H. Urano

ELM pace making and mitigation by pellet injection in ASDEX upgrade

In ASDEX Upgrade, experimental efforts aim to establish pace making and mitigation of type-I edge localized modes (ELMs) in high confinement mode (H-mode) discharges. Injection of small size cryogenic deuterium pellets (~(1.4?mm)2 ? 0.2?mm ? 2.5 ? 1019?D) at rates up to 83?Hz imposed persisting ELM control without significant fuelling, enabling for investigations well inside the type-I ELM regime. The approach turned out to meet all required operational features. ELM pace making was realized with the driving frequency ranging from 1 to 2.8 times the intrinsic ELM frequency, the upper boundary set by hardware limits. ELM frequency enhancement by pellet pace making causes much less confinement reduction than by engineering means like heating, gas bleeding or plasma shaping. Confinement reduction is observed in contrast to the typical for engineering parameters. Matched discharges showed triggered ELMs ameliorated with respect to intrinsic counterparts while their frequency was increased. No significant differences were found in the ELM dynamics with the available spatial and temporal resolution. By breaking the close correlation of ELM frequency and plasma parameters, pace making allows the establishment of fELM as a free parameter giving enhanced operational headroom for tailoring H-mode scenarios with acceptable ELMs. Use was made of the pellet pace making tool in several successful applications in different scenarios. It seems that further reduction of the pellet mass could be possible, eventually resulting in less confinement reduction as well.

Neoclassical tearing mode control using electron cyclotron current drive and magnetic island evolution in JT-60U

The results of stabilizing neoclassical tearing modes (NTMs) with electron cyclotron current drive (ECCD) in JT-60U are described with emphasis on the effectiveness of the stabilization. The range of the minimum EC wave power needed for complete stabilization of an m/n = 2/1 NTM was experimentally identified for two regimes using unmodulated ECCD to clarify the NTM behaviours with different plasma parameters: 0.2 < jEC/jBS < 0.4 for Wsat/dEC ~ 3 and Wsat/Wmarg ~ 2, and 0.35 < jEC/jBS < 0.46 for Wsat/dEC ~ 1.5 and Wsat/Wmarg ~ 2. Here, m and n are the poloidal and toroidal mode numbers; jEC and jBS the EC-driven current density and bootstrap current density at the mode rational surface; Wsat, Wmarg and dEC the full island width at saturation, marginal island width and full-width at half maximum of the ECCD deposition profile, respectively. Stabilization of a 2/1 NTM using modulated ECCD synchronized with a mode rotation of about 5 kHz was performed, in which it was found that the stabilization effect degrades when the phase of the modulation deviates from that of the ECCD at the island O-point. The decay time of the magnetic perturbation amplitude due to the ECCD increases by 50% with a phase shift of ±50° from the O-point ECCD, thus revealing the importance of the phasing of modulated ECCD. For near X-point ECCD, the NTM amplitude increases, revealing a destabilization effect. It was also found that modulated ECCD at the island O-point has a stronger stabilization effect than unmodulated ECCD by a factor of more than 2.

Momentum transport and plasma rotation profile in toroidal direction in JT-60U L-mode plasmas

The characteristics of momentum transport and plasma rotation in the toroidal direction are studied, using near-perpendicular neutral beam injection (PERP-NBI), co tangential and counter (CTR) tangential NBI in JT-60U. Diffusive and non-diffusive terms of momentum transport are evaluated from the transient analysis by using the momentum source modulation. Fast ion losses due to the toroidal field ripple, which locally induces the edge CTR rotation, are used as a novel momentum source. Parameter dependence of these transport coefficients i.e. the toroidal momentum diffusivity (?) and the convection velocity (Vconv), and the relation between momentum and heat diffusivities (?i) are investigated in L-mode plasmas systematically. The toroidal momentum diffusivity increases with increasing heating power and decreases with increasing plasma current. The relation of ? and ?i to some non-dimensional parameters is investigated. A clear dependence of ?/?i on normalized plasma pressure (?N) is observed. It is also found that toroidal rotation velocity profiles in the case with and without external torque input can be almost reproduced by ? and Vconv estimated from the transient momentum transport analysis at low ? (?N < 0.4).

