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Physics of Plasmas 01/2012; 19:102503. · 2.15 Impact Factor
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Y. Narushima,
F. Castejón,
S. Sakakibara,
K.Y. Watanabe,
S. Ohdachi,
Y. Suzuki,
T. Estrada,
F. Medina,
D. López-Bruna,
M. Yokoyama,
M. Yoshinuma,
K. Ida,
S. Nishimura,
LHD Experiment Group,
TJ-II Experiment Group
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ABSTRACT: The dynamics of a magnetic island is studied by focusing on the poloidal flows in the helical devices LHD and TJ-II. An experimental result implying the temporal increment of the E × B poloidal flow prior to the magnetic island transition from growth to healing is observed. The direction of the poloidal flow is in the electron-diamagnetic direction in LHD and in the ion-diamagnetic direction in TJ-II. From the magnetic diagnostics, it is observed that a current structure flowing in the plasma moves ~π rad poloidally in the electron-diamagnetic direction during the transition in LHD experiments. These experimental observations from LHD and TJ-II show that the temporal increment of the poloidal flow is followed by the transition (growth to healing) of the magnetic island regardless of the flow direction and suggests the fact that a significant poloidal flow affects the magnetic island dynamics.
Nuclear Fusion 07/2011; 51(8):083030. · 4.09 Impact Factor
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K Toi,
F Watanabe,
T Tokuzawa,
K Ida,
S Morita,
T Ido,
A Shimizu,
M Isobe,
K Ogawa,
D A Spong, [......],
Y Oka,
R Sakamoto,
T Shimozuma,
Y Takeiri,
K Tanaka,
K Tsumori,
I Yamada,
M Yoshinuma,
K Kawahata,
A Komori
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ABSTRACT: Reversed-shear Alfvén eigenmodes were observed for the first time in a helical plasma having negative q₀'' (the curvature of the safety factor q at the zero shear layer). The frequency is swept downward and upward sequentially via the time variation in the maximum of q. The eigenmodes calculated by ideal MHD theory are consistent with the experimental data. The frequency sweeping is mainly determined by the effects of energetic ions and the bulk pressure gradient. Coupling of reversed-shear Alfvén eigenmodes with energetic ion driven geodesic acoustic modes generates a multitude of frequency-sweeping modes.
Physical Review Letters 10/2010; 105(14):145003. · 7.37 Impact Factor
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ABSTRACT: A drift-magnetohydrodynamic theory is applied to a background anisotropic pressure equilibrium state to generate a drift corrected ballooning mode equation. The ratio of the mode frequency to the hot particle drift frequency constitutes the critical expansion parameter. The fast particles thus contribute weakly to the instability driving mechanism and also to the diamagnetic drift stabilisation. This equation is used to model the inward-shifted Large Helical Device (LHD) configuration. In the single-fluid limit, a weakly ballooning unstable band that encompasses a third of the plasma volume develops in the core of the plasma at low 〈βdia 〉 that becomes displaced towards the edge of the plasma at the experimentally achieved 〈βdia 〉 ≃ 5%. Finite diamagnetic drifts (mainly due to the thermal ions) effectively stabilise these ballooning structures at all values of 〈βdia 〉. The validity of the large hot particle drift approximation is verified for hot to thermal ion density ratios that remain smaller than 2% (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Beiträge aus der Plasmaphysik 07/2010; 50(8):713 - 717.
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ABSTRACT: The effect of the poloidal flow on the magnetic island is studied. The self-healing of the magnetic island has been observed in the Large Helical Device (LHD) and TJ-II. In TJ-II plasmas, the magnetic island is healed in case the radial electric field is positive. In the LHD experiment, on the other hand, the radial electric field is negative when the magnetic island is healed. The relation between the magnitudes of the E×B flow and ion diamagnetic flow is important to consider the effect of the poloidal flow on magnetic island (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Beiträge aus der Plasmaphysik 07/2010; 50(6‐7):529 - 533.
