Major Next Steps for
Fusion Energy
Based on the Spherical Tokamak Design
By John Greenwald,Princeton
Plasma Physics Laboratory
August 24, 2016
Based on the Spherical Tokamak Design
By John Greenwald,
August 24, 2016
Among the top puzzles in the development of fusion energy is the best shape
for the magnetic facility — or “bottle” — that will provide the next steps in
the development of fusion reactors. Leading candidates include spherical
tokamaks, compact machines that are shaped like cored apples, compared with the
doughnut-like shape of conventional tokamaks. The spherical design
produces high-pressure plasmas — essential ingredients for fusion reactions —
with relatively low and cost-effective magnetic fields.
A possible next step is a device called a Fusion Nuclear Science Facility
(FNSF) that could develop the materials and components for a fusion reactor.
Such a device could precede a pilot plant that would demonstrate the ability to
produce net energy.
Spherical tokamaks as
excellent models
Spherical tokamaks could be
excellent models for an FNSF, according to a paper published online in the
journal Nuclear Fusion on
August 16. The two most advanced spherical tokamaks in the world today are the
recently completed National Spherical Torus Experiment-Upgrade (NSTX-U) at the
U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL), and the
Mega Ampere Spherical Tokamak (MAST), which is being upgraded at the
Culham Centre for Fusion Energy in the United Kingdom
“We are opening up new options for future plants,” said Jonathan Menard,
program director for the NSTX-U and lead author of the paper, which discusses
the fitness of both spherical tokamaks as possible models. Support for this
work comes from the DOE Office of Science.
The 43-page paper considers the spherical design for a combined next-step
bottle: an FNSF that could become a pilot plant and serve as a forerunner for a
commercial fusion reactor. Such a facility could provide a pathway leading from
ITER, the international tokamak under construction in France
A key issue for this bottle is the size of the hole in the center of the
tokamak that holds and shapes the plasma. In spherical tokamaks, this hole can
be half the size of the hole in conventional tokamaks. These differences,
reflected in the shape of the magnetic field that confines the superhot plasma,
have a profound effect on how the plasma behaves.
Designs for the Fusion
Nuclear Science Facility
First up for a next-step device would be the FNSF. It would test the
materials that must face and withstand the neutron bombardment that fusion
reactions produce, while also generating a sufficient amount of its own fusion
fuel. According to the paper, recent studies have for the first time identified
integrated designs that would be up to the task.
These integrated capabilities include:
• A blanket system able to breed tritium, a rare isotope — or form — of
hydrogen that fuses with deuterium, another isotope of the atom, to generate
the fusion reactions. The spherical design could breed approximately one
isotope of tritium for each isotope consumed in the reaction, producing tritium
self-sufficiency.
• A lengthy configuration of the magnetic field that vents exhaust heat
from the tokamak. This configuration, called a “divertor,” would reduce the
amount of heat that strikes and could damage the interior wall of the tokamak.
• A vertical maintenance scheme in which the central magnet and the blanket
structures that breed tritium can be removed independently from the tokamak for
installation, maintenance, and repair. Maintenance of these complex nuclear
facilities represents a significant design challenge. Once a tokamak operates with
fusion fuel, this maintenance must be done with remote-handling robots; the new
paper describes how this can be accomplished.
For pilot plant use, superconducting coils that operate at high temperature
would replace the copper coils in the FNSF to reduce power loss. The plant
would generate a small amount of net electricity in a facility that would be as
compact as possible and could more easily scale to a commercial fusion power
station.
High-temperature
superconductors
High-temperature superconductors could have both positive and negative
effects. While they would reduce power loss, they would require additional
shielding to protect the magnets from heating and radiation damage. This would
make the machine larger and less compact.
Recent advances in high-temperature superconductors could help overcome
this problem. The advances enable higher magnetic fields, using much thinner
magnets than are presently achievable, leading to reduction in the
refrigeration power needed to cool the magnets. Such superconducting magnets
open the possibility that all FNSF and associated pilot plants based on the
spherical tokamak design could help minimize the mass and cost of the main
confinement magnets.
For now, the increased power of the NSTX-U and the soon-to-be-completed
MAST facility moves them closer to the capability of a commercial plant that
will create safe, clean and virtually limitless energy. “NSTX-U and MAST-U will
push the physics frontier, expand our knowledge of high temperature plasmas,
and, if successful, lay the scientific foundation for fusion development paths
based on more compact designs,” said PPPL Director Stewart Prager.
Twice the power and five
times the pulse length
The NSTX-U has twice the power and five times the pulse length of its
predecessor and will explore how plasma confinement and sustainment are
influenced by higher plasma pressure in the spherical geometry. The MAST
upgrade will have comparable prowess and will explore a new, state-of-the art
method for exhausting plasmas that are hotter than the core of the sun without
damaging the machine.
“The main reason we research spherical tokamaks is to find a way to produce
fusion at much less cost than conventional tokamaks require,” said Ian Chapman,
the newly appointed chief executive of the United Kingdom Atomic Energy
Authority and leader of the UK
The ability of these machines to create high plasma performance within
their compact geometries demonstrates their fitness as possible models for
next-step fusion facilities. The wide range of considerations, calculations and
figures detailed in this study strongly support the concept of a combined FNSF
and pilot plant based on the spherical design. The NSTX-U and MAST-U devices
must now successfully prototype the necessary high-performance scenarios.
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. Results of PPPL research have ranged from a portable nuclear
materials detector for anti-terrorist use to universally employed computer
codes for analyzing and predicting the outcome of fusion experiments. 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
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