N. Oyama

Japan Atomic Energy Agency, Muramatsu, Niigata, Japan

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Publications (189)378.17 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Simulations with an integrated code TOPICS‐IB showed that a small pellet can significantly reduce the ELM energy loss by penetrating deeply into the pedestal and triggering high‐n ballooning modes localized near the pedestal top, with conditions; the injection from the low‐field‐side with a speed fast enough to approach the pedestal top when the pedestal pressure is about 95% of natural ELM onset. The effectiveness of the above suitable conditions of pellet injection for ELM pacing has been confirmed by JT‐60U and then ITER simulations. The pellet particle content required for ELM pacing is larger for the pedestal plasma with higher density and farther from the stability boundary of ideal ballooning mode near the pedestal top. For an ITER standard scenario, the required pellet particle content is about a few % of pedestal particle content, which gives the physics background to the present design value. Simulations also showed that fueling pellets can be injected from the high‐field‐side just after ELM pacing pellets without disturbing the pacing. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
    Contributions to Plasma Physics 06/2014; 54. · 0.93 Impact Factor
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    Nuclear Fusion 01/2014; 54:013015. · 2.73 Impact Factor
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    ABSTRACT: The energy loss caused by the edge-localized mode (ELM) needs to be reduced for ITER operations with ELMy H-mode plasmas. The reduction in ELM energy loss by pellet injection for ELM pacing is studied by an integrated core/scrape-off layer/divertor transport code TOPICS-IB with a magnetohydrodynamic stability code and a pellet model taking account of the E × B drift of the pellet plasma cloud. It is found that the energy loss can be significantly reduced by a pellet injected to the pedestal plasma equivalent to that at the middle timing in the natural ELM cycle, whose pressure height is only about 5% lower than that of the natural ELM onset. In this case, pellet injection from the low-field side enables a small pellet, with about 1-2% of pedestal particle content and a speed high enough to approach the pedestal top, to reduce the energy loss significantly. With the above suitable conditions for ELM pacing, a pellet penetrates deep into the pedestal and triggers high-n ballooning modes with localized eigenfunctions near the pedestal top, where n is the toroidal mode number. Under suitable conditions, ELM pacing with reduced energy loss is successfully demonstrated in simulations, in which the gas puff reduction and the enhancement of divertor pumping can compensate for the core density increase due to additional particle fuelling by the pacing pellet.
    Nuclear Fusion 12/2013; 53(12):3009-. · 2.73 Impact Factor
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    ABSTRACT: Operating ITER in the reference inductive scenario at the design values of Ip = 15 MA and QDT = 10 requires the achievement of good H-mode confinement that relies on the presence of an edge transport barrier whose pedestal pressure height is key to plasma performance. Strong gradients occur at the edge in such conditions that can drive magnetohydrodynamic instabilities resulting in edge localized modes (ELMs), which produce a rapid energy loss from the pedestal region to the plasma facing components (PFC). Without appropriate control, the heat loads on PFCs during ELMs in ITER are expected to become significant for operation in H-mode at Ip = 6–9 MA; operation at higher plasma currents would result in a very reduced life time of the PFCs.
    Nuclear Fusion 03/2013; 53(4):043004. · 2.73 Impact Factor
  • N. Aiba, N. Oyama
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    ABSTRACT: Numerical stability analysis of edge-localized MHD mode is performed to identify the origin of small-amplitude 'grassy ELMs' on the basis of current understanding of kinetic effects on ballooning mode stability. These qualitative and quantitative analyses show that short wavelength ballooning mode can play an important role in a grassy ELM stability even when kinetic effects are taken into account. After showing the importance of kinetic effects for discussing grassy ELM physics, impacts of plasma parameters important for realizing a grassy ELM plasma experimentally are investigated numerically from the viewpoint of the edge-localized MHD stability including these kinetic effects. These analyses show that low plasma ellipticity is preferable to realize a grassy ELM plasma due to destabilizing ballooning mode by preventing access to the second stability region of the ballooning mode.
