Albert G. Petschek

The Development of Thermals from Rest

Abstract Conventional techniques for releasing a thermal in laboratory experiments induce enough initial motion to affect seriously the thermals subsequent evolution. We have invented a mechanism for releasing thermals from very close to a state of rest. This allows the examination of the transient behavior of thermals previous to the development of self-similarity. A thermal starting from rest exhibits a much smaller entrainment rate than a self-similar thermal for a distance from its starting point of at least six initial diameters. Since we find that thermals typically penetrate four initial diameters or less in a stably stratified environment, this has potentially great significance for atmospheric convection. Numerical simulations using an axisymmetric, two fluid model aid in the interpretation of these results.

Superconductor imaging surface magnetometry

The presence of superconducting surfaces in the vicinity of current sources may be interpreted in terms of image theory. This concept has both experimental and theoretical practicality. Experimentally, sensing coils for magnetic detection, when placed near such surfaces, perform in a gradiometric fashion. In order to explain this effect explicitly, a theoretical treatment of the magnetic fields in the presence of superconducting surfaces and coils is presented. Expressions are derived for planar and spherical geometries that approximate practical experimental situations. These expressions may be used to predict the expected gradiometric response of a coil as a function of the positions of the source and coil relative to the surface.

The Motion of Bomb Debris Following the Starfish Test

Theoretical computations are presented which identify some of the more important processes affecting the debris expansion following the Starfish explosion. A MHD shock wave is formed, through which the moving debris and air plasma is coupled to stationary plasma. The major component of the stationary plasma is produced by the explosion through ionization of neutral air by X rays and by collisions. The detailed behavior of a model MHD shock system with collisional effects is studied by means of a Lagrangian Computer program, and the computed results are compared with photographic data. The data and computations are in good qualitative agreement as to the behavior of the downward and east-west expansion. The upward expansion is slowed partly by the processes mentioned, but it is ultimately stopped and reversed through the action of conduction currents flowing in the F1 layer in a region connected to the moving plasma by way of the field lines.

Stability of DPD and SPH

It is shown that DPD (Dual Particle Dynamics) and SPH (Smoothed Particle Hydrodynamics) are conditionally stable for Eulerian kernels and linear fields. This result is important because it is highly desirable to move and change neighbors where the material deformation is large. For higher dimensions (than 1D), stability for general neighborhoods is shown to require a two-step update, such a predictor-corrector. Co-locational methods (all field variables calculated on every particle) benefit from the completeness property also. We show that SPH with corrected derivatives is conditionally stable. Linear completeness of interpolations is shown to assert itself as a powerful ally with respect to stability as well as accuracy.

Gamma-burst emission from neutron-star accretion

A model for emission of the hard photons of gamma bursts is presented. The model assumes accretion at nearly the Eddington limited rate onto a neutron star without a magnetic field. Initially soft photons are heated as they are compressed between the accreting matter and the star. A large electric field due to relatively small charge separation is required to drag electrons into the star with the nuclei against the flux of photons leaking out through the accreting matter. The photon number is not increased substantially by bremsstrahlung or any other process. Instability in an accretion disc might provide the infalling matter required.

Positrons from gamma bursts

We have proposed that the mechanism for the production of the hard spectrum of photons observed in the typical gamma burst is associated with the charge separation and electron‐photon heating of Eddington limited accreation onto a neutron star. The charge separation occurs because of the radiation stress on the electrons and the gravitational stress on the ions and amounts to some 100 to 1000 volts cm−1 depending upon whether the accreting matter is dynamically decelerated by radiation or is accreted in quasi‐static, steady flow. A positron produced in pair formation by the high energy part of the photon spectrum will be accelerated by both the radiation stress and the electrostatic field. Since for electrons these two stresses are equal and opposite, a positron will attain an energy significantly greater than the purely radiation driven free expansion limit where for these conditions 1/√1−β2=γre]≂[(mp MG/r)/mec2]1/4≂4. This value of γrel is near the threshold for pair production on additional in‐falling ...

Neutron oscillation experiments using ultracold neutrons

The figure of merit for a neutron oscillation experiment using ultracold neutrons is shown to be competitive with that for beam experiments.

Decoherence bounds on the capabilities of cold trapped ion quantum computers

Using simple physical arguments we investigate the capabilities of a quantum computer based on cold trapped ions of the type recently proposed by Cirac and Zoller. From the limitations imposed on such a device by decoherence due to spontaneous decay, laser phase coherence times, ion heating and other possible sources of error, we derive bounds on the number of laser interactions and on the number of ions that may be used. As a quantitative measure of the possible performance of these devices, the largest number which may be factored using Shors quantum factoring algorithm is determined for a variety of species of ion.

Fluctuations in Multiple Capture Processes

The fluctuation in the number of many‐fold captures by an element exposed to a high neutron flux is computed. It is found that the fluctuations are identical to those that would be obtained by selecting nuclei at random from an infinite supply with the appropriate average composition.