Nuclear fusion breakthrough breathes life into the overlooked Z-pinch approach

Nuclear fusion holds untold potential as a source of power, but to recreate the colliding atomic nuclei taking place inside the Sun and generate inexhaustible amounts of clean energy scientists will need to achieve remarkable things. Tokamak reactors and fusion stellarators are a couple of the experimental devices used in pursuit of these lofty goals, but scientists at the University of Washington (UW) are taking a far less-frequented route known as a Z-pinch, with the early signs pointing to a cheaper and more efficient path forward.

In order to mimic the conditions inside the Sun, where hydrogen atoms smash together to form helium atoms and release gargantuan amounts of energy with no harmful by-products, we need a whole lot of heat and a whole lot of pressure.

Forming a stream of plasma and holding it in place long enough for these nuclear reactions to occur, either in a twisted loop or a neat donut shape, are the techniques employed by devices like Germany's wonky Wendelstein 7-X fusion reactor and China's Experimental Advanced Superconducting Tokamak. But this approach has its drawbacks, relating to the magnetic coils needed to suspend the ring of plasma, as study author Uri Shumlak explains to New Atlas.

"Magnetic field coils drive fusion devices to larger size and larger costs," he says. "The coils are also particularly sensitive to neutron damage, which requires more shielding, further driving size and costs."

A more efficient way to achieve these streams of plasma may be what is known as a Z-pinch confinement system. Rather than intricate webs of expensive magnetic coils, these systems pin the plasma in place with an electromagnetic field generated within the plasma itself. Z-pinch systems have been referred to as the dark horse of the nuclear fusion race, as the upside is a far simpler plasma configuration. The downside, however, is that instabilities cause distortions in the plasma that quickly cause it to hit the walls of the container vessel and collapse.

"Compressing and confining a plasma with magnetic fields in a Z-pinch configuration is prone to instabilities since the plasma can escape between the parallel magnetic field lines," Shumlak tells us. "The magnetic field forms circular loops around the plasma column which confines the plasma radially, but the plasma can form bulges, like an aneurysm, which locally weakens the magnetic field and allows the bulge to grow."

These problems have plagued the Z-pinch approach since its inception in the 1950s and effectively drove the development of tokamaks and stellarators, but the UW researchers say it still has something to give.

The reason for this, is that they believe they have figured out a way to stop the distortions that occur in the plasma and cause it to collapse. Through making slight adjustments to the behavior of the plasma by inducing what is known in fluid dynamics as sheared axial flow, the researchers were able to break new ground in their 50-cm-long (20-in) Z-pinch plasma column.

"The primary innovation is using plasma flows, specifically sheared axial flows," Shumlak tells us. "The sheared flow stabilizes the plasma by constantly smoothing the plasma surface and preventing the bulges from developing."

While the potential of sheared axial flow in Z-pinch plasma streams has been explored for years, the researchers write that this is the first time that they have produced "evidence of fusion neutron generation from a sheared-flow stabilized Z-pinch." More specifically, their flowing plasma was held in place 5,000 times longer than a static plasma, and they were able to observe energetic neutrons that are the telltale signs of nuclear fusion.

While enthused by the breakthrough, given the unstable history of Z-pinch confinement systems and uncertainty of nuclear fusion research as a whole, they are taking a cautiously optimistic outlook.

"As a scientist, I would state that we do not know for certain if this advance will lead to a new dawn," says Shumlak. "However, the results are particularly encouraging. Sheared flow stabilization of the Z-pinch approach has been thoroughly investigated, and the predictions of scaling to current performance have been demonstrated. The evidence is described in our Physical Review Letters article. If our current scientific understanding continues to hold, then we should be able to reach even higher performance."

The research was published in the journal Physical Review Letters.

Source: University of Washington