Dr Lewis Reid PhD MPhys

Research Associate Mechanical, Materials & Aerospace Eng

Research

Laser wakefield acceleration

Photo of a gas jet used for laser wakefield accelerator. The path of the laser is shown in red, travelling left to right and the accelerated electrons and emitted x-rays are shown in yellow and magenta. The plume of gas, timed for the arrival of the laser, from the nozzle is shown in blue. The outlet of the nozzle is just 2 mm in diameter and can accelerate particles to the same energy conventional accelerator more than 1 meter long.
Photo of a gas jet used for laser wakefield accelerator. The path of the laser is shown in red, travelling left to right and the accelerated electrons and emitted x-rays are shown in yellow and magenta. The plume of gas, timed for the arrival of the laser, from the nozzle is shown in blue. The outlet of the nozzle is just 2 mm in diameter and can accelerate particles to the same energy conventional accelerator more than 1 meter long.

I use ultra high intensity laser pulses as the drivers for plasma based particle accelerators called the laser wakefield accelerator. A short duration laser pulse is focused into a gas target which is instantaneously ionised becoming a plasma. The force due to the radiation pressure of the laser pushes electrons away from it creating an electron density wave in the plasma. Electrons can become trapped in this wave and begin to "surf" it, and experience the GV/m electric fields from the separation of charged particles in the plasma. These high electric fields allow the accelerators to be extremely compact, making them around 1000 times smaller than conventional accelerator technology.

Some of the highest peak power lasers on the planet are required to form the waves in the plasma necessary to accelerate electrons. Laser powers of 10-1000 TW are required which are thousands of times more powerful than the entire national grid, albeit for a very short period of time! The laser pulses must fit inside the plasma wave so pulse durations of <100 fs are routinely used. Due to the strict laser requirements, laser wakefield acceleration experiments are conducted at large national scale laboratories such as the Central Laser Facility at the Rutherford Appleton laboratory and CLARA, at the Daresbury laboratory.

I am also interested in plasma targetry development to improve the shot to shot repeatability of these accelerators.

PIC simulations

Snapshot of simulation using the code fbpic showing an intense laser pulse (pink to pale blue contours) driving an electron density wave (yellow to green colormap) in its wake. The shape of the electric field which can be used to accelerate particles is shown in red. The peak amplitude of this field is > 600 GV/m, more than 1000 times what is achievable with conventional particle accelerator technology.
Snapshot of simulation using the code fbpic showing an intense laser pulse (pink to pale blue contours) driving an electron density wave (yellow to green colormap) in its wake. The shape of the electric field which can be used to accelerate particles is shown in red. The peak amplitude of this field is > 600 GV/m, more than 1000 times what is achievable with conventional particle accelerator technology.

I am working on Particle-in-Cell (PIC) simulations of the interaction between high power laser pulses and plasma to model experimental work that has already been completed as well as assisting the planning for future experiments so that key parameters can be optimised in advanced of the short experimental slots awarded at laser and accelerator facilities. Since laser-plasma interactions is highly non-linear, numerically solving equations or using theory along does not capture all of the physics and insufficient to replicate the results of experiments. This makes simulations necessary to to gain a better insight of the experimental work that I am involved with.

The PIC technique models plasma as particles which interact via their own electric and magnetic fields as well as external fields, for example from a laser pulse. This method is particularly effective for simulating phenomena where different processes occur over a wide range of timescales. The window of plasma being simulated is split into a number of pixels (or cells) which is populated with a given number of macroparticles which represents a larger number of real particles. The propagation of a laser through a plasma is computed iteratively inside a loop. First, the electromagnetic field vectors at each particle position are collected. Secondly, the particles are pushed by these fields. Thirdly, updated particle currents and current densities are deposited on the grid and finally the Maxwell equations are solved to find the fields for the next loop iteration.

These simulations are computationally intensive so supercomputers are required so that a sufficiently high spatial and temporal resolution can be achieved to correctly capture the physics. Particle in cell codes are highly parallelisable so computing resources can be used efficiently. I use a combination of the University of Liverpool Barkla cluster (both CPU and GPU nodes), ARCHER, part of the UK national supercomputing service and the SCARF cluster at Rutherford Appleton laboratory which was built to support experiments at STFC facilities.

Fibre amplifier lasers

Optical table of fibre laser in the Liverpool laboratory
Optical table of fibre laser in the Liverpool laboratory

I am involved in developing new laser sources for a number of applications in accelerator science.
Currently, I'm building a chirped pulse amplification based laser system which amplifies pulses inside optic fibres where the core is doped with Ytterbium. We are aiming to create short duration (~ 250 fs) laser pulses with high energy by daisy-chaining many fibres together or using the coherent combination of multiple photonic crystal fibres. The use of optic fibres allow laser pulses to be created with high repetition rates to give both high average and peak powers, which we ultimately wish to use to drive laser plasma wakefield acceleration at higher repetition rates than can be achieved with Ti:Sapphire systems.