  Magnetic field is unstable in a medium with time-independent chiral
conductivity. Owing to the chiral anomaly, the electromagnetic field and the
medium exchange helicity which results in time-evolution of the chiral
conductivity. Using the fastest growing momentum and helicity state of the
vector potential as an ansatz, the time-evolution of the chiral conductivity
and magnetic field is solved analytically. The solution for the hot and cold
equations of state shows that the magnetic field does not develop an
instability due to he helicity conservation. Moreover, it develops a peak only
if a significant part of the \emph{initial} helicity is stored in the medium.
The initial helicity determines the height and position of the peak.
