MARAUDER plasma jet

Overview

The Magnetically Accelerated Ring to Achieve Ultra-high Density and Reach-through (MARAUDER) is a large-scale pulsed-power device operated by the Air Force Research Laboratory (AFRL) in Albuquerque, New Mexico. Its primary function is to generate, accelerate, and study high-density, high-velocity magnetized plasma jets, specifically compact toroids known as spheromaks. MARAUDER is a central facility for investigating the physics of Plasma-Jet-Driven Magneto-Inertial Fusion (PJMIF), an alternative approach to achieving controlled thermonuclear fusion.

In the PJMIF scheme, an array of plasma jets, like those produced by MARAUDER, are directed to converge symmetrically on a separate, magnetized target plasma. The immense kinetic energy of the jets is converted into thermal energy upon collision, creating a solid, imploding "plasma liner" that compresses the target to fusion conditions. This approach combines features of both magnetic confinement fusion (MCF), which uses magnetic fields to insulate a plasma, and inertial confinement fusion (ICF), which uses rapid compression. The goal is to reach the intermediate density and confinement time regime required for Magneto-Inertial Fusion (MIF), potentially offering a lower-cost and more compact path to a fusion reactor compared to large-scale devices like tokamaks.

MARAUDER's research informs the fundamental science of plasma liner formation, jet propagation, and stability, which are critical for the viability of PJMIF. The experimental data it produces is used to validate and refine complex magnetohydrodynamic (MHD) simulations that model the behavior of these extreme plasma states.

Physics / Mechanism

MARAUDER operates as a magnetized coaxial plasma gun, a device designed to use electromagnetic forces to accelerate plasma. The core hardware consists of two concentric, cylindrical electrodes (a central inner electrode and a larger outer electrode) and a magnetic coil system. The process of forming and accelerating a plasma jet can be broken down into several stages:

Gas Injection and Pre-ionization: A fast gas valve injects a puff of neutral gas (typically argon or neon) into the annular space between the electrodes. A pre-ionization system then creates an initial low-density plasma, providing a conductive path for the main electrical discharge.
Spheromak Formation: A magnetic field, known as the "stuffing flux," is generated by a solenoid coil located inside the inner electrode. When the main capacitor bank is discharged across the electrodes, a high-current radial discharge flows through the pre-ionized gas. This current interacts with the pre-existing stuffing flux, causing the magnetic field lines to stretch, twist, and eventually reconnect behind the plasma. This process forms a self-contained, closed-flux magnetic structure known as a spheromak. This toroid of plasma contains its own toroidal and poloidal magnetic fields, which are essential for its stability during flight.
Acceleration: The main discharge current continues to flow behind the newly formed spheromak. This current interacts with its own azimuthal magnetic field, generating a powerful J × B (Lorentz) force that accelerates the spheromak down the length of the coaxial electrodes. MARAUDER is designed with contoured, or "shaped," electrodes that help focus and compress the plasma as it accelerates. The device is powered by a multi-megajoule capacitor bank capable of delivering a peak current of approximately 5 MA.
Ejection and Propagation: The spheromak is ejected from the muzzle of the gun at high velocity, typically exceeding 50 km/s, with some experiments achieving over 100 km/s. The mass of the ejected plasma is typically in the range of 0.5 to 2.0 milligrams. Once in the drift chamber, the spheromak propagates as a self-contained entity, held together by its internal magnetic fields. The stability and integrity of the jet during this free-flight phase are critical for PJMIF applications, as the jet must remain coherent until it impacts the target.

Diagnostics used to study the plasma jets include magnetic probes to measure field strength, interferometers to measure electron density, fast-framing cameras for imaging, and spectrometers to analyze temperature and composition.

Historical Development

The conceptual work behind using plasma jets for fusion originated in the 1970s, but the specific PJMIF concept gained traction in the early 2000s. The development of MARAUDER at AFRL is part of a broader research program into high-energy-density plasmas with dual-use applications in defense and energy.

