Spatiotemporal optical injection in laser-plasma accelerators

Thesis

Laser-driven plasma wakefield accelerators (LWFAs) can accelerate charged particles to multi-gigaelectron volt energies over several centimetres due to the strong electric fields sustained in a plasma. Compared to conventional machines, which are often metres to kilometres-long, plasma-based accelerators have the potential to be the next generation of accelerators for particle physics research, in addition to providing convenient and cost-effective sources of high-energy electrons and radiation for applications in medicine, biology, chemistry and security. To be viable, there are challenges that remain to be addressed, particularly regarding the methods used to inject the witness electrons to be accelerated.

Controlled injection techniques aim to reliably generate electron bunches with desirable and repeatable bunch properties over long-term operation, by controlling how electrons from the plasma enter the wakefield. Optical injection techniques trigger injection using additional laser pulses.

In this thesis, we explore five different optical injection techniques. First, we compare three different effects arising from spatiotemporal couplings in ultrashort laser pulses. We show that when a radially-chirped laser pulse is focused by an achromatic lens, it forms a simultaneous space-time focus which exhibits a controllable group velocity, like a flying focus, and that unlike its linearly-chirped counterpart, it does not exhibit pulse-front tilt at the focus. We show that simultaneous space-time foci offer no higher degree of axial localisation of the intensity, contrary to as is often stated.

Accounting for novel polarisation states of the laser pulse, we present a new derivation of the wakefield quantities, to find that radially-polarised laser beams (RPLBs) may enhance electron trapping in LWFAs. We show that RPLBs have a higher degree of axial localisation compared to their linearly-polarised Gaussian counterparts.

Finally, we demonstrate injection and acceleration of 10 - 100 pC, 600 MeV electron bunches using optically-generated shocks. This scheme may be promising for future light-source applications, due to the betatron oscillations that seem to be induced in a crossed-channel configuration.

Authors: Archer, E

• DOI: 10.5287/ora-vjp014p9d
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: electron injection; simultaneous space-time focusing; laser-plasma accelerators; spatiotemporal couplings; ultrashort laser pulse; polarisation
• Subjects: Laser plasmas; Laser pulses, Ultrashort; Laser-plasma interactions; Electron accelerators