  Betatron x-ray sources from laser-plasma accelerators combine compactness,
high peak brightness, femtosecond pulse duration and broadband spectrum.
However, when produced with Terawatt class lasers, their energy was so far
restricted to a few kilo-electronvolt (keV), limiting the range of possible
applications. Here we present a simple method to increase the energy and the
flux by an order of magnitude without increasing the laser energy. The orbits
of the relativistic electrons emitting the radiation were controlled using
density tailored plasmas so that the efficiency of the Betatron source is
significantly improved.
