1 Confinement Studies on TJ-II Stellarator with OH Induced Current F. Castejón, D. López-Bruna, T. Estrada, J. Romero and E. Ascasíbar Laboratorio Nacional de Fusión Asociación Euratom/Ciemat para Fusión Av. Complutense, , Madrid (Spain)

2 Outline »TJ-II characteristics »Description of the experiments: General observations »Effect of plasma current on confinement: Negative currents Positive currents »Discriminating E  and I p effects »Discussion and conclusions

3 TJ-II stellarator Rotational transform: 0.9 ≤  (0)  ≤ 2.1 OH coils allow inducing toroidal current to modify  profile. Rotational transform: 0.9 ≤  (0)  ≤ 2.1 OH coils allow inducing toroidal current to modify  profile. R = 1.5 m, ≤ 0.22 m B 0 < 1.2 T ECRH, P heating ≤ 600 kW ≤ m -3 Helical axis stellarator with high flexibility TF COILS HX,CC COILSPLASMA OH COILS VF COILS

4 TJ-II Characteristics TJ-II is a very suitable device to study the influence of Rotational Transform and Magnetic Shear on confinement, since: –It has high flexibility –It is an almost shearless device: the plasma current modifies strongly the Vacuum Rotational Transform Profile. –Non inductive Current Drive (ECCD) allows one to discriminate the influence of plasma current and toroidal electric field on confinement. ECRH plasmas: low density, low  ( %), Te>Ti and low collisionality. In TJ-II: negative (positive) toroidal currents diminish (increase)  value and causes negative (positive) magnetic shear. Shear Definition: à la tokamak:

5 Description of the experiments Plasma current induced by OH coils: Different ramps (different loop voltages)

6 Description of the experiments  Ip=+8.5 kA Ip=-0.6 kA Ip=-8.2 kA Rotational transform 100_44_64  profile is strongly modified by plasma current.  crosses the several low order resonances that appear as jumps in the central ECE channels*. * T. Estrada et al. Plasma Phys. Control. Fusion 44 (2002) 1

7 General observations Distinctive “signature” with OH induction: –Transitions of central ECE signals (due to rational  (0) sweeping) –Confinement changes observed in Density and SXR signals. –Small jumps for given values of Ip (Vloop) High reproducibility MHD events clearly distinguishable in Mirnov coils. Higher confinement with I p 0, up to I p ≈ 6 kA Weaker effects for low densities

8 Negative current Strong negative current is induced from t=1100 ms: dI OH /dt=-28 kA/s Confinement is improved as can be seen in: –Line density –SXR –Central ECE channels. Two discharges with different densities are compared: –Different currents are due to bootstrap (more negative for shot #7045, in black) –The effect of OH current is stronger for higher density.

9 Moderate negative magnetic shear Substantial improvement (between 30 and 50 %) of the energy content for negative plasma current (negative magnetic shear)* * J.A. Romero et al Submitted to Nuclear Fusion

10 Negative current Evolution of line density vs. current in three discharges with different densities: The more negative the current the higher the density. (In the lowest density case the maximum value of loop voltage has been limited to avoid XR emission)

11 Positive current Strong Positive current is induced from t=1120 ms: dI OH /dt=28 kA/s Confinement is first degraded and then restored. Two discharges with different densities are compared: –As before, the effect of OH current is stronger for higher density.

12 Positive current Evolution of line density vs. current in three discharges with different densities: # 8681 (low), # 8682 (middle), #8683 (moderate). Effects are stronger for higher densities. The confinement is first degraded and then restored for plasma current larger than about 6 kA.

13 Positive current For high positive currents the confinement is similar to the case without OH.

14 Dynamical evolution Evolution of line density profiles (interferometer) during the current evolution (up to 5 kA; t=1250 ms): Phase of negative current: confinement improvement. Phase of positive current: confinement degradation.

15 Sign of magnetic shear OH experiments: plasma currents up to +/- 10 kA have been induced. Negative plasma currents (negative magnetic shear): Improvement of confinement. Positive plasma current (positive magnetic shear): Confinement degradation up to Ip ≈ 6 kA. For higher plasma currents the confinement is restored. OH experiments: plasma currents up to +/- 10 kA have been induced. Negative plasma currents (negative magnetic shear): Improvement of confinement. Positive plasma current (positive magnetic shear): Confinement degradation up to Ip ≈ 6 kA. For higher plasma currents the confinement is restored. Non-symmetric dependence of confinement on the sign of the shear

16 Discriminating E  and I p Question: Are the changes in confinement due the toroidal electric field field (particle orbit modification) or to the current (magnetic shear)? Estimation of drift (average on a magnetic surface) shows that it is too low to justify the improvement (degradation) of confinement. Experiments performed to uncouple toroidal current and toroidal field using ECCD.

17 Discriminating E  and I p : ExB drift Calculated in 3D Geometry* Range ± 0.07 m/s for 1Volt Flux averaged is s*0.01m/s for 1 Volt. Too small (two orders of magnitude) to account for overall observed effect, considering: But it can affect trapped particle confinement. *J. Guasp and M. Liniers. Informe Ciemat 946, Madrid, 2000

18 Discriminating E  and I p Ip Confinement Improvement time If the field is the cause. ECCD Ip EE Confinement Iprovement Iboot Confinement Improvement time if the shear is the cause. EE

19 Discriminating E  and I p N // =0.2, 0, -0.2, Without OH Four discharges with similar density. For the discharges with OH the loop voltage is the same: dI OH /dt=5.5 kA/s (t< 1170ms) dI OH /dt=-32.4 kA/s (t> 1170ms) ECCD is varied from negative to positive values. The transition happens for similar values of the current. The main responsible is the current (the shear).

20 Discussion and Conclusions Current (and magnetic shear) main responsible of confinement changes during OH experiments on TJ-II. Weak, if any, effects of toroidal electric field. Effect of shear on confinement depending on the sign. Look for causes with non symmetric dependence on the shear sign.

21 Discussion and Conclusions Possible causes of transport modification (non exhaustive list): Effect of local shear sign on drift waves*? -> Problem: for these low  ’s the modification of local shear seems to be very weak. Trapped electron modes**: should appear for Te>>Ti, for low collisionality and large fraction of trapped particles. These conditions happen in TJ-II (Fraction of trapped particles about 35 %†) Modification of particle orbits due to changes in magnetic topology ‡. (Explanation for the restoring of confinement for high positive currents?) *N. Nadeem, T. Rafiq and M. Persson. Phys. Plasmas 8 (2001) 4375 ** N. Domínguez, B. Carreras, V. Lynch and P. Diamond. Phys. of Fluids B 4 (1992) 2894 † J. Guasp and M. Liniers. Informes Técnicos Ciemat 946. Madrid, ‡ J. Guasp and M. Liniers. Informes Técnicos Ciemat 951. Madrid, 2000.

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