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Nuclear Fusion

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ISSN (Print) 0029-5515 - ISSN (Online) 1741-4326
Published by Institute of Physics (IOP)

[72 journals]

Physics of radiation-driven islands near the tokamak density limit
- Authors: D.A. Gates; L. Delgado-Aparicio R.B. White
  First page: 063008
  Abstract: In previous work (Gates and Delgado-Aparicio 2012 Phys. Rev. Lett. 108 [http://dx.doi.org/10.1103/PhysRevLett.108.165004] 165004 ), the onset criterion for radiation-driven islands (Rebut et al 1985 Proc. 10th Int. Conf. on Plasma Physics and Controlled Nuclear Fusion Research 1984 (London, UK, 1984) vol 2 (Vienna: IAEA) p 197) in combination with a simple cylindrical model of tokamak current channel behaviour was shown to be consistent with the empirical scaling of the tokamak density limit (Greenwald et al 1988 Nucl. Fusion 28 [http://dx.doi.org/10.1088/0029-5515/28/12/009] 2199 ). A number of the unexplained phenomena at the density limit are consistent with this novel physics mechanism. In this work, a more formal theoretical underpinning, consistent with cylindrical tearing mode theory, is developed for the onset criteria of these modes. The appropriate derivation of the radiation-driven addition to the m...
  Citation: Nuclear Fusion
  PubDate: 2013-05-07T23:00:00Z
  Issue No: Vol. 53, No. 6 (2013)
Low- n magnetohydrodynamic edge instabilities in quiescent H-mode plasmas with a safety-factor plateau
- Authors: L.J. Zheng; M.T. Kotschenreuther P. Valanju
  First page: 063009
  Abstract: Low- n magnetohydrodynamic (MHD) modes in the quiescent high confinement mode (H-mode) pedestal are investigated in this paper. Here, n is the toroidal mode number. The low collisionality regime is considered, so that a safety-factor plateau arises in the pedestal region because of the strong bootstrap current. The JET-like (Joint European Torus) equilibria of quiescent H-mode discharges are generated numerically using the VMEC code. The stability of this type of equilibria is analysed using the AEGIS code, with subsonic rotation effects taken into account. The current investigation extends the previous studies of n = 1 modes to n = 2 and 3 modes. The numerical results show that the MHD instabilities in this type of equilibria have characteristic features of the infernal mode. We find that this type of mode tends to prevail when the safety-factor value in the shear-free region is slightly larger than an integer. In this case the frequencies ( ω
  Citation: Nuclear Fusion
  PubDate: 2013-05-07T23:00:00Z
  Issue No: Vol. 53, No. 6 (2013)
Collisionality scaling of main-ion toroidal and poloidal rotation in low torque DIII-D plasmas
- Authors: B.A. Grierson; K.H. Burrell, W.M. Solomon, R.V. Budny J. Candy
  First page: 063010
  Abstract: In tokamak plasmas with low levels of toroidal rotation, the radial electric field E r is a combination of pressure gradient and toroidal and poloidal rotation components, all having similar magnitudes. In order to assess the validity of neoclassical poloidal rotation theory for determining the poloidal rotation contribution to E r , D α emission from neutral beam heated tokamak discharges in DIII-D (Luxon 2002 Nucl. Fusion 42 [http://dx.doi.org/10.1088/0029-5515/42/5/313] 614 ) has been evaluated in a sequence of low torque (electron cyclotron resonance heating and balanced diagnostic neutral beam pulse) discharges to determine the local deuterium toroidal rotation velocity. By invoking the radial force balance relation the deuterium poloidal rotation can be inferred. It is found that the deuterium poloidal flow exceeds the neoclassical value in plasmas with collisionality ##IMG##
  Citation: Nuclear Fusion
  PubDate: 2013-05-07T23:00:00Z
  Issue No: Vol. 53, No. 6 (2013)
Intrinsic rotation generation in NSTX ohmic H-mode plasmas
- Authors: Jong-Kyu Park; Ronald E. Bell, Stanley M. Kaye, Wayne M. Solomon, Benoit P. LeBlanc, Ahmed Diallo, Jonathan E. Menard, Shigeyuki Kubota the NSTX Research Team
  First page: 063012
  Abstract: Intrinsic rotation generation was observed and investigated in NSTX ohmic plasmas, by utilizing passive views of charge exchange recombination diagnostics. Focus was placed on ohmic L–H transitions to minimize the effects by other momentum exchange and sources such as the NTV torque by intrinsic error fields. Results indicated that intrinsic rotation generation in the edge is well correlated with ion temperature gradient change, compared with much weaker correlations with electron temperature gradient or density gradient change. This is consistent with a corresponding theory of residual stress, and the measured torque could be directly compared with the theoretical prediction using χ i as a free parameter. However, an uncertainty on the order of diamagnetic rotation exists in many places across measurement and theory, as will be discussed in detail in this paper.