    Nuclear Fusion 10/2012; 52(11):114002. · 2.73 Impact Factor
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    ABSTRACT: The first detailed measurements of ion-impurity dynamics for NBI-heated ELMy H-modes at the edge of the JT-60U tokamak are reported. We investigated the ability of external momentum/power input to modify and control the radial electric field, Er, and pedestal structures. The relationship between Er and pedestal structures of ion-impurity density, ni, and temperature, Ti, during the ELMing H-mode phase for various momentum input directions (i.e. co-, balanced- and counter-NBI) and input powers from perpendicular NBI are compared with the ELM-free phase. The observed trend is that the edge Er-well width increases in the co-NBI discharge, while the Er value at the base of the Er-well becomes more negative in the counter-NBI discharge. The scale length for both ni and Ti in the pedestal is ~2 cm and values are ~1 for both ELM-free and ELMing phases with different magnitudes of Er (and/or Er shear). Characteristics of the turbulent density fluctuation, in addition to a uniform toroidal MHD oscillation (i.e. n = 0), during both ELM-free and ELMing phases are also reported.
    Nuclear Fusion 10/2012; 52(11):114010. · 2.73 Impact Factor
  • Physical Review Letters 10/2012; 109(14). · 7.73 Impact Factor
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    ABSTRACT: Recent DIII-D experiments have investigated the effects of localized magnetic field perturbations, using coils that approximate the magnetization of the test blanket modules (TBMs) in one ITER port. In H-mode discharges, compensation of the TBM field using an applied n=1 field yielded only partial recovery of the plasma rotation, and the compensation field that maximized plasma rotation differed significantly from the field that reduced the resonant magnetic response to a very low value. These results provide insight into the effects of error fields, and suggest an important role for non-resonant magnetic braking. In addition, measurements of localized heat deposition with the TBM field are being compared to orbit following calculations of fast ion loss, and a new fast ion detector has confirmed earlier observations of reduced 1 MeV triton confinement.
    10/2012;
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    ABSTRACT: The dependence of the ion-temperature-gradient scale length on the hydrogen isotope mass was examined in conventional H-mode plasmas in JT-60U tokamak. While identical profiles for density and temperature were obtained for hydrogen and deuterium plasmas, the ion conductive heat flux necessary for hydrogen to sustain the same ion temperature profile was two times that required for deuterium, resulting in a clearly higher ion heat diffusivity for hydrogen at the same ion-temperature-gradient scale length. On the other hand, the ion-temperature-gradient scale length for deuterium is less than that for hydrogen at a given ion heat diffusivity.
    Physical Review Letters 09/2012; 109(12). · 7.73 Impact Factor
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    ABSTRACT: The relation between toroidal rotation velocities (Vt) in the core and edge regions is investigated in H-mode plasmas with a small external torque input from the viewpoint of momentum transport. The toroidal rotation velocity in the core region (core-Vt) gradually varies on a timescale of ~20 ms after a rapid change in the toroidal rotation velocity in the edge region (edge-Vt) at the L–H transition. This timescale of ~20 ms is consistent with a transport timescale using the momentum diffusivity (χ) and convection velocity (Vconv). In steady state, a linear correlation between the core- and edge-Vt is observed in H-mode plasmas when the ion pressure gradient (∇Pi) is small. This relation between core- and edge-Vt is also explained by momentum transport. The Vt profiles with a large ∇Pi are reproduced in the core region of r/a ~ 0.2–0.7 by adopting a residual stress term 'Πres = αkχ∇Pi' proposed in this paper. Here r/a is the normalized plasma radius and αk1 is a radial constant. Using this formula, Vt profiles are reproduced over a wide range of plasma conditions. Parameter dependences of the edge-Vt are investigated at a constant ripple loss power, ripple amplitude and plasma current. A reduction in the CTR-rotation is observed with decreasing ion temperature gradient (∇Ti). Here CTR refers to the counter-IP direction.
