  We show that, in a two-dimensional (2d) ideal fluid (also applies to a column
of quasi-2d non-neutral plasma in an axial magnetic field), large elliptical
vortices in a finite disk are stable. The stability is established by
comparison between energy of elliptical and symmetrical states to satisfy a
sufficient condition, without dynamical eigen-analysis. Analytical small
ellipticity expansion of energy and exact numerical values for finite
ellipticity are both obtained. The expansion indicates stable linear l=2
diocotron modes for large vortices (or plasma columns). Numerical simulations
of the 2d Euler equation are also performed. They not only confirm the
sufficient condition, but also show that the stability persists to smaller
vortex sizes. The reason why decaying l=2 modes were obtained by Briggs,
Daugherty, and Levy [Phys. Fluids 13, 421 (1970)] using eigen-analysis is also
discussed.
