P. J. Lomas

Forschungszentrum Jülich, Jülich, North Rhine-Westphalia, Germany

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Publications (310)470.04 Total impact

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    ABSTRACT: Runaway electrons have been created using injections of high-Z noble gases at JET. Their features are close to runaways created in the same way the carbon wall. Runaway beam suppression was achieved with massive gas injection only if fired before the beginning of the current quench. Injections of up to 4300 Pa.m3 on an already accelerated runaway beam were ineffective
    3rd Runaway Electron Meeting (REM-2015), Pertuis,France; 06/2015
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    ABSTRACT: This paper reports the progress made at JET-ILW on integrating the requirements of the reference ITER baseline scenario with normalized confinement factor of 1, at a normalized pressure of 1.8 together with partially detached divertor whilst maintaining these conditions over many energy confinement times. The 2.5 MA high triangularity ELMy H-modes are studied with two different divertor configurations with D-gas injection and nitrogen seeding. The power load reduction with N seeding is reported. The relationship between an increase in energy confinement and pedestal pressure with triangularity is investigated. The operational space of both plasma configurations is studied together with the ELM energy losses and stability of the pedestal of unseeded and seeded plasmas. The achievement of stationary plasma conditions over many energy confinement times is also reported.
    Plasma Physics and Controlled Fusion 03/2015; 57(3). DOI:10.1088/0741-3335/57/3/035004 · 2.39 Impact Factor
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    ABSTRACT: The JET tokamak is unique amongst present fusion devices in its capability to operate at high plasma current, providing the closest plasma parameters to ITER. The physics benefits of high current operation have to be balanced against the risks to the integrity of the machine due to high force disruptions. The installation of the ITER-Like Wall (ILW) has added risks due to the thermal characteristics of the metal Plasma Facing Components. This paper describes the operational aspects of the scientific development of high current H-mode plasmas with the ILW, focusing on disruption prediction, avoidance and amelioration. The development yielded baseline H-mode plasmas up to 4 MA/3.74 T, comparable to the maximum current achieved in JET in Carbon-Wall (CFC) conditions with similar divertor geometry.
    Fusion Engineering and Design 02/2015; DOI:10.1016/j.fusengdes.2015.01.014 · 1.15 Impact Factor
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    ABSTRACT: External magnetic perturbations are typically utilized in tokamak devices with two operational or experi- mental purposes: 1) correction of intrinsic 3-D error fields and 2) mitigation or suppression of edge localized modes (ELMs). At Joint European Torus (JET), dedicated coils are used for the generation of these toroidally asymmetric perturbations. While error fields exist even in the absence of plasma, in ELM mitigation experiments, the external fields are meant to slightly ergodize the magnetic topology in the plasma periphery hence reducing the drive for the destabilization of these instabilities. The control of the magnetic field produced by these coils is achieved by controlling the current flowing in them. The real- time system responsible for this control recently underwent a number of functional improvements since its original implemen- tation utilizing the present voltage-controlled voltage sources. This paper describes the overall system, built-in functionality, and control algorithms and presents preliminary experimental results along with performance assessment studies. In particular, the main improvements are: 1) the possibility of automatically reducing the current references in case the plasma amplifies the applied perturbation; 2) a real-time limitation of d I/d t to reduce the electromotive force in machine protection diagnostic systems; 3) implementation of a model predictive controller as an alternative to the proportional integral derivative; and 4) the possibility of adapting the current references, in real time, using an external system. The result is a flexible control system contributing toward state-of-the-art physics research at JET’s international and dynamic scientific environment.
    IEEE Transactions on Plasma Science 02/2015; 43(2-2):650-664. DOI:10.1109/TPS.2015.2388674 · 0.95 Impact Factor
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    ABSTRACT: The JET Shape Controller (SC) uses nine distinct circuits, powering the JET poloidal field coils, to control in real time the coil currents, and the plasma shape, current and position. The control scheme presently used [1] is based on a Multiple Input Multiple Output (MIMO) controller, which is designed to decouple the inductive coupling of the different coils. Achieving such a decoupling, the SC allows the user to tune independently the time response of each circuit. As a matter of fact the intended decoupling algorithm has been incorrectly coded in the JET SC system. This paper describes the modelling and experimental activities performed to correct the code error, and to improve the performance on a subset of the controlled parameters.