    Nuclear Fusion 01/2012; 52(2):023024. · 2.73 Impact Factor
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    ABSTRACT: The effects of toroidal field (TF) ripple on the edge pedestal characteristics were examined in the TF ripple scan experiments at the plasma current Ip of 1.1 MA in JET and JT-60U. The TF ripple amplitude δR was defined as a value averaged over the existing ripple wells at the separatrix on the outer midplane. By the installation of ferritic inserts (FIs), δR was reduced from 1% to 0.6% at 3.2T (0.5% at 2.2 T) in JT-60U. In JET, δR was varied from 0.08% to 1% by feeding different currents to the odd and even set of coils out of 32 TF coils. The pedestal pressure pped was similar for the cases before and after the installation of FIs in JT-60U. Similarly, no clear difference in pped was also observed in the variation of δR in JET. The core and edge toroidal rotation clearly shifted in the counter-direction by increased δR. However, there were no changes in the spatial profiles of electron density, electron temperature and ion temperature. By the installation of FIs in JT-60U, the ELM frequency fELM decreased by ~20%, while the ELM energy loss increased by 50–150%. The increased ELM loss power by 30% suggests a reduction of inter-ELM transport with the reduced δR. In JET, fELM increased only slightly with increased δR while the edge toroidal rotation frequency decreased as δR increased. From the inter-machine similarity experiment at 1.1 MA, TF ripple less than 1% does not strongly affect the pedestal pressure. However, in the single TF ripple scan at the higher Ip of 2.6 MA in JET, it clearly decreases with the increased δR, accompanying with a strong density pump out at large TF ripple. These results suggests that the effect of TF ripple on H-mode properties becomes stronger in the plasmas with higher Ip or lower edge collisionality of ripple diffusion.
    Nuclear Fusion 10/2011; 51(11):113004. · 2.73 Impact Factor
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    ABSTRACT: The paper compares the transport properties of a set of dimensionless identity experiments performed between JET and JT-60U in the advanced tokamak regime with internal transport barrier, ITB. These International Tokamak Physics Activity, ITPA, joint experiments were carried out with the same plasma shape, toroidal magnetic field ripple and dimensionless profiles as close as possible during the ITB triggering phase in terms of safety factor, normalized Larmor radius, normalized collision frequency, thermal beta, ratio of ion to electron temperatures. Similarities in the ITB triggering mechanisms and sustainment were observed when a good match was achieved of the most relevant normalized profiles except the toroidal Mach number. Similar thermal ion transport levels in the two devices have been measured in either monotonic or non-monotonic q-profiles. In contrast, differences between JET and JT-60U were observed on the electron thermal and particle confinement in reversed magnetic shear configurations. It was found that the larger shear reversal in the very centre (inside normalized radius of 0.2) of JT-60U plasmas allowed the sustainment of stronger electron density ITBs compared with JET. As a consequence of peaked density profile, the core bootstrap current density is more than five times higher in JT-60U compared with JET. Thanks to the bootstrap effect and the slightly broader neutral beam deposition, reversed magnetic shear configurations are self-sustained in JT-60U scenarios. Analyses of similarities and differences between the two devices address key questions on the validity of the usual assumptions made in ITER steady scenario modelling, e.g. a flat density profile in the core with thermal transport barrier? Such assumptions have consequences on the prediction of fusion performance, bootstrap current and on the sustainment of the scenario.
    Nuclear Fusion 06/2011; 51(7):073020. · 2.73 Impact Factor
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    ABSTRACT: The effect of core pressure gradient just inside the top of the pedestal on the energy loss due to type-I edge localized modes (ELMs) is studied. An analysis of the experimental data from JT-60U shows that the ELM energy loss normalized by the pedestal stored energy increases with the pressure gradient inside the top of the pedestal normalized by the pedestal pressure gradient. The dependence of normalized ELM energy loss on the normalized pressure gradient inside the top of the pedestal is similar to that predicted by the integrated code TOPICS-IB. The stability of linear ideal MHD modes is analysed using experimental profiles. It is found that the steep core pressure gradient inside the top of the pedestal broadens the eigenfunction profiles of unstable modes inwards. The TOPICS-IB simulation predicted that this broadening can enhance the ELM energy loss.
