BEGIN:VCALENDAR VERSION:2.0 PRODID:-//linuxsoftware.nz//NONSGML Joyous v1.4//EN BEGIN:VEVENT SUMMARY:Exploring the role of sheared flows in the L-H transition DTSTART:20241209T090000Z DTEND:20241209T110000Z DTSTAMP:20260920T143645Z UID:55ff190d-72d6-4c98-97f4-0c2dc5d78163 SEQUENCE:1 CREATED:20241206T104525Z DESCRIPTION: Transport in toroidally confined plasmas is primarily by turb ulent transport\, which leads to substantial losses. This is partially mit igated when operating in the H-mode regime\, which is characterized by ste ep edge gradients and a marked reduction in turbulent transport. Due to it s superior confinement properties\, H-mode is regarded as the preferred op erational regime for fusion reactors. Consequently\, extensive efforts hav e been devoted to understanding the mechanisms behind the L-H transition\, with a particular focus on the role of sheared flows in suppressing turbu lence.The first goal of this work was to evaluate whether a threshold valu e for perpendicular velocity shear is required to access H-mode. Doppler b ackscattering measurements were analyzed and the perpendicular velocity pr ofiles fitted using least squares splines\, with average shear flow calcul ated across different regions at the plasma edge. Results indicate that no consistent threshold value exists across the dataset studied.The second g oal of this study was to leverage the dithered L-H transition to achieve e nhanced radial and temporal resolution of the transition\, allowing detail ed tracking of the evolution of the radial electric field\, turbulence\, a nd density profiles throughout the L-H-L cycle. Dithers were synchronized and conditionally averaged\, providing robust statistical significance to the analysis. Our findings reveal that the reduction in fluctuation levels is accompanied first by a steepening of the density profile\, followed on ly later by a deepening of the perpendicular velocity well. These results challenge the traditional view that turbulence suppression is driven prima rily by an increase in sheared flows. LAST-MODIFIED:20241206T104525Z LOCATION:Sala P3 Pavilhão de Matemática\, Piso 1 URL:http://df.vps.tecnico.ulisboa.pt/en/events/exploring-the-role-of-shear ed-flows-in-the-l-h-transition/ X-ALT-DESC;FMTTYPE=text/html:

Transport in toroi dally confined plasmas is primarily by turbulent transport\, which leads t o substantial losses. This is partially mitigated when operating in the H- mode regime\, which is characterized by steep edge gradients and a marked reduction in turbulent transport. Due to its superior confinement properti es\, H-mode is regarded as the preferred operational regime for fusion rea ctors. Consequently\, extensive efforts have been devoted to understanding the mechanisms behind the L-H transition\, with a particular focus on the role of sheared flows in suppressing turbulence.

The first goal of this work was to evaluate whether a thresh old value for perpendicular velocity shear is required to access H-mode. D oppler backscattering measurements were analyzed and the perpendicular vel ocity profiles fitted using least squares splines\, with average shear flo w calculated across different regions at the plasma edge. Results indicate that no consistent threshold value exists across the dataset studied.

The second goal of this study was to l everage the dithered L-H transition to achieve enhanced radial and tempora l resolution of the transition\, allowing detailed tracking of the evoluti on of the radial electric field\, turbulence\, and density profiles throug hout the L-H-L cycle. Dithers were synchronized and conditionally averaged \, providing robust statistical significance to the analysis.

Ou r findings reveal that the reduction in fluctuation levels is accompanied first by a steepening of the density profile\, followed only later by a de epening of the perpendicular velocity well. These results challenge the tr aditional view that turbulence suppression is driven primarily by an incre ase in sheared flows.

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