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

Abstract

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.

MHD marking using the MSE polarimeter optics in ILW JET plasmas

“…The first example is in hybrid development experiments. A very common feature of those plasmas, is the occurrence of an internal kink-like n = 1, m = 1 in its initial phase, which shortly develops to tearing activity, as is the case of shot #82722 [4]. The behavior of this MHD is closely related with impurity peaking at the plasma center, mainly tungsten in JET ILW plasmas.…”

“…It is important to note, that those spectrograms does not show the full MHD pattern, as one could observe on Mirnov coil data spectrograms for instance. In JET hybrid or baseline ILW operational scenarios, it is often found n > 1 tearing activity, which can degrade plasma performance and is closely related with high Z impurities, accumulated at the plasma center [4], [5]. An example of those observations is depicted in figure 1, in JET hybrid plasma regimes for JET pulse #83533 (PNBI = 22.5 MW, IP = 2.0 MA, BT = 2.3 T), where a n = 1 MHD can be observed on spectrograms from MSE channel 17 (figure 1a), with plasma location close to 3.128 m, and Mirnov coil data ( figure 1 b).…”

Modelling of Tungsten Behavior in Plasma of T-10 Tokamak

“…Интерес к вольфраму как примеси связан прежде всего с перспективами его использования в качестве материала ди-верторных пластин ИТЭР. В работах [1][2][3] на других уста-новках отмечена аккумуляция вольфрама в центре плазмы, что с учётом его высокой излучательной способности соз-даёт серьёзную проблему, связанную с аномальными ра-диационными потерями на ионах вольфрама из центра шнура.…”

Line Spectrum of Tungsten Ions at Low Ionization Stages in Large Helical Device in Wavelength Range of 300 - 2400 Å Measured Using 20 cm Normal Incidence VUV Spectrometers

“…Considering transport processes of tungsten impurity in ITER, first, neutral tungsten atoms are released from the divertor plates, then tungsten ions at lower ionization stages are transported in the edge plasmas, and finally tungsten ions at higher ionization stages are distributed in the confinement region. In H-mode plasmas in tungsten-wall machines, it has been frequently observed that highly ionized tungsten ions are accumulated in the plasma core region with a high concentration, which results in a large radiation loss and a deterioration of the energy confinement property [4,5]. Therefore, diagnostics for tungsten impurity ions in magneticallyconfined high-temperature plasmas have been intensively conducted, such as visible spectroscopy for neutral tungsten atoms in the wavelength range around 4000 Å and extreme ultraviolet (EUV) spectroscopy for highly-ionized tungsten ions in the wavelength range around 15 -70 Å [6].…”