摘要:

The boundary layer arising in the radial vicinity of a tokamak limiter is examined, with special reference to the TEXT tokamak [Nucl. Fusion27, 1125 (1987); Phys. Fluids27, 2956 (1984)]. It is shown that sheath structure depends upon the self‐consistent effects of ion guiding‐center orbit modification as well as the radial variation ofE×B‐induced toroidal rotation. Reasonable agreement with experiment is obtained from an idealized model, which, however simplified, preserves such self‐consistent effects. It is argued that the radial sheath, which occurs whenever confining magnetic field lines lie in the plasma boundary surface, is an object of some intrinsic interest. It differs from the more familiar axial sheath because magnetized charges respond very differently to parallel and perpendicular electric fields.

ISSN:0899-8221    

DOI:10.1063/1.859067

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The anisotropic loss‐cone distribution of fusion‐born alpha particles in a mirror system is capable of destabilizing low‐frequency plasma waves. A one‐and‐one‐half‐dimensional quasilinear diffusion model is developed to study the nonlinear evolution of the alpha distribution function resulting from enhanced diffusion in velocity space and radial position driven by the unstable Alfve´n wave. It is found that pitch‐angle scattering of the resonant alphas into the loss cone causes significant losses while radial losses are negligible for a reactor‐size plasma. For a typical tandem‐mirror reactor, the alpha plasma heating is greatly reduced and ignition in the central cell cannot be achieved.

ISSN:0899-8221    

DOI:10.1063/1.859068

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

A rapid growth of magnetic fluctuations resulting from them=1 andm=2 modes and succeeding stop of rotation of these modes have been found as the precursor phenomena of current termination in the STP‐3(M) [Trans. Inst. Elec. Eng. Jpn.107‐B, 469 (1987)] reversed field pinch. By deepening the field reversal at the wall, these precursors disappear and the current duration becomes much longer. It is found that sudden current termination is caused by a rapid growth of resistive tearing modes mainly because of nonlinear coupling of them=1 mode.

ISSN:0899-8221    

DOI:10.1063/1.859205

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Stable, steady‐state plasma equilibrium is generated in an axisymmetric configuration. The diamagnetic current is maintained by electromagnetic fields rotating in the electron diamagnetic sense. The stable, free‐standing equilibrium exists for all values of beta up to 0.98, limited only by the available rf power. The plasma is centered on the axis well removed from the metal chamber walls, the limiters, and the rf antenna. The equilibrium pressure balance,J×B=grad p, is quantitatively verified. Radial profiles of pressure and magnetic field show that the equilibrium is that of a rigid rotor.

ISSN:0899-8221    

DOI:10.1063/1.859069

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Images of the visible light emission from the inner wall region of the TFTR tokamak [M. G. Belletal., inPlasmaPhysicsandControlledNuclearFusionResearch1988, Proceedings of the 12th International Conference, Nice, France (IAEA, Vienna, in press)] have been made using a rapidly gated, intensified TV camera. Strong ‘‘filamentation’’ of the neutral deuterium D&agr;light is observed when the camera gating time is <100 &mgr;sec during neutral‐beam‐heated discharges. These turbulent filaments vary in position randomly versus time and have a poloidal wavelength of ≊3–5 cm, which is much shorter than their parallel wavelength of ≊100 cm. A second and new type of edge fluctuation phenomenon, which is called a ‘‘merfe,’’ is also described. Merfes are a regular poloidal pattern of toroidally symmetric, small‐scale marfes that move away from the inner midplane during the current decay after neutral beam injection. Some tentative interpretations of these two phenomena are presented.

ISSN:0899-8221    

DOI:10.1063/1.859070

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Measurements of the spatial distribution of the rf electric field intensities and phases induced in a free‐electron laser (FEL) amplifier operating in the collective (Raman) regime are reported. The studies are carried out at a microwave frequency of ∼10 GHz in a FEL using a mildly relativistic electron beam of ∼160 keV energy and 1– 4 A current. The probing of the ponderomotive (space‐charge) and electromagnetic waves is accomplished by means of small movable electric dipole antennas inserted into the interaction region.

ISSN:0899-8221    

DOI:10.1063/1.859071

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The free‐electron laser (FEL)‐induced modifications to the vacuum waveguide modes are calculated for low‐frequency FEL’s. Typically, the mode modifications are large and exhibit complicated axial behaviors. In addition to the wave component with a near vacuum wavenumber, components at two upshifted wavenumbers must be analyzed. Electron beam surface charges and currents are also important. At low gain, effects from all three roots of the FEL dispersion relation must be included. The dominant modification is due to the electron beam space‐charge wave.

ISSN:0899-8221    

DOI:10.1063/1.859072

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The dispersion relation is derived for the interaction of a sheet beam propagating between parallel plates in a planar wiggler magnetic field. Instead of the usual free electron laser (FEL) mode, one is considered in which the radiation field is perpendicular to the quiver velocity and couples either to betatron or plasma oscillations. In the cold beam limit, coupling to betatron oscillations leads to an absolute instability. This mode competes with the conventional FEL and can disrupt the beam unless suppressed. The effects of energy spread are investigated, and a critical energy spread is found that suppresses the instability. For ultrarelativistic beams, this energy spread is an exponentially decreasing function of the beam energy. For moderately relativistic high current beams, it is proportional to the square root of the beam current. In this regime, there is a limit to the current that can propagate stably in the FEL circuit. Coupling to plasma oscillations also leads to an instability. However, the instability associated with this mode can be made convective with a suitable choice of parameters, and thus is not particularly dangerous to the conventional FEL. It is, however, interesting because the dispersion relation is characteristic of a second harmonic FEL, even though the wiggler is planar.

ISSN:0899-8221    

DOI:10.1063/1.859073

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Erosion rates for a relativistic electron beam propagating through a low‐density plasma channel in the ion‐focused regime are computed via axisymmetric particle simulation. Steady‐state inductive (Ohmic) erosion rates are found that are in quantitative agreement with new theoretical results, which correctly account for the relativistic dynamics of electrons at the beam head. In cases where the finite emittance of the beam is expected to be the dominant loss mechanism, the beam does not develop steady‐state erosion, as suggested by previous analyses. Asymptotically, such a beam is characterized by a low‐emittance population at the beam head, a long rise length, and an erosion rate that tends toward zero.

ISSN:0899-8221    

DOI:10.1063/1.859074

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

A numerical model has been developed for calculating x‐ray line emission from spherical plasma flows and has been applied to a laser‐heated, ablating aluminum target. The results are in good agreement with previous experimental work. The intensity ratios of the dominant lines are predicted in good agreement with experiment. A novel effect, not yet observed, is predicted in the interaction of nearby lines, here an H&agr;doublet: through Doppler shift caused by plasma flow one line absorbs the other. This line obscuring effect has potential applications to diagnosing details of ablative flows in x‐ray lasing and inertial confinement fusion experiments. The main features of the model are plasma parameters obtained through a Lagrangian hydrodynamics model, radiative transport by multienergy and multi‐impact parameter ray groups, and self‐consistent, time‐dependent computation of ion and ion energy‐level populations. Approximately 100 energy groups per line resolve line structure and show Doppler effects. The scalings involved in extending this work to other plasma conditions and materials are discussed.

ISSN:0899-8221    

DOI:10.1063/1.859075

出版商:AIP    

年代:1989

数据来源: AIP