Taylor's (1935) dissipation surrogate reinterpreted

New results from direct numerical simulation of decaying isotropic turbulence show that Taylor’s expression for the viscous dissipation rate ε = CεU3/L is more appropriately interpreted as a surrogate for the inertial energy flux. As a consequence, the well known dependence of the Taylor prefactor Cε on Reynolds number Cε(RL)→Cε,∞ can be understood as corresponding to the onset of an inertial range

Electron beam cooling in intense focussed laser pulses

In the coming years, a new generation of high-power laser facilities (such as the Extreme Light Infrastructure) will become operational, for which it is important to understand how the interaction with intense laser pulses affects the bulk properties of relativistic electron bunches. At such high field intensities, we expect both radiation reaction and quantum effects to have a dominant role to play in determining the dynamics. The reduction in relative energy spread (beam cooling) at the expense of mean beam energy predicted by classical theories of radiation reaction has been shown to occur equally in the longitudinal and transverse directions, whereas this symmetry is broken when the theory is extended to approximate certain quantum effects. The reduction in longitudinal cooling suggests that the effects of radiation reaction could be better observed in measurements of the transverse distribution, which for real-world laser pulses motivates the investigation of the angular dependence of the interaction. Using a stochastic single-photon emission model with a (Gaussian beam) focussed pulse, we find strong angular dependence of the stochastic heating

Noise-related polarization dynamics for femto and picosecond pulses in normal dispersion fibers

We report how the complex intra-pulse polarization dynamics of coherent optical wavebreaking and incoherent Raman amplification processes in all-normal dispersion (ANDi) fibers vary for femto and picosecond pump pulses. Using high temporal resolution vector supercontinuum simulations, we identify deterministic polarization dynamics caused by wavebreaking and self-phase modulation for femtosecond pulses and quasi-chaotic polarization evolution driven by Raman amplification of quantum noise for picosecond pulses. In contrast to cross-phase modulation instability, the Raman-based polarization noise has no power threshold and is reduced by aligning the higher energy polarization component with the lower index axis of the fiber. The degree of polarization stability is quantified using new time domain parameters that build on the spectrally averaged degree of coherence used in supercontinuum research to quantify the output spectral stability. We show that the spectral coherence is intrinsically linked to polarization noise, and that the noise will occur in both polarization maintaining (PM) and non-PM fibers, spanning a broad range of pulse energies, durations, and fiber birefringence values. This analysis provides an in-depth understanding of the nonlinear polarization dynamics associated with coherent and incoherent propagation in ANDi fibers

Controlled generation of ultra-short electron bunches using density modulation

Stimulated electron self-injection in the laser wakefield accelerator (LWFA) using density downramps is well known and regularly used to produce high energy electron bunches. The use of density gradients not only to stimulate injection but also control the properties of the injected electron bunch was recently presented by Tooley et al. [Phys. Rev. Lett. 119 , 044801 (2017)], in which the authors put forward a model for controlling the velocity of the back of the bubble and compared to 2D and 3D particle-in-cell (PIC) data. This model is discussed and used to identify suitable LWFA parameters for ultra-short injection and repeated injection of multiple bunches. Quasi-3D PIC data is used to demonstrate injection of multiple bunches well separated in energy

Streaming instabilities in converging geometry

We present an investigation into counter-streaming electron beams converging towards, and diverging from, a single point in two dimensions, leading to two-stream and current filamentation instabilities, which have radial and azimuthal density modulations, respectively. Using a semi-analytical approach and numerical simulations, we find no evidence for the two-stream instability in this geometry, but show that the system is unstable to the development of current filamentation

Controlling the group velocity of an intense laser pulse using a pre-pulse

The accelerating structure of the laser wakefield accelerator (LWFA) is dynamic and highly sensitive to the local laser and plasma properties. It can expand and contract as it responds to the evolution of the laser and plasma fields. As a result, the position of, and environment within, the LWFA bubble are usually time dependent, which is not ideal for stable acceleration. Variations can have a negative impact on electron bunch properties, and are deleterious for ion channel lasers and plasma wigglers. We demonstrate how a laser pre-pulse improves the stability of the LWFA, and controls the evolution of the laser group and bubble velocity, which are important for determining LWFA dephasing and ultimately the electron bunch energy