First results of NSTX-U research operations

Researchers from the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratories (PPPL) and collaborating institutions presented results from research on the National Spherical Torus Experiment Upgrade (NSTX-U) during October at the 26th International Atomic Energy Agency Conference (IAEA) in Kyoto, Japan. The four-year upgrade doubled the magnetic field strength, plasma current and heating power capability of the predecessor facility and made the NSTX-U the most powerful fusion facility of its kind. Here are first results of the upgrade and related IAEA research presentations.

Physics results of the first 10-weeks of NSTX-U operation

The NSTX-U delivered important physics and operational results during its first research campaign under the run coordination leadership of Program Director Jon Menard and Head of Experimental Research Operations Stefan Gerhardt. Principal results and achievements included:

• Quickly surpassing the maximum magnetic field strength and pulse duration of its predecessor prior to the upgrade.
• Achieving high plasma confinement, or H-mode, on just the eighth day of the 10 weeks of experiments. H-mode is a superior regime for fusion performance.
• Reducing plasma instabilities with beams from a second neutral beam injector that was installed to increase the heating of the plasma. This device fired beams at different angles than the first injector which generated the initial instabilities.
• Changing the propagation direction of other instabilities using the second neutral beam injector. This result is consistent with the new beam significantly modifying the distribution of energetic ions. Providing increased flexibility in the distribution of energetic ions was a major scientific motivation for the new beam.
• Advancing development of methods to prevent plasma disruptions and to ramp down plasma when disruptions can no longer be avoided. Such methods will be critical for ITER, the international fusion experiment under construction in France, and for all future tokamaks.
• Identifying and learning to correct conditions called error fields that are common to tokamaks and can hinder the performance of fusion plasmas. 
• Commissioning all magnetic diagnostics needed to gauge control of the plasma and demonstrating advanced diagnosis of the surface composition of the reactor walls.

Researchers now look forward to continuing their investigation of key issues needed to develop fusion energy when NSTX-U repairs are completed and the spherical tokamak resumes operation.

The popular “predator-prey” model cannot explain the transition to H-mode on the NSTX tokamak

A key challenge to the production of magnetic fusion energy is preventing heat from escaping the core of the superhot plasma held in doughnut-shaped devices called tokamaks. Researchers have long known that a slight increase in heating power can reduce turbulence near the edge of the tokamak, shifting the plasma to an H-mode (high confinement) regime that reduces energy leakage.

But what causes this disappearing turbulence? Researchers at PPPL have found that a popular explanation known as the “predator-prey” model cannot account for the reduction. It posits that the turbulence dumps its energy into a benign spinning of the plasma called “mean poloidal flows” that does not transport heat. For this to happen, the reduction in turbulent energy must roughly equal the increased energy of the mean flows.

To test this theory, the PPPL physicists used a gas puff imaging (GPI) diagnostic that let them directly see turbulent plasma fluctuations in the edge region of PPPL's National Spherical Torus Experiment (NSTX), the laboratory's flagship fusion facility, which has since been upgraded. By pumping small amounts of neutral gas into the plasma, they caused the neutrals to interact with the plasma and glow. A fast camera recorded the glow and revealed how the turbulence evolved in space and time.

The researchers were also able to infer the velocity of the plasma, enabling evaluation of energy in both the turbulence and the mean flows. This showed that the energy in the mean flows was never more than a few percent of the energy in the turbulence before the shift to H-mode.

With this result, the mystery of the H-mode deepens again. However, by ruling out the predator-prey model, the findings may refocus efforts on other contenders. This could increase the chances of identifying the physics behind the mysterious H-mode, and facilitate the ability to employ it for the success of future fusion reactors.

PPPL and DIII-D Advance Understanding of Sheared Rotation that Helps Stabilize Fusion Plasmas

New measurements and simulations of plasma rotation in the DIII-D National Fusion Facility are advancing our understanding and predictive capability for the self-organized “intrinsic rotation” in tokamaks.

It is commonly understood that improved plasma confinement and stability in tokamaks comes through generation of sheared plasma flow, in which one part of the hot gas flows faster than the other. Such flows are typically generated by injection of neutral beam particles that spin the plasma. It has long been assumed that without these beams, there may not be much rotation or shear.

However, the present work shows that simply heating the plasma can cause it to generate a sheared flow.. The model shows that heating the core can cause the outer region of the plasma to flow in one direction, while the core flows in the other. Causing this shear is intrinsic torque, a twisting force that produces the rotation.

We have a first quantitative understanding, through simulations of plasma turbulence, of how these processes happen. In these simulations, performed by the GTS code, the turbulence generates a torque that causes the plasma to spin. The plasma flow is generated by the variation of the turbulence with radius, causing the plasma to accelerate from rest and drive differential flow, like the atmospheric jet stream or the bands of Jupiter.

This flow represents the balance between the intrinsic torque driven by the turbulence, and the viscosity of the plasma that keeps the gas from spinning arbitrarily fast. Simulations with GTS are able to predict the plasma rotation, and it agrees very well with the observed rotation in both shape and magnitude.

The key remaining challenge is how to project these processes to ITER, the international tokamak under construction in France, which requires very large simulations that push the limits of present high-performance computing.

A unique model for finding the equilibria of magnetically perturbed plasmas

Among the key challenges for producing magnetic fusion energy is controlling instabilities known as “edge localized modes” (ELMs). These disturbances at the edge of fusion plasmas can damage components that face the plasma in doughnut-shaped devices called tokamaks that house fusion reactions.

A popular method for coping with this problem uses “resonant magnetic perturbations” (RMPs) to deal with the instabilities. These perturbations, produced by specialized magnetic coils, have mitigated ELMs in tokamaks today. However, researchers are unable to predict how the perturbations will affect ITER, the huge international tokamak under construction in France, for which ELMs could become a major problem.

Now PPPL physicists, together with researchers at the Max Planck Institute for Plasma Physics in Germany, have produced a promising new model for comprehending the processes involved. It departs from conventional theory that has failed to find solutions that can be realized in physical terms.

The older models employed mathematically ill-posed questions, said PPPL physicist Stuart Hudson. Such questions lacked solutions, he said.

The new model takes a novel look at the relationship between the two major forces inside tokamaks — the pressure of the plasma that fuels fusion reactions and the strength of the magnetic field that confines the plasma in place. The inclusion of RMPs transforms the problem into finding states in which the pressure force and the magnetic force are balanced within so-called three-dimensional boundaries.

The model suggests that a mathematical “discontinuous boundary” should be imposed on the problem. This leads to a “discontinuous solution” that can be used to determine the balance between the two forces.

With that solution in hand, the model finds that RMPs will penetrate into the core of the plasma, whereas conventional theory holds that they will be stopped short of the core. At the same time, pressure inside the core will amplify the perturbations, according to the model, and give them greater impact.

These findings could serve as a tool for future operators of ITER. Knowing the depth of RMP penetration, and the fact of its amplification, could lead to much improved modeling of the response of ITER to perturbations used to mitigate the ELMs the giant tokamak will experience.

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PPPL, on Princeton University's Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas — ultra-hot, charged gases — and to developing practical solutions for the creation of fusion energy. The Laboratory is managed by the University for the U.S. Department of Energy's Office of Science, which is the largest single supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov(link is external).