New Paths to Fusion Energy
Staircase to the stars:
Turbulence in fusion plasmas may not be all bad
Turbulence, the swirling eddies and currents that jostle fluids and air, is traditionally seen as disruptive of efforts to capture and control on Earth the fusion energy that powers the sun and stars. Now a discovery by scientists at PPPL and General Atomics has found that enhanced turbulence in the edge of the plasma may actually improve the thermal insulation required to achieve fusion energy.
The surprise findings, published in Physical Review Letters, came through simulation of high-power experiments on the DIII-D National Fusion Facility that General Atomics operates in San Diego. Scientists led by PPPL physicist Arash Ashourvan used an advanced turbulence code developed by General Atomics researchers Jeff Candy and Emily Belli to extract the information. The simulation showed that the normally quiet edge of the plasma became strongly turbulent — causing the edge to take on a staircase shape and the core temperature to increase. The discovery challenged previous assumptions that enhanced turbulence would allow heat to escape from the core of the plasma.
These findings could lead to improved performance of future tokamaks — magnetic fusion facilities such as ITER, which is under construction in France to demonstrate the practicality of fusion energy. "This discovery is very good news,” said PPPL physicist Raffi Nazikian, who discovered the plasma staircase. “As we explore larger-scale plasmas required to produce net electricity, we may find that enhanced turbulence and staircase structures form more readily than in present experiments, with possible beneficial effects for fusion.”
Fiery sighting: A new physics of eruptions that damage fusion experiments
Sudden bursts of heat that can damage the inner walls of tokamak fusion experiments are a hurdle that operators of the facilities must overcome. Such bursts, called “edge localized modes (ELMs),” occur in doughnut-shaped tokamaks that house the hot, charged plasma that is used to replicate on Earth the power that drives the sun and stars.
Researchers at PPPL have now observed a possible and previously unknown process that can trigger the damaging ELMs. Working together, physicists Ahmed Diallo, an experimentalist, and Julien Dominski, a theorist, pieced together data from the DIII-D National Fusion Facility that General Atomics operates for the DOE in San Diego, to uncover a trigger for a particular type of ELM that does not fit into present models of ELM plasma destabilization.
Their findings, reported in Physical Review Letters, could shed light on the variety of mechanisms leading to the onset of ELMs and could broaden the portfolio of ELM suppression tools. The observations began as an effort to unravel puzzling data detected by probes of magnetic field and plasma density fluctuations during DIII-D experiments. The data showed the eruption of ELMs following periods of unusual quiescence. “These were special cases that didn’t follow a standard model,” said Diallo. “We started digging into this together,” Dominski said. “It was a most interesting collaboration.”
In roughly six months of joint research, the physicists uncovered previously unseen correlations of fluctuations in the DIII-D experiments. These correlations revealed the formation of two modes — or waves — at the edge of the plasma that coupled together to generate a third mode. The newcomer then moved toward the wall of the tokamak — created a radial distortion in technical terms — that triggered bursts of low-frequency ELMs.
While the findings open a door on a method for triggering ELMs, they do not fully explain the process. The two physicists thus seek to analyze more data sets. “If we can fully understand how the triggering works we can block and reverse it,” Diallo said.
Batten down the hatches: Preventing heat leaks to help create a star on Earth Upgrade
Creating a star on Earth requires a delicate balance between pumping enormous amounts of energy into plasma to make it hot enough for fusion to occur and preventing that heat from escaping. Now, PPPL physicists have identified a method by which instabilities can be tamed and heat can be prevented from leaking from the plasma, giving scientists a better grasp on how to optimize conditions for fusion in devices known as tokamaks.
The findings provide new insight into the loss of heat that makes fusion reactors less efficient. Results could benefit the operation of ITER, the multinational fusion facility being built in France to demonstrate the practicality of fusion energy.
The research began as an exercise to analyze plasma in the National Spherical Torus Experiment-Upgrade (NSTX-U), PPPL’s flagship fusion device. “These simulations were originally meant to help explain observations made on NSTX-U,” said Elena Belova, a principal research physicist at PPPL and lead author of a paper reporting the results in Physics of Plasmas.
A second neutral beam, designed to help heat the plasma, also quieted instabilities that might cause heat to leak out
The analysis demonstrated a significant fact, originally discovered by PPPL physicist Eric Fredrickson. Belova said it showed “that a second neutral beam, designed to help heat the plasma, also quieted instabilities that might cause heat to leak out of the plasma and reduce the efficiency of the plasma heating.”
The results confirmed and extended observations made during NSTX-U experiments. “It turns out that the new beam was stabilizing all the modes, or plasma instabilities, of this type that were being driven by the original beam,” said Belova. “That’s a good thing.”
These findings extend previous research into how instabilities affected plasma temperature. “Elena Belova's prior work on NSTX had shown how these instabilities could be responsible for electron heat loss in NSTX,” said Amitava Bhattacharjee, the head of the PPPL theory department. “Her new work using fresh data from NSTX-U shows how to cure this heat loss mechanism, and it has very interesting implications for ITER.”
