  We show the possible excitation of a phonon laser instability in an
ultra-cold atomic gas confined in a magneto-optical trap. Such an effect
results from a negative Landau damping of the collective density perturbations
in the gas, leading to the coherent emission of phonons. This laser instability
can be driven by a blue-detuned laser superimposed to the usual red-detuning
laser beams which usually provide the cooling mechanism. Threshold conditions,
instability growth rates and saturation levels are derived. This work
generalizes, on theoretical grounds, the recent results obtained with single
ion phonon laser, to an ultra-cold atomic gas, where real phonons can be
excited. Future phonon lasers could thus adequately be called phasers.
