Net energy gain in direct laser acceleration due to enhanced dephasing induced by an applied magnetic field

Abstract

Even in the situation where an electron interacts with a single plane wave, the well-known dynamical adiabaticity can be broken when an applied magnetic field is present, which will act to increase the dephasing rate of the electron during the interaction. Here we demonstrate this for the case where there is a uniform static magnetic field which is oriented either parallel or perpendicular to the electric field of the incident plane wave, and perpendicular to the direction of its propagation. The described energy gain phenomenon has direct relevance to laser-plasma interactions that involve external magnetic fields generated by laser-driven capacitor coils.

Underdense relativistically thermal plasma produced by magnetically assisted direct laser acceleration

“…While conventional direct laser acceleration in a plane wave (i.e., with B 0 = 0) is reversible, the addition of a weak magnetic field transverse to the laser propagation direction slowly rotates the forward electron momentum into transverse momentum and introduces multi-cycle and half-cycle opportunities for irreversible acceleration. During electron interaction with the short pulse, momentum rotation by the applied magnetic field changes the dephasing rate R = γ − p x /mv φ (where v φ is the phase velocity) over many cycles, allowing the electron to retain energy after the laser has passed [20]. Substantial momentum rotation also occurs during a single half laser cycle when the interaction is sufficiently long [21], and can impart higher net energy than the multi-cycle mechanism.…”

“…In the two-laser configuration, preheating is provided by the short pulse through many-cycle magnetically assisted DLA [20] [illustrated for a single electron in Fig. 1(b)].…”