The halo currents originated during the plasma disruption make more demanding the structural requirements of the tokamak machine vacuum chambers. The IGNITOR [1] vacuum chamber (VC) has been designed since many years when the halo current phenomenon was not yet observed. Hence, the wall thickness was established on the basis of the loads originated during a typical Vertical Displacement Events (VDE) followed by the thermal and current quenches. On the light of the new plasma disruption scenario that envisages the production of a relevant amount at halo current during the current quench the IGNITOR VC has been re-verified, as first attempt, by a linear elastic analysis. The results of the linear elastic analysis did not comply with the allowable limits imposed by ASME III code. So, avoiding to modify the VC geometry which would have an heavy impact on the other machine components it has been decided to verify the VC structural integrity by means of a dynamic elastic-plastic analysis still considering the ASME III code rules. Being the analysis non linear and the various load components distributed with different periodicity, the modelling of the entire (360 deg) VC structure has been necessary and the loads have been applied simultaneously. In this way, it has been demonstrate that the IGNITOR VC is capable to withstand, according to the ASME III code, several thousands of cycles under plasma disruption condition including an average halo current equal to 3MA, which is the 25% of the nominal plasma current, with a toroidal peaking factor equal to 2. The halo current value has been assumed on the basis at relevant experimental data.

The paper will illustrate the main results of the analysis which demonstrates how the elastic-plastic approach could mitigate the structural requirement for the vacuum vessel of tokamak machines. In the IGNITOR particular case, the maximum plastic deformation is below 0.5% and the maximum permanent displacement is below 2mm. On the contrary, to satisfy the elastic limits the wall thickness of the VC should be increased by almost a factor 1.5, thereby affecting the whole machine design.

[1]Coppi B. et al. The IGNITOR Experiment and its perspective F1-CN-64/GP-14 IAEA 1996.