Each time a shot of plasma is created at Zap Energy, Pi-En Tsai’s scintillators have a front row seat. Located at the head of canister-shaped detectors, Tsai and her colleagues use them to measure the millions and millions of neutrons traveling outward from that plasma in the company’s fusion devices, all within fractions of a second.

Quantifying and tracing that barrage is a key indicator of the plasma’s performance.

“Neutrons tell the most straightforward story of the fusion reaction in the plasma,” says Tsai, a senior scientist leading such measurement efforts at Zap Energy. “They can help us better understand what we’re doing, which can lead us on the right course toward commercializing fusion.”

Because neutrons are invisible to the naked eye, the scintillators convert them into visible light, revealing their paths, quantity and distribution. Other instruments can provide data at specific points and times, but the dynamics of a moving plasma make neutron measurement a central metric for fusion reactions.

“We hope to produce a long and stable Z pinch, and be able to scale up our neutron yields," Tsai said of the company’s fusion approach.

Neutrons’ key role within the fusion power plant

Inside future Zap Energy fusion devices, a reaction between hydrogen isotopes known as deuterium and tritium will produce both an energetic helium nucleus and an energetic neutron. Because it's neutrally charged, the neutron isn’t confined by electric or magnetic fields and will head outward. The neutrons leaving the device can be measured, and their quantity and location are an indication of how much fusion is occurring — and where. In a commercial-grade fusion system, the rush of neutrons will be harvested for their energy — the more neutrons, the better.

Tsai doesn’t only measure those neutrons for science. She’s also Zap’s radiation safety officer, ensuring they’re safely handled and don’t do any unwanted damage.

Her colleagues are struck by a depth of knowledge that comes from more than a decade as both a trained nuclear engineer and radiation physicist.

“She possesses both an impressive expertise and an ability to explain it in a way that helps ensure other scientists within different fields can understand,” said Chelsea Liekhus-Schmaltz, Zap Energy’s lead research data scientist. “That’s going to be very important, as we move forward as a team to scale our efforts toward commercial fusion.”

‘Something that could save lives’

When Tsai, who is from Taiwan, first enrolled in National Tsing Hua University there, she considered herself something of a skeptic when it came to nuclear energy. But within her first engineering coursework, she quickly became hooked, learning of some of the medical breakthroughs made possible using radiotherapy.  

Boron Neutron Capture Therapy, for instance, uses a beam of neutrons that are captured by boron-carrying cancer cells, causing a nuclear reaction that kills the cells.

“It made me feel connected to something meaningful,” she said of coursework focusing on medical applications. “That I could contribute my efforts for something that could save lives.”

So Tsai became an expert in measuring radiation, attaining her doctorate in nuclear engineering in 2015 from the University of Tennessee. While there, she also studied how astronauts will be affected by solar and cosmic rays in journeys through space. She was named one of the “Rising Stars of Nuclear Science and Engineering” by the Massachusetts Institute of Technology.

‘An experimental spirit’

Upon receiving her PhD in 2015, Tsai went to work for the Japan Atomic Energy Agency, where she developed a new detector system aiming to provide more nuclear data.

After more than three years in Japan, she desired a chance for something more pragmatic – “a little closer to real life.” She took back-to-back jobs in California at national labs, monitoring radiation in particle accelerators and protecting people from it.

It was there that she heard the classic trope that fusion is always a few decades out of reach. She now knows the world is closer than ever to achieving fusion as a power source.

“I’m hoping that, with a little push, it can finally be achieved,” she said.  

She joined Zap Energy in January 2022. As the company’s radiation safety officer, Tsai says fusion systems are on par with particle accelerators when it comes to such safety. Indeed, it’s a conclusion the U.S. Nuclear Regulatory Commission recently announced it agrees with.

Tsai’s job fits well with her life’s passions. At home, she has surrounded herself in the greenery of house plants. She’s currently propagating many kinds of vegetation, including growing a species of Aglaonema from tissue cultures. She also enjoys taking leftover seeds from her garden, or even those found underneath street trees, to grow new specimens.  

“I have an experimental spirit,” Tsai explained.