  We demonstrate a 13-fold increase in hard x-ray bremsstrahlung (10 - 200 keV)
emitted by a copper plasma created by 100 fs, 806 nm pulses at
$10^{14}-10^{15}$ Wcm$^{-2}$. This enhancement is achieved by roughening the
target surface with copper nanoparticles of ~15 nm size. A simple model that
invokes local field modifications by surface plasmon excitation and `lightning
rod' effects explains the observed enhancement quantitatively and provides
pointers to the design of structured surfaces for maximizing the emission.
