Researchers from Berkeley Lab’s Accelerator Technology & Applied Physics Division are working with colleagues from the Lab’s Physics and Engineering Divisions to create a source of high-energy muons for a powerful new imaging technique capable of penetrating solid objects much thicker than is possible with current technologies. The work could pave the way for a powerful new imaging tool for applications in scientific research, scanning for hazardous, radioactive, and explosive materials and probing deep into mountains and underground structures, such as mines and bunkers.

The “Laser-plasma muon source: BELLA-µ” project is supported by funding from the Defense Advanced Research Projects Agency (DARPA)’s Muons for Science and Security Program (MuS2). The four-year project (split into two phases) seeks to create a compact and transportable source of muons with energies in the tens to 100 giga-electron volts (GeV) range.

The first phase of BELLA-µ “will focus on proof-of-concept experiments for efficiently creating muons in the 10 GeV energy range,” says the project’s director, Jeroen van Tilborg, a staff scientist and deputy director of experiments in the BELLA Center at ATAP.

“The project’s second phase seeks to push this demonstration to 20 GeV and lay a credible path for a compact source of 100 GeV (or greater) muons for use in a transportable imaging device.”

Muons are fundamental particles similar to electrons but about 200 hundred times heavier. Since they are much more deeply penetrating than X-rays, gamma rays, and protons, muon imaging can probe structures made from solid rock or concrete walls tens to hundreds of meters thick—well beyond the capabilities of current imaging techniques. By detecting changes in the density and composition of these objects, muon imaging can provide accurate three-dimensional images of their interiors.

However, while muons are naturally produced by cosmic rays colliding with particles in the Earth’s atmosphere, their numbers are insufficient for many applications. Producing a compact source of terrestrial muons with energies in the 100 GeV range (and in sufficient numbers) to create accurate images is, therefore, very challenging, says van Tilborg.

While conventional accelerators can accelerate particles to high energies, he says accelerating muon-producing electrons to 100 GeV-energies “would require an accelerator kilometers-long.”

Harnessing the power of laser-plasma acceleration

The BELLA-µ team will use advanced laser-plasma accelerators, or LPAs, to produce high-energy electrons to create a compact source of high-energy muons. LPAs can sustain accelerating gradients a thousand times those of conventional accelerators and can accelerate electrons to GeV-energy levels over centimeters. These electrons are fired at a metal composite target to produce high-energy muons.

To achieve this, the researchers will undertake modeling and simulation studies during the project’s first phase to optimize the LPA system and conduct experiments to produce muon beams with energies in the 10 GeV range on the BELLA Petawatt laser facility. The BELLA Center recently published a significant milestone toward this goal by accelerating electrons to 8 GeV in just over 20 cm as part of the ongoing program funded by the Department of Energy (DOE) High Energy Physics (HEP) program for developing LPAs for use in high-energy particle physics colliders to explore fundamental physics.

“What’s important for the first phase of the project from the LPA side,” explains Anthony Gonsalves, a staff scientist and associate deputy director for experiments at BELLA, who is leading the project’s plasma acceleration work, “is to produce electron beams with as high an energy as possible—10 GeV and possibly beyond—for muon production.”

Gonsalves says they plan to keep the high-intensity laser pulses (that accelerate the electrons) focused over tens of centimeters in a “plasma channel” to accomplish this. “We will have to carefully tune the laser pulses and plasma channel in novel ways to achieve 10 GeV beams with the available laser energy.”

He adds that comparing simulation results with experimental data will identify the optimum efficiency conditions, thereby defining “the laser and plasma needs for a future transportable system.”

In the program’s second phase, the team aims to demonstrate proof of concept toward a 100-GeV system by developing a two-stage acceleration system at 20 GeV, in which each 10 GeV stage is driven by roughly half of the power of the BELLA Petawatt laser and an independent plasma channel.

“This,” says van Tilborg, “will lay the groundwork for multiple stages capable of sequentially boosting the electrons by 10 GeV to a final beam energy of 100 GeV and thereby validating the path to a scalable 100 GeV transportable muon source.”

The BELLA Center has previously performed a proof-of-principle for a two-stage acceleration experiment at lower (tens of terawatt) laser power under the DOE HEP program.

“This allowed us to develop critical building blocks such as plasma mirrors for laser coupling and active plasma lenses for inter-stage electron beam transport,” says Gonsalves. “The BELLA-µ project will allow us to demonstrate optimized guiding and acceleration in two stages, meaning maximum energy boost in each stage.”

At the same time, he adds, a usable source would have to operate at a much higher pulsing rate than current lasers allow. The program is advancing designs for combining many fiber lasers and related laser technologies to drive the source at thousands of pulses per second (kilohertz) with high efficiency. This technology is foundational to future applications of LPAs at DARPA, HEP, Accelerator R&D and Production, and a key area of national research and ATAP initiatives.

To meet the MuS2 program’s requirements, the Lab has partnered with researchers from other national laboratories, universities, and industry, including Lawrence Livermore National Lab, the University of Maryland, the University of Michigan, Colorado State University, RadiaSoft, RadiaBeam Technologies, and Northrop Grumman Corporation.

The BELLA-µ team will leverage ATAP’s leadership in LPAs and lasers and the world-class facilities at BELLA, which houses four LPA systems, including the Petawatt laser and a state-of-the-art fiber laser combining laboratory. This experienced team includes Eric Buice and Haris Muratagic from the Lab’s Engineering Division; Kei Nakamura, Stanimir Kisyov, Alex Picksley, Davide Terzani, Carl Schroeder, Carlo Benedetti, Tong Zhou, and Eric Esarey from BELLA; and John Valentine, head of the Lab’s Office for National and Homeland Security.

The project will also benefit from state-of-the-art muon detectors developed by Timon Heim, Maurice Garcia-Sciveres, and colleagues from Berkeley Lab’s Physics Division and established relationships with its partners, who bring world-class laser, LPA, systems engineering, and applications knowledge to the project.

“To create a new generation of active muon sources, we brought together a team of scientific and engineering experts and world leaders in their fields, who are moving forward quickly with initial indications of muon generation meeting phase 1 goals ahead of schedule,” says ATAP Division Director Cameron Geddes, the project’s principal investigator. “The work could lay the foundations for a powerful new tool for breakthrough science, imaging, and applications in radiation and hazardous materials detection and national security.”


To learn more …

Making Muons for Scientific Discovery, National Security



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