arXiv subjects

R. Hollerbach

Publications and source records attributed to R. Hollerbach.

21 records · Page 2Linked to original sources

arXiv2005

New type of magneto-rotational instability in cylindrical Taylor-Couette flow

We study the stability of cylindrical Taylor-Couette flow in the presence of combined axial and azimuthal magnetic fields, and show that adding an azimuthal field profoundly alters the previous results for purely axial fields. For small magnetic Prandtl numbers Pm, the critical Reynolds number Re_c for the onset of the magneto-rotational instability becomes independent of Pm, whereas for purely axial fields it scales as Pm^{-1}. For typical liquid metals, Re_c is then reduced by several orders of magnitude, enough that this new design should succeed in realizing this instability in the laboratory.

astro-ph↗

arXiv2005

The stability of MHD Taylor-Couette flow with current-free spiral magnetic fields between conducting cylinders

We study the magnetorotational instability in cylindrical Taylor-Couette flow, with the (vertically unbounded) cylinders taken to be perfect conductors, and with externally imposed spiral magnetic fields. The azimuthal component of this field is generated by an axial current inside the inner cylinder, and may be slightly stronger than the axial field. We obtain an instability beyond the Rayleigh line, for Reynolds numbers of order 1000 and Hartmann numbers of order 10, and independent of the (small) magnetic Prandtl number. For experiments with R_out = 2 R_in = 10 cm and Omega_out = 0.27 Omega_in, the instability appears for liquid sodium for axial fields of ~20 Gauss and axial currents of \~1200 A. For gallium the numbers are ~50 Gauss and ~3200 A. The vertical cell size is about twice the cell size known for nonmagnetic experiments.

astro-ph↗

arXiv2002

The influence of Hall drift on the magnetic fields of neutron stars

We consider the evolution of magnetic fields under the influence of Hall drift and Ohmic decay. The governing equation is solved numerically, in a spherical shell with r_i/r_o = 0.75. Starting with simple free decay modes as initial conditions, we then consider the subsequent evolution. The Hall effect induces so-called helicoidal oscillations, in which energy is redistributed among the different modes. We find that the amplitude of these oscillations can be quite substantial, with some of the higher harmonics becoming comparable to the original field. Nevertheless, this transfer of energy to the higher harmonics is not sufficient to significantly accelerate the decay of the original field, at least not at the R_B = O(100) parameter values accessible to us, where this Hall parameter R_B measures the ratio of the Ohmic timescale to the Hall timescale. We do find clear evidence though of increasingly fine structures developing for increasingly large R_B, suggesting that perhaps this Hall-induced cascade to ever shorter lengthscales is eventually sufficiently vigorous to enhance the decay of the original field. Finally, the implications for the evolution of neutron star magnetic fields are discussed.

astro-ph↗