How do you keep a fusion plasma that's hotter than the sun from damaging vital components near it?
This is just one question the Material Plasma Exposure eXperiment (MPEX) at the Department of Energy's Oak Ridge National Laboratory will answer in the march to fusion power. To get the full story on MPEX from the people who know it best, we asked members of the ORNL MPEX science team about the project's goals, unique capabilities, and how it will impact the future of fusion energy research. Join us as we explore why the world needs MPEX.
Though the plasma within a fusion reactor is mostly confined within powerful intersecting magnetic fields, there are parts of the device, called plasma-facing components (PFCs), that interact with the plasma and need to withstand extreme heat, particle bombardment and radiation damage for months or years on end without breaking down. Developing the next generation of PFC materials is one of the most critical technical hurdles to overcome in enabling commercial fusion energy, and one that the lab is tackling head-on.
Designing materials that are capable of surviving within a fusion reactor is a chicken-and-egg situation - you can't build a fusion reactor without robust materials, but you can't make the robust materials without a reactor-like device to test them first. That's where MPEX comes in. MPEX is a first-of-its-kind linear plasma facility designed to recreate the extreme conditions found at the edge of a fusion reactor. It can simulate a material's entire lifetime of plasma exposure in just a few weeks, enabling researchers to rapidly test and refine candidate materials for use in PFCs.
When MPEX begins operations in 2028, it will help researchers design, model and refine the building blocks of future fusion reactors. The knowledge gained will not only answer today's plasma-material science questions, but it will also guide the design, engineering and operation of tomorrow's fusion power plants.
The members of the MPEX science team interviewed for this Q&A are:
- Zeke Unterberg, fusion materials R&D lead and interim MPEX chief scientist
- Josh Larson, postdoctoral research associate
- Jake Nichols, plasma physicist
- Gayatri Dhamale, plasma-material interaction scientist
These Q&As are also available in video format on YouTube. Watch the full playlist here.
Q: What research questions will MPEX help answer?
Unterberg: Fusion has been compared to replicating a star here on Earth. Unlike a star that has space all around it that acts as a containment vessel, we don't have the luxury of all that space here on Earth, so we need to come up with a way to put "a star in a jar," as we sometimes describe it. But we need to make the jar out of something that doesn't melt or otherwise fail in the first few minutes after you turn on your fusion reactor. MPEX will be one of the key facilities answering that.
MPEX will reproduce the physics that occurs in the area where a high-energy plasma touches a material's surface. It is here that electric fields and ions, or charged atoms, are formed and interact with the electric fields. It is very hard to understand the details in these big, complex fusion reactor systems, so MPEX will be a test bed that will look at these phenomena.
Larson: Inside a fusion reactor, materials will be bombarded with incredible amounts of heat, plasma particles and neutrons, specifically in a region called the divertor, which helps control the temperature in the reactor and pumps out byproducts created by the plasma to keep it clean and stable. Understanding the lifetime and degradation of materials in this region is key to understanding the operation of future fusion reactors. MPEX aims to provide a testbed that can emulate this plasma environment and act as a sort of plasma rocket nozzle to blast materials to simulate their lifetimes within a fusion reactor. These experiments will help us understand the physics and materials science necessary to construct reactors that can withstand the harsh plasma environment of a fusion power plant.
Q: How can MPEX help accelerate the development of commercial fusion energy?
Unterberg: MPEX will act as a simulator so we can more rapidly turn around experiments and stress-test materials before they are used in actual fusion reactors. All these things give us experience and understanding of plasma-material interactions and will inform what to do in a more tightly regulated fusion reactor.
Nichols: Where MPEX will really shine is what's called plasma fluence, which is basically the number of plasma particles per square meter that can hit a surface over time. Once MPEX gets to full power, a plasma-facing component that has been in MPEX for a few weeks will have seen the same fluence as it would have seen over its full lifetime in a fusion power plant. This really accelerates the design-test-iterate cycles that are needed to develop new plasma-facing materials.
Q: How will MPEX discoveries be used?
Larson: Research from MPEX will help guide our understanding of how materials degrade over their lifetimes within a fusion reactor. This, along with MPEX's flexibility to replicate different plasma conditions, will allow for studies probing how fusion plasmas will interact with surfaces. Understanding the fundamentals of these processes is critical to the entire fusion community as we work toward the development of fusion reactors.
