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H. Zushi,
S. Tashima,
M. Ishiguro,
M. Hasegawa,
S. Banerjee,
N. Nishino,
M. Isobe,
K. Hanada,
H. Idei,
K. Nakamura, [......],
T. Maekawa, A. Ejiri,
T. Yamaguchi,
J. Hiratsuka,
Y. Takase,
M. Kikuchi,
A. Fukuyama,
Y. Ueda,
O. Mitarai,
S. Okamura
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ABSTRACT: Non-inductive current start-up via relativistic electron cyclotron resonance interaction is investigated for the high ratio (~10 %) of vertical Bv to toroidal Bt fields and the concave field lines in the QUEST spherical tokamak. In the start-up scenario with an internal poloidal field null (IPN), the fast current start-up rate of 0.3-0.5 MA/sec and correlation with mildly relativistic electrons accelerated due to multiple ECR interaction are observed. In steady state high p equilibrium characterized by the inboard null (Rs ~ 0.7×R0) and p of 1.5 is achieved, where p are the inverse aspect ratio and poloidal beta, respectively. Relaxation oscillations in this equilibrium and confinement of the energetic electrons are discussed.
24th IAEA Fusion Energy Conference, San Diego, USA; 10/2012
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ABSTRACT: In order to measure the poloidal rotation velocity, a Doppler reflectometer has been developed in Large Helical Device (LHD). A remotely controlled antenna tilting system has been installed in an LHD vacuum vessel. A synthesizer is used as the source, and the operation microwave frequency ranges are ka-band and V-band. In LHD last experimental campaign we obtained the Doppler shifted signal, which was consistent with CXRS measurements.
The Review of scientific instruments 10/2012; 83(10):10E322. · 1.52 Impact Factor
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ABSTRACT: This paper presents an experimental demonstration to determine electron temperature (T(e)) with unknown spectral sensitivity (transmissivity) in a Thomson scattering system. In this method, a double-pass scattering configuration is used and the scattered lights from each pass (with different scattering angles) are measured separately. T(e) can be determined from the ratio of the signal intensities without knowing a real chromatic dependence in the sensitivity. Note that the wavelength range for each spectral channel must be known. This method was applied to the TST-2 Thomson scattering system. As a result, T(e) measured from the ratio (T(e,r)) and T(e) measured from a standard method (T(e,s)) showed a good agreement with <∣T(e,r) - T(e,s)∣∕T(e,s)> = 7.3%.
The Review of scientific instruments 02/2012; 83(2):023507. · 1.52 Impact Factor
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T. Wakatsuki, A. Ejiri,
H. Kakuda,
O. Meneghini,
S. Shiraiwa,
Y. Takase,
J. C. Wright,
T. Ambo,
K. Hanashima,
J. Hiratsuka,
Y. Nagashima,
T. Sakamoto,
R. Shino,
M. Sonehara,
T. Yamaguchi,
O. Watanabe
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ABSTRACT: Plasma current start‐up experiments using the lower hybrid wave (LHW) excited by a new dielectric loaded waveguide array antenna are planned on TST‐2. The wave excitation efficiency of this antenna is investigated using a versatile FEM solver package, COMSOL. Propagation and absorption of the LHW are calculated by iterating between the TORLH full wave code and the CQL3D Fokker‐Planck code. Wave absorption is very weak for Maxwellian plasma, but becomes stronger as the quasilinear deformation of the distribution function develops.
AIP Conference Proceedings. 12/2011; 1406(1):431-434.
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ABSTRACT: In order to measure the internal structure of density fluctuations using a microwave reflectometer, the broadband frequency tunable system, which has the ability of fast and stable hopping operation, has been improved in the Large Helical Device. Simultaneous multipoint measurement is the key issue of this development. For accurate phase measurement, the system utilizes a single sideband modulation technique. Currently, a dual channel heterodyne frequency hopping reflectometer system has been constructed and applied to the Alfvén eigenmode measurements.
