摘要:

A Zakharov‐type set of two differential equations is derived, describing the interaction of upper‐hybrid waves with low‐frequency density perturbations in the case of Boltzmann distributed electrons. It is shown that low‐frequency self‐interaction terms lead to the formation of a traveling solution in the form of a solitary double vortex. This solution can be identified as a new nonlinear mode, related to the driven Hasegawa–Mima modon.

ISSN:0899-8221    

DOI:10.1063/1.859135

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

For a two‐dimensional, stationary, unmagnetized plasma flow around an obstacle, the basic equations of a plasma (Poisson’s equation and equations of continuity and momentum) around the near‐wake of the obstacle are shown to be self‐similar in a limited sense in the case of a constant ion flow velocity (vi,z=constant). For hypersonic plasma flow with Mach numbers 4.0 to 17.5, the evidence of the limited self‐similarity is shown experimentally for the distributions of the plasma potential, the plasma density, and ion velocity in the near‐wake of the obstacle. A metallic or glass obstacle is used. Charge imbalance is found around the near‐wake along the boundary with the hypersonic plasma flow. The remarkable effects of nonzero net space charge on the limited self‐similarity are investigated both experimentally and theoretically.

ISSN:0899-8221    

DOI:10.1063/1.859136

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The plasma–sheath equation for a collisionless plasma with a finite‐temperature particle source in a nonuniform open magnetic field is formulated. The plasma equation is solved analytically and the plasma–sheath equation is also solved numerically for various profiles of the magnetic field. The potential formed in the plasma depends considerably on the profile of the magnetic field strength; the potential drop in the presheath increases and the ion distribution function widens as the magnetic field strength decreases in the direction of the plasma flow. Application of the generalized Bohm criterion shows that the solution of the plasma equation always satisfies this criterion when the magnetic field monotonically decreases in the outside direction. The potential drop in the sheath is almost independent of the magnetic field profile, and the ion energy at a wall for a decreasing magnetic field is slightly large when compared with that for a uniform magnetic field.

ISSN:0899-8221    

DOI:10.1063/1.859137

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

A convective instability that leads to the filamentation of large‐amplitude Alfve´n waves and generates nonoscillatory spatially growing density fluctuations is investigated by using a two‐fluid plasma model. Based on linear perturbation analysis for a four‐wave parametric coupling process, the dispersion relations of the instability for both a right‐ and left‐hand circularly polarized Alfve´n pump are derived. They are then solved numerically for the dependences of the threshold fields and growth rates on the wavenumberkof the nonoscillatory mode, the beta of the plasma, and the frequency of the pumps, for both right‐ and left‐hand circularly polarized Alfve´n pumps. The relevance of the proposed instability with some observations in the space plasmas is discussed.

ISSN:0899-8221    

DOI:10.1063/1.859138

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Temporal and spatial measurements of electron heat transport are made in the University of California Davis AURORA device (J. H. Rogers, Ph.D. dissertation, University of California, Davis, 1987). In AURORA, a microwave pulse heats a region of underdense, collisional, plasma (n/ncr≲1, wherencr=1.8×1010cm−3is the critical density,Te0≊0.15 eV, and the electron scattering mean free path &lgr;⊥≳2 cm). In this region, strong thermal heating(Tc≲0.7 eV) as well as suprathermal heating (Th≊3 eV) is observed. The strong heating results in a steep temperature gradient that violates the approximations of classical heat diffusion theory (LT/&lgr;⊥≳3 for thermal electrons, whereLT=Tc(∂Tc/∂z)−1is the cold electron temperature scale length. The time evolution of the electron temperature profile is measured using Langmuir probes. The measured relaxation of the temperature gradient after the microwave pulse is compared to calculations using the Fokker–Planck International code [Phys. Rev. Lett.49, 1936 (1982)] and the multigroup, flux‐limited, target design codelasnex[Comm. Plasma Phys.2, 51 (1975)]. The electron distribution function at the end of the microwave pulse is used as initial conditions for both codes. The Fokker–Planck calculations are found to agree very well with the measurements. However, the flux‐limited diffusion calculations do not agree with the measurements for any value of the flux limiter.

