The Pegasus program focuses on the unique features of an extremely low aspect ratio spherical tokamak (ST) that enable world-relevant fusion studies in a university-based experiment. The present program mission is to compare, contrast, and develop solenoid-free startup and possibly sustainment techniques for next-step fusion energy systems. The Pegasus research team is comprised of 1 faculty member, 4 scientists, 4 engineers, 10 graduate students, and several undergraduate students. Students have been actively involved in the design, construction, and operation of Pegasus since its inception.
Shown is a high speed movie of a Pegasus discharge. The bright spots at the start of the discharge are the locations at which electron current is injected into the vacuum vessel to provide a source of DC helicity injection. This is one of the many unique methods of plasma startup being explored at Pegasus.
A grand challenge of science today is to develop the scientific understanding of the properties of high temperature plasmas and to apply that knowledge to support the future development of fusion energy. Harnessing fusion would provide a virtually limitless, environmentally friendly source of power to the world. The Pegasus Toroidal Experiment conducts research that is part of a worldwide effort to enhance our understanding of plasma behavior and magnetic confinement. Click here to learn more about fusion: Fusion Basics
Spherical Tokamak Research
When taken to low aspect ratio (A < 2), a tokamak is referred to as a spherical tokamak (ST). The Pegasus Experiment is an extremely low aspect ratio ST. We use magnetic fields to create a donut-shaped trap in which we create plasmas for study. This trap prevents the extremely hot plasma from touching and melting the inner walls of its container. In comparison with conventional tokamaks with larger central holes, ST’s can sustain a much higher ratio of plasma pressure to applied pressure (β). High relative pressure plasmas are relevant to our basic understanding of plasma behavior. In addition, the compact nature of the ST provides a relatively low-cost path to studying the properties of advanced fusion concepts. Pegasus is part of a worldwide effort to explore the ST as a plasma confinement concept and study the unique physical effects the ST produces.
History of the Pegasus Program
The precursor to Pegasus was the Madison EDUcational Small Aspect-ratio (MEDUSA) tokamak. MEDUSA was a small-scale ST designed and built to observe ohmically produced ST plasmas on a laboratory scale. First conceived as an undergraduate project in early 1992, it eventually served as a testbed for Pegasus during the time Pegasus was being constructed. Major parameters were R = 12 cm, a = 8 cm, IP = 10-40 kA, BT = 0.2-0.45 T, Δtpulse = 1-2 ms, and〈ne〉≈ 5×1019 m-3. The purpose of research on MEDUSA was twofold. One objective was to understand the consumption of ohmic flux by the plasma and the mechanisms responsible for current penetration during startup. The other was to understand the processes responsible for internal reconnection events (IRE’s) in ST’s. Experiments conducted on the machine showed that startup efficiency improved with increasing loop voltage and toroidal field. They also showed that double tearing modes were an important mechanism for current penetration that improved magnetic flux consumption. The study of IRE’s revealed important physical precursors for the first time in an ST.
Pegasus Phase I (1998-2002)
Phase I of the Pegasus Toroidal Experiment was the initial incarnation of the experiment. The facility was a spherical tokamak designed to explore the physics limits of plasma operation as the aspect ratio A approaches unity. Its aim was to characterize plasma behavior and stability limits in this unique operating space. The Phase I facility employed a unique high-stress central solenoid Ohmic for heating and current drive with simple magnet current waveforms. The power supplies consisted mainly of commutated capacitor banks, with a few switched DC power supplies. Major experimental parameters were R = 0.2-0.45 m, A = 1.1-1.2, IP = 0.1-0.4 MA, BT ≤ 0.08 T, Δtpulse = 0.02-0.04 s, and〈βT〉≫ 0.1. This phase of the experiment demonstrated ready access to low-A physics with ohmic heating. It showed important features of these plasmas, including βT up to 20%, βN up to 5, electron density up to the Greenwald limit, low magnetic shear in the plasma center, and significant paramagnetism (up to 50%). A “soft limit” on achievable IP and toroidal field utilization, due to the onset of large low-order internal resonant tearing modes and a reduction in available ohmic flux at reduced toroidal field, was discovered at IP/ITF ∽ 1. Experiments of Pegasus-I also began the exploration of the edge kink stability boundary, finding instability at q95 = 5, which was significantly larger than would be expected for a conventional tokamak. Pegasus-I was a successful first effort to study high-pressure plasmas on an extremely low-A ST while maintaining good confinement and stability.
