A new electro-produced pulsed slow positron facility
A new electro-produced pulsed slow positron facility is now in operation at the Institute for Storage Ring Facilities at Aarhus (ISA). Around 30,000 positrons/pulse are routinely produced at a frequency of 12 Hz. With the use of a buncher a pulse length of around 10 ns has been obtained. A program of positron trapping and cooling is in progress with the possibility to efficiently produce a slow positron microbeam.
We present an asymptotic form of the Boussinesq equations in the limit of small P ´ eclet numbers i.e. when the time scale of motions is much larger than the time scale of ther- mal diffusion. We find that, in this limit, the effects of thermal diffusion and stable stratification combine in a single physical process. This process is an anisotropic dissipation (not effective for horizontal motions) which acts primarily on large scale mo- tions. The small-P´ eclet-number approximation presents also the great practical interest to avoid the numerical difficulty induced by the huge separation between the diffusive and dynamical time scales. The relevance of this approximation to study the flow dynamics within the stellar radiative zones is considered.