The Review of scientific instruments 10/2010; 81(10):10D906. · 1.52 Impact Factor
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ABSTRACT: The poloidal eigenmode of the geodesic acoustic mode (GAM) is studied in the limit of high safety factor. In this limit, the poloidal gyroradius cannot be treated as a perturbation or as an expansion parameter. Analytical expressions for the poloidal structure of the GAM potential, the radial wavenumber dependence of the frequency, the phase velocity, and the group velocity are obtained. The spatial structure of the poloidal eigenmode including the higher-order gyroradius effect is revealed theoretically.
Journal of Plasma Physics 11/2009; 75(06):721 - 729. · 0.94 Impact Factor
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ABSTRACT: Zonal flows (ZFs) are excited transiently by geodesic acoustic modes (GAMs) in toroidal plasmas. When GAMs have radially inward and outward propagating components, these components interact to excite the zero frequency modes (ZFs) transiently, in addition to exciting higher harmonics of GAM. A fluid model is used to study this transient excitation. The Reynolds stress is evaluated from the action conservation equation, and a set of equations to analyze nonlinear couplings among GAMs and ZFs is derived. GAMs are driven by the drift wave (DW) turbulence, and ZFs are transiently driven by nonlinear self-interaction of GAMs. The dependences of the peak amplitudes of ZFs on coupling coefficients are explained theoretically. A new energy path from the DW turbulence to ZFs is found under a situation in which there is only energy transfer from the DW turbulence to GAMs.
Plasma Physics and Controlled Fusion 07/2009; 51(8):085002. · 2.42 Impact Factor
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Y Nagashima,
K Itoh,
A Fujisawa,
K Shinohara,
S-I Itoh,
T Ido,
M Yagi,
K Hoshino, A Ejiri,
Y Takase,
K Uehara,
Y Miura
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ABSTRACT: We present a detailed investigation of the geodesic acoustic eigenmode (GAEM) profile in the vicinity of the last closed flux surface (LCFS) in the JFT-2M tokamak. Envelope and spectral peak frequency of the GAEM vanish near the LCFS obtained by an equilibrium reconstitution code. Spatial changes in mean plasma gradients (the electron temperature, electron density and floating potential) are correlated with the boundary. This does not contradict that the GAEM can have a node at the LCFS. Discussions of an inverse problem and the application of this GAEM measurement to an experimental test of the LCFS location calculated by the equilibrium reconstitution code are presented.
Plasma Physics and Controlled Fusion 05/2009; 51(6):065019. · 2.42 Impact Factor
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A. Ejiri,
Y. Takase,
T. Oosako,
T. Yamaguchi,
Y. Adachi,
O. Watanabe,
Y. Nagashima,
B.I. An,
H. Kobayashi,
H. Kurashina,
H. Hayashi,
H. Matsuzawa,
K. Yamada,
H. Tojo,
T. Masuda,
M. Sasaki,
R. Kumazawa,
H. Kasahara,
F. Shimpo
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ABSTRACT: Non-inductive plasma current start-up by EC and RF power was carried out on the TST-2 device. Low frequency RF (21 MHz) sustainment was demonstrated, and the obtained high βp spherical tokamak configuration has similar equilibrium values as the EC (2.45 GHz) sustained plasma. Equilibrium analysis revealed detailed information on three discharge phases: (i) in the initial current formation phase, the plasma current increases with the stored energy, and the current is in the same order as that predicted by theory. (ii) In the current jump phase, the current density profile, which is peaked near the outboard boundary, is not deformed but increases slowly and the initial closed flux surface appears when the current reaches a maximum. (iii) In the current sustained phase, equilibrium is characterized by the hollowness of the current density profile, and it determines the fraction of the current inside the last closed flux surface to the total current. Both EC and RF injections show a similar equilibrium. While MHD instabilities often terminate the RF sustained plasma, no such phenomenon was observed in the EC sustained plasma.
