Scaling law for interchange transport in magnetically confined electron-positron plasmas with ion impurity

Interchange instabilities are a critical issue for confinement and transport in (otherwise exceptionally stable) planned magnetically confined quasi-neutral electron-positron plasma experiments. An additional ion impurity concentration can signifcantly affect the instability. The dispersion relation for interchange modes in perturbed inhomogeneously magnetized electron-positron plasmas in the presence of impurity ions is derived from full-f isothermal gyrofluid equations. The model includes arbitrary ion concentration and Debye screening effects. The resulting interchange growth rate of localized density perturbations is related to the cross-field propagation velocity and associated transport, which is reduced by an additional ion concentration compared to pure electron-positron plasmas. A simplified scaling relation is obtained from the exact analysis and compared with previous pure electron-positron plasma limits and empirical scalings based on simulation data.