摘要:

One of the most interesting reversed‐field pinch (RFP) experimental results occurs in discharges where the toroidal current is slowly raised after toroidal field reversal is established. These discharges exhibit the RFP dynamo effect in its most unambiguous form showing the strong nonlinear coupling of the external poloidal and toroidal magnetic field circuits via the plasma‐magnetic‐field configuration. Three‐dimensional, single‐fluid resistive magnetohydrodynamic simulations of these discharges are reported. It is found that the appropriate nonlinear coupling needed to explain the experimental observations is provided by one or more helical resistive modes of long wavelength.

ISSN:0031-9171    

DOI:10.1063/1.865462

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The kinetic equation governing the evolution of a nonaxially symmetric velocity distribution function of plasma particles in a uniform magnetic field, including the effects of collisions as well as quasilinear diffusion, is presented and analyzed. It is shown that, if the test particles’ orbits are straight lines, the kinetic equation reduces to a standard Fokker–Planck form with friction, collisional diffusion, and quasilinear diffusion coefficientsallexpressible in terms of scalar potentials. These potentials are simple generalizations of Rosenbluth potentials and can easily be used in numerical solutions of the Fokker–Planck equation.

ISSN:0031-9171    

DOI:10.1063/1.865463

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

By drawing an electron current through a plasma to a variably biased conducting button, the electrostatic ion cyclotron instability is excited, consistent with predictions of Drummond and Rosenbluth [Phys. Fluids5, 1507 (1962)]. Recent results show that a nonuniform steady‐state electric field reaches axially into the plasma. Our results indicate that electrons are accelerated along magnetic field lines and decelerated by collisions. The point in space where the electron drift speed exceeds a critical speed (vc∼0.2vte) determines the instability amplitude and frequency. This initiation point moved further into the plasma as the externally controlled electric field was raised. This experiment demonstrates the importance of applying biases so thate&fgr;/T≲1, near threshold, to observe the linear electrostatic ion cyclotron instability.

ISSN:0031-9171    

DOI:10.1063/1.865464

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Azimuthally symmetric (m=0) radio‐frequency (rf) waves for zero and for finite axial wavenumberkare investigated in a low‐compression linear theta pinch. The (k=0) modes occur spontaneously following the implosion phase of the discharge. For thek≠0 modes, a novel 100 MW, 1 MHz current drive is used to excite the plasma column in the vicinity of the lowest fast magnetoacoustic mode at various filling pressures. Phases, amplitudes, and radial mode structure are studied for both thek=0 modes and the externally driven (k≠0) modes. In the first case, the damping is determined from thee‐folding time of the decaying oscillations. In the latter case, the phases and amplitudes indicate a broad resonance structure, from which we extract the damping constant. The energy deposition of the externally driven rf wave leads to a radial expansion of the plasma column, as observed by axial interferometry and by excluded flux measurements. These experimental results are compared with the predictions of two MHD‐like ion‐kinetic models. The characteristic and resonant frequencies, as well as the oscillatory radial mode structure, can be understood within the ideal MHD description. It is found that the viscous kinetic model overestimates the observedk=0 damping by at least an order of magnitude, while both this and the ion Landau‐damping model (with and without ion‐cyclotron effects) underestimate the (k≠0) damping by at least an order of magnitude. The experimentally observed damping and wave‐energy deposition are consistent with the magnitude of the rf oscillations. The efficiency of the rf energy deposition is at least 27%.

ISSN:0031-9171    

DOI:10.1063/1.865465

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Ion acceleration via wave–particle interaction is experimentally studied in a pulsed ion‐beam–plasma system including nonlinear waves of two different modes. Wave fronts of the two modes are observed to accelerate separately a portion of the upstream plasma ions. To interpret the observed maximum energy of the accelerated ions, it is found that the Boltzmann relation for electrons is not valid for large amplitude waves (&dgr;N/N≳0.3) and it should be replaced by the equation given by Forslund and Shonk [Phys. Rev. Lett.25, 1699 (1970)].

ISSN:0031-9171    

DOI:10.1063/1.865466

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

An electron beam injected into a conducting medium produces inductive electric fields. These fields drive conduction currents that, for stable beams, oppose the beam current and thereby reduce the total (net) current. For unstable beams that undergo large transverse displacements, the inductive fields can reverse direction and drive conduction current parallel to the beam current. An analytic and numerical treatment of the latter effect is presented to explain current enhancement as observed for electron beams propagating in gases at pressures above 10 Torr.

ISSN:0031-9171    

DOI:10.1063/1.865467

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Filamentation stability properties of a self‐pinched hollow electron beam propagating through a collisional plasma channel are investigated within the framework of linearized Vlasov–Maxwell equations, assuming that the beam is thin and that the equilibrium and perturbed space‐charge fields are neutralized by background plasma. It is further assumed that the perturbations are well tuned withk&bgr;bc+l&ohgr;band satisfyla≪R0, wherelandkare the azimuthal and axial wavenumbers, &bgr;bcand &ohgr;bare the axial velocity and the rotational frequency of the beam, and 2aandR0are the thickness and mean radius of the beam. From the stability analysis, two distinctive unstable mechanisms are identified: the return‐current driven instability and the resistively driven instability. It is also found that high‐l‐mode perturbations are easily stabilized by a spread in the canonical angular momentum. Making use of a linearized particle‐in‐cell code, numerical simulations are performed. The agreement between the analytical results and those of simulations is excellent.

ISSN:0031-9171    

DOI:10.1063/1.865468

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Gravity waves propagating at the surface of a fluid of infinite depth are considered. The problem is formulated in terms of a series expansion attributable to Havelock [Proc. R. Soc. London Ser. A95, 38 (1919)]. The series is truncated after a finite number of terms and the unknown coefficients are found by collocation. It is shown that this simple numerical procedure yields accurate results for waves of arbitrary steepness.

ISSN:0031-9171    

DOI:10.1063/1.865469

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The classical viscous instability calculation of Choukeetal. [Trans. AIME216, 188 (1959)], further clarified by Saffman and Taylor [Proc. R. Soc. London, Ser. A245, 312 (1958)] in the context of flow in a Hele–Shaw cell, is reconsidered and the importance of the wetting condition and the associated interfacial pressure jump is established. Important qualitative changes appear in the resulting dispersion relation when this effect is included. The wavelength of maximum growth is increased and the growth rate is reduced, however, the critical wavelength for instability remains unchanged. Comparison of the theoretical results with some recent experiments appears to provide support for the new effect.

ISSN:0031-9171    

DOI:10.1063/1.865997

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Quasi‐instantaneous and ensemble averaged transverse vorticity time series were obtained in a weakly excited, single‐stream mixing layer using a four‐wire array. These data indicate that absolute values of instantaneous vorticity can be approximately an order of magnitude greater than the ensemble (or phase) averaged vorticity.

ISSN:0031-9171    

DOI:10.1063/1.865470

出版商:AIP    

年代:1986

数据来源: AIP