New Paths to Fusion Energy

Physicists Raffi Nazikian, left, and Craig Petty at the DIII-D tokamak

The new research targets instabilities called Edge Localized Modes (ELMs) that develop at the periphery of plasma, the free-floating electrons and atomic nuclei, or ions, that fuel fusion reactions.

Steady as she goes: Scientists tame damaging plasma instabilities and pave the way for efficient fusion on Earth

Controlling the hot plasma gas that fuels fusion reactions is essential to harvesting the fusion process that powers the sun and stars to produce virtually limitless energy on Earth. Recent experiments have shown scientists how to tame a plasma instability to produce efficient and steady state operation of ITER, the international experiment under construction in France.

The findings, developed by researchers led by physicist Raffi Nazikian of PPPL and Craig Petty of General Atomics, stem from experiments on the DIII-D National Fusion Facility that General Atomics operates for the DOE in San Diego. The results build on earlier work led by DIII-D scientists that demonstrated the conditions needed for steady-state operation of the core of ITER plasmas and established techniques to control these plasma instabilities.

The new research targets instabilities called Edge Localized Modes (ELMs) that develop at the periphery of plasma, the free-floating electrons and atomic nuclei, or ions, that fuel fusion reactions. Such instabilities can cause periodic heat bursts that can damage plasma-facing components in a tokamak. The experiments suppressed large ELMs and left small benign ELMS in the plasma.

To suppress large ELMs, researchers produce small magnetic ripples known as resonant magnetic perturbations (RMPs) that distort the smooth doughnut shape of tokamak plasmas. In the recent experiments, the scientists found that increasing the overall pressure of the plasma makes it far more responsive to the ripples to better control ELMs and produce the conditions needed for steady-state ITER operation.

The higher pressure also increases a self-generated, or “bootstrap,” current that forms inside tokamak plasmas; this current can combine with particle beams and microwaves to drive and sustain the plasma indefinitely in a so-called steady state. When researchers projected the recent DIII-D results to ITER, they found that the higher plasma pressure and bootstrap current, together with additional sources of current from particle beams and microwaves, could create a fully sustainable steady-state regime that generates four-to-five times more power than it will take to heat the ITER plasma and drive the current. ☀︎

Halos look good on angels, but could damage fusion energy devices

Halo currents — electrical currents that flow from the hot, charged plasma that fuels fusion reactions and strike the walls of fusion facilities — could damage the walls of devices like ITER, the international experiment under construction in France to demonstrate the feasibility of fusion power.

Such halos occur during plasma disruptions and transfer large amounts of magnetic energy from the plasma to the vessel walls. Recent findings, gleaned from a database of 800 plasma discharges from five doughnut-shaped tokamaks around the world, compared the behavior of halo currents as they rotate in different-sized machines.

The results provide insight into the likelihood of rotating halo currents in ITER, an issue that operators of what will be the world’s largest tokamak must face. “These findings provide scientific guidance to the ITER team as they work to address the disruption problem,” said physicist Clayton Myers, lead PPPL author of a paper that reported the study in the journal Nuclear Fusion.

Myers, now a researcher at Sandia National Laboratories, added that the study “places additional importance on the reliability of ITER’s disruption mitigation system that can alleviate the effects of halo current rotation, which we cannot rule out the possibility of occurring in ITER.” ☀︎

Cutaway of the lower divertor region of the National Spherical Torus Experiment in a simulated disruption. The red and blue colors indicate halo currents into the walls of the tokamak. The rope-like structures show the paths of three magnetic field lines that intersect the walls

Physicist Zhirui Wang

For high-performance fusion, follow the bouncing particles

A key challenge in fusion research is maintaining the stability of the hot, charged plasma that fuels fusion reactions. PPPL physicists have recently found that drifting particles in the plasma can forestall instabilities that reduce the pressure crucial to high-performance fusion reactions inside tokamaks.

