  When a fs duration and hundreds of kA peak current electron beam traverses
the vacuum and high-density plasma interface a new process, that we call
relativistic transition radiation (R-TR) generates an intense $\sim100$ as
pulse containing $\sim$ TW power of coherent VUV radiation accompanied by
several smaller fs duration satellite pulses. This pulse inherits the radial
polarization of the incident beam field and has a ring intensity distribution.
This R-TR is emitted when the beam density is comparable to the plasma density
and the spot size much larger than the plasma skin depth. Physically, it arises
from the return current or backward relativistic motion of electrons starting
just inside the plasma that Doppler up-shifts the emitted photons. The number
of R-TR pulses is determined by the number of groups of plasma electrons that
originate at different depths within the first plasma wake period and emit
coherently before phase mixing.
