We report a mechanism of nonisothermal dendritic flux penetration in superconducting films. Our numerical and analytical analysis of coupled nonlinear Maxwell and thermal diffusion equations shows that dendritic flux pattern formation results from spontaneous branching of propagating flux filaments due to nonlocal magnetic flux diffusion and positive feedback between flux motion and Joule heating. The branching is triggered by a thermomagnetic edge instability, which causes stratification of the critical state. The resulting distribution of thermomagnetic microavalanches is not universal, because it depends on a spatial distribution of defects. Our results are in good agreement with experiments on Nb films.

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The penetration of magnetic flux into a thin film of YBa2Cu3O7−x is studied when the external field is ramped slowly. In this case the flux penetrates in bursts or avalanches. The size of these avalanches is distributed according to a power law with an exponent of τ = 1.29(2). The additional observation of finite-size scaling of the avalanche distributions, with an avalanche dimension D = 1.89(3), gives strong indications towards self-organized criticality in this system. Furthermore we determine exponents governing the roughening dynamics of the flux surface using some universal scaling relations. These exponents are compared to those obtained from a standard roughening analysis.

Multi-channel far-infrared HL-2A interferometer-polarimeter Rev. Sci. Instrum. 83, 10E336 (2012) Collinearity alignment of probe beams in a laser-based Faraday effect diagnostic Rev. Sci. Instrum. 83, 10E320 (2012) Spatial heterodyne Stokes vector imaging of the motional Stark-Zeeman multiplet Rev. Sci. Instrum. 83, 10D510 (2012) Far-infrared polarimetry diagnostic for measurement of internal magnetic field dynamics and fluctuations in the C-MOD Tokamak (invited) Rev. Sci. Instrum. 83, 10E316 (2012) First results from the J-TEXT high-resolution three-wave polarimeter-interferometer Rev. Sci. Instrum. 83, 10E306 (2012) Additional information on Rev. Sci. Instrum. A new imaging polarimeter for magneto-optical investigations is described. Improvements over conventional magneto-optics are: it ͑i͒ is insensitive to uneven illumination, ͑ii͒ determines also the sign of the magnetic field, and ͑iii͒ has significantly improved sensitivity at small magnetic fields. The typical root-mean-square ͑rms͒ noise level is 0.7 mT Hz Ϫ1/2 for a single pixel, corresponding to a polarization rotation of 0.03 deg Hz Ϫ1/2 . With limited temporal and spatial averaging, the rms error in magnetization profiles can be reduced to Ͻ10 T, corresponding to 4ϫ10 Ϫ4 deg. Time resolution is 12 frames per second. Demonstration of the performance of the polarimeter is given for measurements of the local field above superconductors and for measurements of the rotation angle of sugar dissolved in water.

Using advanced magneto-optics, the field and current distributions in superconducting thin film YBa 2 Cu 3 O 7 rings in an externally applied magnetic field are studied experimentally. The observations are in close agreement with numerical calculations. During the initial flux penetration and field reversal a highly nonuniform current distribution is observed. In particular, concentric counterrotating current loops occur during field reversal. We explore implications of these results for the determination of critical currents and penetration fields from bulk magnetization measurements.