Bose-Glass behaviour in Bi_{2}Sr_{2}Ca_{1-x}Y_{x}Cu_{2}O_{8} crystals with columnar defects: experimental evidence for variable-range hopping

We report on vortex transport in Bi_{2}Sr_{2}Ca_{1-x}Y_{x}Cu_{2}O_{8} crystals irradiated at different doses of heavy ions. We show evidence of a flux-creep resistivity typical of a variable-range vortex hopping mechanism as predicted by Nelson and Vinokur.Comment: 5 pages LaTeX2e (uses elsart.cls), 1 Encapsulated PostScript figur

A portable platform for accelerated PIC codes and its application to GPUs using OpenACC

We present a portable platform, called PIC_ENGINE, for accelerating Particle-In-Cell (PIC) codes on heterogeneous many-core architectures such as Graphic Processing Units (GPUs). The aim of this development is efficient simulations on future exascale systems by allowing different parallelization strategies depending on the application problem and the specific architecture. To this end, this platform contains the basic steps of the PIC algorithm and has been designed as a test bed for different algorithmic options and data structures. Among the architectures that this engine can explore, particular attention is given here to systems equipped with GPUs. The study demonstrates that our portable PIC implementation based on the OpenACC programming model can achieve performance closely matching theoretical predictions. Using the Cray XC30 system, Piz Daint, at the Swiss National Supercomputing Centre (CSCS), we show that PIC_ENGINE running on an NVIDIA Kepler K20X GPU can outperform the one on an Intel Sandybridge 8-core CPU by a factor of 3.4

Finite β\beta Effects on Short Wavelength Ion Temperature Gradient Modes

The electromagnetic effect is studied on the short wavelength branch of the ion temperature gradient mode in the linear regime for the first time using a global gyrokinetic model. The short wavelength ion temperature gradient mode growth rate is found to be reduced in the presence of electromagnetic perturbations at finite plasma $\beta$. The effect on real frequency is found to be weak. The threshold value of $\eta_i$ is found to increase for the mode as the magnitude of $\beta$ is increased. The global mode structure of the short wavelength branch of the ion temperature gradient mode is compared with the conventional branch. The magnetic character of the mode, measured as the ratio of mode average square values of electromagnetic potential to electrostatic potential, is found to increase with increasing values of the plasma $\beta$. The mixing length estimate for flux shows that the maximum contribution still comes from the long wavelengths modes. The magnitude of the flux decreases with increasing $\beta$.Comment: 23 pages,23 figure