| 1. |
- Abel, I, et al.
(author)
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Overview of the JET results with the ITER-like wall
- 2013
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In: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 53:10, s. 104002-
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Journal article (peer-reviewed)abstract
- Following the completion in May 2011 of the shutdown for the installation of the beryllium wall and the tungsten divertor, the first set of JET campaigns have addressed the investigation of the retention properties and the development of operational scenarios with the new plasma-facing materials. The large reduction in the carbon content (more than a factor ten) led to a much lower Z(eff) (1.2-1.4) during L- and H-mode plasmas, and radiation during the burn-through phase of the plasma initiation with the consequence that breakdown failures are almost absent. Gas balance experiments have shown that the fuel retention rate with the new wall is substantially reduced with respect to the C wall. The re-establishment of the baseline H-mode and hybrid scenarios compatible with the new wall has required an optimization of the control of metallic impurity sources and heat loads. Stable type-I ELMy H-mode regimes with H-98,H-y2 close to 1 and beta(N) similar to 1.6 have been achieved using gas injection. ELM frequency is a key factor for the control of the metallic impurity accumulation. Pedestal temperatures tend to be lower with the new wall, leading to reduced confinement, but nitrogen seeding restores high pedestal temperatures and confinement. Compared with the carbon wall, major disruptions with the new wall show a lower radiated power and a slower current quench. The higher heat loads on Be wall plasma-facing components due to lower radiation made the routine use of massive gas injection for disruption mitigation essential.
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| 2. |
- Romanelli, F, et al.
(author)
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Overview of the JET results
- 2011
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In: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 51:9
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Journal article (peer-reviewed)abstract
- Since the last IAEA Conference JET has been in operation for one year with a programmatic focus on the qualification of ITER operating scenarios, the consolidation of ITER design choices and preparation for plasma operation with the ITER-like wall presently being installed in JET. Good progress has been achieved, including stationary ELMy H-mode operation at 4.5 MA. The high confinement hybrid scenario has been extended to high triangularity, lower ρ*and to pulse lengths comparable to the resistive time. The steady-state scenario has also been extended to lower ρ*and ν*and optimized to simultaneously achieve, under stationary conditions, ITER-like values of all other relevant normalized parameters. A dedicated helium campaign has allowed key aspects of plasma control and H-mode operation for the ITER non-activated phase to be evaluated. Effective sawtooth control by fast ions has been demonstrated with3He minority ICRH, a scenario with negligible minority current drive. Edge localized mode (ELM) control studies using external n = 1 and n = 2 perturbation fields have found a resonance effect in ELM frequency for specific q95values. Complete ELM suppression has, however, not been observed, even with an edge Chirikov parameter larger than 1. Pellet ELM pacing has been demonstrated and the minimum pellet size needed to trigger an ELM has been estimated. For both natural and mitigated ELMs a broadening of the divertor ELM-wetted area with increasing ELM size has been found. In disruption studies with massive gas injection up to 50% of the thermal energy could be radiated before, and 20% during, the thermal quench. Halo currents could be reduced by 60% and, using argon/deuterium and neon/deuterium gas mixtures, runaway electron generation could be avoided. Most objectives of the ITER-like ICRH antenna have been demonstrated; matching with closely packed straps, ELM resilience, scattering matrix arc detection and operation at high power density (6.2 MW m-2) and antenna strap voltages (42 kV). Coupling measurements are in very good agreement with TOPICA modelling. © 2011 IAEA, Vienna.
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| 3. |
- Giroud, C., et al.
(author)
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Integration of a radiative divertor for heat load control into JET high triangularity ELMy H-mode plasmas
- 2012
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In: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 52:6, s. 063022-
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Journal article (peer-reviewed)abstract
- Experiments on JET with a carbon-fibre composite wall have explored the reduction of steady-state power load in an ELMy H-mode scenario at high Greenwald fraction similar to 0.8, constant power and close to the L to H transition. This paper reports a systematic study of power load reduction due to the effect of fuelling in combination with seeding over a wide range of pedestal density ((4-8) x 10(19) m(-3)) with detailed documentation of divertor, pedestal and main plasma conditions, as well as a comparative study of two extrinsic impurity nitrogen and neon. It also reports the impact of steady-state power load reduction on the overall plasma behaviour, as well as possible control parameters to increase fuel purity. Conditions from attached to fully detached divertor were obtained during this study. These experiments provide reference plasmas for comparison with a future JET Be first wall and an all W divertor where the power load reduction is mandatory for operation.
