  In situ particle acceleration is probably occuring in cluster radio haloes.
This is suggested by the uniformity and extent of the haloes, given that
spatial diffusion is slow and that radiative losses limit particle lifetimes.
Stochastic acceleration by plasma turbulence is the most likely mechanism.
Alfven wave turbulence has been suggested as the means of acceleration, but it
is too slow to be important in the cluster environment. We propose, instead,
that acceleration occurs via strong lower-hybrid wave turbulence. We find that
particle acceleration will be effective in clusters if only a small fraction of
the cluster energy density is in this form.
