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{{Short description|Plasma compressor and nuclear fusion system}}
[[File:Z-pinch H-gamma.jpg|thumb|right|300px|A laboratory-scale Z-pinch showing glow from an expanded hydrogen plasma. The pinch current flows through the gas and returns via the bars surrounding the plasma vessel.]]

[[File:TWK pinch.jpg|thumb|right|300px|A desktop-sized inductively coupled current-driven toroidal Z-pinch in a krypton plasma showing an intense glow from a plasma filament.]]

In [[fusion power]] research, the '''Z-pinch''' ('''zeta pinch''') is a type of [[Plasma (physics)|plasma]] confinement system that uses an electric current in the plasma to generate a magnetic field that compresses it (see [[Pinch (plasma physics)|pinch]]). These systems were originally referred to simply as '''pinch''' or '''Bennett pinch''' (after [[Willard Harrison Bennett]]), but the introduction of the [[theta pinch|θ-pinch]] (theta pinch) concept led to the need for clearer, more precise terminology.

The name refers to the direction of the current in the devices, the Z-axis on a [[Cartesian coordinate system|Cartesian three-dimensional graph]]. Any machine that causes a pinch effect due to current running in that direction is correctly referred to as a Z-pinch system, and this encompasses a wide variety of devices used for an equally wide variety of purposes. Early uses focused on fusion research in donut-shaped tubes with the Z-axis running down the inside of the tube, while modern devices are generally cylindrical and used to generate high-intensity [[x-ray]] sources for the study of [[nuclear weapon]]s and other roles. It is one of the first approaches to [[fusion power]] devices, along with the [[stellarator]] and [[magnetic mirror]].

==Physics==
The Z-pinch is an application of the [[Lorentz force]], in which a current-carrying conductor in a magnetic field experiences a force. One example of the Lorentz force is that, if two parallel wires are carrying current in the same direction, the wires will be pulled toward each other. In a Z-pinch machine the wires are replaced by a [[plasma (physics)|plasma]], which can be thought of as many current-carrying wires. When a current is run through the plasma, the particles in the plasma are pulled toward each other by the Lorentz force, thus the plasma contracts. The contraction is counteracted by the increasing gas pressure of the plasma.

As the plasma is electrically conductive, a magnetic field nearby will induce a current in it. This provides a way to run a current into the plasma without physical contact, which is important as a plasma can rapidly erode mechanical [[electrodes]]. In practical devices this was normally arranged by placing the plasma vessel inside the core of a [[transformer]], arranged so the plasma itself would be the secondary. When current was sent into the primary side of the transformer, the magnetic field induced a current into the plasma. As induction requires a ''changing'' magnetic field, and the induced current is supposed to run in a single direction in most reactor designs, the current in the transformer has to be increased over time to produce the varying magnetic field. This places a limit on the product of confinement time and magnetic field, for any given source of power.

In Z-pinch machines the current is generally provided from a large bank of [[capacitor]]s and triggered by a [[spark gap]], known as a Marx Bank or [[Marx generator]]. As the conductivity of plasma is fairly good, about that of [[copper]], the energy stored in the power source is quickly depleted by running through the plasma. Z-pinch devices are inherently pulsed in nature.

==History==
===Early machines===
[[File:Kink instability at Aldermaston.jpg|thumb|right|An early photograph of the kink instability in a toroidal pinch – the 3 by 25 pyrex tube at Aldermaston.]]
Pinch devices were among the earliest efforts in fusion power. Research began in the UK in the immediate post-war era, but a lack of interest led to little development until the 1950s. The announcement of the [[Huemul Project]] in early 1951 led to fusion efforts around the world, notably in the UK and in the US (see [[Perhapsatron]], a z-pinch machine at [[LANL]]). Small experiments were built at labs as various practical issues were addressed, but all of these machines demonstrated unexpected instabilities of the plasma that would cause it to hit the walls of the container vessel. The problem became known as the "[[kink instability]]".

