Conductive Heat Transport and Microinstabilities in High-Beta, Magnetized, Weakly Collisional Plasma

Yerger, Evan Lowell Princeton University ProQuest Dissertations & Thes 2023 해외박사(DDOD)

The regulation of conductive heat transport in high-β, weakly collisional, magnetized plasma by microscale electromagnetic instabilities is investigated. The transport mediated by these instabilities can be a small fraction of the value expected by particle collisions and is particularly important to the macroscale physics of a number of astrophysical systems, including the intracluster medium of galaxy clusters, radiatively inefficient accretion flows, and the solar wind. Heat conduction ultimately determines how thermal energy is distributed throughout these systems and can have implications for their large-scale stability and structure. Transport in high-β, weakly collisional, magnetized plasma is generally anisotropic, (i.e. rapid along the field and suppressed perpendicular to it), corresponding to perturbations in the distribution function that can drive waves unstable. For example, a temperature gradient oriented along a mean magnetic field can induce a heat flux that provides the free energy for a number of kinetic microscale instabilities, including the electron-heat-flux-driven whistler instability (HWI), whistler heat-flux instability (WHFI), ion-heat-flux-driven slow mode instability (HSI), and ion-heat-flux-driven gyrothermal instability (GTI). Each of these instabilities has an associated wave mode that can back-react on the transport by scattering the relevant particle species and reducing the free energy in the distribution function - and thus the associated transport - to a value that is marginally stable.I provide analytic arguments showing that many of these instabilities have marginally stable values of heat flux that scale with β; in particular, ∼β −1 in the case of the HWI and HSI, and ∼β −1/2 in the case of the GTI. While these arguments lend confidence that these instabilities will exist in astrophysical plasmas, they do not guarantee that the instabilities will in fact be able to limit the heat flux to marginally stable values. Fortunately, first-principles numerical simulations can both assess whether transport saturates at the marginally stable values as well as detail the specific physical mechanisms that result in this saturation. The main thrust of the numerical analysis presented in this thesis pertains to the HWI. Previous analytical and numerical studies have shown that the heat flux for the saturated HWI scales as the analytic calculations predict: ∝βe−1 . These numerical studies, however, had limited scale separation and, consequently, large fluctuation amplitudes, which calls into question the relevancy of the presented analytic arguments in explaining the simulation results, and vice-versa. I ran a number of electromagnetic particle-in-cell (PIC) simulations of the HWI for two distinct initial conditions across a range of βe and temperature-gradient length scales in an attempt to increase scale separation and lower fluctuation amplitudes. I found that, even out to the largest heretofore performed simulations, the steady-state heat flux scales as ∼βe−1 . I also present preliminary results using hybrid-PIC simulations of ion-heat-flux-driven micro-scale instabilities, which show the presence of the GTI for the first time; however, the saturated value of the heat flux is yet to be ascertained.In order to investigate how the HWI saturates as a function of βe and scale separation, I used a number of methods to infer from the simulations an effective collision operator describing interactions between electrons and the HWI fluctuations. The first method leverages a Chapman-Enskog expansion of the kinetic equation under the assumption that the waves pitch-angle scatter particles in a frame moving at their phase velocity. Using distribution functions measured in simulations, the collision operator is inverted to solve for the dependence of the pitch-angle scattering frequency as a function of velocity-space variables. The second method employed is the construction of a quasilinear operator from the electromagnetic spectrum measured in simulations, under the assumption of purely resonant wave-particle scattering. Under the further assumption that the wave phase speed is much less than the thermal velocity of the species, I demonstrate that this operator is equivalent to the operator assumed in the first method. Finally, I employ data from a large number of tracked particles in simulations to construct a Fokker-Planck operator, which I show also adopts the form of a pitch-angle scattering operator in the frame of the wave phase velocity. All of the methods presented in this thesis are either novel or are significant refinements of existing methods, and are therefore results in their own right.The exact form of the collision frequency differs between each of the methods; however, each result motivates different key physical ingredients, which I incorporate and synthesize into a model for the pitch-angle scattering frequency of the saturated HWI. The model, which is that of a resonance-broadened quasi-linear operator, shows that the HWI can regulate heat flux to the observed ∼βe−1 scaling in the limit of astrophysical scale separation. Resonance broadening is a crucial ingredient for this extrapolation. I argue that without this feature, electrons with sub-thermal parallel velocities do not have any physical means by which to be scattered by waves, resulting in a large, unphysical heat flux that would deviate from the scaling. Finally, I show that the presence of resonance broadening also explains much of the difference between the model operators that I have inferred from simulations, and that the entirety of the analysis represents a self-consistent picture.

