N. Oyama

Japan Atomic Energy Agency, Muramatsu, Niigata, Japan

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Publications (195)451.11 Total impact

  • N. Hayashi · N. Aiba · T. Takizuka · N. Oyama
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    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(4-6). DOI:10.1002/ctpp.201410040 · 0.84 Impact Factor
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    ABSTRACT: Since its inception in 2002, the International Tokamak Physics Activity topical group on Integrated Operational Scenarios (IOS) has coordinated experimental and modelling activity on the development of advanced inductive scenarios for applications in the ITER tokamak. The physics basis and the prospects for applications in ITER have been advanced significantly during that time, especially with respect to experimental results. The principal findings of this research activity are as follows. Inductive scenarios capable of higher normalized pressure (beta(N) >= 2.4) than the ITER baseline scenario (beta(N) = 1.8) with normalized confinement at or above the standard H-mode scaling are well established under stationary conditions on the four largest diverted tokamaks (AUG, DIII-D, JET, JT-60U), demonstrated in a database of more than 500 plasmas from these tokamaks analysed here. The parameter range where high performance is achieved is broad in q(95) and density normalized to the empirical density limit. MHD modes can play a key role in reaching stationary high performance, but also define the limits to achieved stability and confinement. Projection of performance in ITER from existing experiments uses empirical scalings and theory-based modelling. The status of the experimental validation of both approaches is summarized here. The database shows significant variation in the energy confinement normalized to standard H-mode confinement scalings, indicating the possible influence of additional physics variables absent from the scalings. Tests using the available information on rotation and the ratio of the electron and ion temperatures indicate neither of these variables in isolation can explain the variation in normalized confinement observed. Trends in the normalized confinement with the two dimensionless parameters that vary most from present-day experiments to ITER, gyroradius and collision frequency, are significant. Regression analysis on the multi-tokamak database has been performed, but it appears that the database is not conditioned sufficiently well to yield a new scaling for this type of plasma. Coordinated experiments on size scaling using the dimensionless parameter scaling approach find a weaker scaling with normalized gyroradius than the standard H-mode scaling. Preliminary studies on scaling with collision frequency show a favourable scaling stronger than the standard H-mode scaling. Coordinated modelling activity has resulted in successful benchmarking of modelling codes in the ITER regime. Validation of transport models using these codes on present-day experiments is in progress, but no single model has been shown to capture the variations seen in the experiments. However, projection to ITER using these models is in general agreement with the favourable projections found with the empirical scalings.
    Nuclear Fusion 01/2014; 54(1):013015. DOI:10.1088/0029-5515/54/1/013015 · 3.06 Impact Factor
  • N. Hayashi · N. Aiba · T. Takizuka · N. Oyama
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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-. DOI:10.1088/0029-5515/53/12/123009 · 3.06 Impact Factor
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    ABSTRACT: The dependence of heat transport, edge pedestal and confinement on isotopic composition was investigated in conventional H-mode plasmas. Identical profiles for the electron density, electron temperature and ion temperature were obtained for hydrogen and deuterium plasmas, whereas the required power clearly increased for hydrogen, which resulted in a reduction in heat diffusivity for deuterium. The determination of identical temperature profiles, despite the different heating power, suggested that the characteristics of heat conduction essentially differ for hydrogen and deuterium, even at the same scale length of temperature gradient. The self-regulating physics mechanism determining the overall H-mode confinement was also addressed. The inverse of the ion temperature gradient (ITG) scale length, or del T-i/T-i, which is required for a given ion heat diffusivity, increased by a factor of approximately 1.2 for deuterium compared with that for hydrogen. The relationship between edge pedestal pressure and global beta(p) holds true consistently regardless of the difference in the isotopic composition. A higher value of beta(p) was obtained for deuterium because of its smaller ITG scale length and because of the additional stored energy in the thermal and fast ion components, the latter due to an increase in the slowing down time with an increase in isotopic mass.
    Nuclear Fusion 08/2013; 53(8):083003. DOI:10.1088/0029-5515/53/8/083003 · 3.06 Impact Factor
