  The concept of available energy of a collisionless plasma is discussed in the
context of magnetic confinement. The available energy quantifies how much of
the plasma energy can be converted into fluctuations (including nonlinear ones)
and is thus a measure of plasma stability, which can be used to derive linear
and nonlinear stability criteria without solving an eigenvalue problem. In a
magnetically confined plasma, the available energy is determined by the density
and temperature profiles as well as the magnetic geometry. It also depends on
what constraints limit the possible forms of plasma motion, such as the
conservation of adiabatic invariants and the requirement that the transport be
ambipolar. A general method based on Lagrange multipliers is devised to
incorporate such constraints in the calculation of the available energy, and
several particular cases are discussed. In particular, it is shown that it is
impossible to confine a plasma in a Maxwellian ground state relative to
perturbations with frequencies exceeding the ion bounce frequency.
