Showing papers by "V. Igochine published in 2016"

Optimized tomography methods for plasma emissivity reconstruction at the ASDEX Upgrade tokamak.

Abstract: The soft X-ray (SXR) emission provides valuable insight into processes happening inside of high-temperature plasmas. A standard method for deriving the local emissivity profiles of the plasma from the line-of-sight integrals measured by pinhole cameras is the tomographic inversion. Such an inversion is challenging due to its ill-conditioned nature and because the reconstructed profiles depend not only on the quality of the measurements but also on the inversion algorithm used. This paper provides a detailed description of several tomography algorithms, which solve the inversion problem of Tikhonov regularization with linear computational complexity in the number of basis functions. The feasibility of combining these methods with the minimum Fisher information regularization is demonstrated, and various statistical methods for the optimal choice of the regularization parameter are investigated with emphasis on their reliability and robustness. Finally, the accuracy and the capability of the methods are demonstrated by reconstructions of experimental SXR profiles, featuring poloidal asymmetric impurity distributions as measured at the ASDEX Upgrade tokamak.

Experimental investigation of the radial structure of energetic particle driven modes

Abstract: Alfven eigenmodes (AEs) and energetic particle modes (EPMs) are often excited by energetic particles (EPs) in tokamak plasmas. One of the main open questions concerning EP driven instabilities is the non-linear evolution of the mode structure. The aim of the present paper is to investigate the properties of beta-induced AEs (BAEs) and EP driven geodesic acoustic modes (EGAMs) observed in the ramp-up phase of off-axis NBI heated ASDEX Upgrade (AUG) discharges. This paper focuses on the changes in the mode structure of BAEs/EGAMs during the non-linear chirping phase. Our investigation has shown that in the case of the observed down-chirping BAEs the changes in the radial structure are smaller than the uncertainty of our measurement. This behaviour is most probably the consequence of the fact that BAEs are normal modes, thus their radial structure strongly depends on the background plasma parameters rather than on the EP distribution. In the case of rapidly upward chirping EGAMs the analysis consistently shows shrinkage of the mode structure. The proposed explanation is that the resonance in the velocity space moves towards more passing particles which have narrower orbit widths.

Experimental investigation of the radial structure of energetic particle driven modes

Abstract: Alfven eigenmodes (AEs) and energetic particle modes (EPMs) are often excited by energetic particles (EPs) in tokamak plasmas. One of the main open questions concerning EP driven instabilities is the non-linear evolution of the mode structure. The aim of the present paper is to investigate the properties of beta-induced AEs (BAEs) and EP driven geodesic acoustic modes (EGAMs) observed in the ramp-up phase of off-axis NBI heated ASDEX Upgrade (AUG) discharges. This paper focuses on the changes in the mode structure of BAEs/EGAMs during the non-linear chirping phase. Our investigation has shown that in case of the observed down-chirping BAEs the changes in the radial structure are smaller than the uncertainty of our measurement. This behaviour is most probably the consequence of that BAEs are normal modes, thus their radial structure strongly depends on the background plasma parameters rather than on the EP distribution. In the case of rapidly upward chirping EGAMs the analysis consistently shows shrinkage of the mode structure. The proposed explanation is that the resonance in the velocity space moves towards more passing particles which have narrower orbit widths.

Non-monotonic growth rates of sawtooth precursors evidenced with a new method on ASDEX Upgrade

Abstract: This paper describes a new method to derive, from soft x-ray (SXR) tomography, robust estimates of the core displacement, growth rate and frequency of a 1/1 sawtooth crash precursor. The method is valid for very peaked SXR profiles and is robust against both the inversion algorithm and the presence of tungsten in a rotating plasma. Three typical ASDEX Upgrade crashes are then analysed. In all cases a postcursor is observed, suggesting incomplete reconnection. Despite different dynamics, in all three cases the growth rate of the core displacement shows similar features. First, it is not constant, supporting the idea of non-linear growth. Second, it can be divided into clearly identified phases with quasi-constant growth rates, suggesting sudden change of growth regime rather than smooth transitions. Third, its evolution is non-monotonic, with phases of accelerated growth followed by damped phases. This damping is interpreted for two cases respectively as an effect of fast ions and of mode coupling, based on the result of a MHD simulation. The mode frequency is observed in all cases to be closely related to the plasma bulk rotation profile, with little or no visible effect of the electron diamagnetic drift frequency. The onset criterion could not be clearly identified and it is shown that the role of the pressure gradient is not as expected from a naive extrapolation of the linear stability theory.

The dynamics of a neoclassical tearing mode (NTM) influenced by energetic ions on EAST

Abstract: In the 2014 year's campaign of experimental advanced superconducting tokamak (EAST), a series of Magnetohydrodynamics (MHD) instabilities were observed as the launching of Neutral Beam Injection (NBI), the most interesting one of which is the neoclassical tearing mode (NTM). Evidence clearly shows that a kink mode present after a strong sawtooth-like (ST-like) crash leaves a perturbation near the location of the magnetic island, providing the initial seed. The interaction of energetic ions makes the magnetic island oscillate both in island width and in rotation frequency. Analysis indicates that the bulk plasma still dominates the dynamics of NTM, and the orbit excursion of energetic ions induces a polarization current and modifies the width and rotation frequency of the neoclassical magnetic island.

