The authors have obtained an equation for small oscillations describing the excitation of ideal small scale MHD modes in a finite pressure plasma with allowance for inertial effects in arbitrary magnetic configurations. This equation is used to analyse the plasma stability in an ASPERATOR NP-4 type trap. It is shown that in the currentless regime both ideal ballooning and Mercier modes may be excited in this trap. It is established that these modes are absolutely unstable because of the growth with increasing plasma pressure of the magnetic 'hill' in the device. The growth rates of the excited modes are determined. In unstable regimes, a relative improvement in confinement is observed when, with increasing pressure, a transition may occur from regimes with larger growth rates to regimes with smaller growth rates. Such a transition is connected with finite plasma pressure effects when the destabilizing action of finite pressure, leading to a growth of the 'magnetic hill' and to an absolute instability of Mercier modes, is partially compensated for by the stabilizing action connected with the growth of the shear value with increasing plasma pressure.
Nonlinear Magnetohydrodynamics