Multiple Rotation Regimes in Jet

1. Multiple Rotation Regimes in Jet Massimo De Benedetti Associazione Euratom-ENEA Thanks to F. Nave, M. Gryaznevich Presented to: Easter Workshop on Turbulence and Reconnection Torino 23,24,25 April 2003

2. Summary • Overview of error field modes • Fitzpatrick’s model • Setup of the high beta EF experiments • Observation of multiple rotation regimes • Description of the momentum transport barrier model • Problems that can be solved using the MTB model • Future data analysis • Proposals of future experiments to confirm this model • Conclusions Easter Workshop

3. Features of EF mode experiments • Plasma stable to tearing modes ’<0. • Low beta, L-mode plasma. • Moderate NB power for CX measurements: plasma rotates in the ion diamagnetic direction. • EF induced by SC with 2/1 main harmonic. • Non linear response of plasma to the EF at penetration. • Slowing down of plasma rotation is measured by CX. • The mode spins up and disappears when EF is removed. Easter Workshop

4. Fitzpatrick’s theory(Fitzpatrick, Nuclear Fusion, 33, (1993),1049) • An external EF is amplified by the plasma if the plasma does not rotate: B=Bvac(-2m/rs’0). This amplification factor is typically 4-6. • When the plasma rotates the external field is suppressedand becomes: Two competing torques act on the island: • EM torque that slows down rotation • Viscous torque that tries to restore the natural frequency w0 Easter Workshop

5. Torque balance equation • Electro magnetic torque • Viscous torque If momentum transfer depends only on r, not on v’ Easter Workshop

6. Aim of the experiment • Reproduce DIII-D scaling. • Distinguish between NTM and EF modes. • Observe a transition between the two kind of modes. • Can these modes lower the beta limit for ITER? • H mode • Ip=0.95 MA, Bt=1T • 8 MW PNB  14 MW. • Developed 3/2 NTM • Sub-critical to 2/1 NTM • Only one pair of SC • Ramp-up either EF or PNB • Slow ramp down of PNB Easter Workshop

7. Experiment results • Shot 53606 • 3 rotation regimes. • A very resilient intermediate regime: frequency does not change even with increasing EF. • Same behavior on all CX channels and for low frequency MHD modes. • Slight confinement deterioration at intermediate rotation. • Locking occurs only when the EF has been removed. • ELM frequency doubles. Transition from type I to type III? Easter Workshop

8. Momentum transport barrier model • Transport barriers can develop due to shear flows that suppresses coherent turbulent structures. • In our model, viscosity depends on the velocity shear as: • We have a non-linear equation to solve. • There is hysteresis in the solution for v’. . Easter Workshop

9. Momentum transfer model • In the experiment the momentum is injected by the neutral beams. • The momentum transfer is given by: Easter Workshop

10. Solution of the N. S. equation • An MTB is a region of reduced viscosity. • When momentum transfer is sufficiently high an MTB develops inside the resonant surface. • The viscous torque is proportional to the v’ jump at the rational surface: • When EM torque is sufficiently high a region of low viscosity develops around the resonant surface. This region moves outward Easter Workshop

11. Results with the MTB • The viscous torque consists now of two lines with different slopes. • We have 3 stable rotation regimes. • The intermediate regime is very resilient. • The model explains well the experimental observations. • The mode locking is still puzzling: can the system jump from the medium drag regime to the high drag one by reducing the error field? Easter Workshop

12. Other observations • The MTB model explains rotation regimes in Jet NTM experiments. • We have 3 rotation regimes and a mode locking when the beam power falls below the threshold to create an MTB (H->L transition?). • Observations of 3 rotation regimes have been made on DIII-D in RWM experiments. However in this case we need a modified version of Fitzpatrick’s theory. Easter Workshop

13. Future experiments and work • Observe the transition from high-drag to medium-drag and from medium-drag to low-drag by reducing the EF. • Observe the same transition by stepping up the beam power. • Verify that this model depends on rotation and not on beta (repeat at higher Ip). • Repeat experiments at various NB power to measure when we have the intermediate rotation regime. • Do the same at different ne. • Repeat with a different ratio of PNB/PICRH • Verify to what extent the effect depends on the L->H transition. • Repeat experiments with an ITB in optimised shear. • Measure turbulence fluctuations at the barrier: try to observe barrier location. • Repeat exercise for RWM. • How does bifurcation look like with 3 parameters: Br, PNB,rs? Easter Workshop

14. Conclusions • EF experiments at high beam power show the existence of three stable rotation regimes. • The intermediate regime is very resilient to external EM torques. • The MTB model combined with Fitzpatrick’s theory explains qualitatively the observation. • Similar observations have already been made in NTM experiments on Jet and for RWM experiments on DIII-D. • Other experiments are needed to confirm the model. Easter Workshop