Fast action: A novel device may provide rapid control of plasma disruptions in a fusion facility
Scientists seeking to capture and control fusion energy face the risk of disruptions — sudden events that can halt fusion reactions and damage facilities called tokamaks that house them. Researchers at PPPL and the University of Washington have developed a novel prototype for rapidly controlling disruptions before they take full effect.
The device, called an “electromagnetic particle injector” (EPI), fires a high-velocity projectile from a pair of electrified rails into a plasma on the verge of disruption. The projectile, called a “sabot,” releases a payload of material into the center of the plasma that radiates, or spreads out, the energy stored in the plasma, reducing its impact on the interior of the tokamak.
This process may prove faster and may allow payloads to penetrate more deeply into the plasma than today’s most developed techniques. Current systems release pressurized gas or gas-propelled shattered pellets using a gas valve into the plasma, but with velocity limited by the mass of the gas particles. “The primary advantage of the EPI concept over gas-propelled systems is its potential to meet short-warning time scales,” said Roger Raman, a University of Washington physicist on long-term assignment to PPPL and lead author of a Nuclear Fusion paper that describes the new system.
The risk of disruptions is particularly great for ITER, the large international tokamak under construction in France to demonstrate the feasibility of fusion power. ITER’s dense, high-power discharges of the plasma that fuels fusion reactions will make it difficult for current gas-propelled methods to penetrate deeply enough into the plasma to take good effect. Tests of the EPI prototype show that it can deliver a payload of correctly sized particles in fewer than 10 milliseconds, compared with 30 milliseconds for gas-propelled systems. Results so far provide a degree of confidence that an effective EPI system can be developed to mitigate powerful ITER disruptions.
“The primary advantage of the EPI concept over gas-propelled systems is its potential to meet short-warning time scales”
Powder, not gas: A safer, more effective way to create a star on Earth
A major issue with operating ring-shaped fusion facilities known as tokamaks is keeping the plasma that fuels fusion reactions free of impurities that could reduce the efficiency of the reactions. PPPL scientists have now found that sprinkling a type of powder into the plasma could aid in harnessing the ultra-hot gas within a tokamak facility to produce heat to create electricity without producing greenhouse gases or long-term radioactive waste.
“The main goal of the experiment was to see if we could lay down a layer of boron using a powder injector,” said PPPL physicist Robert Lunsford, lead author of the paper reporting the results in Nuclear Fusion. “So far, the experiment appears to have been successful.”
The boron prevents the element tungsten from leaching out of the tokamak walls and cooling the plasma, making fusion reactions less efficient. Using powder to provide boronization is far safer than using boron gas, the method used today. The gas, called “diborane,” is explosive and requires everyone to leave the building housing the tokamak during the process.
However, “if you could just drop some boron powder into the plasma, that would be a lot easier to manage,” Lunsford said. “While diborane gas is explosive and toxic, boron powder is inert. This new technique would be less intrusive and definitely less dangerous.” Another advantage is that while physicists must halt tokamak operations during the boron gas process, boron powder can be added to the plasma while the machine is running.
In the future, Lunsford and other scientists in the group hope to conduct experiments to determine where, exactly, the powder goes after it has been injected into the plasma. Physicists believe that the powder flows to the top and bottom of the tokamak chamber, the same way the plasma flows. “But it would be useful to have that hypothesis backed up by modeling so we know the exact locations within the tokamak that are getting the boron layers,” Lunsford said.
Blowing bubbles: PPPL scientist confirms novel way to launch and drive current in fusion plasmas
An obstacle to generating fusion reactions inside facilities called tokamaks are the pulses that produce the current in plasma that helps create confining magnetic fields. Such pulses, generated by an electromagnet that runs down the center of the tokamak, would make the steady-state creation of fusion energy difficult to achieve. To address the problem, physicists have developed a technique known as transient coaxial helicity injection (CHI) to create a current that is not pulsed.
Now, PPPL physicist Fatima Ebrahimi has used high-resolution computer simulations to investigate the practicality of the CHI technique. Her simulations show that CHI could produce the current continuously in larger, more powerful tokamaks than exist today to produce stable fusion plasmas.
“Stability is the most important aspect of any current-drive system in tokamaks,” said Ebrahimi, author of a paper reporting the findings in Physics of Plasmas. “If the plasma is stable, you can have more current and more fusion, and have it all sustained over time.”
The CHI technique replaces an electromagnet called a solenoid that induces current in today’s tokamaks. CHI produces the critical current by generating magnetic bubbles, or plasmoids, in the plasma. The new high-resolution simulations confirm that a parade of plasmoids marching through the plasma in future tokamaks could create the current that produces the confining fields. The simulations further showed that the plasmoids would stay intact even when buffeted by three-dimensional instabilities.
