In a previous question, it was established that antimatter could be "mined" through positron-emitting isotopes.
By gathering positrons from decaying positron-emitting isotopes (potassium-40 and aluminum-26, for example) that could be found around young stars, you would have antimatter that was far less expensive than synthesized antimatter. This could allow for an antimatter power system that puts out more energy than was put into making its fuel.
Assuming that proper containment and isolation of positrons was already figured out, is there is a way to manipulate the decay of positron-emitting isotopes such as Potassium-40? Either by increasing the rate of decay or manipulating the chance of beta plus decay?