Nonlinear dynamics of runaway electron induced wave instabilities can significantly modify the runaway distribution critical to tokamak operations. Here we present a fully kinetic simulation of runaway-driven instabilities toward nonlinear saturation in a warm plasma where collisional damping is subdominant. It is found that the slow-X modes grow an order of magnitude faster than the whistler modes, and they parametrically decay to produce whistlers much faster than those directly driven by runaways. These parent-daughter waves, as well as secondary and tertiary wave instabilities, initiate a chain of wave-particle resonances that strongly diffuse runaways to the backward direction. This reduces almost half of the current carried by high-energy runaways, over a time scale orders of magnitude faster than experimental shot duration. These results beyond quasilinear analysis may impact anisotropic energetic electrons broadly in laboratory, space, and astrophysics.

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• Electromagnetic waves & oscillations
• Magnetic confinement fusion
• Nuclear fusion
• Plasma fusion
• Plasma instabilities
• Plasma interactions
• Plasma stability
• Plasma waves
• Wave-wave, wave-particle interactions
• Magnetically confined plasmas
• Magnetized plasma
• First-principles calculations in plasma physics
• Fokker-Planck & Vlasov model
• Particle-in-cell methods
• Plasma kinetic theory