When ITER, the international fusion experiment, fires up its first largely self-heating plasma next decade, a top priority will be avoiding or mitigating violent disruptions that can damage the giant machine. Scientists at PPPL have built and successfully simulated the prototype of a novel device to mitigate the consequences of a damaging disruption before one can proceed.

The risk of disruptions faces all doughnut-shaped facilities called “tokamaks,” devices widely used in the effort to harvest on Earth the fusion energy that powers the sun and stars. Tokamaks confine the fuel called plasma that feeds fusion reactions with powerful magnetic fields and heat it to many times the temperature of the sun, causing the free atomic nuclei in the plasma to merge and release vast energy. The goal: to produce a safe and clean source of power for generating electricity.  

Disruptions occur when the magnetic bottle used to confine the superhot plasma becomes unstable, leading large electromagnetic forces and thermal loads to slam against the tokamak’s walls. A mitigation system cannot stop the disruption, which is like a sudden tear in the skin of a gas balloon, but it can minimize the damage it causes.

The simulated railgun-like device, called an “electromagnetic particle injector” (EPI), is designed to mitigate the problem by firing a high-speed projectile of material that radiates away the energy in the core of the plasma at the first sign of a disruption. The payload will cool and shut down the reaction in a controlled manner to avoid damage to the walls of the reactor chamber.

Researchers modeled the pellet injector with a PPPL fusion code that describes plasma as a fluid that conducts electricity. “This has been a very challenging simulation,” said physicist Cesar Clauser, a postdoctoral researcher at Lehigh University conducting research at PPPL and the first author of a paper describing the modeling process in Nuclear Fusion. “Disruption mitigation systems will be extremely important for future fusion devices,” he said.

The pellet injector could serve as a speedier alternative to the system planned for ITER, which calls for shattering frozen gas pellets against a metal plate inside the tokamak to spread fusion reaction-cooling shards into the edge of the plasma. However, “the electromagnetic system is 10 times faster,” said Roger Raman, a University of Washington physicist on long-term assignment to PPPL and a principal designer of the EPI.

PPPL, a leader in the drive to reproduce the clean, carbon-free, and climate-benefiting fusion energy that drives the sun and stars, is extending its global reach. PPPL is strengthening a long-term collaboration with the Culham Centre for Fusion Energy in the U.K. Culham’s Mega Ampere Spherical Tokamak-Upgrade (MAST-U) fusion device complements the flagship National Spherical Torus Experiment-Upgrade (NSTX-U) at PPPL. 

The two cored-apple-shaped facilities are the most prominent spherical devices in the world. Their design could produce cost-effective fusion power and become the model for a fusion pilot plant as an attractive economic alternative to the design of larger and more widely used doughnut-shaped conventional tokamaks operating around the world.

“MAST-U is exciting because it’s essentially a new machine and provides an opportunity to participate in experiments that may be similar to ones that we want to run on NSTX-U,” said PPPL physicist Jack Berkery, coordinator of spherical research collaborations. “There’s definitely an advantage to having two machines that are similar but not exactly the same. People can do experiments on one machine and repeat them on the other to see if there’s a difference and try to figure out why.” 

The two spherical tokamaks explore different capacities. “MAST-U is concentrating on the divertor, the tokamak region that exhausts waste heat,” he said. Whereas with NSTX-U, “we want to see if plasma confinement improves when the temperature grows hotter.”

PPPL’s MAST-U team. From left: Physicists Jason Parisi, Andreas Kleiner, Jack Berkery, Máté Lampert, Vinícius Duarte and Doménica Corona. (Photo by Kiran Sudarsanan/PPPL Office of Communications.)

“The collaboration between the NSTX-U and MAST-U teams is hugely beneficial for spherical tokamak research,” said physicist James Harrison of the UK Atomic Energy Authority (UKAE). “Through our joint efforts to understand key physics issues using the unique capabilities of our researchers and facilities, we are preparing firm foundations for developing future economical fusion energy sources.” 