Multimachine Data–Based Prediction of High-Frequency Sensor Signal Noise for Resistive Wall Mode Control in ITER

Abstract: The high-frequency noise measured by magnetic sensors, at levels above the typical frequency of resistive wall modes, is analyzed across a range of present tokamak devices including DIII-D, JET, MAST, ASDEX Upgrade, JT-60U, and NSTX. A high-pass filter enables identification of the noise component with Gaussian-like statistics that shares certain common characteristics in all devices considered. A conservative prediction is made for ITER plasma operation of the high-frequency noise component of the sensor signals, to be used for resistive wall mode feedback stabilization, based on the multimachine database. The predicted root-mean-square n = 1 (n is the toroidal mode number) noise level is 10(4) to 10(5) G/s for the voltage signal, and 0.1 to 1 G for the perturbed magnetic field signal. The lower cutoff frequency of the Gaussian pickup noise scales linearly with the sampling frequency, with a scaling coefficient of about 0.1. These basic noise characteristics should be useful for the modeling-based design of the feedback control system for the resistive wall mode in ITER.

Role of MHD Dynamo in the Formation of 3D Equilibria in Fusion Plasmas

Abstract: This work investigates the formation of helical core equilibria in toroidal fusion plasmas, focusing on the role of dynamo, or magnetic flux pumping mechanisms in determining the equilibrium current profile. Dynamo effects determine the safety factor profile of the final 3D equilibrium, with important consequences on MHD stability and transport. We compare experimental results from multiple machines (RFX-mod, MST, AUG, DIII-D) and nonlinear MHD modelling. Two paradigmatic cases of helical state formation are considered and common physics is identified, by direct measurements of dynamo effects and MHD simulations: spontaneous formation in high-current reversed-field pinch (RFP) plasmas [1] and the hybrid scenario in high-beta tokamak plasmas [2]. Helical cores form in both cases, either spontaneously via saturation of MHD modes, or due to the marginally-stable ideal MHD response to external 3D fields. Direct measurements of the dynamo emf associated to 3D plasma distortions will be presented for a database of helical RFP plasmas from RFX-mod and MST, covering a wide range of plasma parameters. Similar measurements were also done in helical states forming in response to external 3D fields in Ohmic RFX-mod tokamak plasmas and in DIII-D high-beta hybrid plasmas. Experimental results qualitatively agree with nonlinear MHD modelling performed with the codes SpeCyl [3], PIXIE3D [4], and NIMROD [5]. They indicate that central current is redistributed by a dominantly electrostatic MHD dynamo. The underlying physics common to RFP and tokamak is thus revealed: a helical core displacement modulates parallel current density along flux tubes, which requires a helical electrostatic potential to build up, giving rise to a helical dynamo flow. Similar results were also recently obtained with the M3D-C1 code [6]. [1] R. Lorenzini et al., Nature Phys. 5, 570 (2009); [2] T.C. Luce et al., Nucl. Fusion 54, 013015 (2014); [3] D. Bonfiglio et al., Phys. Rev. Lett. 94, 145001 (2005); [4] D. Bonfiglio et al., Plasma Phys. Control. Fusion 57, 044001 (2015); [5] J.R. King, C.R. Sovinec, V.V. Mirnov, Phys. Plasmas 19, 055905 (2012); [6] S.C. Jardin et al., Phys. Rev. Lett. 115, 215001 (2015).

Upgraded equilibrium reconstruction by coupling of an extended set of measurements with current diffusion modelling at ASDEX Upgrade

Abstract: The omnipresent ill-posedness of tokamak equilibrium reco nstructions can be mitigated by an integrated approach of combining a comprehensive set of i nternal and external measurements with a predictive model of the expected current distributio n. The goal is to overcome the need for non-physical regularization (smoothing) of the source profiles entering the Grad-Shafranov equation by a sufficiently informative set of measurements a d physical modelling. An extended set of data allows to validate their mutual compatibility du e to partially redundant information. Grad-Shafranov Equation The GSE describing an ideal magneto-hydrodynamic equilibr ium in axisymmetric tokamak geometry reads

Entrainment of MHD modes in ASDEX Upgrade using rotating non-axisymmetric perturbation fields

Abstract: In recent ASDEX Upgrade experiments the entrainment of a fast rotating (around 2-5 kHz) m=2, n=1 tearing mode to a slowly rotating perturbation field (5-10Hz range) applied by in-vessel coils have been investigated. The ultimate goal of these experiments is active disruption avoidance by externally applied magnetic perturbations. In this work we start investigating the physics basis of this approach and our initial results can be considered encouraging. This experiment used a set of 16 active perturbation coils, called B-coils, distributed along the toroidal angle on the low field side in an upper and a lower row of 8 coils, almost symmetric with respect to the midplane [1]. In 2015 new AC power supplies became available with 4 individual controllers, which made these experiments possible. An improved H-mode plasma at moderate elongation with βΝ between 2 and 3 was selected as the target configuration. The plasma current was 0.8 MA, the toroidal magnetic field 2.5 T. The input NBI power was in the interval 5 < PNBI < 13 MW , while the ECRH power in the range 1.5 < PECRH < 3 MW. The ECRH deposition was such that the plasma rotation in the core was reduced in order to have relatively slowly rotating tearing modes. An example is shown in Fig.1, where it can be seen that the 2/1 tearing activity is mostly in the range 5-10 KHz. From the figure it is also clear that the mode is