Results of experiments to simulate radiant heating of propellant in a nuclear light bulb engine using a D-C arc radiant energy source

Simulating radiant heating of propellant stream of nuclear light bulb engin

Explaining the statistical properties of Fast Radio Bursts with suppressed low-frequency emission

The possibility of Fast Radio Burst (FRB) emission being suppressed at low frequencies, resulting in a cutoff of the average rest-frame spectrum, has been raised as an explanation for the lack of detections at meter wavelengths. We examine propagation effects that could cause this suppression, and find that a low-frequency spectral cutoff may be generic regardless of the specific FRB emission mechanism. We then illustrate the effects of a low-frequency spectral cutoff on the statistics of FRBs, given a cosmological source population. The observed FRB rate peaks at a specific frequency under a variety of assumptions. Observations at lower frequencies are more sensitive to high-redshift events than observations above the maximal-rate frequency, and therefore result in more sharply broken fluence distributions. Our results suggest that the absence of low-frequency FRBs, and the differences between the Parkes and the Australian Square Kilometre Array (ASKAP) FRB samples, can be fully explained by suppressed low-frequency FRB emission.Comment: 12 pages, 4 figures, submitted to AAS Journal

Plasmonic and Photonic Designs for Light Trapping in Thin Film Solar Cells

Thin film solar cells are promising to realize cheap solar energy. Compared to conventional wafer cells, they can reduce the use of semiconductor material by 90%. The efficiency of thin film solar cells, however, is limited due to insufficient light absorption. Sufficient light absorption at the bandgap of semiconductor requires a light path more than 10x the thickness of the semiconductor. Advanced designs for light trapping are necessary for solar cells to absorb sufficient light within a limited volume of semiconductor. The goal is to convert the incident light into a trapped mode in the semiconductor layer. In this dissertation, a critical review of currently used methods for light trapping in solar cells is presented. The disadvantage of each design is pointed out including insufficient enhancement, undesired optical loss and undesired loss in carrier transport. The focus of the dissertation is light trapping by plasmonic and photonic structures in thin film Si solar cells. The performance of light trapping by plasmonic structures is dependent on the efficiency of photon radiation from plasmonic structures. The theory of antenna radiation is used to study the radiation by plasmonic structures. In order to achieve efficient photon radiation at a plasmonic resonance, a proper distribution of surface charges is necessary. The planar fishnet structure is proposed as a substitution for plasmonic particles. Large particles are required in order to resonate at the bandgap of semiconductor material. Hence, the resulting overall thickness of solar cells with large particles is large. Instead, the resonance of fishnet structure can be tuned without affecting the overall cell thickness. Numerical simulation shows that the enhancement of light absorption in the active layer is over 10x compared to the same cell without fishnet. Photons radiated from the resonating fishnet structure travel in multiple directions within the semiconductor layer. There is enhanced field localization due to interference. The short circuit current was enhanced by 13.29%. Photonic structures such as nanodomes and gratings are studied. Compared to existing designs, photonic structures studied in this dissertation exhibited further improvements in light absorption and carrier transport. The nanodome geometry was combined with conductive charge collectors in order to perform simultaneous enhancement in optics and carrier transport. Despite the increased volume of semiconductor material, the collection length for carriers is less than the diffusion length for minority carriers. The nanodome geometry can be used in the back end and the front end of solar cells. A blazed grating structure made of transparent conductive oxide serves as the back passivation layer while enhancing light absorption. The surface area of the absorber is increased by only 15%, indicating a limited increase in surface recombination. The resulting short circuit current is enhanced by over 20%. The designs presented in the dissertation have demonstrated enhancement in Si thin film solar cells. The enhancement is achieved without hurting carrier transport in solar cells. As a result, the enhancement in light absorption can efficiently convert to the enhancement in cell efficiency. The fabrication of the proposed designs in this dissertation involves expensive process such as electron beam lithography. Future work is focused on optical designs that are feasible for cheap fabrication process. The designs studied in this dissertation can serve as prototype designs for future work

