Editing Stellarator (section)

== Recent results ==
[[Image:Wendelstein 7-X experimental field line visualization.jpg|thumb|Visualization of magnetic field lines in Wendelstein 7-X]]

=== Optimization to reduce transport losses ===
The goal of magnetic confinement devices is to minimise [[Stellar structure#energy transport|energy transport]] across a magnetic field. Toroidal devices are relatively successful because the magnetic properties seen by the particles are averaged as they travel around the torus. The strength of the field seen by a particle, however, generally varies, so that some particles will be trapped by the [[Magnetic mirror|mirror effect]]. These particles will not be able to average the magnetic properties so effectively, which will result in increased energy transport. In most stellarators, these changes in field strength are greater than in tokamaks, which is a major reason that transport in stellarators tends to be higher than in tokamaks.

University of Wisconsin electrical engineering Professor David Anderson and research assistant John Canik proved in 2007 that the [[Helically Symmetric Experiment|Helically Symmetric eXperiment]] (HSX) can overcome this major barrier in plasma research. The HSX is the first stellarator to use a quasisymmetric magnetic field. The team designed and built the HSX with the prediction that [[quasisymmetry]] would reduce energy transport. As the team's latest research showed, that is exactly what it does. "This is the first demonstration that quasisymmetry works, and you can actually measure the reduction in transport that you get", said Canik.<ref>{{cite journal |last1=Canik |first1=J.M. |s2cid=23140945 |display-authors=etal |date=2007 |title=Experimental Demonstration of Improved Neoclassical Transport with Quasihelical Symmetry |journal=[[Physical Review Letters]] |volume=98 |issue=8 |page=085002 |bibcode=2007PhRvL..98h5002C |doi=10.1103/PhysRevLett.98.085002 |pmid=17359105}}</ref><ref>{{cite news |last1=Seely |first1=R. |date=12 April 2011 |title=UW scientists see a future in fusion |url=http://host.madison.com/wsj/news/local/education/university/uw-scientists-see-a-future-in-fusion/article_586ecb6a-63a4-11e0-870a-001cc4c002e0.html |work=Wisconsin State Journal}}</ref>

The newer [[Wendelstein 7-X]] in Germany was designed to be close to [[omnigeneity]] (a property of the magnetic field such that the mean radial drift is zero), which is a necessary but not sufficient condition for quasisymmetry;<ref>{{cite web |title=Omnigeneity |url=http://fusionwiki.ciemat.es/wiki/Omnigeneity |website=FusionWiki |access-date=31 January 2016}}</ref> that is, all quasisymmetric magnetic fields are omnigenous, but not all omnigenous magnetic fields are quasisymmetric. Experiments at the Wendelstein 7-X stellarator have revealed turbulence-induced anomalous diffusion.