Edge-localized mode

An edge-localized mode (“ELM”) is a disruptive instability occurring in the edge region of a tokamak plasma due to the quasi-periodic relaxation of a transport barrier previously formed during an L → H transition (i.e., in H-mode). This phenomenon was first observed in the ASDEX tokamak in 1981.[1]

The development of edge-localized modes poses a major challenge in magnetic fusion research with tokamaks, as these instabilities 1.) can damage wall components (in particular divertor plates) by ablating them away due to their extremely high energy transfer rate (GW/m2);[2] and 2.) can potentially couple or trigger other instabilities, such as the resistive wall mode (RWM) or the neoclassical tearing mode (NTM) [3]

Simulation and modeling

In 2006 an initiative (called 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 have a second ELM. In 2015, results of the first simulation to demonstrate repeated ELM cycling was published.[4] A key element to obtaining repeated relaxations was to include diamagnetic effects in the model equations. Diamagnetic effects have also been shown to expand the size of the parameter space in which solutions of repeated sawteeth can be recovered compared to a resistive MHD model.[5]

Prevention and control

Research involving prevention of edge localized mode formation is underway. A paper was recently published that suggested a novel method of countering this phenomenon by injecting static magnetic noisy energy into the containment field as a containment-stabilization regime; this may decrease ELM amplitude. ASDEX Upgrade has had some success using pellet injection to increase the frequency and thereby decrease the severity of ELM bursts.

Control in practice

Since 2003 DIII-D has experimented with Resonant magnetic perturbations to control ELMs.[6]

As of late 2011, several research facilities have demonstrated active control or suppression of ELMs in tokamak plasmas. For example, the KSTAR tokamak uses specific asymmetric three-dimensional magnetic field configurations to achieve this goal.[7][8]

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. Lee, Chris (13 September 2018). "A third dimension helps Tokamak fusion reactor avoid wall-destroying instability". Ars Technica. Retrieved 2018-09-17.
  3. 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.
  4. 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.
  5. 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.
  6. T.E. Evans; et al. (2004). "Suppression of Large Edge-Localized Modes in High-Confinement DIII-D Plasmas with a Stochastic Magnetic Boundary" (Submitted manuscript). Physical Review Letters. 92 (23): 235003. Bibcode:2004PhRvL..92w5003E. doi:10.1103/PhysRevLett.92.235003. PMID 15245164.
  7. Kwon, Eunhee (2011-11-10). "KSTAR announces successful ELM suppression". Retrieved 2011-12-11.
  8. 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.

Further reading
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