摘要:

Neoclassical ion viscosity of a steady tokamak plasma with large mass flow is calculated in a general geometry. In addition to the usual ‘‘resonant’’ contribution, the ‘‘nonresonant’’ contribution has been evaluated. It is found that the nonresonant contribution is not small compared to the resonant contribution when the poloidal rotation speed is large. The viscosity evaluation has been reduced to one‐dimensional integrals. Furthermore, simple analytic formula for the viscosity coefficients are suggested. The velocity strain, which is the source of the viscosity, is also analyzed in detail.

ISSN:0899-8221    

DOI:10.1063/1.860553

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Data from several diagnostics employed on the LargesExperiment (LSX) field‐reversed theta pinch were analyzed to seek correlation between plasma distortions and the confinement properties of the field‐reversed configurations (FRC’s) formed. In particular, an array ofB&thgr;probes was used to determine separatrix movement, which might indicate the existence of low‐order modes, such as a tilt instability. No correlation between the quality of confinement and signal was observed. The parameters, equal to the average number of ion gyroradii inside the separatrix, has been postulated as a measure of FRC stability with values above 2, leading to instability and loss of confinement. However, the confinement observed in experiments conducted over a large range ofs(1<s<8) appeared to correlate more with the shape of the equilibrium radial density profile produced during formation rather thans. Flatter profiles correlated with poorer confinement.

ISSN:0899-8221    

DOI:10.1063/1.860554

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A tokamak with slightly imperfect magnetic surfaces should have a microscopically filamented current structure. If so, its equilibrium has an analog in the dynamics of interacting charged rods. Then there will be a natural current profile, analogous to thermal equilibrium of the rods (and the natural profile can be calculated by conventional statistical mechanics). This would account for the phenomenon of profile consistency or resilience in tokamaks. In addition to the natural profiles, this filamentary model also predicts an anomalous inward flux of both heat and particles in a tokamak, as well as an anomalous diffusion. These ‘‘inward‐pinch’’ components are related to the current gradient.

ISSN:0899-8221    

DOI:10.1063/1.860555

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Magnetic islands play an important role in determining plasma transport in toroidal magnetic confinement systems. The magnetic error fields and the associated islands can be theoretically described by a Hamiltonian approach. Experiments have been conducted on the Compact Auburn Torsatron [Fusion Technol.18, 281 (1990)] to test the applicability and limits of the first‐order Hamiltonian theory to magnetic islands. A novel set of helical trim coils with the same spatial periodicity as an &igr;– = 1/2 magnetic island were used to create a perturbation field of variable magnitude and phase. Extensive experimental parametric studies of the magnetic island were conducted and compared with theory. It was found that the first‐order Hamiltonian theory is applicable for the complete range of experimental error fields.

ISSN:0899-8221    

DOI:10.1063/1.860556

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

In this work the effect of ray stochasticity on the multipass absorption of lower‐hybrid waves, used to drive current in tokamaks, is considered. In toroidal geometry, stochasticity arises as an intrinsic property of the Hamiltonian ray trajectories for lower‐hybrid waves. Based on the wave kinetic equation, a diffusion equation is derived, with damping and sources, for the wave energy density in the stochastic layer. This equation is solved simultaneously with the electron Fokker–Planck equation to describe the quasilinear flattening of the electron distribution function and the subsequent modification of the wave damping. Steady‐state solutions of this system (obtained numerically) indicate that the spectral gap is filled in a self‐regulating manner, so that the boundaries of the diffused wave spectrum are independent of the level of ray stochastic diffusion. This allows the development of a simple (semianalytic) model for the self‐consistent wave spectrum and the radial profile of absorbed power.   

ISSN:0899-8221    

DOI:10.1063/1.860557

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

In this article, a general study of diffusional effects is made numerically as well as theoretically with the aim to reproduce not only the current scaling of ion diodes but also the divergence of the ion beam and physical features such as virtual cathode movement due to the diamagnetic effect. It has been observed experimentally that the ion current exceeds that quantity which would be expected in the presence of a strong magnetic field. It is demonstrated that with the introduction of a diffusional fieldE&thgr;into the stationary 21/2‐dimensional particle‐in‐cell code based on boundary‐fitted coordinates [T. Westermann, Nucl. Instrum. Methods A263, 271 (1988)] the simulation results agree well with experiment. Thisadhocmodel is supported theoretically by linear stability analysis of the stationary state.

ISSN:0899-8221    

DOI:10.1063/1.860956

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Grid image refractometry (GIR) is proposed as a technique for determining the two‐dimensional density profiles of long scale‐length laser‐produced plasmas. Its distinctive feature is that an optical probe beam is broken up into ‘‘rays’’ by being passed through a grid before traversing the plasma. The refraction angles of the rays are measured by imaging the plasma at two or more object planes and are integrated to yield the phase front. For cylindrically symmetric plasmas the density profile is then determined using Abel inversion. The feasibility of GIR is illustrated by an experiment in which a thick CH target was irradiated with ∼100 J of 527 nm radiation and diagnosed with a 20 ps, 263 nm probe. The resulting density profile is substantially larger than any that have previously been reported using interferometry and compares quite closely with hydrodynamic simulations.

ISSN:0899-8221    

DOI:10.1063/1.860558

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Transverse instabilities are investigated for the slab analog of the Bennett equilibrium of a magnetically self‐focused relativistic electron beam propagating through a plasma. Two kinds of plasma are considered: a channel matched to the beam and a broad uniform plasma. The high‐frequency electrostatic transverse two‐stream instability and the lower‐frequency resistive‐hose instability are treated analytically with a distributed tune (‘‘mass’’) model. Asymptotic growth is computed for each instability in various regimes, and analytic results are benchmarked against numerical solutions of the linearized system and particle‐in‐cell simulations. It is found that in the limit of large collisionless skin depth the electrostatic hose is the more virulent mode of beam breakup. Numerical examples are discussed for parameters in the range considered for plasma focusing of the asymmetric beams of interest in high‐energy physics.

ISSN:0899-8221    

DOI:10.1063/1.860559

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

In this paper, results of a set of numerical simulations applicable to the problem of particle acceleration by the beating of high‐energy laser waves are presented. The simulations have periodic or open‐ended boundary conditions. The growth rates of the parametric instabilities that are present in both situations are identical. In all cases they yield a destruction of the large amplitude plasma wave in a very short time. The amplitude of the saturation level is shown to be strongly dependent on initial conditions. Finally, some results in the term of the evolution of the distribution function of the background plasma in the context of strong turbulence are shown.

ISSN:0899-8221    

DOI:10.1063/1.860560

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Time‐ and two‐dimensional (2‐D) space‐resolved monochromatic imaging is used to study the dynamics of Ne‐like populations in the electron collisionally excited germanium x‐ray laser. Large‐scale variations in then=3 level population, specifically the upper level of the 2s2p63d‐2s2p63plaser transition, are observed in plasmas created in uniform and nonuniform irradiation conditions, and the degree of homogeneity along the amplification axis is related to the x‐ray gain. The influence of inhomogeneities on the core‐excited 3d‐3px‐ray lasers is discussed.

ISSN:0899-8221    

DOI:10.1063/1.860561

出版商:AIP    

年代:1993

数据来源: AIP