Scientific Achievement

Various simplified analytical and computer simulation models are used to understand better the underlying fundamental physics of laser-driven ion acceleration, which is challenging to decipher from the results of experiments alone. In an attempt to bridge the gap between theory, simulations, and experiments, researchers from the BELLA Center and the Advanced Modeling Program in the Accelerator Technology & Applied Physics (ATAP) Division at Lawrence Berkeley National Laboratory (Berkeley Lab) explored the effect of target expansion before the arrival of the peak of the laser pulse and its consequences for laser ion acceleration.

Significance and Impact

Laser ion acceleration is achieved when high-intensity lasers irradiate targets of different thicknesses, densities, and compositions. The resulting interaction is highly non-linear, preventing its study using the first principles of plasma physics. Instead, multiple phenomenological and computer simulation models are used. These models are usually based on simplifying assumptions, which help to understand the physics of the acceleration process but significantly complicate the comparison with the experiment. In this study, ATAP researchers tailored their models and computer simulations to capture some of the effects that might be important in the experiment on the Magnetic Vortex Acceleration (MVA) (see figure) of ions from the foam targets.

The basic laser ion acceleration mechanisms in the plane of dimensionless parameters characterize the laser amplitude and the target transparency.

These targets are tens of microns thick, and their low density keeps them at the transparency threshold for laser radiation. One of the most consequential simplifications is connected with the target density profile, namely, neglecting the target expansion before the arrival of the peak of the laser pulse. This expansion is due to the irradiation by the laser pre-pulse. The researchers determined that the stronger the target expansion, the less efficient the ion acceleration since the ions can no longer be injected into the accelerating electric field, which is generated in a different part of an expanding target from the ion source. They also studied the properties of the ion beam accelerated by MVA. They found it has extremely low emittance, which is ideal for the staged ion acceleration recently explored at ATAP (see Boosting Intense Ion Beam Energies with Hollow-Channel Laser-Plasma Stages).

Research Details

Laser ion acceleration is considered one of the main applications of high-power laser systems, such as BELLA’s petawatt laser. This is due to numerous envisioned and already implemented applications of accelerated ion beams in fundamental physics, material science, medicine, biology, and national security. Laser ion acceleration offers compact, cost-efficient, and highly flexible sources of energetic ions. Laser ion acceleration utilizes different laser pulses (in terms of intensity, duration, and wavelength) and different targets (in terms of density, thickness, and composition), which results in many possible combinations and acceleration mechanisms.

The MVA mechanism describes the laser ion acceleration from foam targets. It generates mega-tesla magnetic fields and tens of teravolts per meter electric fields. The MVA has several advantages in terms of accelerated ion beam properties and the efficiency of transforming laser energy into ion beam energy. These include low and achromatic divergence of ion beams and almost 10% of laser energy transfer into ion beam energy. This resulted in much interest in the experimental study, including several campaigns at the BELLA Center.

Magnetic Vortex Acceleration

The MVA mechanism is realized when an intense laser pulse can drill a hole through a tens-of-micron-thick target, usually a dense gas or foam. While propagating inside the target, the laser generates extreme electron and ion currents, which give rise to MT magnetic fields. When the laser exits the target from the back side, the magnetic fields expand along the back of the target as a vortex.

This vortex interaction with the back of the target leads to the appearance of tens of TV/m electric fields, which can accelerate ions to hundreds of MeV energies according to the analytical estimates and computer modeling of the optimized laser-target interaction. The “optimized” means here that the targets have intact front and back surfaces before the arrival of the laser pulse, which is focused on a particular spot to ensure maximum energy transfer from the laser to the ions. This is usually not the case in actual laser experiments. The ATAP researchers tailored analytical and computer simulation models to account for that.

Target expansion and maximum ion energy decrease

The computer modeling for this paper was done using open-source particle-in-cell code WarpX, the 2022 Gordon Bell-Prize winning plasma simulation code spearheaded by the AMP team and their collaborators, one of the most popular simulation tools in the laser plasma acceleration community. The simulation model was tailored to include the effect of the target expansion due to the interaction with a low-intensity part of the laser pulse. Different scale lengths of the target expansion were considered to account for the various capabilities of an actual laser system in shaping the laser pulses it delivers to the target.

Ion acceleration at the back of the near critical density target due to the generation of the magnetic vortex.

The results of the computer modeling show the robustness of the MVA mechanism since hole-drilling through the targets, electron, and ion current generation, and the formation of extreme magnetic and electric fields was observed for all considered cases of target expansion. However, the energy of the accelerated ion beam decreased with a stronger target expansion. The MVA relies on precisely injecting ions into an accelerating electric field in a small several-micron scale volume over a hundred femtoseconds. The expansion of the target partially breaks this synchronization between the ions and the fields that accelerate them. This is a significant result since it will guide future experiments regarding what maximum ion energy can be expected and how the laser needs to be shaped to increase it.

Ion beam properties

ATAP researchers also examined the properties of the accelerated ion beam, inspired by previous research on staged ion acceleration, which required high-quality ion beams to operate. MVA-generated ion beams have very small source sizes and low divergence, which makes them ideal for staged acceleration. This opens one more potential application for laser ion acceleration.

Contact: Stepan Bulanov

Researchers: Sahel Hakimi, Stepan S. Bulanov, Axel Huebl, Lieselotte Obst-Huebl, Kei Nakamura, Anthony Gonsalves, Thomas Schenkel, Jeroen van Tilborg, Jean-Luc Vay, Carl B. Schroeder, Eric Esarey, and Cameron R. Geddes

Funding: This work was supported by the U.S. Department of Energy (DOE) Office of Science, Offices of Fusion Energy Science, LaserNetUS, High Energy Physics, and DARPA. This research used resources from OLCF through the ASCR Leadership Computing Challenge program and NERSC through the DOE Office of Science NERSC award.

Publication: Sahel Hakimi, Stepan S. Bulanov, Axel Huebl, Lieselotte Obst-Huebl, Kei Nakamura, Anthony Gonsalves, Thomas Schenkel, Jeroen van Tilborg, Jean-Luc Vay, Carl B. Schroeder, Eric Esarey, and Cameron R. Geddes. “Dephasing of ion beams as Magnetic Vortex Acceleration regime transitions into a bubble-like field structure,” https://doi.org/10.48550/arXiv.2409.09156 (accepted for publication in Physics of Plasmas)

For more information on ATAP News articles, contact caw@lbl.gov.