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Comparative study between the reflective optics and lens based system for microwave imaging system on KSTAR

Published in Review of Scientific Instruments, 2010

Recently, two-dimensional microwave imaging diagnostics such as the electron cyclotron emission imaging (ECEI) system and microwave imaging reflectometry (MIR) have been developed to study magnetohydrodynamics instabilities and turbulence in magnetically confined plasmas. These imaging systems utilize large optics to collect passive emission or reflected radiation. The design of this optics can be classified into two different types: reflective or refractive optical systems. For instance, an ECEI/MIR system on the TEXTOR tokamak [Park et al., Rev. Sci. Instrum. 75, 3787 (2004)] employed the reflective optics which consisted of two large mirrors, while the TEXTOR ECEI upgrade [B. Tobias et al., Rev. Sci. Instrum. 80, 093502 (2009)] and systems on DIII-D, ASDEX-U, and KSTAR adopted refractive systems. Each system has advantages and disadvantages in the standing wave problem and optical aberrations. In this paper, a comparative study between the two optical systems has been performed in order to design a MIR system for KSTAR.

Recommended citation: W. Lee et al., "Comparative study between the reflective optics and lens based system for microwave imaging system on KSTAR", Review of Scientific Instruments 81, 10D932 (2010).

Microwave imaging reflectometry studies for turbulence diagnostics on KSTAR

Published in Review of Scientific Instruments, 2010

The first prototype microwave imaging reflectometry (MIR) system [H. Park et al., Rev. Sci. Instrum. 74, 4239 (2004)] clearly demonstrated the shortcomings of conventional reflectometry when the probe beam encountered a large amplitude and/or high fluctuation wavenumber at the reflection layer in laboratory tests, the distinctive advantages shown in these tests were not fully realized in the plasma operation. To understand the discrepancies, the MIR system performance has been thoroughly investigated at POSTECH. In this paper, a possible cause of the MIR performance degradation on TEXTOR will be presented together with a concept of multifrequency MIR system design that will be developed for KSTAR.

Recommended citation: H. K. Park et al., "Microwave imaging reflectometry studies for turbulence diagnostics on KSTAR", Review of Scientific Instruments 81, 10D933 (2010).

Microwave imaging reflectometry system for KSTAR

Published in Plasma and Fusion Research, 2011

A new microwave imaging reflectometry (MIR) system for KSTAR is being developed based on the experience gained via the TEXTOR proof-of-principle system [H. Park et al., Rev. Sci. Instrum. 74, 4239 (2003)] which aimed to measure the poloidal image of the electron density fluctuations essential for transport studies. The KSTAR system will adopt a multi-frequency probe beam source in the range of 90∼ 100GHz (X-mode case), which will enable the measurement of 2-D (radial and poloidal) fluctuations of the multiple cut-off layers, simultaneously. The optical system of the MIR system will be combined with the 2nd ECEI system (identical to the first ECEI system [GS Yun et al., Rev. Sci. Instrum. 81, 10D930 (2010)]) on KSTAR. The design of the launching and receiving optics of the MIR system will be constrained in order to maintain the performance of the ECEI system and thus it is necessary to consider sharing the zoom lens of the ECEI system. This stringent constraint is a challenge considering the tight wavefront matching requirement to obtain proper images for a wide range of cut-off layers within the focal depth. This paper discusses the details of the MIR system design that is compatible with the 2nd ECEI system on KSTAR.

Recommended citation: W. Lee et al., "Microwave imaging reflectometry system for KSTAR", Plasma and Fusion Research 6, 2402037-2402037 (2011).

