View source for Physics:Edge-localized mode

An '''edge-localized mode''' (ELM) is a [[Plasma stability|plasma instability]] occurring in the edge region of a [[Physics:Tokamak|tokamak]] [[Physics:Plasma|plasma]] due to periodic relaxations of the edge transport barrier in [[Physics:High-confinement mode|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 [[Physics:ASDEX Upgrade|ASDEX tokamak]] in 1981.<ref>{{Cite journal |last1=F. |first1=Wagner |last2=A.R. |first2=Field |last3=G. |first3=Fussmann |last4=J.V. |first4=Hofmann |last5=M.E. |first5=Manso |last6=O. |first6=Vollmer |last7=José |first7=Matias |date=1990 |title=Recent results of H-mode studies on ASDEX |journal=13th International Conference on Plasma Physics and Controlled Nuclear Fusion |pages=277–290 |hdl=10198/9098}}</ref> [[Physics:Diamagnetism|Diamagnetic]] effects in the model equations expand the size of the parameter space in which solutions of repeated [[Physics:Tokamak sawtooth|sawteeth]] can be recovered compared to a resistive [[Physics:Magnetohydrodynamics|MHD]] model.<ref>{{Cite journal |last1=Halpern |first1=F D |last2=Leblond |first2=D |last3=Lütjens |first3=H |last4=Luciani |first4=J-F |date=2010-11-30 |title=Oscillation regimes of the internal kink mode in tokamak plasmas |journal=Plasma Physics and Controlled Fusion |volume=53 |issue=1 |pages=015011 |doi=10.1088/0741-3335/53/1/015011 |s2cid=122868427 |issn=0741-3335}}</ref> An ELM can expel up to 20 percent of the reactor's energy.<ref name=":0">{{Cite web |last=Choi |first=Charles Q. |title=Controlled chaos may be the key to unlimited clean energy |url=https://www.inverse.com/innovation/nuclear-fusion-instabilities |access-date=2022-10-26 |website=Inverse |date=20 October 2022 |language=en}}</ref>
== Issues ==
ELM is a major challenge in [[Physics:Magnetic confinement fusion|magnetic fusion]] research with tokamaks, as these instabilities can:

* Damage wall components (in particular [[Physics:Divertor|divertor]] plates) by ablating them away due to their extremely high energy transfer rate (GW/m<sup>2</sup>);<ref>{{Cite news |last=Lee |first=Chris |date=13 September 2018 |title=A third dimension helps Tokamak fusion reactor avoid wall-destroying instability |language=en-us |work=[[Ars Technica]] |url=https://arstechnica.com/science/2018/09/a-third-dimension-helps-tokamak-fusion-reactor-avoid-wall-destroying-instability/ |access-date=2018-09-17}}</ref> 
* Potentially couple or trigger other instabilities, such as the resistive wall mode (RWM) or the neoclassical tearing mode (NTM).<ref>{{cite journal |last1=Leonard |first1=A.W. |date=11 September 2014 |title=Edge-localized modes in tokamaks |journal=[[Physics:Physics of Plasmas|Physics of Plasmas]] |volume=21 |issue=9 |page=090501 |bibcode=2014PhPl...21i0501L |doi=10.1063/1.4894742 |osti=1352343}}</ref>

==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.<ref>{{cite journal
 |author=T.E. Evans  |display-authors=0  |date=2008
 |title=RMP ELM suppression in DIII-D plasmas with ITER similar shapes and collisionalities
 |journal=Nucl. Fusion |volume=92 |issue=48 |pages=024002 |doi=10.1088/0029-5515/48/2/024002 |bibcode=2008NucFu..48b4002E |url=https://iopscience.iop.org/article/10.1088/0029-5515/48/2/024002 |hdl=11858/00-001M-0000-0026-FFB5-4 |s2cid=54039023 |hdl-access=free }}</ref>
* Injecting pellets to increase the frequency and thereby decrease the severity of ELM bursts ([[Physics:ASDEX Upgrade|ASDEX Upgrade]]).{{cn|date=March 2018}}
* Multiple small-scale ELMs (000s/s) in [[Physics:Tokamak|tokamaks]] to prevent the creation of large ones, spreading the associated heat over a larger area and interval<ref name=":1" />
* Increase the plasma [[Physics:Density|density]] and, at high densities, adjusting the topology of the magnetic field lines confining the plasma.<ref>{{cite web|url=https://physicsworld.com/a/fusion-reactor-instabilities-can-be-optimized-by-adjusting-plasma-density-and-magnetic-fields/|publisher=Physics World|title=Fusion-reactor instabilities can be optimized by adjusting plasma density and magnetic fields|date=Nov 4, 2022}}</ref>

==History==
In 2003 DIII-D begn experimenting with [[Physics:Resonant magnetic perturbations|resonant magnetic perturbations]] to control ELMs.<ref name="Evans-2004">
{{cite journal
 |author=T.E. Evans  |display-authors=0  |date=2004
 |title=Suppression of Large Edge-Localized Modes in High-Confinement DIII-D Plasmas with a Stochastic Magnetic Boundary
 |journal=[[Physics:Physical Review Letters|Physical Review Letters]] |volume=92 |issue=23 |pages=235003 |doi=10.1103/PhysRevLett.92.235003 |bibcode = 2004PhRvL..92w5003E
 |pmid=15245164|url=http://juser.fz-juelich.de/search?p=id:%22PreJuSER-38026%22 }}</ref>

