The beta value of a fusion plasma is defined as the ratio of the thermal plasma pressure p to the magnetic field pressure B2 / 2 µ0 that has to be externally applied (µ0 beeing the vacuum permeability). It is expressed as beta = 2 µ0 p / B2. Since for fusion the plasma pressure has to be kept as high as possible and for cost reasons the external magnetic field B should be as small as possible, a high beta value is desirable.

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