Strong-field Plasma Interactions Research @ Lancaster

A good week for the group as two new papers are published at almost the same time!

Here’s one about laser pulse compression and power amplification in a laser-driven plasma wakefield accelerator. Interesting experimental observations in their own right, but also critical to the evolution of the accelerating structure in a plasma and opens up the possibility to control electron acceleration in a plasma through tailoring of the laser pulse shape.

And here’s one about using active feedback to control the laser pulse shape and enhance x-ray production from a clustered gas jet. This is somewhat linked to the other paper but instead of trying to be clever and optimise the pulse shape ourselves, we just give over control to a genetic algorithm to do the hard work for us. We then have a cup of tea while the experiment optimises itself, and then return to take all the credit! We hope to implement these techniques in all future facilities based on high-intensity laser-plasma interactions to improve and stabilise performance, but it also allows us to discover the important physics by examining the best (and worst) results.

Back in January, we were part of a facility lead experiment using the Gemini laser at the Central Laser Facility. The aim of this experiment was to demonstrate the spacial and temporal resolution of x-rays generated by laser wakefield accelerated electrons. X-rays were created via two mechanisms: betatron oscillations inside the plasma accelerator (plasma wiggler) and Bremsstrahlung radiation from the interaction of electrons with a solid converter. Images from the experiment demonstrating the spatial resolution of these sources are shown below. The first shows a resolution grid imaged using the plasma wiggler. The smallest features which can be resolved are around 3 μm – comparable to micro-focus tube source resolution –  and we suspect that limit is imposed by the detector, not the source.

A composite image of several shots using the bremsstrahlung source is shown here too. The smallest feature resolved in this image is 200 μm thick, although again the resolution seemed to be limited by the detector. This is still very good when compared against other sources with similar photon energy. 

We are currently half way through an electron acceleration experiment at the Central Laser Facility’s Gemini Laser, lead by our colleagues at Imperial College London. We aim to drive two laser wakefield accelerators independently, using two 200 TW laser beams and then couple the electron beam from the first stage into the second to boost the energy. The experiment is ongoing, so hoping for some good results soon!

A 5-week experiment using the Central Laser Facility’s VULCAN Target Area West (TAW) laser has just come to an end.  The experiment explored the evolution of magnetic fields formed between two high intensity laser pulses at the surface of a solid target by probing the dynamic fields with a broad energy spread proton beam.  TAW is the perfect facility for these studies providing multiple high intensity pulses for generation of the reconnection fields and the probing proton beam.  In addition to study of the magnetic fields, angularly resolved measurements of the electron beam accelerated from the reconnecting region have allowed us to observe directional, non-thermal components to the energy spectra.  Further analysis of this in combination with the proton probing images will help us to understand the details of this highly dynamic process.

Alberto Martinez de la Ossa, based at DESY, Hamburg, lead a recent publication describing an impressive theoretical study on high quality electron beam injection in a electron beam driven plasma wakefield (https://link.aps.org/doi/10.1103/PhysRevAccelBeams.20.091301). This mechanism will be experimentally studied in the FLASHForward project, a unique experiment in the world which aims to drive a free-electron laser from a plasma accelerated electron beam.

An experiment at the Central Laser Facility’s Gemini laser has just come to an end after 2 months. A 2 GeV laser-driven electron source was setup, using 10 J of laser energy per pulse and we characterised a very bright synchrotron like x-ray source with photon energies in the 10s of keV range generated by the ‘betatron‘ mechanism. We were using this beam for diffraction, imaging and absorption spectroscopy along with our collaborators at Michigan, Imperial College, Strathclyde, York, Lawrence Livermore Laboratory, Lund, ELI, LOA, Swiss Light Source and Shanghai. A great team and very exciting work!

Congratulations Daniel on his new article in PRL, looking at laser depletion in photon-electron collisions!

After a few weeks of intense experimental time at the CLF, we have produced 100,000 electron beams with particle energies in the 100s of MeV range! We have been using active feedback to tune these beams, as well as the x-rays and optical emission from the plasma. So much data to analyse but definitely lots of interesting results and a great step forward for plasma accelerators.

On our first experiment as a group and we have our first electrons beams. We are using the smaller of the two target areas of the wonderful Gemini laser and using just 450 mJ of energy we are accelerating electrons to 100s of MeV. We’ll be back in the new year to build on this and begin generating 5 beams per second and run our optimisation routines.

Welcome to the SPIRAL group website. We will be adding content over the coming weeks to give an overall idea of who we are and what we are doing!