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

Divertor Heat Load in ASDEX Upgrade L-Mode in Presence of External Magnetic Perturbation

Power exhaust is one of the major challenges for a future fusion device. Applying a non-axisymmetric external magnetic perturbation is one technique that is studied in order to mitigate or suppress large edge localized modes which accompany the high confinement regime in tokamaks. The external magnetic perturbation brakes the axisymmetry of a tokamak and leads to a 2D heat flux pattern on the divertor target. The 2D heat flux pattern at the outer divertor target is studied on ASDEX Upgrade in stationary L-Mode discharges. The amplitude of the 2D characteristic of the heat flux depends on the alignment between the field lines at the edge and the vacuum response of the applied magnetic perturbation spectrum. The 2D characteristic reduces with increasing density. The increasing divertor broadening $S$ with increasing density is proposed as the main actuator. This is supported by a generic model using field line tracing and the vacuum field approach that is in quantitative agreement with the measured heat flux. The perturbed heat flux, averaged over a full toroidal rotation of the magnetic perturbation, is identical to the non-perturbed heat flux without magnetic perturbation. The transport qualifiers, power fall-off length $\lambda_q$ and divertor broadening $S$, are the same within the uncertainty compared to the unperturbed reference. No additional cross field transport is observed.Comment: 23 pages, 28 figures. This is an author-created, un-copyedited version of an article submitted for publication in Plasma Physics and Controlled Fusion. IoP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from i

Exploring fusion-reactor physics with high-power electron cyclotron resonance heating on ASDEX Upgrade

The electron cyclotron resonance heating (ECRH) system of the ASDEX Upgrade tokomak has been upgraded over the last 15 years from a 2MW, 2 s, 140 GHz system to an 8MW, 10 s, dual frequency system (105/140 GHz). The power exceeds the L/H power threshold by at least a factor of two, even for high densities, and roughly equals the installed ion cyclotron range of frequencies power. The power of both wave heating systems together (>10MW in the plasma) is about half of the available neutral beam injection (NBI) power, allowing significant variations of torque input, of the shape of the heating profile and of Qe/Qi, even at high heating power. For applications at a low magnetic field an X3-heating scheme is routinely in use. Such a scenario is now also forseen for ITER to study the first H-modes at one third of the full field. This versatile system allows one to address important issues fundamental to a fusion reactor: H-mode operation with dominant electron heating, accessing low collisionalities in full metal devices (also related to suppression of edge localized modes with resonant magnetic perturbations), influence of Te/Ti and rotational shear on transport, and dependence of impurity accumulation on heating profiles. Experiments on all these subjects have been carried out over the last few years and will be presented in this contribution. The adjustable localized current drive capability of ECRH allows dedicated variations of the shape of the q-profile and the study of their influence on non-inductive tokamak operation (so far at q$_{95}$>5.3). The ultimate goal of these experiments is to use the experimental findings to refine theoretical models such that they allow a reliable design of operational schemes for reactor size devices. In this respect, recent studies comparing a quasi-linear approach (TGLF) with fully non-linear modeling (GENE) of non-inductive high-beta plasmas will be reported

Radiation transport modelling for the interpretation of oblique ECE measurements

The electron cyclotron emission (ECE) diagnostic provides routinely electron temperature (Te) measurements. At ASDEX Upgrade an electron cyclotron forward model, solving the radiation transport equation for given Te and electron density profile, is used in the framework of integrated data analysis. With this method Te profiles can be obtained from ECE measurements even for plasmas with low optical depth. However, due to the assumption of straight lines of sight and an absorption coefficient in the quasi-perpendicular approximation this forward model is not suitable for the interpretation of measurements by ECE diagnostics with an oblique line of sight. Since radiation transport modelling is required for the interpretation of oblique ECE diagnostics we present in this paper an extended forward model that supports oblique lines of sight. To account for the refraction of the line of sight, ray tracing in the cold plasma approximation was added to the model. Furthermore, an absorption coefficient valid for arbitrary propagation was implemented. Using the revised model it is shown that for the oblique ECE Imaging diagnostic at ASDEX Upgrade there can be a significant difference between the cold resonance position and the point from which most of the observed radiation originates

Investigation of the coupling properties of the ion cyclotron fast wave under applied magnetic perturbations and MHD phenomena in ASDEX Upgrade

