gyrofluid model has a different formulation of nonlinear polarisation (the generalisation of “vorticity” to a two-fluid setting) than a fluid or MHD model, full correspondence in the regime of validity of the latter models has been shown [30]. In this sense the model is a superset of global reduced MHD and electromagnetic microturbulence. The global dynamics is in the reduced MHD regime [31] due to low absolute edge beta values ( β = 8 πp/B 2 ∼ 10 − $^{3}$) but the small scales depend on treating the ion gyroradius to arbitrary order. The model does not assume instabilities to occur at any particular scale, but when they occur the resulting spread of the spectrum is found to reach ρ$_{i}$ within a few eddy turnover times of the onset of nonlinearity in every case computed. 

The work reported herein addressed the following points. The relationship of the MHD dynamics to the ideal threshold is much discussed but in the gyrofluid model several mech- anisms of microinstability are also present. The threshold issue is obscured by the existence of ion temperature gradient (ITG) drift instabilities at all beta values, with a smooth tran- sition between them. Nevertheless, varying character in the saturation phases is observed at different values. We also address the issue of possible “rho-star” dependence (here the local rho-star is given by ρ$_{s}$/L$_{T}$ where ρ$_{s}$ is ρ$_{i}$ evaluated at T$_{i}$ = T$_{e}$ and L$_{T}$ is the scale length of the T$_{e}$ profile, both evaluated at the mid-pedestal location in minor radius) which would indicate a role for diamagnetic drifts. The gradient is sufficiently steep that ρ$_{s}$/L$_{T}$ > L$_{T}$ /qR so that none of these effects can be ordered small. We examine the necessary resolution to obtain converged cases, which gives the spectral range actively involved in nonlinear satura- tion. The main instability at experimentally relevant beta is the ideal ballooning instability but the main saturation process is energy exchange with broadband electromagnetic drift wave/ITG turbulence which the instability itself generates. Due mostly to this, the results turn out to be outside of both the magneto-hydrodynamic (MHD) and collisional Braginskii paradigms, on which most previous approaches have been based. On the other hand, the GEMR model still uses delta-f equations, so the actual profile phenomenology in the SOL region is not well represented, and as in other work with GEMR we concentrate on the properties of the nonlinear dynamics [32]. 

Following sections give the details of the GEMR model, a discussion of the physical difference between ITG and MHD dynamics and their relative roles, the methods used to pre-set the axisymmetric equilibrium state so that the computations are done in the absence of axisymmetric oscillations whose decay times can be slower than the IBM rise time, the 

