Editing Tokamak (section)

=== Plasma disruptions ===
Tokamaks are subject to events known as "disruptions" that cause confinement to be lost in [[millisecond]]s. There are two primary mechanisms. In one, the "vertical displacement event" (VDE), the entire plasma moves vertically until it touches the upper or lower section of the vacuum chamber. In the other, the "major disruption", long wavelength, non-axisymmetric [[magnetohydrodynamics|magnetohydrodynamical]] instabilities cause the plasma to be forced into non-symmetrical shapes, often squeezed into the top and bottom of the chamber.<ref name=Kruger>{{cite journal | last1 = Kruger | first1 = S. E. | last2 = Schnack | first2 = D. D. | last3 = Sovinec | first3 = C. R. | year = 2005 | title = Dynamics of the Major Disruption of a DIII-D Plasma | url = http://www.scidac.gov/FES/FES_FusionGrid/pubs/kruger-phys-plasma-2005.pdf | journal = Phys. Plasmas | volume = 12 | issue = 5 | page = 056113 | doi = 10.1063/1.1873872 | bibcode = 2005PhPl...12e6113K | access-date = 5 January 2012 | archive-date = 27 February 2013 | archive-url = https://web.archive.org/web/20130227121923/http://www.scidac.gov/FES/FES_FusionGrid/pubs/kruger-phys-plasma-2005.pdf }}</ref>

When the plasma touches the vessel walls it undergoes rapid cooling, or "thermal quenching". In the major disruption case, this is normally accompanied by a brief increase in plasma current as the plasma concentrates. Quenching ultimately causes the plasma confinement to break up. In the case of the major disruption the current drops again, the "current quench". The initial increase in current is not seen in the VDE, and the thermal and current quench occurs at the same time.<ref name=Kruger/> In both cases, the thermal and electrical load of the plasma is rapidly deposited on the reactor vessel, which has to be able to handle these loads. ITER is designed to handle 2600 of these events over its lifetime.<ref name=Putvinski>[http://w3fusion.ph.utexas.edu/ifs/iaeaep/talks/s11-i11-putvinski-sergei-ep-talk.pdf Runaway Electrons in Tokamaks and Their Mitigation in ITER] {{Webarchive|url=https://web.archive.org/web/20210308000033/http://w3fusion.ph.utexas.edu/ifs/iaeaep/talks/s11-i11-putvinski-sergei-ep-talk.pdf |date=8 March 2021 }}, S. Putvinski, ITER Organization</ref>

For modern high-energy devices, where plasma currents are on the order of 15 mega[[ampere]]s in [[ITER]], it is possible the brief increase in current during a major disruption will cross a critical threshold. This occurs when the current produces a force on the electrons that is higher than the frictional forces of the collisions between particles in the plasma. In this event, electrons can be rapidly accelerated to relativistic velocities, creating so-called "runaway electrons" in the [[relativistic runaway electron avalanche]]. These retain their energy even as the current quench is occurring on the bulk of the plasma.<ref name=Putvinski/>

When confinement finally breaks down, these runaway electrons follow the path of least resistance and impact the side of the reactor. These can reach 12 megaamps of current deposited in a small area, well beyond the capabilities of any mechanical solution.<ref name=Kruger /> In one famous case, the [[Tokamak de Fontenay aux Roses]] had a major disruption where the runaway electrons burned a hole through the vacuum chamber.<ref name=Putvinski/>

