Generalized description of the efficient electron acceleration in ion channels by multi-PW lasers*

Direct laser acceleration (DLA) is a mechanism of electron acceleration by relativistic laser pulses propagating through underdense plasmas. It is based on the so-called betatron resonance between the simultaneous electron oscillations in the background field of an ion channel and in the field of the laser pulse. This mechanism provides a high total charge of accelerated electrons (100s of nC was already achieved in experiments). Plasma interaction with 10 PW laser pulses has the potential to provide electrons with energies up to 10 GeV through DLA. Apart from being a source of high-energy electrons, it also can provide high flux gamma-ray radiation. This mechanism of electron acceleration is naturally present and affects the electron heating in the interaction of lasers with preplasma of solid targets, which is important to study for high- intensity pulses of next generation.

We present analytical scaling laws for electron energies as a function of the plasma density and the laser intensity. Furthermore, we discuss the effects of varying plasma density on acceleration. We derive conserved quantities that allow to predict scaling laws for arbitrary slowly varying profiles. Analytical predictions are in good agreement with the simplified test-particle model as well as with Quasi-3D PIC simulations performed with the code OSIRIS. Furthermore, we provide guide for laser focusing in order to optimize future DLA-based electron and radiation sources that can be directly verified at multi-PW laser facilities [1].