The Z machine is the largest X-ray generator in the world and is designed to test materials in conditions of extreme temperature and pressure. Operated by Sandia National Laboratories, it gathers data to aid in computer modeling ofnuclear weapons. The Z machine is located at Sandia’s main site in Albuquerque, New Mexico.
The Z machine fires a very powerful electrical discharge (20 million amperes over less than 100 nanoseconds) into an array of thin, parallel tungsten wires called a liner (pictured here). The high electrical current vaporizes the wires, which are transformed into a cylindrical plasma curtain. Simultaneously, the current density induces a powerful magnetic field, and their combination creates Lorentz forces that radially compress the plasma into a z-pinch process. The imploding plasma produces a high temperature and an X-ray pulse that 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 axis of current flow 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%. Marx generators are slowly charged with energy prior to firing.
Sandia announced the fusing of small amounts of deuterium in the Z machine on April 7, 2003.
Proposed model of a 1 petawatt LTD-based z-pinch accelerator.
104 m diameter, 70 megaamperes, 24 megavolts.
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. 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 (7 to 12 GPa).
At the beginning of 2006, the Z machine produced plasmas with announced temperatures in excess of 2 billion kelvins (2 GK, 2×109 K) or 3.6 billion °F, even reaching a peak at 3.7 GK or 6.6 billion °F. It was achieved in part by replacing the tungsten wires with 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). The Guinness Book Of Records listed it as the highest human-achieved temperature (the Relativistic Heavy Ion Collider at Brookhaven National Laboratory has produced higher temperatures, but only within nuclear matter). 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.
A $60 million (raised to $90 million) retrofit program called ZR (Z Refurbished) was announced in 2004 to increase its 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 the previous 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. The newer Z machine can now shoot 27 million amperes (instead of 18 million amperes previously) 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 may reach with the same record holder stainless steel wire-array liner used in 2005 is not yet known.
Sandia’s roadmap includes another 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 using a Russian Linear Transformer Driver (LTD) replacing the current Marx generators. After 8 to 10 years of operation, ZN would become a transmutation pilot plant capable of a fusion shot every 100 seconds.
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. As of 2012 Fusion shot simulations at 60 to 70 million amperes are showing a 100 to 1000 fold return on input energy. Tests at the Z machine’s current design maximum of 26-27 million amperes are set to begin in 2013 were proof of concept simulation verification, and Fusion break even, will be pursued.
The ultra-high temperatures reached in 2006 (2.66 to 3.7 billion kelvins) are much higher than those required for the classical hydrogen, deuterium and tritium fusion previously considered. 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, and thus no radioactivity or nuclear waste, so they open for the first time the possibility of human-made clean aneutronic fusion. Edited from Z Machine.
