Orbital angular momentum of a $\pi$-pulse emission by dense relativistic cold electron beam

TL;DR

This paper models the transfer of orbital angular momentum in free-electron laser emission using a quantum fluid approach, revealing how a $$-pulse can amplify and compress radiation while conserving angular momentum.

Contribution

It introduces a set of coupled equations for OAM transfer in FELs and demonstrates a $$-pulse solution governed by a transverse overlap factor R.

Findings

Orbital angular momentum transfer obeys total angular momentum conservation.

Amplification and compression of radiation are achieved in the model.

Energy exchange efficiency depends on the transverse overlap factor R.

Abstract

Using a quantum fluid model, a set of three paraxial coupled equations to describe the FEL instability is derived. These equations are solved numerically considering Laguerre-Gaussian modes as the initial conditions to study the transfer of orbital angular momentum (OAM) between them, which satisfies the total angular momentum conservation as matching condition. The amplification and compression of the output radiation is observed and using the separation of variables method, the set of coupled equations, which describes a $\pi$-pulse solution, is obtained in such a way that a transverse overlapping factor, R, governs the energy exchange efficiency of the process.