Beta (plasma Physics) - Beta

Beta

Given that the magnets are a dominant factor in reactor design, and that density and temperature combine to produce pressure, the ratio of the magnetic energy to the pressure of the plasma naturally becomes a useful figure of merit when comparing MCF designs. In effect, the ratio illustrates how effectively a design confines its plasma. This ratio, beta, is widely used in the fusion field:

is normally measured in terms of the total magnetic field. However, in any real-world design, the strength of the field varies over the volume of the plasma, so to be specific, the average beta is sometimes referred to as the "beta toroidal". In the tokamak design the total field is a combination of the external toroidal field and the current-induced poloidal one, so the "beta poloidal" is sometimes used to compare the relative strengths of these fields. And as the external magnetic field is the driver of reactor cost, "beta external" is used to consider just this contribution.