Prospects for radio detection of ultra-high energy cosmic rays and neutrinos

The origin and nature of the highest energy cosmic ray events is currently the subject of intense investigation by giant air shower arrays and fluorescent detectors. These particles reach energies well beyond what can be achieved in ground-based particle accelerators and hence they are fundamental probes for particle physics as well as astrophysics. Because of the scarcity of these high-energy particles, larger and larger ground-based detectors have been built. The new generation of digital radio telescopes may play an important role in this, if properly designed. Radio detection of cosmic ray showers has a long history but was abandoned in the 1970's. Recent experimental developments together with sophisticated air shower simulations incorporating radio emission give a clearer understanding of the relationship between the air shower parameters and the radio signal, and have led to resurgence in its use. Observations of air showers by the SKA could, because of its large collecting area, contribute significantly to measuring the cosmic ray spectrum at the highest energies. Because of the large surface area of the moon, and the expected excellent angular resolution of the SKA, using the SKA to detect radio Cherenkov emission from neutrino-induced cascades in lunar regolith will be potentially the most important technique for investigating cosmic ray origin at energies above the photoproduction cut-off. (abridged)Comment: latex, 26 pages, 17 figures, to appear in: "Science with the Square Kilometer Array," eds. C. Carilli and S. Rawlings, New Astronomy Reviews, (Elsevier: Amsterdam

An investigation into the replacement of mercury by indium iodide or tin halides in high-intensity discharge lamps

In this study we investigated the possibility to replace mercury by metalhalide molecules in the filling of CMH lamps, since mercury is a poisonous substance. Indium iodide was considered a viable alternative, because atomic indium has a high electron-atom collisional cross section.Several lamps were created, in which the indium iodide content was varied, as well as the addition of sodium iodide and dysprosium iodide. The spectra of the lamps were measured in an integrating sphere and their energy balances were determined. The spectra showed that indium iodide emits a broad continuum in the visible and infrared part of the spectrum. There was also a complete absence of ultraviolet radiation. The plasma propertiesin the core of the discharge were determined from the integrating-sphere measurements. Atomic indium lines were used to calculate the core temperature, and a simplified model, developed by Elenbaas, was used to calculate the atomic indium and electron densities. This way, we determined that the broad band continuum was not caused by bremsstrahlung or recombination radiation, but rather by molecular radiation emitted by InI molecules.Varying the indium iodide content in the lamps confirmed that indium iodide increased the resistivity of the lamps and indeed functioned as a buffer gas. However, the application of the Elenbaas model on the integratingsphere measurements suggested that indium iodide led to contraction of the arc column. A seperate side-on setup was created to measure the spectrum as a function of the lateral position in the lamp. A series of measurements over the total lateral width of the lamp were Abel transformed in order to create a radial temperature profile, which showed contraction of the arc. This was also confirmed by photos taken with a cross polarized filter. Contraction by indium iodide can lead to arc instability and limits the use of additives in CMH lamps.A different set of lamps was created that contained tin bromide and tin iodide as an alternative to mercury. These were only measured in the integrating sphere. The spectra of these lamps showed the same trends as the indium iodide lamps. We therefore concluded that neither indium iodide nor tin halides are a suitable alternative to mercury. In this study we investigated the possibility to replace mercury by metalhalide molecules in the filling of CMH lamps, since mercury is a poisonous substance. Indium iodide was considered a viable alternative, because atomic indium has a high electron-atom collisional cross section.Several lamps were created, in which the indium iodide content was varied, as well as the addition of sodium iodide and dysprosium iodide. The spectra of the lamps were measured in an integrating sphere and their energy balances were determined. The spectra showed that indium iodide emits a broad continuum in the visible and infrared part of the spectrum. There was also a complete absence of ultraviolet radiation. The plasma propertiesin the core of the discharge were determined from the integrating-sphere measurements. Atomic indium lines were used to calculate the core temperature, and a simplified model, developed by Elenbaas, was used to calculate the atomic indium and electron densities. This way, we determined that the broad band continuum was not caused by bremsstrahlung or recombination radiation, but rather by molecular radiation emitted by InI molecules.Varying the indium iodide content in the lamps confirmed that indium iodide increased the resistivity of the lamps and indeed functioned as a buffer gas. However, the application of the Elenbaas model on the integratingsphere measurements suggested that indium iodide led to contraction of the arc column. A seperate side-on setup was created to measure the spectrum as a function of the lateral position in the lamp. A series of measurements over the total lateral width of the lamp were Abel transformed in order to create a radial temperature profile, which showed contraction of the arc. This was also confirmed by photos taken with a cross polarized filter. Contraction by indium iodide can lead to arc instability and limits the use of additives in CMH lamps.A different set of lamps was created that contained tin bromide and tin iodide as an alternative to mercury. These were only measured in the integrating sphere. The spectra of these lamps showed the same trends as the indium iodide lamps. We therefore concluded that neither indium iodide nor tin halides are a suitable alternative to mercury