Some scientists left their laser alone all day, and this is what happened

That’s right, I’ve been learning about generating clickbait!

Taking a break from my usual jumble-sale philosophy and eclectic-humour-based blogging (that’s right, this is generally supposed to be funny), this time there’s a dose of Science for your enjoyment. It’s unusual academic practice but a colleague and I recently wrote a paper together. An enjoyable and productive experience, and great to see it out in the world this month.

I was asked to write a PR piece for the paper’s release. I thought it would be easy, being such an expert blogger and all that, but once I had finished my colleague was kind enough to point out that it broke every one of the journal’s guidelines for promotional material.

Not wanting to waste the written word, here it is! And hopefully it provides an insight in to some of the things I’ve been working on:

Ordinarily a plasma, not unlike a metal, responds to applied electric fields by rearranging its electrons to screen them out, something commonly known as the Faraday Effect. The same effect occurs when a charge imbalance arises within the plasma itself, an effect first described by Debye in 1923. However, in a number of laboratories scientists have found that this effect can be overcome on microscopic scales by firing an ultra-intense laser or particle beam driver in to the plasma. The driver forces electrons out of its path, creating a cavity behind it containing only heavy and immobile positively charged ions. The electric field within this cavity, which travels at almost the speed of light behind the driver, can exceed 100 GV/m, around one thousand times more than can be sustained in a conventional particle accelerator. Electrons placed in this micro-cavity can, therefore, gain energy at a rapid rate.

While laser driven plasma accelerators (LPAs) have made great progress in pushing the frontiers of high energy and bright electron beams from compact sources, implementation of them as workhorse accelerators producing consistent beams over long timescales has made slower progress. In a recent experiment at DESY we optimised the interaction of our highly reproducible laser system with a 1 mm-scale plasma. The 10 trillion Watt, short pulse laser beam was focussed to a spot of only 8 micrometres in size, and we were able to faithfully reproduce the peak electric fields in the focus to within 2% on a shot-to-shot basis. Electrons were injected in to the plasma cavity using ionisation injection, a simple and popular technique where the core of electrons of nitrogen are ionised only close to the centre of the cavity by the peak of the laser pulse. Using this technique electrons were trapped in the plasma cavity and accelerated to approx. 80 MeV (~80 GV/m gradient) at a frequency of 2.5 Hz for 8 hours, itself a first in high frequency LPA, without a single failure to register an accelerated bunch. Over the full 72,000 shots the mean beam charge detected per shot and the charge between 70 and 80 MeV did not change at any resolvable level.

While this represents field-leading stability it was not possible to remove all jitters. By continuously monitoring the laser and plasma parameters on every laser shot we determined that our main source of shot-to-shot fluctuations were changes in the plasma density, which were strongly correlated with the accelerated charge per shot. A series of simulations was used to uncover the underlying physical mechanisms behind this effect, which was driven by the nonlinear interplay between the properties of the laser and the plasma cavity.

At higher plasma densities the plasma squeezed the laser pulse down to a smaller spot, increasing its intensity and, perhaps counter-intuitively, increasing the volume of the laser pulse that was intense enough to ionise the core nitrogen electrons. This matched the measured trend of higher charges at higher densities (a 1% increase in density caused a 3% increase in beam charge), while also resulting in higher emittance beams i.e. beams with worse focussability. This highlights the importance of reproducing the interaction conditions with high fidelity.

Teams at DESY are working constantly on building and improving high repetition rate, high power lasers that deliver pulses with best-in-class consistency, as well as developing plasma targets designed to deliver constant plasma density profiles while withstanding the conditions created by high frequency, intense drive beams. By integrating the knowledge gained from this experiment we hope to bring forward the advent of high repetition rate, reliable LPAs as ultra-compact electron beam accelerators.

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