Capillary waveguide improvements for laser plasma accelerators

  • J. Daniels

Student thesis: Master


Compared to conventional particle accelerators, laser plasma accelerators (LPAs) have accelerating gradients that are orders of magnitude larger. This means that fundamentally these devices can be kept significantly smaller, making LPAs promising for future applications such as seed for a free electron laser or high resolution X-ray sources. On the path to widespread application of the technique several scientific and technological advances are still required.In this thesis three aspects of the development and improvement of an LPA are treated. First, special attention is paid to the capillary gas discharges that can be used to create the required plasma for an LPA. In particular, the electric circuit is optimized for smooth, short current pulses. This is done by modeling the system and then applying the modification experimentally. The key component introduced in the circuit is a saturable inductor, which suppresses oscillations depending on their frequency. A discharge pulse is obtained that can provide smooth and dependable discharge dynamics in the capillary.The main body of this work focuses on the plasma channel that is key to a successful LPA. This plasma channel guides the incoming laser pulses so that they remain focused over a longer distance, increasing the interaction length with the particles to be accelerated. Simulations and experiments are described that study the use of longitudinal interferometry in the development of a diagnostic for the on-axis electron density in the channel and the width of this plasma channel, for low laser intensities. The basis of this technique relies on the fact that the group velocity of the laser pulse is lower at higher plasma densities, introducing a delay on the order of femtoseconds. The measurement of this group velocity dispersion (GVD) is demonstrated to yield a measure for the on-axis density but direct comparison to an alternative technique is necessary to benchmark the method. Similarly, GVD measurements can be used to probe the channel width by changing the input spot size. In the experiment this introduces higher order laser modes which influence the result. These effects are studied in simulations and confirmed in experiments, concluding that for more reliable measurements a single-mode input would be ideal, in combination with a different technique such as a laser-centroid based diagnostic to increase the accuracy of the measurement.Finally, many possible applications of LPAs such as X-ray imaging would benefit tremendously from an increased repetition rate; whereas current-day LPAs operate at 1 Hz-level repetition-rates, the feasibility of using capillary targets for kHz repetition rates is investigated. To cope with the increased heat load, the performance of diamond is compared to the more conventional material sapphire. By exposing the target to 1.3 million discharges, it is shown that erosion of the diamond is indeed significantly less. In addition, comparison with previous experimental results shows that the current experiment causes an order of magnitude more erosion of the substrate. This suggests that the experimental conditions should be adjusted to increase the lifetime of the targets. More research is necessary to directly compare the quality of laser guiding in diamond and sapphire capillaries. While the diamond is shown to establish a waveguide, it is not as symmetric as the one in the conventional sapphire capillary. This observation is attributed to the non-ideal flatness of the diamond surfaces, allowing for small gas leaks that result in reduced plasma temperature and pressure. Additional research with more suitable diamond substrates is recommended to verify this hypothesis.These three topics each have expanded the knowledge of what happens to a gas-filled capillary when a discharge strikes and the effect on a laser pulse. Understanding these processes is essential in improving LPA operating parameters and performance, with the final goal to make the technology suitable for use in a multitude of applications.
Date of Award31 Aug 2013
Original languageEnglish
SupervisorW.P. Leemans (Supervisor 1), O.J. (Jom) Luiten (Supervisor 2) & G.J.H. Brussaard (Supervisor 2)

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