X-ray imaging & applications
Electrons are not only accelerated longitudinally in laser-driven plasma waves, but at high enough laser intensities can be accelerated transversely too. The intense electromagnetic fields of the plasma wave violently ‘wiggle’ the relativistic electron beam, causing it to emit synchrotron radiation in the x-ray region of the electromagnetic spectrum, known as betatron radiation.
The large relativistic factor of the electron beams cause the radiation to be highly directional, emitted in a narrow beam a few degrees wide. The photon energy is high - tens of keV - and the x-ray pulse length very short - several femtoseconds. These factors combine to make the peak brightness of the beam extremely large, comparable to dedicated synchrotron x-ray sources. All of this is occurs over a distance of a few centimetres, drastically shorter than conventional magnetic wigglers.
Groups of researchers around the world have used these unique x-ray sources in a plethora of applications - measuring the spectral properties of materials on femtosecond timescales, or taking pictures of solid materials under extreme conditions. The source of the x-rays is a tiny micron-scale region in the centre of the plasma wave, which means the x-ray beam has very favourable imaging properties, allowing images to be recorded at high resolution.This fact also means the beam becomes spatially coherent, allowing tiny deflections of the beam to be visualised. In light of these properties experiments have recently begun using betatron beams for clinical imaging applications, suitable for both transparent and opaque tissues.
Abundant sources of high brightness, spatially coherent ultrafast x-rays may fundamentally change the way we conduct science, repeating the impact that the advent of the first x-ray tubes had a hundred years ago.