Zhirui Wang led the findings based on data from the National Spherical Torus Experiment (NSTX) prior to its upgrade. The data show how particles that drift and bounce within the magnetic fields that confine the plasma can stabilize its pressure and performance. Such particles become trapped and bounce back and forth within a portion of the fields instead of circling around inside the tokamak. By bouncing and drifting, the particles can dissipate energy that might destabilize the plasma and interfere with fusion reactions.

Overall results of these findings could lead to improved achievement of high-performance fusion plasmas in present-day tokamaks and in ITER, the international experiment under construction in France to demonstrate the feasibility of fusion power. ☀︎

Hot and hotter: New model improves confidence in the performance of ITER

Scientists seeking to bring to Earth the fusion that powers the sun and stars must first make the plasma that fuels fusion reactions superhot. The task calls for heating the plasma, the state of matter composed of free electrons and atomic nuclei, also called ions, to many times the temperature of the core of the sun.

ITER, the international fusion facility under construction in France, will heat both electrons and ions. What will this mix do to the temperature and density of the plasma that are crucial to fusion production?

New findings by physicists at PPPL and the DIII-D National Fusion Facility that General Atomics operates for the DOE indicates that understanding the combined heating can improve the production of fusion in ITER and other next-generation fusion facilities. “This shows what happens when electron heating is added to ion heating,” said PPPL physicist Brian Grierson, who led testing of a computer model that projected the DIII-D results to ITER.

The new model, created by Gary Staebler of General Atomics, investigated the DIII-D experimental results in conditions mimicking those expected in ITER. Diagnostics supplied by the University of Wisconsin-Madison and the University of California, Los Angeles measured the resulting turbulence, or random fluctuations and eddies, that took place in the plasma.

The model focused on the impact of electron heating on the mix and revealed turbulence that modified how particles and heat leaked from the plasma. Results indicated that studying such turbulence will be essential to dealing with its impact on the transport of heat, particles and momentum in ITER and next-generation tokamaks that will have both ion and electron heating. ☀︎

Physicists Brian Grierson of PPPL, left, and Gary Staebler of General Atomics

Eugene Evans studies the field-reversed configuration device

New simulations confirm efficiency of fusion ash-removal process in plasma device

Just as fire produces ash, the combining of light elements in fusion reactions can produce material that interferes with the reactions or creates undesirable side reactions. PPPL scientists have now found evidence suggesting that a process could remove the unwanted ash, make the fusion process more efficient, and produce less radioactivity within a type of fusion facility known as a field-reversed configuration (FRC) device.

FRCs confine plasma that fuels fusion reactions in a magnetic bubble enclosed in nearly linear magnetic fields that extend between two endpoints. This makes FRCs potentially suitable as fusion-powered rocket engines for spacecraft propulsion.

Research on ash removal began about five years ago when Princeton undergraduate student Matt Chu-Cheong and Samuel Cohen, principal investigator of the Laboratory’s FRC experiments, started thinking about how the ash particles created in future FRC reactors could be removed.

Calculations suggested that the unwanted particles would slowly migrate into the “scrape-off layer” (SOL) between the hot core plasma bubble and the far cooler surfaces of the vessel. Passing in and out of the relatively cool SOL region, the ash particles would lose energy and slow down, much as spacecraft reduce their speed by dipping into the atmosphere of a planet. Eventually, the ash particles would lose enough speed to remain in the scrape-off layer and be funneled to an exhaust system that removed them from the plasma.

“This could be a neat way to remove fusion products from the core and prevent them from building up,” said Princeton graduate student Eugene Evans, lead author of a paper in Physics of Plasmas that examined the processes. Evans formed a hypothesis and performed extensive computer simulations that produced data suggesting that ash particles in an FRC reactor would in fact be removed from the plasma fast-enough to keep them from interfering with fusion reactions and producing radioacivity.

The results were extremely encouraging. “My main reaction was relief that the simulations worked out, that our previous estimates were okay, and that at least in these simulations we saw no reason why this process wouldn’t work,” Evans said. “In other words, so far, so good.” ☀︎

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