Direct laser acceleration of electrons: energy gain optimization and application to gamma-ray sources and pair plasmas

This thesis investigates direct laser acceleration (DLA) of electrons in underdense plasmas and demonstrates how to optimize the mechanism to produce multi-GeV, high-charge electron beams for applications such as gamma-ray generation, neutron production, and laboratory pair-plasma studies. In DLA, electrons oscillate within a plasma channel while continuously interacting with the laser field, enabling substantial energy transfer and allowing hundreds of nanocoulombs of charge to reach relativistic energies. The work combines analytical theory, test-particle models, and large-scale quasi-3D particle-in-cell simulations.

The quasi-3D method makes multi-millimeter propagation tractable while retaining essential three-dimensional physics, allowing systematic scans of density, spot size, and laser power. A key contribution is a new scaling law predicting the maximum electron energy attainable in DLA. The analysis shows that optimal acceleration occurs when electrons oscillate at a specific resonant displacement from the channel axis, maximizing sustained interaction with the laser’s transverse field.

This leads to an optimal focusing condition that ties laser power, focal spot, and plasma density together. Simulations indicate that 10-PW lasers can drive electrons to ≈8 GeV with conversion efficiencies of tens of percent. The thesis further extends DLA theory to smoothly varying plasma density profiles typical of experimental conditions. A new approximate invariant is identified, enabling prediction of the maximum energy even when the density evolves. Density tailoring is shown to reduce acceleration length and mitigate radiation-reaction losses at extreme energies.

Finally, the work explores secondary radiation and particle production. DLA electrons generate bright, collimated gamma rays via betatron emission, with analytically predicted critical energies matching simulations from 100-TW to 10-PW regimes. Two positron-generation pathways are demonstrated: Bethe–Heitler production using high-Z targets and a simple single-laser scheme reaching the strong-field QED regime. Overall, the thesis establishes DLA as a powerful method for producing multi-GeV electrons, brilliant gamma-ray beams, and relativistic pair plasmas, offering a comprehensive framework for future multi-petawatt experiments