Engineering topics
Fisher, Peter
Publications and source records attributed to Fisher, Peter.
Task F - Lepton Quark Studies
Final report on a project to commission and operate a dark matter detector, and to develop the DarkLight experiment to search for massive photon-like particles that could be evidence for dark matter.
A Quasi-Periodic Linear Feeder for the Impurity Granular Injection on DIII-D
Injection of solid nonfuel pellets has been actively used as a tool for pacing and mitigation of edge localized modes (ELMs). In DIII-D, effective ELM pacing has been demonstrated by the high-frequency injection of lithium and carbon submillimeter spheres, using the impurity granule injector (IGI). This device injects granules into the plasma at speeds up to 150 m/s, through impact with a rotating impeller. In the IGI, high-frequency granule delivery was accomplished through a vibrational granule dropper, in which high time-average rates are obtained at the cost of lack of period control. Here, we present a new in-line granule feeder, capable of delivering granules of size 0.2-2 mm with no restriction of material properties, at quasi-periodic rates of up to 150 Hz, for 0.7-mm diameter lithium granules (600 Hz using 0.3-mm granules). The new dropper mechanism combines two piezo-in-line units; one which feeds the impeller and one which circulates granules that are filtered out of the feeder path. A remotely adjustable filter eliminates granules that are stacked, oversized, or side by side allowing the formation of a single moving granule injection line. The granules fall off the in-line feeder exit one at a time, achieving a quasi-periodic delivery rate proportional to the exit speed. At higher rates, the periodicity deteriorates. This behavior was studied using high-speed cameras and electrostatic measurements, and it was found that at drop rates <60 Hz, the granule delivery period has a variation of ±25% which appears to be caused by gaps which develop in the last centimeter of the injection line, as granules exit off the moving track. The linear feeder concept is robust against bridge instabilities and clogging issues, thanks to the simple diverter filter and constant recirculation of granules. Furthermore, the open-top design of the device facilitates refilling the device from separate reservoirs and has easy access for directly monitoring operation and adjustment. This paper describes the details of the in-line feeder design, along with several design iterations. The goal is a robust in-vacuum mechanism that can deliver granule flow ranging from a single particle to a line of particles at 150 per second, using different sizes and materials from the same apparatus.
Measurement of the directional sensitivity of Dark Matter Time Projection Chamber detectors
The dark matter time projection chamber (DMTPC) is a direction-sensitive detector designed to measure the direction of recoiling 19 F and 12 C nuclei in low-pressure CF 4 gas using optical and charge readout systems. In this paper, we employ measurements from two DMTPC detectors, with operating pressures of 30–60 torr, to develop and validate a model of the directional response and performance of such detectors as a function of recoil energy. Using our model as a benchmark, we formulate the necessary specifications for a scalable directional detector with sensitivity comparable to that of current-generation counting (nondirectional) experiments, which measure only recoil energy. Furthermore assuming the performance of existing DMTPC detectors, as well as current limits on the spin-dependent WIMP-nucleus cross section, we find that a 10–20 kg scale direction-sensitive detector is capable of correlating the measured direction of nuclear recoils with the predicted direction of incident dark matter particles and providing decisive (3σ) confirmation that a candidate signal from a nondirectional experiment was indeed induced by elastic scattering of dark matter particles off of target nuclei.
Direction sensitive neutron detector
A neutron detector includes a pressure vessel, an electrically conductive field cage assembly within the pressure vessel and an imaging subsystem. A pressurized gas mixture of CF 4 , 3 He and 4 He at respective partial pressures is used. The field cage establishes a relatively large drift region of low field strength, in which ionization electrons generated by neutron-He interactions are directed toward a substantially smaller amplification region of substantially higher field strength in which the ionization electrons undergo avalanche multiplication resulting in scintillation of the CF.sub.4 along scintillation tracks. The imaging system generates two-dimensional images of the scintillation patterns and employs track-finding to identify tracks and deduce the rate and direction of incident neutrons. One or more photo-multiplier tubes record the time-profile of the scintillation tracks permitting the determination of the third coordinate.
Toward high performance radioisotope thermophotovoltaic systems using spectral control
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Thermophotovoltaic energy generation
Inventive systems and methods for the generation of energy using thermophotovoltaic cells are described. Also described are systems and methods for selectively emitting electromagnetic radiation from an emitter for use in thermophotovoltaic energy generation systems. In at least some of the inventive energy generation systems and methods, a voltage applied to the thermophotovoltaic cell (e.g., to enhance the power produced by the cell) can be adjusted to enhance system performance. Certain embodiments of the systems and methods described herein can be used to generate energy relatively efficiently.