摘要:

Neoclassical resonant‐plateau transport in a minimum‐Banchored tandem mirror is calculated in an experimentally observed case where a flux tube of equipotential contours is not circular at the central cell.

ISSN:0899-8221    

DOI:10.1063/1.858975

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

A numerical hydrodynamics chemistry model to simulate the laser–target interaction experiment at the Naval Research Laboratory’s PHAROS [LaserInteractionandRelatedPlasmaPhenomena(Plenum, New York, 1986), Vol. 7, p. 857] is presented. Both laser–target and debris–background interactions are modeled, solving mass continuity, total momentum, and separate ion and electron internal energy equations. The model is appropriate for background densities≥1 Torr. To accurately treat both the early‐time planar ablation and the later spherical expansion of the blast wave, as well as the rear‐side shock front, an oblate spheroidal coordinate system was adopted. The aluminum target ablates into and interacts with an ambient nitrogen gas, filling the facility chamber. The simulation models the target continuously from the solid state to the state of a highly ionized nonequilibrium plasma, including all charge states of aluminum and all charge states of the nitrogen background. The laser beam has a wavelength of 1 &mgr;, a ∼5 nsec full width at half‐maximum (FWHM), an intensity at the target surface ∼1013W/cm2, and total energy varying from 20–100 J. The model accurately reproduces the measured time‐of‐flight profile and the mass of ablated aluminum. Expansion of the blast wave in the model follows the ideal Sedov relation until radiation losses force a deviation due to a failure in the constant energy assumption. In the shock wave region the simulations show electron density of a few times 1018cm−3, temperatures ranging from 10–20 eV, and dominant nitrogen species of N+3and N+4, all in agreement with experimental measurement. A calculated profile of electron density both in the shock and in the cavity region agree closely with experiment and imply an average aluminum charge state of 11 times ionized in the cavity out to late times, as predicted by the simulation described in this paper.The simulation suggests, also, that observed rear‐side structuring is a result of a deceleration Rayleigh–Taylor instability. The model is capable of providing detailed predictions, which are presented, as to profiles of charge states, densities, and temperatures as a function of time; these predictions are not yet tested by experimental measurement.

ISSN:0899-8221    

DOI:10.1063/1.858976

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

An analytical model is developed to describe the parameters of the fuel and the pusher at peak compression when a two‐layer spherical shell target is imploded by a single pressure pulse. The entropy generated by shock waves during the implosion is calculated and an approximate description of the process of central hot spot formation is given. The stagnation stage of a compressible shell is described and scaling laws for the final stage are found. Comparisons with available simulation data are presented and the effect of the thermal conduction in the formation of the central spark is discussed.

ISSN:0899-8221    

DOI:10.1063/1.858977

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The nonuniform (nonlinear) equilibria of the classical (short circuit) Pierce diode and the extended (series RLC external circuit) Pierce diode are described, and the spectrum of oscillations (stable and unstable) about these equilibria are worked out. It is found that only the external capacitance alters the equilibria, though all elements alter the spectrum. In particular, the introduction of an external capacitor destabilizes some equilibria that are marginally stable without the capacitor. Computer simulations are performed to test the theoretical predictions for the case of an external capacitor only. It is found that most equilibria are correctly predicted by theory, but that the continuous set of equilibria of the classical Pierce diode at Pierce parameters (&agr;=&ohgr;pL/v0) that are multiples of 2&pgr; are not observed. This appears to be a failure of the simulation method under the rather singular conditions rather than a failure of the theory.

ISSN:0899-8221    

DOI:10.1063/1.858925

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The existence of the strange attractor discovered by Godfrey [Phys. Fluids30, 1553 (1987)] in the neighborhood of &agr;=3&pgr; for the Pierce diode is verified, and his numerical results are refined. The theory of Feigenbaum for the sequence of period‐doubling bifurcations [J. Stat. Phys.19, 25 (1975)] is tested with good agreement, despite the strong assumptions made by that theory. The evolution of this attractor is then followed as an external capacitance is introduced, producing a family of bifurcation diagrams. Examination of these diagrams produces one result that should be of interest to mathematical physicists: The existence of an unstable equilibrium in the neighborhood of the strange attractor is strongly implicated in both the existence and destruction of the attractor. The reversibility of the equations of evolution is also discussed, but no clear‐cut conclusion is reached.

