Bifurcation theory for the L-H transition in magnetically confined fusion plasmas

The mathematical field of bifurcation theory is extended to be applicable to 1-dimensionally resolved systems of nonlinear partial differential equations, aimed at the determination of a certain specific bifurcation. This extension is needed to be able to properly analyze the bifurcations of the radial transport in magnetically confined fusion plasmas. This is of special interest when describing the transition from the low-energy-confinement state to the high-energy-confinement state of the radial transport in fusion plasmas (i.e., the L-H transition), because the nonlinear dynamical behavior during the transition corresponds to the dynamical behavior of a system containing such a specific bifurcation. This bifurcation determines how the three types (sharp, smooth, and oscillating) of observed L-H transitions are organized as function of all the parameters contained in the model

Determination of localized heat transport in fusion plasmas

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Technical note on the linearity and power dependence of the diffusion coefficient in W7-AS

\u3cp\u3eTransient electron temperature measurements of a step power experiment at W7-AS are reassessed by direct comparison of the up- and downward responses of the electron temperature. The analysis shows that the response at some distance to the center behaves linearly and the model predicted responses based on a power-dependent diffusion coefficient that vary from the measured step responses.\u3c/p\u3

Predictive transport analysis of JET and AUG hybrid scenarios

Hybrid scenarios in present machines are characterized by improved confinement compared to the IPB98(y,2) empirical scaling law expectations. A number of possibilities explaining this improvement have been proposed: reduction in deleterious MHD, pedestal confinement improvement, rotational shear turbulence suppression, increased turbulent thresholds due to qprofile shaping, and stiffness reduction at low magnetic shear [1, 2, 3]. This work concentrates on isolating the impact of increased s/q at outer radii (where s is the magnetic shear) on core confinement in low-triangularity JET and ASDEX Upgrade (AUG) experiments. This is carried out by predictive heat and particle transport modelling using the integrated modelling code CRONOS [4] coupled to the GLF23 turbulent transport model [5]. This work aims to validate recent predictions of the ITER hybrid scenario also employing CRONOS/GLF23, where a high level of confinement and resultant fusion power sensitivity to the s/q profile was found [6]

Separation of transport in slow and fast time-scales using modulated heat pulse experiments (hysteresis in flux explained)

\u3cp\u3eOld and recent experiments show that there is a direct response to the heating power of transport observed in modulated ECH experiments both in tokamaks and stellarators. This is most apparent for modulated experiments in the Large Helical Device (LHD) and in Wendelstein 7 advanced stellarator (W7-AS). In this paper we show that: (1) this power dependence can be reproduced by linear models and as such hysteresis (in flux) has no relationship to hysteresis as defined in the literature; (2) observations of 'hysteresis' (in flux) and a direct response to power can be perfectly reproduced by introducing an error in the estimated deposition profile as long as the errors redistribute the heat over a large radius; (3) non-local models depending directly on the heating power can also explain the experimentally observed Lissajous curves (hysteresis); (4) how non-locality and deposition errors can be recognized in experiments and how they affect estimates of transport coefficients; (5) from a linear perturbation transport experiment, it is not possible to discern deposition errors from non-local fast transport components (mathematically equivalent). However, when studied over different operating points non-linear-non-local transport models can be derived which should be distinguishable from errors in the deposition profile. To show all this, transport needs to be analyzed by separating the transport in a slow (diffusive) time-scale and a fast (heating/non-local) time-scale, which can only be done in the presence of perturbations.\u3c/p\u3

Real-time control of neoclassical tearing modes and its integration with multiple controllers in the TCV Tokamak

\u3cp\u3ePreliminary integrated control of NTMs, beta and model-estimated q profiles has been demonstrated experimentally in TCV for the first time. An upgrade of the supervision layer is foreseen. Dedicated NTM tests show that density affects the triggering of NTMs through global q profile modifications with central co-ECCD - too low or too high density will hinder the triggering. More detailed simulations are ongoing to further clarify these effects.\u3c/p\u3

Heat pulse propagation studies around magnetic islands induced by the Dynamic Ergodic Divertor in TEXTOR

Since the efficiency of the tearing mode suppression by heating depends on the electron heat diffusivity it is important to know if the electron heat transport coefficients inside the island are reduced compared with the ambient plasma. With that aim, modulated ECRH has been employed for heat pulse propagation studies in and around magnetic islands at the TEXTOR tokamak. The combination of its special hardware tools of the Dynamic Ergodic Divertor to generate tearing modes, the ECRH system for producing heat pulses and the electron cyclotron emission imaging (ECE-Imaging) diagnostic for its analysis offered a direct view of the perturbed two-dimensional heat flow in around the magnetic island. Inside m/n = 2/1 and m/n = 3/1 islands with a flattened temperature profile, the electron heat transport is shown to be strongly reduced with respect to the surrounding plasma. Inside the islands a heat pulse diffusion coefficients chi(e) similar to 0.4 m(2) s(-1) was derived, while outside the island it is an order of magnitude larger chi(e) > 3 m(2) s(-1). In contrast, power balance calculations of strongly heated islands show that the electron transport is similar to the surrounding plasma. These results suggest that the heat transport inside a magnetic island is also governed by a critical gradient-like behaviour, similar to the bulk plasma.X1121sciescopu

Recent results on electron cyclotron current drive and MHD activity in RTP

The RTP tokamak (R = 0.72 m, a = 0.164 m, B-phi < 2 5.T, I-p = < 150 kA) is equipped with three gyrotrons (2 x 60 GHz, 180 kW, 100 ms each; 1 x 110 GHz, 500 kW, 200 ms) for electron cyclotron heating (ECH) and current drive (ECCD). The power from one of the 60 GHz gyrotrons is launched via an adjustable mirror from the high field side (HFS) in the 1X-mode. The power of both other gyrotrons is sent in perpendicularly to the toroidal magnetic field from the low field side (LFS). A comprehensive set of high-resolution multichannel plasma diagnostics is available to study the detailed behaviour of various plasma phenomena. First, recent diagnostic innovations are briefly discussed. Then, new physics results are presented for ohmic and EC heated plasmas. ECCD, slide-away discharges, discharges with a hollow temperature profile and MHD phenomena, including sawteeth and disruptions. are treated