  Several simulations of turbulence in the Large Plasma Device (LAPD) [W.
Gekelman et al., Rev. Sci. Inst. 62, 2875 (1991)] are energetically analyzed
and compared with each other and with the experiment. The simulations use the
same model, but different axial boundary conditions. They employ either
periodic, zero-value, zero-derivative, or sheath axial boundaries. The linear
stability physics is different between the scenarios because the various
boundary conditions allow the drift wave instability to access different axial
structures, and the sheath boundary simulation contains a conducting wall mode
instability which is just as unstable as the drift waves. Nevertheless, the
turbulence in all the simulations is relatively similar because it is primarily
driven by a robust nonlinear instability that is the same for all cases. The
nonlinear instability preferentially drives $k_\parallel = 0$ potential energy
fluctuations, which then three-wave couple to $k_\parallel \ne 0$ potential
energy fluctuations in order to access the adiabatic response to transfer their
energy to kinetic energy fluctuations. The turbulence self-organizes to drive
this nonlinear instability, which destroys the linear eigenmode structures,
making the linear instabilities ineffective.
