Impact of energetic particle orbits on long range frequency chirping of BGK modes

Long range frequency chirping of Bernstein-Greene-Kruskal modes, whose existence is determined by the fast particles, is investigated in cases where these particles do not move freely and their motion is bounded to restricted orbits. An equilibrium oscillating potential, which creates different orbit topologies of energetic particles, is included into the bump-on-tail instability problem of a plasma wave. With respect to fast particles dynamics, the extended model captures the range of particles motion (trapped/passing) with energy and thus represents a more realistic 1D picture of the long range sweeping events observed for weakly damped modes, e.g. global Alfven eigenmodes, in tokamaks. The Poisson equation is solved numerically along with bounce averaging the Vlasov equation in the adiabatic regime. We demonstrate that the shape and the saturation amplitude of the nonlinear mode structure depends not only on the amount of deviation from the initial eigenfrequency but also on the initial energy of the resonant electrons in the equilibrium potential. Similarly, the results reveal that the resonant electrons following different equilibrium orbits in the electrostatic potential lead to different rates of frequency evolution. As compared to the previous model [Breizman B.N. 2010 Nucl. Fusion 50 084014], it is shown that the frequency sweeps with lower rates. The additional physics included in the model enables a more complete 1D description of the range of phenomena observed in experiments.Comment: Submitted to Nuclear Fusion 25/01/201

Tunable loop microstrip (TLM) coil array with decoupling capacitors

The tunable loop microstrip (TLM) coil array with capacitive decoupling circuits can achieve more than -20dB element isolation. The decoupling circuit is quite simple and easy to be adjusted, so it is suitable for array with large number of elements. It is not only used for receive-only mode, but an excellent choice for transceiver as well for parallel MRI.published_or_final_versio

A novel RF coil: tunable loop microstrip (TLM) coil

A new tunable loop microstrip (TLM) RF coil for MRI is presented. It is a novel approach to use tuning capacitor on microstrip ring resonator. The resonant frequency and Q are analyzed based on microstrip theory. The experiment results show its advantages of higher Q and less frequency shift with loading effect than conventional microstrip coils terminated with open or short circuit. Because of its tunable characteristics, this new TLM coil can be easily applied to high and particularly to ultra-high filed MRI systems.published_or_final_versio

A novel RF coil: tunable loop microstrip (TLM) coil

A new tunable loop microstrip (TLM) RF coil for MRI is presented. It is a novel approach to use tuning capacitor on microstrip ring resonator. The resonant frequency and Q are analyzed based on microstrip theory. The experiment results show its advantages of higher Q and less frequency shift with loading effect than conventional microstrip coils terminated with open or short circuit. Because of its tunable characteristics, this new TLM coil can be easily applied to high and particularly to ultra-high filed MRI systems.published_or_final_versio

Microstrip Loop Array (MLA) for Parallel Imaging

The microstrip loop array (MLA), an improved version of planar strip array, is proposed which take advantage of loop geometry. Without much loss of the well-decoupling benefit of strip line arrays, the proposed microstrip loop arrays have well-localized sensitivity profiles which are more suitable for parallel imaging. Simulation and experiment results show that microstrip loop coils have sensitivity profiles like conventional loop coils, while the mutual coupling is 10-20dB less at same distances. A sensitivity-encoded cardiac imaging was performed to demonstrate the suitability of MLA for parallel imaging.published_or_final_versio

Optimization of Regularization Parameter for GRAPPA Reconstruction

published_or_final_versio

Investigation of power loss of Bi-2223 HTS tapes used for MRI gradient coil

Bi-2223 High Temperature Superconducting (HTS) tape was investigated for the first time as the material for MRI gradient coil. Due to the low resistance and high critical current, an HTS tape gradient has the great advantages of more compact construction, higher gradient strength, lower coil heating and longer gradient continuous rating over copper wires. We measured the power loss in Bi-2223 tapes from 200Hz to 2000Hz at gradient operating frequencies. It shows 2-4 orders lower loss than copper wires. A prototype HTS tape gradient coil was fabricated and its loss performance was evaluated.published_or_final_versio

Supra-oscillatory critical temperature dependence of Nb-Ho bilayers

We investigate the critical temperature Tc of a thin s-wave superconductor (Nb) proximity coupled to a helical rare earth ferromagnet (Ho). As a function of the Ho layer thickness, we observe multiple oscillations of Tc superimposed on a slow decay, that we attribute to the influence of the Ho on the Nb proximity effect. Because of Ho inhomogeneous magnetization, singlet and triplet pair correlations are present in the bilayers. We take both into consideration when solving the self consistent Bogoliubov-de Gennes equations, and we observe a reasonable agreement. We also observe non-trivial transitions into the superconducting state, the zero resistance state being attained after two successive transitions which appear to be associated with the magnetic structure of Ho.Comment: Main article: 5 pages, 4 figures; Supplementary materials: 4 pages, 5 figure