Te/Ti effects on JET energy confinement properties

Lately the question has been raised if a modification of the energy-confinement scaling law with respect to the electron to ion temperature ratio, Te/Ti, is required. Theoretically, like in e.g. the Weiland model, the confinement is thought to degrade with Te/Ti and studies of the hot-ion (Ti>/Te) mode seems to corroborate this. In this paper, it is shown that due to a number of effects that cancel each other out, the energy confinement time remains constant for Te/Ti>~1. The numerical study relies on a series of JET shots specifically designed to reveal an effect of Te/Ti in the hot-electron (Te>Ti) mode. A distinct effort was made to keep all current scaling-law parameters constant, including the total heating power. The effects that provide the constant confinement times have therefore nothing to do with the global properties of the plasma, but are rather due to variations in the temperature gradients which affects the transport locally.Comment: 12th International Congress on Plasma Physics, 25-29 October 2004, Nice (France

MHD-calibrated ELM model in simulations of ITER

12th International Congress on Plasma Physics, 25-29 October 2004, Nice (France)Self-consistent simulations of ITER have been carried out using the JETTO integrated modeling code in which theory motivated models are used for the H-mode pedestal and for the stability conditions that lead to the ELM crashes. Transport is described by combining the anomalous Mixed Bohm/gyro-Bohm model with the NCLASS neoclassical transport model. In the pedestal region, the anomalous transport is suppressed and the neoclassical transport dominates. The steep edge pressure gradient and large bootstrap current can lead to an MHD instability, which results in an ELM crash. In the simulations, an ELM crash can be triggered either by a pressure-driven ballooning mode or by a current-driven peeling mode. The equilibrium and MHD stability analyses codes, HELENA and MISHKA, are used to evaluate the edge stability of the plasma just prior to an ELM crash in order to calibrate and confirm the validity of the stability criteria used to trigger ELMs in the JETTO simulations. The MHD stability analyses include infinite-n ideal ballooning modes, finite-n ballooning modes, and low-n kink/peeling modes. At the reference design point (with 40 MW auxiliary heating), the ion temperature at the top of the pedestal before each ELM crash is determined to be approximately 4 keV, with 19 keV at the plasma center. The alpha heating power is approximately 130 MW, which results in a fusion Q of 15.5. It is observed in the simulations that if the auxiliary heating power is increased, the ion temperature at the top of the pedestal and the central ion temperature remain nearly unchanged. As a consequence, there is nearly the same alpha power production so that the fusion Q decreases with increasing auxiliary heating power. It is also found in these simulations that all of the ELM crashes are triggered by finite-n ballooning modes and that the pedestal shear and pressure gradient indicate access to second stability. Access to second stability allows high pressure gradients within the pedestal and, consequently, the high temperatures at the top of the pedestal that are needed to produce fusion Q values in ITER of 10 or greater

Effect of plasma response on the fast ion losses due to ELM control coils in ITER

Mitigating edge localized modes (ELMs) with resonant magnetic perturbations (RMPs) can increase energetic particle losses and resulting wall loads, which have previously been studied in the vacuum approximation. This paper presents recent results of fusion alpha and NBI ion losses in the ITER baseline scenario modelled with the Monte Carlo orbit following code ASCOT in a realistic magnetic field including the effect of the plasma response. The response was found to reduce alpha particle losses but increase NBI losses, with up to 4.2% of the injected power being lost. Additionally, some of the load in the divertor was found to be shifted away from the target plates toward the divertor dome.</p

Confinement properties of high density impurity seeded ELMy H-mode discharges at low and high triangularity on JET

The design value for ITER is based on operation at n/nGW = 0.85, βn = 1.8 and H98(y, 2) = 1. These values have been routinely achieved in JET in argon seeded ELMy H-mode discharges in different divertor configurations and with different triangularities. Two main scenarios are emerging from the experiments. First, low triangularity (Δu = 0.19) in septum configuration. In this case large D2 fuelling rates lead to confinement degradation towards L-mode. The seeding of Ar during the D2 fuelling phase gives rise to a density close to the Greenwald value. After the switch-off of the D2 gas fuelling ('afterpuff' phase), the confinement recovers to H-mode quality whereas the density stays near the value reached at the end of the main fuelling phase and Zeff stays close to or below 2. Acting on the refuelling of Ar and D2 in the 'afterpuff' phase allows us to improve the stationarity of the high performance phase while maintaining up to the end of the heating phase the good confinement, density and radiation level. Second, high triangularity (δu = 0.45) in vertical target configuration. In this case large fuelling rates do not lead to strong confinement degradation and the D2 fuelling is applied continuously throughout the discharge. A radiated power fraction of up to 70%, H98(y, 2) = 0.9 at βn = 2.1 and n = 1.15nGW - together with the formation of a radiating mantle and moderate Zeff - are achieved in this scenario. Furthermore, there are indications of significantly reduced heat load on the divertor target plates.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Recent progress in L-H transition studies at JET: Tritium, Helium, Hydrogen and Deuterium

We present an overview of results from a series of L-II transition experiments undertaken at JET since the installation of the ITER-like-wall (JET-ILW), with beryllium wall tiles and a tungsten divertor. Tritium, helium and deuterium plasmas have been investigated. Initial results in tritium show ohmic L-H transitions at low density and the power threshold for the L-H transition (P-LH) is lower in tritium plasmas than in deuterium ones at low densities, while we still lack contrasted data to provide a scaling at high densities. In helium plasmas there is a notable shift of the density at which the power threshold is minimum ((n) over bar (e,min)) to higher values relative to deuterium and hydrogen references. Above (n) over bar (e,min) (He) the L-H power threshold at high densities is similar for D and He plasmas. Transport modelling in slab geometry shows that in helium neoclassical transport competes with interchange-driven transport, unlike in hydrogen isotopes. Measurements of the radial electric field in deuterium plasmas show that E-r shear is not a good indicator of proximity to the L-H transition. Transport analysis of ion heat flux in deuterium plasmas show a non-linearity as density is decreased below (n) over bar (e,min). Lastly, a regression of the JET-ILW deuterium data is compared to the 2008 ITPA scaling law