  Citation: Nuclear Fusion
  PubDate: 2013-05-07T23:00:00Z
  Issue No: Vol. 53, No. 6 (2013)
Basic investigations of electrostatic turbulence and its interaction with plasma and suprathermal ions in a simple magnetized toroidal plasma
- Authors: A. Fasoli; F. Avino, A. Bovet, I. Furno, K. Gustafson, S. Jolliet, J. Loizu, D. Malinverni, P. Ricci, F. Riva, C. Theiler, M. Spolaore N. Vianello
  First page: 063013
  Abstract: Progress in basic understanding of turbulence and its influence on the transport both of the plasma bulk and of suprathermal components is achieved in the TORPEX simple magnetized torus. This configuration combines a microwave plasma production scheme with a quasi-equilibrium generated by a toroidal magnetic field, onto which a small vertical component is superimposed, simulating a simplified form of tokamak scrape-off layers. After having clarified the formation of blobs in ideal interchange turbulence, TORPEX experiments elucidated the mechanisms behind the blob motion, with a general scaling law relating their size and speed. The parallel currents associated with the blobs, responsible for the damping of the charge separation that develops inside them, hence determining their cross-field velocity, have been measured. The blob dynamics is influenced by creating convective cells with biased electrodes, arranged in an array on a metal limiter. Depending on the biasing scheme, rad...
  Citation: Nuclear Fusion
  PubDate: 2013-05-07T23:00:00Z
  Issue No: Vol. 53, No. 6 (2013)
Effect of re-entering fast ions on NBI heating power in high-beta plasmas of the Large Helical Device
- Authors: Ryosuke Seki; Kiyomasa Watanabe, Hisamichi Funaba, Yasuhiro Suzuki, Yutaka Matsumoto, Kiyotaka Hamamatsu, Satoru Sakakibara Satoshi Ohdachi
  First page: 063016
  Abstract: We calculate the heating power of the neutral beam injection (NBI) in a 〈 β 〉 = 4.8% high-beta discharge achieved in the Large Helical Device (LHD). We investigate the difference of the heating efficiency and the heating power profile with and without the re-entering fast ion effects. When the re-entering fast ion effects are taken into account, the heating efficiency of the co-injection of the NBI (co-NBI case) is improved and it is about 1.8 times larger than that without the re-entering effects. In contrast, the heating efficiency with the re-entering effects in the counter-injection of the NBI (ctr-NBI case) scarcely differs from that without the re-entering ones. We also study the re-entering fast ion effects on the transport properties in the LHD high-beta discharges. It is found that the tendency of the thermal conductivities on the beta value is not so much sensitive with and without the re-entering effects. In addition, we investigate the difference in the re-enteri...
  Citation: Nuclear Fusion
  PubDate: 2013-05-07T23:00:00Z
  Issue No: Vol. 53, No. 6 (2013)
Steady-state operation using a dipole mode ion cyclotron heating antenna and 77 GHz electron cyclotron heating in the Large Helical Device
- Authors: T. Mutoh; T. Seki, R. Kumazawa, K. Saito, H. Kasahara, R. Seki, S. Kubo, T. Shimozuma, Y. Yoshimura, H. Igami, H. Takahashi, M. Nishiura, M. Shoji, J. Miyazawa, Y. Nakamura, M. Tokitani, N. Ashikawa, S. Masuzaki, H. Idei, G. Nomura, A. Murakami, R. Sakamoto, G. Motojima, Y.P. Zhao, J.G. Kwak, Y. Takeiri, H. Yamada, O. Kaneko, A. Komori the LHD Experiment Group
  First page: 063017
  Abstract: The steady-state operation of high-performance plasmas in the Large Helical Device (LHD) has progressed since the 2010 IAEA Conference in Korea by means of a newly installed ion cyclotron heating (ICH) antenna (HAS antenna) and an improved electron cyclotron heating (ECH) system. The HAS antenna can control the launched parallel wave number and heat the core plasma efficiently in the case of dipole mode operation. Understanding of the physics and technology of wave heating, particle and heat flow balances, and plasma–wall interactions in LHD has also improved. The heating power of steady-state ICH and ECH exceeded 1 MW and 500 kW, respectively, and a higher density helium plasma with minority hydrogen ions was maintained using the HAS antenna and new 77 GHz gyrotrons. As a result, plasma performance improved, e.g. electron temperature of more than 2 keV at a density of more than 2 × 10 19 m −3 became possible for more than 1 min. Heat flow balance and particle...
