The <a href="https://en.wikipedia.org/wiki/Bussard_ramjet" class="extiw" title="w:Bussard ramjet">Bussard Interstellar Ramjet</a> engine concept uses interstellar hydrogen scooped up from its environment as the spacecraft passes by to provide propellant mass. The hydrogen is then ionized and collected by an electromagentic field. In this image, an onboard laser is used to heat the plasma, and the laser or electron beam is used to trigger fusion pulses thereby creating propulsion.
NASA
<a rel="nofollow" class="external free" href="http://nix.ksc.nasa.gov/info?id=MSFC-9906399&amp;orgid=11">http://nix.ksc.nasa.gov/info?id=MSFC-9906399&amp;orgid=11</a>