Collisionless m=1 tearing mode

The instability of a magnetically confined plasma against macroscopic modes is analyzed in collisionless regimes where magnetic reconnection occurs because of finite electron inertia and the ion gyroradius ${\mathrm{\ensuremath{\rho}}}_{\mathit{i}}$ replaces the skin depth d as the width of the mode boundary layer. Growth rates \ensuremath{\gamma}/${\mathrm{\ensuremath{\omega}}}_{\mathit{A}}$\ensuremath{\sim}(d/${\mathit{r}}_{\mathit{s}}$)(${\mathrm{\ensuremath{\rho}}}_{\mathit{i}}$/d${)}^{2/3}$ are found, with ${\mathrm{\ensuremath{\omega}}}_{\mathit{A}}$ the Alfv\'en frequency and ${\mathit{r}}_{\mathit{s}}$ the radius of the reconnecting surface. For typical JET parameters, ${\ensuremath{\gamma}}^{\mathrm{\ensuremath{-}}1}$\ensuremath{\sim}50--100 \ensuremath{\mu}s, which compares favorably with the observed instability growth time of internal plasma relaxations.