Equinox: A Real-Time Equilibrium Code and its Validation at JET

The real-time reconstruction of the plasma magnetic equilibrium in a Tokamak is a key point to access high performance regimes. Indeed, the shape of the plasma current density profile is a direct output of the reconstruction and has a leading effect for reaching a steadystate high performance regime of operation. The challenge is thus to develop methods and algorithms that reconstruct the magnetic equilibrium in the perspective to use these outputs for feedback control purposes. But in present days tokamaks only the shape of the plasma boundary is routinely identifiable in real-time in less than few milliseconds using a set of magnetic and diamagnetic coils spread around the vessel. This information is mainly used for controlling the plasma shape in realtime during a plasma discharge using coils current in a feedback control loop. The idea is to achieve a required shape and to maintain it in a stationary manner in order to avoid for example sudden termination of the plasma when the plasma touches the first wall. In JET the so-called XLOC code is used routinely for plasma shape control. Based on this JET flux boundary code confinement parameters are deducted like the diamagnetic energy, the internal inductance and plasma separatrix geometry in less than 1ms. But with this algorithm it is not possible to compute the internal magnetic flux configuration which is needed if we want to analyze the phenomenon occurring in the interior of the plasma. In this case the only way to get access to the current density profile is to use off-line codes that can compute accurately the profile but with no possibility to act in real time on it. This is rather a strong limitation because we know from the analysis performed that the shape of the current density profile is one of the key element to enhance the plasma performance. We have seen in particular that non monotonic current density profiles can trigger enhanced particles and heat confinement. On top of this the current density profile has a resistive diffusion time and any variation of the current drive systems takes some time to be efficient. So it is clear that by controlling in real time such a profile, taking into account the effect of disturbances that tends to adversely affect the time behaviour of the controlled variables, we insure stability but also performance.
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