Accelerators of subatomic particles are among the largest and most powerful instruments of scientific discovery. At the energy frontier, where accelerators continue to unravel the structure of matter and forces that shape our understanding of the Universe, a proposed electron-positron linear collider is expected to require 20 km-long accelerators that produce trillion-electron-volt (TeV) beams. High-energy electron accelerators have already driven a revolution in materials science and biology by powering intense sources of radiation, from X-rays to the terahertz (THz) range in wavelengths and energies. Recent machines such as Stanford’s Linac Coherent Light Source are using 15 giga-electro-volt, or GeV, electrons from a kilometer-scale accelerator to generate unprecedented X-ray brightness. Other applications include medical radiotherapy and imaging, as well as MeV photon beams to probe for concealed nuclear material.
Accelerators developed during the past half-century use metallic cavities that shape radio-frequency electromagnetic waves to produce accelerating fields. The electrical breakdown of these cavities limits the maximum accelerating field and therefore the machine’s minimum size for a given energy. In turn, size is a major factor for both cost and location. To scale beyond TeV energies, and to provide brighter (more intense) beams from smaller radiation sources, accelerator scientists are developing machines that greatly increase the accelerating fields, and hence the energy achieved in a given length.
Studying the smallest of particles requires the biggest of “microscopes”
By the late 1960s, machines on the energy frontier, like the Two-Mile Linac of the Stanford Linear Accelerator Center (above left), were already geographic features. Present and future machines, like the notional Future Circular Collider, will be larger still. The FCC is shown above, right in the context of CERN’s Large Hadron Collider, whose 27-km (17-mile) tunnel alone — built for the LHC’s predecessor, the Large Electron-Positron collider — ranked among Europe’s largest civil engineering projects.
BELLA explores a fundamentally different scheme for accelerating electrons that holds the promise of relatively smaller, less costly accelerator facilities. We hope that it will someday enable a new generation of the showpiece colliders of high-energy physics, and meanwhile, also improve the numerous uses of smaller particle accelerators in science, medical treatment, security, and industrial applications.
BELLA research is primarily focused on the development and application of compact laser-plasma accelerators, or LPAs, which achieve electron energies at or above GeV in distances of only centimeters. This is an accelerating gradient thousands of times higher than is achievable through conventional techniques. This high gradient gives the potential to reduce the size of future accelerators for high-energy (particle) physics by more than an order of magnitude. Nearer-term applications for these compact machines are anticipated, such as nuclear nonproliferation and security, free-electron lasers, and cancer treatment. For many of these applications, the intrinsically short (femtosecond) bunch duration of LPAs is also important.
How LPAs work
As mentioned previously, RF accelerators use accelerating structures — resonant metallic cavities in which radio-frequency power sets up electric fields that impart velocity to charged particles. The LPA’s analogue to those accelerating structures is created when the radiation pressure of an intense laser pulse displaces electrons in a plasma channel, initiating plasma oscillations and resulting in a “wake” consisting of a succession of alternating positively and negatively electrically charged regions behind the laser. This is analogous to the wake of a boat, which can carry a surfer. An electron beam located at the appropriate phase behind the laser will be focused transversely and also accelerated longitudinally to (or decelerated from) high energies over a very short distance.
The details of electron beam acceleration and focusing, as well as propagation of the laser pulse, depend critically on the longitudinal and transverse profile of the laser pulse and of the plasma; these factors must be controlled and understood.
The BELLA Center drives advancement in LPAs via experimental and theoretical study of the interaction of intense laser pulses with gas, plasma and solid targets, and related applications and technologies.
Accelerating structures: Traditional linacs use electric fields, set up by powerful radio waves in resonating cavities, to give the beam an energy kick. Precise cavity separation gives the beam its next kick at the right time as its energy increases. In an LPA, the “structure” is a wake field left behind by a laser pulse as it goes through a plasma; electrons can surf on this wake. Here are cavities from a late 1950s heavy-ion linac and a laser-plasma accelerating module. Our research areas is “staging”—stringing together multiple LPA modules to achieve higher energies—and the acceleration of ions.
BELLA is part of the Accelerator Technology and Applied Physics Division of the Lawrence Berkeley National Laboratory.