The diffusion of electrons in velocity space induced by a spectrum of electrostatic waves is numerically investigated. From the scatter of particles, the diffusion coefficient is calculated and compared with the quasilinear value. A strong disagreement is found at high electric field amplitudes, narrow spectra and short (spatial) lengths of the interaction region, which are however usual in Lower Hybrid Heating and Current Drive experiments in tokamaks. In particular, the (velocity) interaction region becomes significantly broader than the Landau resonance region for electric field amplitudes above .
Velocity diffusion in plasma waves excited by electron beams