  A dielectric laser accelerator, operating at optical frequencies and GHz
pulse rate, is expected to produce attosec electron bunches with a moderate
beam current at high energy. For relativistic electrons, the attosec bunch has
a spatial length of a few nanometers, which is well suited for generating
high-brightness superradiance in the VUV, EUV, and x-ray spectra. Our study
shows that the brilliance of coherent undulator radiation driven by a
short-bunch beam with 1~10 fC bunch charge from a dielectric laser accelerator
is comparable to or higher than that of a synchrotron in the 0.1 ~ 3 keV photon
energy range, even though the beam power of the dielectric laser accelerator is
about a million times lower than that of a synchrotron. When the brilliance
under comparison is normalized to the electron beam power, the proposed
coherent undulator radiation source becomes the brightest source on earth
across the whole VUV, EUV, and soft x-ray spectrum.
