學術演講


<專題討論>2019/3/28(四)14:10綜合大樓2樓48218教室演講 : 張存續 教授 (清華大學物理系)

張貼者:Dabby Lu

講題: The Impossible is Possible: Using TM Modes for the Electron Cyclotron Maser
摘要: First, I will provide a brief overview about what we are doing at National Tsing Hua University (NTHU). Our research main themes include: (a) Frequency-tunable, high-power terahertz gyrotrons; (b) Microwave/material interaction and characterization, (c) Microwave physics and applications. Then, I will focus on the physics of the TM-mode gyrotrons. TM (transverse magnetic) modes have long been considered as the unsuitable modes for the electron cyclotron maser (ECM). This study, however, reveals that certain TM modes might be suitable for gyrotrons-ECM based devices. The findings are encouraging and imply that TM modes might be advantageous to the gyrotron backward-wave oscillators. 

2019/3/19(二)14:10綜合大樓1樓48111B教室演講 : Prof. Masayoshi Nagata(Graduate School of Engineering, University of Hyogo, Japan)

張貼者:2019年3月18日 上午12:12Dabby Lu

講題: Coaxial Helicity Injection and Compact Torus Injection Experiments by Magnetized Coaxial Plasma Gun
摘要: The magnetized coaxial plasma gun (MCPG) can produce successfully the compact torus (CT) of a spheromak configuration and spherical torus (ST) plasmas. Thus, the MCPG is well utilized in many fusion plasmas and basic plasma physics experiments. 
Coaxial Helicity Injection (CHI) and Compact Torus Injection (CTI) are attractive fusion applications of the MCPG technology. The CHI is one of non-inductive current drive (plasma start-up and sustainment) methods in spheromak and spherical torus (ST) plasmas. The CHI current drive has been successfully demonstrated in SSPX (LLNL), HIST (U. Hyogo), HIT-II (U. Washington), NSTX (PPPL) and SPECTOR (General Fusion) devices. In the HIST experiments, we have investigated MHD relaxation, dynamo and magnetic reconnection which are related to mechanism of current drive and formation of closed flux surfaces. One of recent topics of CHI on STs is the study of the fast reconnection based on plasmoid instabilities. Intensive internal magnetic field measurements on HIST have recently verified the flux closure due to plasmoid-driven reconnection. 
The CT injection is used for the central fueling, density profile control, plasma rotation, disruption mitigation, ELM-like pulsed heat flux irradiation and so on. The advanced fueling technology should be developed for the fusion reactor. So far, the deep fueling by CT injection has been successfully demonstrated in JFT-2M (JAERI and U. Hyogo), QUEST (Kyusyu U. and U. Hyogo), TdeV (CFFTP, Canada) and STOR-M (U. Saskatchewan, Canada) tokamak devices. We have been conducting pioneering research in this area. The CT is a high density plasma (>1022 m-3) and is magnetically confined by internally self-generated magnetic fields. Thus the CT can be accelerated by the MCPG to a high velocity (> 300 km/s). The penetration mechanism of an injected CT plasmoid into the core plasma is associated with the magnetic reconnection physics.
In this seminar, we will introduce firstly the fundamental of CT/ST magnetic configurations and how to use the MCPG, and then talk about the recent topics of CHI and CTI experiments.

<專題討論>2019/3/21(四)14:10綜合大樓2樓48218教室演講 : 張滋芳 特任助理教授 (日本名古屋大學宇宙地球環境研究所)

張貼者:2019年3月14日 下午10:45Dabby Lu   [ Ann-Chen Jang 已於 2019年3月17日 下午5:52 更新 ]

