FHFIs in terms of frequency, ω_r/|Ω_e| (left), and growth rate, γ/|Ω_e| (right), as functions of wave number kc/ω_pe, obtained with DIS-K for Maxwellian electron populations and different strahl speeds, u_s. The reference case (u_s = 0.02c) is compared with unstable solutions derived for lower drifts down to 0.0175c and also higher ones up to 0.025c. The other parameters used are indicated in the title.