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J. Stober

Bio: J. Stober is an academic researcher from Max Planck Society. The author has contributed to research in topics: ASDEX Upgrade & Tokamak. The author has an hindex of 48, co-authored 350 publications receiving 7328 citations.


Papers
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Journal ArticleDOI
TL;DR: In this paper, local edge parameters on the ASDEX upgrade tokamak are investigated at the L-mode to H-mode transition, during phases with various types of edge-localized modes (ELMs), and at the density limit.
Abstract: Local edge parameters on the ASDEX Upgrade tokamak are investigated at the L-mode to H-mode transition, during phases with various types of edge-localized modes (ELMs), and at the density limit. A scaling law for the boundary electron temperature, , is found which describes the H-mode threshold for deuterium-puffed discharges with favourable ion -drift direction. The region of stable operation is bounded by type I ELMs near the ideal ballooning limit and by a minimum temperature necessary to avoid thermal instability of the plasma edge. Stationary operation with type III ELMs imposes an upper limit on the edge temperature. Within the entire range of boundary densities investigated , both L-mode and H-mode are found to be accessible. During type I ELMy H-mode, a relation of global confinement with the edge pressure gradient is found which is connected with a loss of the favourable density dependence predicted by the ITER-92P and ITER-93H ELMy H-mode scalings. At high density, better confinement is achieved in H-modes with an edge pressure gradient below the ideal ballooning limit, e.g. during type III ELMy H-mode with impurity-seeded radiation.

201 citations

Journal ArticleDOI
TL;DR: In this paper, small ELMs magnetic precursors are observed with a frequency of ≈ 30 kHz and dominant mode numbers of toroidally 3 and 4 and poloidally ≥ 14.
Abstract: H modes with good confinement and small ELMs with the characteristics of type II or grassy ELMs have been observed on ASDEX Upgrade. Such an ELM behaviour is essential to minimize erosion of the divertor tiles in any next step device. For the first time, operation with this favourable ELM type could be demonstrated close to the Greenwald density. Even for such high densities, energy confinement times were close to recent H mode scalings. High density even seems to be favourable, since steady state pure type II ELMy H mode phases on ASDEX Upgrade are obtained only above e/GW ≥ 0.85. Additional requirements are q95 ≥ 4.2 and an equilibrium close to a double null configuration with an average triangularity δ = 0.40. For these small ELMs magnetic precursors are observed with a frequency of ≈ 30 kHz and dominant mode numbers of toroidally 3 and 4 and poloidally ≥ 14.

168 citations

Journal ArticleDOI
O. Gruber1, A. Kallenbach1, Michael Kaufmann1, K. Lackner1, V. Mertens1, J. Neuhauser1, F. Ryter1, H. Zohm1, M. Bessenrodt-Weberpals1, K. Büchl1, S. Fiedler1, A. R. Field1, Ch. Fuchs1, Carmen García-Rosales1, G. Haas1, Albrecht Herrmann1, W. Herrmann1, S. Hirsch1, W. Köppendörfer1, Peter Lang1, G. Lieder, K. F. Mast, C. S. Pitcher, M. Schittenhelm1, J. Stober1, W. Suttrop1, M. Troppmann1, M. Weinlich1, M. Albrecht1, M. Alexander1, K. Asmussen1, M. Ballico, K. Behler1, K.H. Behringer1, H.-S. Bosch1, Marco Brambilla1, A. Carlson1, D. P. Coster1, L. Cupido, H.J. DeBlank1, S. De Pena Hempel1, S. Deschka1, C. Dorn1, R. Drube1, R. Dux1, A. Eberhagen, W. Engelhardt1, H. U. Fahrbach1, H. U. Feist1, D. Fieg, G. Fußmann1, O. Gehre1, J. Gernhardt1, P. Ignacz, B. Jüttner1, W. Junker1, T. Kass1, K. Kiemer1, H. Kollotzek1, M. Kornherr, K. Krieger1, B. Kurzan1, R. Lang, Martin Laux1, M. E. Manso, M. Maraschek1, H. M. Mayer, Patrick J. McCarthy1, D. Meisel, R. Merkel1, H. D. Murmann1, B. Napiontek1, Dirk Naujoks1, G. Neu1, R. Neu1, J.-M. Noterdaeme1, G. Pautasso1, W. Poschenrieder, Gerhard Raupp1, Harald Richter, T. Richter1, H. Röhr1, J. Roth1, Neil A. Salmon, H. Salzmann1, W. Sandmann1, H. B. Schilling1, Harald Schneider1, Ralf Schneider1, Wolf-Dieter Schneider1, K. Schonmann1, G. Schramm1, U. Schumacher, J. Schweinzer1, U. Seidel1, F. Serra, Ana Elisa Bauer de Camargo Silva, M. Sokoll1, E. Speth1, A. Stäbler1, K.-H. Steuer1, B. Streibl1, W. Treutterer1, M. Ulrich, P. Varela, H. Vernickel, O. Vollmer, H. Wedler, U. Wenzel1, F. Wesner1, R. Wunderlich1, D. Zasche1, H. P. Zehrfeld1 
TL;DR: Feedback-controlled puffing of neon and deuterium has been applied to control the edge-localized-mode behavior and the target plate power deposition during high-power H -mode discharges in ASDEX Upgrade.
Abstract: Feedback-controlled puffing of neon and deuterium has been applied to control the edge-localized-mode behavior and the target plate power deposition during high-power H -mode discharges in ASDEX Upgrade. A regime has been found in which more than 90% of the heating power is lost through radiation and divertor detachment occurs, without deterioration of the energy confinement. The plasma remains in the H mode, exhibiting small-amplitude, high-frequency ELM's, which do not penetrate to the target plates in the strike zone region.

