  A dynamic mitigation mechanism of the two-stream instability is discussed
based on a phase control for plasma and fluid instabilities. The basic idea for
the dynamic mitigation mechanism by the phase control was proposed in the paper
[Phys. Plasmas 19, 024503(2012)]. The mitigation method is applied to the
two-stream instability in this paper. In general, instabilities appear from the
perturbations, and normally the perturbation phase is unknown. Therefore, the
instability growth rate is discussed in fluids and plasmas. However, if the
perturbation phase is known, the instability growth can be controlled by a
superimposition of perturbations imposed actively. For instance, a perturbed
driver induces a perturbation to fluids or plasmas; if the perturbation induced
by the perturbed driver is oscillated actively by the driver oscillation, the
perturbation phase is known and the perturbation amplitude can be controlled,
like a feedforward control. The application result shown in this paper
demonstrates that the dynamic mitigation mechanism works well to smooth the
non-uniformities and mitigate the instabilities in plasmas.
