Enhancement of magnetic diagnostics for JET

In view of extended performance of JET, as foreseen in the experimental campaigns starting in 2005, a number of enhancements of the machine are in course of implementation. The enhancement project of the magnetic diagnostics, under the direct responsibility of the Association EURATOM-ENEA, aims at the design, procurement, assembly, installation and commissioning of new sets of magnetics coils inside and outside the vessel, in order to substantially improve the current capabilities of the JET magnetics. Furthermore, analytical and numerical studies are needed for justifying/validating the lay-out of the new sensors (near term action) and for improving the detailed plasma equilibrium reconstruction (longer term action). The activities are conducted in close interaction with the JET operator, which is responsible for the operation of the machine and for the remote handling operations needed for the installation. The most important tasks requested of the magnetic diagnostics and accompanying interpretation software are: i) fast real time control of the vertical position for highly elongated plasmas; ii) slow real time control of the plasma current and shape; iii) provide experimental input for equilibrium reconstruction and interpretation codes; iv) measurement of MHD modes, with reasonable spatial and temporal resolution. The system should contribute to better integration of various diagnostics for real time profile control, and could also provide experimental input for ITER relevant studies on non axisymmetric equilibria. With respect to the above tasks, the present magnetic system has a number of inadequacies, to be removed with the new system, because: i) a number of sensors are faulty and their replacement is questionable; ii) there is lack of sensors near the top of the plasma, so secondary top X-points are difficult to diagnose; iii) the accuracy of the reconstruction provided by the present set is not fully satisfactory. iv) the equilibrium reconstruction codes can produce large errors. The new system is intended to improve the performance of the magnetics, with respect to the following functions: i) after pulse reconstruction of equilibrium map (1KHz); ii) slow real time control of the plasma current and shape (1-5 KHz): IP, betapol, li, current moments, gaps, strike points); iii) detection of MHD modes (1 MHz); iv) fast real time control of the vertical position for highly elongated plasmas (10 KHz); v) ex-vessel measurement of iron saturation level, for iron modelling definition and validation. The system mainly consists of in-vessel and ex-vessel sensors. The in-vessel sensor system is a set of two field component coils, located as near as possible to the plasma. With the exception of coils to be dedicated to MHD studies, all coils are at relatively low frequency (up to 50 kHz), which allows robust mechanical properties. Coils are generally grouped in rails, in order to ease RH in vessel installation. The final location of the sensors might be object of further optimisation. The ex-vessel sensor system is dedicated to the detection of iron characteristics. The whole system will include the following subsystems: • Divertor pick-up coils: 6 coil pairs (R- and Z-component) in module 3, at Octant 2; subsystem replicated in module 5 for redundancy, at Octant 2; • Inner lower coils: 1 coil pair (R- and Z-component) in module 3; subsystem replicated in module 5 for redundancy; • Outer poloidal array at Octants 4 and 8: 7+7 coil pairs (slow coils for normal component and fast coils for MHD studies for tangential coils) in octants 4 and 8; • Top coils: 4 coil pairs (tangential and normal component) in octant 4; subsystem replicated in octant 8 for redundancy; • Upper outer coils: 1 coil pair (tangential and normal component) in octant 4; subsystem replicated in octant 8 for redundancy; • Ex-vessel coils: 2 sets (top and bottom) of “Limb” probes, 3 flux loops (45º spanning between 2 limbs or across 1 limb), 3 pick-up coils (1 component normal to iron), 3 Hall effect sensors (1 component normal to iron), 2 sets (top and bottom) of “Collar” probes, 1 pick-up coil (2 components), 1 Hall effect sensor (2 components) Top coils and outer poloidal coils are accommodated into rails, terminated by a multi-pin socket/plug for connection to the wiring going along conduits to the feed-throughs. The engineering design will be based on remote handling installation.