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A fully implicit particle-in-cell method for handling the $v_\parallel$-formalism of electromagnetic gyrokinetics has been implemented in XGC. By choosing the $v_\parallel$-formalism, we avoid introducing the non-physical skin terms in Amp\`{e}re's law, which are responsible for the well-known ``cancellation problem" in the $p_\parallel$-formalism. The $v_\parallel$-formalism, however, is known to suffer from a numerical instability when explicit time integration schemes are used due to the appearance of a time derivative in the particle equations of motion from the inductive component of the electric field. Here, using the conventional $\delta f$ scheme, we demonstrate that our implicitly discretized algorithm can provide numerically stable simulation results with accurate dispersive properties. We verify the algorithm using a test case for shear Alfv\'{e}n wave propagation in addition to a case demonstrating the ITG-KBM transition. The ITG-KBM transition case is compared to results obtained from other $\delta f$ gyrokinetic codes/schemes, whose verification has already been archived in the literature.

We have extended our study of the competition between the drive and stabilization of plasma microinstabilities by sheared flow to include electromagnetic effects at low plasma $\beta$ (the ratio of plasma to magnetic pressure). The extended system of characteristic equations is formulated, for a dissipative fluid model developed from the gyrokinetic equation, using a twisting mode representation in sheared slab geometry and focusing on the ion temperature gradient mode. Perpendicular flow shear convects perturbations along the field at the speed we denote as $Mc_s$ (where $c_s$ is the sound speed). $M > 1/ \sqrt{\beta}$ is required to make the system characteristics unidirectional and inhibit eigenmode formation, leaving only transitory perturbations in the system. This typically represents a much larger flow shear than in the electrostatic case, which only needs $M>1$. Numerical investigation of the region $M < 1/\sqrt{\beta}$ shows the driving terms can conflict, as in the electrostatic case, giving low growth rates over a range of parameters. Also, at modest drive strengths and low $\beta$ values typical of experiments, including electromagnetic effects does not significantly alter the growth rates. For stronger flow shear and higher $\beta$, geometry characteristic of the spherical tokamak mitigates the effect of an instability of the shear Alfv\'{e}n wave, driven by the parallel flow shear., Comment: 23 pages, 12 figures