ISSN:0899-8221    

DOI:10.1063/1.859139

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

A unified basis from which to study the transport of tokamaks at low collisionality is provided by specializing the ‘‘generalized Balescu–Lenard’’ collision operator to toroidal geometry. Explicitly evaluating this operator, ripple, turbulent, and neoclassical transport coefficients are obtained, simply by further specializing the single operator to different particular classes of fluctuation wavelength and mode structure. For each class of fluctuations, the operator possesses a diffusive, test‐particle contributionD, and in addition a dynamic drag termF, which makes the operator self‐consistent, and whose presence is accordingly essential for the resultant fluxes to possess the appropriate conservation laws and symmetries. These properties, well known for axisymmetric transport, are demonstrated for one type of turbulent transport, chosen for definiteness, by explicit evaluation of both the ‘‘anomalous diffusion’’ term arising fromD, as well as the closely related ‘‘anomalous pinch’’ term coming fromF. The latter term is neglected by test‐particle calculations, but is shown to have an important impact on the predicted fluxes.

ISSN:0899-8221    

DOI:10.1063/1.859140

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

An existing pseudospectral code for solving the three‐dimensional equations of ‘‘reduced’’ magnetohydrodynamics is extended by adding a temperature equation. Resistivities and thermal conductivities are given their (isotropic) Braginskii temperature dependences, and are advanced self‐consistently. Realistic‐looking sawtooth oscillations are observed at modest Lundquist and Reynolds numbers. However, the oscillations are excited upon, and relax back to, a helical (rather than an axisymmetric) current channel.

ISSN:0899-8221    

DOI:10.1063/1.859141

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Tokamak sawtooth oscillations are studied with a nonreduced, fully toroidal, resistive magnetohydrodynamic (MHD) model that includes Ohmic heating, and parallel and perpendicular thermal conductions. Effects of perpendicular transport in producing different types of sawteeth, varying from simple, periodic oscillations to giant sawteeth with temperature modulations of order unity, and compound sawteeth with multiple relaxations, are demonstrated. Some of the recent experimental observations from large tokamaks, such as the fast crash times and a presumed topological anomaly in the x‐ray tomography pictures, thought to be inconsistent with the Kadomtsev reconnection model, are examined and possible explanations are offered.

ISSN:0899-8221    

DOI:10.1063/1.859142

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Magnetohydrodynamic (MHD) stability and nonlinear evolution of them=1 mode are studied in toroidal geometry for safety factor profiles having an inflection point at theq=1 surface. For ideally stable cases, linear growth rates of the resistivem=1 mode are decreased by any combination of the following variations: decreasing the aspect ratio, decreasing shear at theq=1 surface, increasingS, decreasingq0, and/or decreasing beta. Transitions from resistive kink to tearing mode to complete stability are accompanied by increasing localization as the mode is stabilized. The ideal stability of them=1 mode for low‐shear inflection‐point profiles is broken at very low, but finite, beta by the appearance of a radially localized ideal interchange mode. As beta is increased, them=1 mode becomes a global ideal internal kink. These ideal modes are stabilized by decreasing the aspect ratio and, to a lesser extent, by increasing shear at theq=1 surface. Nonlinear evolution of them=1 mode is found to follow the Kadomtsev complete reconnection process or to be stable, depending on the linear stability of the mode. However, all nonlinear calculations were carried out at Lundquist numberS=105and hence contain substantial resistive kink contributions. Consequently, nonlinear evolution in them=1 tearing mode regime is not examined. Even for cases in which ideal effects dominate the linear mode, Kadomtsev reconnection is observed. The nonlinear stability of linearly stable modes was also uniformly observed, even for several cases having large initial perturbations. Hence complete linear stabilization resulting from a combination of toroidal effects and low shear at theq=1 surface can suppress sawteeth, as described by the Kadomtsev reconnection process, in tokamaks having safety factor profiles withq0<1 at the magnetic axis; but, for theSvalues considered, nonlinear calculations of unstable modes result in complete Kadomtsev reconnection.

ISSN:0899-8221    

DOI:10.1063/1.859003

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Resistive ‘‘infernal’’ modes are considered that are just below the ideal internal mode threshold. Nearest the ideal threshold, a resistive continuation of the ideal infernal mode, whose growth rate scales as &eegr;3/13(&eegr; being the resistivity), is seen. Farther away is the conventional tearing mode with a &eegr;3/5scaling. These scalings are found both numerically and analytically.

ISSN:0899-8221    

DOI:10.1063/1.859004

出版商:AIP    

年代:1989

数据来源: AIP