Transient response of global heat energy transport in a quadrupling Co2 oulse experiment

박소은 Graduate School, Yonsei University 2022 국내석사

대기와 해양을 통한 극 방향의 열 에너지 수송은 전 지구의 에너지 균형을 이루는 데에 있어 중요한 역할을 하며 이를 파악하는 것은 지역적 기후변화를 이해하는 데에 필수적이다. 파리협정에 근거하여 탄소 배출 경감을 위한 노력이 지속됨에 따라 배출 감소 시기에 대한 전구 열 에너지 수송 변화를 파악할 필요가 있다. 본 연구에서는 즉각적인 이산화탄소 4 배 증감 실험을 통해 강제력이 주어져 있는 기간과 이산화탄소 농도가 회복된 직후의 특정 기간 동안 열 에너지 수송이 산업화 이 전 대비 어떻게 변화하였는지 분석하고자 하였다. 강제력이 주어져 있는 기간 동안 대기의 연직 안정도 증가에 따라 해들리 순환이 약화되었음에도 불구하고 증가한 비습으로 인 해 대기의 극 방향 수송이 강화되었다. 동일한 시기에 해양에서는 북대서양 자오선 역전 순환(AMOC)의 감소가 극 방향 수송을 약화시켰다. 이산화탄소 농도가 회복된 이후에는 아노말리 관점에서 대기 와 해양 상층의 수송이 전반적으로 북쪽을 향하였는데, 이는 두 반구 사이의 에너지 불균형을 해소 하려는 반응이다. 이와 반대로 해양 심층에서는 남쪽 수송이 지배적으로 나타났으며, 이는 강제력이 주어져 있던 시기보다 더욱 약화된 북대서양 자오선 역전 순환 세기 변화에 따른 결과이다. 두 시기에 대해 공통적으로 대기와 해양 사이의 열 에너지 수송 반응이 반대로 나타나는 것은 비야크네스 보상(Bjerknes Compensation) 과정으로 일컬어진다. 이는 대기와 해양에서의 남북 방향 에너지 수송 총합을 일정하게 유지하려는 기후시스템의 기작이다. The poleward heat energy transport through the atmosphere and ocean plays an important role in balancing Earth's energy. Identifying how meridional transport of heat energy occurs is essential to understanding regional climate change. In this study, the changes in meridional transport compared to pre-industrial period are analyzed using an immediate quadrupling CO2 experiment, focusing on forcing period and specific duration right after the CO2 concentration is recovered. The poleward transport in atmosphere is enhanced despite the slowdown of Hadley circulation due to an increased specific humidity while the abrupt forcing is switched on. During the same period, oceanic transport is reduced because of the change in Atlantic Meridional Overturning Circulation (AMOC). After the recovery of CO2, the atmosphere shows anomalous northward transport which is a response to release hemispheric thermal asymmetry. On the other hand, the further weakened AMOC induces overall southward transport in ocean anomalously. For both periods, the atmosphere and ocean have changed in opposite direction and such response is called "Bjerknes Compensation", which is the mechanism of climate system to maintain the constancy in total meridional heat transport.

