24aA13
Interaction between turbulence and shear flows during L-H transitions in TJ-II
plasmasT. Estrada1, T. Happel2, C. Hidalgo1, E. Ascasíbar1, E. Blanco1 and the TJ-II Team1

Laboratorio Nacional de Fusión. As. Euratom-CIEMAT, 28040, Madrid, Spain
2
MPI für Plasmaphysik, As. Euratom-IPP, 85748 Garching, Germany

The interaction between plasma turbulence and flows has been measured during the L-H transition in TJII plasmas by means of Doppler reflectometry. The results show that mean sheared flow is not the uniqueelement to explain the reduction of turbulence at the transition, oscillating sheared flows have to be also
considered. Besides, close to the L-H transition threshold conditions turbulence and flows display anoscillatory behaviour with a characteristic predator-prey relationship. These results are consistent withmodels predicting L-H transition triggered by zonal flows. Finally, the characterization of the H-L backtransition has revealed signatures of spreading of the turbulence from the plasma core to the edge barrier.

1. Introduction
Spontaneous transitions from the Low to theHigh confinement mode (L-H transitions) areobserved in fusion plasmas where a reduction in
the density turbulence, ñe, is found linked to apronounced shear in the radial electric field, Ershear. The Doppler reflectometry (DR) techniqueoffers the possibility to measure these magnitudes
simultaneously and with good spatiotemporal
resolution allowing the investigation of thephysical mechanisms involved in the transition. In
particular, the Doppler reflectometer installed at
the TJ-II stellarator was carefully designed tofulfil the optimization requirements for a good
spectral resolution [1] and has been used to study
the L-H transition physics in TJ-II plasmas.
In TJ-II, spontaneous L-H transitions areobserved in pure NBI heated plasmas under Li
coated wall conditions [2]. ”Fast” as well as“gradual” L-H transitions are observed dependingon the heating power level and/or magnetic
configuration, facilitating a detailed study of thephysical mechanisms involved in the L-Htransition. Furthermore, impurity accumulation
during the H-mode and the concomitant increasein the radiation losses occasionally bring theplasma into the H-L back-transition conditions,allowing its characterization and the comparison
with the L-H transition.
2. Experimental Results
2.1 Turbulence and flows at L-H transition
L-H transitions show a pronounced reductionin ñe in the edge plasma region where the Er-shear
develops. However, high temporal resolution
measurements show that the reduction in ñe
precedes the increase in the mean Er-shear.
Furthermore, simultaneous with the reduction in

turbulence, an increase in the Er-shear low
frequency oscillations is measured [3]. Theseobservations indicate that mean sheared flow is
not the unique element to explain the reduction ofturbulence at the transition, oscillating shearedflows have to be also considered. These results areconsistent with models predicting L-H transition
triggered by zonal flows [4].
2.2 Turbulence and flows close to the L-H
transition threshold conditions
Further experimental evidence supporting thecentral role of zonal flows has been observed
close to the L-H transition threshold conditions. In
these cases, pronounced oscillations in both, Ershear and fluctuation level are measured at theplasma edge. This oscillatory behaviour isdisplayed in fig. 1. It shows a characteristic
predator-prey relationship between turbulence andflows, with the flow –the predator– following theturbulence –the prey– with a phase delay of 90º in
a limit-cycle way (see fig. 2) [5]. The turbulenceflow oscillation-pattern repetition frequency
decreases as the plasma density increases, and theEr oscillation amplitude decreases while that of
the turbulence level increases. These observations
can be explained based on the collisional dampingof flows, which eventually sets the turbulence
level. The coupling found between turbulence andflows follows that found in the predator-preytheory model [4]. In this model the turbulence
driven zonal flow triggers the transition byregulating the turbulence until the mean flow ishigh enough to suppress turbulence effectively.
Due to the self-regulation between turbulence andflows, the transition is marked by an oscillatory
behaviour with a characteristic predator-prey
relationship between turbulence and zonal flows.

AD-4

10-5

r

10

S (a. u.)|E | (kV/m)

105–where the key quantity to the control of
turbulence spreading was found to be the Ershearing rate– and point to the possible role ofradial spreading of turbulence in determining thewidth of transport barriers.

-6169.5

170.0

170.5

171.0

-5

time (ms)Fig. 1. The time evolution of Er (solid line) and density
fluctuation level (broken line) showing a predator-preyrelationship.

HH --> L
L back-10

r

169.0

0

E (kV/m)

0168.5

10-15a)-20
0.7#23473, t=170-170.4 ms

100.85

0.9
time to H-Lback-transition
(ms)
7-6
2-1
1-0-4

H/L

80.8!

0(dB)

20.75b)1ñr

|E | (kV/m)

12-8

63-710-610-510S (a.u.)-4

10Fig. 2. Relation between Er and density fluctuationlevel showing a limit-cycle behaviour. The timeinterval between consecutive points is 12.8 µs.

2.3 Turbulence and flows at the H-L backtransition
H-L back-transition experiments at TJ-II haverevealed signatures of spatial spreading of plasma
turbulence [6]. In these experiments, DR
measurements show a gradual decrease in Er andEr-shear as the plasma approaches the backtransition and a fast drop in the Er-shear right atthe back-transition (see fig. 3.a). The turbulence
level shows a distinct behaviour at both sides of
the Er-shear position. Outside the Er-shear theturbulence level remains almost unchanged as theback-transition is approached and increases in avery fast time scale right at the transition.
Contrary, the turbulence level measured at innerradial positions increases gradually, reachingvalues right before the back-transition comparable
to those measured at the L-mode (see fig. 3.b).
These experimental results suggest the following
scenario: Radial spreading of the turbulence,braked during the H-mode by the high Er-shear,becomes noticeable as Er-shear declines andproduces a gradual increase in the turbulence levelat the internal radial positions, reaching the Ershear location right before the H-L backtransition. The consequence is a gradual retreat of
the transport barrier quantified as the width of theturbulence reduction region. These experimental
results resemble simulation studies reported in [7]

-12
0.7

0.75

0.8!

0.85

0.9

Fig. 3. Profiles of (a) Er measured during the H-mode(red circles), right before the H–L back-transition (bluesquares) and in the L-mode (green diamonds), and (b)turbulence reduction (ñH/ñL) measured 7, 2 and 1 ms
before the back-transition.

3 Conclusions
The interaction between turbulence and flows
measured in TJ-II plasmas, both, during the L-H
transition and in plasma conditions close to the LH transition threshold, is consistent with models
predicting L-H transitions triggered by zonalflows. H-L back-transition results point to thepossible role of radial spreading of turbulence indetermining the width of transport barriers.
Acknowledgments
This work has been partially funded by theSpanish Ministry of Science and Innovation undercontract number ENE2010-18409.
References[1] T. Happel et al., Rev. Sci. Intrum. 80 (2009)
073502[2] J. Sánchez et al., Nuclear Fusion 49 (2009) 104018[3] T. Estrada et al., Plasma Phys. Control. Fusion 51(2009) 124015[4] E.-J. Kim and P.H. Diamond. Phys. Plasmas 10(2003) 1698
[5] T. Estrada et al., EPL (Europhysics Letters) 92(2010) 25001[6] T. Estrada et al., Nuclear Fusion 51 (2011) 032001[7] W.X. Wang et al., Phys. Plasmas 14 (2007)
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