Unterberg: MPEX will provide an opportunity to stress-test materials that we could potentially use to armor fusion reactor vessels. What works, what doesn't work, why it doesn't work, how does it fail - these are all things that will be tested on MPEX. With that knowledge, we could engineer new materials based on the outcome of these tests that could then be used in a fusion reactor in the future.
Q: Will MPEX impact the design of fusion power plants?
Nichols: Absolutely! While there are a bunch of fusion prototype facilities being built right now, one of the things that we will have to further develop before we can scale up to widespread fusion power plants is plasma-facing materials. As the community develops these next-gen materials over the coming years, they will be tested right here in MPEX.
Unterberg: MPEX will not only impact the design by determining whether a material has potential or not, it will also allow fusion companies to bring their materials here and quality-test them for plasma-material interactions, as well as investigate some of the fundamental physics questions that still remain.
Q: What will MPEX do that no other device before has been able to do?
Dhamale: Current linear plasma devices are not able to study the full lifetime of plasma-facing components due to their inability to replicate the damage caused by neutron irradiation and the material erosion that can cause structural integrity failures. MPEX will add new capabilities to address these limitations by utilizing a new plasma source system and exposing previously irradiated materials to fusion reactor-grade plasmas.
MPEX will utilize a novel concept developed and tested on Proto-MPEX - the predecessor device to MPEX - that combines a high-power plasma source with both microwave electron heating and ion cyclotron heating. This will allow researchers to fine-tune the plasma density and temperature in front of the target to create conditions like those expected in a reactor. MPEX will also be able to undock the target chamber and transfer the plasma-exposed material to a nearby analysis station while maintaining vacuum.
Unterberg: From a technical point of view, MPEX is designed to operate for one million seconds. That's about two weeks of continuous on-time. Typically, fusion-grade plasmas last for something like 100-500 seconds, up to 1,000 seconds at most, which is roughly 15 minutes. MPEX will be almost three orders of magnitude greater than any other linear plasma device that has operated up to this point. That will be very unique in terms of how materials withstand bombardment by high-energy plasmas for two weeks versus 15 minutes.
Q: Why is ORNL the best place for MPEX?
Dhamale: We have the expertise for successful MPEX operations, which involves fusion plasma experts in theory, modeling and experimentation; experts on neutron irradiation at the High Flux Isotope Reactor (HFIR), materials scientists who can develop the plasma-facing materials and analyze material characteristics, superconducting magnet experts and a world-class, high-performance computing facility with Frontier, the world's first exascale supercomputer.
Nichols: Not many facilities worldwide are capable of testing neutron-irradiated materials. We've leveraged ORNL's expertise in nuclear safety to enable MPEX to accept neutron-irradiated samples. So, we'll be able to do experiments where we move samples back and forth between HFIR and MPEX to start answering these questions.
Q: Who will benefit from the key discoveries made at MPEX?
Unterberg: The biggest beneficiary is any organization - whether a private company that wants to start their own fusion energy device or a national program developing a fusion pilot plant - that needs solutions to protect the walls of their device. MPEX can inform those efforts and is informing them even today.
Dhamale: The MPEX User Research Forum brings the plasma-material interaction community together to help shape the science program and strengthen the collaborator network. We're actively collaborating with various laboratories and universities in the United States and internationally with the fusion research community. These efforts aim to accelerate the commercialization of fusion reactor technology, ultimately providing U.S. consumers with an abundant amount of energy.
Q: What does success look like for MPEX in 10 or 15 years?
Larson: Success for MPEX means we've made headway in understanding how these materials react in the plasma environment, and that it does exactly what we've been saying it will do, which will inform our movement forward in designing fusion reactors and fusion power plants. We'll have developed more robust materials and have a better understanding of how we need to maintain or replace these materials over the lifetime of what would then be first-of-their-kind fusion pilot power plants.
The High Flux Isotope Reactor and the Oak Ridge Leadership Computing Facility, home of the Frontier exascale supercomputer, are Department of Energy Office of Science user facilities.
ORNL is committed to supporting U.S. energy needs by pursuing strategic research that advances a wide variety of affordable, abundant and competitive nuclear technologies, and strengthens national security. The lab's scientific expertise and world-class facilities are often the first step in advancing nuclear energy innovations.
UT-Battelle manages ORNL for the Department of Energy's Office of Science, the single largest supporter of basic research in the physical sciences in the United States. The Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit energy.gov/science. - Sean Simoneau
