Direct laser acceleration, ion channel, near-critical-density plasmas, radiography and nuclear applications

Resonances are prevalent in daily life. A brigade of soldiers marching at a particular pace across a bridge may cause strong resonances that even break the latter. This indicates strong energy transfer. Such a phenomenon may be very useful for accelerating charged particles.

In high-intensity laser-plasma interactions, a laser pulse may drive an ion channel (bare ions with background electrons expelled out radially) by the enormous light pressure. Injected electrons are thus subject to both the laser oscillation and the betatron oscillation (i.e., the bouncing motion due to the ion-focusing force). The frequency of the latter is normally small compared to the laser. But when the electrons have a high speed, the Doppler effect reduces the laser frequency witnessed by electrons. At proper conditions the two oscillation freuqencies match, resulting in strong resonances and energy tranfer from laser to electrons, so-called direct laser acceleration.

While the physics is straightforward, finding the proper conditions for electrons to be trapped in the resonances is non-trivial because the process is highly nonlinear. We provide a penetrating analysis into this process by questioning the usual paraxial approximation (i.e., electrons have negligible pitch angles relative to the forward axis). Instead, we use the angle as an explicit parameter and unify the dynamics (including arbitrary harmonics) within a single framework. This theory importantly can be used to predict the trapping conditions, a crucial issue for any advanced accelerators.

F.-Y. Li, P. K. Sing, S. Palaniyappan, and C.-K. Huang, Phys. Rev. Accel. Beams 24, 041301 (2021).