Dynomak

Two colored model drawings, blue on the left, and red on the right, showing a series of more tangled chaotic disordered spirals in blue, changing to less tangled, more orderly spirals in red.
A model showing the formation of a spheromak from chaotic start. A dynomak is a spheromak formed by injecting magnetic flux.

Dynomak is a spheromak[1] fusion reactor concept developed by the University of Washington using U.S. Department of Energy funding.[2][3]

A dynomak is defined as a spheromak that is started and maintained by magnetic flux injection. A dynomak is formed when an alternating current is used to induce a magnetic flux into plasma. A transformer uses the same induction process to create a secondary current. Once formed, the plasma inside a dynomak relaxes into its lowest energy state, while conserving overall flux.[4][5] This is termed a Taylor state and inside the machine what is formed is a plasma structure called a spheromak. A dynomak is a kind of spheromak that is started and driven by externally induced magnetic fields.

Technical roots[edit]

Spheromak formation
Spheromak formation

Plasma is a fluid that conducts electricity, which gives it the unique property that it can be self-structured into smoke-ring-like objects such as field-reversed configurations and spheromaks. A structured plasma has the advantage that it is hotter, denser and more controllable which makes it a good choice for a fusion reactor.[6] But forming these plasma structures has been challenging since the first structures were observed in 1959 [7][8][9] because they are inherently unstable.

In 1974, Dr. John B Taylor proposed that a spheromak could be formed by inducing a magnetic flux into a loop plasma. The plasma would then relax naturally into a spheromak also termed a Taylor state.[10][5] This process worked if the plasma:

  • Conserved the total magnetic flux
  • Minimized the total energy

These claims were later checked by Marshall Rosenbluth in 1979.[11] In 1974, Dr. Taylor could only use results from the ZETA pinch device to back up these claims. But, since then, Taylor states have been formed in multiple machines including:

  • Compact Torus Experiment (CTX) at Los Alamos. The CTX operated from ~1979 to ~1987 at Los Alamos. It reached electron temperatures of 4.6 million kelvin [12] ran for 3 microseconds [13] and had a plasma to magnetic pressure ratio of 0.2.[14]
  • Sustained Spheromak Physics Experiment (SSPX) at Livermore was a more advanced version of the CTX that was used to measure the relaxation process that led to a Taylor state. The SSPX was working at Livermore from 1999 until 2007.[15]
  • Caltech Spheromak Experiment at Caltech was a small experiment run by Dr. Paul Bellans’ lab at Caltech from ~2000 to ~2010.
  • Helicity Injected Torus-Steady Inductive (HIT-SI) at the University of Washington was run by Dr. Jarboe from 2004 to 2012 and was the precursor to the dynomak. This machine created 90 kiloamps of stable plasma current over several (<2) microseconds.[16] This machine also showed the first demonstration of Imposed-Dynamo Current Drive (IDCD) in 2011.[17] The IDCD breakthrough enabled Dr. Jarboes’ group to envision the first reactor-scale version of this machine; called the dynomak.

The dynomak evolved from the Helicity Injected Torus-Steady Inductive (HIT-SI) (~2004 to ~2012) experiment that was operated by the University of Washington. The HIT-SI machine went through several upgrades, the HIT-SI3 (~2013 to ~2020) and HIT-SIU (Post ~2020) were both variations on the same machine.[18] These machines demonstrated that an inductive current could be used to make and sustain the Spheromak plasma structure.

Magnetic induction drive[edit]

A comparison of a dynomak and an AC transformer.
A comparison of a dynomak and an AC transformer shows how magnetic flux is needed to drive both machines.
A comparison of the HIT-SI (left) and HIT-SI3 (right) shows different kinds and forms of flux injectors.
A comparison of the HIT-SI (left) and HIT-SI3 (right) shows different kinds and forms of flux injectors.

By definition, a dynomak is a plasma structure that is started, formed, and sustained using magnetic flux injection. Electric transformers use a similar process; a magnetic flux is created on the primary loop, and this makes an alternating current on the secondary side. Because of Faraday's law of induction, only a changing magnetic field can induce a secondary current – this is why a direct current transformer cannot exist. In a dynomak, magnetic induction is used to create a plasma current inside a plasma filled chamber. This gets the plasma moving and the system eventually relaxes into a Taylor state or spheromak. The relaxation process involves the flow of magnetic helicity (a twist in the field lines) from the injectors into the center of the machine.[19]

Supporters of this heating approach have argued that induction is 2-3 orders of magnitude more efficient than radio frequency (RF) or neutral beam heating.[19][20] If this is true, it gives a dynomak several distinct advantages over other fusion approaches like tokamaks or magnetic mirrors. But this is an open area of research; below are some examples of how effective inductive drive is in creating plasma current inside a dynomak.