===Stabilized pinch===
By 1953 the "stabilized pinch" seemed to solve the problems encountered on earlier devices. Stabilized pinch machines added external magnets that created a toroidal magnetic field inside the chamber. When the device was fired, this field added to the one created by the current in the plasma. The result was that the formerly straight magnetic field was twisted into a helix, which the particles followed as they traveled around the tube driven by the current. A particle near the outside of the tube that wanted to kink outward would travel along these lines until it returned to the inside of the tube, where its outward-directed motion would bring it back into the centre of the plasma.

Researchers in the UK started construction of [[ZETA (fusion reactor)|ZETA]] in 1954. ZETA was by far the largest fusion device of its era. At the time, almost all fusion research was classified, so progress on ZETA was generally unknown outside the labs working on it. However US researchers visited ZETA and realized that they were about to be outpaced. Teams on both sides of the Atlantic rushed to be the first to complete stabilized pinch machines.

ZETA won the race, and by the summer of 1957 it was producing bursts of [[neutron]]s on every run. Despite the researchers' reservations, their results were released with great fanfare as the first successful step on the path to commercial fusion energy. However, further study soon demonstrated that the measurements were misleading, and none of the machines were near fusion levels. Interest in pinch devices faded, although ZETA and its cousin [[Sceptre (fusion reactor)|Sceptre]] served for many years as experimental devices.

===Fusion-based propulsion===
{{Further|Fusion rocket}}
A concept of Z-pinch fusion propulsion system was developed through collaboration between [[NASA]] and private companies.<ref>{{cite web|url=http://www.icarusinterstellar.org/papers/ASPW2010-Z-Pinch(RAdams).pdf|title=Conceptual Design of a Z-Pinch Fusion Propulsion System|last=Adams|first=R|archive-url=https://web.archive.org/web/20140630081723/http://www.icarusinterstellar.org/papers/ASPW2010-Z-Pinch(RAdams).pdf|archive-date=2014-06-30|access-date=2014-05-20}}</ref> The energy released by the Z-pinch effect would accelerate lithium propellant to a high speed, resulting in a [[specific impulse]] value of 19400 s and [[thrust]] of 38&nbsp;kN. A magnetic nozzle would be required to convert the released energy into a useful impulse. This propulsion method could potentially reduce interplanetary travel times. For example, a mission to Mars would take about 35 days one-way with a total burn time of 20 days and a burned propellant mass of 350 tonnes.<ref>{{Cite journal
| last1 = Miernik | first1 = J.
| last2 = Statham | first2 = G.
| last3 = Fabisinski | first3 = L.
| last4 = Maples | first4 = C. D. 
| last5 = Adams | first5 = R.
| last6 = Polsgrove | first6 = T.
| last7 = Fincher | first7 = S.
| last8 = Cassibry | first8 = J.
| last9 = Cortez | first9 = R.
| last10 = Turner | first10 = M.
| last11 = Percy | first11 = T.
| year = 2013
| doi = 10.1016/j.actaastro.2012.02.012
| title = Z-Pinch fusion-based nuclear propulsion
| journal = Acta Astronautica
| volume = 82
| issue = 2
| pages = 173–82
|bibcode = 2013AcAau..82..173M }}{{Subscription required}}</ref>

===Tokamak===
Although it remained relatively unknown for years, Soviet scientists used the pinch concept to develop the [[tokamak]] device. Unlike the stabilized pinch devices in the US and UK, the tokamak used considerably more energy in the stabilizing magnets, and much less in the plasma current. This reduced the instabilities due to the large currents in the plasma, and led to great improvements in stability. The results were so dramatic that other researchers were skeptical when they were first announced in 1968. Members of the still-operational ZETA team were called in to verify the results. The tokamak became the most studied approach to controlled fusion.

