In simplest terms, the probability that a given fusion reaction. However, it is expressed as an area. That area is the effective target area that a stationary fuel ion presents to an oncoming projectile fuel ion. It is determined experimentally and therefore takes into account all of the difficult to calculate quantum effects.

Several interesting things pop out of this: first, cross-section is not constant; it depends on projectile energy. Second, cross-section peaks at a given energy for each fuel combination. Assuming the target ion is at rest, the total cross-section in barns (sigma), as a function of E, the energy of the projectile in keV, can be fitted by the following equation:

sigmaT(E) = {A5+{(A4-A3E)2+1}-1A2}/{E{exp(A1E-1/2)-1}}

The coefficients Aj are called Duane coefficients and vary for each fuel combination. Duane coefficients for several common reactions may be found on pp 45 of the 1998 Revised NRL Plasma Physics Formulary. I may post them later as an addendum to this writeup.

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