Nuclear Fusion 05/2009; 49(6):065010. · 4.09 Impact Factor
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D. A. Gates,
J. Ahn,
J. Allain,
R. Andre,
R. Bastasz,
M. Bell,
R. Bell,
E. Belova,
J. Berkery,
R. Betti, [......],
K. Wong,
J. Wright,
Z. Xia,
X. Xu,
D. Youchison,
G. Yu,
H. Yuh,
L. Zakharov,
D. Zemlyanov,
S. Zweben
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ABSTRACT: The mission of the National Spherical Torus Experiment (NSTX) is the demonstration of the physics basis required to extrapolate to the next steps for the spherical torus (ST), such as a plasma facing component test facility (NHTX) or an ST based component test facility (ST-CTF), and to support ITER. Key issues for the ST are transport, and steady state high b operation. To better understand electron transport, a new high-k scattering diagnostic was used extensively to investigate electron gyro-scale fluctuations with varying electron temperature gradient scale length. Results from n = 3 braking studies are consistent with the flow shear dependence of ion transport. New results from electron Bernstein wave emission measurements from plasmas with lithium wall coating applied indicate transmission efficiencies near 70% in H-mode as a result of reduced collisionality. Improved coupling of high harmonic fast-waves has been achieved by reducing the edge density relative to the critical density for surface wave coupling. In order to achieve high bootstrap current fraction, future ST designs envision running at very high elongation. Plasmas have been maintained on NSTX at very low internal inductance li [?] 0.4 with strong shaping (k [?] 2.7, d [?] 0.8) with bN approaching the with-wall b-limit for several energy confinement times. By operating at lower collisionality in this regime, NSTX has achieved record non-inductive current drive fraction fNI [?] 71%. Instabilities driven by super-Alfvenic ions will be an important issue for all burning plasmas, including ITER. Fast ions from NBI on NSTX are super-Alfvenic. Linear toroidal Alfven eigenmode thresholds and appreciable fast ion loss during multi-mode bursts are measured and these results are compared with theory. The impact of n > 1 error fields on stability is an important result for ITER. Resistive wall mode/resonant field amplification feedback combined with n = 3 error field control was used on NSTX to maintain plasma rotation with b above the no-wall limit. Other highlights are results of lithium coating experiments, momentum confinement studies, scrape-off layer width scaling, demonstration of divertor heat load mitigation in strongly shaped plasmas and coupling of coaxial helicity injection plasmas to ohmic heating ramp-up. These results advance the ST towards next step fusion energy devices such as NHTX and ST-CTF.
Nuclear Fusion. 01/2009; 49(10):104016.
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ABSTRACT: In order to measure the internal structure of fluctuation, the broadband frequency tunable system, which has the ability of fast and stable hopping operation, is applied in the Large Helical Device. One of the important issues of density fluctuation measurements using this reflectometer is the study of energetic particle driven magnetohydrodynamics instability. During one plasma discharge, the launching frequency changes from one frequency to another frequency, which this operation is called as frequency hopping, and the cutoff position can be scanned in the wide area. As a hopping source, a synthesizer is used because it has a quite stable and low phase noise. The frequency component of the source output is multiplied to V-band (50-75 GHz) region for plasma measurements in extraordinary mode polarization. Also this system has a heterodyne detection with single side band frequency modulation for sensitive phase and amplitude measurement. We can obtain the radial profile of Alfvén eigenmodelike oscillation in a neutral beam injected plasma.
The Review of scientific instruments 11/2008; 79(10):10F109. · 1.52 Impact Factor
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ABSTRACT: A method to determine the poloidal mode number m in a spherical tokamak based on magnetic probe data was developed. Perturbed magnetic fields at Mirnov coils are calculated for distributed helical filamentary currents on rational surfaces assuming the maximum current amplitude, m and n (toroidal mode number), and the toroidal location of the filaments. These free parameters were determined from the best fit to the measured signals. The residual error was reduced by a factor of 2 by introducing helical filaments instead of toroidal filaments. Using this method, m/n=2/1 and 3/2 modes were identified in Mega-Ampere Spherical Tokamak discharges, and the time evolution of the tearing modes was derived.