Effectiveness of a dynomak drive[21][20]
Induction power Drive frequency Plasma current Machine
3 megawatts 5.8 kHz 12 kiloamps HIT-SI (2006)
6 megawatts 14.7 kHz 38 kiloamps HIT-SI (2011)

A dynomak uses injectors, which are curved arms that are attached to the main chamber. An alternating current is applied around the curve of these arms, which creates the magnetic flux that drives a dynomak. The University of Washington experimented with two and three numbers of injectors. The phase of the alternating current is offset to allow continuous injection of flux into a dynomak. Injector count effects offset angle: The drive current, and thus injectors, are offset by 90 degrees with two injectors, and by 60 degrees with three injectors.

Advantages[edit]

Because the spheromak plasma structure forms naturally, supporters have argued that it has several inherent advantages, these are listed below.

  • Because the structure forms naturally, it may avoid the kink, interchange, and other plasma instabilities that normally plague structures. For this reason, it has been argued that a dynomak can pressurize and heat its plasma up to the Mercer limit on beta number.[22] If true, this could ultimately shrink a reactor when compared to other fusion approaches.
  • Supporters have argued that the inductive drive is 2-3 orders of magnitude more efficient then RF heating or neutral beam heating.[19][20] But this is an open area of research.
  • A dynomak has no central solenoid, in contrast to a tokamak, lowering cost and power needs for a reactor.
  • It has been argued that this machine does not need added heating hardware such as neutral beam injection.

As of 2014 plasma densities reached 5x1019 m−3, temperatures of 60 eV, and maximum operation time of 1.5 ms.[citation needed] No confinement time results were available. At those temperatures, fusion, alpha heating, or neutron production do not occur.

Commercialization[edit]

Once the technical principals were proven in the HIT-SI machine, Dr. Jarboe challenged his students in a University of Washington class to come up with a fusion reactor based on this approach.[2] The students designed the dynomak as a reactor-level power plant that built on discoveries made from the HIT-SI and earlier machines.

Eventually, these students formed CT Fusion as a spin off from the University of Washington, to commercialize the dynomak in 2015.[23] The company has exclusive rights to 3 University of Washington patents and raised over $3.6 million from 2015 to 2019 in public and private funding.[24] The acronym CT stands for Compact Toroid, which is what spheromaks were referred to for decades. The company has received funding as part of an ARPA-E funding award for fusion. CT Fusion shut down in 2023.[25]

Unlike other fusion reactor designs (such as ITER), a dynomak can be, according to its engineering team, comparable in costs to a conventional coal plant.[2] A dynomak is calculated to cost a tenth of ITER and produce five times more energy at an efficiency of 40 percent. A one gigawatt dynomak would cost US$2.7 billion compared to US$2.8 billion for a coal plant.[26]

Design[edit]

Dynomak incorporates an ITER-developed cryogenic pumping system. Spheromak use an oblate spheroid instead of a tokamak configuration, with no central core, or large, complex superconducting magnets as in many tokamaks, e.g., ITER. The magnetic fields are produced by putting electric fields into the center of the plasma using superconducting tapes wrapped around the vessel, such that the plasma contains itself.[26]

A dynomak is smaller simpler and cheaper to build than a tokamak, such as ITER, while producing more power. The fusion reaction is self-sustaining as excess heat is drawn off by a molten salt blanket to power a steam turbine.[26] The prototype was about one tenth the scale of a commercial project, and can sustain plasma efficiently. Higher output would require larger scale, and higher plasma temperature.[2]

Criticisms[edit]

A dynomak relies on a copper wall to conserve and direct the magnetic flux that is injected into the machine. This wall butts up against the plasma, creating the possibility of high conduction losses through the metal. The HIT-SI coated the inside of the copper wall with an aluminum-oxide insulator to reduce these losses, but this could still be a major loss mechanism if the machine goes to fusion reactor conditions.[27]

Further, the injection of magnetic helicity into the field forces the machine to break the magnetic flux surfaces that hold and sustain the plasma structure. The breaking of these surfaces has been cited as a reason that a dynomaks' heating mechanism does not work as efficiently as predicted.

Lastly, a dynomak has a complex chamber geometry, which complicates and presents challenges for maintenance and vacuum forming.

