Z-Pinch

• Hottest Temperature on Earth
• The Focus Fusion Perspective
• More Notes on the MSN Article

Hottest Temperature on Earth

Headlines March 8, 2006 read: “Record set for hottest temperature on Earth: Scientists produce gas more than 100 times hotter than the sun.”

The article says:

Scientists have produced superheated gas exceeding temperatures of 2 billion degrees Kelvin, or 3.6 billion degrees Fahrenheit. This is hotter than the interior of our sun, which is about 15 million degrees Kelvin, and also hotter than any previous temperature ever achieved on Earth, they say.

The only thing is, they’re not sure how they did it.

 

One thing that puzzles scientists is that the high temperature was achieved after the plasma’s ions should have been losing energy and cooling. Also, when the high temperature was achieved, the Z machine was releasing more energy than was originally put in, something that usually occurs only in nuclear reactions.

The Focus Fusion Perspective:

Hydrogen-boron fuel requires high ion energies or temperatures for fusion reactions. Significant burn only starts at one billion degrees or 100keV and 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, 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 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. The resulting plasma, now with a 20 MA current coursing through it, then contracts toward the axis, compressed by the current’s powerful magnetic field. In the DPF in contrast, the electrodes 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, the Texas DPF experiments achieved in the best shot 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 form 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 factor 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 plasm focus, by plasmoids or hotspots are heating 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, so rapid pulsed operation is precluded.

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.

Notes on the MSN Article above:

Bob Steinke notes: The statement in the article that, “the Z machine was releasing more energy than was originally put in” is confusing. It took some digging, but I found out what they really meant.

When they pulse the Z machine it starts with a certain amount of energy from a capacitor bank (just like in focus fusion) and this energy gets divided up between the magnetic field energy and kinetic energy of the particles in the plasma. The researchers doing the experiment assumed that only the kinetic energy gets turned into heat, and the magnetic energy just goes away without heating the plasma. It turns out their assumption was wrong. As the magnetic field tries to decay it causes electric currents in the plasma and some of the magnetic field energy gets converted to heat (just like in focus fusion.) Even though the Z-pinch machine is different than focus fusion it’s good to see independent confirmation that this magnetic field to heat conversion can make the kind of temperatures we want.

The statement that “the Z machine was releasing more energy than was originally put in” really meant “more energy than was in the kinetic energy of the particles” If you assume heat energy comes only from kinetic energy this is a mystery. When you realize that heat energy can come from magnetic field energy the mystery goes away. The machine did NOT release more energy than was originally in the capacitor bank.