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F. Watanabe,
K. Toi,
S. Ohdachi,
S. Sakakibara,
S. Morita,
H. Funaba,
T. Minami,
K. Narihara, Y. Narushima,
C. Suzuki,
K. Tanaka,
T. Tokuzawa,
K.Y. Watanabe,
I. Yamada,
LHD Experiment Group
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ABSTRACT: In the Large Helical Device (LHD), nonlinear evolution of strongly destabilized edge MHD modes, which are thought to be resistive interchange modes, induce repetitive bursts of magnetic fluctuations, soft X-ray (SX) fluctuations, and extreme ultra-violet (XUV) fluctuations, and generates a train of sharp spikes in H emission signals. These H spikes exhibit the character of edge localized mode (ELM). The ELM activities are also observed in high beta or high density L-mode plasmas having steep pressure gradient at plasma edge as well as H-mode plasmas with the L-H transition. The responsible instabilities for ELMs in LHD plasmas are thought to be resistive interchange modes, and are clearly different from those in a tokamak where peeling-ballooning modes are the candidates. The repetition frequency of ELMs (fELM) increases with the increase of external heating power, of which the character is similar to that of type I ELMs in tokamaks. The product of fELM and the relative ELM amplitude (δH/H) is roughly in proportion to the input heating power, where the ELM induced loss energy (ΔWp) increases with δH/H (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Beiträge aus der Plasmaphysik 07/2010; 50(6‐7):651 - 655.
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S. Ohdachi,
R. Sakamoto,
J. Miyazawa,
T. Morisaki,
S. Masuzaki,
H. Yamada,
K.Y. Watanabe,
V.R. Jacobo,
N. Nakajima,
F. Watanabe, [......],
S. Sakakibara,
Y. Suzuki, Y. Narushima,
I. Yamada,
T. Mianami,
K. Narihara,
K. Tanaka,
T. Tokuzawa,
K. Kawahata,
LHD Experiment Group
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ABSTRACT: A core density collapse (CDC) phenomenon is a rapid collapse events observed in super dense core (SDC) plasma with internal diffusion barrier (IDB) in the Large Helical Device (LHD). By CDC, the central beta is decreased by up to 50%. The collapse starts from the edge region of the plasma. CDCs appear with plasma parameters where the high–n ballooning modes are unstable at ϱ ∼ 0.8. With less collisional conditions, m = 1 type oscillations are observed with similar beta profile. The origin of the m = 1 oscillations is not clarified (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Beiträge aus der Plasmaphysik 07/2010; 50(6‐7):552 - 557.
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ABSTRACT: Ideal MHD yields at best inconclusive predictions about the stability of the LHD heliotron for β ≥ 3%. We investigate the impact of the drift stabilization of ballooning modes for the inward-shifted LHD configuration (vacuum magnetic axis R0 ~ 3.5 m). The background equilibrium is considered anisotropic in which the neutral beam ions contribute about 1/4 fraction of the total diamagnetic beta, βdia. A drift corrected ballooning mode equation obtained from the linearized gyrokinetic equation is expanded assuming that the hot particle drifts are much larger than the mode frequency. The fast particle pressure gradients contribute weakly to both the instability drive and the diamagnetic drift stabilization (which is dominated by the thermal ion diamagnetic drifts) for βdia [0, 4.8]%. In the single-fluid limit (diamagnetic drifts ignored), the thermal pressure gradients drive ballooning modes in a broad region encompassing the outer 60–90% of the plasma volume at βdia ≈ 4.8%. To stabilize these modes, we find that diamagnetic drift corrections must be invoked (mainly due to the thermal ions). The energetic ion diamagnetic drifts play a role only for low wave number values, kα ≤ 8. It has been verified that the fast particle drift ordering imposed by the model is amply satisfied for on-axis hot particle to thermal density Nh0/Ni0 ≈ 1% even at high βdia.
Nuclear Fusion 01/2010; 50(2):025009. · 4.09 Impact Factor
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Computer Physics Communications. 01/2009; 180:1524-1533.