Mechanisms of Polar-Amplified Warming: Understanding Hemispheric and Seasonal Asymmetries

Hahn, Lily C. ProQuest Dissertations & Theses University of Wash 2023 해외박사(DDOD)

The Arctic has warmed four times faster than the global average in recent decades. This polar-amplified warming has wide-ranging impacts on local ecosystems and global climate, yet there is still debate over the drivers of polar amplification, how they interact with each other, and their relative importance. Moreover, future warming uncertainty is larger for the poles than for any other region on Earth.I motivate key research questions in Chapter 1 before examining contributions to polar warming and its uncertainty in the latest generation of climate models in Chapter 2. I find that a larger albedo feedback than previously shown contributes comparably to temperature feedbacks towards greater warming in the Arctic than in the tropics or Antarctic. The albedo feedback and its positive covariance with other polar feedbacks are also the dominant source of model uncertainty in polar warming. I also show that increased atmospheric moisture transport to the poles is the third largest contributor to Arctic amplification and the largest contributor to projected Antarctic warming.In Chapter 3, I complement these comprehensive climate models with idealized models and theory to identify a central mechanism for seasonality in Arctic warming. As frozen sea ice melts and transitions to open ocean, the increasing thermal inertia of the surface layer slows the rate of seasonal cooling, which can alone produce the observed pattern of peak Arctic warming in early winter.In Chapter 4, I investigate the seasonality of projected changes in poleward heat and moisture transport to the Arctic, which have mainly been studied in the annual-mean or in winter. This seasonality can be explained by down-gradient diffusion of anomalies in dry and moist static energy, yielding a winter peak in reduced dry heat transport and a summer peak in increased moisture transport. I hypothesize that even for compensating annual-mean changes in moist and dry heat transports, their opposite seasonality will produce non-compensating impacts on Arctic warming.Despite its winter peak, Arctic warming is impacted by forcing in all seasons, highlighting a need for further research to investigate how seasonality in poleward heat transport and other feedbacks mediates their effect on Arctic warming. I present preliminary results and future work addressing this question in Chapter 5, in addition to summarizing our conclusions.

Time Domain Reflectance for Thermal Conductivity of Electronic Materials

Warkander, Sorren Victoria University of California, Berkeley ProQuest Disser 2022 해외박사(DDOD)

Electronic materials represent a vast category with wide-ranging properties. Though definitionally, their electronic properties are of interest, study of their thermal properties provides additional important information. Whether to find materials with desirable thermal conductivity-high to dissipate heat or low to limit heat transport-or because the mechanisms underlying heat transport provide insight into the fundamental physics of the material, thermal characterization allows better understanding of such materials.In the study of the thermal properties of materials, optical pump-probe methods are a key tool. Laser-based measurements allow probing of small areas without requiring microfabrication, and the use of pulsed lasers and modulated beams allows measurement of high-speed effects and so small sample areas. Time domain thermoreflectivity (TDTR), which uses modulated pulsed laser beams, stands as a key tool for measurement of the thermal properties of both bulk and thin film samples. This work seeks to provide a detailed introduction to the technique and the mathematical analysis required for such measurements, apply TDTR to interesting materials systems, and push beyond the limits of traditional TDTR with the development of a transducerless time domain reflectance (tTDR) technique which uses the same equipment.Study of thermal properties can provide new insights into the fundamental properties of materials. Metallic vanadium dioxide nanobeams have a much lower thermal conductivity than would be expected given their electrical conductivity, implying that electrons are more effective at carrying charge than heat. This result is not always seen in other sample geometries, and TDTR measurements allow characterization of thin film samples, allowing further study. Unfortunately, the samples measured in this work did not provide high enough electrical conductivity to draw conclusions about electronic thermal transport, but this stands as an interesting line of investigation. One category of electronic materials of increasing interest is that of two-dimensional materials, whose ultra-thin nature offers new properties and applications. However, it also makes study of their thermal properties challenging and makes traditional TDTR infeasible. By contrast, tTDR is well suited to characterize their properties, and in this work initial measurements were made on suspended molybdenum disulfide. In addition to the fundamental properties of the materials, other effects can be engineered. For example, creating bilayers with twists between layers causes the development of moire patterns which have novel properties. Their in-plane thermal conductivities are not well characterized, and tTDR provides an avenue for changing that. Preparation of such samples is very challenging, thus far preventing systematic study as a function of twist angle, but initial measurements demonstrated that tTDR is an applicable tool for such systems. Time domain reflectance measurements, both in the form of TDTR and tTDR, are powerful tools for the characterization of a wide range of materials, including many electronic materials. This work seeks to offer insight into and expand their applications.

