  To better understand the interaction of global tearing modes and
microturbulence in the Madison Symmetric Torus (MST) reversed-field pinch
(RFP), the global gyrokinetic code \textsc{Gene} is modified to describe global
tearing mode instability via a shifted Maxwellian distribution consistent with
experimental equilibria. The implementation of the shifted Maxwellian is tested
and benchmarked by comparisons with different codes and models. Good agreement
is obtained in code-code and code-theory comparisons. Linear stability of
tearing modes of a non-reversed MST discharge is studied. A collisionality scan
is performed to the lowest order unstable modes ($n=5$, $n=6$) and shown to
behave consistently with theoretical scaling. The nonlinear evolution is
simulated, and saturation is found to arise from mode coupling and transfer of
energy from the most unstable tearing mode to small-scale stable modes mediated
by the $m=2$ tearing mode. The work described herein lays the foundation for
nonlinear simulation and analysis of the interaction of tearing modes and
gyroradius-scale instabilities in RFP plasmas.

Plasma Physics