  Transport of fast electron in overdense plasmas is of key importance in high
energy density physics. However, it is challenging to diagnose the fast
electron transport in experiments. In this article, we study coherent
transition radiation (CTR) generated by fast electrons on the back surface of
the target by using 2D and 3D first-principle particle-in-cell (PIC)
simulations. In our simulations, aluminium target of 2.7 g/cc is simulated in
two different situations by using a newly developed high order implicit PIC
code. Comparing realistic simulations containing collision and ionization
effects, artificial simulations without taking collision and ionization effects
into account significantly underestimate the energy loss of electron beam when
transporting in the target, which fail to describe the complete characteristics
of CTR produced by electron beam on the back surface of the target. Realistic
simulations indicate the diameter of CTR increases when the thickness of the
target is increased. This is attributed to synergetic energy losses of high
flux fast electrons due to Ohm heatings and colliding drags, which appear quite
significant even when the thickness of the solid target only differs by
micrometers. Especially, when the diagnosing position is fixed, we find that
the intensity distribution of the CTR is also a function of time, with the
diameter increased with time. As the diameter of CTR is related to the speed of
electrons passing through the back surface of the target, our finding may be
used as a new tool to diagnose the electron energy spectra near the surface of
solid density plasmas.
