Edge-localized mode

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[7]
  • Increase the plasma density and, at high densities, adjusting the topology of the magnetic field lines confining the plasma.[8]

History

In 2003 DIII-D begn experimenting with resonant magnetic perturbations to control ELMs.[9]

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.[10][11]

In 2015, results of the first simulation to demonstrate repeated ELM cycling was published.[12]

In 2022, researchers began testing the small ELM hypothesis at JET to assess the utility of the technique.[7][3]

See also

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. hdl: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. doi:10.1088/0741-3335/53/1/015011. ISSN 0741-3335. S2CID 122868427.
  3. Choi, Charles Q. "Controlled chaos may be the key to unlimited clean energy". Inverse. Retrieved 2022-10-26.
  4. Lee, Chris (13 September 2018). "A third dimension helps Tokamak fusion reactor avoid wall-destroying instability". Ars Technica. Retrieved 2018-09-17.
  5. Leonard, A.W. (11 September 2014). "Edge-localized modes in tokamaks". Physics of Plasmas. 21 (9): 090501. Bibcode:2014PhPl...21i0501L. doi:10.1063/1.4894742. OSTI 1352343.
  6. T.E. Evans; et al. (2008). "RMP ELM suppression in DIII-D plasmas with ITER similar shapes and collisionalities". Nucl. Fusion. 92 (48): 024002. doi:10.1088/0029-5515/48/2/024002. hdl:11858/00-001M-0000-0026-FFB5-4.
  7. 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. arXiv:2110.12664. doi:10.1103/PhysRevLett.129.165001. PMID 36306746. S2CID 239768831.
  8. "Fusion-reactor instabilities can be optimized by adjusting plasma density and magnetic fields". Physics World. Nov 4, 2022.
  9. T.E. Evans; et al. (2004). "Suppression of Large Edge-Localized Modes in High-Confinement DIII-D Plasmas with a Stochastic Magnetic Boundary". Physical Review Letters. 92 (23): 235003. Bibcode:2004PhRvL..92w5003E. doi:10.1103/PhysRevLett.92.235003. PMID 15245164.
  10. Kwon, Eunhee (2011-11-10). "KSTAR announces successful ELM suppression". Retrieved 2011-12-11.
  11. 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". Nature Physics. 14 (12): 1223–1228. Bibcode:2018NatPh..14.1223P. doi:10.1038/s41567-018-0268-8. ISSN 1745-2473. OSTI 1485109. S2CID 125338335.
  12. 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" (PDF). Plasma Physics and Controlled Fusion. 57 (1): 014020. doi:10.1088/0741-3335/57/1/014020. ISSN 0741-3335. S2CID 44243673.

Further reading
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