Early 1990s: The predecessor to MARAUDER, the Compact Toroid Accelerator (CTA), was developed at AFRL to explore the acceleration of spheromaks for various applications.
Late 1990s - Early 2000s: Based on lessons from CTA, the MARAUDER device was designed and constructed. Its larger scale and higher energy capacity were intended to push plasma jet parameters into a regime relevant for MIF experiments. Early experiments focused on demonstrating the formation of stable spheromaks and achieving high velocities.
Mid-2000s: A key milestone was the successful demonstration of high-velocity (>50 km/s) argon plasma jets with sufficient density and coherence. Research during this period, led by figures like Dr. John T. Cassibry and others in the AFRL community, focused on characterizing jet performance and understanding the underlying MHD physics through a combination of experiment and simulation. A significant achievement was demonstrating that the electrode geometry could be tailored to optimize acceleration and focusing.
2010s: Research shifted towards understanding the physics of jet merging and liner formation. While MARAUDER itself is a single-jet device, its experiments are designed to provide the data needed to model a multi-jet array. Studies on jet stability, particularly the tilt and shift instabilities common to spheromaks, became a major focus. The development of advanced diagnostics allowed for more detailed measurements of internal magnetic field structure and plasma parameters during flight.
Late 2010s - Early 2020s: The program continued to refine jet formation techniques and explore different gas types and electrode configurations. The data from MARAUDER became foundational for the design of next-generation PJMIF facilities and for validating the simulation codes used by commercial fusion ventures entering the field.

Current Status

As of 2026, MARAUDER remains an active and crucial experimental platform at AFRL. It is one of the world's leading facilities for generating the high-performance plasma jets required for PJMIF. Current research campaigns are focused on several key areas:

Jet Stability and Coherence: Improving the stability of the spheromak as it propagates over long distances. This involves optimizing the initial formation process and the magnetic field geometry to suppress destructive MHD instabilities.
Advanced Diagnostics: Implementing and refining non-invasive diagnostic techniques to measure the internal properties of the plasma jet without disturbing its flight. This includes techniques like Faraday rotation to map the internal magnetic field.
Simulation Benchmarking: Providing high-fidelity experimental data to benchmark and validate sophisticated MHD simulation codes, such as MACH2 and Lsp. These codes are essential for designing a full-scale PJMIF compression system, which would involve the symmetric convergence of dozens of jets.
Alternative Propellants: Investigating the use of different gases (e.g., neon, deuterium) to tailor the jet's properties, such as its mass, velocity, and atomic number, for optimal liner formation and compression dynamics. The choice of propellant impacts the radiative properties of the resulting plasma liner.

MARAUDER's results continue to be published in leading journals like Physics of Plasmas and presented at major conferences, providing a public-sector knowledge base that supports the broader MIF research community.

Notable Implementations

While MARAUDER is a unique government research facility, its work has directly inspired and informed several commercial and academic efforts in the PJMIF space.

General Fusion: This Canadian company, based in Richmond, British Columbia, is a prominent developer of MTF technology. While their approach uses mechanically driven pistons to compress a liquid metal liner rather than a plasma liner, the underlying physics of compressing a magnetized target shares common principles. Their target is a spheromak, similar in nature to the plasma jets accelerated by MARAUDER.
Helion: Based in Everett, Washington, Helion's approach involves colliding two high-velocity compact toroids (specifically, Field-Reversed Configurations) head-on. Although their accelerator technology and plasma configuration differ from MARAUDER's, the fundamental concept of using the kinetic energy of accelerated plasmoids for fusion is a shared theme.
University of Alabama in Huntsville (UAH): Researchers at UAH's Propulsion Research Center collaborate closely with AFRL and use data from MARAUDER to develop and validate computational models of PJMIF. This academic partnership is crucial for advancing the theoretical understanding of plasma liner physics.
HyperV Technologies Corp.: This company has also been involved in developing plasma gun technology for fusion applications. Their work on the Plasma Liner Experiment (PLX) at Los Alamos National Laboratory was a direct effort to demonstrate the formation of a spherical plasma liner by merging multiple plasma jets, a concept MARAUDER was built to investigate at the single-jet level.