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| 4. |
- Maddison, G. P., et al.
(author)
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Contrasting H-mode behaviour with deuterium fuelling and nitrogen seeding in the all-carbon and metallic versions of JET
- 2014
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In: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 54:7, s. 073016-
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Journal article (peer-reviewed)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. Attention is focused upon a common high-triangularity, single-null divertor configuration at 2.5 MA, q(95) approximate to 3.5 yielding the most robust all-C performance. Contrasting results between the alternative linings are found firstly in unseeded plasmas, for which purity is improved and intrinsic radiation reduced in the ITER-like wall (ILW) but normalized energy confinement is approximate to 30% lower than in all-C counterparts, owing to a commensurately lower (electron) pedestal temperature. Divertor recycling is also radically altered, with slower, inboard-outboard asymmetric transients at ELMs and spontaneous oscillations in between them. Secondly, nitrogen seeding elicits opposite responses in the ILW to all-C experience, tending to raise plasma density, reduce ELM frequency, and above all to recover (electron) pedestal pressure, hence global confinement, almost back to previous levels. A hitherto unrecognized role of light impurities in pedestal stability and dynamics is consequently suggested. Thirdly, while heat loads on the divertor outboard target between ELMs are successfully reduced in proportion to the radiative cooling and ELM frequency effects of N in both wall environments, more surprisingly, average power ejected by ELMs also declines in the same proportion for the ILW. Detachment between transients is simultaneously promoted. Finally, inter-ELM W sources in the ILW divertor tend to fall with N input, although core accumulation possibly due to increased particle confinement still leads to significantly less steady conditions than in all-C plasmas. This limitation of ILW H-modes so far will be readdressed in future campaigns to continue progress towards a fully integrated scenario suitable for D-T experiments on JET and for 'baseline' operation on ITER. The diverse changes in behaviour between all-C and ILW contexts demonstrate essentially the strong impact which boundary conditions and intrinsic impurities can have on tokamak-plasma states.
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| 5. |
- Solano, E. R., et al.
(author)
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Observation of Confined Current Ribbon in JET Plasmas
- 2010
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In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 104:18, s. 185003-
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Journal article (peer-reviewed)abstract
- We report the identification of a localized current structure inside the JET plasma. It is a field-aligned closed helical ribbon, carrying current in the same direction as the background current profile (cocurrent), rotating toroidally with the ion velocity (corotating). It appears to be located at a flat spot in the plasma pressure profile, at the top of the pedestal. The structure appears spontaneously in low density, high rotation plasmas, and can last up to 1.4 s, a time comparable to a local resistive time. It considerably delays the appearance of the first edge localized mode.
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| 6. |
- Giroud, C., et al.
(author)
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Impact of nitrogen seeding on confinement and power load control of a high-triangularity JET ELMy H-mode plasma with a metal wall
- 2013
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In: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 53:11, s. 113025-
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Journal article (peer-reviewed)abstract
- This paper reports the impact on confinement and power load of the high-shape 2.5MA ELMy H-mode scenario at JET of a change from all carbon plasma-facing components to an all metal wall. In preparation to this change, systematic studies of power load reduction and impact on confinement as a result of fuelling in combination with nitrogen seeding were carried out in JET-C and are compared with their counterpart in JET with a metallic wall. An unexpected and significant change is reported on the decrease in the pedestal confinement but is partially recovered with the injection of nitrogen.
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| 7. |
- Pamela, S. J. P., et al.
(author)
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Simulation of ELMs in JET
- 2010
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In: 37th EPS Conference on Plasma Physics 2010, EPS 2010. - 9781622763313 ; , s. 53-56
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Conference paper (peer-reviewed)
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| 8. |
- Schlenk, T., et al.
