Kinetics of the evaporative cooling of an atomic beam

We compare two distincts models of evaporative cooling of a magnetically guided atomic beam: a continuous one, consisting in approximating the atomic distribution function by a truncated equilibrium distribution, and a discrete-step one, in which the evaporation process is described in terms of successive steps consisting in a truncation of the distribution followed by rethermalization. Calculations are performed for the semi-linear potential relevant for experiments. We show that it is possible to map one model onto the other, allowing us to infer, for the discrete-step model, the rethermalization kinetics, which turns out to be strongly dependent upon the shape of the confining potential.Comment: Submitted to Phys. Rev.

Is the ebay feedback system really efficient ? an experimental study

The eBay Feedback Forum is claimed to be a crucial component of the success of eBay. Many empirical studies have found that this feedback system exerts a deterrent effect on the opportunistic behavior the Internet's anonymity may incite buyers and sellers to adopt. The feedback system in place on eBay is however far from being perfect and may be especially vulnerable to strategic ratings (or nonratings) that might reduce the informational content of feedback profiles. This article aims to examine the efficiency of the eBay feedback system, through a set of experiments based on the trust game. Our experimental design consists of four different treatments. The baseline treatment corresponds to a finite repeated simultaneous trust game. The second treatment, called “eBay rating” is identical to the baseline treatment except that we added a second stage in which the players have the opportunity of rating their partner. In this treatment, each participant is given the choice to either evaluate immediately or wait, knowing that only one rating will be accepted. The third treatment, called "Sequential rating" is identical to the “eBay rating” treatment, except that the order in which players evaluate one another is randomly determined by the computer. Finally in the fourth treatment, called “Simultaneous rating”, both players are required to make their rating decisions simultaneously. Our experimental results indicate that the eBay feedback system could be improved by either constraining partners to leave ratings simultaneously or by predetermining the rating sequence.

Transient growth in stable collisionless plasma

The first kinetic study of transient growth for a collisionless homogeneous Maxwellian plasma in a uniform magnetic field is presented. A system which is linearly stable may display transient growth if the linear operator describing its evolution is non-normal, so that its eigenvectors are non-orthogonal. In order to include plasma kinetic effects a Landau fluid model is employed. The linear operator of the model is shown to be non-normal and the results suggest that the nonnormality of a collisionless plasma is intrinsically related to its kinetic nature, with the transient growth being more accentuated for smaller scales and higher plasma beta. The results based on linear spectral theory have been confirmed with nonlinear simulations.Comment: accepted as a Letter in Physics of Plasma