The versatile Z machine helps safeguard America’s nuclear arsenal while giving scientists new insights into fusion technology.

Many Albuquerque, New Mexico residents do not know it, but around 200 times a year something extraordinary happens on the outskirts of their town. On some days, the conditions of the sun are fleetingly recreated. On others, the physics at the heart of a nuclear explosion are brought momentarily to life.

These little miracles of physics take place inside Sandia National Laboratories’ (Sandia) Z machine, one of the world’s most powerful and sophisticated X-ray generators. Since operations began in 1996, the Z machine has helped safeguard America’s nuclear arsenal, given scientists an insight into the conditions deep within stars and boosted understanding of fusion technology, which one day may prove to be an environmentally friendly power source. Not bad for a device that operates off a regular domestic power supply.

The Z machine’s secret is simple: suck in electricity slowly, and then release it in a brief burst. The outer border of the machine is a circular ring of 36 blocks of capacitors that can be charged in just under two minutes. Then, at the flick of a laser-triggered switch, each pours its electrical energy through metal cylinders and into the heart of the machine. The 36 current pulses are synchronized to within 20 nanoseconds, and when combined deliver 26 million amperes to the center of the machine.

In some experiments, the Sandia team channels that current through a cylindrical array of 300 tungsten wires, each thinner than a human hair. The very large current first vaporizes the wires. Then the magnetic field created by the current pushes the tungsten particles inward, creating a brew that is so hot and dense that electrons are ripped from the atoms. This particle soup, known as plasma, collapses inward at velocities greater than 0.1 percent of the speed of light.

Finally the particles collide, creating temperatures that exceed those of the sun and an X-ray pulse lasting just a few billionths of a second. The power released is immense: over 200 trillion watts, or 80 times the world’s output of electricity. “It’s the very high currents and powers that make the Z machine unique,” said Sandia scientist John Porter. That kind of power can be very useful.

In 2003, Sandia researchers placed a small pellet containing deuterium, an isotope of hydrogen, close to the tungsten wires. When the X-ray pulse heated the pellet it pushed the deuterium atoms together until they fused into atoms of helium. The event marked the first time this method, known as pulsed power, was used to spark a fusion reaction. “It was a big breakthrough for us,” Porter exclaimed.

The technique has enormous potential. Deuterium is plentiful and cheap, and releases energy when it fuses. The hope is one day the reaction could be used to run a zero-carbon power station. But it’s easier to start a fusion reaction than to keep one going. Also, the Z machine lacks the needed punch to power a self-sustaining fusion reaction that produces more fusion energy output than electrical energy input. While further work at Sandia has helped its researchers ignite reactions that produce more energy, net energy gain would require a machine based on the same principles but around four times as powerful, said Porter.

Even so, Sandia researchers have already proven the Z machine’s worth by demonstrating that pulsed power is a candidate for powering a future fusion plant. “They’ve been able to produce proof of principle,” said Per Peterson, a nuclear engineer at the University of California-Berkeley.

When the Z machine is being used for nuclear weapons work, the tungsten wires are replaced with copper ones. These produce a higher-frequency X-ray pulse that mimics that given off in a nuclear explosion. That allows weapons researchers to investigate the possibility that, during military action, one of these pulses could interfere with the electronics in nearby unused nuclear devices. At Sandia, researchers monitor the effect the pulses have on materials found in electronic circuits. The results can be used to improve simulations of the effect of X-rays on circuits in real weapons.

The behavior of the bombs themselves also can be probed using the Z machine. To do so, researchers swap the wires for two sheets of copper and reverse the direction of current through one of them, so that the plates are pushed apart. When the current flows, the pressure experienced by the plates is terrifying: 8 million times greater than the atmospheric pressure at sea level and twice that at the earth’s center.

Since pressures like that also are present at the heart of a nuclear explosion, the Z machine gives weapons scientists a rare chance to examine their theories of how materials respond to these extreme pressures. To avoid detonating weapons for test purposes, scientists run computer simulations of the conditions within the nuclear components of a weapon. As with the simulation of the electrical circuits, these tests need real data to ensure accuracy. So Sandia researchers place samples of plutonium on the copper plates and observe how the material reacts to the pressure. This data gives weapons researchers a better idea of what could happen on the battlefield with respect to the safety, security and reliability of a warhead’s plutonium core.