Complex sentence: All of Bussard's Polywell experiments were based on deuterium-deuterium (2 H + 2 H) which has a high reaction cross section at a low temperature relative to other fuels.
Split into simpler sentences: All of Bussard's Polywell experiments were based on deuterium-deuterium (2 H + 2 H) reactions. This was because it has a high reaction cross section at a low temperature relative to other fuels.