C. Wigger

Max Planck Institute for Plasma Physics, Arching, Bavaria, Germany

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Publications (8)19.38 Total impact

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    ABSTRACT: The ASDEX Upgrade programme is directed towards physics input to critical elements of the ITER design and the preparation of ITER operation, as well as addressing physics issues for a future DEMO design. After the finalization of the tungsten coating of the plasma facing components, the re-availability of all flywheel-generators allowed high-power operation with up to 20MW heating power at Ip up to 1.2 MA. Implementation of alternative ECRH schemes (140 GHz O2- and X3-mode) facilitated central heating above ne = 1.2×1020 m−3 and low q95 operation at Bt = 1.8 T. Central O2-mode heatingwas successfully used in high P/R discharges with 20MWtotal heating power and divertor load control with nitrogen seeding. Improved energy confinement is obtained with nitrogen seeding both for type-I and type-III ELMy conditions. The main contributor is increased plasma temperature, no significant changes in the density profile have been observed. This behaviour may be explained by higher pedestal temperatures caused by ion dilution in combination with a pressure limited pedestal and hollow nitrogen profiles. Core particle transport simulations with gyrokinetic calculations have been benchmarked by dedicated discharges using variations of the ECRH deposition location. The reaction of normalized electron density gradients to variations of temperature gradients and the Te/Ti ratio could be well reproduced. Doppler reflectometry studies at the L–H transition allowed the disentanglement of the interplay between the oscillatory geodesic acoustic modes, turbulent fluctuations and the mean equilibrium E × B flow in the edge negative Er well region just inside the separatrix. Improved pedestal diagnostics revealed also a refined picture of the pedestal transport in the fully developed H-mode type-I ELM cycle. Impurity ion transport turned out to be neoclassical in between ELMs. Electron and energy transport remain anomalous, but exhibit different recovery time scales after an ELM. After recovery of the pre-ELM profiles, strong fluctuations develop in the gradients of ne and Te. The occurrence of the next ELM cannot be explained by the local current diffusion time scale, since this turns out to be too short. Fast ion losses induced by shear Alfv´en eigenmodes have been investigated by time-resolved energy and pitch angle measurements. This allowed the separation of the convective and diffusive loss mechanisms.
    Nuclear Fusion 09/2011; 51(8):094012. DOI:10.1088/0029-5515/51/9/094012 · 3.24 Impact Factor
  • The Review of scientific instruments 03/2010; 81(3):039903. DOI:10.1063/1.3340955 · 1.58 Impact Factor
  • The Review of scientific instruments 01/2010; 81:-. · 1.58 Impact Factor
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    ABSTRACT: ASDEX Upgrade was operated with a fully W-covered wall in 2007 and 2008. Stationary H-modes at the ITER target values and improved H-modes with H up to 1.2 were run without any boronization. The boundary conditions set by the full W wall (high enough ELM frequency, high enough central heating and low enough power density arriving at the target plates) require significant scenario development, but will apply to ITER as well. D retention has been reduced and stationary operation with saturated wall conditions has been found. Concerning confinement, impurity ion transport across the pedestal is neoclassical, explaining the strong inward pinch of high-Z impurities in between ELMs. In improved H-mode, the width of the temperature pedestal increases with heating power, consistent with a scaling. In the area of MHD instabilities, disruption mitigation experiments using massive Ne injection reach volume averaged values of the total electron density close to those required for runaway suppression in ITER. ECRH at the q = 2 surface was successfully applied to delay density limit disruptions. The characterization of fast particle losses due to MHD has shown the importance of different loss mechanisms for NTMs, TAEs and also beta-induced Alfven eigenmodes (BAEs). Specific studies addressing the first ITER operational phase show that O1 ECRH at the HFS assists reliable low-voltage breakdown. During ramp-up, additional heating can be used to vary li to fit within the ITER range. Confinement and power threshold in He are more favourable than in H, suggesting that He operation could allow us to assess H-mode operation in the non-nuclear phase of ITER operation.
    Nuclear Fusion 09/2009; 49(10):104009. DOI:10.1088/0029-5515/49/10/104009 · 3.24 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: ASDEX Upgrade was operated with a fully W-covered wall in 2007 and 2008. Stationary H-modes at the ITER target values and improved H-modes with H up to 1.2 were run without any boronization. The boundary conditions set by the full W wall (high enough ELM frequency, high enough central heating and low enough power density arriving at the target plates) require significant scenario development, but will apply to ITER as well. D retention has been reduced and stationary operation with saturated wall conditions has been found. Concerning confinement, impurity ion transport across the pedestal is neoclassical, explaining the strong inward pinch of high-Z impurities in between ELMs. In improved H-mode, the width of the temperature pedestal increases with heating power, consistent with a \beta_{{\rm pol,ped}}^{1/2} scaling. In the area of MHD instabilities, disruption mitigation experiments using massive Ne injection reach volume averaged values of the total electron density close to those required for runaway suppression in ITER. ECRH at the q = 2 surface was successfully applied to delay density limit disruptions. The characterization of fast particle losses due to MHD has shown the importance of different loss mechanisms for NTMs, TAEs and also beta-induced Alfven eigenmodes (BAEs). Specific studies addressing the first ITER operational phase show that O1 ECRH at the HFS assists reliable low-voltage breakdown. During ramp-up, additional heating can be used to vary li to fit within the ITER range. Confinement and power threshold in He are more favourable than in H, suggesting that He operation could allow us to assess H-mode operation in the non-nuclear phase of ITER operation.
