An experimental study has been carried out on high-order harmonic generation from the interaction of $45\mathrm{fs}$ Ti:sapphire laser pulses with low-excited silver plasma plumes. The dependence of high-order harmonic intensity $\left(I_H\right)$ on the plasma plume length $\left(L_{\mathrm{med}}\right)$ shows slower than quadratic scaling $I_H\sim L_{\mathrm{med}}^p$ with “$p$”$\approx 0.9$, 0.8, and 0.7 for 21st, 33rd, and 41st harmonics, respectively. The harmonic intensity is observed to decrease with increasing harmonic order, and this decrease is faster for longer plasma plumes. The results are explained from physical considerations of phase mismatch between the driving laser pulse and the harmonic radiation during propagation, and reabsorption of the harmonic radiation in the plasma plume.

We report here a systematic quantitative study on the generation and characteristics of an active muon source driven by the interaction of an electron beam within the energy range of 1–10 GeV from laser wakefield acceleration (LWFA) with a tungsten target, using Monte Carlo simulations. The 10 GeV electron beam, achievable in the near future, from LWFA using femtosecond multi-PW lasers is employed to drive the bright source of muon pairs in a compact setup. We show that a highly directional and intense source of short-pulsed GeV muon pairs ( μ ⁻ μ ⁺ ) have a peak brightness of 5 × 10 ¹⁷ pairs s ^–1 cm ^–2 sr ^–1 and sub-100 ps duration could be produced using a quasi-monoenergetic 10 fs, 10 GeV electron bunch with a 1-mrad divergence and 100 pC charge. The muon pairs are emitted from a point-like source with a well-defined position and timing; the source has a size and geometric emittance of about 1 mm and 40 μ m, respectively. Such muon sources can greatly benefit applications in muon radiography as well as studies on anomalous dipole moments, rare decays of muons, neutrino oscillations, and an injector for a future compact muon collider.

Strongly self-modulated regime of laser wakefield electron acceleration has been experimentally studied using positively/negatively chirped Ti:sapphire laser pulses with duration $\ge 45\mathrm{fs}$, and intensity $\le 1.2\times {10}^{18}\mathrm{W}/{\mathrm{cm}}^2$ interacting with a plasma having density $>5\times {10}^{19}{\mathrm{cm}}^{-3}$. The total charge of the high energy electrons produced from self-injection and acceleration in the wakefield was found to depend strongly and asymmetrically on the magnitude and sign of the chirp, which was also corroborated by the forward Raman scattering signals. Generation of low divergence ($\le 10\mathrm{mrad}$), quasimonoenergetic electron beam was observed with a maximum energy of $\sim 28\mathrm{MeV}$ for a positively chirped laser pulse of duration $\sim 70\mathrm{fs}$. The observations are explained considering faster rise time associated with a positive chirped laser pulse which generates intense seed for stronger modulation of the laser pulse and consequently stronger wakefield excitation leading to higher electron beam charge and energy.

We present a technique, based on a dipole magnet spectrometer containing multiple scintillation screens, to accurately characterize the spectral distribution of a GeV electron beam generated by laser wakefield acceleration (LWFA). An optimization algorithm, along with a numerical code, was developed for trajectory tracking and reconstructing the electron beam angle, divergence, and energy spectrum with a single-shot measurement. The code was validated by comparing the results with the Monte-Carlo simulation of electron beam trajectories. We applied the method to analyze data obtained from laser wakefield acceleration experiments performed using a multi-Petawatt laser to accelerate electron beams to multi-GeV energy. Our technique offers a high degree of accuracy to faithfully characterize electron beams with the nonnegligible shot-to-shot beam pointing fluctuations, particularly in the state-of-the-art multi-GeV LWFA experiments performed to push the energy frontier.

The role of nanosecond duration pre-pulse pedestal (Amplified Spontaneous Emission (ASE) pre-pulse) in the propagation of 45 fs, 4 TW Ti:Sapphire laser pulse through a helium gas jet target has been investigated. We observed that the pre-pulse pedestal of about 1 ns duration and intensity 3 × 10¹² W/cm² creates pre-formed plasma with optical guiding channel like structure in the gas-jet at density around 3 × 10¹⁹ cm⁻³. Guiding of the 45 fs laser pulse (I_L = 3 × 10¹⁸ W/cm²) in the pre-formed plasma channel, over a distance much longer than the Rayleigh length was also observed. The guiding of the laser pulse resulted in the generation of high energy electron beam by laser wake-field acceleration of self-injected electrons. The accelerated electron beam was quasi-monoenergetic with peak energy up to 50 MeV, low divergence in the range of 3–6 mrad, and bunch charge up to 100 pC.

The generation of relativistic electron beams with quasi-thermal energy distribution (maximum energy: 30 MeV) by the interaction of a Ti:sapphire laser pulse of 200 fs duration, focussed to an intensity of ∼2.1 × 10 ¹⁸ W cm ⁻² , with an underdense (electron density ∼3.6 × 10 ¹⁹ to ∼1.1 × 10 ²⁰ cm ⁻³ ) He gas-jet plasma was observed. We observed two stages of self-focusing of the laser pulse in the plasma. Two groups of accelerated electrons associated with these two stages of laser channeling were also observed, and are attributed to the betatron resonance acceleration mechanism. This is supported by 2D PIC simulations performed using the EPOCH code and a detailed theoretical analysis. Further, the generation of quasi-monoenergetic (ΔE/E ∼ 10%–20%) electron beams with a peak energy of ∼17–22 MeV was also observed, even with such long laser pulses, although with a low probability of occurrence.