In the last 30 days

• Open/close Proliferation risks of magnetic fusion energy: clandestine production, covert production and breakout

  A. Glaser and R.J. Goldston 2012 Nucl. Fusion 52 043004 Tag this article

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  Nuclear proliferation risks from magnetic fusion energy associated with access to weapon-usable materials can be divided into three main categories: (1) clandestine production of weapon-usable material in an undeclared facility, (2) covert production of such material in a declared facility and (3) use of a declared facility in a breakout scenario, in which a state begins production of fissile material without concealing the effort. In this paper, we address each of these categories of risks from fusion. For each case, we find that the proliferation risk from fusion systems can be much lower than the equivalent risk from fission systems, if the fusion system is designed to accommodate appropriate safeguards.

• Open/close Toroidal velocity shear Kelvin–Helmholtz instabilities in strongly rotating tokamak plasmas

  I.T. Chapman et al 2012 Nucl. Fusion 52 042005 Tag this article

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  Strong sheared toroidal flows are predicted to drive parallel velocity shear Kelvin–Helmholtz oscillations unstable in spherical tokamak plasmas. Numerical simulations that include rotation in the equilibrium suggest that future spherical tokamaks will have an upper limit in uni-directional injected neutral beam heating in order to keep the sheared toroidal rotation below the critical level for the onset of the global parallel velocity shear Kelvin–Helmholtz instability.

• Open/close Resonant features of energy and particle transport during application of resonant magnetic perturbation fields at TEXTOR and DIII-D

  O. Schmitz et al 2012 Nucl. Fusion 52 043005 Tag this article

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  In this paper, results of a direct comparison of TEXTOR and DIII-D experiments with resonant magnetic perturbation (RMP) fields are presented. This comparison of resistive L-mode plasmas at TEXTOR with highly conductive H-mode plasmas at DIII-D is useful to identify generic physics mechanisms during application of RMP fields with a strong field line pitch angle alignment in the plasma edge.

  A reduction in the pedestal electron pressure p _e with increasing extension of the vacuum modelled stochastic layer and p _e recovery with decreasing layer width is found caused by a q ₉₅ resonant reduction in the edge (0.8 < Ψ _N < 0.95) electron temperature T _e( q ₉₅) on both devices. For RMP edge-localized mode (ELM) suppressed H-mode plasmas at DIII-D, the gradients ∇ T _e and nominal values of T _e are reduced in this edge region while increasing in the pedestal (0.95 < Ψ _N < 1.0) with RMP field applied and both are highly dependent on q ₉₅. In contrast, an increase in the central ion temperature with strong steepening of the ion temperature profile at mid-radius is found—again being highly dependent on q ₉₅. However, these resonant thermal transport effects are only seen in high triangularity plasmas revealing a strong shape dependence of the thermal transport. In contrast to the highly q ₉₅ dependent thermal transport features, the reduction of n _e—known as density pump out—shows a much weaker dependence on q ₉₅. We show the potential to reduce the RMP induced particle pump out by fine tuning of the RMP spectral properties. At low resonant field amplitudes enhanced particle confinement is seen in high-field side limited L-mode discharges on both devices while higher resonant field amplitudes yield particle pumps out.

• Open/close The effect of non-axisymmetric wall geometry on ¹³C transport in ASDEX Upgrade

  J. Miettunen et al 2012 Nucl. Fusion 52 032001 Tag this article

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  We present the first results of 3D simulations of global ¹³C transport in ASDEX Upgrade (AUG) indicating that the deposition profile of ¹³C exhibits toroidal asymmetry in the main chamber.

  In 2007, the migration of carbon in AUG was studied with a methane ( ¹³CH ₄) injection experiment (A. Hakola et al and the ASDEX Upgrade Team 2010 Plasma Phys. Control. Fusion 52 065006). The total amount of deposited ¹³C was estimated by assuming toroidally symmetric deposition. Remarkably, the total number of deposited atoms was observed to be less than 10% of the number of injected atoms.

  The experiment has been simulated with the 3D orbit-following Monte Carlo code ASCOT using both a realistic 3D wall geometry of AUG and a 3D magnetic field with toroidal ripple. The simulations indicate that the non-axisymmetric wall geometry causes notable toroidal asymmetry in the deposition profile in the outer (low-field side) midplane region which can provide a partial explanation for the missing carbon inferred from post-mortem analysis of ¹³C deposition.

• Open/close Tungsten nano-tendril growth in the Alcator C-Mod divertor

  G.M. Wright et al 2012 Nucl. Fusion 52 042003 Tag this article

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  Growth of tungsten nano-tendrils (‘fuzz’) has been observed for the first time in the divertor region of a high-power density tokamak experiment. After 14 consecutive helium L-mode discharges in Alcator C-Mod, the tip of a tungsten Langmuir probe at the outer strike point was fully covered with a layer of nano-tendrils. The thickness of the individual nano-tendrils (50–100 nm) and the depth of the layer (600 ± 150 nm) are consistent with observations from experiments on linear plasma devices. The observation of tungsten fuzz in a tokamak may have important implications for material erosion, dust formation, divertor lifetime and tokamak operations in next-step devices.

