Exploring Experimental Isotope Scaling in Tokamak Transport**

A problem that has not yet been resolved is the scaling of tokamak transport with the ion mass¹. Candidates for explaining this scaling are mainly related to zonal flows². Such flows are central to both the understanding of the nonlinear Dimits shift and the H-mode barrier.  We note that zonal flows are sensitive to the fluid closure and also to the contribution to the Reynolds stress by the pressure.  We will study this effect in detail using our most general electromagnetic description,³ including kinetic ballooning modes as well as peeling modes and collisions. We will also compare this with the effects of fast particles since we have recently combined a theory for fast particles with our usual drift wave model⁴. In particular, it was found in Ref 2 that the effects of collisions may be needed for obtaining the experimental isotope scaling. We expect that collisions may be needed in the outer parts of the H-mode barrier, where our model is usually in the second stability region for MHD ballooning modes.

[1] J.W. Connor and H.R. Wilson, Plasma Phys. Control. Fusion 36, 719 (1994).

[2] S-I- Itoh and K. Itoh, Nuclear Fusion 56, 106028 (2016).

[3] J. Weiland, Stability and Transport in Magnetic Confinement Systems, Springer, New York (2012).

[4] J. Weiland, T. Rafiq and E. Shuster Phys. Plasmas 30, 042517 (2023).