INSTITUTE OF PHYSICS PUBLISHING and INTERNATIONAL ATOMIC ENERGY AGENCY NUCLEAR FUSION Nucl. Fusion 43 (2003) 870–882 PII: S0029-5515(03)67507-1 Real-time control of the q -profile in JET for steady state advanced tokamak operation D. Moreau 1,2 , F. Crisanti 3 , X. Litaudon 2 , D. Mazon 2 , P. De Vries 4 , R. Felton 5 , E. Joffrin 2 , L. Laborde 2 , M. Lennholm 2 , A. Murari 6 , V. Pericoli-Ridolfini 3 , M. Riva 3 , T. Tala 7 , G. Tresset 2 , L. Zabeo 2 , K.D. Zastrow 5 and contributors to the EFDA-JET Workprogramme 1 EFDA-JET Close Support Unit, Culham Science Centre, Abingdon, OX14 3DB, UK 2 Euratom-CEA Association, CEA-DSM-DRFC Cadarache, 13108, St Paul lez Durance, France 3 Euratom-ENEA Association, C.R. Frascati, 00044 Frascati, Italy 4 Euratom-FOM Association, TEC Cluster, 3430 BE Nieuwegein, The Netherlands 5 Euratom-UKAEA Association, Culham Science Centre, Abingdon, UK 6 Euratom-ENEA Association, Consorzio RFX, 4-35127 Padova, Italy 7 Euratom-Tekes Association, VTT Processes, FIN-02044 VTT, Finland Received 31 October 2002, accepted for publication 13 August 2003 Published 28 August 2003 Online at stacks.iop.org/NF/43/870 Abstract In order to simultaneously control the current and pressure profiles in high performance tokamak plasmas with internal transport barriers (ITB), a multi-variable model-based technique has been proposed. New algorithms using a truncated singular value decomposition (TSVD) of a linearized model operator and retaining the distributed nature of the system have been implemented in the JET control system. Their simplest versions have been applied to the control of the current density profile in reversed shear plasmas using three heating and current drive actuators (neutral beam injection, ion cyclotron resonant frequency heating and lower hybrid current drive). Successful control of the safety factor profile has been achieved in the quasi-steady-state, on a timescale of the order of the current redistribution time. How the TSVD algorithm will be used in the forthcoming campaigns for the simultaneous control of the current profile and of the ITB temperature gradient is discussed in some detail, but this has not yet been attempted in the present pioneering experiments. PACS numbers: 52.55.Fa, 52.55.Wq, 52.25.Fi, and possibly 52.50.Gj, 52.50.Qt, 52.50.Sw 1. Introduction The control of so-called ‘advanced’ plasma regimes [1–3] for steady state high performance tokamak operation is a challenge, in particular because of the non-linear coupling between the current density and the pressure profile, leading to the emergence of and interplay between internal transport barriers (ITB), large bootstrap current fractions and plasma rotation, and weak or negative magnetic shear. In a burning plasma, the alpha-particle power will also be a strong function of these profiles, and, through its effect on the bootstrap current, will be at the origin of a large (though ultra-slow) redistribution of the current density. The possible destabilization of adverse toroidal Alfv´ en eigenmodes (TAE)—such as the drift kinetic modes, which are anticipated to appear at high values of the central safety factor [4]—as well as potential thermal instabilities due to the ITB dynamics will further complicate the issue. A strategy for reaching a high performance plasma state in an advanced tokamak scenario is to preform an optimized current density profile early in the discharge by combining the skin effect (fast Ohmic current ramp) with off-axis non- inductive currents. During this phase of the discharge, the low plasma density and pressure are beneficial for efficient current drive and for avoiding pressure driven instabilities at rational magnetic surfaces, respectively. A slow interchange between the non-inductive current and the bootstrap component must then take place while the density rises together with intense plasma heating. Adequate plasma control will be required in order to maintain the pressure and current profiles close 0029-5515/03/090870+13$30.00 © 2003 IAEA, Vienna Printed in the UK 870