Star Thrust Experiment

Last updated December 13, 2023

The Star Thrust Experiment (STX) was a plasma physics experiment at the University of Washington's Redmond Plasma Physics Laboratory which ran from 1999 to 2001.^[1] The experiment studied magnetic plasma confinement to support controlled nuclear fusion experiments. Specifically, STX pioneered the possibility of forming a Field-reversed configuration (FRC) by using a Rotating Magnetic Field (RMF).

Background

FRCs are of interest to the plasma physics community because of their confinement properties and their small size. While most large fusion experiments in the world are tokamaks, FRCs are seen as a viable alternative because of their higher Beta, meaning the same power output could be produced from a smaller volume of plasma, and their good plasma stability.

History

The STX was built in 1998. The STX was motivated by a discovery from an unrelated experiment; a few years previously, the Large-S Experiment (LSX) had demonstrated the existence of a kinetically stabilized parameter regime which appeared advantageous for a fusion reactor. However, the LSX experiment formed FRCs in a power-hungry, violent way called a theta-pinch.

The US Department of Energy funded the Translation Confinement Sustainment (TCS) program as a follow-on to the LSX program, but it had not yet begun when the STX started operation. The purpose of TCS was to see whether Rotating Magnetic Fields could sustain FRCs born of the theta-pinch method, but the question remained as to whether RMF alone could form FRCs. If so, this was expected to be a lighter, more efficient means of FRC formation.^[1] This was the question that the STX was meant to answer.

The STX was contemporary with the following RMF-FRC experiments: The TCS, the PFRC, and the PV Rotamak.

Relevance to spacecraft propulsion

NASA funded the construction of the experiment.^[1] This is because FRC-based fusion reactors appear to be well-suited to deep-space fusion rockets, especially those formed by RMF.^[2] This concept is similar to the Direct Fusion Drive, a current research project to create a fusion rocket from an RMF-driven FRC fusion reactor.

Apparatus

The STX vacuum vessel was made of quartz, as it needed to be non-conductive to allow the RMF to pass through. It was 3 meters long and 40 centimeters in diameter. The axial magnetic field was created by electromagnetic coils and was 100 Gauss in strength.^[3] The RMF was created by a novel solid-state RF amplifier which was designed to be more powerful and more efficient than preceding Rotamak experiments.^[4] The RMF system as run operated at 350 kHz, at 2 MW of power, far below its design rating.

To measure the plasma's behavior, the STX experiment was fitted with an insertable magnetic probe, an array of diamagnetic loops, an interferometer, visible-light spectroscopy diagnostics, and a triple Langmuir probe.^[5]

Contributions

The STX experiment was able to use RMF to achieve temperatures of 40 eV, which is hotter than the surface of the Sun but still a factor of 500 from the temperatures necessary in a fusion reactor. The STX experiment was able to achieve plasma density of $5\times 10^{12}$ particles per cubic centimeter, which is a factor of 200 from the temperatures necessary in a fusion reactor.^[3]

While the STX was designed to demonstrate the formation of an FRC using RMF,^[1] it had more success in demonstrating the build-up and sustainment of FRCs created via the theta-pinch method.^[3]

Shortcomings

An FRC plasma is harder to heat at low temperature. Because of this, the RMF system on the STX was designed to produce dozens of MW at the beginning of the discharge to rapidly heat the plasma beyond this so-called "radiation barrier" to hundreds of eV of temperature, where the plasma could be more easily sustained.^[1] However, problems with the novel solid-state RF amplifier led to only a fraction of this power being available for heating.^[5] As a result, rather than the hundreds of eV hoped for, only 40 eV of temperature was achieved.

Furthermore, it was initially hoped for that the plasma could be kept away from the walls of the vacuum vessel by using low-resistance loops of copper that fit snugly around the vessel called "flux conservers."^[1] However the plasma was often observed to be in contact with the 40 cm inner diameter quartz vessel.^[3]

Legacy

The findings of STX were used to improve the TCS experiment, which eventually did demonstrate FRC formation solely from RMF. The TCS went on to heat the plasma to 350 eV.^[6]

The idea of using an RMF-driven FRC to create a fusion rocket persists to this day. One example is the Direct Fusion Drive.

Related Research Articles

A stellarator is a plasma device that relies primarily on external magnets to confine a plasma. Scientists researching magnetic confinement fusion aim to use stellarator devices as a vessel for nuclear fusion reactions. The name refers to the possibility of harnessing the power source of the stars, such as the Sun. It is one of the earliest fusion power devices, along with the z-pinch and magnetic mirror.

The stability of a plasma is an important consideration in the study of plasma physics. When a system containing a plasma is at equilibrium, it is possible for certain parts of the plasma to be disturbed by small perturbative forces acting on it. The stability of the system determines if the perturbations will grow, oscillate, or be damped out.

Fusion power is a proposed form of power generation that would generate electricity by using heat from nuclear fusion reactions. In a fusion process, two lighter atomic nuclei combine to form a heavier nucleus, while releasing energy. Devices designed to harness this energy are known as fusion reactors. Research into fusion reactors began in the 1940s, but as of 2023, no device has reached net power.

