other 80% is released as neutrons which, because they cannot be directed by magnetic fields or solid walls, would be difficult to direct towards thrust. Helium-3 is produced via beta decay of tritium, which can be produced from deuterium, lithium, or boron. Even if a self-sustaining fusion reaction cannot be produced, it might be possible to use fusion to boost the efficiency of another propulsion system, such as a VASIMR engine. Confinement alternatives Magnetic To sustain a fusion reaction, the plasma must be confined. The most widely studied configuration for terrestrial fusion is the tokamak, a form of magnetic confinement fusion. Currently tokamaks weigh a great deal, so the thrust to weight ratio would seem unacceptable. NASA's Glenn Research Center proposed a small aspect ratio spherical torus reactor for its "Discovery II" conceptual vehicle design. "Discovery II" could deliver a crewed 172 metric tons payload to Jupiter in 118 days (or 212 days to Saturn) using 861 metric tons of hydrogen propellant, plus 11 metric tons of Helium-3-Deuterium (D-He3) fusion fuel. The hydrogen is heated by the fusion plasma debris to increase thrust, at a cost of reduced exhaust velocity (348–463 km/s) and hence increased propellant mass. Inertial The main alternative to magnetic confinement is inertial confinement fusion (ICF), such as that proposed by Project Daedalus. A small pellet of fusion fuel (with a diameter of a couple of millimeters) would be ignited by an electron beam or a laser. To produce direct thrust, a magnetic field forms the pusher plate. In principle, the Helium-3-Deuterium reaction or an aneutronic fusion reaction could be used to maximize the energy in charged particles and to minimize radiation, but it is highly questionable whether using these reactions is technically feasible. Both the detailed design studies in the 1970s, the Orion drive and Project Daedalus, used inertial confinement. In the 1980s, Lawrence Livermore National Laboratory and NASA studied an ICF-powered "Vehicle for Interplanetary Transport Applications" (VISTA). The conical VISTA spacecraft could deliver a 100-tonne payload to Mars orbit and return to Earth in 130 days, or to Jupiter orbit and back in 403 days. 41 tonnes of deuterium/tritium (D-T) fusion fuel would be required, plus 4,124 tonnes of hydrogen expellant. The exhaust velocity would be 157 km/s. Magnetized target Magnetized target fusion (MTF) is a relatively new approach that combines the best features of the more widely studied magnetic confinement fusion (i.e. good energy