Pegasus Phase II (2003-2019)
The motivation for upgrading the experiment to Phase II was to provide tools to improve control of discharge evolution. Multiple methods were employed to mitigate the tearing modes that bounded performance on Pegasus-I, including the installation of new power supplies designed for flexible waveform control of the magnetic field coils, non-solenoidal startup via Local Helicity Injection (LHI), and Ohmic operations at extremely low toroidal field current assisted by LHI. Other major distinguishing features of Phase II as compared to Phase I were a new low-inductance, high-strength toroidal field conductor bundle, actively switched magnet power supplies, and significantly improved coil sets. The use of LHI began with the installation of two plasma-arc based current injectors installed in the lower divertor to be used for testing helicity injection as a means of startup. This eventually expanded into an extensive technology and physics development campaign to utilize LHI for plasma startup, heating, and sustainment. LHI enabled access to a unique ST operating space with high IN and high βT, which allowed for the study of extreme stability limits for tokamaks. This, along with strong reconnection-driven anomalous ion heating, provided access to tokamak plasmas with βT up to 100%, a world record 2 times higher than that previously achieved in advanced or spherical tokamaks. Experiments on Pegasus-II validated low-A physics predictions through unique measurements of edge current profiles and H-mode behavior, including the L-H power threshold for extremely low A. Pegasus-II was also the first to access and characterize ELM’s (Edge Localized Modes) at A < 1.3. Multiple unstable toroidal magnet modes were measured during ELM crashes, with generally lower toroidal mode numbers than seen at higher A, which was attributed to the larger peeling mode instability drive at low A. First-ever edge current profile measurements through an ELM crash showed current-carrying filaments were generated.
Pegasus Phase III (2020-)
A major upgrade is underway for a new facility, Pegasus-III. The new facility will provide a dedicated U.S. platform that will compare, contrast, and combine different concepts for solenoid-free startup. This approach will allow for a more integrated study of the leading startup and possibly sustainment techniques, including local helicity injection (LHI), transient and sustained coaxial helicity injection (CHI), radiofrequency heating and current drive, external magnetic field induction, and neutral beam injection. The goal of the facility is to develop a validated concept for 1 MA plasma startup on the U.S. flagship facility NSTX-U and beyond.
Pegasus-III will feature many facility enhancements that will allow for the study of several areas of exciting physics and engineering research. Included in the new facility features are a solenoid-free central column, a stronger, high toroidal field assembly, new power systems, active divertor coils, next-generation LHI, coaxial helicity injection (CHI), rf startup and assist, and expanded diagnostics. Projecting LHI to larger facilities requires tests at increasing toroidal field. Pegasus-III will enable investigation of LHI at increased field levels, allowing for the testing of key physics issues such as as core confinement, reconnection and current drive, stochastic edge transport, and current stream stability. Along with these physics developments is the advancement of LHI technology, including the installation and testing of a non-circular injector concept. Increased toroidal field also enables comparative studies of CHI. Included in this are both improved technology development and more physics studies. This will involve improved CHI technology development, including a new electrode configuration, as well as exploration of CHI physics at higher toroidal field. Pegasus-III will also explore RF heating and current drive capabilities with an initial focus on EBW synergies with helicity injection, namely improved startup via electron heating and EBW current drive as a current sustainment mechanism post-helicity injection startup. Collaborators from the Oak Ridge National Laboratory, the Princeton Plasma Physics Laboratory, and the University of Washington are involved in the Pegasus-III project.
Construction of Pegasus-III is underway, and the facility will be operational in 2020. Please see the Technical Overview pages to learn more about Pegasus-III.