Comparison of small edge-localized modes on MAST and ASDEX Upgrade

Abstract

Results from a set of ITPA-coordinated experiments on ASDEX Upgrade and MAST to compare the characteristics of small edge-localized modes (ELMs) are presented. In MAST a small ELM regime is established in connected double null discharges in a limited region of normalised pedestal pressure and collisionality. Type II ELMs on ASDEX Upgrade have high frequency and low energy loss and occur at high triangularity and close to double null. On both devices the transition from type I to small ELMs is connected with a similar threshold value of the pedestal collisionality. For the first time the temporal and spatial evolution of the filament structures observed during these small ELMs has been studied. The radial and toroidal velocities of the filaments in these small ELMs are slower compared to type I ELMs on both devices. The observations are compatible with the filaments in small ELMs originating closer to the last closed flux surface than is the case in type I ELMs. The toroidal mode number of the small ELMs, derived from the temporal evolution of the filaments, is typically a factor of two larger than for type I ELMs. The small ELMs on MAST have sufficient similarities to type II ELMs on ASDEX Upgrade that they should be classified as the same.

Effect of resonant magnetic perturbations with toroidal mode numbers of 4 and 6 on edge-localized modes in single null H-mode plasmas in MAST

“…The toroidal mode number of the ELMs is also unaffected and the mode number for the natural and mitigated ELMs is consistent with what is expected from type I ELMs i.e. the increased ELM frequency in the mitigated stage is not due to a transition to type III ELMs which in MAST have been shown previously to have a higher mean mode number (n=20) and wider distribution of mode numbers (from n=5 to 30) [22].…”

Kinetic Instabilities that Limitβin the Edge of a Tokamak Plasma: A Picture of anH-Mode Pedestal

“…The crossing of this threshold may trigger a significant transport event in the edge plasma. n = 25 coincides with the toroidal mode number of the peelingballooning mode found marginally unstable at the end of this ELM cycle [8], and is in the observed range for ELM filaments in MAST [30]. Conclusions:-Plasma equilibria have been reconstructed from the spherical tokamak MAST, with sufficient resolution in time and space to capture plasma evolution during the short period between ELMs.…”

Understanding the effect resonant magnetic perturbations have on ELMs

“…These small/no ELM regimes show considerable reduction of instantaneous ELM heat load and their operational space can be characterised, in a similar way to the RMP type I suppressed regimes, in a plot of collisionality versus plasma density as a function of the Greenwald density (see Figure 8). [71]. For a fixed plasma shape there is a clear density (ν*) access threshold.…”

“…Associated with a stable type II regime is a large radial flux of particles, where the particle flux is larger than that in a type I regime [72]. In fact, if the particle flux is not sufficient a mixed type I/II regime results [71] presumably because the pedestal continues to evolve until it reaches the peeling-ballooning boundary and results in a type I ELM. The mode responsible for type II ELMs appears to be driven unstable by changes in shape, density and possibly the change in neutral fuelling location as the second strike point becomes more important and has a higher toroidal mode number and very regular mode structure [71].…”