Bright polarised x-ray flashes from dense plasmas

Creating a plasma dominated by strong-field quantum electrodynamic (SFQED) effects is a major goal of new multi-PW laser facilities. This is motivated by the fact that the fundamental dynamics of such plasmas are poorly understood and play an important role in the electrodynamics of extreme astrophysical environments such as pulsar magnetospheres. The most obvious observable for which such a regime has been reached is the production of a bright flash of x-rays, but distinguishing this from hard x-rays induced by bremsstrahlung radiation is a major challenge. Here we show that the photons from the x-ray flash are highly polarised relative to the unpolarised background, indicating that the SFQED plasma has indeed been produced. For a laser of intensity >1021Wcm−2 impinging on a solid Al target, the photons of the flash with energy >10keV are >65% polarised.