The Review of scientific instruments 11/2008; 79(10):10F120. · 1.52 Impact Factor
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Y Adachi, A Ejiri,
Y Takase,
O Watanabe,
T Oosako,
H Tojo,
S Kainaga,
T Masuda,
M Sasaki,
J Sugiyama,
T Yamaguchi
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ABSTRACT: Parametric decay instability (PDI) is often observed in the TST-2 spherical tokamak during high harmonic fast wave heating by rf pickup probes. The frequency spectrum exhibits lower and upper sideband peaks in addition to the pump wave at f(0)=21 MHz. Two types of PDI are observed. One is the well-known decay into the ion-cyclotron quasimode (nf(ci)) and the ion Bernstein wave (f(0)-nf(ci)). The other is a newly found decay with the sideband frequency between f(0) and f(0)-f(ci). The frequency difference between this sideband and the pump increases in proportion to B(t). Moreover, high-speed visible light measuring systems with photomultiplier tubes or hybrid photodetectors viewing the plasma core detected oscillation of light emission at around f(0).
The Review of scientific instruments 11/2008; 79(10):10F507. · 1.52 Impact Factor
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Y Adachi, A. Ejiri,
Y Takase,
T. Oosako,
T Masuda,
J. Sugiyama,
H. Tojo,
T Yamaguchi,
O Watanabe,
S. Kainaga,
M Sasaki
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ABSTRACT: TST-2 is a spherical tokamak (ST) device, and High Harmonic Fast Wave (HHFW) injection experiments were performed to heat electrons in the spherical tokamak. In TST-2, electron heating was observed in some discharges, but when Parametric Decay Instability (PDI) occurred the injected wave energy is transferred to two daughter modes. PDI during HHFW injection in the NSTX ST device creates modes, which are presumably an Ion Bernstein wave (IBW) and a quasi mode at ion cyclotron frequency (ICQM). A similar decay was observed for low toroidal magnetic field discharges in TST-2. For higher toroidal magnetic field discharges, we found another type of decay, in which another peak in the frequency domain was detected between a pump wave (i.e., HHFW) frequency and the IBW frequency. Experiments are performed to identify this new peak. RF pickup probes are used to measure the magnetic field oscillations outside the closed magnetic surface region. Major results are as follows. The frequency of the decay waves coupled with ICQM increases with the toroidal magnetic field as expected. When Bt varies from 0.13 to 0.18 T, the frequency difference of the decay wave and the pump wave increases from 1.5 to 1.9 MHz in proportion to Bt- The new unidentified wave shows a similar dependence, and its frequency difference varies from 0.54 to 0.70 MHz in proportion to Bt- The width of this new peak is broader than that of IBWs peak. These results suggest that the new peak corresponds to the decay wave coupled with Alfven mode, because the fact that the frequency of Alfven mode is in proportional to the magnetic field, is consistent with the experimental results.
Plasma Science, 2008. ICOPS 2008. IEEE 35th International Conference on; 07/2008
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ABSTRACT: The response of microwave reflectometry for various configurations is studied theoretically. Based on the Kirchhoff integral (i.e. physical optics), analytic formulae for the measured electric field are obtained. Phase and power sensitivities for weak fluctuations are derived. It was found that the effective spot size, which is a combination of the real spot size and the Fresnel zone, of the launching and the receiving beams is an important parameter, determining the sensitivities. Fringe jump and phase runaway phenomena are attributed to two parameters: one is proportional to the normalized wavenumber of the fluctuations, and the other is proportional to the plasma shift (i.e. misalignment). In the case of frozen turbulence, a complex power spectrum of the measured field becomes a universal quantity, because of easy correction of the configuration effect. Using this feature, an ideal complex amplitude signal (i.e. free from diffraction effects) can be reconstructed.
Plasma Physics and Controlled Fusion 04/2008; 50(6):065003. · 2.42 Impact Factor
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ABSTRACT: The radial eigenmode of the geodesic acoustic mode (GAM) is investigated, taking into account the finite ion Larmor radius effect. The radial eigenfunction and the eigenfrequency (real frequency and damping rate) are obtained numerically. The eigenfrequency is found to be not continuous, but discrete. In addition, we obtain an analytic expression for the real eigenfrequency, which shows good agreement with numerical results. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Beiträge aus der Plasmaphysik 03/2008; 48(1‐3):68 - 72.
