Controlled High-Repetition Plasma Based Electron Accelerators

The grant proposes development of laser-driven plasma based accelerators both from the fundamental viewpoint, such as demonstrating energy scaling and understanding injection mechanisms, but also proposes technological development of plasma accelerators in areas such as reproducibility and generating ultracompact accelerators. First the already ground-breaking work on state-of-the-art 100 TW scale laser systems will be extended. A complete characterisation of the interaction promises to elucidate the complex set of interactions that lead to mononenergetic electron beam production in this regime. Continuing improvements in the laser parameters promises to extend the energy range obtainable in this scheme to beyond the GeV level, (to which we are already tantalisingly close). We will also study the interaction of the single beam at high intensity (>10^20 Wcm^-2) in underdense plasma (i.e. not in the bubble matched regime as required for high energy mononenergetic beams), by using tighter focusing (~ f/3 as opposed to ~f/20 for monoenergetic beam production) on the same > 100 TW lasers. Though this limits the maximum energy that can be produced, such interactions promise to produce extremely high efficiency of conversion of laser energy into relativistic electrons, and extremely high current density beams. We also propose to extend our studies to more advanced schemes for dephasing and trapping electrons into a wakefield accelerator. A number of different methods will be investigated for reproducibility and low energy spread (which is of utmost importance for many applications). These include density modulation, and optical injection mechanisms. The ability to separate injection and acceleration phase, dramatically reduces the requirements on the driving laser pulse, hence allowing the wakefield to be driven at lower plasma densities, and thus potentially to much higher energy. These experiments will continue to make use of the Astra Gemini laser system at RAL, (which on optimisation should offer twin 15 J laser pulses at 30 fs), the Hercules laser at the Univesity of Michigan (which offers PW powers in an ultrashort pulse), as well as the Lund Laser Centre, which offers the highest beam quality laser at the 1J 30 fs level. The final major aim of this grant will be development towards producing a laser system of ultrashort pulse duration (< 10 fs i.e. ~ few laser cycles) at moderate pulse energy. This will allow high repetition rate operation of compact accelerators to be developed, whilst also opening the possibility of producing an affordable dedicated system for studying compact plasma accelerators. The demonstration of narrow energy spread beams of electrons produced with such a system would have important technological implications.