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Y. Narushima,
K.Y. Watanabe,
S. Sakakibara,
K. Narihara,
I. Yamada,
Y. Suzuki,
S. Ohdachi,
N. Ohyabu,
H. Yamada,
Y. Nakamura,
the LHD Experimental Group
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ABSTRACT: The dynamics of the magnetic island structure in the plasma are investigated in plasmas with a wide range of beta and collisionality. The perturbed magnetic field is diagnosed by a toroidal array of flux loops installed in the vacuum vessel on the Large Helical Device (LHD). It is found that the magnetic island grows with beta at relatively low beta values. In contrast, when the beta exceeds a critical value, the sign of the perturbed magnetic field suddenly reverses and its strength saturates to the magnetic field perturbation required to cancel the external perturbation. This suggests spontaneous healing of the magnetic island.
Nuclear Fusion 06/2008; 48(7):075010. · 4.09 Impact Factor
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ABSTRACT: Three states of a magnetic island are observed when the magnetic shear at the rational surface is modified using inductive current associated with the neutral beam current drive in the Large Helical Device. One state is the healed magnetic island with a zero island width. The second state is the saturated magnetic island with partial flattening of the T(e) profile. The third state is characterized by the global flattening of the T(e) profile in the core region. As the plasma assumes each of the three states consecutively through a bifurcation process a clear hysteresis in the relation between the size of the magnetic island and the magnetic shear is observed.
Physical Review Letters 03/2008; 100(4):045003. · 7.37 Impact Factor
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S Sakakibara,
K Y Watanabe,
Y Suzuki, Y Narushima,
S Ohdachi,
N Nakajima,
F Watanabe,
L Garcia,
A Weller,
K Toi,
I Yamada,
K Tanaka,
T Tokuzawa,
K Ida,
H Yamada,
A Komori,
O Motojima
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ABSTRACT: In the Large Helical Device, the volume averaged beta value β dia of 5%, which is the highest value in all heliotron/stellarators and relevant to the reactor requirement, was achieved by optimizing the magnetic configuration from the viewpoint of magneto-hydrodynamic (MHD) characteristics, transport and heating efficiency of the neutral beam. This beta value was instantaneously obtained by pellet injection and maintained for more than 10τ E , whereas the steady-state plasma with a maximum β dia of 4.8% was sustained for 85τ E by gas-puff fueling. While it is theoretically predicted that stochastization of the peripheral magnetic field structure develops with an increment of β dia , no serious degradation of the global confinement has been observed in the present β dia range. The several low-order MHD activities located in the periphery were enhanced with the beta value and sometimes affect the local profiles. The amplitude of the mode in the periphery strongly depends on the magnetic Reynolds number, which is close to that of the growth rate and/or the radial mode width of the resistive interchange instability.
Plasma Phys. Control. Fusion. 01/2008; 50:124014-10.
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H Yamada,
R Sakamoto,
J Miyazawa,
M Kobayashi,
T Morisaki,
S Masuzaki,
S Ohdachi,
M Goto,
K Ida,
S Sakakibara, [......],
M Yokoyama,
S Yoshimura,
M Yoshinuma,
S Imagawa,
O Kaneko,
K Kawahata,
T Mutoh,
N Ohyabu,
A Komori,
O Motojima
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ABSTRACT: A high density regime with an internal diffusion barrier (IDB) has been extended to the helical divertor (HD) configuration in the Large Helical Device (LHD). Avoidance of the local enhancement of neutral pressure is necessary to enable IDB formation, which is consistent with earlier works by using the Local Island Divertor (LID) with efficient active pumping. The central pressure reached 1.3 times atmospheric pressure, where ne(0) = 6 × 1020 m−3 and Te(0) = 660 eV. The plasmas with an IDB are located in the plateau collisionality regime. The significant impurity effect has not been observed throughout the discharges in spite of the existence of a negative radial electric field. A central pressure limiting event is observed in the plasmas with an IDB using the HD. During this event which is referred to as the core density collapse (CDC), particles are flushed out from the core on the time scale of a few hundreds of microseconds. The suppression of the Shafranov shift by vertical elongation (κ) is effective to mitigate CDC. At κ = 1.2, the central β value is increased up to 6.6% at 1 T.
