Parallel and Poloidal Sheared Flows close to Instability Threshold in the TJ-II Stellarator

1. Parallel and Poloidal Sheared Flows close to Instability Threshold • in the TJ-II Stellarator • M. A. Pedrosa, C. Hidalgo, B. Gonçalves*, E. Ascasibar, T. Estrada, J. A. Jiménez, A. López-Fraguas • I. Pastor, and the TJ-II team • Laboratorio Nacional de Fusión por Confinamiento Magnético • Asociación EURATOM-CIEMAT, 28040-Madrid, SPAIN • *EURATOM-IST, 1049-001 Lisbon, Portugal

2. M. Angeles: INTRODUCTION • The operational flexibility of TJ-II makes it an ideal device to study the onset of fluctuations and related phenomena close to instability thresholds. The absence of magnetic well (i.e. magnetic hill) in TJ-II gives rise to instabilities. • Evidence of sheared parallel flow linked to poloidal sheared flow has been previously found in the proximity of the LCFS in tokamaks. Similar results are reported in the TJ-II stellarator. Parallel flows are modified as the magnetic configuration becomes unstable (i.e. the magnetic well is reduced). • The transition to improved regimes implies non-monotonic relations between gradients and transport. Strong dynamical coupling between density gradients and turbulent transport has been experimentally found in the boundary region of TJ-II plasmas.

3. Experimental Set-up TJ-II is a four-field-period low-magnetic-shear stellarator. A fast movable Langmuir probe array is inserted into the plasma edge region at a velocity ≈ 1 m/s. <R> = 1.5 m <a> ≤ 0.22 m B0 ≤ 1.2 T <ne>≈ (0.5 - 1.2) x 1019 m-3 PECRH ≤ 600 kW fECRH = 53.2 GHz Pulse length ≤ 250 ms

4. Langmuir and Mach Probes Measurements of edge plasma profiles in one shot. Data obtained in two separated radial positions. Measurements of Mach Number profiles. Ion saturation current Is nTe1/2 Floating potential Vf ≈ Vp - Te Mach Number  v/cs

5. Parallel and Poloidal Flows • A steep gradient in the Ion Saturation Current profile is observed close to the shear layer. • The Floating Potential changes sign close to the shear layer. • Shear of parallel flow appears close to the poloidal velocity shear layer. • The resulting radial gradient dvphase/dr is in the range of 105 s-1, comparable to the inverse of the correlation time of fluctuations (t ≈ 10  s). • The presence of sheared flows with shearing rates close to the critical value modify plasma turbulence in the plasma boundary. • In some plasma conditions a reduction in the level of fluctuations close to the velocity shear layer has been obtained.

6. Parallel and Poloidal Flows

7. Radial Correlation Reduction The radial correlation decreases in the proximity of the velocity shear layer. Velocity shear layer suppresses low frequency fluctuations. radially inwards Shear Layer

8. Parallel Flows (100_40_90) Shear of parallel flow appears close to the poloidal velocity shear layer. ≈ 4 104 m/s ≈ -2 104 m/s The absolute value of the Mach Number (M= 0.4xln(IsatCt / IsatCo) could be affected by differences in the probe areas as an offset.

9. Magnetic Configurations It is possible in TJ-II to compare different magnetic configurations with a close value of the rotational transform and different magnetic well. A region having magnetic well in the bulk of the plasma can coexist with a region having magnetic hill in the plasma edge.

10. TJ-II Flexibility Vacuum Magnetic Configurations with different magnetic well 0.6 % 0.2 % 2.4 %

11. Fluctuations Changes Strong change in fluctuations is obtained for the most unstable configuration (reduced magnetic well).

12. Edge Instabilities The strong events observed in the most unstable configuration are observed simultaneously in the H, ECE and line average density signals, similarly to ELMs events characteristics.

13. Radial Velocity The measured radial time delay in the propagation of large transport events implies radial velocities up to 500 - 1000 m/s. This velocity seems very similar to the radial propagation of ELMs in tokamaks (e.g. JET).

14. Magnetic Well Effects in Profiles Electron density profiles measured by reflectometry show differences in shape depending on the magnetic well value.

15. Parallel Flows and Instability • Although no changes have been observed in the poloidal velocity shear, parallel flows are affected by the degree of instability in the plasma boundary region.

16. Transport and Gradients • As the density gradient increases above the most probable gradient, • the ExB turbulent driven transport increases • the system perform a relaxation which tends to drive the plasma back to the marginal stable situation which minimized the size of transport events.

17. Radial Velocity and Electric Fields • The radial velocity is close to 20 m/s for small deviations from the averaged gradient but increases up to 1000 m/s for large transport events (reduced magnetic well). • The effective radial velocity is consistent with the ExB drift velocity.

18. CONCLUSIONS • Comparative studies for different magnetic configurations with different magnetic well show changes in density and potential fluctuations in the plasma edge suggesting the existence of an instability threshold. • The radial velocity obtained for the large events associated to the magnetic well-induced instability is in the range of 1000 m/s. • The bursty beaviour of turbulent transport is linked to a departure from the most probable radial gradient. • Sheared poloidal end parallel flows are linked in the proximity of the LCFS of TJ-II in agreement with previous results obtained in tokamaks. • Parallel flow affected by the degree of instability although no relevant modification has been found in the poloidal flow as the magnetic well changes.