  Magnetized plasma columns and extended magnetic structures with both
footpoints anchored to a surface layer are an important building block of
astrophysical dissipation models. Current loops shining in X-rays during the
growth of plasma instabilities are observed in the corona of the Sun and are
expected to exist in highly magnetized neutron star magnetospheres and
accretion disk coronae. For varying twist and system sizes, we investigate the
stability of line-tied force-free flux tubes and the dissipation of twist
energy during instabilities using linear analysis and time-dependent force-free
electrodynamics simulations. Kink modes ($m=1$) and efficient magnetic energy
dissipation develop for plasma safety factors $q\lesssim 1$, where $q$ is the
inverse of the number of magnetic field line windings per column length.
Higher-order fluting modes ($m>1$) can distort equilibrium flux tubes for $q>1$
but induce significantly less dissipation. In our analysis, the characteristic
pitch $\tilde{\mu}_0$ of flux tube field lines determines the growth rate
($\propto \tilde{\mu}_0^3$) and minimum wavelength of the kink instability
($\propto \tilde{\mu}_0^{-1}$). We use these scalings to determine a minimum
flux tube length for the growth of the kink instability for any given
$\tilde{\mu}_0$. By drawing analogies to idealized magnetar magnetospheres with
varying regimes of boundary shearing rates, we discuss the expected impact of
the pitch-dependent growth rates for magnetospheric dissipation in magnetar
conditions.