Open Challenges

Despite significant progress, several scientific and engineering challenges must be overcome for the PJMIF concept, informed by MARAUDER, to succeed.

Jet Stability: The spheromak jet is susceptible to a tilt instability, where the entire plasma ring can tumble during flight. While shaping of the electrodes and magnetic fields can mitigate this, ensuring perfect stability over the required propagation distance in a reactor setting remains an open question.
Symmetric Implosion: Achieving a highly symmetric implosion with an array of 30 or more discrete jets is a major challenge. Any asymmetry in the arrival time, mass, or velocity of the jets can lead to non-uniform compression, preventing the target from reaching the required densities and temperatures for ignition. The transition from a set of discrete jets to a uniform, spherical liner must be highly efficient.
Standoff Distance: In a reactor, the plasma guns must be positioned far enough from the fusion target to avoid being damaged by the neutron flux and debris from the fusion event. This requires the jets to propagate over several meters while remaining coherent and focused, a significant challenge that MARAUDER's experiments are designed to address.
Target-Liner Interaction: The physics of how the imploding plasma liner interacts with the magnetized target plasma is complex. Issues such as mixing between the liner and target material could cool the target and quench the fusion reaction. Understanding and controlling this interface is critical to achieving a high energy gain, or Q_plasma.
Repetition Rate: Like all pulsed fusion concepts, a future PJMIF reactor would need to operate at a repetition rate of several Hertz (cycles per second) to be economically viable. The pulsed-power systems and plasma guns would need to be engineered for high reliability and rapid recharging, which is beyond the scope of single-shot experimental devices like MARAUDER.

Outlook

The credible 5-15 year trajectory for MARAUDER and the PJMIF concept it supports involves a methodical progression from single-jet physics to multi-jet integration experiments. In the near term (5 years), MARAUDER will likely continue to focus on optimizing single-jet performance, particularly in achieving higher velocities and densities while maintaining stability. This period will also see increased integration of experimental results with high-performance computing to create predictive models of jet behavior.

Looking further ahead (5-10 years), the knowledge gained from MARAUDER and complementary experiments like the former PLX project will inform the design and construction of a next-generation, multi-jet facility. Such a device would be the first to test the formation of a fully spherical plasma liner by merging dozens of jets, directly addressing the critical challenge of implosion symmetry. This would be a major step towards demonstrating the scientific feasibility of the PJMIF concept.

Within a 15-year timeframe, a successful multi-jet experiment could lead to a full integrated test facility that introduces a magnetized target plasma. This would aim to demonstrate significant compression and neutron yield, validating the core principles of PJMIF and potentially achieving conditions close to the Lawson criterion. The ultimate success of this approach depends on solving the key challenges of stability, symmetry, and standoff, with MARAUDER providing the foundational single-jet physics understanding required for each subsequent step.

References

Experimental results for an argon plasma-jet-driven, magneto-inertial-fusion relevant, spherically imploding plasma liner — Physics of Plasmas (2017)
Stagnation of a high-velocity plasma jet in the MARAUDER experiment — Physics of Plasmas (2008)
Spheromak formation in a coaxial gun with a stabilizing conducting wall — Physics of Plasmas (2012)
Plasma-Jet-Driven Magneto-Inertial Fusion — Journal of Fusion Energy (2017)
Development of a Formative Stage of a Plasma Liner for Plasma-Jet-Driven Magneto-Inertial Fusion — Physical Review Letters (2012)
Overview of the Plasma Liner Experiment–Alpha (PLX-α) Project — IEEE Transactions on Plasma Science (2021)
Numerical Investigation of the Effects of Electrode Geometry on a Coaxial Plasma Accelerator — AIAA Propulsion and Energy Forum (2018)
Magnetized coaxial gun spheromak experiment — Review of Scientific Instruments (1996)