Saturation level of the magnetic fluctuations. Panel a: wave saturation intensity as function of n_cr/n_i. Blue circles present the results from 1D3V PIC simulations. The red dashed line follows the scaling from Eq. (17) that balances the momentum loss by CRs with momentum gained by waves. The data point at n_cr/n_i = 10⁻³ is a lower limit as the simulation ended before a saturated state was reached. Panel b: same but varying m_i/m_e at fixed n_cr/n_i = 10⁻² (blue symbols) and n_cr/n_i = 5 × 10⁻³ (red symbols).