  The dynamics of relativistic runaway electrons are analyzed using the
relativistic Fokker-Planck equation including deceleration due to the
synchrotron radiation and radial diffusion loss caused by stochastic magnetic
fluctuations (SMFs). SMFs are treated as friction force in [J. Martin-Solis et
al., Phys. Plasmas 6, 3925 (1999)]. However, we think SMFs act as a porter in
configuration space, but not directly affect the runaway electrons (REs) in
momentum space. Both critical electric fields for sustainment of the existing
REs and for avalanche onset are enhanced, and the modified avalanche growth
rate is reduced by the combined effects of SMFs and synchrotron radiation as
compared to the case with only synchrotron radiation [P. Aleynikov et al.,
Phys. Rev. Lett. 114, 155001 (2015)].