• Open/close Summary of the International Workshop on Magnetic Fusion Energy (MFE) Roadmapping in the ITER Era; 7–10 September 2011, Princeton, NJ, USA

  G.H. Neilson et al 2012 Nucl. Fusion 52 047001 Tag this article

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  With the ITER project now well under way, the countries engaged in fusion research are planning, with renewed intensity, the research and major facilities needed to develop the science and technology for harnessing fusion energy. The Workshop on MFE Roadmapping in the ITER Era was organized to provide a timely forum for an international exchange of technical information and strategic perspectives on how best to tackle the remaining challenges leading to a magnetic fusion DEMO, a nuclear fusion device or devices with a level of physics and technology integration necessary to cover the essential elements of a commercial fusion power plant. Presentations addressed issues under four topics: (1) Perspectives on DEMO and the roadmap to DEMO; (2) Technology; (3) Physics-Technology integration and optimization; and (4) Major facilities on the path to DEMO. Participants identified a set of technical issues of high strategic importance, where the development strategy strongly influences the overall roadmap, and where there are divergent understandings in the world community, namely (1) the assumptions used in fusion design codes, (2) the strategy for fusion materials development, (3) the strategy for blanket development, (4) the strategy for plasma exhaust solution development and (5) the requirements and state of readiness for next-step facility options. It was concluded that there is a need to continue and to focus the international discussion concerning the scientific and technical issues that determine the fusion roadmap, and it was suggested that an international activity be organized under appropriate auspices to foster international cooperation on these issues.

• Open/close A new model of the L–H transition in tokamaks

  W. Fundamenski et al 2012 Nucl. Fusion 52 062003 Tag this article

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  A new model of the L–H transition in tokamaks is proposed, based on the criterion that the transition occurs when plasma turbulence and shear Alfvén waves compete in the vicinity of the last-closed flux surface. The model is used to predict the scaling of the H-mode access power, P _L–H, with magnetic and plasma variables. The predictions are in good agreement with the experimental scaling of P _L–H with plasma size, density, magnetic field and edge safety factor (plasma current). They are also qualitatively consistent with the dependence of P _L–H on ion mass and charge, limiter versus divertor plasmas, single versus double null configurations, the divertor leg length and H–L versus L–H hysteresis. Most notably, the model explains the appearance of the minimum in P _L–H with plasma density (in terms of the transition between sheath limited and conduction limited scrape-off layer (SOL) transport) and correctly predicts the scaling of the density minimum with magnetic field and Greenwald number. Finally, the effect of toroidal field reversal is included by making the normalized correlation length of the eddy and maximum safety factor in the edge-SOL layer functions of B  × ∇ B direction.

• Open/close Direct evidence of eddy breaking and tilting by edge sheared flows observed in the TEXTOR tokamak

  I. Shesterikov et al 2012 Nucl. Fusion 52 042004 Tag this article

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  In this letter, we present direct evidence of eddy breaking and tilting events observed at the edge of the TEXTOR tokamak using a 2D gas puff imaging diagnostic. The occurrence of eddy breaking depends on the magnitude of the E _r  ×  B flow shearing rate. The results confirm the theoretical predictions about the influence of E _r  ×  B sheared flows in tilting and splitting turbulence eddies, and consequently, reducing the turbulent transport.

• Open/close Message from the Editor

  Paul Thomas 2012 Nucl. Fusion 52 040201 Tag this article

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  Whilst travelling to Vienna to hand over the Editorship of Nuclear Fusion to Ron Stambaugh, I jotted down a few ideas to put in a farewell message. Somewhat unsurprisingly, I find them almost identical to the remarks that Chris Schueller made in handing over to me five years ago. Both of us served in this role for five years, which seems like a good timescale to replace presidents and editors alike; just to allow a fresh approach. In addition, since I have been directly involved in ITER, I have found it increasingly difficult, due to time pressure, to give the journal the attention that it deserves. Therefore, I am very pleased that Ron Stambaugh has agreed to take over as Editor. Not only does he bring the experience as a leading figure in the US fusion programme but, in addition, he has all the contacts, worldwide, from his leadership of the ITPA. I am completely assured that the journal is in a highly competent pair of hands.

  Such a farewell should not lack my heartfelt thanks to all of those who have made being Editor of Nuclear Fusion so enjoyable and stimulating; readership, authors, referees, the Editorial Board and the NF Office alike. I wish Ron all the best for his tenure and have offered such help and support as I am able to give.

• Open/close Chapter 2: Plasma confinement and transport

  E.J. Doyle (Chair Transport Physics) et al 2007 Nucl. Fusion 47 S18 Tag this article

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  The understanding and predictive capability of transport physics and plasma confinement is reviewed from the perspective of achieving reactor-scale burning plasmas in the ITER tokamak, for both core and edge plasma regions. Very considerable progress has been made in understanding, controlling and predicting tokamak transport across a wide variety of plasma conditions and regimes since the publication of the ITER Physics Basis (IPB) document ( 1999 Nucl. Fusion 39 2137–2664). Major areas of progress considered here follow. (1) Substantial improvement in the physics content, capability and reliability of transport simulation and modelling codes, leading to much increased theory/experiment interaction as these codes are increasingly used to interpret and predict experiment. (2) Remarkable progress has been made in developing and understanding regimes of improved core confinement. Internal transport barriers and other forms of reduced core transport are now routinely obtained in all the leading tokamak devices worldwide. (3) The importance of controlling the H-mode edge pedestal is now generally recognized. Substantial progress has been made in extending high confinement H-mode operation to the Greenwald density, the demonstration of Type I ELM mitigation and control techniques and systematic explanation of Type I ELM stability. Theory-based predictive capability has also shown progress by integrating the plasma and neutral transport with MHD stability. (4) Transport projections to ITER are now made using three complementary approaches: empirical or global scaling, theory-based transport modelling and dimensionless parameter scaling (previously, empirical scaling was the dominant approach). For the ITER base case or the reference scenario of conventional ELMy H-mode operation, all three techniques predict that ITER will have sufficient confinement to meet its design target of Q = 10 operation, within similar uncertainties.