The occurrence of major disruptions in running tokamaks has always been rather high, of the order of a few percent of the total numbers of the shots. In currently operated tokamaks, the damage is often large but rarely dramatic. In the ITER tokamak, it is expected that the occurrence of a limited number of major disruptions will definitively damage the chamber with no possibility to restore the device.<ref>{{Cite conference|conference=MFE Roadmapping in the ITER Era |last=Wurden|first=G. A.|date=9 September 2011 |url=http://advprojects.pppl.gov/ROADMAPPING/presentations/MFE_POSTERS/WURDEN_Disruption_RiskPOSTER.pdf |title=Dealing with the Risk and Consequences of Disruptions in Large Tokamaks |archive-url=https://web.archive.org/web/20151105232903/http://advprojects.pppl.gov/ROADMAPPING/presentations/MFE_POSTERS/WURDEN_Disruption_RiskPOSTER.pdf|archive-date=5 November 2015}}</ref><ref>{{cite journal |display-authors=4 | last1 = Baylor | first1 = L. R. | last2 = Combs | first2 = S. K. | last3 = Foust | first3 = C. R. | last4 = Jernigan | first4 = T.C. | last5 = Meitner | first5 = S. J. | last6 = Parks | first6 = P. B. | last7 = Caughman | first7 = J. B. | last8 = Fehling | first8 = D. T. | last9 = Maruyama | first9 = S. | last10 = Qualls | first10 = A. L. | last11 = Rasmussen | first11 = D. A. | last12 = Thomas | first12 = C. E. | year = 2009 | title = Pellet Fuelling, ELM Pacing and Disruption Mitigation Technology Development for ITER | url = http://www-pub.iaea.org/MTCD/Meetings/FEC2008/it_p6-19.pdf | journal = Nucl. Fusion | volume = 49 | issue = 8| page = 085013 | doi = 10.1088/0029-5515/49/8/085013 | bibcode = 2009NucFu..49h5013B | s2cid = 17071617 }}</ref><ref>{{cite journal |display-authors=4 | last1 = Thornton | first1 = A. J. | last2 = Gibsonb | first2 = K. J. | last3 = Harrisona | first3 = J. R. | last4 = Kirka | first4 = A. | last5 = Lisgoc | first5 = S. W. | last6 = Lehnend | first6 = M. | last7 = Martina | first7 = R. | last8 = Naylora | first8 = G. | last9 = Scannella | first9 = R. | last10 = Cullena | first10 = A. | last11 = Mast Team | first11 = Thornton | year = 2011 | title = Disruption mitigation studies on the Mega Amp Spherical Tokamak (MAST) | journal = J. Nucl. Mater. | volume = 415 | issue = 1| pages = S836–S840 | doi = 10.1016/j.jnucmat.2010.10.029 | bibcode = 2011JNuM..415S.836M }}</ref> The development of systems to counter the effects of runaway electrons is considered a must-have piece of technology for the operational level ITER.<ref name=Putvinski/>

A large amplitude of the central current density can also result in [[Tokamak sawtooth|internal disruptions]], or sawteeth, which do not generally result in termination of the discharge.<ref>{{cite journal | last1 = von Goeler | first1 = S. | last2 = Stodiek | first2 = W. | last3 = Sauthoff | first3 = N. | year = 1974 | title = Studies of internal disruptions and m= 1 oscillations in tokamak discharges with soft – x-ray techniques | doi = 10.1103/physrevlett.33.1201 | journal = Physical Review Letters | volume = 33 | issue = 20| page = 1201 | bibcode = 1974PhRvL..33.1201V }}</ref>

Densities over the Greenwald limit, a bound depending on the plasma current and the minor radius, typically leads to disruptions.<ref>{{Cite journal |last=Greenwald |first=Martin |date=2002-08-01 |title=Density limits in toroidal plasmas |url=https://iopscience.iop.org/article/10.1088/0741-3335/44/8/201 |journal=Plasma Physics and Controlled Fusion |volume=44 |issue=8 |pages=R27–R53 |doi=10.1088/0741-3335/44/8/201|hdl=1721.1/93996 |hdl-access=free }}</ref><ref>{{Cite web |title=Greenwald limit |url=https://wiki.fusion.ciemat.es/wiki/Greenwald_limit |website=FusionWiki}}</ref> It has been exceeded up to factors of 10,<ref>{{Cite journal |last1=Hurst |first1=N. C. |last2=Chapman |first2=B. E. |last3=Sarff |first3=J. S. |last4=Almagri |first4=A. F. |last5=McCollam |first5=K. J. |last6=Den Hartog |first6=D. J. |last7=Flahavan |first7=J. B. |last8=Forest |first8=C. B. |date=2024-07-29 |title=Tokamak Plasmas with Density up to 10 Times the Greenwald Limit |url=https://link.aps.org/doi/10.1103/PhysRevLett.133.055101 |journal=Physical Review Letters |volume=133 |issue=5 |pages=055101 |doi=10.1103/PhysRevLett.133.055101|pmid=39159104 |osti=2370426 }}</ref> but it remains an important concept describing the phenomenology of the transition of the plasma flow, which still needs to be understood.<ref>{{Cite journal |last1=Gates |first1=D. A. |last2=Delgado-Aparicio |first2=L. |date=2012-04-20 |title=Origin of Tokamak Density Limit Scalings |url=https://link.aps.org/doi/10.1103/PhysRevLett.108.165004 |journal=Physical Review Letters |language=en |volume=108 |issue=16 |page=165004 |doi=10.1103/PhysRevLett.108.165004 |pmid=22680727 |issn=0031-9007}}</ref>