Z-pinch

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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. Z-pinch H-gamma.jpg
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.
A desktop-sized inductively coupled current-driven toroidal Z-pinch in a krypton plasma showing an intense glow from a plasma filament. TWK pinch.jpg
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 confinement system that uses an electric current in the plasma to generate a magnetic field that compresses it (see pinch). These systems were originally referred to simply as pinch or Bennett pinch (after Willard Harrison Bennett), but the introduction of the θ-pinch (theta pinch) concept led to the need for clearer, more precise terminology.

Contents

The name refers to the direction of the current in the devices, the Z-axis on a 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 weapons 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, 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 capacitors 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

An early photograph of the kink instability in a toroidal pinch - the 3 by 25 pyrex tube at Aldermaston. Kink instability at Aldermaston.jpg
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 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 neutrons 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 served for many years as experimental devices.

Fusion-based propulsion

A concept of Z-pinch fusion propulsion system was developed through collaboration between NASA and private companies. [1] 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 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. [2]

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. [3] [4]

In 2018, a sheared-flow stabilized Z-pinch demonstrated neutron generation. It was built by a fusion company, Zap Energy, Inc., [5] a spin-out from the University of Washington, [6] and funded by strategic and financial investors and grants from the Advanced Research Projects Agency – Energy (ARPA–E). [7] [8] Flow stabilized plasma remained stable 5,000 times longer than a static plasma. [9] 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)×105 neutrons/pulse. Plasma temperatures of 1–2 keV (12–24 million °C) and densities of approximately 1017 cm−3 with 0.3 cm pinch radii were measured. [10]

Experiments

A Z-pinch machine at UAM, Mexico City. Pinchseta.JPG
A Z-pinch machine at UAM, Mexico City.

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), NSTRI (Iran).

See also

Related Research Articles

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<span class="mw-page-title-main">Plasma stability</span> Degree to which disturbing a plasma system at equilibrium will destabilize it

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<span class="mw-page-title-main">Fusion power</span> Electricity generation through nuclear fusion

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<span class="mw-page-title-main">Reversed field pinch</span> Magnetic field plasma confinement device

A reversed-field pinch (RFP) is a device used to produce and contain near-thermonuclear plasmas. It is a toroidal pinch which uses a unique magnetic field configuration as a scheme to magnetically confine a plasma, primarily to study magnetic confinement fusion. Its magnetic geometry is somewhat different from that of the more common tokamak. As one moves out radially, the portion of the magnetic field pointing toroidally reverses its direction, giving rise to the term reversed field. This configuration can be sustained with comparatively lower fields than that of a tokamak of similar power density. One of the disadvantages of this configuration is that it tends to be more susceptible to non-linear effects and turbulence. This makes it a useful system for studying non-ideal (resistive) magnetohydrodynamics. RFPs are also used in studying astrophysical plasmas, which share many common features.

<span class="mw-page-title-main">Magnetic confinement fusion</span> Approach to controlled thermonuclear fusion using magnetic fields

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<span class="mw-page-title-main">Field-reversed configuration</span> Magnetic confinement fusion reactor

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<span class="mw-page-title-main">Spheromak</span>

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<span class="mw-page-title-main">ZETA (fusion reactor)</span> Experimental fusion reactor in the United Kingdom

ZETA, short for Zero Energy Thermonuclear Assembly, was a major experiment in the early history of fusion power research. Based on the pinch plasma confinement technique, and built at the Atomic Energy Research Establishment in the United Kingdom, ZETA was larger and more powerful than any fusion machine in the world at that time. Its goal was to produce large numbers of fusion reactions, although it was not large enough to produce net energy.

<span class="mw-page-title-main">Pinch (plasma physics)</span> Compression of an electrically conducting filament by magnetic forces

A pinch is the compression of an electrically conducting filament by magnetic forces, or a device that does such. The conductor is usually a plasma, but could also be a solid or liquid metal. Pinches were the first type of device used for experiments in controlled nuclear fusion power.

Sceptre was a series of early fusion power devices based on the Z-pinch concept of plasma confinement, built in the UK starting in 1956. They were the ultimate versions of a series of devices tracing their history to the original pinch machines, built at Imperial College London by Cousins and Ware in 1947. When the UK's fusion work was classified in 1950, Ware's team was moved to the Associated Electrical Industries (AEI) labs at Aldermaston. The team worked on the problems associated with using metal tubes with high voltages, in support of the efforts at Harwell. When Harwell's ZETA machine apparently produced fusion, AEI quickly built a smaller machine, Sceptre, to test their results. Sceptre also produced neutrons, apparently confirming the ZETA experiment. It was later found that the neutrons were spurious, and UK work on Z-pinch ended in the early 1960s.

Derek Charles Robinson FRS was a physicist who worked in the UK fusion power program for most of his professional career. Studying turbulence in the UK's ZETA reactor, he helped develop the reversed field pinch concept, an area of study to this day. He is best known for his role in taking a critical measurement on the T-3 device in the USSR in 1969 that established the tokamak as the primary magnetic fusion energy device to this day. He was also instrumental in the development of the spherical tokamak design though the construction of the START device, and its follow-on, MAST. Robinson was in charge of portions of the UK Atomic Energy Authority's fusion program from 1979 until he took over the entire program in 1996 before his death in 2002.

