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Confinement issues


Further investigations are required in order to assess the stability of the m0=1, n0=1 internal modes that can be excited under ignition conditions when the edge safety factor qy is close to the reference value adopted for Ignitor and βp≤0.3. The amplitude reached by the m=2 harmonic could be an additional concern that could not be sufficiently smoothed by the stabilizing fast α particle effects, which are too weak to be effective in the Ignitor’s standard low temperature ignition scenario. It would therefore be important to ensure that the central value of q0 is kept > 1 during most of the current ramp. Furthermore, at the highest plasma currents the qy ≤1 region should not be a large fraction of the plasma volume (≤10% of the total volume) and a class of q profiles should be chosen which are characterized by relatively low magnetic shear between the q=1 and the q=2 resonant surfaces. Providing a moderate ICRH power during current ramp could be envisaged as a method, among others, to control the q profiles.
A second problem for the plasma confinement could be represented by the macroscopic instabilities involving magnetic reconnection (e.g., m=3, n=2 modes) which are driven unstable by the trapped high energy particle population
Off-normal events such as plasma vertical disruption events (VDE) and associated halo currents (HC) also require a detailed analysis. A full current VDE is, in fact, the worst expected event for the plasma chamber integrity, due to the fast (few ms) transfer to the chamber of the whole plasma thermal and magnetic energy with an intrinsic poloidal (and possibly toroidal) asymmetry. This fast VDE can begin with a loss of vertical stabilization capability (due, for instance, to a failure in the vertical control system or to the appearance of unstable, uncontrollable MHD modes) that results in a slowly downward (or upward) moving plasma. The plasma current slowly decreases while the plasma column moves and the safety factor decreases due to the plasma shrinking. When the safety factor at the edge goes below 2, the chance of arising unstable modes increases and eventually the plasma suddenly loses all its thermal energy in the so-called thermal quench.
When the plasma boundary comes in contact with the wall during a disruption, the poloidal halo currents flow from the plasma into the vessel and then again to the plasma along open field lines that are in contact with the wall. The halo current can also be toroidally asymmetric, applying relevant radial forces to the vessel, as well as axisymmetric pressures

1) B. Coppi et al, Nucl. Fusion 41, 1253 (2001)
2) B. Coppi et al, Overview Paper OV/P-02, Proceedings of the 24 th IAEA Fusion Energy Conference, San Diego, US, 8-13/10/2012

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