International Doctoral College in Fusion Science and Engineering
 Thesis catalogue
Ion-mass and flow-shear dependence of turbulent transport in stellarator plasmas
PhD Code: 2016-DC-11:
  • Host institute 1: FP5-Universität Stuttgart (Home University)
  • Host institute 2: FP4-Universidad Carlos III de Madrid (Host University) - AM01-Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (Host Institution)
  • Host institute 3: AM06-Institute of Plasma Physics, v.v.i. Acad. of Sci. of the CZRep
Research fields:
  • F3. Stellarator and reversed field pinch research and advanced concepts
  • Prof. Thomas Hirth (promotor) - Dr. Mirko Ramish (mentor)
  • Prof. Ramón Martin Solis (co-promotor) - Dr. Carlos Hidalgo (mentor)
  • Dr. Martin Hron (mentor)
Contact Person and email: Mirko Ramisch -

Subject description
Background: Previous studies at the stellarator TJ-K have shown the link between zonal-flow (ZF) amplitude and reduction in turbulent transport. Since the turbulent transport is a nonlinear quantity incorporating density and potential fluctuations, shear flows can predominantly act on either their amplitudes or their mutual coupling as to reduce transport. Via mixing-length estimates, the amplitudes are related to vortex sizes, which suggests an increase of transport with the ion mass. Experimentally, the increase of turbulent-structure sizes with the ion mass could be shown in the tokamaks TEXTOR and ISTTOK [1-2] and in the stellarators TJ-K and TJ-II [2-3]. In contrast, tokamak D plasmas where observed to reveal a better confinement than H plasmas. This isotope effect was recently observed to become manifest in a reduction of the L-H power threshold in the range of 50% when using D instead of H. A reduced L-H power threshold with ion mass would have great impact in the development of ITER plasma operation scenarios. The isotope effect is not fully understood, yet. Regarding the structure-size scaling, the density-potential coupling constitutes a key agent in this process. Moreover, this coupling determines the efficiency of ZF drive. In fact, a systematic increase in the amplitude of long-range correlations showed up in the transition from H to D dominated tokamak plasmas [1-2], but not in the TJ-II stellarator [2]. Differences in the level of isotope effect in tokamaks and stellarators could be a consequence of the stronger damping or less efficient drive of ZFs in non- optimized stellarators than in tokamaks. [1] Y. Xu et al, Phys. Rev. Lett. 110, 265005 (2013) [2] B. Liu et al., Nucl. Fusion (2015) submitted [3] M. Ramisch et al., Phys. Plasmas 12, 032504 (2005)

Expected outcomes

Objective: The objective is to investigate the ion-mass dependence of zonal-flow properties and density-potential coupling in stellarators as contribution to unravelling the physics of the isotope effect. Complement flow-shear scaling studies of turbulent-transport components will be carried out, where in toroidal geometry the influence of self- excited zonal flows and externally imposed shear flows will be distinguished. Zonal flows will be detected on the basis of long-range correlations or directly using Langmuir-probe arrays. Large-scale zonal structures are then correlated with fluctuations on small scales relevant for transport. Spectral decomposition of turbulent transport, thereby, give insight into the relevant scales affected by flow shear and into the change in kind and strength of density-potential coupling. Time line and mobility scheme (research need to be performed for at least six month in two different countries): Home university: FP5-Universität Stuttgart Host institution: AM01-Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas; Host university FP4-Universidad Carlos III de Madrid Host institution: AM06-Institute of Plasma Physics, v.v.i. Acad. of Sci. of the CZRep


Original document: 2016-DC-11

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