Quantitative nanoscale thermal characterization of nano materials and devices by enabling low-noise null-point scanning thermal microscopy

Hwang, Kwangsuk 고려대학교 대학원 2015 국내박사

The application of conventional scanning thermal microscopy (SThM) is severely limited by three major problems: (i) distortion of the measured signal due to heat transfer through the air, (ii) the unknown and variable value of the tip-sample thermal contact resistance, and (iii) perturbation of the sample temperature due to the heat flux through the tip-sample thermal contact. Recently, we proposed null-point scanning thermal microscopy (NP SThM) as a way of overcoming these problems in principle by tracking the thermal equilibrium between the end of the SThM tip and the sample surface. However, in order to obtain high spatial resolution, which is the primary motivation for SThM, NP SThM requires an extremely sensitive SThM probe that can trace the vanishingly small heat flux through the tip-sample nano-thermal contact. Herein, we derive a relation between the spatial resolution and the design parameters of a SThM probe, optimize the thermal and electrical design, and develop a batch-fabrication process. We also quantitatively demonstrate significant sensitivity improvement, lower measurement noise, and higher spatial resolution of the fabricated SThM probes. By utilizing the exceptional performance of these fabricated probes, we show that NP SThM can be used to obtain a quantitative temperature profile with nanoscale resolution independent of the changing tip-sample thermal contact resistance and without perturbation of the sample temperature or distortion due to the heat transfer through the air. we rigorously re-derived the principal equation of null-point scanning thermal microscopy (NP SThM) in terms of measuring the thermal properties to explain how this technique, which has already been proven to resolve the major problems of conventional SThM and be able to quantitatively measure the temperature profile, can be effectively utilized for quantitatively measuring the local thermal resistance with a nanoscale spatial resolution. Using NP SThM, we measured the relative change in the thermal conductivity of suspended chemical vapor deposition (CVD)-grown graphene disks with radii of 50?3680 nm and estimated the absolute value of the thermal conductivity of these disks in a diffusive regime. We performed a theoretical analysis to demonstrate that the relative changes and absolute values of the thermal conductivity of the graphene disks were consistent. Finally, by using the NP SThM, we profile the undisturbed temperature distribution around the electrically heated 100 nm-wide platinum nano-heater patterned on SOI wafer and the local spreading thermal resistance qualitatively at the same time. Comparison of the experimental temperature and thermal resistance profiles with those obtained from diffusion equation explains why the local temperature gradient as well as the absolute temperature is higher than the modeling results around the nano-heater. The quantitative data obtained in this study would be an essential reference data for the validation of the theoretical model for thermal analysis in nanoelectronic devices. Furthermore, since NP SThM can profile φ simultaneously as well as the undisturbed temperature, NP SThM would be widely applicable in the analysis of the energy transport/conversion in the nano-devices and nano-materials.

Experimental and Numerical Investigation of Thermal Dispersion and its Significance for Designing Groundwater Heat Pump (GWHP) Systems

