School of Electrical and Electronics Engineering

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    Auto-resonant peniotron amplifier with a down tapered DC magnetic field
    (Taylor & Francis, 1990-02) Musyoki, Stephen; Yokoo, Kuniyoshi; Sato, Nabuyi; Ono, Shoichi
    High efficiency operation of an auto-resonant peniotron amplifier operating with a down tapered DC magnetic field is discussed and the basic design concepts for a 200 GHz experimental prototype tube are described.
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    Auto-resonant eniotron oscillator using a magnetron type cavity
    (1991) Musyoki, Stephen; Yokoo, Kuniyoshi; Sato, Nobuyuki; Ono, Shoichi
    In an ideal auto-resonant eniotron oscillator (ARPO), all the electrons are able to give almost all their kinetic energy to an electromagnetic wave, resulting in a conversion efficiency of nearly 100%. However, the electric field strength required to drive a highly efficient interaction is difficult to achieve in an ordinary rectangular or circular waveguide cavity because of the low interaction coupling between electrons and the electromagnetic wave in the cavity. In this aper, we describe our investigation of an ARPO with a magnetron type cavity in lace of that above. In this cavity, the coupling becomes sufficiently strong to confirm experimentally the romising features of an ARPO. Additionally, the experiments should be useful for developing highly efficient medium-power oscillators in the microwave region. Here we resent our rocedure for designing such an experimental tube and our computer simulation results for its operational characteristics
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    Design of a high power, 10 GHz auto-resonant peniotron amplifier
    (1992) Musyoki, Stephen; Sakamoto, Keishi; Watanabe, Akihiko
    The autoresonant peniotron amplifier is a suitable source of high power RF radiation because of its high gain, high power, high frequency and high efficiency operation features. In this report we present our simulation results of a 10 GHz, 2.2 GW autoresonant peniotron amplifier with an electron energy conversion efficiency of 72.5 % and a gain of about 58 dB
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    Design of a high power, 2.75 GHz relativistic peniotron oscillator
    (1992-10) Musyoki, Stephen; Sakamoto, Keishi; Watanabe, Akihiko
    In the peniotron oscillator, the kinetic energy of relativistic electrons can be efficiently converted to an electromagnetic wave. This feature makes the peniotron a very attractive source of high power microwaves, which is required in heating of fusion plasma in tokamaks and high gradient particle accelerators. In this report we present the design of a 2.75 GHz relativistic peniotron oscillator which is capable of generating a microwave radiation of 30 megawatt with an efficiency of about 60 %. The experimental test results of the designed cavity are also presented
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    A new method of emittance measurement for electron beams from the Micro-emitter
    (1994) Ishizuka, Hiroshi; Nakahara, Yuriko; Kawasaki, Sunao; Musyoki, Stephen; Shimizu, Hiroshi; Watanabe, Akihiko; Shiho, Makoto
    Recently a new type of cathode called Micor-emitter has been put in progress. This cathode is a microfabricated field emitter having the characteristics of very low emittance and high brightness. We cannot measure the emittance of the cathode with conventional methods like the pepper-pot method. The reasons are: (1) the angle between the electron orbit and the axis is very small and (2) we cannot focus the electron beam in the vacuum or on the surface of the material since the current density of the cathode is extremely high. For the emittance measurement for such low emittance and high brightness cathode, we need to expand the beam, and measure the beam's cross section without any slits or apertures. We studied and proposed a new emittance measurement method for the Micro-emitter.
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    Emittance measurement of high-brightness microbeams
    (Japanese Journal of Applied Physics, 1994) Musyoki, Stephen; Ishizuka, Hiroshi; Nakahara, Yuriko; Kawasaki, Sunao; Shimizu, Hiroshi; Watanabe, Akihiko; Shiho, Makoto
    Arrays of microtriodes have recently become available due to the development of microfabricated field-emission electron sources. Computer simulation has shown that the brightness of beams emitted by them is significantly higher than that of the common microbeams, and possible application of the accelerated beam to free electron lasers has been discussed. Experimentation on beam generation has started, but methods for diagnosing the beam have not yet been established. Difficulty is predicted, because of the high brightness, in applying the conventional methods of emittance measurement. In this paper we propose a new method that determines the emittance without using apertures. The cross section of a converging beam is elongated by a quadrupole lens, and parameters of the emittance ellipse are obtained from the beam size on a screen when changing either the strength or the axial position of the quadrupole lens.
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    Simulation of the influence of secondary electrons on the performance of potential depressed collectors for TWTs
    (2000) Musyoki, Stephen; Takahashi, M; Uchikawa, T
    Significant differences have been found between measured and simulated data for collectors designed with codes which disregard the influence of secondary electrons. To solve this problem, various codes have been developed. These codes assume that secondary electrons are only emitted in a single direction and ignore their spatial distribution. We have developed a code in which secondary electron yield is calculated from an empirical formula and the emitted secondary electrons are assumed to have cosine spatial distribution. To represent this distribution, for each primary beam point of impact, the emitted secondary beam is split into 3 beams; one beam normal to the impact surface and the other two at an angle to the normal. The normal beam carries half of the current and the rest of the current is divided equally between the side beams. The code uses a two dimensional, axis symmetric Finite Element Method (FEM) for modeling. FEM has been chosen because of its ability to model accurately arbitrary shaped structures. We designed a ku-band (14.25 GHz), 2-stage TWT by using the developed code.
