摘要:

The effect of a turbulent flow on the rate of reconnection of magnetic field lines by a tearing instability is calculated numerically. The reconnection rate is shown to be reduced by the turbulent flow. The turbulent flow transfers energy from the growing tearing instability into shorter wavelength electromagnetic modes that are stable, thereby reducing the rate of reconnection of magnetic field lines by the longer wavelength tearing instability. The numerical results presented contradict the prediction of a turbulence theory of Diamondetal. [Phys. Fluids27, 1449 (1984)] that a turbulent flow causes an anomalous diffusivity (resistivity) which increases the growth rate of tearing modes.

ISSN:0899-8221    

DOI:10.1063/1.860931

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A theoretical and experimental study of Trivelpiece–Gould waves propagating in a magnetized plasma column is presented in this paper. In the experiments, these waves are excited by a radio frequency (rf) source, which also serves to create the plasma. Observation of nonlinear effects includes space and time recurrence effects, a wave spectrum containing a large number (up to 25) harmonics, and low‐frequency sidebands. The theoretical model explains the recurrence effects as a consequence of multiple nonlinear interactions between the fundamental wave and its harmonics. A good agreement is found between theory and the experiments.

ISSN:0899-8221    

DOI:10.1063/1.860975

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A quasiperiodic route to chaos is investigated experimentally and theoretically in a current‐carrying unstable ion sheath. A quasiperiodic regime with two frequencies is realized by a coherent instability and an external periodic oscillation. Chaos appears directly from the quasiperiodic regime when the amplitude of the external oscillation is increased, making nonlinear effects more important. A correlation dimension and the largest Lyapunov exponent, obtained experimentally in the chaotic regime, confirm that the observed chaos is deterministic and the attractor is strange. A period‐doubling route to chaos is also observed after the instability is quenched with the external oscillation by increasing the amplitude of the external oscillation further. These experimental results are in fair agreement with a theory that describes ion dynamics in the ion sheath.

ISSN:0899-8221    

DOI:10.1063/1.860932

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Even though there have been many experimental observations of various types of double layers (DL), laboratory data thus far have only produced special examples of the more general case of multiple magnetized double layers. A series of stationary, fully magnetized (ions and electrons), two‐dimensional (both parallel and transverse to the magnetic field) weak double layers (DL’s) with three, four, or more potential steps has been observed in a laboratory experiment for the first time. They may play important roles in the auroral zone, the physics of solar flares, high‐power gas lasers, and laser pellet ablation plasmas. The transverse electric fields are, on average, larger than the parallel ones, and the transverse spatial size scales with ion gyroradius. The total number of DL potential drops depends on the location and what particular path is followed across any given potential profile. Electron beams acquire additional directed energy across these DL’s along with heating of the bulk distribution and large fluctuating energy density.

ISSN:0899-8221    

DOI:10.1063/1.860933

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Nonlinear gyrofluid equations are derived by taking moments of the nonlinear, electrostatic gyrokinetic equation. The principal model presented includes evolution equations for the guiding centern,u∥,T∥, andT⊥along with an equation expressing the quasineutrality constraint. Additional evolution equations for higher moments are derived that may be used if greater accuracy is desired. The moment hierarchy is closed with a Landau damping model [G. W. Hammett and F. W. Perkins, Phys. Rev. Lett.64, 3019 (1990)], which is equivalent to a multipole approximation to the plasma dispersion function, extended to include finite Larmor radius effects (FLR). In particular, new dissipative, nonlinear terms are found that model the perpendicular phase mixing of the distribution function along contours of constant electrostatic potential. These ‘‘FLR phase‐mixing’’ terms introduce a hyperviscositylike damping ∝k⊥2‖&Fgr;kk×k’‖, which should provide a physics‐based damping mechanism at highk⊥&rgr; which is potentially as important as the usual polarization drift nonlinearity. The moments are taken in guiding center space to pick up the correct nonlinear FLR terms and the gyroaveraging of the shear. The equations are solved with a nonlinear, three‐dimensional initial value code. Linear results are presented, showing excellent agreement with linear gyrokinetic theory.

ISSN:0899-8221    

DOI:10.1063/1.860934

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Analysis of the generalized cross‐field current instability is carried out in which cross‐field drift of both the ions and electrons and their temperatures are permitted to vary in time. The unstable mode under consideration is the electromagnetic generalization of the classical modified‐two‐stream instability. The generalized instability is made of the modified‐two‐stream and ion‐Weibel modes. The relative importance of the features associated with the ion‐Weibel mode and those of the modified‐two‐stream mode is assessed. Specific applications are made to the Earth’s neutral sheet prior to substorm onset and to the Earth’s bow shock. The numerical solution indicates that the ion‐Weibel mode dominates in the Earth’s neutral sheet environment. In contrast, the situation for the bow shock is dominated by the modified‐two‐stream mode. Notable differences are found between the present calculation and previous results on ion‐Weibel mode which restrict the analysis to only parallel propagating waves. However, in the case of Earth’s bow shock for which the ion‐Weibel mode plays no important role, the inclusion of the electromagnetic ion response is found to differ little from the previous results which treats ions responding only to the electrostatic component of the excited waves.

