M. Romanelli

Tungsten transport in JET H-mode plasmas in hybrid scenario, experimental observations and modelling

The behaviour of tungsten in the core of hybrid scenario plasmas in JET with the ITER-like wall is analysed and modelled with a combination of neoclassical and gyrokinetic codes. In these discharges, good confinement conditions can be maintained only for the first 2?3?s of the high power phase. Later W accumulation is regularly observed, often accompanied by the onset of magneto-hydrodynamical activity, in particular neoclassical tearing modes (NTMs), both of which have detrimental effects on the global energy confinement. The dynamics of the accumulation process is examined, taking into consideration the concurrent evolution of the background plasma profiles, and the possible onset of NTMs. Two time slices of a representative discharge, before and during the accumulation process, are analysed with two independent methods, in order to reconstruct the W density distribution over the poloidal cross-section. The same time slices are modelled, computing both neoclassical and turbulent transport components and consistently including the impact of centrifugal effects, which can be significant in these plasmas, and strongly enhance W neoclassical transport. The modelling closely reproduces the observations and identifies inward neoclassical convection due to the density peaking of the bulk plasma in the central region as the main cause of the accumulation. The change in W neoclassical convection is directly produced by the transient behaviour of the main plasma density profile, which is hollow in the central region in the initial part of the high power phase of the discharge, but which develops a significant density peaking very close to the magnetic axis in the later phase. The analysis of a large set of discharges provides clear indications that this effect is generic in this scenario. The unfavourable impact of the onset of NTMs on the W behaviour, observed in several discharges, is suggested to be a consequence of a detrimental combination of the effects of neoclassical transport and of the appearance of an island.

L to H mode transition: on the role of Zeff

In this paper, the nature of the primary instability present in the pedestal forming region prior to the transition into H mode is analysed using a gyrokinetic code on JET-ILW profiles. The linear analysis shows that the primary instability is of resistive nature, and can therefore be stabilized by increased temperature, hence power. The unstable modes are identified as being resistive ballooning modes. Their growth rates decrease for temperatures increasing towards the experimentally measured temperature at the L–H transition. The growth rates are larger for lower effective charge Zeff. This dependence is shown to be in qualitative agreement with recent and past experimental observations of reduced Zeff associated with lower L–H power thresholds.

Modelling of plasma performance and transient density behaviour in the H-mode access for ITER gas fuelled scenarios

ITER operations require effective fuelling of the core plasma for conditions in which neutral dynamics through the scrape-off layer is expected to significantly affect the efficiency of gas penetration. On the basis of the previous analysis for stationary conditions, pellets are foreseen to provide core fuelling of high-Q DT scenarios. In this paper we present a modelling study of the gas fuelling efficiency in ITER providing an estimate of the maximum plasma density achievable with gas fuelling only in various DT reference scenarios. Dynamical integrated core-edge plasma simulations for various phases of ITER DT H-mode discharges have been carried out with the JINTRAC suite of codes. Simulations of the L-mode phase show that divertor detachment sets the maximum density achievable at the separatrix by deuterium-tritium gas fuelling. The maximum volume-average density is achieved for 15 MA/5.3 T and it is close to the requirement for stationary application of neutral beam injection heating at full power (16.5 MW per injector) and ion energy (1 MeV) compatible with acceptable shine-through loads on the first wall. The achievable density in gas fuelled H-modes is typically a factor of 2–3 larger than in L-modes. The fusion performance of gas fuelled H-modes at 15 MA is typically found to be moderately high (Q ~ 6–8). Sensitivity of the above predictions to modelling assumptions and validation of the physics models are discussed.

Gas analyses of the first complete JET cryopump regeneration with ITER-like wall

Analytical results of a complete JET cryopump regeneration, including the nitrogen panel, following the first ITER-like wall campaign are presented along with the in situ analyses of residual gas. H/D mixtures and impurities such as nitrogen and neon were injected during plasma operation in the vessel to study radiation cooling in the scrape-off-layer and divertor region. The global gas inventory over the campaign is incomplete, suggesting residual volatile impurities are remaining on the cryogenic panel. This paper presents results on (i) residual deuterium on the panel which is very loosely related to the campaign, (ii) impurities like nitrogen which stick on the panel, and (iii) the ammonia production which can be observed by mass spectrometry.

