Z machine

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Operation overview

The Z machine fires a very powerful electrical discharge (several tens million-ampere for less than 100 nanoseconds) into an array of thin, parallel tungsten wires called a "liner". The high electrical current vaporizes the wires, transformed into a cylindrical plasma curtain. Simultaneously, the current density induces a powerful magnetic field and their combination creates Lorentz forces which radially compress the plasma into a z-pinch process. The imploding plasma produces a high temperature and an X-ray pulse which can create a shock wave in a target structure. The target structure is placed in a cavity inside the wires called a hohlraum. The powerful fluctuation in the magnetic field (an "electromagnetic pulse") also generates electric current in all of the metallic objects in the room (see picture at upper right). The vertical cylinder's axis is conventionally termed the "z"-axis, hence the name "Z machine".

Originally designed to supply 50 terawatts of power in one fast pulse, technological advances resulted in an increased output of 290 terawatts, enough to study nuclear fusion. Z releases 80 times the world's electrical power output for about seventy nanoseconds; however, only a moderate amount of energy is consumed in each test (roughly twelve megajoules) - the efficiency from wall current to X-ray output is about 15% [http://www.sandia.gov/media/z290.htm] [Young and Freedman. University Physics. 12th Edition. ] . Marx generators are slowly charged with energy prior to firing.

Sandia announced the fusing of deuterium in the Z machine on April 7, 2003. [ [http://www.sandia.gov/news-center/news-releases/2003/nuclear-power/Zneutrons.html Z produces fusion neutrons, Sandia scientists confirm] , Sandia's press release (April 7, 2003).] This application could result in an efficient method to ignite a nuclear fusion reaction starting from a small capsule of deuterium. Unfortunately many technical difficulties — for instance the small quantity of deuterium that can be contained in the hohlraum and the practical impossibility to transfer the compressed capsule to a larger nuclear fuel reservoir — prevent the machine to be used this way, for the moment.

Besides being used as an X-ray generator, the Z machine propelled small plates at 34 kilometres a second, faster than the 30 kilometres per second that Earth travels in its orbit around the Sun, and three times Earth's escape velocity. [ [http://www.sandia.gov/news-center/news-releases/2005/nuclear-power/z-saturn.html Z fires objects faster than Earth moves through space] , Sandia's press release (June 6, 2005).] It also successfully created a special, hyperdense "hot ice" known as ice VII, by quickly compressing water to pressures of 70,000 to 120,000 atmospheres. [ [http://www.sandia.gov/news/resources/releases/2007/z-ice.html Ice created in nanoseconds by Sandia’s Z machine] , Sandia's press release (March 15, 2007).]

At the end of 2005, the Z machine produced plasmas with announced temperatures in excess of 2 billion kelvin (2 GK, 2×10⁹ K) or 3.6 billion °F, even reaching a peak at 3.7 billion K or 6.6 billion °F. [ [http://www.sandia.gov/news-center/news-releases/2006/physics-astron/hottest-z-output.html Sandia’s Z machine exceeds two billion degrees Kelvin] , Sandia's press release (March 8, 2006).] It was achieved in part by replacing the tungsten wires by thicker steel wires. This temperature, which enables a 10% to 15% efficiency in converting electrical energy to soft x-rays, was much higher than anticipated (3 to 4 times the kinetic energy of the incoming wires on axis). Thus far, it is currently the highest man-made temperature ever achieved according to The Guinness Book Of Records. The origin of this extra energy still remains unexplained, but it has been theorized that small-scale MHD turbulence and viscous damping would convert magnetic energy into thermal energy of the ions, which then would transfer their energy to the electrons through collisions. [http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PRLTAO000096000007075003000001 Abstract of Malcolm Haines' paper] published in Physical Review Letters 7, Vol.96 (February 24, 2006).] [http://www.mhdprospects.com/pdf/comments_on_haines_paper.pdf Analysis of Malcolm Haines' paper] by Jean-Pierre Petit (June 25, 2006).]