    Fusion Engineering and Design 02/2015; DOI:10.1016/j.fusengdes.2015.01.035 · 1.15 Impact Factor
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    ABSTRACT: The baseline type-I ELMy H-mode scenario has been re-established in JET with the new tungsten MKII-HD divertor and beryllium on the main wall (hereafter called the ITER-like wall, JET-ILW).The first JET-ILW results show that the confinement is degraded by 20–30% in the baseline scenarios compared to the previous carbon wall JET (JET-C) plasmas. The degradation is mainly driven by the reduction in the pedestal temperature. Stored energies and pedestal temperature comparable to the JET-C have been obtained to date in JET-ILW baseline plasmas only in the high triangularity shape using N2 seeding.This work compares the energy losses during ELMs and the corresponding time scales of the temperature and density collapse in JET-ILW baseline plasmas with and without N2 seeding with similar JET-C baseline plasmas. ELMs in the JET-ILW differ from those with the carbon wall both in terms of time scales and energy losses. The ELM time scale, defined as the time to reach the minimum pedestal temperature soon after the ELM collapse, is ∼2 ms in the JET-ILW and lower than 1 ms in the JET-C. The energy losses are in the range ΔW ELM/W ped ≈ 7–12% in the JET-ILW and ΔW ELM/W ped ≈ 10–20% in JET-C, and fit relatively well with earlier multi-machine empirical scalings of ΔW ELM/W ped with collisionality. The time scale of the ELM collapse seems to be related to the pedestal collisionality. Most of the non-seeded JET-ILW ELMs are followed by a further energy drop characterized by a slower time scale ∼8–10 ms (hereafter called slow transport events), that can lead to losses in the range ΔW slow/W ped ≈ 15–22%, slightly larger than the losses in JET-C. The N2 seeding in JET-ILW significantly affects the ELMs. The JET-ILW plasmas with N2 seeding are characterized by ELM energy losses and time scales similar to the JET-C and by the absence of the slow transport events.
    Nuclear Fusion 02/2015; 55(2). DOI:10.1088/0029-5515/55/2/023007 · 3.24 Impact Factor
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    Journal of Nuclear Materials 01/2015; DOI:10.1016/j.jnucmat.2014.12.034 · 2.02 Impact Factor
  • Nuclear Fusion 01/2015; 55(1). DOI:10.1088/0029-5515/55/1/013019 · 3.24 Impact Factor
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    ABSTRACT: The JET ITER-like Wall (ILW) provides the same plasma facing component configuration as ITER during its active phase: beryllium in the main chamber and tungsten in the divertor. Moving from a carbon-based wall to an all metals wall requires some operational adjustment. The reduction in radiation at the plasma edge and in the divertor can lead to high power loads on the plasma facing components both in steady state and in transients and requires the development of radiative scenarios and the use of massive gas injection to mitigate disruptions. These tools are even more important now because an all metal wall is much less forgiving to thermal overloading the carbon based wall used to be. Here the impact of the first 11 months of operation on the ILW plasma facing components is discussed.
    Fusion Engineering and Design 10/2014; DOI:10.1016/j.fusengdes.2014.01.045 · 1.15 Impact Factor
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    ABSTRACT: The JET scrape-off layer (SOL) has been characterized with a reciprocating probe in inner wall (IW), and outer wall (OW), limited plasmas. Experiments revealed that SOL profiles are substantially broader (by a factor of ∼5–7.5 in the power e-folding length) for IW limited than in OW limited plasmas. Results are consistent with the larger radial turbulent transport found for IW limited plasmas. Major differences are observed between IW and OW limited plasmas on the density and electron temperature e-folding lengths, parallel flow, radial turbulent transport as well as on the temporal and spatial characteristics of the fluctuations. Experimental findings on JET suggest that the differences in the SOL characteristics for both configurations are due to a combination of a poloidal asymmetry in radial transport with a reduced cross-field transport across the last closed flux surface associated with the confinement improvement observed for OW limited plasmas.The dependence of the SOL power e-folding length on the main plasma parameters was also investigated for IW limited plasmas and a modest negative dependence on both the plasma current and the line-averaged density found. Finally, it is shown that the SOL radial transport and the amplitude of the fluctuations increase with plasma current and decrease with line-averaged density for IW limited plasmas.