    Nuclear Fusion 09/2009; 49(10):104009. · 3.24 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: ASDEX Upgrade was operated with a fully W-covered wall in 2007 and 2008. Stationary H-modes at the ITER target values and improved H-modes with H up to 1.2 were run without any boronization. The boundary conditions set by the full W wall (high enough ELM frequency, high enough central heating and low enough power density arriving at the target plates) require significant scenario development, but will apply to ITER as well. D retention has been reduced and stationary operation with saturated wall conditions has been found. Concerning confinement, impurity ion transport across the pedestal is neoclassical, explaining the strong inward pinch of high-Z impurities in between ELMs. In improved H-mode, the width of the temperature pedestal increases with heating power, consistent with a \beta_{{\rm pol,ped}}^{1/2} scaling. In the area of MHD instabilities, disruption mitigation experiments using massive Ne injection reach volume averaged values of the total electron density close to those required for runaway suppression in ITER. ECRH at the q = 2 surface was successfully applied to delay density limit disruptions. The characterization of fast particle losses due to MHD has shown the importance of different loss mechanisms for NTMs, TAEs and also beta-induced Alfven eigenmodes (BAEs). Specific studies addressing the first ITER operational phase show that O1 ECRH at the HFS assists reliable low-voltage breakdown. During ramp-up, additional heating can be used to vary li to fit within the ITER range. Confinement and power threshold in He are more favourable than in H, suggesting that He operation could allow us to assess H-mode operation in the non-nuclear phase of ITER operation.
    Nuclear Fusion 09/2009; 49(10):104009. · 3.24 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: ASDEX Upgrade was operated with a fully W-covered wall in 2007 and 2008. Stationary H-modes at the ITER target values and improved H-modes with H up to 1.2 were run without any boronization. The boundary conditions set by the full W wall (high enough ELM frequency, high enough central heating and low enough power density arriving at the target plates) require significant scenario development, but will apply to ITER as well. D retention has been reduced and stationary operation with saturated wall conditions has been found. Concerning confinement, impurity ion transport across the pedestal is neoclassical, explaining the strong inward pinch of high-Z impurities in between ELMs. In improved H-mode, the width of the temperature pedestal increases with heating power, consistent with a \beta_{{\rm pol,ped}}^{1/2} scaling. In the area of MHD instabilities, disruption mitigation experiments using massive Ne injection reach volume averaged values of the total electron density close to those required for runaway suppression in ITER. ECRH at the q = 2 surface was successfully applied to delay density limit disruptions. The characterization of fast particle losses due to MHD has shown the importance of different loss mechanisms for NTMs, TAEs and also beta-induced Alfven eigenmodes (BAEs). Specific studies addressing the first ITER operational phase show that O1 ECRH at the HFS assists reliable low-voltage breakdown. During ramp-up, additional heating can be used to vary li to fit within the ITER range. Confinement and power threshold in He are more favourable than in H, suggesting that He operation could allow us to assess H-mode operation in the non-nuclear phase of ITER operation.
    Nuclear Fusion 09/2009; 49(10):104009. · 3.24 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: ASDEX Upgrade was operated with fully W-covered wall in 2007 and 2008. Stationary H-modes at the ITER target values and improved H-modes with H up to 1.2 were run without any boronisation. The boundary conditions set by the full W-wall (high enough ELM frequency, high enough central heating and low enough power density arriving at the target plates) require significant scenario development, but will apply to ITER as well. D retention has been reduced and stationary operation with saturated wall conditions has been found. Concerning confinement, impurity ion transport across the pedestal is neoclassical, explaining the strong inward pinch of high-Z impurities in between ELMs. In improved H-mode, the width of the temperature pedestal increases with heating power, consistent with a β pol,ped 1/2 scaling. In the area of MHD instabilities, disruption mitigation experiments using massive Ne injection reach volume averaged values of the total electron density close to those required for Runaway suppression in ITER. ECRH at the q=2 surface was successfully applied to delay density limit disruptions. The characterisation of fast particle losses due to MHD has shown the importance of different loss mechanisms for NTMs, TAEs and BAEs. Specific studies addressing the first ITER operational phase show that O1 ECRH at the HFS assists reliable low-voltage breakdown. During ramp-up, additional heating can be used to vary l i to fit within the ITER range. Confinement and power threshold in He 2 OV/2-3 are more favourable than in H, suggesting that He operation could allow to assess H-mode operation in the non-nuclear phase of ITER operation.
    22nd IAEA Fusion Energy Conference, Geneva 2008; 10/2008