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    Influence of graphite morphology, matrix structure on gray iron machining
    (2000) Marwanga, Reuben O; Robert, C. Voigt; Paul, H. Cohen
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    Processing of Semi-Solid Gray Cast Iron Using the Cooling Plate Technique
    (2003) Muumbo, Alex; Takita, Mitsuharu; Nomura, Hiroyuki
    A semi-solid processing technique combining a cooling plate and various mold materials is developed to produce high quality gray cast iron components. Flow behavior of semi-solid slurry along an inclined cooling plate is studied to establish the effect of plate orientation on the integrity of cast products. The concept of multiple-stage cooling is discussed by considering cooling rates at different stages of processing along the cooling plate and in sand, graphite and metallic molds to show the significance of cooling rate in determining component microstructure. The morphology of microstructure in cast components is discussed and image analysis results presented. A refined microstructure of primary austenite and graphite, and their characteristics associated with the use of different mold materials are reported. Angle of inclination of plate is shown to influence the morphology and quantity of precipitated phases. Finally, the effects of cooling plate and mold material on Vickers hardness and mechanical strength of cast components are discussed.
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    Mechanical properties and microstructure of semi-solid processed cast iron
    (2003) Muumbo, Alex; Hiroyuki, Nomura; Mitsuharu, Takita
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    Casting of semi-solid cast iron slurry using combination of cooling slope and pressurisation
    (2004) Muumbo, A; Nomura, H; Takita, M
    A system developed at Nagoya University combining a cooling slope, gravity pouring and pressurisation was utilised to cast components from hypoeutectic cast iron in the semi-solid state. Investigation of resulting microstructure and attendant defects was carried out, which revealed significant improvements in primary particle size refinement and globularity in components produced through semi-solid casting (SSC) when compared with normal gravity die casting (GDC) of super-heated melt. Sub-surface anomalies such as chilling tendency and skin-deep shrinkage cavities were unavoidable in components cast under pressure in a metallic mould at room temperature but, increasing the mould preheat temperature reduced the anomalies whereas increased pressurisation improved melt feeding and drastically reduced porosity attributed to gas entrapment and solidification shrinkage
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    A precast concrete technology for affordable housing in Kenya. Pt. 2
    (2008) Shitote, SM; Nyomboi, T; Muumbo, A; Wanjala, RS; Khadambi, EL; Orowe, J; Sakwa, F; Apollo, A
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    Thermal removal of mercury in spent powdered activated carbon from Toxecon process
    (2009) Okwadha, GDO; Li, Jin; Ramme, Bruce; Michaud, Dave; Kollakowsky, Dave
    This research developed and demonstrated a technology to liberate Hg adsorbed onto powdered activated carbon (PAC) by the TOXECON process using pilot-scale high temperature air slide (HTAS) and bench-scale thermogravimetric analyzer (TGA). The HTAS removed 65, 83, and 92% of Hg captured with PAC when ran at 900°F , 1,000°F , and 1,200°F , respectively, while the TGA removed 46 and 100% of Hg at 800°F and 900°F , respectively. However, addition of CuO–Fe2O3 mixture and CuCl catalysts enhanced Hg removal and PAC regeneration at lower temperatures. CuO–Fe2O3 mixture performed better than CuCl in PAC regeneration. Scanning electron microscopy images and energy dispersive X-ray analysis show no change in PAC particle aggregation or chemical composition. Thermally treated sorbents had higher surface area and pore volume than the untreated samples indicating regeneration. The optimum temperature for PAC regeneration in the HTAS was 1,000°F . At this temperature, the regenerated sorbent had sufficient adsorption capacity similar to its virgin counterpart at 33.9% loss on ignition. Consequently, the regenerated PAC may be recycled back into the system by blending it with virgin PAC.
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    Steady state inferential modeling of temperature and pressure in an air-swept coal pulverizing ball mill
    (Elsevier, 2009) Makokha, AB; Moys, Michael H; Couvas, Costa; Muumbo, AM
    Mill discharge temperature and differential pressure have a strong effect on efficiency and safety of a coal fired power plant. Therefore, it is imperative that they are closely monitored and controlled during mill operation to keep their levels within a predetermined safe and efficient operating range regardless of the rate at which the raw coal is fed to the mill. One way to achieve this is through a control schedule that compares the value obtained from the process to the stored set-point value to determine if there is any deviation that requires correction. This paper describes a steady state model that could be used alongside conventional controllers as an on-line shadow that provides inferential estimates of desired temperatures and pressure drops in the mill circuit which can be continuously compared with the actual values for adjustment. This would not only help to avoid the difficulties associated with direct measurement but also provide a means for early detection of drifts and failing sensors and serve as a temporal back-up for the out-of-order sensors. The model was tested using industrial data collected from four ball mills at a coal fired power plant in South Africa and the results show a reasonable agreement between the measured data and model predictions both qualitatively and quantitatively within a 5% error margin. The model outputs were found to be highly sensitive to the variation in mill loading, the primary air (PA) flow and the mill channel dimensions. Therefore, for validity of this model, accurate determination of all significant parameters is essential. For now, the model is only valid for the ball mills involved in the current study, but with availability of data it can be reproduced elsewhere.
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