Plasmonic focusing was investigated in symmetry broken nanocorrals under linearly polarized illumination. Near-field optical measurements of the perpendicular electric field show that a single subwavelength spot size of 320 nm can be generated. The interference pattern within the corral can be controlled by changing the polarization of optical excitation and the degree of symmetry breaking. The intensity enhancement factor was investigated using finite-difference time-domain simulations and confirmed by analytical calculations taking into account the plasmon damping and multiple reflections against the corral wall.
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High temporal (10 µs) and spatial (4 mm) resolution electron density profiles can now be automatically obtained on a between-shot basis from profile reflectometry data on the DIII-D tokamak. Fast, automated analysis is made possible by a new between-shot analysis package, details of which are presented. Analysis of X-mode data covering ∼0-3 × 10 19 m −3 with 1 ms duration data bursts every 5 ms for 5-6 s, for a total of ∼1000 profiles, takes 4-5 min, allowing the profiles to be viewed and used between shots. Additional details are provided of how the system density coverage at the above temporal and spatial resolution was recently expanded to (0-6.4) × 10 19 m −3 using a novel simultaneous, dual-polarization (O-and X-mode) measurement technique. Automatic (but not between-shot) analysis for the complete O-and X-mode data set (1.5 GB for each discharge, about 100 000 profiles at 25 µs temporal resolution) is available after the experiment. The improved capabilities of the system are illustrated using new results from a variety of areas such as edge localized mode dynamics, L-H transition and pedestal physics, impurity deposition and ITB studies, and direct comparisons with Thomson data are presented.
Feedback control has become a crucial tool in the research on magnetic confinement of plasmas for achieving controlled nuclear fusion. We present the first experimental results from a novel feedback control system that, for the first time, employs a graphics processing unit (GPU) for microsecond-latency, real-time control computations. The system was tested on the HBT-EP tokamak using an adaptive control algorithm for control of rotating magnetic perturbations. The algorithm assumes that perturbations of known shape are rotating rigidly, but dynamically derives and updates the rotation frequency to improve phase and gain accuracy of the control signals. Experiments were set up to control four rotating n = 1 perturbations at different poloidal angles. The perturbations are treated as coupled in frequency but independent of amplitude and phase, so that the system effectively controls a helical n = 1 perturbation with unknown poloidal spectrum. The control system suppresses the amplitude of the dominant 8 kHz mode by up to 60%. Deviation from the optimal feedback phase combines suppression with a speed up or slow down of the mode rotation frequency. The feedback performance is found to exceed previous results obtained with an FPGA-and Kalman-filter based control system without requiring any tuning of system model parameters.
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