Resonant excitation of plasma wakefields in long plasma channels

Laser-plasma accelerators (LPAs) promise a route towards accelerating electrons to GeV-scale energies in centimetre-scale distances. However, the ultra-short laser pulse systems typically used for LPAs are currently limited in repetition rate to ≲ 1 Hz. Future laser-plasma accelerators (LPAs) will need to work at the kilohertz level to be useful for the most demanding applications in medicine, industry and research.
This thesis focuses on methods to achieve LPAs operating at the kilohertz level repetition rates. First, hydrodynamic optical-field-ionised (HOFI) plasma channels are studied, which have been shown to be capable of guiding laser pulses over hundreds of millimetres at low plasma densities [1] and are able to operate at kilohertz repetition rates [2]. In this thesis, limitations of the standard optical setups used to form HOFI plasma channels are investigated. Refractive axicon lenses are found to induce significant chromatic and non-linear effects on the ultra-short channel-forming laser pulse. The use of a thin phase plate is shown to overcome this problem. An alternative to the axicon, an axilens, is studied and a parameter regime for an axilens is found that achieves a longitudinally-extended focus over 300mm with highly uniform axial-intensity and with spot-size variations at the 15% level.
Next, the transmission losses of HOFI plasma channels are studied. It is found that increasing the radius of the initial ionised plasma column is an effective way to deepen the resulting plasma channel, thereby reducing the expected energy leakage from the guided beam as it propagates. Higher-order Bessel modes and random phase plates are investigated as methods to increase the radius of the initial plasma column. Higher-order Bessel modes are shown in calculations to maximise the efficiency of energy conversion from the channel-forming laser pulse to the initial plasma column. Hydrodynamic simulations show that Bessel modes of order 1 and 2 could generate plasma channel profiles with increased channel depth compared to the standard zeroth-order Bessel mode.
In the second part of the thesis, an experimental campaign relevant to the plasma-modulated plasma accelerator (P-MoPA) scheme is presented [3, 4]. Trains of laser pulses, consisting of ∼ 10 pulses spaced at 170 fs with total energy (2.5 +- 0.5) J, are guided over 100 mm-long propagation distances in a HOFI plasma channel. Measurements of the spectrum of the transmitted pulse train show that a plasma wakefield is resonantly excited when the on-axis channel density is tuned to match the plasma period to the separation of pulses in the train. The resonant plasma density is shown to scale with pulse train spacing as expected. Resonant wakefield excitation is not observed when the pulse train is replaced by a temporally-smooth pulse of the same energy and same (1 ps full-width-half-maximum) total duration. Comparison of the experimental results with fluid calculations suggests that wakefield amplitudes up to 10 GV/m were excited by the pulse trains at resonance. These results constitute the first experimental demonstration of the accelerator stage of the P-MoPA scheme [4].

Advisor: Hooker, S
Walczak, R
Pattathil, R