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Roger Jaspers

Roger Jaspers

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Roger Jaspers

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  1. TP-8.2: Investigation of rational q effects on ITB formation and expansion &TP-8.4: T-10, TEXTOR, HL-2A similarity experiments Roger Jaspers

  2. TP-8.2: rational q effect on ITB’s • Machines involved: DIII-D, JET, T-10, TEXTOR • Aim: • Understanding the physics of rational q effects in triggering ITB by: • Poloidal flow changes (JET) • Exact qmin -value at which improvement occurs (JET) • Test theories of ExB shear flow associated with zonal flows (DIII-D) • Compare strong shear with weak/zero shear (DIII-D) • Fluctuations (T10/TEXTOR) R. Jaspers - Progress/Plans TP8.2

  3. TP-8.2: rational q effect on ITB’s • Status/Results: • DIII-D: Balanced vs co beams • JET: no progress (except for diagnostics: poloidal rotation measurements by CXRS and Alfvén cascades by magnetics and reflectometry) • T-10: ECRH switch off experiments (8.4) • TEXTOR: test paleo-classical model R. Jaspers - Progress/Plans TP8.2

  4. TP-8.2: rational q effect on ITB’s • Status/Results: DIII-D Max Austin R. Jaspers - Progress/Plans TP8.2

  5. Recent experiments in DIII-D with balanced beam injection tested role of ExB shear 2006-09-12 Balanced NBI Shot • In previous experiments with all co beams, plasma was marginal for core barrier formation due to large ExB shear from high v • A model of ITB formation is that zonal flow effects near integer qmin “kick” the marginal plasma into ITB regime • Balanced NBI case shows only transient confinement improvement near integer qmin and no core barrier formation

  6. Reduced tor. rotation and lack of barrier formation in accordance with model Co Injection Balanced Injection • Balanced injection case confirms that sufficient background ExB shear is required for barrier formation but is not necessary for transient “zonal flow” changes onset of ITB

  7. Reduction of turbulent fluctuation levels seen near integer qmin in balanced NBI case • Very similar to reductions near integer qmin seen in co-NBI discharges • Fluctuation drop starts before time of qmin = integer • Measurement location for BES data shown is near/outside of qmin radius

  8. Strong TAE modes and drop in core ion temperature are absent in balanced NBI case 5 MW Co Injection 5 MW Balanced Injection • Implies TAE are responsible for drop in core Ti, v, but not linked to transient confinement improvement TAE NO TAE drop no drop Relevance: Previously it was suggested that TAE-induced fast ion loss and consequent Er variation could trigger transport changes near integer qmin time

  9. Future explorations • DIII-D work will focus on understanding the operating regime required for observing rational q effects on transport • Why are rational q effects not seen in: (a) reverse shear discharges created with ECH? (b) reverse shear H-mode discharges? • Considering experiments to test this, also GYRO code runs • Upcoming 2007 experiments to look for rational-q related transport effects in reverse shear discharges on other tokamaks: • NSTX – D. Stutman • C-MOD – M. Austin (piggyback on M. Porkolab exp.)

  10. TP-8.2: rational q effect on ITB’s • Status/Results: T-10 KseniaRazumova R. Jaspers - Progress/Plans TP8.2

  11. T-10 • Central Te does not drop for more than 60 ms (~2xtE) after switch off of off-axis ECRH • ECRH further away from centre results in longer transient ITB’s • Depends on current diffusion time. • Te even increase after ECRH switch off

  12. TP-8.4:T10, TEXTOR, HL-2A similarity • Status/Results: TEXTOR Dick Hogeweij R. Jaspers - Progress/Plans TP8.2

  13. TEXTOR • No new data since gyrotron was not available (repaired now!) • Previous data: Same effect as in T-10 • Now modeled with Paleo-classical model

  14. Paleo-classical Model • Ingredients: • Key hypothesis: • the electron guiding centers diffuse with small bundles of poloidal magnetic flux on the magnetic (”skin”) diffusion time scale. • Parallel equilibration length along field lines determines transport • Electron transport: • Dominant unless fluctuations induced transport takes over (>1 keV)

  15. TEXTOR - PC model gives reasonable agreement with experiment !

  16. TEXTOR - Increased shear triggers sawtooth and ends ITB.

  17. RTP • ECRH deposition scan: staircase profile • Also here PC model gives satisfactory agreement !

  18. TP-8.4: HL-2A • Plans: • Use ECRH flexibility • Repeat TEXTOR experiment with one gyrotron used for HPP measurements • Use one gyrotron for ECCD to change magnetic shear • Determine shear dependence in ce • Experimental campaign: Sept. 2007-Feb 2008 R. Jaspers - Progress/Plans TP8.2