149 citations

Journal ArticleDOI
TL;DR: In this article, the effect of plasma shape on high density ELMy H-modes, with geometry of the magnetic boundary similar to that envisaged for the standard Q = 10 operation in ITER, was investigated.
Abstract: We present the results of experiments in JET to study the effect of plasma shape on high density ELMy H-modes, with geometry of the magnetic boundary similar to that envisaged for the standard Q = 10 operation in ITER. The experiments described are single lower null plasmas, with standard q profile, neutral beam heating and gas fuelling, with average plasma triangularity ? calculated at the separatrix ~0.45-0.5 and elongation ?~1.75. In agreement with the previous results obtained in JET and other divertor Tokamaks, the thermal energy confinement time and the maximum density achievable in steady state for a given confinement enhancement factor increase with ?. The new experiments have confirmed and extended the earlier results, achieving a maximum line average density ne~1.1nGR for H98~0.96. In this plasma configuration, at 2.5?MA/2.7?T (q95~2.8), a line average density ~95%?nGR with H98 = 1 and ?N~2 are obtained, with plasma thermal stored energy content Wth being approximately constant with increasing density, as long as the discharge maintains Type I ELMs, up to nped~nGR (and ne~1.1nGR). A change in the Type I ELMs behaviour is observed for pedestal densities nped70%?nGR, with their frequency decreasing with density (at constant Psep), enhanced divertor D? emission and increased inter-ELM losses. We show that this change in the ELM character at high pedestal density is due to a change in transport and/or stability in the pedestal region, with the ELMs changing from Type I to mixed Type I and Type II. The similarity of these observations with those in the Type II ELM regime in ASDEX?Upgrade and with other small ELM regimes in DIII-D, JT-60U and Alcator C-MOD is discussed. Finally, we present the first results of experiments by studying in more detail the effects of the plasma boundary geometry, in particular by investigating separately the effect of the upper and lower triangularity, at high average ?. We show that the changes to the lower ? (or of the radial position of the x-point) affect the pedestal parameters, the size of ELM energy losses as well as the global energy confinement of the plasma.

147 citations

Journal ArticleDOI
R. Dux1, Rudolf Neu1, A. G. Peeters1, G. V. Pereverzev1, A. Mück, F. Ryter1, J. Stober1 
TL;DR: In this article, the transport of silicon has been investigated for various heating scenarios in ASDEX Upgrade H-mode discharges, and the diffusion coefficient D is either mainly neoclassical or anomalous depending on the heating method.
Abstract: The transport of silicon has been investigated for various heating scenarios in ASDEX Upgrade H-mode discharges. Inside of r≈a/4, the diffusion coefficient D is either mainly neoclassical or anomalous depending on the heating method. For all investigated scenarios with NBI-heating and off-axis ECRH or off-axis ICRH, the diffusion coefficient is approximately neoclassical, and the effective heat diffusion coefficient χeff is below the neoclassical ion heat diffusion χi,neo in the plasma core. When central ECRH is added, χeff is above χi,neo, and D strongly increases by a factor of 3–10, i.e. becomes predominantly anomalous. For central ICRH, D is above the neoclassical level by a factor of 2. For radii outside of r≈a/4, D is always anomalous and increases towards the plasma edge. For ra/4, we find a clear scaling of D in terms of χeff, where D is about equal or above χeff. A strong inward drift parameter v/D is only observed in the core and only for cases, when the diffusion coefficient is neoclassical. With central wave heating, the drift parameter decreases to small values.

133 citations


Cited by
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[...]

