Beam-plasma system and its X-ray directivity

TL;DR

This study numerically investigates the evolution of electron distributions in beam-plasma systems with return currents, examining how magnetic fields influence X-ray emission directivity and electron anisotropy.

Contribution

It provides new insights into how magnetic fields affect electron distribution functions and X-ray directivity in beam-plasma interactions using 3D particle-in-cell simulations.

Findings

Electron distribution broadens perpendicular to the beam at low magnetic fields.

Stronger magnetic fields extend the distribution along the beam direction.

X-ray directivity is closer to isotropic than simple beaming models.

Abstract

An evolution of the electron distribution function in the beam-plasma system with the return current is computed numerically for different parameters. The X-ray bremsstrahlung corresponding to such an electron distribution is calculated and the directivity of the X-ray emission is studied. For computations of the electron distribution functions we used a 3-D particle-in-cell electromagnetic code. The directivity of the X-ray emission was calculated using the angle-dependent electron-ion bremsstrahlung cross-section. It was found that the resulting electron distribution function depends on the magnetic field assumed along the electron beam propagation direction. For small magnetic fields the electron distribution function becomes broad in the direction perpendicular to the beam propagation due to the Weibel instability and the return current is formed by the electrons in a broad and shifted bulk of the distribution. On the other hand, for stronger magnetic fields the distribution is more extended in the beam-propagation direction and the return current is formed by the electrons in the extended distribution tail. In all cases, the anisotropy of the electron distribution decreases rapidly due to fast collisionless processes. However, the magnetic field reduces this anisotropy decrease. The X-ray directivity shows the same trend and it is always closer to the isotropic case than that in a simple beaming model.