Relativistic laser pulses can accelerate electrons up to energies of several GeV during their interaction with gaseous targets through the direct laser acceleration (DLA) mechanism. While the electrons are accelerated to high energies, they oscillate transversely to the laser propagation direction, emitting radiation. We demonstrate using particle-in-cell (PIC) simulations that the high accelerated electron charge enables DLA sources to emit photons/0.1% bandwidth (BW) at energies of hundreds of MeV when interacting with multi-petawatt (PW) laser pulses. We provide an analytical estimate of the expected critical frequency for the DLA betatron spectrum which is in strong agreement with PIC simulations. We also show that using gas jets of low density ( cm−3) is beneficial for the brightness of the source, since low plasma density produces collimated radiation. If the laser pulse is focused to an optimal spot size that results in the highest cut-off energies, the conversion efficiency from laser to radiation can reach up to a few percent, which makes DLA a promising high-brilliance source of gamma-ray radiation.
This paper explores the emission of high-energy betatron radiation from electrons accelerated by powerful laser pulses interacting with low-density plasmas. The simulations show that this 'direct laser acceleration' can produce collimated gamma-ray beams with high brightness, making it a promising source for various applications.