact and have ∇ ⋅ B=0 satisfied explicitly. From this set of equations it is a simple matter to derive the equations of reduced MHD to any order in the inverse aspect ratio &egr; of the torus and for &bgr;∼&egr; or smaller. This is demonstrated by deriving a reduced set of MHD equations that are correct to fifth order in &egr;. These equations contain the exact equilibrium relation and, as such, can be used to find three‐dimensional stellarator equilibria. In addition, if a subsidiary ordering in &eegr;, the resistivity, is made, the equations of Glasser, Greene, and Johnson [Phys. Fluids8, 875 (1967);19, 567 (1967)] are recovered. This set of reduced equations has been coded by extending the initial value codehilo[Phys. Fluids26, 3066 (1983)]. Results obtained for both ideal and resistive linear stability from the reduced equations are compared with those obtained by solving the full set of MHD equations in a cylinder. Good agreement is shown for both zero and finite‐beta calculations. Comparisons are also made with analytic theory illuminating the present limitations of the latter.

ISSN:0031-9171    

DOI:10.1063/1.865061

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A model of particle transport has been incorporated in the equilibrium and stability codebeta. From the Clebsch potentials of Boozer’s theory a first‐order partial differential equation with constant coefficients is derived relating a guiding center step size &lgr;∥/‖∇s‖ to the parallel gyroradius &rgr;∥. In terms of the Fourier coefficients of &rgr;∥, a formal solution for &lgr;∥is found that exhibits the resonance at rational values of the rotational transform explicitly. Phenomenological reasoning leads to a transport coefficient for both circulating and trapped particles that involves integrals of &lgr;2∥analogous to a term in the Mercier stability criterion depending on the square of the Pfirsch–Schlu¨ter current. Good correlation has been established between plateau values of this transport coefficient and Monte Carlo calculations performed by H. Wobig for the Wendelstein VII‐AS experiment, for a typical Heliac, and for referencel=2 stellarators.

ISSN:0031-9171    

DOI:10.1063/1.865062

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

Several aspects of ion‐acoustic decay (IAD) and oscillating two‐stream (OTS) instabilities are studied by theory and particle simulations in homogeneous plasma for densities 0.5≤ne/nc≤0.8. Linear theory for IAD and OTS is reviewed and compared with particle‐in‐cell simulations. Nonlinear aspects of IAD such as the appearance of ion‐acoustic second harmonic are observed and theoretically investigated. A saturation mechanism for IAD in which the second harmonic plays an important role is proposed. A simple model in which particles move in the fields of coherent waves related to IAD (OTS) is proposed in order to explain heating of the electron distribution function. If the wave amplitudes exceed a certain threshold, large scale stochasticity (LSS) develops allowing appreciable momentum transfer to particles. The appearance of a characteristic Maxwellian tail in the electron distribution function is related to LSS in particle motion. The method presented here should also be useful for analyzing hot particle production in other parametric instabilities.

ISSN:0031-9171    

DOI:10.1063/1.865063

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

One‐dimensional numerical calculations are made to simulate the acceleration of thin foils with high‐power laser beams. The main objective is to study the effects of a realistic equation‐of‐state in contrast to an ideal gas approximation. Shock build‐up and dynamics show marked differences, compression and entropy production are strongly reduced. The thermodynamic state of the accelerated, dense material is analyzed, and the concept of critical entropy is applied. The possibility of keeping the accelerated foils in a condensed state moving at velocities of more than 100 km/sec is discussed.

ISSN:0031-9171    

DOI:10.1063/1.865064

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The diocotron instability of an electron layer in which the electron Larmor radius is of the order of the layer thickness is studied. Remarkably, exact analytical solutions are obtainable in a nontrivial special case. These results allow an examination of the effects of finite Larmor radius for arbitrary ratios of Larmor radius to wavelength and of Larmor radius to layer thickness. In addition, an energy principle which yields a necessary and sufficient condition for instability for general distribution functions is derived.

