Plasma beta

β

The ratio of a plasma's pressure to the pressure of the magnetic field that confines it (the field's 'magnetic pressure', which rises with the square of the field strength). A higher beta means the magnetic field is being used more efficiently to hold a given plasma pressure, and because fusion power rises steeply with plasma pressure, it generally means a smaller or cheaper magnet for the same fusion power. Configurations differ sharply: conventional tokamaks run at a few per cent, spherical tokamaks have reached tens of per cent, and field-reversed configurations run close to 100 per cent. Beta is defined in more than one way, using different parts of the field, so compare two figures only if they use the same definition.

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concept:beta

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