Effect of toroidal field ripple on plasma rotation in JET

Dedicated experiments on TF ripple effects on the performance of tokamak plasmas have been carried out at JET. The TF ripple was found to have a profound effect on the plasma rotation. The central Mach number, M, defined as the ratio of the rotation velocity and the thermal velocity, was found to drop as a function of TF ripple amplitude (3) from an average value of M = 0.40-0.55 for operations at the standard JET ripple of 6 = 0.08% to M = 0.25-0.40 for 6 = 0.5% and M = 0.1-0.3 for delta = 1%. TF ripple effects should be considered when estimating the plasma rotation in ITER. With standard co-current injection of neutral beam injection (NBI), plasmas were found to rotate in the co-current direction. However, for higher TF ripple amplitudes (delta similar to 1%) an area of counter rotation developed at the edge of the plasma, while the core kept its co-rotation. The edge counter rotation was found to depend, besides on the TF ripple amplitude, on the edge temperature. The observed reduction of toroidal plasma rotation with increasing TF ripple could partly be explained by TF ripple induced losses of energetic ions, injected by NBI. However, the calculated torque due to these losses was insufficient to explain the observed counter rotation and its scaling with edge parameters. It is suggested that additional TF ripple induced losses of thermal ions contribute to this effect.

Pedestal conditions for small ELM regimes in tokamaks

Several small/no ELM regimes such as EDA, grassy ELM, HRS, QH-mode, type II and V ELMs with good confinement properties have been obtained in Alcator C-Mod, ASDEX-Upgrade, DIII-D, JET, JFT-2M, JT-60U and NSTX. All these regimes show considerable reduction of instantaneous ELM heat load onto divertor target plates in contrast to conventional type I ELM, and ELM energy losses are evaluated as less than 5% of the pedestal stored energy. These small/no ELM regimes are summarized and widely categorized by their pedestal conditions in terms of the operational space in non-dimensional pedestal parameters and requirement of plasma shape/configuration. The characteristics of edge fluctuations and activities of ideal MHD stability leading to small/no ELMs are also summarized.

Dimensionless parameter dependence of H-mode pedestal width using hydrogen and deuterium plasmas in JT-60U

The characteristics of the spatial width of the H-mode pedestal were investigated in hydrogen and deuterium plasmas in JT-60U. Both the database analysis and the dedicated experiments on the mass scan indicated that the pedestal width depends very weakly on the plasma particle species or . Identical profiles of the edge ion temperature Ti were obtained in the experiments with hydrogen and deuterium plasmas while the profile for the H-mode edge pedestal is not fixed but differs by a factor of ~1.4 (which is the square root of the mass ratio). In addition, an experiment on edge βpol scan was also performed. Higher βpol plasma had higher pedestal Ti value accompanied by greater pedestal width in spite of almost identical at the pedestal. Based on these dimensionless parameter scans, the scaling of the pedestal width was evaluated as .

Effects of ripple loss of fast ions on toroidal rotation in JT-60U

The driving mechanism of toroidal rotation and the momentum transport are studied on JT-60U in relation to the toroidal rotation in the direction antiparallel to the plasma current, i.e. counter (CTR) direction, with near-perpendicular neutral beam (PERP-NB) injection. Fast ion losses due to the toroidal field ripple induce CTR rotation in the peripheral region, and the magnitude of CTR rotation with PERP-NBs reduces by installing the ferritic steel tiles as a consequence of the reduction in the ripple losses. It is also found that toroidal rotation velocity profiles in the core region can be explained by momentum transport considering diffusivity and convective velocity estimated from transient momentum transport analysis in L-mode plasmas.

