Helium Helmholtz

The first simulations of the collapsar scenario have been performed using 2D Newtonian, hydrodynamics (MacFadyen Woosley 1999) exploring the collapse of helium cores of more than 10 M . In their 2D simulation MacFadyen Woosley found the jet to be collimated by the stellar material into opening angles of a few degrees and to transverse the star within 10 s. The accretion process was estimated to occur for a few tens of seconds. In such a model variability in the lightcurve could result for example from (magneto-) hydrodynamic instabilities in the accretion disk that would translate into a modulation of the neutrino emission/annihilation processes or via Kelvin-Helmholtz instabilities at the interface between the jet and the stellar mantle. 

For FDMR experiments it is desirable to be able to apply an external magnetic field. In one of the arrangements the field is provided by a pair of Helmholtz coils outside the cryostat which can be rotated with respect to the sample to obtain the directions of the principal axes of the zero-field tensor. With this approach the exciting laser can be kept focussed on the same sample volume while orienting the field, but the fields are limited to lOmT. In the other set-up the sample holder is inserted into the bore of a fixed superconducting Helmholtz-type magnet immersed in liquid helium. This allows the application of fields up to 1.3 T, but the direction of the magnetic field with respect to the zero-field tensor cannot be varied. 

In the experiment on the helium film, the exposed surfaces of the beaker are in equilibrium with the saturated vapour of the liquid of the bath and will, therefore, be covered by an adsorbed layer of helium. At temperatures below the superfluid is able to flow in the adsorbed layer or film which, therefore, acts as a sort of syphon. The driving force for the motion is the difference in gravitational potential energy of the liquid in the beaker and in the bath, which leads to a difference in the specific Helmholtz functions. 

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