Beta (plasma physics)

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The beta of a plasma, symbolized by β, is the ratio of the plasma pressure (p = n kB T) to the magnetic pressure (pmag = B²/2μ0).

\beta = \frac{p}{p_{mag}} = \frac{nk_BT}{(B^2/2\mu_0)}

Beta is a parameter indicating the relative importance of kinetic to electromagnetic phenomena. In fusion power applications, beta can be thought of as an economic figure of merit. The magnetic field in a fusion device has technological limits such as the critical field of practical superconductors. The fusion power density, which determines the power output of a reactor, scales with the square of the plasma pressure. Therefore the output of a reactor can be said to scale with the square of the average beta. In a tokamak, for example, there is a fairly firm limit on the value beta can have before destructive instabilities occur.

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