    Nuclear Fusion 08/2014; 54(8). DOI:10.1088/0029-5515/54/8/083022 · 3.24 Impact Factor
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    ABSTRACT: The former all-carbon wall on JET has been replaced with beryllium in the main torus and tungsten in the divertor to mimic the surface materials envisaged for ITER. Comparisons are presented between Type I H-mode characteristics in each design by examining respective scans over deuterium fuelling and impurity seeding, required to ameliorate exhaust loads both in JET at full capability and in ITER.
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    21st International Conference on Plasma Surface Interactions , Kanazawa 2014; 06/2014
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    21st International Conference on Plasma Surface Interactions , Kanazawa 2014; 06/2014
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    ABSTRACT: In order to preserve the integrity of large tokamaks such as ITER, the number of disruptions has to be limited. JET has operated previously with a low frequency of disruptions (i.e., disruption rate) of 3.4% [P. C. de Vries et al., Nucl. Fusion 51, 053018 (2011)]. The start of operations with the new full-metal ITER-like wall at JET showed a marked rise in the disruption rate to 10%. A full survey was carried out to identify the root causes, the chain-of-events and classifying each disruption, similar to a previous analysis for carbon-wall operations. It showed the improvements made to avoid various disruption classes, but also indicated those disruption types responsible for the enhanced disruption rate. The latter can be mainly attributed to disruptions due to too high core radiation but also due to density control issues and error field locked modes. Detailed technical and physics understanding of disruption causes is essential for devising optimized strategies to avoid or mitigate these events.
    Physics of Plasmas 04/2014; 21(5). DOI:10.1063/1.4872017 · 2.25 Impact Factor
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    ABSTRACT: The ITER-like wall (ILW) at JET provides a unique opportunity to study the combination of material (beryllium and tungsten) that will be used for the plasma facing components (PFCs) in ITER. Both the limiters (Be) and divertor (CFC W coated and bulk W) have been designed to maximize their power handling capability. During the last experimental campaign (October 2010–July 2011) this capability has been assessed and even challenged in the case of the Be wall. The Be limiters' power handling capability (19 MW m−2 s−1/2), predicted with a simple model, has been proven to be robust by the experiments despite an unexpected power load pattern. This capability has been pushed to its limit leading to Be melt events, which revealed that the power load is toroidally asymmetric. The protection system of the ILW did not prevent melt events mainly because the protection strategy relies on the assumption that the power load is toroidally symmetric. The bulk W divertor target performed as predicted. Operations were constrained by: (i) an energy load limit (60 MJ m−2); (ii) the limited number of cycles of the surface temperature above 1200 °C in order to prevent thermal fatigue. This latter limit has been exceeded about 300 times and no signs of damage or thermal fatigue have been observed by the photogrammetric survey.
    Physica Scripta 04/2014; 2014(T159):014009. DOI:10.1088/0031-8949/2014/T159/014009 · 1.30 Impact Factor
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    ABSTRACT: This paper presents the electromagnetic modeling of the plasma current breakdown phase of the JET tokamak. The first part of this paper models the presence of the JET iron core up-down asymmetry and the effects of the eddy currents in the reconstruction of the magnetic topology needed for the plasma start. The second part describes the approach used to evaluate the ionized particle connection length inside the vacuum chamber at breakdown. The results obtained are validated using JET experimental measurements.
    IEEE Transactions on Magnetics 02/2014; 50(2):937-940. DOI:10.1109/TMAG.2013.2282351 · 1.21 Impact Factor
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    ABSTRACT: In the recent JET experimental campaigns with the new ITER-like wall (JET-ILW), major progress has been achieved in the characterization and operation of the H-mode regime in metallic environments: (i) plasma breakdown has been achieved at the first attempt and X-point L-mode operation recovered in a few days of operation; (ii) stationary and stable type-I ELMy H-modes with βN ~ 1.4 have been achieved in low and high triangularity ITER-like shape plasmas and are showing that their operational domain at H = 1 is significantly reduced with the JET-ILW mainly because of the need to inject a large amount of gas (above 1022 D s−1) to control core radiation; (iii) in contrast, the hybrid H-mode scenario has reached an H factor of 1.2–1.3 at βN of 3 for 2–3 s; and, (iv) in comparison to carbon equivalent discharges, total radiation is similar but the edge radiation is lower and Zeff of the order of 1.3–1.4. Strong core radiation peaking is observed in H-mode discharges at a low gas fuelling rate (i.e. below 0.5 × 1022 D s−1) and low ELM frequency (typically less than 10 Hz), even when the tungsten influx from the diverter is constant. High-Z impurity transport from the plasma edge to the core appears to be the dominant factor to explain these observations. This paper reviews the major physics and operational achievements and challenges that an ITER-like wall configuration has to face to produce stable plasma scenarios with maximized performance.