<span class="mw-page-title-main">Spherical tokamak</span> Fusion power device

A spherical tokamak is a type of fusion power device based on the tokamak principle. It is notable for its very narrow profile, or aspect ratio. A traditional tokamak has a toroidal confinement area that gives it an overall shape similar to a donut, complete with a large hole in the middle. The spherical tokamak reduces the size of the hole as much as possible, resulting in a plasma shape that is almost spherical, often compared to a cored apple. The spherical tokamak is sometimes referred to as a spherical torus and often shortened to ST.

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The Princeton Large Torus, was an early tokamak built at the Princeton Plasma Physics Laboratory (PPPL). It was one of the first large scale tokamak machines, and among the most powerful in terms of current and magnetic fields. Originally built to demonstrate that larger devices would have better confinement times, it was later modified to perform heating of the plasma fuel, a requirement of any practical fusion power device.

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<span class="mw-page-title-main">Theta pinch</span> Fusion power reactor design

Theta-pinch, or θ-pinch, is a type of fusion power reactor design. The name refers to the configuration of currents used to confine the plasma fuel in the reactor, arranged to run around a cylinder in the direction normally denoted as theta in polar coordinate diagrams. The name was chosen to differentiate it from machines based on the pinch effect that arranged their currents running down the centre of the cylinder; these became known as z-pinch machines, referring to the Z-axis in cartesian coordinates.

Zap Energy is an American company that aims to commercialize fusion power through use of a sheared-flow-stabilized Z-pinch. The company is based near Seattle with research facilities in Everett and Mukilteo, Washington. The company aims to scale their technology to maintain plasma stability at increasingly higher energy levels, with the goal of achieving scientific breakeven and eventual commercial profitability.

References

  1. ↑ Adams, R. "Conceptual Design of a Z-Pinch Fusion Propulsion System" (PDF). Archived from the original (PDF) on 2014-06-30. Retrieved 2014-05-20.
  2. ↑ Miernik, J.; Statham, G.; Fabisinski, L.; Maples, C. D.; Adams, R.; Polsgrove, T.; Fincher, S.; Cassibry, J.; Cortez, R.; Turner, M.; Percy, T. (2013). "Z-Pinch fusion-based nuclear propulsion". Acta Astronautica. 82 (2): 173–82. Bibcode:2013AcAau..82..173M. doi:10.1016/j.actaastro.2012.02.012.(subscription required)
  3. ↑ Forbes, Eleanor G.; Shumlak, Uri; McLean, Harry S.; Nelson, Brian A.; Claveau, Elliot L.; Golingo, Raymond P.; Higginson, Drew P.; Mitrani, James M.; Stepanov, Anton D.; Tummel, Kurt K.; Weber, Tobin R. (11 June 2018). "Progress Toward a Compact Fusion Reactor Using the Sheared-Flow-Stabilized Z-Pinch". Fusion Science and Technology. Informa UK Limited. 75 (7): 599–607. doi:10.1080/15361055.2019.1622971. OSTI   1632373. S2CID   198442070.
  4. ↑ Shumlak, U. (27 May 2020). "Z-pinch fusion". Journal of Applied Physics. AIP Publishing. 127 (20): 200901. Bibcode:2020JAP...127t0901S. doi: 10.1063/5.0004228 .
  5. ↑ "Zap Energy". Zap Energy.
  6. ↑ Nelson, B.A.; Conway, B.; Shumlak, U.; Weber, T.R.; Claveau, E.L.; Draper, Z.T.; Forbes, E.G.; Stepanov, A.D.; Zhang, Y.; McLean, H.S.; Higginson, D.P.; Mitrani, J.M.; Tummel, K. (2019). Development of a Compact Fusion Device based on the Flow Z-Pinch (PDF). ARPA–E (Report). Advanced Research Projects Agency. Retrieved 2 April 2021.
  7. ↑ Nelson, Brian (15 November 2018). "Zap Energy: Electrode Technology Development for the Sheared-Flow Z-Pinch Fusion Reactor". ARPA–E. Advanced Research Projects Agency. Retrieved 2 April 2021.
  8. ↑ Nelson, Brian (7 April 2020). "Zap Energy: Sheared Flow Stabilized Z-Pinch Performance Improvement". ARPA–E. Advanced Research Projects Agency. Retrieved 2 April 2021.
  9. ↑ Lavars, Nick (April 12, 2019). "Nuclear fusion breakthrough breathes life into the overlooked Z-pinch approach". New Atlas. Retrieved 2019-04-14.
  10. ↑ Zhang, Y.; Shumlak, U.; Nelson, B. A.; Golingo, R. P.; Weber, T. R.; Stepanov, A. D.; Claveau, E. L.; Forbes, E. G.; Draper, Z. T. (2019-04-04). "Sustained Neutron Production from a Sheared-Flow Stabilized Z Pinch". Physical Review Letters. 122 (13): 135001. arXiv: 1806.05894 . Bibcode:2019PhRvL.122m5001Z. doi:10.1103/PhysRevLett.122.135001. ISSN   0031-9007. PMID   31012637. S2CID   51680710.
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