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    ABSTRACT: In the JT-60U high-beta plasmas above the no-wall beta limit, a triggering of an edge localized mode (ELM) by an energetic particle (EP)-driven mode has been observed. This EP-driven mode is thought to be driven by trapped EPs and it has been named EP-driven wall mode (EWM) on JT-60U (Matsunaga et al 2009 Phys. Rev. Lett. 103 045001). When the EWM appears in an ELMy H-mode phase, ELM crashes are reproducibly synchronized with the EWM bursts. The EWM-triggered ELM has a higher repetition frequency and less energy loss than those of the natural ELM. In order to trigger an ELM by the EP-driven mode, some conditions are thought to be needed, thus an EWM with large amplitude and growth rate, and marginal edge stability. In the scrape-off layer region, several measurements indicate an ion loss induced by the EWM. The ion transport is considered as the EP transport through the edge region. From these observations, the EP contributions to edge stability are discussed as one of the ELM triggering mechanisms.
    Nuclear Fusion 07/2013; 53(7):073046. DOI:10.1088/0029-5515/53/7/073046 · 3.06 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. DOI:10.1088/0029-5515/53/4/043004 · 3.06 Impact Factor
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    ABSTRACT: The effects of local toroidal field (TF) ripples due to ferromagnetic steels used in test blanket modules (TBMs) in ITER on the radial transport of thermal ions located near the top of the pedestal are investigated using a fully three-dimensional magnetic field orbit following Monte Carlo (F3D-OFMC) code. In the simulation, the three-dimensional motion of 20 000 test particles, distributed near the top of the pedestal (psi(N) = 0.91) with the same Maxwellian velocity distribution as the thermal ions at this location, is traced for 1.9 s. In comparison with the number of lost particles in the case without a TBM, the additional loss with three TBM ports expected in ITER is evaluated to be less than 1% of the test particles. The additional losses increase linearly with the number of TBM ports and with the square of the amplitude of the local TF ripple. The poloidal structure of the TF ripple without ferritic inserts and a case with 18 TBM ports are also compared. It is found that cases having the same ripple amplitude at a certain point can have substantially different additional loss rates if the poloidal ripple structure is not the same. The ripple amplitude near the banana tip seems to be the most important factor in determining the radial diffusion of thermal ions.
    Nuclear Fusion 11/2012; 52(11):114013. DOI:10.1088/0029-5515/52/11/114013 · 3.06 Impact Factor
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    ABSTRACT: Energy confinement properties for hydrogen and deuterium H-mode plasmas are investigated. The thermal energy confinement time becomes longer in deuterium by a factor of similar to 1.4 than in hydrogen at a given absorbed power. When the absorbed power is fixed, the values of electron temperature T-e and ion temperature T-i become explicitly higher in deuterium than in hydrogen across the whole range of minor radius while the profiles of electron density n(e) are almost the same. Accordingly, the effective heat diffusivity becomes relatively lower in deuterium than in hydrogen. Despite almost the same power crossing the separatrix, type-I ELM frequency for hydrogen becomes approximately double that of deuterium. When the stored energy is fixed, the spatial profiles of n(e), T-e and T-i become identical for both cases while higher heating power is required in the hydrogen case. The pedestal pressure is about twice as high in deuterium as that in hydrogen at a given absorbed power. The increase of the pedestal temperature is more significant for the deuterium case while the pedestal density is not changed. The poloidal beta at the H-mode pedestal beta(ped)(p) is increased linearly with the increased total poloidal beta beta(TOT)(p) for both cases. The relation between beta(TOT)(p) and beta(ped)(p) is almost identical regardless of the difference of the isotope species.
    Nuclear Fusion 11/2012; 52(11):114021. DOI:10.1088/0029-5515/52/11/114021 · 3.06 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. DOI:10.1088/0029-5515/52/11/114002 · 3.06 Impact Factor
  • Physical Review Letters 10/2012; 109(14). DOI:10.1103/PhysRevLett.109.149901 · 7.51 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. DOI:10.1088/0029-5515/52/11/114010 · 3.06 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.
  • H. Urano · N. Oyama · K. Kamiya · N. Aiba · Y. Kamada · T. Fujita
    Nuclear Fusion 10/2012; 52(10):103012. DOI:10.1088/0029-5515/52/10/103012 · 3.06 Impact Factor
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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). DOI:10.1103/PhysRevLett.109.125001 · 7.51 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. DOI:10.1088/0029-5515/52/2/023024 · 3.06 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. DOI:10.1088/0029-5515/51/11/113004 · 3.06 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 10/2011; 51(10). DOI:10.1088/0029-5515/51/10/103030 · 3.06 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.
    Nuclear Fusion 10/2011; 51(10). DOI:10.1088/0029-5515/51/10/103028 · 3.06 Impact Factor
  • N. Asakura · T. Nakano · O. Naito · N. Oyama
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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 08/2011; 415(1). DOI:10.1016/j.jnucmat.2011.01.128 · 1.87 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. DOI:10.1088/0029-5515/51/7/073020 · 3.06 Impact Factor

Publication Stats

2k Citations
451.11 Total Impact Points


  • 2001–2014
    • Japan Atomic Energy Agency
      • Nuclear Science and Engineering Directorate
      Muramatsu, Niigata, Japan
  • 2002
    • Kyushu University
      • Department of Psychosomatic Medicine
      Hukuoka, Fukuoka, Japan
  • 1997–2002
    • University of Tsukuba
      • Applied Physics
      Tsukuba, Ibaraki, Japan