Evaluation of the imaging properties of Microwave Imaging Reflectometry

Published in Journal of Instrumentation, 2012

Microwave Imaging Reflectometry (MIR) has been developed for unambiguous measurement of electron density fluctuations in fusion plasmas. The loss of phase information limiting the use of conventional reflectometry can be minimized by a large aperture imaging optics and an array of detectors in the MIR embodiment. The evaluation of the optical system is critical for precise reconstruction of the fluctuations. The optical systems of the prototype TEXTOR MIR [2] and newly-designed KSTAR MIR [5] systems have been tested with a corrugated target simulating density fluctuations at the cut-off surface. The reconstructed phase from the MIR system has been compared to the directly measured phase of corrugations taking into account the rotational speed of the target. The effects of optical aberrations and interference between lenses on the phase reconstruction have been investigated by the 2D amplitude measurement of the reflected waves and the diffraction-based optical simulations. (CODE V) A preliminary design of the KSTAR MIR optics has been suggested which can minimize the aberration and interference effects.

Recommended citation: I. Hong et al., "Evaluation of the imaging properties of Microwave Imaging Reflectometry", Journal of Instrumentation 7, C01077 (2012).

A Network Approach to the Transfer Market of European Football Leagues

Published in New Physics: Sae Mulli, 2015

The transfer market for sport players presents an interesting issue both for the people who are directly involved with it and for the many fans. Although attempts have been made to understand it from a perspective of economy and management, an analysis from the view point of a complex system remains in the initial stages. In this research, we analyzed the transfer market of European football leagues as a weighted network in order to understand the detailed transfer patterns. A Google-based standard is used to quantify the value of the 436 transfers that occurred in the summer of 2014. The transfer patterns on a scale of both individual teams and whole leagues validate common sense intuitions about the capitalistic English Premier League. The log-normal distributions for players and teams imply that the network has evolved according to the Yule process. The properties of the network, such as assortativity, strength correlation, and betweenness centrality, provide several significant implications for topology. An assortativity coefficient close to zero represents a randomly-mixed transfer pattern on the league scale, which contradicts the intuitive assumption of disassortativity.

Recommended citation: S. Lee, I. Hong and W.-S. Jung, "A network approach to the transfer market of European football leagues", New Physics: Sae Mulli 65, 402-409 (2015).

Application of gravity model on the Korean urban bus network

Published in Physica A: Statistical Mechanics and its Applications, 2016

Mobility models have been studied to describe the underlying mechanism of human mobility. The mobility patterns in various transportation systems were understood with the gravity model by estimating the traffic as a simple function of population and distance. Compared to most studies on large-scale systems, we focused on the validity and characteristics of gravity model for intraurban mobility. Several variations of gravity model are applied on the urban bus systems of five medium-sized cities in Korea. The gravity model successfully estimates the intraurban traffic without universal exponents for cities. From the change of exponents by predictor types, we figure out the effect by a non-trivial relation between traffic and population in the urban areas.

Recommended citation: I. Hong and W.-S. Jung, "Application of gravity model on the Korean urban bus network", Physica A 462, 48-55 (2016).

A common trajectory recapitulated by urban economies

Published in arXiv preprint, 2018

Is there a general economic pathway recapitulated by individual cities over and over? Identifying such evolution structure, if any, would inform models for the assessment, maintenance, and forecasting of urban sustainability and economic success as a quantitative baseline. This premise seems to contradict the existing body of empirical evidences for path-dependent growth shaping the unique history of individual cities. And yet, recent empirical evidences and theoretical models have amounted to the universal patterns, mostly size-dependent, thereby expressing many of urban quantities as a set of simple scaling laws. Here, we provide a mathematical framework to integrate repeated cross-sectional data, each of which freezes in time dimension, into a frame of reference for longitudinal evolution of individual cities in time. Using data of over 100 millions employment in thousand business categories between 1998 and 2013, we decompose each city’s evolution into a pre-factor and relative changes to eliminate national and global effects. In this way, we show the longitudinal dynamics of individual cities recapitulate the observed cross-sectional regularity. Larger cities are not only scaled-up versions of their smaller peers but also of their past. In addition, our model shows that both specialization and diversification are attributed to the distribution of industry’s scaling exponents, resulting a critical population of 1.2 million at which a city makes an industrial transition into innovative economies.

Recommended citation: I. Hong, M. R. Frank, I. Rahwan, W.-S. Jung and H. Youn, "A common trajectory recapitulated by urban economies", arXiv preprint arXiv:1810.08330, (2018).

