  We consider the general problem of charged particle motion in a strong
electromagnetic field of arbitrary configuration and find a universal
behaviour: for sufficiently high field strengths, the radiation losses lead to
a general tendency of the charge to move along the direction that locally
yields zero lateral acceleration. The relativistic motion along such a
direction results in no radiation losses, according to both classical and
quantum descriptions of radiation reaction. We show that such a radiation-free
direction (RFD) exists at each point of an arbitrary electromagnetic field,
while the time-scale of approaching this direction decreases with the increase
of field strength. Thus, in the case of a sufficiently strong electromagnetic
field, at each point of space, the charges mainly move and form currents along
local RFD, while the deviation of their motion from RFD can be calculated in
order to account for their incoherent emission. This forms a general
description of particle, and therefore plasma, dynamics in strong
electromagnetic fields, the latter can be generated by state-of-the-art lasers
or in astrophysical environments.
