Events at Physics
Events on Monday, February 6th, 2012
- Plasma Physics (Physics/ECE/NE 922) Seminar
- High energy density plasmas generated by radial foils on COBRA
- Time: 12:05 pm
- Place: 2241 Chamberlin
- Speaker: Dr. Pierre-Alexandre Gourdain, Cornell University
- Abstract: High energy density plasmas turn out to be a wonderful platform to study warm dense matter, high yield fusion energy or astrophysical phenomena. The key ingredient to reach megabars of pressures: geometrical focusing. For instance, a flat metallic foil driven by radial currents, which converge onto a central pin, achieves local densities on the order of 1022 electrons/cm-3 and temperatures in the keV range. More precisely, this foil goes through an ablation phase, characterized by an axial collimated plasma jet, a compression phase, forming a central Z-pinch, and finally an expansion phase, where super-Alfvenic flows carry plasma away. Due to large Reynolds (107 to 109) and magnetic Reynolds numbers (101 to 103), radial foil plasmas reproduce more or less astrophysical jets and outflows. Using these laboratory experiments as intuitive guides, it is possible to highlight the physical mechanisms that might cause such astrophysical formations.
- Condensed Matter Theory Group Seminar
- Quantum efficiency of a microwave photon detector based on a current-biased Josephson junction
- Time: 4:30 pm
- Place: 5310 Chamberlin
- Speaker: Amrit Poudel, UW-Madison
- Abstract: We analyze the quantum efficiency of a microwave photon detector based on a current-biased Josephson junction. We consider the Jaynes-Cummings Hamiltonian to describe coupling between the photon field and the junction. We then take into account coupling of the junction and the resonator to the environment. We solve the equation of motion of the density matrix of the resonator-junction system to compute the quantum efficiency of the detector as a function of detection time, bias current, and energy relaxation time. Our results indicate that junctions with modest coherence properties can provide efficient detection of single microwave photons, with quantum efficiency in excess of 80%.
- Host: Perkins