| 1. |
- Hatch, D. R., et al.
(författare)
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Microtearing modes as the source of magnetic fluctuations in the JET pedestal
- 2021
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 61:3
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Tidskriftsartikel (refereegranskat)abstract
- We report on a detailed study of magnetic fluctuations in the JET pedestal, employing basic theoretical considerations, gyrokinetic simulations, and experimental fluctuation data to establish the physical basis for their origin, role, and distinctive characteristics. We demonstrate quantitative agreement between gyrokinetic simulations of microtearing modes (MTMs) and two magnetic frequency bands with corresponding toroidal mode numbers n = 4 and 8. Such disparate fluctuation scales, with substantial gaps between toroidal mode numbers, are commonly observed in pedestal fluctuations. Here we provide a clear explanation, namely the alignment of the relevant rational surfaces (and not others) with the peak in the omega(*) profile, which is localized in the steep gradient region of the pedestal. We demonstrate that a global treatment is required to capture this effect. Nonlinear simulations suggest that the MTM fluctuations produce experimentally-relevant transport levels and saturate by relaxing the background electron temperature gradient, slightly downshifting the fluctuation frequencies from the linear predictions. Scans in collisionality are compared with a simple MTM dispersion relation. At the experimental points considered, MTM growth rates can either increase or decrease with collision frequency depending on the parameters thus defying any simple characterization of collisionality dependence.
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| 2. |
- Frassinetti, Lorenzo, et al.
(författare)
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Role of the separatrix density in the pedestal performance in deuterium low triangularity JET-ILW plasmas and comparison with JET-C
- 2021
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Ingår i: Nuclear Fusion. - : IOP Publishing Ltd. - 0029-5515 .- 1741-4326. ; 61:12
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Tidskriftsartikel (refereegranskat)abstract
- A reduction of the pedestal pressure with increasing separatrix density over pedestal density (n (e) (sep)/n (e) (ped)) has been observed in JET. The physics behind this correlation is investigated. The correlation is due to two distinct mechanisms. The increase of n (e) (sep)/n (e) (ped) till approximate to 0.4 shifts the pedestal pressure radially outwards, decreasing the peeling-balloning stability and reducing the pressure height. The effect of the position saturates above n (e) (sep)/n (e) (ped) approximate to 0.4. For higher values, the reduction of the pedestal pressure is ascribed to increased turbulent transport and, likely, to resistive MHD effects. The increase of n (e) (sep)/n (e) (ped) above approximate to 0.4 reduces backward difference n (e) /n (e), increasing eta (e) and the pedestal turbulent transport. This reduces the pressure gradient and the pedestal temperature, producing an increase in the pedestal resistivity. The work suggests that the increase in resistivity might destabilize resistive balloning modes, further reducing the pedestal stability.
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| 3. |
- Frassinetti, Lorenzo, et al.
(författare)
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Pedestal structure, stability and scalings in JET-ILW : the EUROfusion JET-ILW pedestal database
- 2021
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 61:1
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Tidskriftsartikel (refereegranskat)abstract
- The EUROfusion JET-ILW pedestal database is described, with emphasis on three main issues. First, the technical aspects are introduced, including a description of the data selection, the datasets, the diagnostics used, the experimental and theoretical methods implemented and the main definitions. Second, the JET-ILW pedestal structure and stability are described. In particular, the work describes the links between the engineering parameters (power, gas and divertor configuration) and the disagreement with the peeling-ballooning (PB) model implemented with ideal magnetohydrodynamics equations. Specifically, the work clarifies why the JET-ILW pedestal tends to be far from the PB boundary at high gas and high power, showing that a universal threshold in power and gas cannot be found but that the relative shift (the distance between the position of the pedestal density and of the pedestal temperature) plays a key role. These links are then used to achieve an empirical explanation of the behavior of the JET-ILW pedestal pressure with gas, power and divertor configuration. Third, the pedestal database is used to revise the scaling law of the pedestal stored energy. The work shows a reasonable agreement with the earlier Cordey scaling in terms of plasma current and triangularity dependence, but highlights some differences in terms of power and isotope mass dependence.
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| 4. |
- Stefániková, Estera, et al.
(författare)
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Change in the pedestal stability between JET-C and JET-ILW low triangularity peeling-ballooning limited plasmas
- 2021
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Ingår i: Nuclear Fusion. - : Institute of Physics (IOP). - 0029-5515 .- 1741-4326. ; 61:2
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Tidskriftsartikel (refereegranskat)abstract
- This work discusses the possible mechanisms that have led to the degradation of the pedestal pressure height and pedestal stability of low triangularity peeling-ballooning (PB) limited pla-smas from JET with the carbon wall (JET-C) to JET with the ITER-like wall (JET-ILW). The work shows that the lower pedestal stability of JET-ILW is not directly caused by the presence of a metal wall, but rather due to the operational constraints that require operation with high gas fuelling. Recent results have suggested the possible role of the distance between the density and temperature pedestal positions (n(e)(pos) - T-e(pos), also called the 'relative shift') with increasing gas fuelling rate on the PB stability of JET-ILW baseline plasmas (Stefanikova et al 2018 Nucl. Fusion 58 056010). The work further extends the analysis of the role of the relative shift, showing that it plays an important role in the difference between the pedestal performance of JET-C and JET-ILW. Moreover, the work also shows that the pedestal density n(e)(ped), Z(eff), and pedestal pressure width w(pe) play an important role in this difference. The pedestal structure and stability have been studied both experimentally and by modelling. The modelling shows that the changes in n(e)(pos) - T-e(pos), n(e)(ped), Z(eff), and w(pe) are sufficient to explain the differences in the pedestal performance between JET-C and JET-ILW PB limited discharges. A hypothesis describing the possible mechanisms leading to the degradation of the pedestal pressure and stability from JET-C to JET-ILW in PB limited plasmas is put forward.
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