See also[edit]

References[edit]

  1. ^ D.A. Sutherland, T.R. Jarboe et al., "The dynomak: An advanced spheromak reactor concept with imposed-dynamo current drive and next-generation nuclear power technologies", Fusion Engineering and Design, Volume 89, Issue 4, April 2014, Pages 412–425
  2. ^ a b c d Michelle Ma, "UW fusion reactor concept could be cheaper than coal", University of Washington, October 8, 2014
  3. ^ Evan Ackerman, "Inside the Dynomak", IEEE Spectrum, November 26, 2014
  4. ^ Kaptanoglu, Alan A., et al. "Advanced modeling for the HIT-SI Experiment." arXiv preprint arXiv:2003.00557 (2020).
  5. ^ a b Taylor, J. Brian. "Relaxation of toroidal plasma and generation of reverse magnetic fields." Physical Review Letters 33.19 (1974): 1139.
  6. ^ Goldenbaum, G., J. Irby, Y. Chong, and G. Hart. "Formation of a Spheromak Plasma Configuration." Physical Review Letters 44.6 (1980): 393-96. Web.
  7. ^ Kolb, A.C.; Dobbie, C.B.; Griem, H.R. (1 July 1959). "Field mixing and associated neutron production in a plasma". Physical Review Letters. 3 (1): 5–7.
  8. ^ Tuszewski, M. "Field Reversed Configurations." Nuclear Fusion 28.11 (1988): 2033-092
  9. ^ “Evidence of a hot dense plasma in a theta pinch” Green, 1960
  10. ^ Bellan, Paul (2000). Spheromaks. Imperial College Press. ISBN 978-1-86094-141-2.
  11. ^ Rosenbluth, M. N.; M. N. Bussac. "MHD stability of spheromak." Nuclear Fusion 19.4 (1979): 489
  12. ^ Jarboe, T. R., Wysocki, F.J., Fernandez, J.C., Henins, I., Marklin, G.J., Physics of Fluids B 2 (1990) 1342-1346
  13. ^ "Physics through the 1990s", National Academies Press, 1986, p. 198.
  14. ^ WYSOCKI, F.J., Fernandez, J.C., Henins, I., Jarboe, T.R., Marklin, G.J., Physics Review Letters 21 (1988) 2457-2460
  15. ^ Wood, R. D., et al. "Particle control in the sustained spheromak physics experiment." Journal of nuclear materials 290 (2001): 513-517.
  16. ^ Sieck, P. E., et al. "First Plasma Results from the HIT-SI Spheromak." APS Division of Plasma Physics Meeting Abstracts. Vol. 45. 2003.
  17. ^ Sutherland, D. A., et al. "The dynomak: An advanced fusion reactor concept with imposed-dynamo current drive and next-generation nuclear power technologies."
  18. ^ Morgan, K. D., et al. "High-speed feedback control of an oscillating magnetic helicity injector using a graphics processing unit." Review of Scientific Instruments 92.5 (2021): 053530.
  19. ^ a b c Fisch, Nathaniel J. "Theory of current drive in plasmas." Reviews of Modern Physics 59.1 (1987): 175
  20. ^ a b c Jarboe, T. R., et al. "Recent results from the HIT-SI experiment." Nuclear Fusion 51.6 (2011): 063029
  21. ^ Jarboe, T. R., et al. "Spheromak formation by steady inductive helicity injection." Physical review letters 97.11 (2006): 115003
  22. ^ The Nuclear Fusion Shark Tank - June Call - PSS & CT Fusion. YouTube. Published online June 11, 2019. Accessed May 2, 2022
  23. ^ Dr. Matthew Moynihan. The Nuclear Fusion Shark Tank - June Call - PSS & CT Fusion. YouTube. Published online June 11, 2019. Accessed May 2, 2022.
  24. ^ CTFusion. Fusionenergybase.com. Published 2019. Accessed May 2, 2022. https://www.fusionenergybase.com/organization/ctfusion
  25. ^ Stiffler, Lisa (April 5, 2023). "Energy startup CTFusion folds as co-founders land at rival Zap".
  26. ^ a b c Szondy, David (October 12, 2014). "University of Washington fusion reactor promises "cheaper than coal" energy". newatlas.com. Retrieved October 13, 2016.
  27. ^ Sieck, P. E., et al. "First Plasma Results from the HIT-SI Spheromak." APS Division of Plasma Physics Meeting Abstracts. Vol. 45. 2003
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