Speaker
Description
In two toroidal plasma merging experiments with high guide field ($B_{t}$ >> $B_{p}$), the ion heating energy by magnetic reconnection is shown to scale with the reconnecting magnetic energy ($B_{rec}^{2}$/2$\mu_{0}$) where $B_{rec}$~$B_{p}$. This $B_{rec}^{2}$- scaling of ion heating energy by reconnection can be understood by the fact that in the reconnection downstream the ion energy is mainly in the form of outflow kinetic energy before ions are thermalized in further downstream. The ion outflow velocity is produced mainly by the large $E\times B$ drift velocity associated with large poloidal electric field $E_{z}$, resulting from the formation of quadrupolar electrostatic potential structure in the downstream region and $E_{z}$ depends linearly on $B_{t} B_{rec}$~$B_{t} B_{p}$ as observed in the experiments. Hence, the outflow velocity scales with $B_{rec}$, and thus the ion heating energy scales with $B_{rec}^{2}$. High power ion heating with about 40-50% of $B_{rec}^{2}$/$2\mu_{0}$ can only be achieved when the reconnection current sheet thickness $\delta$ is compressed to smaller than the ion gyroradius $\rho_{i}$. If $\delta$>>$\rho_{i}$, the magnetic reconnection converts only 5-10% of $B_{rec}^{2}$/$2\mu_{0}$ into ion energy. The $B_{rec}^{2}$-scaling of high-power reconnection ion heating provides an efficient way to produce burning plasmas with $T_{i}$ > 10keV by increasing $B_{rec}$ to 0.6T (for $n_e$ ~ $1.5\times 10^{19}m^{-3}$) without using any auxiliary heating methods.