Jon Menard, PPPL deputy director for research, said, “Working closely with the MAST-U team on important issues for spherical tokamaks is very important for the upcoming NSTX-U program and for next-step spherical tokamaks including pilot plants. We are grateful to DOE Fusion Energy Sciences and MAST-U for their support and assistance.”

PPPL scientists have shown that a Laboratory-developed powder dropper can successfully drop boron powder into high-temperature plasma within tokamaks with parts made of a heat-resistant material known as tungsten.

Tokamaks increasingly use tungsten’s high melting point, which is increasingly used to help components withstand the million-degree heat of the fusion process. Boron, a common household cleaner ingredient, shields the tungsten from the plasma to prevent cooling it down. It also absorbs any stray elements like oxygen that may be in the plasma from other sources. These unwanted impurities could cool the plasma and quench the fusion reactions.

“We need a way to deposit boron coatings without turning off the tokamaks’ magnetic field, and that’s what the powder dropper allows us to do,” said Grant Bodner, a postdoctoral researcher at PPPL who led the paper reporting the results in Nuclear Fusion. The research was performed on the W Environment in Steady-State Tokamak (WEST), operated by France’s Atomic Energy Commission (CEA).

PPPL has a number of international collaborations, and the physicists experimented on WEST because its magnets are made of superconducting material that future fusion devices will use. This material conducts electricity with little or no resistance and little excess heat so the magnetic fields can  bottle up the plasma to enable fusion reactions to proceed.

Fusion, the power that drives the sun and stars, combines light elements in the form of plasma — the hot, charged state of matter composed of free electrons and atomic nuclei — that generates massive amounts of energy. Scientists are seeking to replicate fusion on Earth for a virtually inexhaustible supply of power to generate electricity.

“Dropping boron into a tokamak while it is operating is like cleaning your apartment while doing all the other things that you usually do in it,” said CEA scientist Alberto Gallo, who contributed to the research. “It’s very helpful — it means you don’t have to do the cleaning.” 

Upcoming experiments will focus on how much boron is actually coating the tungsten surfaces. “We want to measure these amounts so we can really quantify what we’re doing and extend these results in the future,” Bodner said. 

Physicist Stefano Munaretto of PPPL received leadership roles in two DOE three-year collaborations. Both are designed to improve models used to design and evaluate the performance of spherical tokamaks, compact fusion facilities being explored as possible designs for future fusion power plants.

The first award funds efforts by PPPL and Oak Ridge National Laboratory, another U.S. national laboratory, to develop software tools to allow scientists to simulate the critical flow of heat from fusion plasma. Munaretto will head this effort, to be conducted using engineering design and plasma scenario details from the high magnetic field SPARC tokamak being developed by Commonwealth Fusion Systems (CFS).

The PPPL funding aims to increase the accuracy of plasma simulations and to help scientists tailor the operation of differently shaped tokamaks to be as efficient as possible. “In compact spherical tokamaks like PPPL’s National Spherical Torus Experiment-Upgrade (NSTX-U), which are shaped like cored apples, we are trying to make a plasma as powerful as those in larger, more conventional tokamaks, which look more like doughnuts,” Munaretto said.

“This project represents a significant new collaboration between PPPL and CFS,” said Rajesh Maingi, head of Tokamak Experimental Science at PPPL. “The resulting codes will be applicable not only to NSTX-U but also to spherical tokamak pilot plant and reactor designs.”

The second award involves PPPL and General Atomics, which operates the DIII-D tokamak in San Diego. Munaretto is the principal investigator for the PPPL side of the research, which calls for experiments on the United Kingdom’s Mega-Amp Spherical Tokamak-Upgrade.

“This research matters because it pertains to both spherical tokamaks and ITER,” Munaretto said. “We will be using 3D fields with different shapes to see how each one affects the plasma. Some shapes might suppress ELMs [disruptive instabilities] better and could therefore help tokamaks operate more efficiently.”