Prospects

The ultra high temperatures reached in 2005 (2.66 to 3.7 billion Kelvin) are greatly higher than those required for the classical hydrogen, deuterium and tritium fusion envisaged hitherto. They could allow in theory, if not in practice, the fusion of light hydrogen atoms with heavier atoms such as lithium or boron. These two possible fusion reactions do not produce neutrons thus no radioactivity nor nuclear waste, so they open for the first time the possibility of a man-made clean aneutronic fusion. But it could also represent a potential nuclear proliferation problem, because pure fusion weapons could be made if the little fission A-bomb used classically for ignition of H-bombs were replaced by a Z pinch detonator, fed with a compact pulsed power generator fast enough (a disc generator for example). [ [http://www.princeton.edu/~globsec/publications/pdf/7_2Jones.pdf The Question of Pure Fusion Explosions Under the CTB] , Science & Global Security, Vol.7, Appendix D (1998).]

A $60 million (raised to $90 million) retrofit program called ZR (Z Refurbished) was announced in 2004 to increase the power by 50%. The Z machine was dismantled in July 2006 for this upgrade, including the installation of newly designed hardware and components and more powerful Marx generators. The de-ionized water section of the machine has been reduced to about half its old size while the oil section has been expanded significantly in order to house larger intermediate storage lines (i-stores) and new laser towers, which used to sit in the water section. The refurbishment was completed in October 2007. [ [http://www.sandia.gov/news/resources/releases/2007/zaccelerator.html Successful 'shots' signal re-opening of Sandia's giant Z accelerator] , Sandia's press release (17 October, 2007).] The newer Z machine can now shoot 27 million amperes instead of 18 million amperes previously, still in 95 nanoseconds.The radiated power has been raised to 350 terawatts and the X-ray energy output to 2.7 megajoules. However the maximum temperature the new version could reach with the same record holder stainless steel wire-array liner used in 2005 is not known yet.

Sandia's roadmap includes another future Z machine version called ZN (Z Neutron) to test higher yields in fusion power and automation systems. ZN is planned to give between 20 and 30 MJ of hydrogen fusion power with a shot per hour thanks to Russian Linear Transformer Driver (LTD) replacing the current Marx generators. [ [http://www.sandia.gov/LabNews/070427.html Rapid-fire pulse brings Sandia’s Z method closer to goal of developing high-yield fusion reactor] , Sandia's press release (April 27 , 2007).] After 8 to 10 years of operation, ZN would become a transmutation pilot plant capable of a fusion shot every 100 seconds. [ [http://www.prod.sandia.gov/cgi-bin/techlib/access-control.pl/2006/067148.pdf Z-Inertial Fusion Energy: Power Plant Final Report FY 2006] , Sandia Report SAND2006-7148 (October 2006).]

The next step planned would be the Z-IFE (Z-inertial fusion energy) test facility, the first true z-pinch driven prototype fusion power plant. It is suggested it would integrate Sandia's latest designs using LTDs. Sandia labs recently proposed a conceptual 1 petawatt (10¹⁵ watts) LTD Z-pinch power plant, where the electric discharge would reach 70 million amperes. [W.A. Stygar "et al.", [http://www.sandia.gov/pulsedpower/prog_cap/pub_papers/accelerator_architecture_PRSTAB_2007.pdf Architecture of petawatt-class z-pinch accelerators] (October 2007).]

One should however note all these announced fusion systems make use of the X-ray radiation to heat a hohlraum in order to indirectly ignite fusion reaction of isotopes of hydrogen. The unexpected ultra high temperature achieved recently is not taken into account yet for any possible use of direct aneutronic fusion reactions (p–⁷Li or p–¹¹B) which, if proven practicable, would imply reconsidering some proposed designs.

Z-Pinch Inertial Fusion Energy program

The Sandia Laboratories Z-IFE project [An introduction to the Z-IFE project may be found [http://fire.pppl.gov/fpa05_olson.pdf here] .] aims to solve the practical difficulties in harnessing fusion power. Major problems include producing energy in a single Z-pinch shot, and quickly reloading the reactor after each shot. By their early estimates, an implosion of a fuel capsule every 10 seconds could economically produce 300 MW of fusion energy.

Notes and references

ee also

* Sandia National Laboratories
* Fusion power
* Stockpile stewardship
* Pulsed power
* Inertial fusion power plant

External links

* [http://zpinch.sandia.gov/ Z Machine Home Page]
* [http://observer.guardian.co.uk/life/story/0,6903,416412,00.html "A machine called Z"]
* [http://www.aip.org/enews/physnews/2006/split/767-3.html "Physics News Update"] , February 28, 2006
* [http://www.mystrangenewmexico.com/the-columns/2007/2/22/black-holes-at-home.html "The Z Machine and the Possibility of Miniature Black Holes"]