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 [[Physics:KSTAR|KSTAR tokamak]] used specific asymmetric three-dimensional magnetic field configurations to achieve this goal.<ref name="KSTAR-2011">{{cite web|url=http://www.iter.org/newsline/198/950|title=KSTAR announces successful ELM suppression |author=Kwon, Eunhee |date=2011-11-10|access-date=2011-12-11}}</ref><ref>{{Cite journal |last1=Park |first1=Jong-Kyu |last2=Jeon |first2=YoungMu |last3=In |first3=Yongkyoon |last4=Ahn |first4=Joon-Wook |last5=Nazikian |first5=Raffi |last6=Park |first6=Gunyoung |last7=Kim |first7=Jaehyun |last8=Lee |first8=HyungHo |last9=Ko |first9=WonHa |last10=Kim |first10=Hyun-Seok |last11=Logan |first11=Nikolas C. |last12=Wang |first12=Zhirui |last13=Feibush |first13=Eliot A. |last14=Menard |first14=Jonathan E. |last15=Zarnstroff |first15=Michael C. |date=2018-09-10 |title=3D field phase-space control in tokamak plasmas |journal=[[Physics:Nature Physics|Nature Physics]] |language=En |volume=14 |issue=12 |pages=1223–1228 |bibcode=2018NatPh..14.1223P |doi=10.1038/s41567-018-0268-8 |issn=1745-2473 |osti=1485109 |s2cid=125338335}}</ref>

In 2015, results of the first simulation to demonstrate repeated ELM cycling was published.<ref>{{Cite journal |last1=Orain |first1=François |last2=Bécoulet |first2=M |last3=Morales |first3=J |last4=Huijsmans |first4=G T A |last5=Dif-Pradalier |first5=G |last6=Hoelzl |first6=M |last7=Garbet |first7=X |last8=Pamela |first8=S |last9=Nardon |first9=E |date=2014-11-28 |title=Non-linear MHD modeling of edge localized mode cycles and mitigation by resonant magnetic perturbations |url=https://hal.archives-ouvertes.fr/hal-02184569/file/Orain_F_PPCF_2014%20%281%29.pdf |journal=[[Physics:Plasma Physics and Controlled Fusion|Plasma Physics and Controlled Fusion]] |volume=57 |issue=1 |pages=014020 |doi=10.1088/0741-3335/57/1/014020 |s2cid=44243673 |issn=0741-3335}}</ref> 

In 2022, researchers began testing the small ELM hypothesis at [[Physics:Joint European Torus|JET]] to assess the utility of the technique.<ref name=":1">{{Cite journal |last1=Harrer |first1=G. F. |last2=Faitsch |first2=M. |last3=Radovanovic |first3=L. |last4=Wolfrum |first4=E. |last5=Albert |first5=C. |last6=Cathey |first6=A. |last7=Cavedon |first7=M. |last8=Dunne |first8=M. |last9=Eich |first9=T. |last10=Fischer |first10=R. |last11=Griener |first11=M. |last12=Hoelzl |first12=M. |last13=Labit |first13=B. |last14=Meyer |first14=H. |last15=Aumayr |first15=F. |date=2022-10-10 |title=Quasicontinuous Exhaust Scenario for a Fusion Reactor: The Renaissance of Small Edge Localized Modes |url=https://link.aps.org/doi/10.1103/PhysRevLett.129.165001 |journal=Physical Review Letters |volume=129 |issue=16 |pages=165001 |doi=10.1103/PhysRevLett.129.165001|pmid=36306746 |s2cid=239768831 |arxiv=2110.12664 |bibcode=2022PhRvL.129p5001H }}</ref><ref name=":0" />  

== See also ==
* [[Physics:Resonant magnetic perturbations|Resonant magnetic perturbations]], used to control ELMs
* Plasma instability
* [[Physics:Tokamak|Tokamak]]

==References==
{{reflist}}

==Further reading==
* {{Cite journal|last1=Kirk|first1=A|last2=Liu|first2=Yueqiang|last3=Chapman|first3=I T|last4=Harrison|first4=J|last5=Nardon|first5=E|last6=Scannell|first6=R|last7=Thornton|first7=A J|s2cid=119208710|date=2013-03-06|title=Effect of resonant magnetic perturbations on ELMs in connected double null plasmas in MAST|journal=Plasma Physics and Controlled Fusion|volume=55|issue=4|pages=045007|doi=10.1088/0741-3335/55/4/045007|issn=0741-3335|arxiv=1303.0146|bibcode=2013PPCF...55d5007K}}
*{{Cite journal |last1=Tala |first1=Tuomas |last2=Garbet |first2=Xavier |date=2006 |title=Physics of Internal Transport Barriers |url=https://comptes-rendus.academie-sciences.fr/physique/item/10.1016/j.crhy.2006.06.005.pdf |journal=[[Physics:Comptes Rendus Physique|Comptes Rendus Physique]] |volume=7 |issue=6 |pages=622–633 |doi=10.1016/j.crhy.2006.06.005 |bibcode=2006CRPhy...7..622T |via=Elsevier Science Direct}}

[[Category:Plasma instabilities]]

{{Sourceattribution|Edge-localized mode}}