    Nuclear Fusion 05/2011; 51(7):073015. · 2.73 Impact Factor
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    ABSTRACT: Mechanisms of plasma rotation effects on edge magnetohydrodynamic (MHD) stability are investigated numerically by introducing energies that are distinguished by physics. By comparing them, it is found that an edge-localized MHD mode is destabilized by the difference between the eigenmode frequency and the equilibrium toroidal rotation frequency, which is induced by rotation shear. In addition, this destabilizing effect is found to be effective in the shorter wavelength region. The effect of poloidal rotation on the edge MHD stability is also investigated. Under the assumption that the change in equilibrium by poloidal rotation is negligible, it is identified numerically that poloidal rotation can have both stabilizing and destabilizing effects on the edge MHD stability, which depends on the direction of poloidal rotation. A numerical analysis demonstrates that these effects of plasma rotation in both the toroidal and poloidal directions can play important roles in type-I edge-localized mode phenomena in JT-60U H-mode plasmas.
    Nuclear Fusion 05/2011; 51(7):073012. · 2.73 Impact Factor
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    ABSTRACT: Charge exchange recombination spectroscopy measurements with high spatial and temporal resolution have made the evaluation of the radial electric field (Er) in the JT-60U tokamak peripheral region possible with a better signal-to-noise ratio. In a very low density ( 0.94 started to become negative, down to -40 kV m-1 (similar to Vphi), the steepening of the Ti profile in the same region accelerated. Even though the numerical value of Vphi was considerably smaller than that of Vtheta, its contribution to shear could be significant: while for r/a >= 0.90 Vphi became increasingly negative; it simultaneously became increasingly positive for r/a < 0.90. Clean bifurcations in the edge Er values between weak negative and strong negative ones were observed in the later ELM-free H-mode phase, with essentially the same confinement properties corresponding to two very different E × B shearing rates. The 'normal' ELMy H-mode took place only after the bifurcations. The results indicate that energy confinement improvement may not necessarily be connected to E × B shear suppression of turbulence.
    Nuclear Fusion 05/2011; 51(5). · 2.73 Impact Factor
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    ABSTRACT: The detailed characteristics of the precursor of the type I edge localized mode (ELM) have been studied in JT-60U using diagnostics with high temporal and spatial resolution such as a microwave reflectometer, electron cyclotron emission (ECE) heterodyne radiometer and grating polychromator. Coherent density and temperature precursors have been observed before the collapse phase of type I ELM. The growth rate of the precursor is evaluated to be γ/ωA ~ 10−3 for several edge pedestal conditions. From the phase delay between ECE signals measured at two toroidal locations and the frequency of the precursor, the toroidal mode number is experimentally evaluated as n = 8–10 or 14–16 assuming that the precursor rotates toroidally with the same toroidal rotation speed of carbon impurity. It is found that the dominant n varies with each ELM under the same plasma condition. The ratio of the pressure gradient inside the pedestal (∇pin) to the pressure gradient within the pedestal (∇pped) has been confirmed as an important parameter in determining the ELM energy loss (ΔWELM) normalized to the pedestal stored energy (Wped), ΔWELM/Wped. From the comparison of the reduction rate in the ion temperature profile due to ELMs, a larger reduction rate within the pedestal and a wider ELM affected area are observed in the plasma with larger ∇pin/∇pped. When the plasma near the top of the pedestal on the high-field side is heated by an electron cyclotron wave (ECW) power of 1.57 MW, the ΔWELM/Wped is reduced by ~35%, together with an increase in the ELM frequency. The increasing rate of the ELM frequency with the heating power is about four times larger in the ECW injection case than the natural power dependence observed in the neutral beam injection case.
    Nuclear Fusion 03/2011; 51(3):033009. · 2.73 Impact Factor
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    ABSTRACT: Radiation distribution and divertor detachment in argon impurity seeding discharges were investigated in JT-60U. A large peak in the radiation power profile was observed at the edge. A radiation power in the core plasma (ρ/a⩽0.93–0.95) and power flux to the edge pedestal were evaluated, and transition of ELM characteristics from Type-I to Type-III was seen at an exhaust power of 1.5–1.8 times larger than the L–H threshold power scaling. In the Type-III ELM plasma, the outer divertor plasma became partially detached. Intrinsic carbon impurity rather than the seeding argon dominated the radiated power in the divertor, which was decreased in the detached divertor due to a reduction in carbon influx. During re-attachment, ion saturation current and electron temperature at the outer divertor plasma increased with similar time scales (0.3–0.4s) of decreasing Ar impurity at the edge and the changing ELM activity.