박병학 서울대학교 대학원 2019 국내박사

Groundwater heat pump (GWHP) systems are not always efficient and sustainable. GWHP systems generally generate thermal plumes in the aquifer, which can adversely affect the system performance and the subsurface environment. The prediction of thermal plumes is therefore a matter of great concern in the design stage. The plumes can propagate in saturated porous media by conduction, advection, and dispersion. Thermal dispersion, however, has been underestimated in the subsurface heat transport processes by making use of default values in the numerical simulator or solute dispersivity as thermal dispersivity, without any field-based evaluation. These conventions or assumptions can play a crucial role in assessing thermal impacts of GWHP systems but they have not been fully examined so far. Field and numerical studies were performed on the alluvial aquifer to investigate the importance of thermal dispersivity in GWHP systems. Numerical analysis indicates that thermal dispersivity has a great effect on the temperature distribution as well as the extent of thermal plume. Observed data also show that mechanical thermal dispersion can dominates over thermal diffusion even under the natural groundwater flow. Field-based model was modified to analyze the role of aquifer properties. The sensitivity analysis confirms and advances the previous finding that hydrodynamic parameters affect thermal plume development, and thereby reveals that aquifer properties composing mechanical thermal dispersion are of great importance for predicting thermal plumes of GWHP systems. A laboratory-scale experimental system was designed to control flow and temperature boundary conditions, and investigate the thermal dispersion behavior in porous media under forced flow field by injected water. Transport experiments using two different heat sources as tracers were conducted in saturated coarse sand (d50 = 1.28 mm) at various background flow velocities (Re < 0.52). Experimental and analytical results describe the scatter in the relationships between thermal dispersion coefficients and thermal front velocities with conduction dominant regime, nonuniform flow field, and heterogeneous thermal properties. Numerical results indicate that injected water greatly increases flow velocities and thermal dispersion coefficients, and thus makes the regions near the injection well stay in transition zone or convection dominant regime. Therefore, mechanical thermal dispersion becomes important even at low flow velocity. Additional heat tracer tests were performed to investigate the validity of the general assumption that transverse thermal dispersivity is one–tenth of longitudinal one, and to analyze its impacts on thermal plume propagation. Experimental results confirm that such assumption can be violated. Numerical results show that the effect of dispersivity ratio is time-dependent, anisotropic, and varying with injection rates. These results indicate that the thermal dispersivity ratio can be significant for assessing the long-term environmental impacts of large-scale GWHP plants. Therefore, evaluation and reflection of dispersivity magnitude and ratio are necessary in the design stage of GWHP plants for sustainability. This work describes the significance and role of thermal dispersion when using groundwater as energy resources. The findings of this work can have a significant implication on the heat transport in saturated porous media as well as the efficient and sustainable use of shallow groundwater. 개방형 지열 시스템은 항상 효율적이고 지속 가능한 것은 아니다. 개방형 시스템은 대수층에 열 플룸을 생성시켜 시스템 효율과 지중 환경에 악영향을 미칠 수 있으므로 그에 대한 예측은 설계 단계에서 매우 중요한 문제이다. 생성된 열 플룸은 포화된 다공성 매질 내에서 전도, 이류 및 분산에 의해 이동할 수 있다. 그러나 열 분산은 현장 기반의 평가없이 수치 모델의 기본값이나 용질의 분산도를 열 분산도로 사용함으로써 지중 열 전달 과정에서 과소평가되어왔다. 이러한 관행이나 가정은 개방형 지열 시스템의 열적 영향을 평가하는데 결정적인 역할을 할 수 있지만, 지금까지는 완전히 검증되지 않았다. 충적 대수층에 대한 현장 및 수치 연구를 수행하여 개방형 지열 시스템에서 열 분산도의 중요성을 조사하였다. 수치적 분석은 열 분산도가 열 플룸의 범위뿐만 아니라 온도 분포에도 큰 영향을 미친다는 것을 나타낸다. 관측된 자료는 또한 자연적인 지하수 흐름 하에서도 기계적 열 분산이 열 확산에 비해 우세할 수 있음을 보여준다. 현장 기반 모델을 수정하여 대수층 특성의 역할을 살펴보았다. 민감도 분석을 통해 수력학적 변수들이 열 플룸의 발달에 영향을 준다는 선행 연구 결과를 확인하고 발전시켜, 기계적 열 분산을 구성하는 대수층 특성들이 개방형 시스템의 열 플룸을 예측하는데 매우 중요하다는 것을 밝혔다. 흐름 및 온도에 대한 경계 조건을 제어하고 주입수로 인해 교란된 유동장에서 다공성 매질을 통한 열 분산 거동을 규명하고자 실내실험장치를 제작하였다. 두 개의 다른 열원을 사용하는 추적자 실험을 포화된 조립질 모래(d50 = 1.28 mm)에서 다양한 배경 유속(Re < 0.52)과 함께 수행하였다. 실험 및 해석 모델은 열 분산 계수와 열 전달 속도 사이의 관계에 나타나는 산란을 전도 지배 영역, 불균일한 속도장 및 불균질한 열 물성에 의해 설명할 수 있음을 보여준다. 수치 모의 결과는 주입수가 유속 및 열 분산 계수를 크게 증가시킴으로써 주입정 주변 지역을 전이대나 대류 지배 영역에 머물게 한다는 것을 나타낸다. 따라서 낮은 유속에서도 기계적 열 분산이 중요해진다. 횡 방향 열 분산도가 종 방향 열 분산도의 10 분의 1이라는 일반적인 가정의 타당성을 조사하고 이러한 가정이 열 플룸 전파에 미치는 영향을 분석하기 위해 추가적인 열 추적자 시험을 수행하였다. 실험 결과는 일반적인 가정이 위배될 수 있음을 확인한다. 수치 모델은 열 분산도 비율의 영향이 시간에 의존적이고 이방성을 가지며 주입량에 따라 달라진다는 것을 보여준다. 이러한 결과는 열 분산도 비율이 대규모 시설의 장기 환경 영향 평가에 중요할 수 있음을 나타낸다. 그러므로 개방형 지열 시스템의 설계 단계에서는 지속 가능성을 위해 분산도의 크기와 비율을 평가하고 이를 반영해야 한다. 본 연구는 지하수를 에너지 자원으로 활용할 때의 열 분산의 중요성과 역할에 대해 기술한다. 본 연구의 결과들은 천부 지하수의 효율적이고 지속가능한 사용뿐만 아니라 포화된 다공성 매질에서의 열 거동에도 중요한 함의를 지닌다.

