The experiments were carried out on the Vint-20 ℓ = 1 torsatron with a magnetic configuration characterized by large field inhomogeneity. Alternating electric field with a broad spectrum (Δf ∼ 2 MHz) were excited in the frequency range 2–30 MHz in a microwave discharge plasma with a density ≳ 109 cm−3, an electron temperature ∼ 5 eV, and cold ions. Superbanana diffusion was reduced as a result of the effect on the trapped electrons, and this was reflected in a three-to-fivefold increase in plasma density. The largest decrease in superbanana diffusion was observed at excitation field frequencies 3–4 times greater than the theoretical oscillation frequency value for electrons located between adjacent magnetic mirrors.