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Thesis Opportunities and Research Topics


Transition radiation based longitudinal diagnostics for ultra-short electron bunches

Plasma acceleration is capable of generating ultra-short electron bunches with durations that allows the generation of short X-ray pulses, which can be used to diagnose dynamic processes with resolution on the fs scale. Characterisation of the longitudinal electron bunch profile is therefore very important and the measurement of transition radiation over a wide frequency range provides a possible solution. We propose to combine several spectrometers to permit simultaneous measurements over a wider wavelength range that previously achievable providing a single-shot, high resolution diagnostic.

Controlling the transverse electron beam quality in plasma wakefield accelerators

One unresolved challenge of plasma-based accelerators (PWFA) is the capture of accelerated beams for transport via conventional beam optics. PBA beams intrinsically have a high divergence ~1mrad and a large energy spread ~1-10%. This leads to temporal elongation of the electron bunch and a growth of the initially low emittance ~ µm inside the plasma during the vacuum drift following the plasma target. Tailoring the particle density profile at the transition from plasma into vacuum is a promising method by which the divergence can be decreased, thus improving the beam quality after a drift. To show the feasibility of analytically calculated density ramps in a plasma target computational fluid dynamics (CFD) simulations are performed. Numerical studies and particle-in-cell simulations employing the results from the CFD simulations show a promising mitigation of the divergence.

Single cycle laser probe

Transverse optical probing of plasma accelerators can reveal vital information about the injection and acceleration processes. In order to get the most detailed picture possible, it is important to use very short laser pulses. Probing with typically titanium-sapphire laser pulses (~25 fs in duration) results in significant motion blur. This is improved by using a shorter laser pulse, which allows extra details about the accelerating structure and electron bunch to be observed. We have the opportunity in our laser lab to create pulses as short as 5 fs by using self phase modulation in a gas filled fibre and a chirped mirror compressors. One built and tested, this single cycle laser line will be used to probe plasmas and electron bunches in our plasma acceleration experiments.

Inverse Compton scattering from plasma produced electron bunches

The scattering interaction of a high energy electron with an optical/infrared photon can result in a transfer of some of the energy of the electron to the photon. The resultant energy of the photon is related to the energy of the electron, and will be 100s of KeV for 500 MeV electrons and in the MeV range for GeV electrons. Such photon beams are extremely interesting as photon sources while they also present diagnostic challenges for measuring their properties. We aim to develop diagnostics capable of measuring the photon yield, the angular distribution and the spectrum in order to characterise this source. This can also provide interesting information about the properties of the electron beams produced by our plasma accelerators.

External injection

Part of the research for FLASHForward will look at the external injection of an electron bunch in to a wake generated by a preceding electron bunch. The FLASH accelerator has the ability to generate two independent electron bunches in the same RF bucket. Demonstrating that the first of these bunches generates a wake filed which the second bunch is injected into and accelerated is the first step to multi stage plasma wakefield accelerators.  We are currently working with the FLASH accelerator to understand the properties of the two electron bunches and how they can be adjusted. We will then move onto understanding the effect of the FLASHForward beam line has on these bunches to allow us to design the best experiments to test external injection.

Generation of hollow-core plasma channels

Transverse focusing fields experienced by the electron bunches are responsible for emittance evolution during wakefield acceleration. Simulations have shown that this focusing can be controlled by decoupling transverse and longitudinal fields using a hollow-core plasma channel. We attempt to precisely control the plasma generation by ionising a gas target with a short and intense laser pulse. Various optics can be introduced to change the shape of the laser beam in order to realise different plasma shapes in proof-of-concept experiments.