text: Things can be somewhat improved by using two accelerators firing at each other instead of a single accelerator and a non-moving target. In this case, the second fuel, boron in the example above, is already ionized, so the "ionization drag" seen by the protons entering the solid block is eliminated.
  In this case, however, the concept of a characteristic interaction length has no meaning as there is no solid target. Instead, for these types of system, the typical measure is to use the beam luminosity, L, a term that combines the reaction cross-section with the number of events. The term is normally defined as:
  For this discussion, we will re-arrange it to extract the collisional frequency:
  Each of these collisions will produce 8.7 MeV, so multiplying by  gives the power. To generate N collisions one requires luminosity L, generating L requires power, so one can calculate the amount of power needed to produce a given L through:
  If we set P to 1 MW, equivalent to a small wind turbine, this requires an L of 10 cms. For comparison, the world record for luminosity set by the Large Hadron Collider in 2017 was 2.06 x 10 cms, more than seven orders of magnitude too low.