Plasma Physics and Controlled Fusion 11/2007; 49(12B):B487. · 2.42 Impact Factor
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K Y Watanabe,
Y Suzuki,
T Yamaguchi,
K Narihara,
K Tanaka,
T Tokuzawa,
I Yamada,
S Sakakibara, Y Narushima,
T Morisaki,
N Nakajima,
H Yamada,
K Kawahata,
the LHD Experimental Group
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ABSTRACT: We analyse the change of plasma boundaries due to changes in beta in large helical device plasmas based on electron temperature and density profile measurements, and compare the change with the prediction by a MHD equilibrium code without a priori assumptions of the nested magnetic surfaces, the HINT code. In the open magnetic field line region, which is predicted to exist at high beta, a small but non-zero electron pressure and its gradient are observed. The shift of the geometric centre of the peripheral magnetic surface due to the beta value is systematically consistent with the prediction, which suggests that the HINT code is quite a useful method for identifying the shape and location of the plasma boundary in heliotron plasmas with a divertor configuration. However, in the open field line region, a discrepancy arises because the electron mean free path is much shorter than the connection length of the magnetic field line to a wall, which suggests that the additional effects such as the inertial effect and/or the viscosity in the equilibrium force equation should be taken into account.
Plasma Physics and Controlled Fusion 03/2007; 49(5):605. · 2.42 Impact Factor
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N Ohyabu,
T Morisaki,
S Masuzaki,
R Sakamoto,
M Kobayashi,
J Miyazawa,
M Shoji,
H Funaba,
J H Harris,
Y Hirooka, [......],
K Ida,
T Shimozuma,
H Yamada,
Y Nagayama,
O Kaneko,
T Mutoh,
K Kawahata,
A Komori,
S Sudo,
O Motojima
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ABSTRACT: In local island (m/n = 1/1) divertor discharges in the large helical device a stable super dense core plasma develops when a series of pellets are injected. A core region with a density as high as 4.6 × 1020 m−3 and a temperature of 0.85 keV is maintained by an internal diffusion barrier with a very high density gradient. In a study of island dynamics, we find that an externally imposed large island (m/n = 1/1) as large as 15% of the minor radius is healed when beta at the island exceeds a critical value.
Plasma Physics and Controlled Fusion 11/2006; 48(12B):B383. · 2.42 Impact Factor
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A Isayama,
S Inagaki,
K Y Watanabe, Y Narushima,
S Sakakibara,
H Funaba,
K Ida,
Y Nagayama,
H Yamada,
K Kawahata,
A Komori,
O Motojima,
the LHD Experimental Group
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ABSTRACT: In the Large Helical Device, the structure of localized oscillations at the m/n = 2/1 rational surface during neutral beam (NB) injection in the counter direction (counter-NB injection) has been investigated in detail using an electron cyclotron emission (ECE) diagnostic. Here m and n are poloidal and toroidal mode numbers, respectively. Under the condition of increasing plasma current in the counter direction and nearly constant pressure, the oscillations are observed at larger magnitudes of plasma current (−20 kA T−1). In the NB reversal experiments, where the direction of the NB is changed from the co- to the counter-direction and vice versa, the oscillations were observed only for the former case. These results suggest that the profile and magnitude of the plasma current play an important role in the mode onset. Analysis based on the Mercier criteria shows that for a flat or a hollow current profile an interchange mode becomes more unstable at higher negative plasma current, showing a tendency similar to the experimental observations. In addition, radial displacement due to the localized oscillations evaluated from electron temperature perturbation profiles with the ECE diagnostic agrees well with that predicted by a three-dimensional ideal magnetohydrodynamic stability code.
Plasma Physics and Controlled Fusion 03/2006; 48(4):L45. · 2.42 Impact Factor
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T. Yamaguchi,
K.Y. Watanabe,
S. Sakakibara, Y. Narushima,
K. Narihara,
T. Tokuzawa,
K. Tanaka,
I. Yamada,
M. Osakabe,
H. Yamada,
K. Kawahata,
K. Yamazaki,
LHD Experimental Group
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ABSTRACT: In low density discharges of a Large Helical Device (LHD), anisotropic pressure is expected because the LHD has powerful tangential neutral beam injection systems. We study the behaviour of the ratio of the observed saddle loop flux to the diamagnetic flux, and the results are compared with the predicted beam pressure anisotropy by a Monte Carlo technique and the steady state Fokker–Planck solution. We show the possibility of the degree of pressure anisotropy being estimated by magnetic measurements in the LHD.