A reversed-field pinch (RFP) is a device used to produce and contain near-thermonuclear plasmas. It is a toroidal pinch which uses a unique magnetic field configuration as a scheme to magnetically confine a plasma, primarily to study magnetic confinement fusion. Its magnetic geometry is somewhat different from that of the more common tokamak. As one moves out radially, the portion of the magnetic field pointing toroidally reverses its direction, giving rise to the term reversed field. This configuration can be sustained with comparatively lower fields than that of a tokamak of similar power density. One of the disadvantages of this configuration is that it tends to be more susceptible to non-linear effects and turbulence. This makes it a useful system for studying non-ideal (resistive) magnetohydrodynamics. RFPs are also used in studying astrophysical plasmas, which share many common features.

<span class="mw-page-title-main">Magnetic confinement fusion</span> Approach to controlled thermonuclear fusion using magnetic fields

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, along with inertial confinement fusion.

A field-reversed configuration (FRC) is a type of plasma device studied as a means of producing nuclear fusion. It confines a plasma on closed magnetic field lines without a central penetration. In an FRC, the plasma has the form of a self-stable torus, similar to a smoke ring.

A spheromak is an arrangement of plasma formed into a toroidal shape similar to a smoke ring. The spheromak contains large internal electric currents and their associated magnetic fields arranged so the magnetohydrodynamic forces within the spheromak are nearly balanced, resulting in long-lived (microsecond) confinement times without external fields. Spheromaks belong to a type of plasma configuration referred to as the compact toroids. A spheromak can be made and sustained using magnetic flux injection, leading to a dynomak.

The Madison Symmetric Torus (MST) is a reversed field pinch (RFP) physics experiment with applications to both fusion energy research and astrophysical plasmas.

The polywell is a design for a fusion reactor based on two ideas: heating ions by concentrating (-) charge to accelerate the ions and trapping a diamagnetic plasma inside a cusp field.

The Helically Symmetric Experiment, is an experimental plasma confinement device at the University of Wisconsin–Madison, with design principles that are intended to be incorporated into a fusion reactor. The HSX is a modular coil stellarator which is a toroid-shaped pressure vessel with external electromagnets which generate a magnetic field for the purpose of containing a plasma. It began operation in 1999.

Magnetized Target Fusion (MTF) is a fusion power concept that combines features of magnetic confinement fusion (MCF) and inertial confinement fusion (ICF). Like the magnetic approach, the fusion fuel is confined at lower density by magnetic fields while it is heated into a plasma. As with the inertial approach, fusion is initiated by rapidly squeezing the target to greatly increase fuel density and temperature. Although the resulting density is far lower than in ICF, it is thought that the combination of longer confinement times and better heat retention will let MTF operate, yet be easier to build. The term magneto-inertial fusion (MIF) is similar, but encompasses a wider variety of arrangements. The two terms are often applied interchangeably to experiments.

Compact toroids are a class of toroidal plasma configurations that are self-stable, and whose configuration does not require magnet coils running through the center of the toroid. They are studied primarily in the field of fusion energy, where the lack of complex magnets and a simple geometry may allow the construction of dramatically simpler and less expensive fusion reactors.

TAE Technologies, formerly Tri Alpha Energy, is an American company based in Foothill Ranch, California developing aneutronic fusion power. The company's design relies on an advanced beam-driven field-reversed configuration (FRC), which combines features from accelerator physics and other fusion concepts in a unique fashion, and is optimized for hydrogen-boron fuel, also known as proton-boron and p-B11. It regularly publishes theoretical and experimental results in academic journals with hundreds of publications and posters at scientific conferences and in a research library hosting these articles on its website. TAE has developed five generations of original fusion platforms with a sixth currently in development. It aims to manufacture a prototype commercial fusion reactor by 2030.

Dynomak is a spheromak fusion reactor concept developed by the University of Washington using U.S. Department of Energy funding.

The Princeton field-reversed configuration (PFRC) is a series of experiments in plasma physics, an experimental program to evaluate a configuration for a fusion power reactor, at the Princeton Plasma Physics Laboratory (PPPL). The experiment probes the dynamics of long-pulse, collisionless, low s-parameter field-reversed configurations (FRCs) formed with odd-parity rotating magnetic fields. FRCs are the evolution of the Greek engineer's Nicholas C. Christofilos original idea of E-layers which he developed for the Astron fusion reactor. The PFRC program aims to experimentally verify the physics predictions that such configurations are globally stable and have transport levels comparable with classical magnetic diffusion. It also aims to apply this technology to the Direct Fusion Drive concept for spacecraft propulsion.

The Prairie View (PV) Rotamak is a plasma physics experiment at Prairie View A&M University. The experiment studies magnetic plasma confinement to support controlled nuclear fusion experiments. Specifically, the PV Rotamak can be used as either a spherical tokamak or a field-reversed configuration. Some time between 2015 and 2017, most personnel moved on to advanced career opportunities. In 2017, a Final Report to Department of Energy (DOE) was prepared and submitted by Dr. Saganti of PVAMU on the entire research work supported by DOE for 12 years.

