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# 28th IAEA Fusion Energy Conference (FEC 2020)

May 10 – 15, 2021
Virtual Event
Europe/Vienna timezone
The Conference will be held virtually from 10-15 May 2021

## Limits of RMP ELM Suppression in Double Null Plasmas

May 11, 2021, 8:30 AM
4h
Virtual Event

#### Virtual Event

Regular Poster Magnetic Fusion Experiments

### Speaker

Morgan Shafer (Oak Ridge National Laboratory)

### Description

New DIII-D results may explain why achieving ELM suppression by resonant magnetic fields (RMPs) remains elusive in double null (DN) diverted configurations: the lack of ELM suppression in DN correlates with a damped high-field side response of field-aligned structures that could be indicative of a missing resonant tearing needed to stop inward growth of pedestal. This is found despite favorable conditions for RMP suppression in lower single null (LSN): low $\Omega_{E\times{B}}$ aligned with a resonant surface at the pedestal top at low $n_{e,ped}$. The DN configuration is advantageous for future machine design as it allows improved divertor power handling and particle control, but still needs ELM handling solutions and may not be compatible traditional RMP ELM suppression driven from low-field side coils.

In experiments where the magnetic balance is varied from LSN toward DN, ELM suppression was obtained for $dR_{sep}<-1.7$ cm, where $dR_{sep}$ is defined as the separation between the separatrices from the lower null and upper null at the outboard midplane. In discrete steps of $dR_{sep}$, $q_{95}$ is scanned to find a window in ELM suppression under the model that aligning a resonant surface in a region of low $\Omega_{E\times{B}}$ results in resonant tearing inhibiting the inward growth of the pedestal otherwise leading to an ELM {1}. Results of these scans are shown in Figure 1a where the values of $\Omega_{E\times{B}}$ at resonant surfaces are within $\pm 2\%$ of $\psi_{ped,top}$ (to account for uncertainty in profile fitting). Figure 1b shows $n_{e,ped}$ where ELM suppression was achieved (with the largest value of $n_{e,ped}\sim2.4×10^{19} m^{-3}$). This shows for each ELM-suppressed discharge, a resonant surface is near the pedestal top with $\left|\Omega_{E\times B}\right|<20$ krad/s.

In near balanced DN ($dR_{sep}\sim-0.1$ cm), similar scans show that nominal ELM suppression conditions are demonstrated while still ELMing. ELM suppression is not achieved over a range of $q_{95}$ from 3.4 to 4.1 where it was observed in LSN. This is shown in Figure 1c where $\Omega_{E\times{B}}$ at resonant surfaces aligned within $\pm 2\%$ pedestal top are $\left|\Omega_{E\times B}\right|<10$ krad/s—a tighter range than in LSN. These discharges also achieve a lower value of $n_{e,ped}$ than the highest value suppressed in LSN. The pedestal temperature width is consistently wider in ELMing DN plasmas compared to the ELM suppressed LSN plasmas. This leads to a wider total pedestal pressure, consistent with lacking a mechanism inhibiting the pedestal inward growth.

The 3D plasma response to applied RMPs measured on the high-field side (HFS) drops in plasma shapes transitioning from LSN to DN and recovers in upper single null (USN) as shown in Figure 2. The plasma response on the low field side (LFS) remains relatively constant during the shape transition. The reduced HFS response is found similarly for $n=2,3$ over a range of $q_{95}$ from 3.4 to 5. This feature is found broadly across a range of $\left|dR_{sep}\lt0.1\right|$ cm indicating it is not restricted to exactly balanced DN or specific pedestal conditions.

Linearized single-fluid resistive MHD modeling with M3D-C1 shows relatively good agreement with plasma response measurements transitioning from LSN to DN for both the HFS and LFS indicative of a strong damped of perturbations on the HFS in DN. This is further illustrated in the modeled $T_e$ perturbations in Figure 2 where the perturbations are strongly damping on the HFS in DN. This can be partially understood using a simple geometric model assuming field-aligned resonant perturbations. Field-aligned modes driven from the LFS (as is the case with I-coils in DIII-D) are connected to the HFS through a region of low poloidal field in the presence of a secondary null. This can lead to interference of radially-separated resonant modes on the HFS. In balanced double null, this leads to the strongest interference on the HFS.

Results presented here are consistent with using HFS response as a proxy for local tearing drive responsible for ELM suppression by stopping inward growth of pedestal. This is consistent with previous results showing correlation of HFS response and tearing drive needed for ELM suppression {2}. The benefit of DN in power handling resides in a narrow region of $dR_{sep}$ where exact splitting of the heat flux depends on cross-field drifts and has been shown to balance at $dR_{sep}\sim0.25$ cm (near double null) with the ion $\nabla B$ drift directed to the lower divertor {3}. This region of $dR_{sep}$ lies within the damped HFS response and lack of demonstrable ELM suppression. If this model is correct, we can use it to optimize shape and coil positions to better attempt ELM suppression.

This work was supported in part by the US Department of Energy under DE-AC05-00OR22725, DE-FC02-04ER54698, and DE-AC02-09CH11466.

{1} Snyder P.B. et al 2012 Phys. Plasmas 19 056115; Nazikian R. et al 2015 Phys. Rev. Lett. 114 105002; Paz-Soldan C. et al 2019 Nucl. Fusion 59 056012.
{2} Paz-Soldan C. et al 2015 Phys. Rev. Lett. 114 10500
{3} Leonard A.W. et al 2005 Fusion Sci. Technol. 48 1083

Country or International Organization United States Oak Ridge National Laboratory

### Primary author

Morgan Shafer (Oak Ridge National Laboratory)

### Co-authors

Carlos Paz-Soldan (General Atomics) Todd E. Evans (General Atomics) Nathaniel Ferraro (Princeton Plasma Physics Laboratory) Brendan Lyons (General Atomics) Dr Thomas Osborne (General Atomics) Alan Turnbull (General Atomics)