    Journal of Nuclear Materials 01/2011; 415(1). · 2.02 Impact Factor
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    ABSTRACT: Two integrated core/scrape-off layer/divertor transport codes TOPICS-IB and JINTRAC with links to MHD stability codes are coupled with models of pellet injection to clarify effects of the pellet on the behaviour of edge-localized modes (ELMs). Both codes predict the following two triggering mechanisms. Energy absorption by the pellet and its further displacement due to the E × B drift, as well as transport enhancement by the pellet, are found to be able to trigger the ELM. The ablated cloud of pellet absorbs the background plasma energy and causes a radial redistribution of pressure due to the subsequent E × B drift. Further, the sharp increase in local density and temperature gradients in the vicinity of ablated cloud causes the transient enhancement of heat and particle transport. Both mechanisms produce a region of increased pressure gradient in the background plasma profile within the pedestal, which triggers the ELM. The mechanisms have the potential to explain a wide range of experimental observations.
    Nuclear Fusion 01/2011; · 2.73 Impact Factor
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    ABSTRACT: Experiments at DIII-D investigated the effects of magnetic error fields similar to those expected from proposed ITER test blanket modules (TBMs) containing ferromagnetic material. Studied were effects on: plasma rotation and locking, confinement, L-H transition, the H-mode pedestal, edge localized modes (ELMs) and ELM suppression by resonant magnetic perturbations, energetic particle losses, and more. The experiments used a purpose-built three-coil mock-up of two magnetized ITER TBMs in one ITER equatorial port. The largest effect was a reduction in plasma toroidal rotation velocity v across the entire radial profile by as much as Deltav/v ~ 60% via non-resonant braking. Changes to global Deltan/n, Deltabeta/beta and DeltaH98/H98 were ~3 times smaller. These effects are stronger at higher beta. Other effects were smaller. The TBM field increased sensitivity to locking by an applied known n = 1 test field in both L- and H-mode plasmas. Locked mode tolerance was completely restored in L-mode by re-adjusting the DIII-D n = 1 error field compensation system. Numerical modelling by IPEC reproduces the rotation braking and locking semi-quantitatively, and identifies plasma amplification of a few n = 1 Fourier harmonics as the main cause of braking. IPEC predicts that TBM braking in H-mode may be reduced by n = 1 control. Although extrapolation from DIII-D to ITER is still an open issue, these experiments suggest that a TBM-like error field will produce only a few potentially troublesome problems, and that they might be made acceptably small.
    01/2011;
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    ABSTRACT: We have observed a fishbone-like mode in high-β plasmas above the ideal β limit without a conducting wall. The mode frequency chirps down, and its initial value is close to the precession frequency of trapped fast ions produced by perpendicularly injected neutral beams. This mode can often induce the resistive wall mode (RWM). The mode is observed mainly in the high-βN plasma where an ideal kink-ballooning mode (IKBM) and the RWM are marginally stable. Since this mode and the RWM were simultaneously observed, the mode is attributed to the interaction between the trapped fast ions and a marginally stable IKBM stabilized by a conducting wall. Actually, the MARG2D analysis shows that the IKBM is wall-stabilized and has a real frequency of the same order as the observed mode frequency. From these results, the observed mode was named 'energetic particle driven wall mode (EWM)'. Moreover, the EWM can change the edge localized mode (ELM) behaviour. For example, it was observed that the ELM was synchronized with the EWM. An energy loss due to the EWM-triggered ELM is smaller, and its frequency becomes higher compared with the usual ELM.
    Nuclear Fusion 07/2010; 50(8):084003. · 2.73 Impact Factor

Publication Stats

1k Citations
378.17 Total Impact Points

Institutions

  • 2006–2014
    • Japan Atomic Energy Agency
      • Nuclear Science and Engineering Directorate
      Muramatsu, Niigata, Japan
  • 2005–2008
    • National Institute for Fusion Science
      Tokitsu-chō, Gifu, Japan
  • 2000–2002
    • Kyushu University
      • • Division of Health Care Administration and Management
      • • Department of Psychosomatic Medicine
      Fukuoka-shi, Fukuoka-ken, Japan
  • 1995–2002
    • University of Tsukuba
      • Applied Physics
      Tsukuba, Ibaraki, Japan