    Nuclear Fusion 01/2014; 54:013011. DOI:10.1088/0029-5515/54/1/013011 · 3.24 Impact Factor
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    ABSTRACT: Type I ELMy H-mode operation in JET with the ITER-like Be/W wall (JET-ILW) generally occurs at lower pedestal pressures compared to those with the full carbon wall (JET-C). The pedestal density is similar but the pedestal temperature where type I ELMs occur is reduced and below to the so-called critical type I–type III transition temperature reported in JET-C experiments. Furthermore, the confinement factor H98(y,2) in type I ELMy H-mode baseline plasmas is generally lower in JET-ILW compared to JET-C at low power fractions Ploss/Pthr,08 < 2 (where Ploss is (Pin − dW/dt), and Pthr,08 the L–H power threshold from Martin et al 2008 (J. Phys. Conf. Ser. 123 012033)). Higher power fractions have thus far not been achieved in the baseline plasmas. At Ploss/Pthr,08 > 2, the confinement in JET-ILW hybrid plasmas is similar to that in JET-C. A reduction in pedestal pressure is the main reason for the reduced confinement in JET-ILW baseline ELMy H-mode plasmas where typically H98(y,2) = 0.8 is obtained, compared to H98(y,2) = 1.0 in JET-C. In JET-ILW hybrid plasmas a similarly reduced pedestal pressure is compensated by an increased peaking of the core pressure profile resulting in H98(y,2) ≤ 1.25. The pedestal stability has significantly changed in high triangularity baseline plasmas where the confinement loss is also most apparent. Applying the same stability analysis for JET-C and JET-ILW, the measured pedestal in JET-ILW is stable with respect to the calculated peeling–ballooning stability limit and the ELM collapse time has increased to 2 ms from typically 200 µs in JET-C. This indicates that changes in the pedestal stability may have contributed to the reduced pedestal confinement in JET-ILW plasmas. A comparison of EPED1 pedestal pressure prediction with JET-ILW experimental data in over 500 JET-C and JET-ILW baseline and hybrid plasmas shows a good agreement with 0.8 < (measured pped)/(predicted pped,EPED) < 1.2, but that the role of triangularity is generally weaker in the JET-ILW experimental data than in the model predictions
    Nuclear Fusion 01/2014; 54:043001. DOI:10.1088/0029-5515/54/4/043001 · 3.24 Impact Factor
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    ABSTRACT: In both JET and ASDEX Upgrade (AUG) the plasma energy confinement has been affected by the presence of a metal wall by the requirement of increased gas fuelling to avoid tungsten pollution of the plasma. In JET with a beryllium/tungsten wall the high triangularity baseline H-mode scenario (i.e. similar to the ITER reference scenario) has been the strongest affected and the benefit of high shaping to give good normalized confinement of H-98 similar to 1 at high Greenwald density fraction of f(GW) similar to 0.8 has not been recovered to date. In AUG with a full tungsten wall, a good normalized confinement H-98 similar to 1 could be achieved in the high triangularity baseline plasmas, albeit at elevated normalized pressure beta(N) > 2. The confinement lost with respect to the carbon devices can be largely recovered by the seeding of nitrogen in both JET and AUG. This suggests that the absence of carbon in JET and AUG with a metal wall may have affected the achievable confinement. Three mechanisms have been tested that could explain the effect of carbon or nitrogen (and the absence thereof) on the plasma confinement. First it has been seen in experiments and by means of nonlinear gyrokinetic simulations (with the GENE code), that nitrogen seeding does not significantly change the core temperature profile peaking and does not affect the critical ion temperature gradient. Secondly, the dilution of the edge ion density by the injection of nitrogen is not sufficient to explain the plasma temperature and pressure rise. For this latter mechanism to explain the confinement improvement with nitrogen seeding, strongly hollow Z(eff) profiles would be required which is not supported by experimental observations. The confinement improvement with nitrogen seeding cannot be explained with these two mechanisms. Thirdly, detailed pedestal structure