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.{{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}} 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.{{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}} An ELM can expel up to 20 percent of the reactor's energy.{{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}}

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);{{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}}
Potentially couple or trigger other instabilities, such as the resistive wall mode (RWM) or the neoclassical tearing mode (NTM).{{cite journal |last1=Leonard |first1=A.W. |date=11 September 2014 |title=Edge-localized modes in tokamaks |journal=Physics of Plasmas |volume=21 |issue=9 |page=090501 |bibcode=2014PhPl...21i0501L |doi=10.1063/1.4894742 |osti=1352343}}

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.{{cite journal

|author=T.E. Evans |display-authors=etal |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 }}

Injecting pellets to increase the frequency and thereby decrease the severity of ELM bursts (ASDEX Upgrade).{{cn|date=March 2018}}
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.{{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}}

History

In 2003 DIII-D began experimenting with resonant magnetic perturbations to control ELMs.

{{cite journal

|author=T.E. Evans |display-authors=etal |date=2004

|title=Suppression of Large Edge-Localized Modes in High-Confinement DIII-D Plasmas with a Stochastic Magnetic Boundary

|pmid=15245164|url=http://juser.fz-juelich.de/search?p=id:%22PreJuSER-38026%22 }}

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.{{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}}{{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=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}}

In 2015, results of the first simulation to demonstrate repeated ELM cycling was published.{{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=Plasma Physics and Controlled Fusion |volume=57 |issue=1 |pages=014020 |doi=10.1088/0741-3335/57/1/014020 |s2cid=44243673 |issn=0741-3335}}

In 2022, researchers began testing the small ELM hypothesis at JET to assess the utility of the technique.{{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 }}

Edge-localized mode

Issues

Prevention and control

History

See also

References

Further reading