This book explores the physics of fast ions and fast ion- driven instabilities. It also describes modern theory of near- critical nonlinear wave– particle systems with the particle source and wave damping. Such a theory was developed by H.L. Berk and B.N. Breizman in the mid- 1990s, and it delivered outstanding results successfully explaining the experimentally observed collective phenomena driven by energetic ions. A systematic and step-by-step analysis of resonant interactions between the waves in plasmas and various types of energetic ion populations is presented and analyzed, taking the readers on an exciting journey into the world of nonlinear physics and cutting-edge experiments performed on the world’s major magnetic fusion machines. The phenomena described in this book will be of interest for researchers studying fusion, solar plasma, space plasma, and for a broader realm of scientists working in nonlinear phenomena. Key Features:Features experimental data and the Berk-Breizman theory on nonlinear evolution of energetic particle-driven wavesDescribes in simple terms, the recent advances in the diagnostics of energetic particles and Alfvén wavesPresents a systematic overview of extrapolating results presented in other types of plasmas (e.g., solar and space) and nonlinear systems.