Energy and particle losses during type-I ELMy H-mode in ASDEX Upgrade

Characteristics of edge-localized mode (ELM) energy and particle losses in type-I ELMy H-mode plasmas on ASDEX Upgrade are investigated by a large data set with as much as possible independent variations in engineering parameters. A statistically unbiased estimate for ELM energy and particle losses has been employed to remove the diagnostic noise as much as possible. The data set shows that the ELM energy loss decreases with the pedestal density or collisionality. However, the role of plasma shape, especially, triangularity, as an explicit parameter is revealed. Elevated triangularity leads to lower ELM frequency and larger ELM energy loss significantly exceeding that simply expected from the increased pedestal pressure in high triangularity plasmas at a fixed power. The observed larger ELM energy drop at higher triangularity involves the ELM perturbations of the electron temperature profile across an ELM that extend radially more inward, suggesting that there is a direct effect of plasma shape on ELM energy losses. It is found that the fraction of ELM loss power does not remain constant but the increased pedestal collisionality enhances the transport level between ELMs and reduces the ELM loss power. The ELM particle flux at fixed power rises with increasing gas fuelling rate. On the other hand, at fixed gas puff, an inverse proportionality between ELM frequency and ELM particle loss roughly holds despite of a large variation of power. When the particle flux near the separatrix is enhanced, the increase of ELM frequency and the reduction of ELM loss power caused by the increased collisionality lead to the reduction of ELM energy loss so that the energy balance could be sustained.

Understanding of H mode pedestal characteristics using the multimachine pedestal database

With use of a multimachine pedestal database, essential issues for each regime of ELM types are investigated. They include (i) understanding and prediction of pedestal pressure during type I ELMs, a reference operation mode of a future tokamak reactor; (ii) identification of the operation regime of type II ELMs, which have small ELM amplitude with good confinement characteristics; (iii) identification of the upper stability boundary of type III ELMs for access to the higher confinement regimes with type I or II ELMs; (iv) understanding the relation between core confinement and pedestal temperature in conjunction with the confinement degradation in high density discharges. Both scaling and model based approaches for expressing pedestal pressure are shown to roughly scale the experimental data well and could be used to make initial predictions for a future reactor. q and delta are identified as important parameters for obtaining the type II ELM regime. A theoretical model of type III ELMs is shown to reproduce the upper stability boundary reasonably well. It is shown that there exists some critical pedestal temperature below which the core confinement starts to degrade. It is also shown that it is possible to obtain improved pedestal conditions for good confinement in high density discharges by increasing the plasma triangularity.

Recent progress on the development and analysis of the ITPA global H-mode confinement database

This paper describes the updates to and analysis of the International Tokamak Physics Activity (ITPA) Global H-Mode Confinement Database version 3 (DB3) over the period 1994–2004. Global data, for the energy confinement time and its controlling parameters, have now been collected from 18 machines of different sizes and shapes: ASDEX, ASDEX Upgrade, C-Mod, COMPASS-D, DIII-D, JET, JFT-2M, JT-60U, MAST, NSTX, PBX-M, PDX, START, T-10, TCV, TdeV, TFTR and TUMAN-3M. The database now contains 10382 data entries from 3762 plasma discharges, including data from deuterium–tritium experiments, low-aspect ratio plasmas, dimensionless parameter experiments and plasmas. DB3 also contains an increased amount of data from a range of diverted machines and further data at high triangularity, high density and high current. A wide range of physics studies has been performed on DB3 with particular progress made in the separation of core and edge behaviour, dimensionless parameter analyses and the comparison of the database with one-dimensional transport codes. The errors in the physics variables of the database have also been studied and this has led to the use of errors in variables fits. A key aim of the database has always been to provide a basis for estimating the energy confinement properties of next step machines such as ITER, and so the impact of the database and its analysis on such machines is also discussed.