It is known that as a laser wakefield passes through a downward density transition in a plasma some portion of the background electrons are trapped in the laser wakefield and the trapped electrons are accelerated to relativistic high energies over a very short distance. In this study, by using a two-dimensional (2D) particle-in-cell (PIC) simulation, we suggest an experimental scheme that can manipulate electron trapping and acceleration across a parabolic plasma density channel, which is easier to produce and more feasible to apply to the laser wakefield acceleration experiments. In this study, 2D PIC simulation results for the physical characteristics of the electron bunches that are emitted from the parabolic density plasma channel are reported in great detail.

Non-invasive Gamma ENtrainment Using Sensory stimulation (GENUS) at 40Hz reduced Alzheimers disease (AD) pathology such as amyloid and tau levels, prevented cerebral atrophy and improved performance during behavioral testing in mouse models of AD. We report data from a randomized, placebo-controlled trial (n = 15) in volunteers with probable mild AD after 4 months of one-hour daily 40Hz sensory stimulation (NCT 04055376) to assess safety, compliance, entrainment and possible effects on brain structure, function, sleep activity and cognitive function. 40Hz light and sound GENUS was well-tolerated and compliance was high in both groups. Electroencephalography recordings show that our novel 40Hz GENUS device safely and effectively induced 40Hz entrainment in participants with mild AD. After 3 months of daily stimulation, the 40Hz GENUS group showed reduced ventricular dilation and stabilization of the hippocampal size compared to the control group. Functional connectivity was found to improve in the default mode network as well as with the medial visual network after 3 months of stimulation. Furthermore, actigraphy recordings show that circadian rhythmicity also improved with 40Hz stimulation. Compared to controls, the active group performed better on the face-name association delayed recall test. These results suggest that 40Hz GENUS can be used safely at home daily and shows favorable outcomes on cognitive function, structure and functional MRI biomarkers of AD-related degeneration. These results support further evaluation of GENUS in larger and longer clinical trials to evaluate its potential as a novel disease modifying therapeutic for Alzheimers dementia.

A relativistic electron bunch with a large charge (>2 nC) was produced from a self-modulated laser wakefield acceleration configuration. For this experiment, an intense laser beam with a peak power of 2 TW and a duration of 700 fs was focused in a supersonic He gas jet, and relativistic high-energy electrons were observed from the strong laser-plasma interaction. By passing the electron bunch through a small pinholelike collimator, we could generate a quasimonoenergetic high-energy electron beam, in which electrons within a cone angle of 0.25 mrad (f/70) were selected. The beam clearly showed a narrow-energy-spread behavior with a central energy of 4.3 MeV and a charge of 200 pC. The acceleration gradient was estimated to be about 30 GeV/m. Particle-in-cell simulations were performed for comparison study and the result shows that both the experimental and simulation results are in good agreement and the electron trapping is initiated by the slow beat wave of the Raman backward wave and the incident laser pulse.

Energy exchange between pump and daughter waves during the stimulated Raman scattering process in a plasma is investigated, including the effect of a damping coefficient of electron-ion collision at different initial three-wave phases. To obey the energy and momentum conservations, the resonance conditions are satisfied at an optimal initial phase difference between the interacting waves. The amplitudes of the interacting waves exhibit behaviors such as a parametric oscillator. The variations in initial three-wave phase difference generate a phase mismatch, which enhances the rate of the amplitude variations of the interacting waves. The relativistic mass effect modifies the dispersion relations of the interacting waves, and consequently the energy exchange during the stimulated Raman scattering is affected. The collisional damping in the plasma is shown to have an important effect on the evolution of the interacting waves.

A relativistic electron bunch with a large charge (~2 nC ) was produced from a self-modulated laser wakefield acceleration configuration. In this experiment, an intense laser pulse with a peak power of 2 TW and a duration of 700 fs was focused in a nitrogen gas jet, and multi-MeV electrons were observed from the strong laser-plasma interaction. By passing the electrons through a small pinhole-like collimator of cone f/70, we observed a narrowing in the electron beam's energy spread. The beam clearly showed a small energy-spread behavior with a central energy of 4.8 MeV and a charge of 115 pC. The acceleration gradient was estimated to be about 20 GeV/m.

The dynamics of the laser-produced plasma in a gas filled chamber and in a gas jet were investigated using a relatively low intensity laser (I⩽5×1012 W/cm2) with long pulse duration (7 ns). Visualization methods such as the Mach–Zehnder interferometry and the shadow imaging were employed to characterize the shock velocity, electron densities, and temperatures in the laser-produced plasma. The electron density depletion on the laser propagation axis was observed in a gas jet but not in the gas filled chamber. Qualitative analysis for the density depletion is discussed by employing a fluid theory for the ambipolar diffusion of the plasma species. In the gas filled chamber, increase of the laser energy, and applied gas pressure results in faster and further propagation of the shock front with the distance-time relation of Rs∝t2/5, implicating an instantaneous energy release at a point. However, the plasma produced in a gas jet shows a rather linear relation of Rs∝t at early time (t⩽10 ns).