induction linac

induction linac. An induction linear accelerator is a type of particle accelerator that uses pulsed electromagnetic induction to generate the accelerating voltage for a charged particle beam. Unlike conventional radio frequency linear accelerators, it employs a series of toroidal magnetic cores driven by fast high-voltage pulses to create axial electric fields within accelerating gaps. This design is particularly suited for producing very high-current, short-pulse beams of electrons or ions, finding specialized roles in national security research, materials science, and radiography.

Principle of operation

The fundamental principle relies on Faraday's law of induction, where a changing magnetic flux through a ferrite or metallic glass core induces an electromotive force. A pulsed power supply, often utilizing Marx generator or pulse-forming network technology, drives a primary current through a winding around the core. This rapidly changing current creates a time-varying magnetic field within the core, which in turn induces a strong axial electric field in the accelerating gap that passes through the core's center. The beam, typically from an injector like a field emission cathode or a laser-plasma source, transits these sequential gaps in synchrony with the voltage pulses. The architecture avoids the use of resonant RF cavities, allowing each induction cell to act as a one-to-one transformer where the particle beam itself serves as a secondary winding.

Historical development

The conceptual foundation for using induction for acceleration was laid by the development of the betatron by Donald Kerst at the University of Illinois in the 1940s. The first true induction linear accelerator for electrons was built in the 1960s by a team at Lawrence Livermore National Laboratory, notably including Nicholas Christofilos, as part of the Astron fusion project. Major development was subsequently driven by the needs of the Strategic Defense Initiative and inertial confinement fusion research in the 1970s and 1980s. Pioneering work at Lawrence Berkeley National Laboratory and the Naval Research Laboratory advanced the technology for high-current ion beams. The Dual-Axis Radiographic Hydrodynamic Test facility at the Los Alamos National Laboratory represents a later, large-scale implementation of these principles.

Key components

Essential subsystems include a robust pulsed power system, often based on water dielectric Blumlein lines or spark gap switches, to deliver precise high-voltage pulses. The magnetic cores, made from materials like nickel-zinc ferrite or cobalt-based amorphous metal, must have high magnetic saturation and low loss at the required pulse repetition rates. A high-brightness injector, such as a field emission array or a magnetically insulated diode, provides the initial beam. Beam transport and focusing are managed by a series of solenoid or quadrupole magnet lenses placed between induction cells. Diagnostics like beam current monitors, beam position monitors, and Cherenkov radiation detectors are critical for tuning. A sophisticated trigger generator and timing system synchronizes all pulsed components.

Applications

Primary applications are in the realm of flash radiography, where the high-current, short-pulse beams are used to produce intense bursts of bremsstrahlung X-rays for imaging fast hydrodynamic events, crucial for the stockpile stewardship program at the National Nuclear Security Administration. They are used as drivers for free-electron laser experiments, such as those conducted at the Stanford Linear Accelerator Center. In plasma physics, induction linacs serve as powerful sources for heating plasma and studying warm dense matter. Other uses include serving as injectors for synchrotron light sources, investigating high energy density physics, and potential roles in novel waste transmutation and nuclear propulsion concepts.

Advantages and limitations

Key advantages include an inherent ability to accelerate extremely high beam currents, often exceeding tens of kiloamperes, without suffering from the space charge limitations that plague conventional RF linacs. The modular, non-resonant cell design allows for easy scaling in energy by adding more cells and provides great flexibility in pulse shape and duration. They are relatively immune to beam-loading effects. Significant limitations, however, include a generally lower accelerating gradient compared to advanced superconducting RF structures, leading to longer accelerators for a given final energy. The technology requires complex, high-peak-power pulsed systems with limited repetition rates. Achieving very high beam quality and low emittance is more challenging than with radio frequency quadrupole or drift tube linac designs for ions.

Notable examples

The Advanced Test Accelerator at Lawrence Livermore National Laboratory was a major 50-MeV research machine. The Dual-Axis Radiographic Hydrodynamic Test facility at Los Alamos National Laboratory is a premier dual-axis induction electron accelerator for radiographic testing. The Integrable Optics Test Accelerator at Fermilab incorporates induction-based bunching elements. The Sphinx accelerator at the Centre d'études de Gramat in France is used for radiographic applications. Earlier pioneering machines include the Experimental Test Accelerator at Lawrence Livermore National Laboratory and the Radiographic Linear Accelerator systems developed for the Atomic Weapons Establishment in the United Kingdom.