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In laser-wakefield acceleration, an ultra-intense laser pulse is focused into an underdense plasma to accelerate electrons to relativistic velocities. In most cases, the pulses consist of multiple optical cycles and the interaction is well described in the framework of the ponderomotive force where only the envelope of the laser has to be considered. But when using single-cycle pulses, the ponderomotive approximation breaks down, and the actual waveform of the laser has to be taken into account. In this paper, we use near-single-cycle laser pulses to drive a laser-wakefield accelerator. We observe variations of the electron beam pointing on the order of 10 mrad in the polarization direction, as well as 30% variations of the beam charge, locked to the value of the controlled laser carrier-envelope phase, in both nitrogen and helium plasma. Those findings are explained through particle-in-cell simulations indicating that low-emittance, ultrashort electron bunches are periodically injected off-axis by the transversally oscillating bubble associated with the slipping carrier-envelope phase. [ABSTRACT FROM AUTHOR]

We present two-dimensional (2D) optical Thomson scattering measurements of electron density and temperature in laser-produced plasmas. The novel instrument directly measures n e (x , y) and T e (x , y) in two dimensions over large spatial regions (cm 2 ) with sub-mm spatial resolution, by automatically translating the scattering volume while the plasma is produced repeatedly by irradiating a solid target with a high-repetition-rate laser beam (10 J, ∼10 12 W/cm 2 , 1 Hz). In this paper, we describe the design and motorized auto-alignment of the instrument and the computerized algorithm that autonomously fits the spectral distribution function to the tens-of-thousands of measured scattering spectra, and captures the transition from the collective to the non-collective regime with distance from the target. As an example, we present the first 2D scattering measurements in laser-driven shock waves in ambient nitrogen gas at a pressure of 0.13 mbar. [ABSTRACT FROM AUTHOR]

Driving laser wakefield acceleration with extremely short, near single-cycle laser pulses is crucial to the realization of an electron source that can operate at kHz-repetition rate while relying on modest laser energy. It is also interesting from a fundamental point of view, as the ponderomotive approximation is no longer valid for such short pulses. Through particle-in-cell simulations, we show how the plasma response becomes asymmetric in the plane of laser polarization, and dependent on the carrier-envelope phase (CEP) of the laser pulse. For the case of self-injection, this in turn strongly affects the initial conditions of injected electrons, causing collective betatron oscillations of the electron beam. As a result, the electron beam pointing, electron energy spectrum, and the direction of emitted betatron radiation become CEP dependent. For injection in a density gradient, the effect on beam pointing is reduced and the electron energy spectrum is CEP independent, as electron injection is mostly longitudinal and mainly determined by the density gradient. Our results highlight the importance of controlling the CEP in this regime for producing stable and reproducible relativistic electron beams and identify how CEP effects may be observed in experiments. In the future, CEP control may become an additional tool to control the energy spectrum or pointing of the accelerated electron beam. [ABSTRACT FROM AUTHOR]

Laser-plasma accelerators can produce ultra-short electron bunches in the femtosecond to picosecond duration range, resulting in very high peak dose rates in comparison with clinical accelerators. This unique characteristic motivates their possible application to radiation biology studies to elucidate the effect of high peak dose rates and peculiar temporal structures on the biological response of living cells, which might improve the differential response between tumour and healthy tissues. Electron beams driven by kHz laser systems are an attractive option among laser-plasma accelerators since the high repetition rate can boost the mean dose rate and improve the stability of the delivered dose in comparison with J-class laser accelerators running at few Hz. In this work, we present the dosimetric characterisation of a kHz, low energy laser-driven electron source and preliminary results on in-vitro irradiation of cancer cells. A shot-to-shot dosimetry protocol enabled monitoring of the beam stability and the irradiation conditions for each cell sample. Results of survival assays on HCT116 colorectal cancer cells are in good agreement with previous findings reported in the literature and validate the robustness of the dosimetry and irradiation protocol. [ABSTRACT FROM AUTHOR]