The Translation Confinement Sustainment experiment (TCS) was a plasma physics experiment at the University of Washington's Redmond Plasma Physics Laboratory from 2002 until 2009. The experiment studied magnetic plasma confinement to support controlled nuclear fusion experiments. Specifically, TCS pioneered the sustainment and heating of a Field-Reversed Configuration (FRC) by Rotating Magnetic Field (RMF).

Wendelstein 7-AS was an experimental stellarator which was in operation from 1988 to 2002 by the Max Planck Institute for Plasma Physics (IPP) in Garching. It was the first of a new class of advanced stellarators with modular coils, designed with the goal of developing a nuclear fusion reactor to generate electricity.

The history of nuclear fusion began early in the 20th century as an inquiry into how stars powered themselves and expanded to incorporate a broad inquiry into the nature of matter and energy, as potential applications expanded to include warfare, energy production and rocket propulsion.

References

1 2 3 4 5 6 Miller, Kenneth; Slough, John; Hoffman, Alan (1998). "An overview of the star thrust experiment". AIP Conference Proceedings. AIP. 420: 1352–1358. Bibcode:1998AIPC..420.1352M. doi:10.1063/1.54907.
↑ Slough, John; Miller, Kenneth (1999-06-20). "FRC fusion propulsion system for deep space exploration results from the Star Thrust Experiment (STX)". 35th Joint Propulsion Conference and Exhibit. Reston, Virginia: American Institute of Aeronautics and Astronautics. doi:10.2514/6.1999-2705.
1 2 3 4 Slough, J. T.; Miller, K. E. (2000). "Flux generation and sustainment of a field reversed configuration with rotating magnetic field current drive". Physics of Plasmas. 7 (5): 1945–1950. Bibcode:2000PhPl....7.1945S. doi:10.1063/1.874019. ISSN 1070-664X.
↑ Slough, J. T.; Miller, K. E.; Lotz, D. E.; Kostora, M. R. (2000). "Multimegawatt solid state rf driver for generating rotating magnetic fields". Review of Scientific Instruments. 71 (8): 3210–3213. Bibcode:2000RScI...71.3210S. doi:10.1063/1.1304873. ISSN 0034-6748.
1 2 Elric, Miller, Kenneth (2001). The star thrust experiment, rotating magnetic field current drive in the field reversed configuration (Thesis).{{cite thesis}}: CS1 maint: multiple names: authors list (link)
↑ Guo, H. Y.; Hoffman, A. L.; Milroy, R. D.; Steinhauer, L. C.; Brooks, R. D.; Deards, C. L.; Grossnickle, J. A.; Melnik, P.; Miller, K. E. (2008). "Improved confinement and current drive of high temperature field reversed configurations in the new translation, confinement, and sustainment upgrade device". Physics of Plasmas. 15 (5): 056101. Bibcode:2008PhPl...15e6101G. doi:10.1063/1.2837056. ISSN 1070-664X.

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[Miller_1998-1] 1 2 3 4 5 6 Miller, Kenneth; Slough, John; Hoffman, Alan (1998). "An overview of the star thrust experiment". AIP Conference Proceedings. AIP. 420: 1352–1358. Bibcode:1998AIPC..420.1352M. doi:10.1063/1.54907.

[2] Slough, John; Miller, Kenneth (1999-06-20). "FRC fusion propulsion system for deep space exploration results from the Star Thrust Experiment (STX)". 35th Joint Propulsion Conference and Exhibit. Reston, Virginia: American Institute of Aeronautics and Astronautics. doi:10.2514/6.1999-2705.

[Slough_2000-3] 1 2 3 4 Slough, J. T.; Miller, K. E. (2000). "Flux generation and sustainment of a field reversed configuration with rotating magnetic field current drive". Physics of Plasmas. 7 (5): 1945–1950. Bibcode:2000PhPl....7.1945S. doi:10.1063/1.874019. ISSN 1070-664X.

[4] Slough, J. T.; Miller, K. E.; Lotz, D. E.; Kostora, M. R. (2000). "Multimegawatt solid state rf driver for generating rotating magnetic fields". Review of Scientific Instruments. 71 (8): 3210–3213. Bibcode:2000RScI...71.3210S. doi:10.1063/1.1304873. ISSN 0034-6748.

[Elric_2001-5] 1 2 Elric, Miller, Kenneth (2001). The star thrust experiment, rotating magnetic field current drive in the field reversed configuration (Thesis).{{cite thesis}}: CS1 maint: multiple names: authors list (link)

[6] Guo, H. Y.; Hoffman, A. L.; Milroy, R. D.; Steinhauer, L. C.; Brooks, R. D.; Deards, C. L.; Grossnickle, J. A.; Melnik, P.; Miller, K. E. (2008). "Improved confinement and current drive of high temperature field reversed configurations in the new translation, confinement, and sustainment upgrade device". Physics of Plasmas. 15 (5): 056101. Bibcode:2008PhPl...15e6101G. doi:10.1063/1.2837056. ISSN 1070-664X.

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