Nuclear Fusion 10/2005; 45(11):L33. · 4.09 Impact Factor
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O. Motojima,
K. Ida,
K.Y. Watanabe,
Y. Nagayama,
A. Komori,
T. Morisaki,
B.J. Peterson,
Y. Takeiri,
K. Ohkubo,
K. Tanaka, [......],
Y. Torii,
K. Tsumori,
T. Uda,
A. Wakasa,
T. Watari,
I. Yamada,
S. Yamamoto,
K. Yamazaki,
M. Yokoyama,
Y. Yoshimura
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ABSTRACT: The Large Helical Device is a heliotron device with L = 2 and M = 10 continuous helical coils with a major radius of 3.5–4.1 m, a minor radius of 0.6 m and a toroidal field of 0.5–3 T, which is a candidate among toroidal magnetic confinement systems for a steady state thermonuclear fusion reactor. There has been significant progress in extending the plasma operational regime in various plasma parameters by neutral beam injection with a power of 13 MW and electron cyclotron heating (ECH) with a power of 2 MW. The electron and ion temperatures have reached up to 10 keV in the collisionless regime, and the maximum electron density, the volume averaged beta value and stored energy are 2.4 × 1020 m−3, 4.1% and 1.3 MJ, respectively. In the last two years, intensive studies of the magnetohydrodynamics stability providing access to the high beta regime and of healing of the magnetic island in comparison with the neoclassical tearing mode in tokamaks have been conducted. Local island divertor experiments have also been performed to control the edge plasma aimed at confinement improvement. As for transport study, transient transport analysis was executed for a plasma with an internal transport barrier and a magnetic island. The high ion temperature plasma was obtained by adding impurities to the plasma to keep the power deposition to the ions reasonably high even at a very low density. By injecting 72 kW of ECH power, the plasma was sustained for 756 s without serious problems of impurities or recycling.
Nuclear Fusion 10/2005; 45(10):S255. · 4.09 Impact Factor
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ABSTRACT: A rapid bifurcation of the equilibria with and without a large island (n/m = 1/1) has been observed in the medium to high beta large helical device discharges. A large island imposed by an external resonant field is suddenly suppressed nearly perfectly by the plasma effects when the beta at the ι/2π = 1 surface exceeds a critical value. The critical beta value is nearly proportional to the externally imposed resonant field normalized by the main field strength.
Plasma Physics and Controlled Fusion 08/2005; 47(9):1431. · 2.42 Impact Factor
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Y. Nagayama,
K. Narihara, Y. Narushima,
N. Ohyabu,
T. Hayashi,
K. Ida,
S. Inagaki,
D. Kalinina,
R. Kanno,
A. Komori,
T. Morisaki,
R. Sakamoto,
S. Sudo,
N. Tamura,
T. Tokuzawa,
H. Yamada,
M. Yoshinuma,
LHD experimental group
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ABSTRACT: Magnetic island formation is experimentally investigated in the Large Helical Device. The (m, n) = (1, 1) vacuum magnetic island is generated by using the local island diverter field, where m and n are the poloidal and toroidal mode numbers, respectively. The island width depends on plasma parameters (the electron temperature and the β) and the magnetic axis position. In the case of Rax = 3.53 m the magnetic island in the plasma is larger than that in the vacuum field. Here, Rax is the major radius of the magnetic axis. In the case of Rax = 3.6 m the magnetic island is not generated when the error field is less than the threshold which is increased as the β is increased. Evidence of island current is obtained when the magnetic island is formed due to a small error field. However, the mechanism that generates the island is not yet known.
Nuclear Fusion 07/2005; 45(8):888. · 4.09 Impact Factor