The modulation of the ion cyclotron fast wave coupling to the plasma due to non-axisymmetric changes of the distance antenna-R-cutoff is studied. These changes can arise when magnetic perturbation (MP) fields are used, or when MHD activity is present. The application of MP fields can excite a low field side midplane plasma kink response that amplifies the vacuum perturbation field, leading to appreciable 3D plasma displacements. This effect is studied via NEMEC simulations. Rigid rotation of the MP field is found to produce a coherent antenna loading resistance modulation, suggesting an interplay between the non-axisymmetric magnetic field structure and the wave coupling properties. MHD modes are shown to introduce similar loading resistance oscillations, coherent with the mode rotation frequency. The case of a (2,1) mode is presented

Investigation of the coupling properties of the ion cyclotron fast wave under applied magnetic perturbations and MHD phenomena in ASDEX Upgrade

The modulation of the ion cyclotron fast wave coupling to the plasma due to non-axisymmetric changes of the distance antenna-R-cutoff is studied. These changes can arise when magnetic perturbation (MP) fields are used, or when MHD activity is present. The application of MP fields can excite a low field side midplane plasma kink response that amplifies the vacuum perturbation field, leading to appreciable 3D plasma displacements. This effect is studied via NEMEC simulations. Rigid rotation of the MP field is found to produce a coherent antenna loading resistance modulation, suggesting an interplay between the non-axisymmetric magnetic field structure and the wave coupling properties. MHD modes are shown to introduce similar loading resistance oscillations, coherent with the mode rotation frequency. The case of a (2,1) mode is presented

Non-linear modeling of the threshold between ELM mitigation and ELM suppression by resonant magnetic perturbations in ASDEX upgrade

\u3cp\u3eThe interaction between Edge-Localized Modes (ELMs) and Resonant Magnetic Perturbations (RMPs) is modeled with the magnetohydrodynamic code JOREK using experimental parameters from ASDEX Upgrade discharges. According to the modeling, the ELM mitigation or suppression is optimal when the amplification of both tearing and peeling-kink responses results in a better RMP penetration. The ELM mitigation or suppression is not only due to the reduction of the pressure gradient but predominantly arises from the toroidal coupling between the ELMs and the RMP-induced mode at the plasma edge, forcing the edge modes to saturate at a low level. The bifurcation from ELM mitigation to ELM suppression is observed when the RMP amplitude is increased. ELM mitigation is characterized by rotating modes at the edge, while the mode locking to RMPs is induced by the resonant braking of the electron perpendicular flow in the ELM suppression regime.\u3c/p\u3

The ASDEX upgrade program targeting gaps to fusion energy

Recent experiments in ASDEX Upgrade aimed at improving the physics base for ITER and DEMO to prepare operation and aid the design. In order to increase its exhaust capabilities and operational flexibility, a new bulk W divertor as well as an adjustable cryopump had been installed prior to the 2014 campaign. In experiments with high-field-side pellet injection, central electron densities twice as high as the Greenwald density limit could be achieved without strongly increasing the pedestal density and deleterious effect on confinement. Due to its large installed heating power, a large normalized heat flux P-sep/R = 10 MWm(-1) has been reached, representing two-thirds of the ITER value, under partially detached conditions with a peak target heat flux well below 10 MWm(-2). The divertor load could be further reduced by increasing the core radiation, still keeping the confinement in the range of H-98 y2 approximate to 1. Suppression of edge-localized modes (ELMs) at low collisionality has been observed in a narrow spectral window in contrast to earlier results at high densities. The ITER Q = 10 baseline scenario has been investigated, matching as close as possible the triangularity, the plasma beta, q(95), and the distance to the L-H threshold. It turned out that the ELM frequency is low and consequently the energy ejected by a single ELM is very high and ELM mitigation appears to be difficult. As a possible alternative, a scenario has been developed achieving a similar performance at a lower plasma current (and consequently higher q(95)). Experiments using electron cyclotron current drive (ECCD) with feedback-controlled deposition have allowed successfully testing several control strategies for ITER, including automated control of (3, 2) and (2, 1) neoclassical tearing modes during a single discharge. Concerning advanced scenarios, experiments with central ctr-ECCD have been performed in order to modify the q-profile. A strong reversal of the q-profile could be stationarily achieved and an internal transport barrier could be triggered. In disruption mitigation studies with massive gas injection (MGI), a runaway electron beam could be provoked and mitigated by a second MGI. Ongoing enhancements aim at strengthening the power supplies in order to allow full use of the installed heating power, the exchange of two ion cyclotron resonance heating (ICRH) antennas to reduce the W influx during ICRH, and the upgrading of the electron cyclotron resonance heating (ECRH) system to 7-8 MW for 10 s