講題: Development of space weather research-A new approach to the substorm onset issue
摘要: Magnetospheric storms and substorms are the main manifestations of what are nowadays known as the “space weather” which has a practical impact on modern technology systems and our activities on Earth. Various spacecraft have been investigating the space environment around the Earth to explore the effects of substorms and storms-the THEMIS (Time History of Events and Macroscale Interactions during Substorms) and the ERG (Exploration of energization and Radiation in Geospace) missions for example. In particular, substorms are disturbances in the Earth's magnetosphere that causes energy to be released from the "tail" of the magnetosphere to the high-latitude ionosphere. The enhancement in electron fluxes during substorms can be a few orders greater than the pre-onset conditions. Since a fierce debate on substorm onset triggering has persisted for a long time, it highlights a pressing need for a better understanding of substorms in developing the space weather research. Two categories of substorm onset initiation models have been proposed-the Near-Earth Neutral Line model and the Current Disruption model. After the launch of the THEMIS quintet, some notable results even reported a new scenario – plasma intrusion to the inner plasma sheet. Thus starting a further exploration of substorms is inevitable. We will put forth a proposal to settle one of the center issues in dispute, the definition of substorm onset, by investigating the auroral streamers.

2019/3/15(五)14:00綜合大樓1樓48111B教室演講 : 畢可為 博士(捷克查理大學數學與物理學院)

張貼者:2019年3月11日 下午7:04Dabby Lu

講題:Long-lasting radial IMF events and their influence of dayside magnetosphere
摘要:Long-lasting radial interplanetary magnetic field (IMF) events are one of the special solar wind conditions. These events are usually accompanied with low density, weak magnetic field strength, low temperature, and low fluctuations of each solar wind parameters; however, the unique IMF orientation can cause numerous interesting features in the dayside magnetosphere. When the radial magnetic field transits through the bow shock, the magnetic field will be enhanced in all components. After that, Bx will divert to the Y and Z components and form an asymmetric Bz polarization in the Northern and Southern Hemispheres near the magnetopause. This asymmetry will lead the dayside reconnections occur at different locations and create a complex structure of magnetopause and its boundary sublayers. These sublayers of (1) the inner part of the low-latitude boundary layer (LLBL) on closed magnetic field lines; (2) the outer LLBL on open field lines; (3) the inner part of the magnetosheath boundary layer (MSBL) formed by dayside reconnection in the Southern Hemisphere; and (4) the outer MSBL resulting from lobe reconnection in the Northern Hemisphere are identified by observation.

<專題討論>2019/3/14(四)14:10綜合大樓2樓48218教室演講 : Dr. Alexei V. Dmitriev (Graduate Institute of Space Science, National Central University)

張貼者:2019年3月7日 下午10:37Dabby Lu

講題:Effects of magnetosheath plasma jets in the magnetosphere and radiation belts
摘要:Magnetosheath plasma jets are dense and fast plasma streams, whose energy density in the magnetosheath is higher than the energy of the incident solar wind. Jets can be generated in interaction of interplanetary discontinuities with the bow shock or as a result of foreshock disturbances in the subsolar region. They can move across the streamlines and, thus, interact with the magnetopause. The interaction results in large-scale local magnetopause distortions of ~10 min duration that corresponds to spatial scales of ~10 Re. The magnetopause distortions are translated inside the magnetosphere in the form of geomagnetic pulses, which can be observed globally while a jet is travelling along the magnetopause. It was found that jets with sufficiently high speed and high kinetic b can pierce through the magnetopause resulting in a direct transport of the magnetosheath plasma inside the dayside magnetosphere. The average transport rate was estimated to be ~1029 particles/day and sometimes can achieve values of 1.5 x 1029 particles/hour that is comparable with estimates of the total amount of plasma entering the dayside magnetosphere. In the magnetosphere, the magnetosheath plasma precipitates to the high-latitude ionosphere and produces throat aurora. An increase of conductivity in the dayside ionosphere results in induction of electric field on the nightside where the conductivity is weak. This electric field can penetrate to low latitudes and produce anomalous ExB transport of energetic particles in the inner radiation belt. This scenario allows connecting of interplanetary intermittency in the form of discontinuities to the dynamics of particles in the inner magnetosphere.

