DIII-D experiments demonstrate simultaneous stability measurements and control of resistive wall modes (RWMs) with toroidal mode numbers 𝓃= 1 and 𝓃= 2. RWMs with 𝓃 > 1 are sometimes observed on DIII-D following the successful feedback stabilization of the 𝓃 = 1 mode, motivating the development of multi-n control. A new optimal multi-mode feedback algorithm based on the VALEN physics code has been implemented on the DIII-D tokamak using a real-time GPU installed directly in the DIII-D plasma control system (PCS). In addition to stabilizing RWMs, the feedback can control the stable plasma error field response, enabling compensation of the typically unaddressed DIII-D 𝓃 = 2 error field component. Experiments recently demonstrated this algorithm’s ability to simultaneously control 𝓃 = 1and 𝓃= 2 perturbed fields for the first time in a tokamak, using reactor relevant external coils. Control was maintained for hundreds of wall-times above the 𝓃 = 1 no-wall pressure limit and approaching the 𝓃 = 1 and 𝓃 = 2 ideal-wall limit. Multi-mode feedback also improved the control of the ELM-driven 𝓃 = 1and 𝓃 = 2 fields which further validates the feedback performance. Furthermore, a rotating non-zero target was set for the feedback, allowing stability to be assessed by monitoring the rotating plasma response while maintaining control. This novel technique can be viewed as a closed-loop extension of active MHD spectroscopy, which has been used to validate stability models through comparisons of the plasma response to applied, open-loop perturbations. The closed-loop response measurements are consistent with open-loop MHD spectroscopy data over a range of 𝜷𝑛 approaching the 𝓃 = 1 ideal-wall limit, demonstrating the potential of this technique as a useful tool for measuring stability while maintaining control even as the marginal stability point is approached. These plasma response measurements were then fit to produce both VALEN and single-mode stability models. These models allow for important plasma stability information to be determined and have been shown to agree with experimentally observed RWM growth rates. This improved understanding and control of the 𝓃 = 1 and 𝓃 = 2 RWM will allow for more robust operation above the 𝓃 = 2 no-wall limit.

A Dissertation submitted in fulfillment of the requirements for the degree Master of Science In the Faculty of Sciences at the University of the Witwatersrand, Johannesburg Johannesburg, January 2015 / Three methods, for the analyses of low levels of moisture in gas samples, were developed and optimized. The analytical techniques included Fourier Transform Infrared Spectroscopy (FTIR) and Pulsed Discharge Helium Ionization/Gas Chromatography (PDHID/GC). The methods included the direct analyses of moisture in gas samples using FTIR as well as the analysis of acetylene (C2H2) by FTIR and GC/PDHID. For the latter methods, the purpose was to convert the moisture in a gas sample to C2H2 by hydrolization of the calcium carbide (CaC2) with moisture to C2H2 and then analyze the resulting C2H2 content by FTIR or GC/PDHID. The C2H2 result was then converted back to moisture to obtain the moisture content of the sample. The FTIR moisture method developed provided eleven different wavenumbers for quantitation providing a wide analytical scope, specifically in complex gas matrices, where there is often peak overlap between matrix and moisture. A heated eight meter glass long path gas cell and a mercury cadmium telluride (MCT) detector were utilized. The FTIR method required much greater volumes of sample than the GC method but allowed for direct analysis of moisture without prior conversion to acetylene. Moisture permeation standards were used for calibration and the LOD’s ranged from 0.5 to 1 ppm with quantification possible from 0.5 to 10ppm. For the FTIR C2H2 method various concentration ranges were established from 50 up to 2000 ppm. Three wavenumbers were evaluated for C2H2 and methane was introduced as an internal standard. The use of methane as an internal standard provided better r2 values on the calibration data than for the tests run without internal standard. A gas chromatographic (GC), pulsed discharge helium ionization detector (PDHID) method for the determination of moisture content in small quantities of gases, based on the conversion of the moisture to acetylene (C2H2) prior to analysis, was developed. The method developed on the GC/PDHID for C2H2, provided a quantitation range from 0.6 to 7.7 ppm. Conversion of the moisture to acetylene was achieved by hydrolysing an excess of calcium carbide (CaC2) in a closed reaction vessel with a measured volume of a sample containing a known quantity of moisture. The gaseous reaction mixture was transferred, using helium (He) carrier gas, to a GC/PDHID, set up with “sample injection and heart cut to detector” to prevent matrix disturbances on the PDHID, for analysis. The acetylene concentration values thus obtained were converted back to moisture values and percentage recoveries calculated. A similar conversion process was applied on FTIR. The conversion of moisture to C2H2 using CaC2 was tested and proven to be viable. Quantification was not possible as the available sample holder could not be adequately sealed to prevent air ingress. This led to higher C2H2 values than expected. This process can be optimized by the design and production of a sealed sample holder.

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<p> Part I describes experiments carried out to search for possible products of ternary fission of 235U. Inert gases extracted from neutron-irradiated 235u were analyzed mass-spectrometrically for the presence of stable and radioactive neon and argon isotopes. No evidence for fission product neon or argon was found. Upper limits obtained for the yields are orders of magnitude lower than those suggested by some other studies.</p> <p> Part II describes measurements of relative yields and energy distributions of 3H, 3He, and 4He produced in fission of 235U. A short-range (<8 Mev) component in 4He, not previously established, has been detected in this work. The upper limit obtained for direct formation of 3He is lower by two to four orders of magnitude than the frequency of formation of 3He found in studies of other fissile nuclides.</p> / Thesis / Doctor of Philosophy (PhD)

This study attempts to calculate the areas of the cross section and diameter of the CH3 group, COOH group, the length of the carbon chain, and the longtiudinal distance between carbon atoms for each of the three acids. It also attempts to develop a method of measuring the effective diameter of molecules of gases. Equations for these terms as given by MacDougall and experiments were performed and the data applied to the equation.