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Kim, M.H.,Thatipamula, S.G.,Lee, J.E.,Choi, M.J.,Park, H.K.,Akiyama, T.,Yun, G.S. International Atomic Energy Agency 2018 Nuclear fusion Vol.58 No.9
<P>Using a high-speed and broadband radio frequency (RF) (0.1–1 GHz) spectrum analyzer developed on the KSTAR tokamak, it is found that several distinct stages of RF emission appear at the pedestal collapse in high confinement discharges. Comparison with 2D electron cyclotron emission (ECE) images has revealed that each stage is related to the instantaneous condition at the outboard mid-plane edge. First, high-harmonic ion cyclotron emissions (ICE) are intensified with the appearance of a non-modal filamentary perturbation in the edge within several tens of microseconds before the collapse. Then, the RF emission becomes broad toward high-frequency range (<500 MHz) at the burst onset of the non-modal filament. During the pedestal collapse initiated by the filament burst, rapid chirping (1–3 <I>μ</I>s) appear with additional filament bursts. The strong correlation between the RF spectra and the perturbation structure provides important clues on the stability of edge-localized modes and on the ion dynamics in the plasma boundary.</P>
Dynamic spectra of radio frequency bursts associated with edge-localized modes
Thatipamula, Shekar G,Yun, G S,Leem, J,Park, H K,Kim, K W,Akiyama, T,Lee, S G IOP 2016 Plasma physics and controlled fusion Vol.58 No.6
<P>Electromagnetic emissions in the radio frequency (RF) range are detected in the high-confinement-mode (H-mode) plasma using a fast RF spectrometer on the KSTAR tokamak. The emissions at the crash events of edge-localized modes (ELMs) are found to occur as strong RF bursts with dynamic features in intensity and spectrum. The RF burst spectra (obtained with frequency resolution better than 10 MHz) exhibit diverse spectral features and evolve in multiple steps before the onset and through the ELM crash: (1) a narrow-band spectral line around 200 MHz persistent for extended duration in the pre-ELM crash times, (2) harmonic spectral lines with spacing comparable to deuterium or hydrogen ion cyclotron frequency at the pedestal, (3) rapid onset (faster than ~1 <I>μ</I>s) of intense RF burst with wide-band continuum in frequency which coincides with the onset of ELM crash, and (4) a few additional intense RF bursts with chirping-down narrow-band spectrum during the crash. These observations indicate plasma waves are excited in the pedestal region and strongly correlated with the ELM dynamics such as the onset of the explosive crash. Thus the investigation of RF burst occurrence and their dynamic spectral features potentially offers the possibility of exploring H-mode physics in great detail.</P>
S. H. R. Hosseini,S. Iwasaki,T. Sakugawa,H. Akiyama 한국물리학회 2011 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.59 No.61
The paper reports on production and focusing of micro-underwater shock waves for medical applications. Shock wave focusing has various scientific, industrial and medical applications. For precise shock wave therapies near sensitive organs, such as cranioplasty in the close vicinity of the brain, a micro-shock wave source is required. A half-ellipsoidal cavity with 20.0 mm minor diameter and the ratio of major to minor diameters of 1.41 was designed and constructed as an extracorporeal shock wave (ESW) source. Underwater shock waves were generated by electric discharge produced by a magnetic pulse compression circuit (MPC) and an electrode. Input voltage and input current were measured by using an oscilloscope and a current monitor. The whole sequences of the shock wave generation, propagation, and focusing were observed by time-resolved high speed shadowgraph visualization method. Pressure histories were measured at different stand-off distances by using a PVDF needle and a fiber optic probe hydrophones. A wide range of peak overpressures from 10 to 200 MPa at the focus was obtained, and small focal zone and focal energy flux density were measured. It is concluded that the present compact extracorporeal shock wave generator has appropriate characteristics for application in precise and sensitive medical procedures.