=== Sheared-flow stabilized ===
Sheared-flow stabilizing uses one or more high speed annular flowing plasma layers, surrounding a plasma filament, to stabilize the filament against kink and pinch instabilities.<ref>
{{cite journal |last1=Forbes |first1=Eleanor G. |last2=Shumlak |first2=Uri |last3=McLean |first3=Harry S. |last4=Nelson |first4=Brian A. |last5=Claveau |first5=Elliot L. |last6=Golingo |first6=Raymond P. |last7=Higginson |first7=Drew P. |last8=Mitrani |first8=James M. |last9=Stepanov |first9=Anton D. |last10=Tummel |first10=Kurt K. |last11=Weber |first11=Tobin R. |date=11 June 2018 |title=Progress Toward a Compact Fusion Reactor Using the Sheared-Flow-Stabilized Z-Pinch |journal=Fusion Science and Technology |volume=75 |issue=7 |pages=599–607 |publisher=Informa UK Limited |doi=10.1080/15361055.2019.1622971|osti=1632373 |s2cid=198442070 |url=https://www.osti.gov/biblio/1632373}}</ref><ref>
{{cite journal |last=Shumlak |first=U. |date=27 May 2020 |title=Z-pinch fusion |journal=Journal of Applied Physics |volume=127 |issue=20 |page=200901 |publisher=AIP Publishing |doi=10.1063/5.0004228 |bibcode=2020JAP...127t0901S |doi-access=free}}</ref>

In 2018, a sheared-flow stabilized Z-pinch demonstrated neutron generation. It was built by a fusion company, [[Zap Energy]], Inc.,<ref>{{Cite web |url=https://www.zapenergyinc.com/ |title=Zap Energy |website=Zap Energy}}</ref> a [[University spin-off|spin-off]] from the [[University of Washington]],<ref>{{Cite report |last1=Nelson |first1=B.A. |last2=Conway |first2=B. |last3=Shumlak |first3=U. |last4=Weber |first4=T.R. |last5=Claveau |first5=E.L. |last6=Draper |first6=Z.T. |last7=Forbes |first7=E.G. |last8=Stepanov |first8=A.D. |last9=Zhang |first9=Y. |last10=McLean |first10=H.S. |last11=Higginson |first11=D.P. |last12=Mitrani |first12=J.M. |last13=Tummel |first13=K. |date=2019 |url=https://arpa-e.energy.gov/sites/default/files/Nelson_FuZE_.pdf |title=Development of a Compact Fusion Device based on the Flow Z-Pinch |website=ARPA–E |publisher=Advanced Research Projects Agency |access-date=2 April 2021 |archive-date=19 March 2021 |archive-url=https://web.archive.org/web/20210319010604/https://arpa-e.energy.gov/sites/default/files/Nelson_FuZE_.pdf }}</ref> and funded by strategic and financial investors and grants from the Advanced Research Projects Agency – Energy ([[ARPA-E]]).<ref>{{cite web |last=Nelson |first=Brian |date=15 November 2018 |url=https://arpa-e.energy.gov/technologies/projects/electrode-technology-development-sheared-flow-z-pinch-fusion-reactor |title=Zap Energy: Electrode Technology Development for the Sheared-Flow Z-Pinch Fusion Reactor |website=ARPA–E |publisher=Advanced Research Projects Agency |access-date=2 April 2021 |archive-date=19 March 2021 |archive-url=https://web.archive.org/web/20210319012849/https://arpa-e.energy.gov/technologies/projects/electrode-technology-development-sheared-flow-z-pinch-fusion-reactor }}</ref><ref>{{cite web |last=Nelson |first=Brian |date=7 April 2020 |url=https://arpa-e.energy.gov/technologies/projects/sheared-flow-stabilized-z-pinch-performance-improvement |title=Zap Energy: Sheared Flow Stabilized Z-Pinch Performance Improvement |website=ARPA–E |publisher=Advanced Research Projects Agency |access-date=2 April 2021 |archive-date=18 March 2021 |archive-url=https://web.archive.org/web/20210318055638/https://arpa-e.energy.gov/technologies/projects/sheared-flow-stabilized-z-pinch-performance-improvement }}</ref> Flow stabilized plasma remained stable 5,000 times longer than a static plasma.<ref>{{Cite web |url=https://newatlas.com/nuclear-fusion-breakthrough-z-pinch/59257/ |title=Nuclear fusion breakthrough breathes life into the overlooked Z-pinch approach |last=Lavars |first=Nick |date=April 12, 2019 |website=New Atlas |language=en |access-date=2019-04-14}}</ref> A mix of 20% deuterium and 80% hydrogen by pressure, produced neutron emissions lasting approximately 5 μs with pinch currents of approximately 200 kA during an approximately 16 μs period of plasma quiescence. Average neutron yield was estimated to be (1.25±0.45)×10<sup>5</sup> neutrons/pulse. Plasma temperatures of 1–2 keV (12–24 million °C) and densities of approximately 10<sup>17</sup> cm<sup>−3</sup> with 0.3&nbsp;cm pinch radii were measured.<ref>{{Cite journal |last1=Zhang |first1=Y. |last2=Shumlak |first2=U. |last3=Nelson |first3=B. A. |last4=Golingo |first4=R. P. |last5=Weber |first5=T. R. |last6=Stepanov |first6=A. D. |last7=Claveau |first7=E. L. |last8=Forbes |first8=E. G. |last9=Draper |first9=Z. T. |date=2019-04-04 |title=Sustained Neutron Production from a Sheared-Flow Stabilized Z Pinch |url=https://link.aps.org/doi/10.1103/PhysRevLett.122.135001 |journal=Physical Review Letters |language=en |volume=122 |issue=13 |article-number=135001 |doi=10.1103/PhysRevLett.122.135001 |pmid=31012637 |issn=0031-9007 |arxiv=1806.05894 |bibcode=2019PhRvL.122m5001Z |s2cid=51680710}}</ref>

