The ballooning instability (a.k.a.
ballooning mode instability) is a type of internal pressure-driven plasma instability usually seen in tokamak fusion power reactors[1] or in space plasmas.
[2] It is important in fusion research as it determines a set of criteria for the maximum achievable plasma beta.
[3] The name refers to the shape and action of the instability, which acts like the elongations formed in a long balloon when it is squeezed.
In literature, the structure of these elongations are commonly referred to as 'fingers'.
[4][5][6] The narrow fingers of plasma produced by the instability are capable of accelerating and pushing aside the surrounding magnetic field in order to cause a sudden, explosive release of energy.
Thus, the instability is also known as the explosive instability.
[7][8][9] The dispersion relation is
ω ( ω −
ω
ω
ω
2 ( ω −
{\displaystyle \delta =\beta _{e}/(\omega _{*}pi-\omega _{*}ep)/2(\omega -q_{i}T_{-}\omega _{*}pi)b_{i})/(\omega -\omega _{*}e)-3/2(\omega -\omega _{*}pe)b_{i}/(\omega -\omega -\omega _{*}e)(\omega _{B}e+\omega _{k}e)/2\omega }
The interchange instability can be derived from the equations of the ballooning instability as a special case in which the ballooning mode does not perturb the equilibrium magnetic field.
[2] This special limit is known as the Mercier criterion.
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