“It is a remarkable achievement for any scientist to win two awards from a single funding opportunity announcement, and especially an early career scientist,” added Maingi. “PPPL is very fortunate that Stefano joined the NSTX-U team!”

The Laboratory and Princeton University have sharply refined the application of magnetic fields to improve the performance of doughnut-shaped fusion facilities known as tokamaks. The improved technique protects internal parts from damage by instabilities called “edge-localized modes” (ELMs) and allows tokamaks to operate longer without a pause.

“Our main result is that we showed that our technique can suppress ELMs while maximizing plasma performance,” said Ricardo Shousha, a graduate student in the plasma control group in Princeton University’s Mechanical and Aerospace Engineering Department who is affiliated with PPPL. Shousha is the lead author of a paper reporting the results in Physics of Plasmas.

The researchers used the Korea Superconducting Tokamak Advanced Research (KSTAR) facility to study conditions under which the center of plasma becomes ultra hot and dense. This “H-mode” shape creates a sharp separation between the center and the colder edge of the plasma, which produces the most efficient fusion reactions. But because the temperature and density of the two regions are so different, ELM instabilities form along the boundary, much as thunderstorms can form where hot and cold fronts meet.

These conditions can damage the inner walls and components, requiring the machine to shut down for repairs. The risk is even higher for ITER, the multinational tokamak being built in Cadarache, France, to prove the feasibility of fusion as a large-scale and carbon-free source of energy.

So physicists have a dilemma that Shousha and colleagues addressed by applying magnetic fields to tamp down the ELMs. This reduces instabilities by allowing particles to flow through the boundary but also cools the plasma, making fusion reactions less efficient.

The team therefore combined magnets and a feedback system to determine the weakest magnetic field that can suppress ELMs while minimizing the degradation of H-mode conditions. “That’s the novel part of our research,” Shousha said.

“Being part of PPPL and Princeton University marks a great opportunity for graduate students,” said Egemen Kolemen, an associate professor in Princeton University’s Mechanical and Aerospace Engineering Department with a joint appointment with PPPL. “They can run experiments anywhere in the world and have the chance to control these powerful machines. Where there’s a will, there’s a way.”

PPPL has become the first affiliate to join the Co-design Center for Quantum Advantage (C2QA) led by Brookhaven National Laboratory. C2QA aims to develop tools for quantum computers. Such computing utilizes quantum mechanics to solve complex problems faster than classical computers can solve them.

C2QA is one of five DOE Quantum Information Science (QIS) Research Centers established in support of the National Quantum Initiative Act, which aims to develop the full potential of quantum-based applications. C2QA’s primary focus is on building the tools necessary to create scalable, distributed and fault-tolerant quantum computer systems. 

Such computers have the potential to solve scientific and other kinds of problems that would be practically impossible for traditional supercomputers. However, the current quantum generation — called noisy intermediate-scale quantum — suffers from a high error rate because of noise, faults and loss of quantum coherence.

“This new affiliation allows us to take advantage of PPPL’s expertise in plasma-based material to accelerate C2QA’s work in distributed quantum computing,” said Andrew Houck, C2QA director and joint appointee at Brookhaven and Princeton University, where he is a professor of electrical and computer engineering. “C2QA is committed to adding affiliates to the collaboration where new relationships can be developed and further build the QIS networking ecosystem.”

The PPPL focus will address “plasma-assisted synthesis and doping of quantum-grade diamond,” said David Graves, associate laboratory director for low-temperature plasma surface interactions and a professor in the Princeton Department of Chemical and Biological Engineering. Such diamond can be a useful component of quantum computing.

“In this way,” he said, “we anticipate that the collaboration will ultimately lead to improvements in the purity and quality of plasma-grown diamond substrates for quantum applications.”