The National Spherical Torus Experiment (NSTX) is a

magnetic fusion Magnetic confinement fusion (MCF) is an approach to generate thermonuclear fusion power that uses magnetic fields to confine fusion fuel in the form of a plasma. Magnetic confinement is one of two major branches of controlled fusion research, alon ...

device based on the ''

spherical tokamak A spherical tokamak is a type of fusion power device based on the tokamak principle. It is notable for its very narrow profile, or ''aspect ratio''. A traditional tokamak has a toroidal confinement area that gives it an overall shape similar to ...

'' concept. It was constructed by the

Princeton Plasma Physics Laboratory Princeton Plasma Physics Laboratory (PPPL) is a United States Department of Energy national laboratory for plasma physics and nuclear fusion science. Its primary mission is research into and development of fusion as an energy source. It is know ...

(PPPL) in collaboration with the

Oak Ridge National Laboratory Oak Ridge National Laboratory (ORNL) is a federally funded research and development centers, federally funded research and development center in Oak Ridge, Tennessee, United States. Founded in 1943, the laboratory is sponsored by the United Sta ...

Columbia University Columbia University in the City of New York, commonly referred to as Columbia University, is a Private university, private Ivy League research university in New York City. Established in 1754 as King's College on the grounds of Trinity Churc ...

, and the

University of Washington The University of Washington (UW and informally U-Dub or U Dub) is a public research university in Seattle, Washington, United States. Founded in 1861, the University of Washington is one of the oldest universities on the West Coast of the Uni ...

at Seattle. It entered service in 1999. In 2012 it was shut down as part of an upgrade program and became NSTX-U, U for Upgrade. Like other

magnetic confinement fusion Magnetic confinement fusion (MCF) is an approach to generate thermonuclear fusion power that uses magnetic fields to confine fusion fuel in the form of a plasma (physics), plasma. Magnetic confinement is one of two major branches of controlled fusi ...

experiments, NSTX studies the physics principles of thermonuclear plasmas—ionized gases with sufficiently high temperatures and densities for

nuclear fusion Nuclear fusion is a nuclear reaction, reaction in which two or more atomic nuclei combine to form a larger nuclei, nuclei/neutrons, neutron by-products. The difference in mass between the reactants and products is manifested as either the rele ...

to occur—which are confined in a magnetic field. The spherical tokamak design implemented by NSTX is an offshoot of the conventional

tokamak A tokamak (; ) is a device which uses a powerful magnetic field generated by external magnets to confine plasma (physics), plasma in the shape of an axially symmetrical torus. The tokamak is one of several types of magnetic confinement fusi ...

. Proponents claim that spherical tokamaks have dramatic practical advantages over conventional tokamaks. For this reason the spherical tokamak has seen considerable interest since it was proposed in the late 1980s. However, development remains effectively one generation behind mainline tokamak efforts such as JET. Other major spherical tokamak experiments include the

START Start can refer to multiple topics: * Takeoff, the phase of flight where an aircraft transitions from moving along the ground to flying through the air * Starting lineup in sports * Track and field#Starts use in race, Starts use in sport race * S ...

and

MAST Mast, MAST or MASt may refer to: Engineering * Mast (sailing), a vertical spar on a sailing ship * Flagmast, a pole for flying a flag * Guyed mast, a structure supported by guy-wires * Mooring mast, a structure for docking an airship * Radio mas ...

Culham Culham is a village and Civil parishes in England, civil parish in a bend of the River Thames, south of Abingdon, Oxfordshire, Abingdon in Oxfordshire. The parish includes Culham Science Centre and Europa School UK (formerly the European Sch ...

in the UK.

History

1999–2012

First plasma was obtained on NSTX on Friday, February 12, 1999 at 7:06 p.m. Magnetic fusion experiments use plasmas composed of one or more hydrogen

isotope Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...

s. For example, in 1994, PPPL's Tokamak Fusion Test Reactor (

TFTR The Tokamak Fusion Test Reactor (TFTR) was an experimental tokamak built at Princeton Plasma Physics Laboratory (PPPL) circa 1980 and entering service in 1982. TFTR was designed with the explicit goal of reaching scientific breakeven, the point w ...