  Citation: Nuclear Fusion
  PubDate: 2013-05-13T23:00:00Z
  Issue No: Vol. 53, No. 6 (2013)
Computational study of strong magnetic field generation in a nonspherical, cone-guided implosion
- Authors: H. Nagatomo; T. Johzaki, A. Sunahara, H. Sakagami, K. Mima, H. Shiraga H. Azechi
  First page: 063018
  Abstract: It is known that a strong magnetic field is generated in laser plasma, especially during an asymmetric implosion such as in a cone-guided implosion used for fast ignition of nuclear fusion. As the first approximation, in this paper, the magnetic field for a nonspherical, cone-guided implosion is simulated using temporal evolution equations of the magnetic field coupled with the simulated result of a two-dimensional radiation hydrodynamic simulation for fast ignition. The results show that the magnetic field is generated by the ∇ T e × ∇ n e term, and is compressed by the implosion. In addition, we find that the magnetic field reaches 5 MG at maximum compression, which has not been investigated previously. Also, a high Hall parameter region appears between the cone tip and core plasma. This magnetic field is strong enough to affect the implosion dynamics and the hot electron transport, and should therefore be considered in simulations for fast ignit...
  Citation: Nuclear Fusion
  PubDate: 2013-05-13T23:00:00Z
  Issue No: Vol. 53, No. 6 (2013)
Combination of fast-ion diagnostics in velocity-space tomographies
- Authors: M. Salewski; B. Geiger, S.K. Nielsen, H. Bindslev, M. García-Muñoz, W.W. Heidbrink, S.B. Korsholm, F. Leipold, J. Madsen, F. Meo, P.K. Michelsen, D. Moseev, M. Stejner, G. Tardini the ASDEX Upgrade Team
  First page: 063019
  Abstract: Fast-ion D α (FIDA) and collective Thomson scattering (CTS) diagnostics provide indirect measurements of fast-ion velocity distribution functions in magnetically confined plasmas. Here we present the first prescription for velocity-space tomographic inversion of CTS and FIDA measurements that can use CTS and FIDA measurements together and that takes uncertainties in such measurements into account. Our prescription is general and could be applied to other diagnostics. We demonstrate tomographic reconstructions of an ASDEX Upgrade beam ion velocity distribution function. First, we compute synthetic measurements from two CTS views and two FIDA views using a TRANSP/NUBEAM simulation, and then we compute joint tomographic inversions in velocity-space from these. The overall shape of the 2D velocity distribution function and the location of the maxima at full and half beam injection energy are well reproduced in velocity-space tomographic inversions, if the noise level...
  Citation: Nuclear Fusion
  PubDate: 2013-05-13T23:00:00Z
  Issue No: Vol. 53, No. 6 (2013)
Power requirements for electron cyclotron current drive and ion cyclotron resonance heating for sawtooth control in ITER
- Authors: I.T. Chapman; J.P. Graves, O. Sauter, C. Zucca, O. Asunta, R.J. Buttery, S. Coda, T. Goodman, V. Igochine, T. Johnson, M. Jucker, R.J. La Haye, M. Lennholm JET-EFDA Contributors
  First page: 066001
  Abstract: 13 MW of electron cyclotron current drive (ECCD) power deposited inside the q = 1 surface is likely to reduce the sawtooth period in ITER baseline scenario below the level empirically predicted to trigger neoclassical tearing modes (NTMs). However, since the ECCD control scheme is solely predicated upon changing the local magnetic shear, it is prudent to plan to use a complementary scheme which directly decreases the potential energy of the kink mode in order to reduce the sawtooth period. In the event that the natural sawtooth period is longer than expected, due to enhanced α particle stabilization for instance, this ancillary sawtooth control can be provided from >10MW of ion cyclotron resonance heating (ICRH) power with a resonance just inside the q = 1 surface. Both ECCD and ICRH control schemes would benefit greatly from active feedback of the deposition with respect to the rational surface. If the q = 1 surface can be maintained closer to the ...
  Citation: Nuclear Fusion
  PubDate: 2013-05-07T23:00:00Z
  Issue No: Vol. 53, No. 6 (2013)