Wire tomography in the H-1NF heliac for investigation of fine structure of magnetic islands

Electron beam wire tomography in the H-1NF heliac enables high resolution mapping of vacuum flux surfaces with minimal disruption of the plasma operations schedule. Recent experimental results have proven this technique to be a highly accurate and high resolution method for mapping vacuum magnetic islands. Islands of width as small as delta approximately 8 mm have been measured, providing estimates of the internal rotational transform of the island. Point-to-point comparison of the mapping results with computer tracing, in conjunction with an image warping technique, enables systematic exploration of magnetic islands and surfaces of interest. Recent development of a fast mapping technique significantly reduced the mapping time and made this technique suitable for mapping at higher magnetic fields. This article presents recent experimental results and associated techniques.with support from the Australian Research Council Grant No. DP0344361

Oscillatory Flows Induced by Microorganisms Swimming in Two-dimensions

We present the first time-resolved measurements of the oscillatory velocity field induced by swimming unicellular microorganisms. Confinement of the green alga C. reinhardtii in stabilized thin liquid films allows simultaneous tracking of cells and tracer particles. The measured velocity field reveals complex time-dependent flow structures, and scales inversely with distance. The instantaneous mechanical power generated by the cells is measured from the velocity fields and peaks at 15 fW. The dissipation per cycle is more than four times what steady swimming would require.Comment: 4 pages, 4 figure

Aspirated compressor

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2000.Also available online at the MIT Theses Online homepage .Includes bibliographical references (p. 53).The current motivators in gas turbine aircraft engine design are improved efficiency and reduced engine weight. The drive for improved efficiency leads to high pressure ratio compressors with multiple stages, as thermodynamic efficiency increases with pressure ratio. These compressors are both heavy and expensive. Recent work by Kerrebrock, Merchant, and Schuler, has led to the possibility of achieving high pressure ratios with a reduction in the number of stages. These compressors use aspiration, or suction on the surface of the blades and end-walls, to keep the boundary layer attached under higher loading conditions, reducing losses. The work done by a rotor is related to the change in swirl across the blade and the rotational velocity of the blades. Counter-rotation provides for the largest change in swirl across a series of rotors, facilitating the largest pressure ratio. A three-stage, counter-rotating, compressor was designed to take advantage of these benefits. The tip speeds were kept as high as possible to increase the work done on the flow. The final design has a three-stage pressure ratio of 27, much higher than is currently available in three stages. The blade aspect ratios are smaller than on current compressors because of the large swirl change across each blade row. The flow passage has large contractions across each blade row, causing sharper turns in the flow path than is currently observed. The design consists of the flow passage, blade numbers and locations, rotor speeds, and desired swirl changes through each blade row. These data will allow later efforts to design the blade shapes. A mean line analysis was conducted to design a three stage turbine to drive the compressor. The benefits of counter-rotation are apparent in the turbine, as the turbine is designed without inter-rotor nozzles. Reduced engine weight and improved efficiency are available through the turbine section with the elimination of these nozzles.by Jeffrey H. Freedman.S.M

Trading networks

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/139950/1/ectj12090-sup-0001-onlineappendix.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/139950/2/ectj12090_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/139950/3/ectj12090.pd

Core magnetic islands and plasma confinement in the H-1NF heliac

Plasma confinement in the vicinity of vacuum magnetic islands near the magnetic axis in the H-1NF heliac [S. M. Hamberger et al., Fusion Technol.17, 123 (1990)] has been experimentally studied in a low temperature argon plasma. Experimental results indicate that, under favorable conditions, these low order (m=2)islands near the core of the plasma serve as “pockets” of higher electron density. This results in significant profile modifications including enhancement of the core radial electric field to a large positive value, possibly through an electron-root ambipolar condition. The characteristics of islands are found to be dependent on the plasma collisionality and island width.This research was supported in part by the Australian Research Council Grant No. DP0344361 and the U.S. Department of Energy under Contract No. DE-AC05- 00OR22725 with UT-Battelle, LLC

Lift-off dynamics in a simple jumping robot

We study vertical jumping in a simple robot comprising an actuated mass-spring arrangement. The actuator frequency and phase are systematically varied to find optimal performance. Optimal jumps occur above and below (but not at) the robot's resonant frequency $f_0$. Two distinct jumping modes emerge: a simple jump which is optimal above $f_0$ is achievable with a squat maneuver, and a peculiar stutter jump which is optimal below $f_0$ is generated with a counter-movement. A simple dynamical model reveals how optimal lift-off results from non-resonant transient dynamics.Comment: 4 pages, 4 figures, Physical Review Letters, in press (2012

Tomographic interferometry of a filtered high-current vacuum arc plasma

Tomography of a plasma enables the distribution of electron density to be visualized. We report on the design of two tomographic interferometer systems used to measureplasma electron density distributions in a high-current pulsed cathodic vacuum arc. The method is shown to be capable of microsecond time resolution. The spatial resolution of the quasioptical interferometer operating at 2 mm wavelength is 20 mm and the spatial resolution of the waveguide-based interferometer operating at 8 mm wavelength is 50 mm. In both cases the resolution achieved depends on the launching and receiving geometries. We developed criteria for assessing the tomogram for artifacts arising from limited sampling. First results of the spatial and temporal history of plasma in a high-current vacuum arc guided by a curved magnetic filter are presented and indicate poloidal field fluctuations reminiscent of magnetohydrodynamic instabilities in pinches. The applicability of the tomographic interferometry method to optimize plasma transport through the filter is also demonstrated.This work was in part supported by the Australian Research Council