AIMStar was a proposed antimatter catalyzed nuclear pulse propulsion craft that uses clouds of antiprotons to initiate fission and fusion within fuel pellets. A magnetic nozzle derives motive force from the resulting explosions. The design was studied during the 1990s by Penn State University.
The craft was designed to reach a distance of on the order of 10,000 AU from the Sun, with a travel time of 50 years, and a coasting velocity of approximately 960 km/s after the boost phase (roughly 1/300th of the speed of light). The probe would be able to study the interstellar medium as well as reach Alpha Centauri. 
The project would require more antimatter then we are capable of producing. In addition, some technical hurdles need to be surpassed before it would be feasible. 
ICAN-II a similar concept that uses less antimatter but more fission propellant.
- Lewis, Raymond A; Meyer, Kirby; Smith, Gerald A; Howe, Steven D. "AIMStar: Antimatter Initiated Microfusion For Pre-cursor Interstellar Missions".
- Antimatter Production for Near-term Propulsion Applications http://www.engr.psu.edu/antimatter/papers/nasa_anti.pdf
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