==Experiments==
[[File:Pinchseta.JPG|thumb|A Z-pinch machine at UAM, Mexico City.]]
The ''Z''-pinch machines can be found at [[University of Nevada, Reno]] (USA), [[Cornell University]] (USA), [[University of Michigan]] (USA), [[Sandia National Laboratories]] (USA), [[University of California, San Diego]] (USA), [[University of Washington]] (USA), [[Ruhr University]] (Germany), [[Imperial College]] (United Kingdom), [[École Polytechnique]] (France), [[Weizmann Institute of Science]] (Israel), [[Universidad Autónoma Metropolitana]] (Mexico), [[GLAST (tokamak)|GLAST]] (Pakistan).

==See also==
*[[Z Pulsed Power Facility]]
*[[Pinch (plasma physics)]]

==References==
{{reflist}}

==External links==
* [http://www.sandia.gov/z-machine/ Z Machine] (Sandia Labs)
* [http://www.sandia.gov/pulsedpower/prog_cap/pub_papers/003132j.pdf An Inertial-Fusion Z-Pinch Power Plant Concept] (Sandia Labs)
* [http://www.fusion.ucla.edu/neil/Publications/DevelopmentPathForZ-pinch.pdf Development path for Z-pinch IFE] {{Webarchive|url=https://web.archive.org/web/20080410131448/http://www.fusion.ucla.edu/neil/Publications/DevelopmentPathForZ-pinch.pdf |date=2008-04-10 }}
* [http://www.aps.org/publications/apsnews/200203/oceans-eleven.cfm "Physics of 'Ocean's Eleven'"]
* [http://dorland.pp.ph.ic.ac.uk/magpie/ The MAGPIE project at Imperial College London] {{Webarchive|url=https://web.archive.org/web/20100406085526/http://dorland.pp.ph.ic.ac.uk/magpie/ |date=2010-04-06 }} is used to study wire array Z-pinch implosions.

{{Fusion power}}

[[Category:Magnetic confinement fusion]]
[[Category:English inventions]]