Tanaka, K.,Nagaoka, K.,Murakami, S.,Takahashi, H.,Osakabe, M.,Yokoyama, M.,Seki, R.,Michael, C.A.,Yamaguchi, H.,Suzuki, C.,Shimizu, A.,Tokuzawa, T.,Yoshinuma, M.,Akiyama, T.,Ida, K.,Yamada, I.,Yasuhar IOP 2017 Nuclear fusion Vol.57 No.11
<P>Surveys of the ion and electron heat transports of neutral beam (NB) heating plasma were carried out by power balance analysis in He and H rich plasma at LHD. Collisionality was scanned by changing density and heating power. The characteristics of the transport vary depending on collisionality. In low collisionality, with low density and high heating power, an ion internal transport barrier (ITB) was formed. The ion heat conductivity (<I>χ</I> <SUB>i</SUB>) is lower than electron heat conductivity (<I>χ</I> <SUB>e</SUB>) in the core region at <I>ρ</I> < 0.7. On the other hand, in high collisionality, with high density and low heating power, <I>χ</I> <SUB>i</SUB> is higher than <I>χ</I> <SUB>e</SUB> across the entire range of plasma. These different confinement regimes are associated with different fluctuation characteristics. In ion ITB, fluctuation has a peak at <I>ρ</I> = 0.7, and in normal confinement, fluctuation has a peak at <I>ρ</I> = 1.0. The two confinement modes change gradually depending on the collisionality. Scans of concentration ratio between He and H were also performed. The ion confinement improvements were investigated using gyro-Bohm normalization, taking account of the effective mass and charge. The concentration ratio affected the normalized <I>χ</I> <SUB>i</SUB> only in the edge region (<I>ρ</I> ~ 1.0). This indicates ion species effects vary depending on collisionality. Turbulence was modulated by the fast ion loss instability. The modulation of turbulence is higher in H rich than in He rich plasma.</P>
Vibration isolation system with a compact damping system for power recycling mirrors of KAGRA
Akiyama, Y,Akutsu, T,Ando, M,Arai, K,Arai, Y,Araki, S,Araya, A,Aritomi, N,Asada, H,Aso, Y,Bae, S,Baiotti, L,Barton, M A,Cannon, K,Capocasa, E,Chen, C-S,Chiu, T-W,Cho, K,Chu, Y-K,Craig, K,Dattilo, V,Do Institute of Physics 2019 Classical and quantum gravity Vol.36 No.9
<P>A vibration isolation system called the Type-Bp system used for power recycling mirrors has been developed for KAGRA, the interferometric gravitational-wave observatory in Japan. A suspension of the Type-Bp system passively isolates an optic from seismic vibration using three main pendulum stages equipped with two vertical vibration isolation systems. A compact reaction mass around each of the main stages allows for achieving sufficient damping performance with a simple feedback as well as vibration isolation ratio. Three Type-Bp systems were installed in KAGRA, and were proved to satisfy the requirements on the damping performance, and also on estimated residual displacement of the optics.</P>
Characteristics of second harmonic ECCD on CHS
yasuo Yoshimura,H. Matsushita,H. Igami,K. Nagasaki,K. Matsuoka,K. Ohkubo,K. Nagaoka,M. Isobe,S. Kobayashi,S. Okamura,S. Kubo,S. Nishimura,T. Minami,T. Akiyama,T. Notake,T. Shimozuma,A. Shimizu,C. Suzu 한국물리학회 2006 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.49 No.III
Second harmonic electron cyclotron current drive (ECCD) has been investigated on CHS (Compact Helical System) by using a 2-D steerable EC-beam injection system. The direction of the plasma current, which is the sum of bootstrap current and EC-driven current, is clearly reversed by a reversal of the EC-beam injection direction. The direction of driven current is consistent with expectation in the case of low-field side injection of EC wave. So far, a driven current up to 6 kA has been obtained when bootstrap current is less than 2 kA. It is confirmed that right-hand polarization and low density are effective for ECCD.
Studies on electron Bernstein wave heating in CHS and LHD at NIFS
Y. Yoshimura,H. Igami,S. Ferrando-Margalet,K. Nagasaki,S. Kubo,T. Shimozuma,M. Isobe,C. Suzuki,A. Shimizu,T. Akiyama,C. Takahashi,K. Nagaoka,S. Nishimura,T. Minami,K. Matsuoka,S. Okamura,T. Mutoh 한국진공학회 2007 한국진공학회 학술발표회초록집 Vol.16 No.2