ISSN:0031-9171    

DOI:10.1063/1.865065

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

A relativistically covariant moment description of the relativistic Vlasov equation is presented and is applied to a warm charged particle beam. Two truncation schemes of the moment equations are proposed yielding models that are distinct both mathematically and physically. The first simply neglects the heat flow tensorS &mgr;&ngr;&lgr;and recovers what is essentially the model of Newcomb. The distinctions between Newcomb’s formulation and the formulation presented in this paper are described. The second method incorporates fully one of the relativistic constraints on the moments and assumesS &mgr;&ngr;&lgr;nonzero, but no fourth moments are introduced. The first leads to a hyperbolic system of partial differential equations, whereas the second is partially elliptic and partially hyperbolic. Thus, although the second system appears better on physical grounds, it leads to a mathematically ill‐posed system and cannot be used. The second model is rejected. The first system is used to analyze anew the Bennett pinch and space‐charge neutral steady flows. It is found that the usual assumptions regarding particular elements of the pressure tensor &THgr;ijare incomplete and require modification. For the nonrotating and rigidly rotating Bennett pinch steady flows, it is found that the beam rest frame pressure tensor is diagonal if and only ifallof the diagonal elements are equal. Only when the axial and radial pressures are equal can one invoke the standard assumption of isotropy in a plane perpendicular to the axial guide magnetic field. For rigidly rotating space‐charge neutral steady flows, the same conclusion holds but now from the perspective of the laboratory frame. For both types of steady flows in the case of rotational shear, &THgr;ijis generally nondiagonal and anisotropic in the respective reference frames. An arbitrary warm beam steady flow must satisfy the constraints imposed by the generalized ‘‘equations of state’’ given in this paper in order to be physically legitimate.

ISSN:0031-9171    

DOI:10.1063/1.865066

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The mechanism by which unstable electron plasma waves are converted into electromagnetic waves in a uniform plasma is investigated. Electromagnetic radiation is generated upon injection of an electron beam (500 eV) into a collisionless quiescent magnetoplasma (ne≲1012cm−3,kTe≊2 eV). The emission (&ohgr;0) is observed to peak near the plasma frequency (&ohgr;p) which is well above the cyclotron frequency (&ohgr;c≪&ohgr;p≲&ohgr;0). It is shown that electromagnetic waves (&ohgr;0,k0) are produced by the scattering of electrostatic plasma waves (&ohgr;e,ke) off self‐consistently produced ion‐acoustic waves (&ohgr;i,ki). At low beam intensities the frequency and wave vector matching conditions are experimentally verified (&ohgr;e=&ohgr;i+&ohgr;0,ke=ki+k0&bartil;ki). The emission is found to be polarized, negligible in intensity at &ohgr;0=2&ohgr;p, and its source is localized. The space‐time evolution of the three‐wave interaction is presented. Besides these nonlinear wave–wave interactions the wave–particle interactions are investigated. It is found that the strong Langmuir turbulence exhibits a three‐dimensional character. Cross‐correlation surfaces are measured, and their characteristic scale lengths are found to decrease with increasing wave intensity toward the Debye length (&lgr;D&bartil;20 &mgr;m) although the resolution is probe limited (Lmin&bartil;1 mm). The beam electrons are diagnosed using a novel directional velocity analyzer which is capable of resolving the true three‐dimensional distribution function. After interacting with the intense Langmuir waves, the beam electrons are scattered in velocity space, both parallelandperpendicular to their injection velocity. The background electron distribution is observed to develop an energetic anisotropic tail (E≲50 eV,kTe&bartil;1 eV). The energization of the background electrons coincides with the strong damping of the Langmuir waves. The nonlinear damping is seen as an anomalous ac resistivity produced by large amplitude ion‐density fluctuations (&dgr;ni/ni&bartil;5%) in the electron beam region.

ISSN:0031-9171    

DOI:10.1063/1.865067

出版商:AIP    

年代:1985

数据来源: AIP

摘要:

The flow of electrons produced in the pinch region of a relativistic electron beam (REB) diode is studied by means of the time‐dependentK&agr;emission from a double‐layered anode. The anode materials were Cu and Ni foils of various thicknesses. The experimental data were analyzed using detailed Monte‐Carlo simulation. Results indicate that no anomalous deposition processes seem to occur, and that the angular distribution at the anode can be fairly well represented by an axially symmetric angular distribution with an angle of incidence of 60° to the normal.

ISSN:0031-9171    

DOI:10.1063/1.865068

出版商:AIP    

年代:1985

数据来源: AIP