Experimental aspects of plasma wakefields driven in the linear regime

This thesis describes two main pieces of experimental work and a piece of development work that aims to advance laser wakeﬁeld accelerators that use lower intensity, high repetition rate lasers. The ﬁrst piece of experimental work looked at developing further the capillary discharge waveguide by changing the key parameters that form the plasma channel and measuring the electron density proﬁle by transverse interferometry. The ﬁrst change was from a capacitor discharge circuit to the Compact Marx Bank system. This achieved a reduction in electrical noise but also delivered a driving current pulse which was better matched to the plasma channel and so had less current bounce. The circuit delivered a guiding window of around 200 ns and created an electron density channel proﬁle which supported a laser spot size of 35 µ m over 15 mm. A further change was the use of helium gas rather than hydrogen gas; it was found that helium also formed an electron density proﬁle that could support a focal spot size of 35 µ m over 15 mm. Finally, the material of the capillary was changed from sapphire to diamond and whilst it was not possible to measure the plasma channel formed in this material, it was found that even after 10 000 plasma discharges there was no noticeable effect on the material, whereas sapphire under the same conditions had 0.7 µ m of material ablated.
The developmental work focussed on the design of the targets to be used in laser wakeﬁeld experiments. The capillary discharge waveguide and housing were re-designed for the speciﬁc geometry of an experiment on the ASTRA-Gemini laser at the Rutherford Appleton Laboratory (RAL) and to accommodate a gas cell in the housing of the capillary discharge waveguide. It also raised questions concerning the gas density proﬁle present in the capillary discharge waveguide and so this was modelled using OpenFOAM. It was found that the modiﬁcations had no eﬀect on the initial gas density proﬁle in the simulations. The combined knowledge of the OpenFOAM software and the capillary ischarge waveguide for Gemini were used to create a gas cell for the multi-pulse ASTRA experiment. This gas cell maintained a ﬂat gas density proﬁle ideal for creating the interaction region required in this experiment. The robust design and manufacture of this target proved itself during the experiment in which it was at times taken apart daily but could always be reassembled easily.
The second experiment undertaken was the driving of a multi-pulse wakeﬁeld using the ASTRA laser at RAL. Here, a low amplitude wakeﬁeld was driven by a 55 fs duration, 2.6 × 10 21 Wm −2 peak intensity single pulse; a train of seven pulses, 50 fs each in duration and peak intensity of 0.3 × 10 21 Wm −2 and by two fully compressed pulses each 65 fs in duration and with a peak intensity of 1.3 × 10 21 Wm −2. These wakeﬁelds were measured using Frequency Domain Holograiiphy (FDH) and Temporarily Encoded Spectral Shifting (TESS) to deduce wakeﬁeld amplitudes of 1 % variation of the plasma density. As well as driving a wakeﬁeld, the double pulse experiment suggests the possibility of energy recovery from the wakeﬁeld structure.

Advisor: Hooker, S