ITER Science. Nuclear Fusion. What is Burning Plasma?

Written by, Piers Letcher.

The dream of fusion power depends first and foremost on a self-sustaining fusion reaction, with most of the heating power needed coming from within the reaction itself, rather than from external sources. This is what ITER is all about: trying to create those conditions, in a stable and predictable way, as a starting point towards developing future fusion power stations.

“It’s a bit like a barbecue,” says Simon Pinches, ITER’s Section Leader for Plasma Modelling & Analysis. “At first you often need help to get the barbecue really going well—some people use hot air blowers—but once it’s started, and the charcoal’s really hot, it burns on its own and the barbecue becomes self-heating.”

After ten years in Provence, Pinches knows a lot about barbecues—but he knows a great deal more about plasma physics. He did his doctoral thesis (Nonlinear Interaction of Fast Particles with Alfvén Waves in Tokamaks) in the UK back in the 1990s, and followed that with a decade at the Max Planck Institute for Plasma Physics near Munich, Germany. After four years at the European Joint European Torus, and six years at the UK Atomic Energy Authority, he moved to ITER in 2012.

“In numerical terms, what we’re trying to do here at ITER is to produce plasmas with a fusion gain (Q) equal to or greater than 10,” says Pinches. “So that means if we use input heating power of 50 megawatts, we want to see 500 megawatts of fusion power generated.”

Of that 500 megawatts, 80 percent of the energy is carried by uncharged neutrons, which fly out of the plasma and heat the tokamak’s walls; this is the energy that in future power plants will be harnessed to drive steam turbines. The remaining 20 percent is carried by fast-moving charged alpha particles that cannot escape the plasma. As they collide with other particles they cause the plasma to self-heat, contributing 100 megawatts (or twice the amount provided initially by external sources) to the fusion reaction.

“Before fusion starts occurring, however, we have to get the plasma really, really hot,” says Pinches. “And we’re going to do this in three ways: ohmic heating, neutral particle beam injection, and two sources of high-frequency electromagnetic waves.”

All three types of plasma heating have their parallels in everyday households.

Ohmic heating, used at the very beginning of the process, relies on the same principles as an electrical toaster or fire, using electrical resistance to generate heat—in this case by running current through the plasma.

Neutral beam particle injection works much like the steam attachment on an expresso machine—but on a truly gigantic scale. “The two beam injectors are each bigger than French TGV train carriages, and will account for 33 of the total 50 megawatts of input power,” explains Pinches. “Operating at one mega-electron volt each—similar to the sorts of voltages you see on major power grids—the injectors fire neutral particle beams into the plasma. When the particles hit other particles in the tokamak chamber, they pass on their energy and everything heats up.”

High-frequency electromagnetic waves work on the same basic principle as a microwave oven, but again on an enormous scale. High-intensity beams of electromagnetic radiation—one for ions and one for electrons, providing tens of megawatts of power each in ITER—bring additional heat to the plasma.

Once you have burning plasma, there is then the question of how to manage and control it.

“Once the plasma is self-heating, with two-thirds of the power generated inside the machine and only one-third from outside, it becomes harder to manage,” says Pinches. “But it’s essential that we are able to do so, especially at the scale of ITER. If you lose control of the plasma—for example if it becomes vertically unstable—it can crash into the wall, and that could generate substantial thermal and mechanical loads on the machine’s components.”

“That’s why we have the ITER disruption mitigation system,” he continues, “which gives us extra protection to ameliorate the consequences of such events in a number of ways. One of these is to fire champagne-cork-sized pellets of frozen deuterium, neon or argon into the reactor. The pellets radiate away the energy, cooling the plasma and decreasing disruption loads on the machine.”

“Nonetheless, it’s obviously much better if we avoid those really big loads altogether,” he goes on to add. “That’s why we’ll be starting operations at lower currents and lower magnetic field strengths and powers in the beginning, and gradually working our way up to full power, learning lessons as we go along.”

This is important, as scientists predict that there may be non-linear effects with sudden changes of plasma behaviour as the power levels increase, because factors such as turbulence and instabilities in the plasma may be very different when operating at varying input power levels.

Of particular concern with the burning plasma are the famous Alfvén waves (named after the only Nobel Laureate in plasma physics, Hannes Alfvén, who won the prize in 1970). The particles and the magnetic field within the tokamak create waveforms which could end up expelling charged alpha particles from the plasma in much the same way that surfers bob up and down on small waves—staying in place until a larger wave carries them along with it, pushing them towards the shore at high speed.

“While that’s great if you’re a surfer—indeed, it’s the whole point—this is obviously an undesirable outcome for our charged alpha particles, which are essential to create and maintain the burning plasma,” explains Pinches. “If they are expelled from the plasma, we start to lose power, and the whole thing cools down.”

Working out how to manage this is a key part of Pinches’ job, and he spends a lot of his time working with predictive models, leveraging the power of supercomputing, artificial intelligence and machine learning.

“We are building predictive models for all scenarios via our Integrated Modelling and Analysis Suite (IMAS),” he says. “This is a suite that’s built around a standard data representation with lots of metadata, and it’s a portable software, accessible by all ITER Members.”

The idea is to have a high-fidelity plasma simulator allowing Pinches and his colleagues—and other scientists around the world—to develop and test different plasma scenarios, and have a better picture of what’s going on inside the machine. Detailed simulations are now beginning to happen.

“When we get to First Plasma we can compare the predictions we made, using modelling, with the actual results. We can then tweak our models—long before we get as far as burning plasma and fusion reactions—reworking and retuning them as necessary.”

“That’s what I love about fusion, and that’s why I’ve been fascinated by it ever since I was at university,” says Pinches, in closing. “Fusion is challenging, it’s interesting, and it’s non-linear. But above all, it’s goal-oriented science, with the potential to deliver energy security for all of humankind.”

Self-heating plasmas are the key to producing electricity from fusion energy, allowing for sustained, ongoing fusion reactions, despite strongly reduced input power from external heating sources. The ITER community, including Simon Pinches from the Science Division, is already building the predictive models that—once tailored to reflect actual experimental data—will be important tools for managing and controlling burning plasmas on ITER.

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