Theoretical and Computational Studies of Heat Transport Processes in Molecular Systems

Chen, Renai ProQuest Dissertations & Theses University of Penn 2021 해외박사(DDOD)

There has been growing research interest in the field of nanoscale thermal transport over the past two decades due its importance to a variety of fascinating applications, such as waste heat control, improved electronic functionality, and phononics building blocks. Much of this focus has been on solid-state systems for which advanced experimental characterizations and measurements are readily available. Molecule-based systems, which in principle exhibit no less structural richness than solid state systems and may show excellent energy transport capabilities, have been largely ignored until recently. This is mostly because of the difficulties associated with measuring heat transport on the molecular scale. However, a few recent experimental breakthroughs have brought molecular energy transport process into the spotlight, and at the same time established measurement techniques that can be tested, verified, and explained using theoretical tools. This dissertation examines and explores theoretical approaches for modeling heat transport in molecular systems. Specifically, we have developed a stochastic nonequilibrium molecular dynamics (MD) method which mimics the experimental setting of substrate-bridge-substrate structure, i.e., a molecular junction. We incorporate this approach, along with a quantum Landauer's formalism, into the open-source molecular simulation package--GROMACS, so that it can be applied to molecular systems with different topologies and thermal environments. Our simulations of heat conduction in hydrocarbon-based single molecule junctions yield excellent agreement with the recent state-of-the-art experimental data. Within the capacities of the new method, we have also investigated phononic interference effects in the heat conduction characteristics of benzendithiol molecules. Using the methods developed in this dissertation, we have mapped, for the first time, thermal fluxes down to the atomistic level. In the context of phononic energy transport, we develop a simulation method that integrates quantum effects into classical MD. This hybrid method, once fully implemented, will compensate for the disadvantages of classical approaches at low temperatures and for the difficulty in treating anharmonicites in Landauer-type quantum transport calculations. This method will improve the predictive power of classical heat conduction simulations.The second part of this dissertation explores an intriguing energy transport channel that has been newly discovered termed electron-transfer-induced heat transport (ETIHT), which is distinct from traditional heat transfer mechanisms that rely purely on molecular vibrations. We construct a theoretical model that combines the two energy transport channels (ETIHT and phononic) into one general model and then we show analytically under certain parametric thresholds (e.g. reorganization energies) that ETIHT dominates while other conditions may magnify the phononic contributions. Although the work in this part of the thesis is currently purely theoretical, it may provide useful insights into future organic molecular thermoelectric devices.

