摘要:

It has been suggested that the recently observed fast sawtooth crashes are caused by a low‐shear, pressure‐driven ideal instability. This hypothesis is investigated, using asymptotic methods to solve the toroidal mode equations for a class of equilibria characterized by a low‐shear central region in whichq−1 is small, separated from the wall by a region with finite shear. A dispersion relation that differs significantly from previous results is obtained. An explicit expression for the growth rate is given for a modelqprofile.

ISSN:0031-9171    

DOI:10.1063/1.866750

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A simple analytic model for the estimation of lower‐hybrid‐generated current in the presence of a dc electric field is derived. The model is based on a perturbation solution of the adjoint problem to the Fokker–Planck equation and includes the effect of the continuous wave spectrum. It indicates that the dependence of the current drive efficiency on the dc electric field is determined only by the wave accessibility condition. The model’s results are found to agree with the ASDEX tokamak lower‐hybrid experimental data [Plasma Phys. Controlled Fusion27, 1399 (1985)] as well as the newly developed two‐dimensional Fokker–Planck code results. The predictions lie within an experimental scattering of about 20%.

ISSN:0031-9171    

DOI:10.1063/1.866751

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Density fluctuations have been studied using the multichord interferometer over a wide range of plasma parameters in the ZT‐40M reversed‐field pinch experiment [Nucl. Fusion20, 1255 (1980)]. The total fluctuation power does not strongly depend upon the plasma current, and therefore, the relative fluctuation level decreases with increasing the current. Antisymmetric oscillations in the frequency region from 100–200 kHz are strongly excited in the outer region of the plasma column with increasing current. The maximum beta value attained seems to be inversely proportional to the magnitude of the fluctuations in this frequency region.

ISSN:0031-9171    

DOI:10.1063/1.866752

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Experiments were conducted using a barium plasma in a simple axisymmetric mirror machine that was stabilized against the magnetic curvature‐driven interchange mode by surface line tying. Ion cyclotron resonance heating was applied to heat ions, producing a mirror‐trapped ion population. The ion‐velocity distribution function was diagnosed nonperturbatively by using laser‐induced fluorescence techniques. The rf heated distribution was a non‐Maxwellian infI(v⊥), and a sloshing distribution infI(v∥). When the plasma source was cut off, the plasma decay was studied to determine the confinement properties of the plasma afterglow and the effects of the stabilizing blanket. Mirror‐trapped ions were lost axially at the classical ion–ion collision rate. Analysis of the temporal evolution of radial profiles of ion‐energy density, in the afterglow, indicates that axial loss rates dominanted over radial losses.

ISSN:0031-9171    

DOI:10.1063/1.867006

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Spectroscopic measurements of the ion motion in a fast compression high beta tokamak experiment show paramagnetic ion motion. This motion is initially caused by magnetic field gradient drift, which does not produce any net paramagnetic current opposing the plasma diamagnetic current. After the crowbar of the fast rising toroidal magnetic field a larger paramagnetic polarization drift develops, which contributes to the disappearance of the diamagnetic well.

ISSN:0031-9171    

DOI:10.1063/1.866753

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Self‐focusing and filamentation of short wavelength laser light in highZplasmas of interest to laser fusion are discussed. It is found that self‐focusing behavior is very dependent on the details of the characteristics of the laser beam, the plasma conditions, and the energy transport processes. Laser light absorption and self‐focusing are strongly competitive processes. At. 0.26 &mgr;m wavelength the collisional absorption is often so great that there is no intensity amplification of the beam despite the fact that strong self‐focusing is present. Wide variations are found in laser light penetration, affected by several factors. Diverging optics reduce the likelihood of self‐focusing. Large scale length density gradients have little effect on focusing behavior. The self‐focusing behavior is very dependent on beam shape. Large scale hot spots can have a significant effect on whole beam self‐focusing early in the pulse. The behavior of small scale hot spots can be qualitatively different than the standard picture. The calculations indicate that small scale hot spots do not achieve a steady state in some cases. Sound waves cause chaotic interactions among neighboring hot spots. It is found that sub‐beam size structures are also generated when nonlocal thermodynamic equilibrium (non‐LTE) radiation and atomic physics are used in the calculations. The nature of the heat flux and thermoelectric magnetic field generation are examined with a kinetic model. Stimulated Raman backscattering levels in self‐focused light are significantly reduced for short wavelengths and highZplasmas Landau damping plays an important role in determining the Raman levels. Implications for suprathermal electron production, symmetric illumination, x‐ray conversion efficiency, and laser light absorption are discussed.

ISSN:0031-9171    

DOI:10.1063/1.866754

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The dispersion tensor for linear wave propagation in an arbitrary number of moving plasma species is derived using an approach that is based on Lorentz transformations. This approach requires less computational effort than the direct approach, in which the equation of motion for each plasma species is linearized about an equilibrium state with nonzero velocityvs. As a specific example, the effects of an axial magnetic field and transverse beam temperature on the collisionless Weibel instability are studied.

ISSN:0031-9171    

DOI:10.1063/1.866755

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A mechanism is proposed to greatly increase the space charge limiting current. It consists of making use of the negative ponderomotive energy of some appropriate cavity mode to thwart the buildup of the beam electrostatic energy. For present microwave technology, the calculations show, for instance, that a 70% increase in the limiting current of a 250 kV relativistic electron beam looks achievable.

ISSN:0031-9171    

DOI:10.1063/1.866756

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The free‐electron laser (FEL)–electrostatic electron cyclotron resonance maser (EECRM) hybrids, a new kind of FEL comprised of a relativistic electron beam and an electrostatic centrifugal field, are presented and studied in this paper. In a fluid dynamical linear theory, the dispersion equation for transverse electromagnetic (TEM) mode operation is derived, analyzed, and numerically solved. It is found both analytically and numerically that FEL instability occurs near beam resonance [(&ohgr;l−klv∥)2−&ohgr;2p(&ggr;−2∥−&bgr;2∥&bgr;2⊥&Jgr;˙20/ &OHgr;2l)=0]. However, FEL instability near cyclotron resonance (&ohgr;s−ksv∥=±(2−&bgr;2⊥)1/2&Jgr;˙0) is not found in the TEM mode analysis.

ISSN:0031-9171    

DOI:10.1063/1.867017

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

An extension of earlier computations on deep‐water gravity wave instabilities up to a wave steepness of 0.434 is performed to compare the relative strength of class I versus class II instabilities. The three‐dimensional class II,m=1 instabilities are still predominant. However, as the steepness increases, the difference between the growth rates of two‐dimensional and three‐dimensional perturbations decreases rapidly and becomes negligible around a 0.43 wave steepness.

ISSN:0031-9171    

DOI:10.1063/1.866757

出版商:AIP    

年代:1988

数据来源: AIP