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ABSTRACT: An O-mode reflectometer at a frequency of 25.85 GHz was applied to plasmas heated by the high harmonic fast wave (21 MHz) in the TST-2 spherical tokamak. An oscillation in the phase of the reflected microwave in the rf range was observed directly for the first time. In TST-2, the rf (250 kW) induced density oscillation depends mainly on the poloidal rf electric field, which is estimated to be about 0.2 kV/m rms by the reflectometer measurement. Sideband peaks separated in frequency by ion cyclotron harmonics from 21 MHz, and peaks at ion cyclotron harmonics which are suggested to be quasimodes generated by parametric decay, were detected.
Review of Scientific Instruments 09/2007; 78(8):083502. · 1.37 Impact Factor
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Y Nagashima,
K Itoh,
S-I Itoh,
A Fujisawa,
M Yagi,
K Hoshino,
K Shinohara, A Ejiri,
Y Takase,
T Ido,
K Uehara,
Y Miura,
the group
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ABSTRACT: A method for zonal flow study by using direct density fluctuation measurements is proposed. When ambient drift-wave turbulence is modulated by zonal flows (i.e. in the drift-wave-zonal flow systems), an envelope of the ambient density fluctuations has spectral peaks around zonal flow frequencies. A spectral peak at the geodesic acoustic mode (GAM) frequency is observed in the envelope of the ambient density fluctuations measured in edge plasma of the JFT-2M tokamak. The significant cross-bicoherence is also found between the ambient density fluctuations and its envelope in the GAM frequency. This result demonstrates that we can measure the GAM only by using density fluctuation data. This method provides a possibility of zonal flow research in burning core plasma by density fluctuation diagnostics such as microwave reflectometry.
Plasma Physics and Controlled Fusion 08/2007; 49(10):1611. · 2.42 Impact Factor
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ABSTRACT: Single-particle orbits in a low aspect ratio axisymmetric configuration have been studied to clarify their role in current initiation using electron cyclotron waves. In this study, electron guiding centre orbits are studied neglecting collisions and charge exchange. Current is generated mainly by the toroidal drift of mirror-trapped electrons. Analytic scaling laws for the current and the poloidal field generation are derived, and they are found to be functions of temperature, poloidal field strength, number of electrons and parameters defining the magnetic and geometrical configuration. The current increases with temperature and decreases with the poloidal field strength as long as the temperature is much smaller than the characteristic temperature, whose thermal velocity is defined by the product of poloidal cyclotron frequency and the major radius.
Nuclear Fusion 04/2007; 47(5):403. · 4.09 Impact Factor
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ABSTRACT: Impurity transport of carbon has been studied using a new method combined carbon pellet injection with high-spatial resolution bremsstrahlung measurement on the Large Helical Device
[A. Iiyoshi et al., Fusion Technol. 17, 169 (1990)]
. The carbon pellets are injected into a steady phase in neutral beam heated discharges with a standard configuration of Rax = 3.6 m. The particle transport coefficients (diffusion coefficient D and convective velocity V) are inferred using a diffusive/convective model. The results are compared between hydrogen and helium plasmas. As a result, it is found that the analyzed D has a constant radial profile with almost closed values of 0.2 m2/s in both plasmas. On the other hand, the inward V is required only at the plasma outer region (ρ>0.6) where the electron density gradient exits, and the inward V in helium plasmas (−0.4 m/s at ρ = 0.8 and ne ∼ 4.0×1019 m−3) is nearly half as much as that in hydrogen plasmas (−0.7 m/s). This difference of the inward V between hydrogen and helium plasmas suggests a dependence on the charge state of fuel ions predicted from neoclassical theory.
Physics of Plasmas 09/2006; 13(9):092502-092502-6. · 2.15 Impact Factor