08 Dec 2001-BMJ
TL;DR: There is, I think, something ethereal about i —the square root of minus one, which seems an odd beast at that time—an intruder hovering on the edge of reality.
Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

33,785 citations

Journal ArticleDOI
01 Apr 1988-Nature
TL;DR: In this paper, a sedimentological core and petrographic characterisation of samples from eleven boreholes from the Lower Carboniferous of Bowland Basin (Northwest England) is presented.
Abstract: Deposits of clastic carbonate-dominated (calciclastic) sedimentary slope systems in the rock record have been identified mostly as linearly-consistent carbonate apron deposits, even though most ancient clastic carbonate slope deposits fit the submarine fan systems better. Calciclastic submarine fans are consequently rarely described and are poorly understood. Subsequently, very little is known especially in mud-dominated calciclastic submarine fan systems. Presented in this study are a sedimentological core and petrographic characterisation of samples from eleven boreholes from the Lower Carboniferous of Bowland Basin (Northwest England) that reveals a >250 m thick calciturbidite complex deposited in a calciclastic submarine fan setting. Seven facies are recognised from core and thin section characterisation and are grouped into three carbonate turbidite sequences. They include: 1) Calciturbidites, comprising mostly of highto low-density, wavy-laminated bioclast-rich facies; 2) low-density densite mudstones which are characterised by planar laminated and unlaminated muddominated facies; and 3) Calcidebrites which are muddy or hyper-concentrated debrisflow deposits occurring as poorly-sorted, chaotic, mud-supported floatstones. These

9,929 citations

01 Jan 2007

1,932 citations

Journal ArticleDOI
TL;DR: In this article, the authors review the underlying physical processes and the existing experimental database of plasma-material interactions both in tokamaks and laboratory simulation facilities for conditions of direct relevance to next-step fusion reactors.
Abstract: The major increase in discharge duration and plasma energy in a next step DT fusion reactor will give rise to important plasma-material effects that will critically influence its operation, safety and performance. Erosion will increase to a scale of several centimetres from being barely measurable at a micron scale in today's tokamaks. Tritium co-deposited with carbon will strongly affect the operation of machines with carbon plasma facing components. Controlling plasma-wall interactions is critical to achieving high performance in present day tokamaks, and this is likely to continue to be the case in the approach to practical fusion reactors. Recognition of the important consequences of these phenomena stimulated an internationally co-ordinated effort in the field of plasma-surface interactions supporting the Engineering Design Activities of the International Thermonuclear Experimental Reactor project (ITER), and significant progress has been made in better understanding these issues. The paper reviews the underlying physical processes and the existing experimental database of plasma-material interactions both in tokamaks and laboratory simulation facilities for conditions of direct relevance to next step fusion reactors. Two main topical groups of interaction are considered: (i) erosion/redeposition from plasma sputtering and disruptions, including dust and flake generation and (ii) tritium retention and removal. The use of modelling tools to interpret the experimental results and make projections for conditions expected in future devices is explained. Outstanding technical issues and specific recommendations on potential R&D avenues for their resolution are presented.

1,187 citations

Journal ArticleDOI
TL;DR: A review of recent advances in the area of MHD stability and disruptions, since the publication of the 1999 ITER Physics Basis document (1999 Nucl. Fusion 39 2137-2664), is reviewed in this paper.
Abstract: Progress in the area of MHD stability and disruptions, since the publication of the 1999 ITER Physics Basis document (1999 Nucl. Fusion 39 2137-2664), is reviewed. Recent theoretical and experimental research has made important advances in both understanding and control of MHD stability in tokamak plasmas. Sawteeth are anticipated in the ITER baseline ELMy H-mode scenario, but the tools exist to avoid or control them through localized current drive or fast ion generation. Active control of other MHD instabilities will most likely be also required in ITER. Extrapolation from existing experiments indicates that stabilization of neoclassical tearing modes by highly localized feedback-controlled current drive should be possible in ITER. Resistive wall modes are a key issue for advanced scenarios, but again, existing experiments indicate that these modes can be stabilized by a combination of plasma rotation and direct feedback control with non-axisymmetric coils. Reduction of error fields is a requirement for avoiding non-rotating magnetic island formation and for maintaining plasma rotation to help stabilize resistive wall modes. Recent experiments have shown the feasibility of reducing error fields to an acceptable level by means of non-axisymmetric coils, possibly controlled by feedback. The MHD stability limits associated with advanced scenarios are becoming well understood theoretically, and can be extended by tailoring of the pressure and current density profiles as well as by other techniques mentioned here. There have been significant advances also in the control of disruptions, most notably by injection of massive quantities of gas, leading to reduced halo current fractions and a larger fraction of the total thermal and magnetic energy dissipated by radiation. These advances in disruption control are supported by the development of means to predict impending disruption, most notably using neural networks. In addition to these advances in means to control or ameliorate the consequences of MHD instabilities, there has been significant progress in improving physics understanding and modelling. This progress has been in areas including the mechanisms governing NTM growth and seeding, in understanding the damping controlling RWM stability and in modelling RWM feedback schemes. For disruptions there has been continued progress on the instability mechanisms that underlie various classes of disruption, on the detailed modelling of halo currents and forces and in refining predictions of quench rates and disruption power loads. Overall the studies reviewed in this chapter demonstrate that MHD instabilities can be controlled, avoided or ameliorated to the extent that they should not compromise ITER operation, though they will necessarily impose a range of constraints.

1,051 citations