Resonant Magnetic Perturbation
A Resonant Magnetic Perturbation (RMP) is an externally-induced small perturbation of the equilibrium magnetic field, such that it is resonant with the field at a given magnetic flux surface, usually located in the plasma edge region. Resonance means that the perturbation has the same helicity as the equilibrium magnetic field at the target flux surface. The perturbation is assumed to lead to the ergodization of the magnetic field around the resonant flux surface.
The goal is to stabilize a specific instability, mainly Edge Localized Modes (ELMs), in order to reduce the deleterious effects of the corresponding transport bursts on the wall of the device.  Resonance of the externally imposed field with the field line pitch is assumed to be essential for the generation of a stochastic zone and the desired mitigating effect on the instability.  However, recent results indicate that resonance is not strictly necessary to obtain a mitigating effect on ELMs from external field perturbations. 
- T.E. Evans, et al, Edge stability and transport control with resonant magnetic perturbations in collisionless tokamak plasmas, Nature Physics 2 (2006) 419 - 423
- O. Schmitz, T.E. Evans, M.E. Fenstermacher, et al, Resonant Pedestal Pressure Reduction Induced by a Thermal Transport Enhancement due to Stochastic Magnetic Boundary Layers in High Temperature Plasmas, Phys. Rev. Lett. 103 (2009) 165005
- W. Suttrop, T. Eich, J.C. Fuchs, et al, First Observation of Edge Localized Modes Mitigation with Resonant and Nonresonant Magnetic Perturbations in ASDEX Upgrade, Phys. Rev. Lett. 106 (2011) 225004