April 24, 2024

Adding Enough Fuel to the Fire

Plasma Fusion: Adding Enough Fuel to the Fire

Elevation of LTX-β. The shell is visible, with the inner and outer toroidal gaps indicated, as well as one of the two poloidal cuts (the two poloidal cuts are 180° apart). Poloidal field coils, except the Ohmica coil system, are color coded like blue, yellow, red, green, etc. Nuclear fusion (2024). DOI: 10.1088/1741-4326/ad2ca7

How much fuel can we add to the fire while maintaining control? Metaphorically speaking, that’s the question a team at the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) has been asking itself lately.

Now, they believe they have the answer to a specific scenario. It’s all part of the Lab’s work to bring fusion energy to the power grid.

Building on recent findings that show the promise of coating the inner surface of the container containing a fusion plasma in liquid lithium, researchers have determined the maximum density of neutral or uncharged particles at the edge of a plasma before the edge of the plasma cools and certain instabilities become unpredictable.

Knowing the maximum density of neutral particles at the edge of a fusion plasma is important because it gives researchers a sense of how and how much to fuel the fusion reaction.

The research, which is presented in a new article in the journal Nuclear fusion, includes observations, numerical simulations, and analysis of their experiments inside a fusion plasma vessel called Lithium Tokamak Experiment-Beta (LTX-β).

The unique environment of LTX-β

LTX-β is one of many fusion vessels around the world that hold plasma in a donut shape using magnetic fields. These ships are known as tokamaks. What makes this tokamak special is that its internal walls can be coated almost entirely with lithium. This fundamentally changes the behavior of the wall, as lithium retains a very high percentage of the hydrogen atoms leaving the plasma.

Without lithium, much more hydrogen would bounce off the walls and back into the plasma. In early 2024, the research team reported that this low-hydrogen recycling environment keeps the edge of the plasma hot, making the plasma more stable and providing space for a larger volume of plasma.

“We are trying to show that a lithium wall can enable a smaller fusion reactor, which will translate into higher power density,” said Richard Majeski, principal research physicist at PPPL and head of LTX-β. Ultimately, this research could translate into the cost-effective fusion energy source the world needs.

Now, the LTX-β team has published additional findings showing the relationship between plasma fuel and its stability. Specifically, the researchers discovered the maximum density of neutral particles at the edge of the plasma within LTX-β before the edge begins to cool, potentially leading to stability issues.

The researchers believe they can reduce the likelihood of certain instabilities by keeping the density at the edge of the plasma below the recently defined level of 1 x 1019 I–3. This is the first time such a level has been established for LTX-β, and knowing it is a big step in their mission to prove that lithium is the ideal choice for an inner wall coating in a tokamak because it guides them toward the best practices to feed your plasmas.

In LTX-β, fusion is powered in two ways: using puffs of hydrogen gas from the edge and a beam of neutral particles. Researchers are refining how to use the two methods together to create an ideal plasma that will sustain fusion for a long time in future fusion reactors, while also generating enough power to make it practical for the power grid.

Refining methods to maintain a uniform plasma temperature

Physicists often compare the temperature at its limit with the core temperature to assess how easy it will be to manage. They plot these numbers on a graph and consider the slope of the line. If the temperature in the inner core and outer rim is about the same, the line will be almost flat, which is why they call it a flat temperature profile. If the temperature at the outer edge is significantly lower than the temperature at the inner core, scientists call this a peak temperature profile.

“The team determined the maximum density of neutral particles beyond the edge of a plasma that still allows for a flat edge temperature profile. Going beyond this number of neutrals at the edge will definitely lower the edge temperature and you will end up in a temperature profile of peak,” said Santanu Banerjee, a research physicist at PPPL and lead author of the new paper.

“This same neutral density is the limit for instabilities known as rupture modes. Beyond this density, rupture modes tend to become destabilized, cause threats to the plasma, and can halt the fusion reaction if left unchecked.”

If the instabilities become too large, the fusion reaction will end. To support the power grid, researchers are discovering the best ways to manage a fusion plasma so that the reaction is stable.

Banerjee and Majeski worked with several other researchers on the paper, including PPPL’s ​​Dennis Boyle, Anurag Maan, Nate Ferraro, George Wilkie, Mario Podesta, and Ron Bell.

Work on the project continues. PPPL engineer Dylan Corl is optimizing the direction in which the neutral beam, used to heat the plasma, is injected into the tokamak. “We’re basically creating a new door for this,” Corl said. It uses a 3D model of LTX-β, testing different beam trajectories to ensure the beam doesn’t hit another part of the equipment, such as tools used to measure plasma. “Finding the best angle has been a challenge, but I believe we have it now,” said Corl.

More information:
Santanu Banerjee et al, Investigating the role of edge neutrals in exciting tear mode activity and achieving flat temperature profiles in LTX-β, Nuclear fusion (2024). DOI: 10.1088/1741-4326/ad2ca7

Provided by the Princeton Plasma Physics Laboratory

Quote: Plasma Fusion: Adding Enough Fuel to the Fire (2024, March 28) retrieved March 29, 2024 from https://phys.org/news/2024-03-plasma-fusion-adding-fuel.html

This document is subject to copyright. Other than any fair dealing for private study or research purposes, no part may be reproduced without written permission. Content is provided for informational purposes only.

Leave a Reply

Your email address will not be published. Required fields are marked *