<ref>{{cite journal |last1=Geiger |first1=B. |last2=Wegner |first2=T. |last3=Beidler |first3=C.D. |last4=Burhenn |first4=R. |last5=Buttenschön |first5=B. |last6=Dux |first6=R. |display-authors=1 |date=2019 |title=Observation of anomalous impurity transport during low-density experiments in W7-X with laser blow-off injections of iron |journal=Nuclear Fusion |volume=59 |issue=4 |page=046009 |doi=10.1088/1741-4326/aaff71 |hdl=21.11116/0000-0002-F435-F |s2cid=127842248 |hdl-access=free }}</ref>  The optimized magnetic field of W7-X showed effective control of bootstrap current and reduced neoclassical energy transport, enabling high-temperature plasma conditions and record fusion values but also longer impurity confinement times during turbulence-suppressed phases. These findings highlight the success of magnetic field optimization in stellarators.<ref>{{cite journal |doi=10.1038/s41567-018-0141-9 |title=Magnetic configuration effects on the Wendelstein 7-X stellarator |year=2018 |last1=Dinklage |first1=A. |last2=Beidler |first2=C.D. |last3=Helander |first3=P. |last4=Fuchert |first4=G. |last5=Maaßberg |first5=H. |last6=Rahbarnia |first6=K. |last7=Sunn Pedersen |first7=T. |last8=Turkin |first8=Y. |last9=Wolf |first9=R.C. |last10=Alonso |first10=A. |last11=Andreeva |first11=T. |last12=Blackwell |first12=B. |last13=Bozhenkov |first13=S. |last14=Buttenschön |first14=B. |last15=Czarnecka |first15=A. |last16=Effenberg |first16=F. |last17=Feng |first17=Y. |last18=Geiger |first18=J. |last19=Hirsch |first19=M. |last20=Höfel |first20=U. |last21=Jakubowski |first21=M. |last22=Klinger |first22=T. |last23=Knauer |first23=J. |last24=Kocsis |first24=G. |last25=Krämer-Flecken |first25=A. |last26=Kubkowska |first26=M. |last27=Langenberg |first27=A. |last28=Laqua |first28=H.P. |last29=Marushchenko |first29=N. |last30=Mollén |first30=A. |journal=Nature Physics |volume=14 |issue=8 |pages=855–860 |bibcode=2018NatPh..14..855D |hdl=21.11116/0000-0001-F331-5 |s2cid=256704728 |display-authors=1 |hdl-access=free }}</ref><ref>{{cite journal |doi=10.1038/s41586-021-03687-w |title=Demonstration of reduced neoclassical energy transport in Wendelstein 7-X |year=2021 |last1=Beidler |first1=C. D. |last2=Smith |first2=H. M. |last3=Alonso |first3=A. |last4=Andreeva |first4=T. |last5=Baldzuhn |first5=J. |last6=Beurskens |first6=M. N. A. |last7=Borchardt |first7=M. |last8=Bozhenkov |first8=S.A. |last9=Brunner |first9=K. J. |last10=Damm |first10=H. |last11=Drevlak |first11=M. |last12=Ford |first12=O.P. |last13=Fuchert |first13=G. |last14=Geiger |first14=J. |last15=Helander |first15=P. |last16=Hergenhahn |first16=U. |last17=Hirsch |first17=M. |last18=Höfel |first18=U. |last19=Kazakov |first19=Ye.O. |last20=Kleiber |first20=R. |last21=Krychowiak |first21=M. |last22=Kwak |first22=S. |last23=Langenberg |first23=A. |last24=Laqua |first24=H.P. |last25=Neuner |first25=U. |last26=Pablant |first26=N. A. |last27=Pasch |first27=E. |last28=Pavone |first28=A. |last29=Pedersen |first29=T.S. |last30=Rahbarnia |first30=K. |display-authors=1 |journal=Nature |volume=596 |issue=7871 |pages=221–226 |pmid=34381232 |pmc=8357633 |bibcode=2021Natur.596..221B}}</ref><ref>{{cite journal |last1=Pedersen |first1=T.S. |display-authors=etal |title=Experimental confirmation of efficient island divertor operation and successful neoclassical transport optimization in Wendelstein 7-X |date=2022 |journal=Nuclear Fusion |volume=62 |issue=4 |page=042022 |doi=10.1088/1741-4326/ac2cf5 |s2cid=234338848 |hdl=1721.1/147631 |hdl-access=free }}</ref>

