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http://pesn.com/2006/03/18/9600251_Lawrenceville_and_Sandia_fusion_compared/
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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 |
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| 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) |
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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 effectthe 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 pinchnearly 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.
Its 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.
# # #
Source:
Contact
Eric Lerner <elerner {at} igc.org >
New Jersey
973-736-0522
Aaron Blake <blake_aaron {at} hotmail.com >
Hanscom AFB, Massachusetts
781-862-3292
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See also
Editing by Mary-Sue
Haliburton
Page posted by Sterling
D. Allan March 17, 2006
Last updated March 17, 2011
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