May 30, 2023

Hot enough, dense enough, for long enough: Fusion's triple product

A graphic showing a bar chart of triple product measurements (density, temperature and time) sits to the left of a conceptual Zap Energy fusion device.

A trio of measurements helps compare the performance of fusion energy devices.

The most recent World Survey of Fusion Devices identified over 130 separate systems that are currently in operation, under construction or planned. The physics and technology of these devices varies widely, making it difficult to compare progress and results. A metric called triple product can help.

Simply put, triple product reflects the combination of three measurements: temperature, density and time. Generally, maximizing the three conditions in a plasma made of the right fuels increases the chances for fusion reactions to occur. As a result, the higher the triple product, the greater the energy released.

Zap Energy’s sheared-flow-stabilized Z-pinch fusion balances the three conditions in a distinct and advantageous way. As the company scales fusion plasmas toward commercially-relevant energy gains, understanding and hitting the right blend will be critical.

Zap Energy’s Ben Levitt, VP of R&D, explains fusion’s triple product.

An extreme mix

Regardless of approach, all credible methods for releasing fusion energy require extreme temperatures. Usually that means over 180 million degrees Fahrenheit, far hotter than most stars. That creates the essential conditions where atoms shed electrons and the remaining nuclei become positively charged ions with the potential to fuse.

“The goal is to get the plasma ions to overcome energy barriers and fuse together,” describes Derek Sutherland, a Zap Energy senior research scientist. "But they’ll only fuse if the temperatures are hot enough to maximize the chances of a fusion reaction.”

Density and confinement time, on the other hand, can vary dramatically depending on the fusion approach.

For example, magnetic confinement fusion approaches such as tokamaks and stellarators require high energy confinement times but a lower density of ions in the reaction. Inertial confinement experiments, like the powerful laser-based system at the National Ignition Facility, require a very brief confinement time, but the densities must be extremely high.

Zap Energy’s Z-pinch fusion device (center) must achieve different levels of density, temperature and plasma confinement time – known in fusion as the triple product – than those of a tokamak (at left) and laser-driven inertial confinement systems (at right).  

Zap Energy's triple product

In many ways, Zap Energy’s triple product divides the pack. Plasmas in a sheared-flow-stabilized Z pinch are 100,000 times denser than that of a typical tokamak, but with a far shorter energy confinement time. Zap also requires lower density than an inertial approach.  

“One of the nice things about our technology is that we can target relatively moderate plasma conditions compared to other approaches and potentially even make some trade-offs,” says Sutherland. “Understanding the interplay between the parameters is an important topic for improving performance.”

For Zap to get to higher triple product levels, more electric current must run through the pinch, all while maintaining the pinch’s heat, density and stability. One of the reasons for the company’s success to date has been the team’s history of high triple product gains against low capital costs.

Fuel matters

Triple product misses one key aspect of fusion. The chances that nuclei will overcome their barriers and fuse in a reaction depends not just on its triple product, but also on what mix of materials, or fuels, make up the plasma.

Researchers have long known that one of those fuel types, a mix of the hydrogen isotopes deuterium and tritium, fuses at the most attainable triple product levels. Many fusion approaches, including Zap's, plan to take advantage of that fact. Some concepts plan to use alternative fuel mixes that require a higher triple product. Today, almost all fusion experiments use only deuterium because it’s cheaper, less complex to work with and past tests have shown that it’s possible to calculate the equivalent reaction when tritium is added.  

Regardless of fuel source, all approaches must scale up their triple product levels. That, in turn, will raise the most important metric of all: that of Q, the ratio of how much power is generated by a fusion reaction and the power required to produce it.

“An optimized triple product at high enough temperatures will lead to an increasing Q value,” Sutherland said. “The higher the Q in a commercially attractive package, the closer the world is to harnessing scalable fusion energy.”