) produced a world-record 10.7 megawatts of fusion power from a plasma composed of equal parts of

deuterium Deuterium (hydrogen-2, symbol H or D, also known as heavy hydrogen) is one of two stable isotopes of hydrogen; the other is protium, or hydrogen-1, H. The deuterium nucleus (deuteron) contains one proton and one neutron, whereas the far more c ...

and

tritium Tritium () or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with a half-life of ~12.33 years. The tritium nucleus (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus of the ...

, a fuel mix likely to be used in commercial fusion power reactors. NSTX was a "proof of principle" experiment and therefore employed deuterium plasmas only. If successful it was to be followed by similar devices, eventually including a demonstration power reactor (e.g.

ITER ITER (initially the International Thermonuclear Experimental Reactor, ''iter'' meaning "the way" or "the path" in Latin) is an international nuclear fusion research and engineering megaproject aimed at creating energy through a fusion process s ...

), burning deuterium-tritium fuel. NSTX produced a spherical plasma with a hole through its center (a "cored apple" profile; see

), different from the doughnut-shaped (toroidal) plasmas of conventional

s. The low aspect ratio ''A'' (that is, an ''R''/''a'' of 1.31, with the major radius ''R'' of 0.85 m and the minor radius ''a'' of 0.65 m) experimental NSTX device had several advantages including plasma stability through improved confinement. Design challenges include the toroidal and poloidal field coils, vacuum vessels and

plasma-facing components In nuclear fusion power research, the plasma-facing material (or materials) (PFM) is any material used to construct the plasma-facing components (PFC), those components exposed to the plasma within which nuclear fusion occurs, and particularly t ...

. This plasma configuration can confine a higher pressure plasma than a doughnut tokamak of high aspect ratio for a given, confinement magnetic field strength. Since the amount of fusion power produced is proportional to the square of the plasma pressure, the use of spherically shaped plasmas could allow the development of smaller, more economical and more stable fusion reactors. NSTX's attractiveness may be further enhanced by its ability to trap a high "bootstrap" electric current. This self-driven internal plasma current would reduce the power requirements of externally driven plasma currents required to heat and confine the plasma.

Upgrade 2012–2015

The $94 million NSTX-U (Upgrade) was completed in 2015. It doubles the toroidal field (to 1 Tesla), plasma current (to 2 MA) and heating power. It increases the pulse duration by a factor of five.The Role of the Spherical Tokamak in the U.S. Fusion Energy Sciences Program
Menard, 2012 To achieve this the central stack (CS)

solenoid upright=1.20, An illustration of a solenoid upright=1.20, Magnetic field created by a seven-loop solenoid (cross-sectional view) described using field lines A solenoid () is a type of electromagnet formed by a helix, helical coil of wire whos ...

was widened, and an OH coil, inner poloidal coils, and a 2nd neutral-ion beam line were added. This upgrade consisted of a copper coil installation, not a superconducting coil.

Poloidal coil problem 2016 and Recovery 2016–present

The NSTX-U (Upgrade) was stopped in late 2016 just after its update, due to a failure of one its poloidal coils. The NSTX had been shut down since 2012 and only returned for 10 weeks at the end of 2016 just after it was updated. The origin of this failure is partly attributed to a non-compliance of the chilled copper winding, the manufacture of which had been sub-contracted. After a diagnostic phase requiring the complete dismantling of the device and coils, evaluation of the design, and a redesign of major components including the six inner poloidal coils, a restarting plan was adopted in March 2018, with reactivation scheduled for the end of 2020, though this was later pushed back to 2022. As of 2022, the restart was still delayed due to an insulation problem between the central solenoid and the coils around it.

References

Sources

* – Original source for this article was an earlier version of this page. *
Schematic diagram of NSTX-U Facility Upgraded Components

External links

Overview of The National Spherical Torus Experiment Upgrade
from PPPL {{fusion experiments Tokamaks Princeton Plasma Physics Laboratory