摘要:

An estimate for the coefficient of self‐diffusion can be derived whenever the Eulerian velocity spectrum is known. The result is more general than those previously obtained. A comparison is made with computed test‐particle diffusion in an inviscid two‐dimensional Navier–Stokes fluid.

ISSN:0031-9171    

DOI:10.1063/1.861690

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

The onset of convection in a box of water‐saturated porous media is considered. The effects of viscosity variation due to large temperature differences are included. Critical conditions for three box geometries which are representative of the problem are reported: a thin vertical slab, a flat box, and a tall slender column. The critical Rayleigh number is found to be substantially reduced from the value predicted by constant viscosity calculation. The horizontal planform of the preferred cellular motion is found to be independent of the temperature difference, being a function of the aspect ratios of the box only. The liquid motion is faster near the lower hot surface since the viscosity decreases with depth. The case of very tall slender columns led to the study of a fourth‐order ordinary differential equation with a turning point at the boundary.

ISSN:0031-9171    

DOI:10.1063/1.861695

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

The first iterate solution of the Boltzmann equation is given for an impulsively started, specularly reflecting piston. An approximation for each collision operator that preserves the significant contribution from each of the fourteen collision classes which are generated by the single discontinuity in momentum space is obtained. Five of the sevenfold integrals appearing in the expressions for the first iterate moments are evaluated in closed form and the remaining two integrals are evaluated numerically. This iterative solution is valid for large departures from equilibrium, and, to the order to which the iteration has been carried, to times of the order of one or two mean free times between collisions.

ISSN:0031-9171    

DOI:10.1063/1.861699

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

A model is given for blast waves which are dominated by heat transfer rather than by shock heating and adiabatic expansion. The wave motion is attributed to a net heat flowW(watts/cm2) from the expanding hot core to the surrounding cold gas (density &rgr;1). The surface of the hot core is initially a supersonic heat wave without a shock. It changes into a subsonic heat wave with a leading shock after quickly passing through the detonation mode. A quantitative analysis of the space time trace near the detonation point yields the enthalpyhfand other parameters of the hot core, the thermal response functionhf=f(W) of the background medium, and the blast wave energyE0=const &rgr;1V2sr3H(whereVsis the shock front velocity andrHis the radius of the hot core). The analysis shows the steady transition of such a blast wave from the early stage of a ’’thermal’’ wave with negligible mass motion to the late stage (which approaches ’’classical’’ blast waves) with negligible heat transfer. Applications to laser spark plasmas are discussed.

ISSN:0031-9171    

DOI:10.1063/1.861689

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

Extensive measurements are presented of initial electric breakdown wave speed in nitrogen and argon as a function of local electric field at the wavefront. The results are in basic agreement with the electron fluid dynamic theory of the phenomenon. The apparent gas density dependence over and above theE/pdependence predicted by theory, that has been found by other investigators, is confirmed and has been traced to observational limitations. The results indicate that photo‐ionizing processes are not needed to explain the effects observed over most of the pressure range studied.

ISSN:0031-9171    

DOI:10.1063/1.861691

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

The Wigner–Keck theory of dissociation and recombination is applied to the thermal ionization reaction A+N2→A++e−+N2, where A is an alkali atom. The calculated upper‐bound ionization rate coefficient (ki) is in good agreement with flame measurements and indicates that high ionization rates are theoretically possible. The very large ionization cross sections (&sgr;i∼10−12cm2) are due to (a) ionization from excited states lying within 2kTof the ionization limit, and (b) inelastic energy transfers between electrons and the collision partner. In contradistinction to conventional collision theories, the present work shows thatkiis independent of the mass of A.

ISSN:0031-9171    

DOI:10.1063/1.861692

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

Measurements of the effectiveZof a plasma produced in a coaxial electromagnetic shock tube (ne≈2×1016cm−3,Te≈10 eV,Ti≈300 eV) were made in both deuterium and helium gas. A fast, Ge: In photodetector (4°K) was used to measure the total infrared emission spectrum (20–100 &mgr;m) and a laser interferometer was used to measure electron density. In helium, electron temperatures were measured by line emission intensity ratios; Thomson scattering was used to measureTein deuterium. After extensive discharge cleaning (approximately 100 shots) the emission from the plasma was found to agree with the standard theory of bremsstrahlung [j&ohgr; d&ohgr; ∝ (ne2⟨Z⟩/T1/2e)d&ohgr;] with ⟨Z⟩ =2.2±0.9 for deuterium and ⟨Z⟩=4.7±2.2 for helium. The infrared detector was calibrated absolutely using a CO2laser at 10.6 &mgr;m and a relative response curve. Specific types of impurities present were identified by spectral line emission in the visible region and analysis using mass spectrometry.

ISSN:0031-9171    

DOI:10.1063/1.861693

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

Using 90° Thomson scattering and magnetic probes, the time and space resolved electron temperature and density and magnetic field profiles were obtained for a small, 15 kJ theta pinch. At times after the implosion phase,t≳0.45–0.5 &mgr;sec, the plasma is characterized by a high &bgr;, 0.5≲&bgr;≲1.0,Te/Ti<1, andvE&Vthgr;≲vi(wherevE&Vthgr;is the E×B drift speed andviis the ion thermal speed). The current sheath width, &Dgr;Ls, and plasma resistivity in the sheath &eegr;eff, were obtained from the measured temperature, density, and field profiles at these late times. The results show that &Dgr;Ls≲ 3c/&ohgr;pi≫c/&ohgr;pe, &eegr;eff≈100 &eegr;⊥, and the effective collision frequency, &ngr;′eff, &ohgr;1h≲&ngr;′eff≪&ohgr;pi, where &eegr;⊥is the classical, Spitzer–Ha¨rm resistivity, and &ohgr;1his the lower hybrid frequency. These results indicate that plasma turbulence is an important factor when considering heating and transport, even in the low drift velocity regime. Theoretical estimates based on the lower‐hybrid‐drift instability are in reasonably good agreement with the measured resistivity and ion heating rate.

ISSN:0031-9171    

DOI:10.1063/1.861694

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

The hydrodynamic modification of an expanding plasma by coherent radiation for polarization out of the plane of incidence is investigated theoretically in the absence of parametric and modulational instabilities. The pondermotive force of the incident wave modifies the plasma flow from that of a rarefaction wave into a subsonic flow in the overdense region with a transition to supersonic flow at less than critical density. A constraint imposed at the sonic transition point allows a complete determination of the equilibrium as observed in simulations. The theory predicts that the average flow in the subcritical density region scales as the square root of the incident power.

ISSN:0031-9171    

DOI:10.1063/1.861696

出版商:AIP    

年代:1977

数据来源: AIP

摘要:

The absorption of laser light due to short wavelength ion fluctuations (&lgr;∼10 electron Debye lengths) is considered. The effective collision frequencies and heated electron distributions are computed as a function of plasma density and compared with theory. A simple upper bound to the level of ion turbulence is given, and several sources for the turbulence are discussed briefly. Finally, the absorption due to this mechanism in an inhomogeneous, expanding plasma is calculated by putting the estimate into a fluid code including wave propagation and density profile steepening by laser light pressure. The results are related to some recent experiments with short‐pulse‐length Nd laser light.

ISSN:0031-9171    

DOI:10.1063/1.861697

出版商:AIP    

年代:1977

数据来源: AIP