<專題討論>2019/3/5(二)13:10綜合大樓2樓48218教室演講 : Prof. Michiaki Inomoto(Graduate School of Frontier Sciences, The University of Tokyo, Japan)

張貼者:2019年2月26日 下午6:59Dabby Lu

講題:Torus Plasma Experiment for Exploring Application of Magnetic Reconnection Process on Fusion Plasma
摘要:Magnetically confined torus plasma is a potential candidate as a core plasma of a nuclear fusion reactor. Tokamak-type configuration has shown high performances on plasma confinement, stability, and controllability, and the world’s largest tokamak device ITER is under construction through international cooperation. However, tokamak’s low beta (= plasma pressure / magnetic pressure) limit requires expensive construction cost of fusion power plant. Since lowering the aspect ratio (= major radius / minor radius) of tokamak-type plasma will bring about improvement on the beta limit, “spherical tokamak (ST)”, which is a variety of tokamak configuration with aspect ratio lower than 2, attracts attention and is considered to be a potential cost-effective fusion core plasma. 

The ST concept has difficulty in initial plasma start-up because of its limited space near the geometrical axis. Thus, the center solenoid coil, which is employed in tokamak-type device to drive initial plasma current and to provide initial heating, must be removed or downsized in ST device. Center-solenoid-free start-up methods are investigated in many experimental devices, but the achieved plasma has not satisfied the plasma parameter such as plasma current, density and electron temperature required for a target plasma for additional heating.

Axial merging method is one of the candidates to provide center-solenoid-free start-up of high-beta ST plasma, in which two initially formed STs merge through magnetic reconnection in the presence of the guide (toroidal) magnetic field, which is perpendicular to the reconnection (poloidal) magnetic field. Magnetic reconnection between two STs is capable of heating the electrons in the vicinity of the reconnection point, however, its mechanism has not been identified in laboratory experiments yet. During ST merging start-up, electrons are effectively accelerated near the reconnection point where the reconnection electric field is approximately parallel to the magnetic field and will provide the local electron heating. Recent experimental results from soft X-ray observation in the UTST experiment show transient generation of energetic electrons accelerated along the field line during reconnection.  

2018/12/26(三)13:00綜合大樓1樓48111B教室演講 : 張滋芳 特任助理教授(日本名古屋大學宇宙地球環境研究所)

張貼者:2018年12月20日 下午6:11Dabby Lu   [ Ann-Chen Jang 已於 2018年12月21日 上午12:52 更新 ]

講題:From investigation of magnetospheric substorms to development of space weather research
摘要:THEMIS satellites clustered at substorm orbit together with optical observations from the ground or space make it possible for us to investigate substorm events practically. New constraints on substorm onset phenomena have been firmly built after the launch of THEMIS project. The clearest constraint comes from activations of azimuthally-spaced auroral forms manifested before breakup initiation. Pi2 disturbances are identified from the ground and space prior to substorm onsets and throughout substorm events. On the other hand, substorms inject energetic particles to inner magnetosphere which may become an important source of particles with relativistic energies. ERG satellite observes the entire radiation belts and provides critical data for developing models which give variations of radiation belts and inner magnetosphere. The enhancement in electron fluxes during substorms can be a few orders larger than the pre-onset conditions. Thus substorm-associated fields must be taken into consideration regarding modeling of radiation belt electron populations. We build a substorm injection model and successfully produce the main features of flux evolution captured in multi-satellite observations. Radiation belts models or global magnetosphere models which consider variations at whole MLT and L-shells can include fields of substorms by coupling our model and thus provide more refined results closer to realities to explore local particle accelerations up to relativistic energies, or to even further develop the feasible capabilities related to space weather forecast. In the end of this talk, I will also introduce my future research plans and directions.

