Editing Stellarator (section)

=== Inherent problems ===
Work on the then-new tokamak concept in the early 1970s, notably by [[Tihiro Ohkawa]] at [[General Atomics]], suggested that toroids with smaller ''[[aspect ratio]]s'' and non-circular plasmas would have much-improved performance.{{sfn|Bromberg|1982|p=164}} The aspect ratio is the comparison of the radius of the device as a whole to the radius of the cross-section of the vacuum tube. An ideal reactor would have no hole in the center, minimizing the aspect ratio. The modern [[spherical tokamak]] takes this to its practical limit, reducing the center hole to a single metal post, elongating the cross-section of the tubing vertically, producing an overall shape that is nearly spherical and has a ratio less than 2. The [[Mega Ampere Spherical Tokamak|MAST]] device in the UK, among the most powerful of these designs, has a ratio of 1.3.<ref>{{cite conference |url=https://www.researchgate.net/publication/260451493 |title= The upgrade to the Mega Amp Spherical Tokamak |last1= Stork |first1=Derek |last2= Meyer |first2= Hendrik |date= January 2010 |publisher= |book-title= |pages= |location=Daejon |conference= Proceedings of the 23rd International Conference on Fusion Energy |id=}}</ref>

Stellarators generally require complex magnets to generate the desired field. In early examples, this was often in the form of several different sets of magnets stacked. While modern designs combine these together, the resulting designs often require significant room around them. This limits the size of the inner radius to something much larger than seen in modern tokamaks, so they have relatively large aspect ratios. For instance, W7-X has an aspect ratio of 10,<ref>{{cite journal |first=Friedrich |last=Wagner |journal=Europhysics News |date=1995 |pages=3–5 |title=The W7-X Stellarator Project|volume=26 |issue=1 |doi=10.1051/epn/19952601003 |bibcode=1995ENews..26....3W |url=https://www.europhysicsnews.org/articles/epn/pdf/1995/01/epn19952601p3.pdf|doi-access=free }}</ref> which leads to a very large overall size. There are some new layouts that aim to reduce the aspect ratio, but these remain untested {{asof|2023|lc=yes}} and the reduction is still nowhere near the level seen in modern tokamaks.{{sfn|Landreman|Boozer|2017|p=1}}

In a production design, the magnets would need to be protected from the 14.1&nbsp;MeV [[neutron]]s being produced by the fusion reactions. This is normally accomplished through the use of a [[breeding blanket]], a layer of material containing large amounts of [[lithium]]. In order to capture most of the neutrons, the blanket has to be about 1 to 1.5 meters thick, which moves the magnets away from the plasma and therefore requires them to be more powerful than those on experimental machines where they line the outside of the vacuum chamber directly. This is normally addressed by scaling the machine up to extremely large sizes, such that the ~10&nbsp;centimetre separation found in smaller machines is linearly scaled to about 1 meter. This has the effect of making the machine much larger, growing to impractical sizes.{{sfn|Landreman|Boozer|2017|p=1}} Designs with smaller aspect ratios, which scale more rapidly, would address this effect to some degree, but designs of such systems, like ARIES-CS, are enormous, about 8 meters in radius with a relatively high aspect ratio of about 4.6.<ref>{{cite journal |journal=Fusion Science and Technology |first=F. |last= Najmabadi |date=2008 |volume=54 |issue=3 |title=The ARIES-CS Compact Stellarator Fusion Power Plant |pages=655–672 |url=https://www.tandfonline.com/doi/abs/10.13182/FST54-655 |doi=10.13182/FST54-655|bibcode=2008FuST...54..655N |s2cid=8620401 }}</ref>

The stellarator's complex magnets combine together to produce the desired field shape. This demands extremely tight positioning tolerances which drive up construction costs. It was this problem that led to the cancellation of the US's [[National Compact Stellarator Experiment]], or NCSX, which was an experimental low-aspect design with a ratio of 4.4. To work properly, the maximum deviation in placement across the entire machine was {{val|1.5|u=mm}}. As it was assembled this was found to be impossible to achieve, even the natural sagging of the components over time was more than the allowed limit. Construction was cancelled in 2008, throwing the future of the PPPL into doubt.<ref name=Orbach2008>{{cite web |url=http://ncsx.pppl.gov//DOE_NCSX_052208.pdf |title=Future of the Princeton Plasma Physics Laboratory (PPPL), Statement by Dr. Raymond L. Orbach, Under Secretary for Science and Director, Office of Science, U.S. Department of Energy |date=22 May 2008}}</ref>

Finally, stellarator designs are expected to leak around 5% of the generated [[alpha particle]]s, increasing stress on the plasma-facing components of a reactor.{{sfn|Landreman|Boozer|2017|p=2}}