On Demand Triggering of Edge Localized Instabilities Using External Nonaxisymmetric Magnetic Perturbations in Toroidal Plasmas

The application of nonaxisymmetric magnetic fields is shown to destabilize edge-localized modes (ELMs) during otherwise ELM-free periods of discharges in the National Spherical Torus Experiment (NSTX). Profile analysis shows the applied fields increased the temperature and pressure gradients, decreasing edge stability. This robust effect was exploited for a new form of ELM control: the triggering of ELMs at will in high performance $H$ mode plasmas enabled by lithium conditioning, yielding high time-averaged energy confinement with reduced core impurity density and radiated power.

Figure 1

$D_{\alpha }$ emission (solid line) and current in the RWM coils (dashed line) from discharges with (a) no $n=3$ field and (b)–(d) varying time of $n=3$ field application.

Figure 2

Edge profiles of electron (a) density ($n_e$) and (b) temperature ($T_e$) from a discharge with no 3D field applied (black crosses) and at a time after $n=3$ application but before ELMs (red or gray diamonds).

Figure 3

Comparison of discharges with lithium conditioning only (black line) and with combined lithium and 3D field-induced ELMs (red and blue or gray lines): (a) stored energy $W_{\mathrm{MHD}}$, (b) line-averaged density from Thomson scattering $n_e{}^{\mathrm{ave}}$, (c) total radiated power $P_{\mathrm{rad}}$, and (d)–(f) RWM coil current $I_{\mathrm{RWM}-5}$ and $D_{\alpha }$ emission.