Improving Diamagnetic Flux Temporal Resolution to Measure ELM Energy Loss

When an ELM occurs in a tokamak, a substantial loss of stored thermal energy can occur in a very short time, resulting in a change in the plasma diamagnetism. A diamagnetic loop is therefore an attractive diagnostic for characterizing the change in energy during ELMs. A loop external to the vessel can be used but it is bandwidth-limited by the vessel wall, therefore the signal is severely attenuated above 40 Hz in \hbox{DIII-D}. The temporal resolution can be improved by combining the (slow) diamagnetic signal with a properly scaled internal (fast) toroidal $B_{\rm T}$ signal. The results agree with finely-spaced EFIT equilibrium reconstructions to within 10\% before each ELM, but the diamagnetic calculation often shows up to twice the drop in energy at the ELM. The $B_{\rm T}$ signal reveals the magnetic change completes in 0.5 ms or less with occasional dynamics above 10 kHz. This improved temporal resolution allows comparison of phenomenology in natural vs. pellet-triggered ELMs, and also effects of partial ELM suppression under resonant magnetic perturbation.