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You are here: > News > March 18, 2006

Top 100
Sandia Z-Pinch and Texas A&M Focus Fusion Compared

Billion-degree fusion reported by Eric Lerner, et al. at Texas A&M University in 2001 has been duplicated recently by a variation set-up at Sandia National Laboratories.  The two processes are compared.

click to enlarge

Sandia's Z machine firing
The “arcs and sparks” formed at the water-air interface travel between metal conductors.
(Photo by Randy Montoya)
Click here for story
Photo of hot plasma vortex filaments
Hot plasma vortex filaments pinched together by their own magnetic fields in a plasma focus fusion device.

Photo taken by Winston Bostick & Victorio Nardi using an exposure time of a few nanoseconds. (Ref)
Hydrogen-boron fuel requires high ion energies or temperatures for fusion reactions. Significant burn starts only from one billion degrees Celsius or 100keV.  The highest rate of burn is at 600keV. While Eric Lerner reported achieving ion energies above up to 210 keV in a plasma focus device at Texas A&M university in 2001, these results have remained controversial, with some critics thinking that they were too good to be true.

Now a second experiment, this time at Sandia National Laboratories, has achieved the same and even somewhat higher ion energies, up to 3 billion degrees. The Sandia results were reported in Physical Review Letters, Feb 24 (96, 075003). In the Sandia experiment, as with the plasma focus, the plasma was confined by the pinch effect—the tendency of large currents in plasma to create magnetic fields that compress or pinch them together.

However, there were significant differences from the Texas plasma focus experiments. First, a much larger machine was used. The Z-machine has a peak current and total energy nearly 100 times larger than the Texas dense plasma focus (DPF)—20 mega-amps and 11 mega-joules vs. 1.4 MA and 0.12 MJ. Also, the pinch was formed differently. In the Z-machine, a cylindrical array of fine steel wires is vaporized by the huge pulse of current. Compressed by the powerful magnetic field of a 20-MA current coursing through it, the resulting plasma then contracts toward the axis. In contrast, the electrodes in the DPF are not destroyed, and the pinch gives rise to a tiny plasmoid, only microns across, with high density and very high magnetic fields.

At Sandia, no plasmoids or hot spots were looked for in this experiment, but such hotspots have routinely been observed in earlier z-machine experiments. The plasma, which was 2 cm long, contracted to a minimum radius of 750 microns (0.75 mm). The peak density was 2 x10^20 ions/cm^3 and was maintained for about 5 ns (billionths of a second) while the peak ion energy reached 320 keV. The experimenters measured the ion temperature by detecting the width of certain lines in the optical spectrum. These lines were broadened by the Doppler shifts of the ions traveling at high speed. The higher the speed, and thus the broader the lines, the higher the ion energy. The maximum magnetic field reached was around 50 MG (million gauss).

By comparison, in the best shot, the Texas DPF experiments achieved a much higher field (400 MG), a higher ion density (3x 10^21/cm^3), and longer confinement time (55ns), but not as high an ion energy (55keV). (Other shots had less density but higher ion energies, up to 210 keV.) The temperature-density confinement time product, often considered a “figure of merit” for fusion devices, was 9x10^15 sec-keV/cm^3 for the Texas DPF and 3.2x10^14 sec-keV/cm^3 for the Sandia experiment. In addition, the confinement at Texas was fairly stable, with an average ion making thousands of orbits during the lifetime of the plasmoid, while the much larger Sandia pinch lasted only about a single orbit. However, the Sandia experiment had a far greater portion of the total input energy in the pinch—nearly 25%, as compared with only 0.0017% for Texas.

The iron plasma was able to radiate nearly all the energy in the pinch rapidly as X-rays, since X-ray production increases as the square of the atomic charge. As a result of the fast ion heating and electron cooling, the electrons were much cooler than the ions, reaching temperatures of only around 3.6keV.

“It’s possible that part of the difference in ion and electron energies is due to the magnetic field effect,” comments Focus Fusion Society Executive Director Eric Lerner. Lerner has pointed out the importance of the effect, which slows the transfer of energy from ions to electrons in a high magnetic field. While the fields achieved in the Z-machine are low compared with the fields achieved in the plasma focus, the value of the critical magnetic field for the effect decreases as the atomic mass of the ions increases. For iron ions with an energy of 150 keV, the critical field is 250 MG, and even for a field of 50 MG the magnetic field effect will slow ion heating of electrons by a facto of six.

The high ion energies surprised the Sandi researchers. M.G. Haines of the Imperial College, London, and colleagues at Sandia, interpreted the high ion energies as resulting from microturbulent heating in the plasma. Whether or not this is true, some process seems to be very efficiently converting nearly all the magnetic field energy to thermal energy. An alternative idea is that ion beams generated – as in the plasma focus, by plasmoids or hotspots – are what is causing heating of the plasma ions.

The Sandia machine could potentially be used to burn pB11 fuel, if a pellet containing the fuel were placed at the center of the array. However, there are serious obstacles to a Z-Pinch being used as a practical fusion reactor. For one thing, the Z-Pinch destroys the electrodes with each shot, precluding rapid pulsed operation.

The Sandia result does confirm two important conclusions of focus fusion research: that high ion temperature can be obtained from a pinch machine, and that huge differences between ion and electron temperatures are possible.

# # #


  • LPP Press Release


Eric Lerner <elerner {at} >
New Jersey

Aaron Blake <blake_aaron {at} >
Hanscom AFB, Massachusetts

Related Coverage by PESN

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    Fission Tragedy Could be Averted - The communications industry pumps 25% of their revenue back into research. The result? Slim iPhones and androids and many amazing communications wonders. Meanwhile, we starve energy research for funds with just 0.3% of the energy budget going to R&D, and wring our hands over oil spills and meltdowns. (PESN; March 15, 2011)
  • Focus Fusion poses competition to Tokamak - Purports to be a far more feasible and profoundly less expensive approach to hot fusion, in contrast to what the international project (ITER) in France is pursuing. Lawrenceville Plasma Physics is currently researching and developing the Plasma Focus Device for hydrogen-boron nuclear fusion. (PESN; Nov. 2, 2005)

See also

Editing by Mary-Sue Haliburton
Page posted by Sterling D. Allan March 17, 2006
Last updated December 24, 2014




"It is harder to crack a prejudice than an atom." // "I'd rather be an optimist and a fool than a pessimist and right." -- Albert Einstein

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