A one-dimensional model of a small plasma ion source ͑10 cm long͒ is studied. A hybrid simulation where ions are treated as particles and electrons as a fluid obeying the Boltzmann relation is used to investigate ion heating in the plasma presheath. At low pressure ͑below a few mTorr͒, the ion velocity distribution is Maxwellian in the bulk and becomes a drifting Maxwellian distribution while transiting the presheath. The distribution remains essentially isotropic as the ions are accelerated through the presheath to satisfy the Bohm criterion. At intermediate pressures ͑around 10 mTorr͒, ion-neutral collisions scatter a significant part of the ion kinetic energy from the parallel direction to the perpendicular direction, leading to a net heating of the ions. In addition, the ion velocity distribution becomes distinctly anisotropic. At higher pressure ͑above a few tens of mTorr͒, ion heating is still observed, but yields isotropic ion velocity distributions.
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