ISSN:0899-8221    

DOI:10.1063/1.859199

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

A particle‐in‐cell code has been used to study the electron beam–plasma interaction, withnb<np, in a bounded one‐dimensional system. Sheaths form on the boundaries because electrons leave the system as a result of the beam–plasma interaction heating the plasma electrons. The ions are chosen to be immobile since most time scales are too short for ion motion. The sheaths reflect waves and beam electrons, so that the interaction rapidly reaches a nonlinear stage. Standing waves have been observed. Despite this, the rate at which the beam–plasma interaction heats the plasma is found to agree with linear and quasilinear theory for weak beams in long systems, except whennb≳np/4 or the interaction length,L≳3vb/fpe.

ISSN:0899-8221    

DOI:10.1063/1.858926

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Measurements of millimeter and submillimeter wavelength emission (240 GHz<&ohgr;/2&pgr;<470 GHz) from a free‐electron laser are reported. The laser operates as a superradiant amplifier and without an axial guide magnetic field; focusing and transport of the electron beam through the wiggler interaction region are achieved by means of the bifilar helical wiggler field itself. Approximately 18 MW of rf power has been observed at a frequency of 470 GHz, corresponding to an electronic efficiency of 0.8%. Frequency spectra are measured with a grating spectrometer and show linewidths &Dgr;&ohgr;/&ohgr;∼2%–4%. The experimental results are in very good agreement with nonlinear numerical simulations.

ISSN:0899-8221    

DOI:10.1063/1.858927

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Rotating stars or planets like Earth are sometimes surrounded by a dipolar magnetic field distribution. The thermal plasma forming a corona or an ionosphere around these astrophysical objects diffuses upward along the magnetic field lines and forms a toroidal region filled with this thermal plasma, like the terrestrial plasmasphere. The field‐aligned distribution of this thermal ionospheric plasma is controlled by the gravitational and pseudocentrifugal potential distribution. One can distinguish two extreme types of plasma distribution in this field‐aligned potential: the diffusive equilibrium distribution and the exospheric equilibrium distribution corresponding, respectively, (i) to a saturated, and (ii) to an almost depleted, magnetic flux tube. As a result of pitch angle scattering by Coulomb collisions, an increasing number of ions escaping from the ionosphere are stored in trapped orbits. These trapped particles have magnetic mirror points at high altitudes, i.e., in the low‐density exospheric region. Also as a result of collisions, the field‐aligned density distributions irreversibly evolve from exospheric equilibrium with a highly anisotropic pitch angle (cigarlike) distribution to a diffusive equilibrium distribution characterized by an isotropic pitch angle distribution. It is shown that the suprathermal ions become anisotropic much more slowly than ions of energies smaller than 1 eV.

ISSN:0899-8221    

DOI:10.1063/1.858928

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

In this paper detailed simulations are presented of radiation‐hydrodynamic response of gaseous cylindrical targets irradiated with heavy‐ion beams that will be produced at the Gesellschaft fu¨r Schwerionenforschung, Darmstadt, using a heavy‐ion synchrotron (SIS) [HeavyIonFusion, AIP Conference Proceedings No. 152 (AIP, New York, 1986), p. 23]. The purpose of this work is to explore material conditions for which the thermal radiation effects can be maximized. This is desirable in order to study a number of interesting and important effects including maximization of conversion efficiency of the ion beam energy to thermal radiation and measurement of the target opacity in the SIS experiments. It is expected that the SIS beams will produce a specific deposition power of 10 TW/g. The simulations in this paper show that a temperature of the order of 10 eV could be achieved by the SIS beams using homogeneous, cylindrical Xe targets. It has been shown that with the help of these computer simulations one should be able to measure the target opacity in these experiments within a factor of 3. Also these calculations show that in the SIS experiments one should be able to have a 50% conversion efficiency using a Xe target under optimum conditions. It has been found that the radiation effects will be optimized in the SIS experiments if the initial target density is of the order of 10−3g/cm3. If the initial density is too high (of the order of 10−1g/cm3or more), hydrodynamic effects will dominate, while, on the other hand, if the initial density is too low (of the order of 10−4g/cm3or less), the electron thermal conductivity will take over.

ISSN:0899-8221    

DOI:10.1063/1.858929

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

A linear, electrostatic, stability analysis of a magnetized cross‐field drifting plasma with a sharp boundary is presented. The analysis corrects an error in a previously published sharp boundary theory [Phys. Fluids19, 882 (1976)] and extends another theory [Geophys. Res. Lett.14, 60 (1987)] to include finite electron mass and non‐neutral perturbations. The instability’s long wavelength structure is associated with the classical flute instability, while the peak of the growth rate curve, at much shorter wavelengths, is a Buneman‐like instability.

ISSN:0899-8221    

DOI:10.1063/1.858930

出版商:AIP    

年代:1989

数据来源: AIP