<專題討論>2018/12/20(四)14:10綜合大樓2樓48218教室演講 : 張博宇 助理教授(成功大學電漿所)

張貼者:2018年12月16日 下午6:38Dabby Lu

講題: Exploring space science, generating coherent x rays and neutrons via developing a metallic ion thruster and a pulsed-power system
摘要:Two independent projects are being conducted in Pulsed-Power Generator for Space Science (PGS) laboratory to study space science, generating coherent x rays and neutrons. One is to build a metallic ion thruster using magnetron e-beam bombardments. The other one is to build a pulsed-power system. For the metallic ion thruster, the principle of e-beam evaporations is used to generate ions. Ions are first generated from a metallic target being evaporated and ionized by thermal emission electrons and are accelerated by electric fields to provide the thrust. Different from inert gas used in conventional plasma thrusters, the metal target is in the solid state, high density, easy to be stored and cheap. A mass flow rate of 8.0 ± 1.3 µg/sec using Zn was measured. Therefore, let the parameter β be the ionization rate of the metal vapor, the estimated thrust is β mN with a power of 630 W assuming 5 kV accelerating voltage is used. The thrust is much larger than those provided from existing thrusters with similar powers to date. For the pulsed-power system, 6 kJ of energy is first stored in twenty 1 µF capacitors charged to 50 kV and is discharged to a load in a short period of time to provide high-power output. The preliminary data analysis of discharge test of two capacitors charged to 20 kV showed that a peak current of 30 kA was delivered. The extrapolated peak current of final design from this test is 500 kA with a rise time of 1 µs giving an output power of 10 GW. The system will be used to generate (1) plasma jets for studying space sciences specifically on interactions between solar winds and unmagnetized planets using conical wire arrays; (2) soft x-ray laser using capillary z-pinch; and (3) neutrons using dense plasma focus. Status of developments for both projects and proposed research topics will be introduced.

2018/12/18(二)13:00綜合大樓1樓48111B教室演講 : 林忠義 博士(中央大學天文研究所)

張貼者:2018年12月11日 下午9:29Dabby Lu

講題:The study of comets from ground-based observations to space missions
摘要:With almost a century of experience and several decade observations using ground‐based and space telescopes monitoring in a wavelength band from the cm‐wave radio astronomy to x‐ray, we are confident that we were able to use such data effectively for studying the most of physical properties and chemical abundance of cometary nuclei. However, the chemical composition of cometary nuclei could be different from comet to comet. For instance, SPCs (short period comets) have been subjected more times to solar radiation than LPCs (long period comets), and they should show a lower content of volatile material and a more highly processed surface. In addition, the investigation of the chemical composition is indirect because it considers only molecules that are released from the nucleus surface that can be affected by numerous processes, for instance cosmic ray. Therefore, how the molecular composition and abundance of the coma is related to the one inside the nucleus is still an open issue. 
In this talk, I will present recent study of comets from the ground-based observations to space mission and show how important to link ROSETTA’s incredible detail results to ground‐based observations. The link and direct comparison between 67P and other comets observed in the same ways from Earth will be discussed. Furthermore, I will discuss the interaction between cometary plasma tail and solar wind using our observing data set.

2018/12/11(二)13:00綜合大樓1樓48111B教室演講 : 饒駿頌 博士(Astroparticle Physics Theory Group, DESY)

張貼者:2018年12月4日 下午7:47Dabby Lu

講題:The study of streaming instability in space, astrophysical, and laboratory plasmas
摘要:With important applications to space, astrophysical, and laboratory environments, plasma wave excitation led by the drift energy of streaming particles (streaming instability) is an extensively studied phenomenon in plasma physics.  In the Sun-Earth environment, for instance, the electrostatic streaming instability is generally considered to lie in the origin of the observed electrostatic solitary waves and in the core mechanism of type III solar radio bursts.  As in astrophysical environments, a non-resonant type electromagnetic streaming instability is also proposed as a candidate for the amplification of interstellar magnetic fields in the upstream region of supernova remnant shocks, which plays an important role in the efficiency of cosmic-ray particle acceleration.

In this talk I will present my past studies of streaming instability problems from linear theory and numerical simulation, to the application to laboratory astrophysics experiments. The research topics include the formation of phase-space electron-hole structure in the electron-positron plasma, fire-hose-type electromagnetic streaming instabilities, and cosmic-rays driven magnetic field amplification.  Together with these ongoing projects, I will also discuss potential future approaches within this rich research field.

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