ISSN:0899-8221    

DOI:10.1063/1.860935

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A two‐dimensionalk‐space simulation of long‐wavelength trapped ion mode turbulence is used to investigate Bohm (macroscopic) versus gyro‐Bohm (microscopic) scaling behavior in tokamaks. A nonlinear two‐field model of dissipative trapped ion turbulence evolving trapped ion and trapped electron density fluctuations shows gyro‐Bohm‐like scaling in contrast to earlier simulation work by Saisonetal. [Plasma Phys.20, 1 (1978)]. In fact, nearly all features of trapped ion mode turbulence and transport originally posited by Kadomtsev and Pogutse [ReviewsofPlasmaPhysics(Consultants Bureau, New York, 1970), Vol. 5, p. 379] are recovered. However, the widely used Kadomtsev–Pogutse one‐field approximation to the two‐field dissipative trapped ion model is unexpectedly found to be unrepresentative with much different fluctuation levels and diffusion. Transformation to a new field basis diagonalizing the linear part of the two‐field equations clarifies the importance of the damped branch not contained in the one‐field equation. The two‐field collisionless trapped ion mode equations are transformed into a parameterless single equation for a complex field providing a perfect scaling relation for the dependence on the driving curvature drifts.

ISSN:0899-8221    

DOI:10.1063/1.860936

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

An empirical scaling for the potential difference between the plug potential &Fgr;Pand the floating potential &Fgr;EPof an endplate is obtained in terms of end‐loss electron temperatures. Results from the GAMMA 10 tandem mirror [Phys. Rev. Lett.55, 939 (1985)] indicate that &Dgr;&Fgr;=&Fgr;P−&Fgr;EPscales much better with an effective temperatureTeff≡(1−&bgr;)TeL+&bgr;TeHthan with a single temperatureTeLorTeH, whereTeLandTeHare temperatures of the bulk and the high‐energy tail components of the end‐loss electrons and &bgr; denotes the flux fraction of theTeHcomponent. Moreover, the observed scaling is consistent with a recently developed theoretical model [K. Kuriharaetal., J. Phys. Soc. Jpn.61, 3153 (1992)].

ISSN:0899-8221    

DOI:10.1063/1.860937

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A simple model is developed that isolates the essential features of a magnetically confined plasma undergoing anomalous transport and whose properties are manipulated by externally injected radial currents. The model exhibits the following intrinsic properties (not built‐inapriori): Ohmic confinement improves with density; L‐mode scaling depends on heating power asP−0.6; a transition to H‐mode behavior is induced by radial currents on the order of 10 A. The model is benchmarked against the experiments performed in the Continuous Current Tokamak (CCT) [R. J. Tayloretal., Phys. Rev. Lett.63, 2365 (1989)]; excellent quantitative and qualitative agreement is obtained with most of the observations. Some discrepancies are identified and their significance is elucidated. The new effects explored with the model are H‐modes induced by multiple electrodes, auxiliary heating in the presence of radial currents, and the concept of H mode as a ‘‘phase transition.’’

ISSN:0899-8221    

DOI:10.1063/1.860938

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Magnetic field fluctuations have been measured in the TPE‐1RM20 reversed‐field pinch [inPlasmaPhysicsandControlledNuclearFusionResearch,Proceedingsofthe14thInternationalConference, 1992, Wu¨rzburg (International Atomic Energy Agency, Vienna, 1992) Paper IAEA‐CN‐56/C‐4‐3]. The dominant modes arem=1/n=7–9, resonant near the axis. The fluctuations are coherent, corresponding to nearly continuous rigid body rotation with poloidal and toroidal frequencies off&thgr;=130–170 kHz andf&fgr;=11–14 kHz, respectively. Discrete dynamo events (in which toroidal magnetic flux is generated) of two different types are observed at high values of pinch parameter (&THgr;≳1.6). The first type is triggered by a single, wall‐lockedm=1/n=7 mode. The slowing down of the mode rotation is in rough agreement with a simulation which assumes the effect of induced eddy current in the vacuum liner. In the subsequent relaxation event, the locked mode decays and them=1 spectrum cascades to higher‐nmodes. However, in some cases the saturatedm=1/n=7 mode exists for periods up tot=0.4 msec, resembling single helicity Ohmic states. The second type of event is characterized by phase locking of severalm=1/n=7–9 modes and the appearance of largem=0/n=1–5 modes. Mode rotation continues through the event, similar to the normal machine operating condition.

ISSN:0899-8221    

DOI:10.1063/1.860939

出版商:AIP    

年代:1993

数据来源: AIP