Contrasting H-mode behaviour with deuterium fuelling and nitrogen seeding in the all-carbon and metallic versions of JET

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 ≈ 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 normalised energy confinement is ≈ 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 unrecognised 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 recurrence effects of N in both wall environments, more surprisingly, average power ejected by ELMs and estimated scrape-off-layer pressure ratio during the approach to outboard detachment also both decline in the same proportion for the ILW. Finally, inter-ELM W sources in the ILW divertor tend to fall with N input, although core accumulation possibly due to net ELM erosion 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. ‡ That is, contours are interpolated between the torus-pressure values for each all-C and ILW shot at its position in the normalised confinement versus density plane.

The H-mode pedestal structure and its role on confinement in JET with a carbon and metal wall

“…The triangularity () ranges between  0.22-0.42, see Table 1. These plasmas are predominantly from the JET fuelling and seeding experiment before and after the installation of the Be/W ILW [Maddison 2011, Giroud 2012, Giroud-IAEA-2012, Giroud 2013, Maddison 2014. The primary aim of this experiment is to develop a radiative scenario with the introduction of an impurity to mitigate divertor heat loads.…”

“…After the installation of the Be/W ILW wall Z eff decreases from  2.0 to 1.2 confirming the dominant impurity for JET-ILW plasmas is beryllium as opposed to carbon [Bresinsek 2013]. Furthermore with the introduction of nitrogen for JET-ILW plasmas Z eff increases from  1.2 up to 1.8 [Giroud 2013, Maddison 2014. The EPED1 predictions presented in [Leyland 2013] and the most recent EPED1 runs for the low triangularity JET-ILW plasmas presented in this study use bremsstrahlung line-integral measurements of Z eff .…”

“…Measurements from recent JET campaigns with the ILW offer an invaluable opportunity to investigate how the pedestal structure changes with the presence of a metallic wall and its role on confinement. This paper presents a database consisting of deuterium fuelled and nitrogen seeded Type I ELMy H-mode plasmas on JET with the ILW (JET-ILW) [Maddison 2011, Giroud-IAEA-2012, Giroud 2013, Beurskens 2013, Beurskens 2014, Maddison 2014 with the focus on quantifying the pedestal structure. The pedestal width, gradient and height is determined by fitting a modified hyperbolic tangent (mtanh) function [Groebner 1998] to JET High Resolution Thomson Scattering (HRTS) radial profiles of electron temperature and density [Pasqualotto 2004, Frassinetti 2012, Scannell 2011.…”

“…This can be attributed to a degraded pressure pedestal height [Giroud 2013, Maddison 2014, Joffrin 2014. However, with nitrogen seeding the pressure pedestal height and consequently global performance for JET-ILW plasmas can be partially recovered [Giroud-IAEA-2012, Giroud 2013, Maddison 2014.…”

Nitrogen retention mechanisms in tokamaks with beryllium and tungsten plasma-facing surfaces

“…Impurity seeding is used in tokamaks with metallic plasma-facing materials with the aim to increase the radiated power fraction and mitigate otherwise excessive heat loads to the divertor targets [1]. Nitrogen is currently the preferred seeding species at JET and AUG because of its favorable radiative characteristics [2], the benevolent control behavior [3] and the observed beneficial effect on energy confinement [4,5]. However, nitrogen radicals are chemically reactive and can influence properties of the plasma facing surfaces and because of that affect the hydrogen (tritium) inventory build-up in the reactor vessel.…”