Analysis of the upper ocean structure and heat transport in the north pacific and their variation under climate change

임보영 Graduate School, Yonsei University 2011 국내박사

Validity of local thermal equilibrium assumption on heat transport in porous medium

백지영 서울대학교 대학원 2020 국내석사

In many hydrogeological systems, it has been generally assumed that the temperature of solid and fluid in a porous medium reaches local thermal equilibrium almost immediately in Darcy range (Re < 3). This assumption, also known as local thermal equilibrium (LTE) assumption, however, can be gradually violated with the increase of flow velocity. Numerical and analytical approaches have been proposed to figure out the effect of local thermal non-equilibrium (LTNE), but the experimental evidence about proposed models is insufficient. In this study, a total of 52 transport experiments on a sub-meter scale using both heat and solute as tracers were performed at various background flow velocities (Re < 0.37) to confirm the validity of LTE assumption under natural groundwater flow. Observed fluid EC and temperature time series data were analyzed by mathematical models and derived transport parameters were related by a retardation factor. Consequently, it was experimentally confirmed that the LTE assumption can be violated even in Darcy range and the LTNE can affect heat transport parameter estimation by up to 20 %. This study suggests that the LTE assumption need to be verified in thermogeological systems. 많은 수문학적 시스템에서, 다공성 매질 내 고체와 유체 사이의 온도가 거의 즉시 열 평형의 상태에 도달한다고 일반적으로 가정해오고 있다. 그러나, 국소 열 평형 가정이라 알려진 이 가정은 유속의 증가에 따라 점진적으로 위배되어질 수 있다. 국소 열 비평형에 대한 효과를 파악하기 위한 여러 수치적, 해석적 모델들이 제시되어오고 있으나, 그 모델들에 대한 실험적인 증거들은 아직까지 거의 제시되어 오고 있지 않다. 본 연구에서는, 다르시안 유속에서의 국소 열 평형가정에 대한 유효성을 확인하기 위하여 총 52회 열과 용질을 추적자로 하는 추적자 시험을 다양한 유속 환경 하에서 수행하였다. 실험 중에 관측된 EC와 온도의 시계열 자료는 수학적 모델들을 통해 분석되었으며, 그로 인해 얻어진 변수들을 지연계수로 서로 비교하였다. 그 결과, 다르시안 유속에서도 국소 열 평형 가정이 위배되어질 수 있으며, 그에 의한 영향이 열 속도의 추정에 20 % 까지 영향을 줄 수 있음을 확인하였다. 이 연구는 국소 열 평형가정이 수문지질학적 시스템에서 열의 거동을 해석할 때 검증될 필요가 있음을 시사한다.

Electron Heat Transport Study in KSTAR L-mode Plasmas Using ECRH Modulation

고정민 서울대학교 대학원 2018 국내석사

In this study, we explore the electron heat transport in a simpler approach using a semi-empirical model, the so-called critical gradient model (CGM) that has not been applied to KSTAR before, instead of a complex theoretical approach. KSTAR experiments were designed to perform the perturbative analysis using electron cyclotron resonance heating (ECRH) modulation as well as power balance analysis. We determined the free-parameters of the semi-empirical model that can reproduce the experimental results by simulations using ASTRA. The ranges of free-parameters obtained through this process from KSTRA experiments are compared and analyzed with those obtained from other devices such as JET and ASDEX Upgrade. The dependences on the plasma parameters are conducted.