Control of the resistive wall mode with internal coils in the DIII–D tokamak (original) (raw)

New coils were installed inside the vacuum vessel of the DIII-D device for producing nonaxisymmetric magnetic fields. These " Internal-Coils" are predicted to stabilize the Resistive Wall Mode (RWM) branch of the long-wavelength external kink mode with plasma beta close to the ideal wall limit. Feedback using these new Internal-Coils was found to be more effective when compared with using the External-Coils located outside the vacuum vessel, because the location inside the vessel allows faster response and their geometry also couples better to the helical mode structure. A proper choice of feedback gain increased the plasma beta above the no-wall limit to C 0.9, where C is a measure of achievable beta above no-wall limit defined as (no-wall.limit )/( ideal.wall.limitno.wall.limit ). The feedback system with Internal-Coils can suppress the RWM up to the normalized growth rate w ~ 10 ( w is the resistive flux penetration time of the wall). The feedback-driven dynamic error field correction helps to stabilize the RWM by reducing the rotational drag for rot > crit , where rot is the angular rotation frequency of plasma and crit is the critical value for the rotational stabilization. When rot < crit /2, the feedback system must stabilize the RWM mainly through direct magnetic control of the mode. The estimated crit / A is 2.5% by the MARS-F code analysis with experimentally observed profiles, where A is the Alfvén angular rotational frequency at q = 2 surface. The MARS-F code also predicts that for successful RWM magnetic feedback control the power supply characteristic time should be a fraction of the growth time of the targeted RWM.