In the future, Ebrahimi plans to simulate CHI startup while including more physics about the plasma. “That’s a little bit harder,” she says, “but the news right now is that these simulations show that CHI is a reliable current-drive technique that could be used in fusion facilities around the world as they start to incorporate stronger magnetic fields.”
Discovered: A new way to measure the stability of next-generation magnetic fusion devices
Scientists seeking to capture on Earth the fusion that powers the sun and stars must control the hot, charged plasma that fuels fusion reactions. A key task for scientists who confine the plasma in magnetic fields in tokamaks is mapping the shape of the fields — a process known as measuring the stability of the plasma. PPPL researchers have proposed a new measurement technique to avoid problems expected when mapping the fields on large and powerful future tokamaks.
Such tokamaks, including ITER, the international experiment under construction in France to demonstrate the feasibility of harnessing fusion energy, will produce neutron bombardments that could damage the diagnostics that today map the fields in current facilities. PPPL thus proposes use of an alternative diagnostic system that could operate in high-neutron environments.
The system, called “Electron Cyclotron Emission (ECE),” measures the temperature of the electrons cycling around the field lines. “By using an ECE system, we can learn about the plasma temperature and about fluctuations in the plasma,” said Andrew “Oak” Nelson, a graduate student in plasma physics at PPPL and first author of a Plasma Physics and Controlled Fusion paper that reports the research.
The proposed method combines ECE data with a fast-camera image used to measure the boundary of the plasma. The combination provides “diagnostics which can be robustly designed in high-neutron environments,” according to the paper. The system requires use of only a single diagnostic — a useful capability for many tokamaks including ITER, since combining many different diagnostics can be highly problematic.
New technique could streamline design of intricate fusion device
Stellarators, twisty machines that house fusion reactions, rely on complex magnetic coils that are challenging to design and build. A PPPL physicist has now developed a mathematical technique to help simplify the design of the coils, making stellarators a potentially more cost-effective facility for producing fusion energy.
A key benefit of stellarators is their production of highly stable plasmas that are less liable to the damaging disruptions that tokamaks can incur. But the complexity of stellarator coils has been a factor holding back development of such facilities.
“Our main result is that we came up with a new method of identifying the irregular magnetic fields produced by stellarator coils,” said physicist Caoxiang Zhu, lead author of a paper reporting the results in Nuclear Fusion. “This technique can let you know in advance which coil shapes and placements could harm the plasma’s magnetic confinement, promising a shorter construction time and reduced costs.”
Stellarator coils must be constructed and arranged around the vacuum chamber very precisely, since deviations from the best coil arrangement create bumps and wiggles in the magnetic field that degrade the magnetic confinement and allow the plasma that fuels reactions to escape. These problematic magnetic fields can easily be caused by misplacement of the magnetic coils.
“In this paper,” said Zhu, “we propose a new mathematical method to rapidly identify dangerous coil deviations that could appear during fabrication and assembly.”
The method relies on a Hessian matrix, a mathematical tool that allows researchers to determine which variations of the magnetic coils can make the plasma change its properties. “The idea is to figure out which perturbations you really have to control or avoid, and which you can ignore,” Zhu said. This technique could make possible ways to identify an optimal coil arrangement that no one had considered before, he said.
Machine ready to see if magic metal – lithium – can help bring the fusion that lights the stars to Earth
Lithium, the light silvery metal used in everything from pharmaceutical applications to batteries that power your smart phone, could also help harness on Earth the fusion energy that lights the sun and stars. Lithium can maintain the heat and protect the walls inside doughnut-shaped tokamaks that house fusion reactions, and will be used to produce tritium, the hydrogen isotope that will combine with its cousin deuterium to fuel fusion in future reactors.
Researchers at PPPL have completed a three-year upgrade of the Lithium Tokamak Experiment — now called the Lithium Tokamak Experiment-Beta (LTX-β) — a small tokamak that will test the ability of the metal to maintain the heat and protect the walls of the now-more-powerful tokamak.
The upgrade installed a neutral beam injector to heat, fuel and increase the density of the plasma. Other improvements include an increase in the magnetic field that confines the plasma, and installation of new lithium systems. The improvements bring conditions in the experiment closer to those in a fusion reactor, said Dick Majeski, principal investigator of the experiment.
The novel device, which uses a coating of lithium to cover the interior wall of the small tokamak, is now ready to exploit the full capability of the upgrade. Prior to the upgrade it had become the first tokamak to keep temperature constant from the hot, central core of the plasma to the normally cool outer edge.
The upgrade will now test whether the machine can maintain good confinement and constant temperature in far hotter plasmas, with stronger magnetic fields. The neutral beam will keep the density from dropping and demonstrate whether the hotter and more energetic plasma can still be controlled.
Collaborating on the LTX-β are scientists from eight research centers across the country: Oak Ridge and Lawrence Livermore National Laboratories; Princeton University; University of California, Los Angeles; University of Wisconsin-Madison; University of Washington; and University of Tennessee, Knoxville.