(author)
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Controllable Magnetic Doping of the Surface State of a Topological Insulator
- 2013
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In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 110:12, s. 126804-
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Journal article (peer-reviewed)abstract
- A combined experimental and theoretical study of doping individual Fe atoms into Bi2Se3 is presented. It is shown through a scanning tunneling microscopy study that single Fe atoms initially located at hollow sites on top of the surface (adatoms) can be incorporated into subsurface layers by thermally activated diffusion. Angle-resolved photoemission spectroscopy in combination with ab initio calculations suggest that the doping behavior changes from electron donation for the Fe adatom to neutral or electron acceptance for Fe incorporated into substitutional Bi sites. According to first principles calculations within density functional theory, these Fe substitutional impurities retain a large magnetic moment, thus presenting an alternative scheme for magnetically doping the topological surface state. For both types of Fe doping, we see no indication of a gap at the Dirac point. DOI: 10.1103/PhysRevLett.110.126804
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| 9. |
- Liang, Y., et al.
(author)
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Mitigation of type-I ELMs with n=2 fields on JET with ITER-like wall
- 2013
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In: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 53:7, s. 073036-
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Journal article (peer-reviewed)abstract
- Mitigation of type-I edge-localized modes (ELMs) was observed with the application of an n = 2 field in H-mode plasmas on the JET tokamak with the ITER-like wall (ILW). Several new findings with the ILW were identified and contrasted to the previous carbon wall (C-wall) results for comparable conditions. Previous results for high collisionality plasmas (nu*(e,ped) similar to 2.0) with the C-wall saw little or no influence of either n = 1 or n = 2 fields on the ELMs. However, recent observations with the ILW show large type-I ELMs with a frequency of similar to 45 Hz were replaced by high-frequency (similar to 200 Hz) small ELMs during the application of the n = 2 field. With the ILW, splitting of the outer strike point was observed for the first time during the strong mitigation of the type-I ELMs. The maximal surface temperature (T-max) on the outer divertor plate reached a stationary state and has only small variations of a few degrees due to the small mitigated ELMs. In moderate collisionality (nu*(e,ped) similar to 0.8) H-mode plasmas, similar to previous results with the C-wall, both an increase in the ELM frequency and density pump-out were observed during the application of the n = 2 field. There are two new observations compared with the C-wall results. Firstly, the effect of ELM mitigation with the n = 2 field was seen to saturate so that the ELM frequency did not further increase above a certain level of n = 2 magnetic perturbations. Secondly splitting of the outer strike point during the ELM crash was seen, resulting in mitigation of the maximal ELM peak heat fluxes on the divertor region.
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| 10. |
- Versloot, T W, et al.
(author)
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Comparison between dominant NB and dominant IC heated ELMy H-mode discharges in JET
- 2011
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In: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 51:10, s. 103033-
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Journal article (peer-reviewed)abstract
- The experiment described in this paper is aimed at characterization of ELMy H-mode discharges with varying momentum input, rotation, power deposition profiles and ion to electron heating ratio obtained by varying the proportion between ion cyclotron (IC) and neutral beam (NB) heating. The motivation for the experiment was to verify if the basic confinement and transport properties of the baseline ITER H-mode are robust to these changes, and similar to those derived mostly from dominant NB heated H-modes. No significant difference in the density and temperature profiles or in the global confinement were found. Although ion temperature profiles were seen to be globally stiff, some variation of stiffness was obtained in the experiment by varying the deposition profiles, but not one that could significantly affect the profiles in terms of global confinement. This analysis shows the thermal plasma energy confinement enhancement factor to be independent of the heating mix, for the range of conditions explored. Moreover, the response of the global confinement to changes in density and power were also independent of heating mix, reflecting the changes in the pedestal, which is in agreement with globally stiff profiles. Consistently, the pedestal characteristics (pressure and width) and their dependences on global parameters such as density and power were the same during NB only or with predominant IC heating.
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