An interesting discussion of Bussard ramjet interesteller propulsion technology here at Centauri Dreams and Adam Crowl:
Robert Bussard put the interstellar ramjet into the public eye back in 1960 in a paper proposing that a starship moving fast enough would be able to use the hydrogen between the stars as a source of fuel, enabling a constant acceleration at one g.
One of the key problems with Bussard ramjets is the speed of the fusion reactions:
Physicist Daniel Whitmire tackled this problem in a 1975 paper that proposed using hydrogen for fuel but exploiting a catalytic nuclear reaction chain instead of straight proton burning.
It’s all interesting stuff, but even if the Bussard ramjet is unfeasible as a propulsion system it may be feasible as a brake for interstellar spacecraft.