In the laser-wakefield scheme for electron acceleration, a driver (either a laser pulse or a high-energy particle beam) propagates through tenuous plasma and creates a wake wave, which is then used to accelerate an electron bunch. The advantage of this scheme is that very high electron energies (1-10 GeV and beyond) can be achieved even within modestly sized institutions.
To sustain wakefield generation and electron acceleration over appreciable distances, it is imperative to prevent divergence of the driving laser pulse during its propagation.
This can be done by either firing the laser pulse along a preformed plasma channel (which guides light in the same way a glass fibre does), or by using a laser pulse of such intensity that it guides itself through relativistic effects. Channel-guided propagation is currently the method of choice when using laser pulses of 10-40 TW, while self-guiding becomes viable for higher intensities (100 TW or more).
Particle-in-cell simulations are currently under way to investigate both channel-guided and self-guided propagation. The channel-guided simulations are performed in support of recent experiments on the CLF Astra laser (up to 15 TW) by Simon Hooker et al. from Oxford University, while the self-guided simulations are conducted to predict the results of future experiments on the recently commissioned Astra Gemini laser (up to 500 TW).
A particular feature of laser-plasma interaction at very high intensity is the so-called bubble regime, in which the driving laser pulse sweeps almost all plasma electrons out of its way to create a single-period wake wave, the so-called ~~bubble~~.
This structure has proved particularly advantageous for the production of electron bunches having not only a high mean energy but also a low energy spread. We are currently conducting large-scale particle-in-cell simulations to study the details of bubble formation and electron acceleration by bubbles, and to investigate how the final electron energy scales with various laser and plasma parameters. A typical "bubble" is shown in the image above, which depicts the electron density of a single-period wake, driven by a 500 TW laser pulse propagating to the right, in an underdense plasma. The image shows that most of the bubble is almost devoid of electrons (red areas), while there is a sizable population of trapped electrons along the bubble axis (black areas).
In addition to electron acceleration, we are also studying photon acceleration of laser light by plasma wakefields. The process of photon acceleration causes characteristic modulations to the spectrum of either the driving pulse or a low-intensity ~~witness~~ pulse that cannot be attributed to other processes such as parametric instabilities. Photon acceleration has potential as a single-shot diagnostic to visualise wakefields in experiments.
