Edge-localized mode explained

An edge-localized mode (ELM) is a plasma instability occurring in the edge region of a tokamak plasma due to periodic relaxations of the edge transport barrier in high-confinement mode. Each ELM burst is associated with expulsion of particles and energy from the confined plasma into the scrape-off layer. This phenomenon was first observed in the ASDEX tokamak in 1981.^[1] Diamagnetic effects in the model equations expand the size of the parameter space in which solutions of repeated sawteeth can be recovered compared to a resistive MHD model.^[2] An ELM can expel up to 20 percent of the reactor's energy.^[3]

Issues

ELM is a major challenge in magnetic fusion research with tokamaks, as these instabilities can:

• Damage wall components (in particular divertor plates) by ablating them away due to their extremely high energy transfer rate (GW/m²);^[4]
• Potentially couple or trigger other instabilities, such as the resistive wall mode (RWM) or the neoclassical tearing mode (NTM).^[5]

Prevention and control

A variety of experiments/simulations have attempted to mitigate damage from ELM. Techniques include:

• Application of resonant magnetic perturbations (RMPs) with in-vessel current carrying coils can eliminate or weaken ELMs.^[6]
• Injecting pellets to increase the frequency and thereby decrease the severity of ELM bursts (ASDEX Upgrade).
• Multiple small-scale ELMs (000s/s) in tokamaks to prevent the creation of large ones, spreading the associated heat over a larger area and interval
• Increase the plasma density and, at high densities, adjusting the topology of the magnetic field lines confining the plasma.^[7]

History

In 2003 DIII-D began experimenting with resonant magnetic perturbations to control ELMs.^[8]

In 2006 an initiative (Project Aster) was started to simulate a full ELM cycle including its onset, the highly non-linear phase, and its decay. However, this did not constitute a “true” ELM cycle, since a true ELM cycle would require modeling the slow growth after the crash, in order to produce a second ELM.

As of late 2011, several research facilities had demonstrated active control or suppression of ELMs in tokamak plasmas. For example, the KSTAR tokamak used specific asymmetric three-dimensional magnetic field configurations to achieve this goal.^{[9] [10]}

In 2015, results of the first simulation to demonstrate repeated ELM cycling was published.^[11]

In 2022, researchers began testing the small ELM hypothesis at JET to assess the utility of the technique.^[12]

See also

• Resonant magnetic perturbations, used to control ELMs
• Plasma instability
• Tokamak

Further reading

• Kirk. A. Liu. Yueqiang. Chapman. I T. Harrison. J. Nardon. E. Scannell. R. Thornton. A J. 119208710. 2013-03-06. Effect of resonant magnetic perturbations on ELMs in connected double null plasmas in MAST. Plasma Physics and Controlled Fusion. 55. 4. 045007. 10.1088/0741-3335/55/4/045007. 0741-3335. 1303.0146. 2013PPCF...55d5007K.
• Tala . Tuomas . Garbet . Xavier . 2006 . Physics of Internal Transport Barriers . . 7 . 6 . 622–633 . 10.1016/j.crhy.2006.06.005 . 2006CRPhy...7..622T . Elsevier Science Direct.

Notes and References

1. F. . Wagner . A.R. . Field . G. . Fussmann . J.V. . Hofmann . M.E. . Manso . O. . Vollmer . José . Matias . 1990 . Recent results of H-mode studies on ASDEX . 13th International Conference on Plasma Physics and Controlled Nuclear Fusion . 277–290 . 10198/9098.
2. Halpern . F D . Leblond . D . Lütjens . H . Luciani . J-F . 2010-11-30 . Oscillation regimes of the internal kink mode in tokamak plasmas . Plasma Physics and Controlled Fusion . 53 . 1 . 015011 . 10.1088/0741-3335/53/1/015011 . 122868427 . 0741-3335.
3. Web site: Choi . Charles Q. . Controlled chaos may be the key to unlimited clean energy . 2022-10-26 . Inverse . 20 October 2022 . en.
4. News: Lee . Chris . 13 September 2018 . A third dimension helps Tokamak fusion reactor avoid wall-destroying instability . en-us . . 2018-09-17.
5. Leonard . A.W. . 11 September 2014 . Edge-localized modes in tokamaks . . 21 . 9 . 090501 . 2014PhPl...21i0501L . 10.1063/1.4894742 . 1352343.
6. T.E. Evans . etal . 2008 . RMP ELM suppression in DIII-D plasmas with ITER similar shapes and collisionalities . . 92 . 48 . 024002 . 10.1088/0029-5515/48/2/024002 . 2008NucFu..48b4002E . 11858/00-001M-0000-0026-FFB5-4 . 54039023 . free .
7. Web site: Fusion-reactor instabilities can be optimized by adjusting plasma density and magnetic fields. Physics World. Nov 4, 2022.
8. T.E. Evans . etal . 2004 . Suppression of Large Edge-Localized Modes in High-Confinement DIII-D Plasmas with a Stochastic Magnetic Boundary . . 92 . 23 . 235003 . 10.1103/PhysRevLett.92.235003 . 2004PhRvL..92w5003E . 15245164.
9. Web site: KSTAR announces successful ELM suppression . Kwon, Eunhee . 2011-11-10. 2011-12-11.
10. Park . Jong-Kyu . Jeon . YoungMu . In . Yongkyoon . Ahn . Joon-Wook . Nazikian . Raffi . Park . Gunyoung . Kim . Jaehyun . Lee . HyungHo . Ko . WonHa . Kim . Hyun-Seok . Logan . Nikolas C. . Wang . Zhirui . Feibush . Eliot A. . Menard . Jonathan E. . Zarnstroff . Michael C. . 2018-09-10 . 3D field phase-space control in tokamak plasmas . . En . 14 . 12 . 1223–1228 . 2018NatPh..14.1223P . 10.1038/s41567-018-0268-8 . 1745-2473 . 1485109 . 125338335.
11. Orain . François . Bécoulet . M . Morales . J . Huijsmans . G T A . Dif-Pradalier . G . Hoelzl . M . Garbet . X . Pamela . S . Nardon . E . 2014-11-28 . Non-linear MHD modeling of edge localized mode cycles and mitigation by resonant magnetic perturbations . . 57 . 1 . 014020 . 10.1088/0741-3335/57/1/014020 . 44243673 . 0741-3335.
12. Harrer . G. F. . Faitsch . M. . Radovanovic . L. . Wolfrum . E. . Albert . C. . Cathey . A. . Cavedon . M. . Dunne . M. . Eich . T. . Fischer . R. . Griener . M. . Hoelzl . M. . Labit . B. . Meyer . H. . Aumayr . F. . 2022-10-10 . Quasicontinuous Exhaust Scenario for a Fusion Reactor: The Renaissance of Small Edge Localized Modes . Physical Review Letters . 129 . 16 . 165001 . 10.1103/PhysRevLett.129.165001. 36306746 . 239768831 . 2110.12664 . 2022PhRvL.129p5001H .