There are about 0.002 stars per cubic light-year, or the average distance to the nearest star is 5 light years. To the nearest G star is 11 light years. ISM is both propellant and fuel. However, pp fusion reaction too weak. Need pd, dd or dt reaction and deuterium is rare.
Later analyses suggest insufficient energy from fusion.
Assume crew of 150, mass of ship 10,000 metric tons. Assume 0.1 H atoms per cubic cm. Ramscoop radius for constant acceleration so as the density changes the radius must change. As the ram moves against the ISM, it will build up a shockwave that ionizes the material. The dense plasma will conduct current and guide magnetic field lines.