Plasma accelerators utilize the enormous electric fields formed within plasma waves to accelerate charged particles to high energies in a fraction of the distance needed in a conventional particle accelerator. You can find further details of our research programme in this area on our Research page.
Our lead academics are all members of the John Adams Institute (JAI), working in Oxford's sub-departments of Particle Physics and in Atomic & Laser Physics. We collaborate closely with JAI at Imperial College and with groups at DESY, Jena, MPQ and LBNL. Some experiments are undertaken in our laboratories in Oxford, at facilities based at the Rutherford Appleton Laboratory (just outside Oxford), or with our collaborators in the USA and Europe.
Our work on laser-driven plasma accelerators is in four areas: (i) investigation of techniques for controlling the injection of electrons into the plasma wakefield; (ii) development of new techniques for driving plasma accelerators, such as multi-pulse laser wakefield acceleration; (iii) development of techniques for driving the intense driving laser pulse over 100s of mm; and (iv) development of applications of laser-driven plasma accelerators, particularly their application to the generation of x-rays. We pursue these goals by both experiment and numerical modelling.
Projects available to start in 2021
We are offering two DPhil projects to start in October 2021, as outlined below.
1. X-ray sources driven in all-optical plasma channels
Conventional electron-beam-driven light sources (i.e. synchrotrons and free-electron lasers) use electron bunches with energies of a few GeV. An Oxford-Berkeley collaboration were the first to generate electron beams with comparable energy from a laser-plasma accelerator. Reaching this energy requires the driving laser pulse, which has an intensity of around 1018 W / cm2, to be guided over several centimetres — well beyond the distance over which diffraction occurs.
In the first GeV-scale experiments, the laser pulse was guided in a plasma channel — a gradient refractive index waveguide made from plasma — generated by a capillary discharge. The drawback of this approach is that the discharge structure can be damaged by the driving laser pulse. The Oxford group has recently developed a new type of plasma channel generated by auxiliary laser pulses. Since they are free-standing, these channels are immune to laser damage, and hence they are very promising stages for future multi-GeV plasma accelerators operating at kilohertz pulse repetition rates.
In this project we will investigate further developments of these hydrodynamic optical-field-ionized (HOFI) plasma channels, and their application to the generation of incoherent keV X-rays via the transverse oscillation of the electron bunch in the plasma wakefield.
2. Multi-pulse laser wakefield accelerators
In a laser wakefield plasma accelerator, a short, intense laser pulse is used to drive a longitudinal density wave (a ‘plasma wave’) in a plasma. The electric fields (which constitute a ‘laser wakefield') within this wave are about 1000 times greater than the accelerating fields employed in a conventional, radio-frequency accelerator — and hence laser-plasma accelerators can generate high-energy beams from a very compact accelerator stage. Laser-driven plasma accelerators have already been demonstrated to generated electron beams with energies of several GeV.
To date, most work has been done with single driving pulses. These must have an energy of order 1 J and a duration shorter than the plasma period, which is around 100 fs. These demanding parameters can be generated by Ti:sapphire laser laser systems. However, Ti:sapphire lasers have very low efficiencies (< 0.1%) and (at these pulse energies) are limited to pulse repetition rates below 10 Hz.
Many potential applications of laser-plasma accelerators — such as light sources and future particle colliders — require operation at much higher pulse repetition rates (at least in the kilohertz range) and much higher ‘wall-plug’ efficiencies. New types of laser are becoming available which can meet these requirements, but they generate pulses in the picosecond range, which are too long to drive a plasma wave. If the output pulses of these lasers could be modulated, with a modulation spacing equal to the plasma period, then they could be used to resonantly excite the plasma wave in a plasma accelerator. We have recently shown that this is possible in a proof-of-principle experiment which employed temporally-stretched Ti:sapphire laser pulses.
In this project we will investigate methods for modulating long, high-energy laser pulses to form a train of short, low energy pulses. We will investigate multi-pulse laser wakefield accelerators (MP-LWFA) driven in this way, and will seek to demonstrate electron acceleration in a MP-LWFA for the first time.