analysis in JET high triangularity baseline plasmas have shown that the fuelling of either deuterium or nitrogen widens the pressure pedestal. However, in JET-ILW this only leads to a confinement benefit in the case of nitrogen seeding where, as the pedestal widens, the obtained pedestal pressure gradient is conserved. In the case of deuterium fuelling in JET-ILW the pressure gradient is strongly degraded in the fuelling scan leading to no net confinement gain due to the pedestal widening. The pedestal code EPED correctly predicts the pedestal pressure of the unseeded plasmas in JET-ILW within +/- 5%, however it does not capture the complex variation of pedestal width and gradient with fuelling and impurity seeding. Also it does not predict the observed increase of pedestal pressure by nitrogen seeding in JET-ILW. Ideal peeling ballooning MHD stability analysis shows that the widening of the pedestal leads to a down shift of the marginal stability boundary by only 10-20%. However, the variations in the pressure gradient observed in the JET-ILW fuelling experiment is much larger and spans a factor of more than two. As a result the experimental points move from deeply unstable to deeply stable on the stability diagram in a deuterium fuelling scan. In AUG-W nitrogen seeded plasmas, a widening of the pedestal has also been observed, consistent with the JET observations. The absence of carbon can thus affect the pedestal structure, and mainly the achieved pedestal gradient, which can be recovered by seeding nitrogen. The underlying physics mechanism is still under investigation and requires further understanding of the role of impurities on the pedestal stability and pedestal structure formation.
    Plasma Physics and Controlled Fusion 12/2013; 55(12):124043. DOI:10.1088/0741-3335/55/12/124043 · 2.39 Impact Factor
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    ABSTRACT: The aim of the JET ITER-like Wall Project was to provide JET with the plasma facing material combination now selected for the DT phase of ITER (bulk beryllium main chamber limiters and a full tungsten divertor) and, in conjunction with the upgraded neutral beam heating system, to achieve ITER relevant conditions. The design of the bulk Be plasma facing components had to be compatible with increased heating power and pulse length, as well as to reuse the existing tile supports originally designed to cope with disruption loads from carbon based tiles and be installed by remote handling. Risk reduction measures (prototypes, jigs, etc) were implemented to maximize efficiency during the shutdown. However, a large number of clashes with existing components not fully captured by the configuration model occurred. Restarting the plasma on the ITER-like Wall proved much easier than for the carbon wall and no deconditioning by disruptions was observed. Disruptions have been more threatening than expected due to the reduced radiative losses compared to carbon, leaving most of the plasma magnetic energy to be conducted to the wall and requiring routine disruption mitigation. The main chamber power handling has achieved and possibly exceeded the design targets.
    Fusion Engineering and Design 11/2013; 88(6-8). DOI:10.1016/j.fusengdes.2013.01.084 · 1.15 Impact Factor

Publication Stats

2k Citations
470.04 Total Impact Points

Institutions

  • 2013
    • Forschungszentrum Jülich
      • Zentralabteilung für Chemische Analysen (ZCH)
      Jülich, North Rhine-Westphalia, Germany
  • 2002–2013
    • Max Planck Institute for Plasma Physics
      • Max Planck Institute for Plasma Physics, Greifswald
      Arching, Bavaria, Germany
    • Massachusetts Institute of Technology
      Cambridge, Massachusetts, United States
  • 2012
    • Technical University of Lisbon
      • Institute for Plasma Research and Nuclear Fusion (IPFN)
      Lisboa, Lisbon, Portugal
  • 2002–2012
    • Culham Centre for Fusion Energy
      Abingdon-on-Thames, England, United Kingdom
  • 1989–2004
    • General Atomics
      San Diego, California, United States
  • 2003
    • Oak Ridge National Laboratory
      • Fusion Energy Division
      Oak Ridge, Florida, United States
  • 1991–1995
    • University of Toronto
      Toronto, Ontario, Canada