IOP PUBLISHING and INTERNATIONAL ATOMIC ENERGY AGENCY NUCLEAR FUSION Nucl. Fusion 48 (2008) 115003 (9pp) doi:10.1088/0029-5515/48/11/115003 Bicoherence during confinement transitions in the TJ-II stellarator B.Ph. van Milligen, T. Kalhoff, M.A. Pedrosa and C. Hidalgo Asociaci´ on EURATOM-CIEMAT para Fusi´ on, Avda. Complutense 22, 28040 Madrid, Spain E-mail: boudewijn.vanmilligen@ciemat.es Received 3 July 2008, accepted for publication 29 August 2008 Published 25 September 2008 Online at stacks.iop.org/NF/48/115003 Abstract Bicoherence was computed for a number of discharges having confinement transitions, either induced by biasing or spontaneous, at the TJ-II stellarator. The bicoherence was computed for a variety of signals obtained using a reciprocating Langmuir probe system, but the main emphasis here is on the analysis of the poloidal electric field (E θ ). During biasing, the auto-bicoherence of E θ was significant only in a narrow radial range, in contrast to the fluctuation level of E θ and the linear coherence between E θ and E r (the radial electric field), which were affected over a very broad radial extension. With spontaneous transitions, significant bicoherence was again detected only in a narrow radial range, associated with the position of the flow shear layer. The observations are consistent with the standard sheared flow model for confinement transitions. The temporal asymmetry A of E θ was also computed. During biasing, the region of strongly modified asymmetry was located inwards from the bicoherence maximum. This suggests that the detected bicoherence is not merely due to a change in the temporal shape of the signal. PACS numbers: 52.55.Hc, 52.35.Ra (Some figures in this article are in colour only in the electronic version) 1. Introduction In the flexible heliac TJ-II, confinement transitions are observed, which can be either induced (by applying biasing) or spontaneous. The improved confinement state is characterized mainly by a higher (energy and particle) confinement time, which is reflected in, e.g., a higher value of the electron density and correspondingly higher values of the density gradient near the edge. The confinement transition is associated with the formation of a sheared flow layer in the edge plasma [1]. Sheared flow is an important theoretical ingredient of models attempting to explain such confinement transitions [2]. Recently, both diagnostic and theoretical developments have sparked renewed efforts directed towards the detection of the effects predicted by such theories [3, 4]. The theories assume that sheared flow is responsible for the establishment of an edge transport barrier, by means of a negative feedback between the sheared flow amplitude and the turbulence amplitude. On the other hand, the sheared flow itself is presumed to be generated by the turbulent fluctuations via an inverse energy cascade (from high to low wave-numbers). The detection of this inverse cascade is, however, not trivial [5]. Thus, most efforts in this respect have centred on the detection of mode coupling by means of bicoherence, using various turbulent signals (but mostly Langmuir probes) [5–10]. The goal of this paper is to study the bicoherence during forced and spontaneous confinement transitions at TJ-II. As pointed out in [5], not much is known about the behaviour of the bicoherence during confinement transitions. Such transitions are characterized by a critical point (a bifurcation). The experimental difficulty resides in the fact that the critical point is unstable and, typically, plasmas are either far below or far above the critical condition, and if the transition is made, it occurs very fast. The stellarator TJ-II is in a unique position to investigate this issue due to the specific magnetic configuration of this device: the transitions at TJ-II are ‘soft’ and the plasma can remain close to the transition for some time and cross the critical point ‘slowly’, thus allowing a study of the evolution of parameters during the transition. The outline of this paper is as follows: first, we discuss the experimental set-up and methods used, and then we present the results from biasing experiments and spontaneous transitions. Finally, we present a discussion and draw some conclusions. 0029-5515/08/115003+09$30.00 1 © 2008 IAEA, Vienna Printed in the UK