=== Proof of divertor concepts ===

At Wendelstein 7-X, the island [[divertor]] has been successful in stabilizing detached plasma scenarios and reducing the [[Heat flux|heat fluxes]] on divertor targets.<ref>{{cite journal |title=O. Schmitz et al Nucl. Fusion 61, 016026 (2021) |journal=Nuclear Fusion |date=3 September 2020 |volume=61 |issue=1 |doi=10.1088/1741-4326/abb51e |osti=1814444 |s2cid=225288529 |url=https://www.osti.gov/biblio/1814444 |last1=Schmitz |first1=Oliver |last2=Feng |first2=Yuhe |last3=Jakubowski |first3=Marcin |last4=König |first4=Ralf |last5=Krychowiak |first5=Maciej |last6=Otte |first6=Matthias |last7=Reimold |first7=Felix |last8=Barbui |first8=Tullio |last9=Biedermann |first9=Christoph |last10=Bozhenkov |first10=Sergey A. |last11=Brezinsek |first11=Sebastijan |last12=Buttenschön |first12=Birger |last13=Brunner |first13=Kai Jakob |last14=Drewelow |first14=Peter |last15=Effenberg |first15=Florian |last16=Flom |first16=Erik |last17=Frerichs |first17=Heinke |last18=Ford |first18=Oliver P. |last19=Fuchert |first19=Golo |last20=Gao |first20=Yu |last21=Gradic |first21=Dorothea |last22=Grulke |first22=Olaf |last23=Hammond |first23=Kenneth |last24=Hergenhahn |first24=Uwe |last25=Höfel |first25=Udo |last26=Knauer |first26=Jens P. |last27=Kornejew |first27=Petra |last28=Kremeyer |first28=Thierry |last29=Niemann |first29=Holger |last30=Pasch |first30=Ekkehard |hdl=21.11116/0000-0007-A4DC-8 |display-authors=1 |hdl-access=free }}</ref><ref>{{cite journal |last1=Jakubowski |first1=M. |display-authors=etal |date=2021 |title=Overview of the results from divertor experiments with attached and detached plasmas at Wendelstein 7-X and their implications for steady-state operation |journal=Nuclear Fusion |volume=61 |issue=10 |doi=10.1088/1741-4326/ac1b68 |s2cid=237408135 |url=https://publikationen.bibliothek.kit.edu/1000140073/133022842 |doi-access=free }}</ref> This topology has multiple adjacent counter-streaming flow regions that can reduce the flow speed parallel to magnetic field lines, leading to substantial heat flux mitigation.<ref>{{cite journal |author1-last=Perseo |author1-first=V. |author2-first=F. |author2-last=Effenberg |author3-first=D. |author3-last=Gradic |author4-first=R. |author4-last=König |author5-first=O.P. |author5-last=Ford |author6-first=F. |author6-last=Reimold |author7-first=D.A. |author7-last=Ennis| author8-first=O. |author8-last=Schmitz |author9-first=T. Sunn |author9-last=Pedersen |display-authors=1 |title=Direct measurements of counter-streaming flows in a low-shear stellarator magnetic island topology |journal=Nuclear Fusion |volume=59 |number=12 |date=2019 |doi=10.1088/1741-4326/ab4320|osti=1572710 |s2cid=203087561 |doi-access=free }}</ref> Radiative power exhaust by impurity seeding has been demonstrated in island divertor configurations, resulting in stable plasma operation and reduced divertor heat loads.<ref>{{cite journal |title=First demonstration of radiative power exhaust with impurity seeding in the island divertor at Wendelstein 7-X |author=F. Effenberg |display-authors=etal |journal=Nucl. Fusion |volume=59 |number=10 |page=106020 |year=2019 |doi=10.1088/1741-4326/ab32c4 |s2cid=199132000 |url=https://hal.archives-ouvertes.fr/hal-03740994/file/w7x-impurity-seeding-power-exhaust-florian-effenberg.pdf }}</ref> This makes the island divertor a promising solution for future detachment control in high-performance scenarios and upgrades towards a metal divertor.<ref>{{cite journal|title=First feedback-controlled divertor detachment in W7-X: Experience from TDU operation and prospects for operation with actively cooled divertor |author=M. Krychowiak |display-authors=etal |journal=Nucl. Mater. Energy |volume=34 |page=101363 |year=2023 |doi=10.1016/j.nme.2023.101363|osti=1957530 |s2cid=255694619 |doi-access=free}}</ref> The edge magnetic structure in quasi-omnigenous and helically symmetric stellarators, like W7-X and HSX, has a significant impact on particle fueling and exhaust. It has been shown that the magnetic island chain can be used to control plasma fueling from the recycling source and active gas injection.<ref>{{cite journal |author1-first=L. |author1-last=Stephey |author2-first=A. |author2-last=Bader |author3-first=F. |author3-last=Effenberg |author4-first=O. |author4-last=Schmitz |author5-first=G. |author5-last=Wurden |author6-first=D. |author6-last=Anderson |author7-first=F. |author7-last=Anderson |author8-first=C. |author8-last=Biedermann |author9-first=A. |author9-last=Dinklage |author10-first=Y. |author10-last=Feng |author11-first=H. |author11-last=Frerichs |author12-first=G. |author12-last=Fuchert |author13-first=J. |author13-last=Geiger |author14-first=J. |author14-last=Harris |author15-first=R. |author15-last=König |author16-first=P. |author16-last=Kornejew |author17-first=M. |author17-last=Krychowiak |author18-first=J. |author18-last=Lore |author19-first=E. |author19-last=Unterberg |author20-first=I. |author20-last=Waters |display-authors=1 |title=Impact of magnetic islands in the plasma edge on particle fueling and exhaust in the HSX and W7-X stellarators |journal=Physics of Plasmas |volume=25 |issue=6 |year=2018 |doi=10.1063/1.5026324 |hdl=21.11116/0000-0001-6AE2-9 |osti=1439332 |s2cid=125652747 |hdl-access=free }}</ref>

=== MUSE ===
The MUSE device at [[Princeton Plasma Physics Laboratory]] uses primarily off-the-shelf parts such as 10000 [[permanent magnets]] to build a stellarator for use in research. The magnets are embedded in a [[3D printing|3D printed]] nylon matrix. It adopted the magnetic [[surface charge]] method. Peak internal stress was found to be less than 7&nbsp;MPa. It is the first quasi-axisymmetric experiment.<ref>{{cite web |last=Wang |first=Brian |date=2024-04-22 |title=MUSE Nuclear Fusion Stellerator Made with Off the Shelf Parts and 3D Printed Shell {{!}} NextBigFuture.com |url=https://www.nextbigfuture.com/2024/04/muse-nuclear-fusion-stellerator-made-with-off-the-shelf-parts-and-3d-printed-shell.html |access-date=2024-04-25 |language=en-US }}</ref>