The effect of a metal wall on confinement in JET and ASDEX Upgrade

Abstract

In both JET and ASDEX Upgrade (AUG) the plasma energy confinement has been affected by the presence of a metal wall by the requirement of increased gas-fuelling to avoid tungsten pollution of the plasma. In JET with a beryllium/tungsten wall the high triangularity baseline H-mode scenario (i.e. similar to the ITER reference scenario) has been strongest affected and the benefit of high shaping to give good normalised confinement of H 98~1 at high Greenwald density fraction of f GW ~0.8 has disappeared. In AUG with a full tungsten wall a good normalised confinement H 98~1 could be achieved in the high triangularity baseline plasmas, albeit at elevated normalised pressure β N >2. The confinement lost with respect to the carbon devices can be largely recovered by the seeding of nitrogen in both JET and AUG. This suggests that the absence of carbon in JET and AUG with a metal wall may have affected the achievable confinement. Three mechanisms have been tested that could explain the effect of carbon or nitrogen (and the absence thereof) on the plasma confinement. First it has been seen in experiments and by means of non-linear gyrokinetic simulations (with the GENE code), that nitrogen seeding does not significantly change the core temperature profile peaking and does not affect the critical ion temperature gradient. Secondly, the dilution of the edge ion density by the injection of nitrogen is not sufficient to explain the plasma temperature and pressure rise. For this latter mechanism to explain the confinement improvement with nitrogen seeding, strongly hollow Z eff profiles would be required, which is not supported by experimental observations. The confinement improvement with nitrogen seeding cannot be explained with these two mechanisms. Thirdly, detailed pedestal structure analysis in JET high triangularity baseline plasmas have shown that fuelling of either deuterium or nitrogen widens the pressure pedestal. However, this only leads to a confinement benefit in the case of nitrogen seeding where as the pedestal widens, the obtained pedestal pressure gradient is conserved. In the case of deuterium fuelling the pressure gradient is strongly degraded in the fuelling scan leading to no net confinement gain due to the pedestal widening. The pedestal code EPED correctly predicts the pedestal pressure of the unseeded plasmas within +/-5%, however it does not capture the complex variation of pedestal width and gradient with fuelling and impurity seeding. Also it does not predict the observed increase of pedestal pressure by nitrogen seeding. Ideal peeling ballooning MHD stability analysis shows that the widening of the pedestal leads to a down shift of the marginal stability boundary by only 10-20%. However the variations in the pressure gradient observed in the experiment is much larger and spans a factor of more than two. As a result the experimental points move from deeply unstable to deeply stable on the stability diagram in a deuterium fuelling scan. In AUG-W nitrogen seeded plasmas, also a widening of the pedestal...

Progress in preparing scenarios for operation of the International Thermonuclear Experimental Reactor

“…Compared with results with a C-dominated wall, the operation with the full-W wall in AUG is restricted to higher densities f GW > 0.75 and confinement is on average reduced by 5%-10%. 38 Thus, the lowest H-factors obtained in AUG can be attributed to high gas puff levels required to avoid W-accumulation. For the extrapolation to ITER however, the results of AUG-W with an intact, fresh boronisation and thus higher H 98(y,2) are most relevant as the boron-coated main chamber walls in AUG-W together with the tungsten divertor (typically not affected during a boronisation process) simulate more closely the situation in ITER of a Be main-chamber-wall and a tungsten divertor.…”

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

“…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.…”

“…A multi-machine review [Beurskens 2013] of three possible mechanisms which could account for the changes in performance observed on JET and AUG concluded that the improvement in performance is not due to an improvement in core confinement nor can it be accounted for due to ion dilution. Instead it is most likely the change in pedestal structure results in the improvement in global performance.…”

“…More specifically, with increasing Nitrogen seeding for JET-ILW plasmas the pedestal widens and the peak gradient increases both contributing towards an increase pedestal pressure. Furthermore [Beurskens 2013] compares the measurements to preliminary results from the predictive pedestal structure model, EPED.…”

“…In [Beurskens 2013] only the pressure pedestal structure is discussed for high triangularity JET plasmas. This paper extends the JET-ILW pedestal analysis by considering a wider dataset of fuelling and seeding plasmas, incorporating more high triangularity JET-ILW plasmas as well as including low triangularity JET-ILW plasmas.…”