Effect of tungsten off-axis accumulation on neutral beam deposition in JET rotating plasmas

Evidence for low field side accumulation of tungsten is often observed in bolometry and soft x-ray emissivities of highly rotating JET ITER-like wall (ILW) plasmas. Poloidal variation of the density of high-Z impurities, such as tungsten, in the core of NBI heated plasmas is expected from neoclassical theory due to charge displacement and parallel electric field generated by the centrifugal force. We calculate the poloidally asymmetric distribution of tungsten using fluid equations?and a 1D transport simulation with the JETTO/SANCO code. Peaking of tungsten on the outboard side of the plasma is found and verified with soft x-ray and bolometry measurements. We then study the effect of a poloidally asymmetric tungsten distribution on the distribution of the NBI heat source by simulations with the Monte Carlo code ASCOT. The simulations show that the poloidally asymmetric tungsten profile redistributes the fast NBI ions radially through shifting their ionization profile and poloidally through enhanced pitch-angle scattering at high energy. The amplitude of the redistribution is in the order of 10% for the highest nW/ne ratios of??10?4 measured in recent JET H-mode plasmas. As a result of the scattering of the beam particles, the core heat deposition is changed less than 10%, which does not have a significant impact to the performance of JET plasmas. The modelling is in qualitative agreement with measurements by the vertical neutron camera that sees a broadening in the 2.5?MeV neutron profile when tungsten peaks on the outboard side of the plasma.

Dimensionless scalings of confinement, heat transport and pedestal stability in JET-ILW and comparison with JET-C

Three dimensionless scans in the normalized Larmor radius rho*, normalized collisionality nu* and normalized plasma pressure beta have been performed in JET with the ITER-like wall (JET-ILW). The n ...

A novel flexible field-aligned coordinate system for tokamak edge plasma simulation

Abstract Tokamak plasmas are confined by a magnetic field that limits the particle and heat transport perpendicular to the field. Parallel to the field the ionised particles can move freely, so to obtain confinement the field lines are “closed” (i.e. form closed surfaces of constant poloidal flux) in the core of a tokamak. Towards, the edge, however, the field lines intersect physical surfaces, leading to interaction between neutral and ionised particles, and the potential melting of the material surface. Simulation of this interaction is important for predicting the performance and lifetime of future tokamak devices such as ITER. Field-aligned coordinates are commonly used in the simulation of tokamak plasmas due to the geometry and magnetic topology of the system. However, these coordinates are limited in the geometry they allow in the poloidal plane due to orthogonality requirements. A novel 3D coordinate system is proposed herein that relaxes this constraint so that any arbitrary, smoothly varying geometry can be matched in the poloidal plane while maintaining a field-aligned coordinate. This system is implemented in BOUT++ and tested for accuracy using the method of manufactured solutions. A MAST edge cross-section is simulated using a fluid plasma model and the results show expected behaviour for density, temperature, and velocity. Finally, simulations of an isolated divertor leg are conducted with and without neutrals to demonstrate the ion-neutral interaction near the divertor plate and the corresponding beneficial decrease in plasma temperature.

On the validity of drift-reduced fluid models for tokamak plasma simulation

Drift-reduced plasma fluid models are commonly used in plasma physics for analytic studies and simulations, so the validity of such models must be verified for the regions of parameter space in which tokamak plasmas exist. By deriving and comparing the linear dispersion relations for the drift-wave instability for both a drift-reduced model and a full-velocity model, the importance of the physics lost with the drift-reduction is examined. This analysis is generalised for typical tokamak parameter spaces and is then applied directly to JET data. It is found that drift-reduced models are generally more applicable to the edge plasma ( 30% error) particularly at mid-radius. The effect of drift-wave mode number and wavelength also play a key role in determining the accuracy of drift-reduced models.

EFD-C(13)03/37 Modelling of the Effect of the ITER-like Wall on NBI Heating in JET

T. Koskela1, O. Asunta1, P. Belo4, M. O’Mullane3, M. Romanelli2, S. Sipilä1 and JET-EFDA contributors∗ JET-EFDA, Culham Science Centre, Abingdon, OX14 3DB, UK 1 Aalto University School of Science, PO Box 14100, FI-00076 AALTO, Finland 2 Association Euratom-CCFE, Culham Science Centre, Abingdon, Oxon. OX14 3DB, UK 3 Department of Physics, University of Strathclyde, Glasgow G4 0NG, UK 4 Associacao EURATOM/IST, IPFN Laboratorio Associado, IST,P-1049-001 Lisboa, Portugal