Berkeley Lab

Plasma Sources

An enabling technology for high-energy, high-quality beams from laser-plasma accelerators


An advanced plasma target for LPA integrates a DeLaval gas jet nozzle (vertical, at left) for injection control into a 3-cm long capillary discharge (horizontal line) which guides the laser. This produced beams with record stability and tunability.

In a laser plasma accelerator (LPA), an intense laser pulse propagating through a plasma excites a plasma wave. The plasma both supports high electric fields that accelerate particles and guides the intense laser pulse allowing acceleration over long distances. Precise control over the plasma density, and spatial tailoring, are critical to increase the beam energy and quality of LPAs. BELLA researchers have a history of innovation in this field.

Transverse tailoring of the density profile to form a plasma waveguide mitigates diffraction and keeps the laser intense over longer distances. This in turn allows for higher energy gain of the accelerated particles. BELLA Center researchers achieve this in multiple ways, including laser heating and subsequent expansion of the plasma; and thermal cooling at the walls of a capillary pre-filled with plasma.

The laser heating technique allowed for high-quality 100 MeV-level beams to be produced for the first time, and the capillary method allowed for a longer and lower density plasma source that produced GeV electron beams for the first time. Looking to the future, BELLA Center researchers are investigating a hybrid plasma waveguide that will allow more control over the plasma properties.

Tailoring of the plasma source in the longitudinal direction can also yield significant benefits. Optimum injection and acceleration require different density profiles. BELLA Center researchers employ a plasma in which the density profile with a short high-density section designed to enable injection of electrons into the wake followed by a long lower density channel to further accelerate the beam. This allows for unprecedented control over LPA electron beams. Longitudinal density transitions can control the wake phase velocity to trigger and control trapping. Gradual density taper can be used to control phasing in the wake and increase energy gain.

Future work that will enable LPA applications include plasma sources to mitigate dephasing, optimize focusing forces in the plasma wakefield, minimize emittance growth, and achieve high repetition rates.