Measuring national capability over big science’s multidisciplinarity: A case study of nuclear fusion research

Published in PLoS ONE, 2019

In the era of big science, countries allocate big research and development budgets to large scientific facilities that boost collaboration and research capability. A nuclear fusion device called the ``tokamak’’ is a source of great interest for many countries because it ideally generates sustainable energy expected to solve the energy crisis in the future. Here, to explore the scientific effects of tokamaks, we map a country’s research capability in nuclear fusion research with normalized revealed comparative advantage on five topical clusters – material, plasma, device, diagnostics, and simulation – detected through a dynamic topic model. Our approach captures not only the growth of China, India, and the Republic of Korea but also the decline of Canada, Japan, Sweden, and the Netherlands. Time points of their rise and fall are related to tokamak operation, highlighting the importance of large facilities in big science. The gravity model points out that two countries collaborate less in device, diagnostics, and plasma research if they have comparative advantages in different topics. This relation is a unique feature of nuclear fusion compared to other science fields. Our results can be used and extended when building national policies for big science.

Recommended citation: H. Kim, I. Hong and W.-S. Jung, "Measuring national capability over big science’s multidisciplinarity: A case study of nuclear fusion research", PLoS ONE 14, e0211963 (2019).

Gravity model explained by the radiation model on a population landscape

Published in PLoS ONE, 2019

Understanding the mechanisms behind human mobility patterns is crucial to improve our ability to optimize and predict traffic flows. Two representative mobility models, i.e., radiation and gravity models, have been extensively compared to each other against various empirical data sets, while their fundamental relation is far from being fully understood. In order to study such a relation, we first model the heterogeneous population landscape by generating a fractal geometry of sites and then by assigning to each site a population independently drawn from a power-law distribution. Then the radiation model on this population landscape, which we call the radiation-on-landscape (RoL) model, is compared to the gravity model to derive the distance exponent in the gravity model in terms of the properties of the population landscape, which is confirmed by the numerical simulations. Consequently, we provide a possible explanation for the origin of the distance exponent in terms of the properties of the heterogeneous population landscape, enabling us to better understand mobility patterns constrained by the travel distance.

Recommended citation: I. Hong, W.-S. Jung and H.-H. Jo, "Gravity model explained by the radiation model on a population landscape", PLoS ONE 14, e0218028 (2019).

Impossible by Conventional Means: Ten Years on from the DARPA Red Balloon Challenge

Published in arXiv preprint, 2020

Ten years ago, DARPA launched the ‘Network Challenge’, more commonly known as the ‘DARPA Red Balloon Challenge’. Ten red weather balloons were fixed at unknown locations in the US. An open challenge was launched to locate all ten, the first to do so would be declared the winner receiving a cash prize. A team from MIT Media Lab was able to locate them all within 9 hours using social media and a novel reward scheme that rewarded viral recruitment. This achievement was rightly seen as proof of the remarkable ability of social media, then relatively nascent, to solve real world problems such as large-scale spatial search. Upon reflection, however, the challenge was also remarkable as it succeeded despite many efforts to provide false information on the location of the balloons. At the time the false reports were filtered based on manual inspection of visual proof and comparing the IP addresses of those reporting with the purported coordinates of the balloons. In the ten years since, misinformation on social media has grown in prevalence and sophistication to be one of the defining social issues of our time. Seen differently we can cast the misinformation observed in the Red Balloon Challenge, and unexpected adverse effects in other social mobilisation challenges subsequently, not as bugs but as essential features. We further investigate the role of the increasing levels of political polarisation in modulating social mobilisation. We confirm that polarisation not only impedes the overall success of mobilisation, but also leads to a low reachability to oppositely polarised states, significantly hampering recruitment. We find that diversifying geographic pathways of social influence are key to circumvent barriers of political mobilisation and can boost the success of new open challenges.

Recommended citation: A. Rutherford, M. Cebrian, I. Hong and I. Rahwan, "Impossible by Conventional Means: Ten Years on from the DARPA Red Balloon Challenge", arXiv preprint arXiv:2008.05940, (2020).