Hyunwoo Jung

Dynamics of mode competition in a gigawatt-class magnetically insulated line oscillator

An axial mode competition is observed in a 1GHz magnetically insulated line oscillator operating at gigawatt power level with a pulse duration of 130ns. A fast-growing axial mode adjacent to desired π-mode starts up first and hops to the slow-growing and stable π mode. The dynamics of the mode competition is found to be strongly dependent on the time-varying axial velocity of the magnetically insulated electron beam. The experimental observation is verified by the particle-in-cell simulation using a time-frequency analysis.

Transmission of gigawatt-level microwave using a beam-rotating mode converter in a relativistic backward wave oscillator

Gigawatt-level circularly polarized radiation was transmitted using a coaxial beam rotating antenna in an X-band relativistic backward wave oscillator. The mode conversion from the TM01 mode to the circularly polarized TE11-like mode was experimentally and numerically shown. The simulated radiation pattern was in good agreement with the measured radiation pattern.

Policy for physical education and school sport in England, 2003–2010: vested interests and dominant discourses

Background: The salience of physical education and school sport (PESS) in England changed dramatically in the 2000s in terms of central government investment and political interests. The government put in place the physical education, school sport and club links strategy and the physical education and sport strategy for young people for a wide-ranging array of social objectives. Although policy research relating to PESS has centred on the sport policy-making process and the role of government or agents, including teachers, has been growing from the 2000s, this paper argues for the need to explore the social construction and constitution of school knowledge underpinned and influenced by particular dominant vested interests and their associated discourses to understand certain pedagogical implications for young people. Method: Applying the educational policy sociology approach adapted from Basil Bernsteins work on the social construction of pedagogic discourse, the focus of this paper was to identify the main discourses which constructed and constituted policy for PESS from 2003 to 2010 in England. Qualitative content analysis on six policy documents and 467 media articles was conducted. Findings: This paper identifies five discourses constructing and constituting policy for PESS during the period under study: sport, health, citizenship, lifelong participation and Olympic legacy. These are sources of policy for PESS that were constructed in Bernsteins re-contextualising field. This paper also seeks to show the complexity of policies and strategies for PESS in that they are anchored in a web of significations in terms of complex connections between elements of discourses. It can be argued that as a structure-in-dominance, policies for PESS reinforced competitive sport-based conceptions of physical education and, arguably, created a limited universe of possibilities, of what was thinkable, for and as PESS. Conclusions: This paper argues that the inclusions and exclusions of discourses from policy for PESS are all politically charged, and will have an impact on the quality of young peoples education and their life chances in the future. Furthermore, this paper proposes that we need to explore in further depth the processes of how to maximise the possibilities of realising quality PESS in order for young people to learn citizenship, foster health improvement and facilitate lifelong participation in physical activities.

A comparison of cranial fluctuating asymmetry between the two sexes in a Joseon Dynasty population of Korea.

SUMMARY Cranial fluctuating asymmetry (FA) has been used to examine developmental stress levels in various populations. In bioarchaeological studies, developmental stress can be an important factor for inferring standard of living in a population. However, the crania of a Joseon Dynasty population have only been studied in terms of its morphological characteristics. In this regard, the cranial FA of the two sexes from a Joseon Dynasty population of Korea were compared here for examining their aspects of living conditions in relation to developmental stress and socio-cultural factors. In this study, 77 individuals (39 males and 38 females) who belong to the 15th to the early-20th centuries from Seoul and Gyeonggi province in Korea were investigated. Nineteen three-dimensional landmarks on the vault and basicranium were collected using MicroScribe G2X. The coordinate data were aligned into a common coordinate plane by Procrustes least-squares superimposition. Procrustes ANOVA was adopted to evaluate FA at a population level. The FA of males and females were compared with t-test using SPSS 18. There was statistically insignificant difference in FA between the two sexes in the Joseon Dynasty population. The result was interpreted to reflect both relatively high developmental stress and sex-related discrimination in the population. It was postulated that relatively high developmental stress could increase difference in FA between the two sexes in the prenatal term compared to the postnatal term because sex-related discrimination cannot be practiced before birth and males have lower stress resistance than females. Then, the difference between the two sexes in the cranial FA could be decreased during postnatal development related to sex-related discrimination, resulting in insignificant FA difference of the two sexes in spite of high developmental stress of the Joseon Dynasty population. The results will be useful for comparing and reconstructing living conditions of ancient Korean populations.

Pulse shortening by RF breakdown in relativistic backward wave oscillator

Relativistic backward wave oscillator (RBWO) is the suitable source for generating high power electromagnetic wave (HPEM). The RBWO has a high efficiency by means of the interaction region between a backward wave and an electron beam. However, the pulse shortening problem occurs frequently in experiments of RBWO because of the RF breakdown on rippled slow wave structure. Experimental study is done 500 MW RBWO in progress.

Experimental study on hundreds of MW at X-band for relativistic backward wave oscillator (RBWO)

BWO-circuit was designed to generate hundreds MW electromagnetic wave on X-band in this study. A RF coupler is designed and fabricated to measure the power of an electromagnetic wave which is generated in the slow wave structure of a relativistic backward wave oscillator (RBWO) before this wave is radiated through an antenna. To pickup about −63dB of total output power and avoid a RF breakdown in this coupler, a 2mm × 5mm slot is made in a cylindrical waveguide. This coupler is installed before the radiation antenna and the measured value is compared with the power which is measured after the radiation. In the end, the radiation pattern was identified by using of fluorescent lamps.

Effect of anode plasma on pulse shortening in magnetically insulated transmission line oscillator

A competition between two axial mode of TM01 is observed in the experiment of a L-band magnetically insulated transmission line oscillator (MILO) (D. H. Kim, et al., 2007). This mode competition can be affected by anode plasma formation during the operation. The transient time between two modes becomes shorter with additional anode plasma in MAGIC2D simulation.

High power THz radiation from a cylindrical grating structure using a high current relativistic electron beam

A Smith-Purcell radiation source using a grating structure modified in a cylindrical manner was considered to utilize a high power relativistic annular electron beam. The output characteristics were confirmed by numerical simulations assuming a 500 keV-5 kA electron beam. The experiment is being prepared.

Experimental Study on Dynamics of Mode Competition in Magnetically Insulated Transmission Line Oscillator (MILO)

A mode competition phenomenon of gigawatts-nanonoseconds magnetically insulated transmission line oscillator (MILO) is experimentally investigated. A pulsed power system of 500 kV-35 kA and 130 ns pulse length is used for generating the relativistic electron beam. The microwave with peak power of 2.8 GW, pulse duration of 70 ns, and frequency of 1.14 GHz is observed. A spatiotemporal measurement using RF B-dotprobe is in a reasonable agreement with a numerical simulation using a particle-in-cell (PIC) code confirming mode competition between axial modes.

Experimental Study on X-band Relativistic Backward Wave Oscillator (RBWO)

A Relativistic backward wave oscillator (RBWO) is a reliable source of a high power electromagnetic wave (HPEM). The RBWO is capable of high efficiency by a relatively long interaction region between a backward wave and an electron beam. But the pulse shortening problem occurs frequently in experiments of RBWO because of the mode competition, the RF breakdown and so on. After this mode competition between axial modes of TM01, the RF power is vanished. There also occurs a mode competition between TM01 and TM02 when the radii of the hollow electron beam are smaller than a critical value. Mode competition which is one of the main causes of pulse shortening is found to be very sensitive to the position of the electron beam in this RBWO device. Experimental study is done on 100MW RBWO and in progress on X-band hundreds MWRBWO.