Safety factor (q)

q

In a tokamak, the number of times a magnetic field line travels around the machine the long way (toroidally) for each time it goes around the short way (poloidally): a measure of how tightly the field is twisted. It changes from the centre of the plasma to the edge. It controls which instabilities the plasma is vulnerable to: if q at the edge falls to about 2, the plasma almost always disrupts, which limits how much current a tokamak of a given size and field can carry. This lowercase q is unrelated to fusion gain, which is written as capital Q; the letters are shared by coincidence.

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concept:safety-factor

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