Bobbili Sanyasi Rao

We demonstrate high brightness betatron x-ray generation from chirped laser pulses driven plasma accelerator. It is shown that positively chirped laser pulse leads to the initiation and enhancement of collective oscillations of electrons inside plasma bubble, due to associated pulse front tilt (PFT). The PFT causes transverse drift of the bubble with respect to the laser axis, which results in high brightness x-ray generation. At an optimum chirp, enhanced x-ray emission of >108 photons/pulse/sr in 0.1% BW with a critical energy of ∼18 keV was observed by a factor >2 in comparison to the case of no chirp. The role of collective oscillation in enhancing x-ray emission is also validated in the Geant4 simulations.

Laser wakefield accelerators rely on relativistically moving micrometer-sized plasma cavities that provide extremely high electric field >100 GV/m. Here, we demonstrate transverse shaping of the plasma cavity to produce controlled sub-GeV electron beams, adopting laser pulses with an axially rotatable ellipse-shaped focal spot. We showed the control capability on electron self-injection, charge, and transverse profile of the electron beam by rotating the focal spot. We observed that the effect of the elliptical focal spot was imprinted in the profiles of the electron beams and the electron energy increased, as compared to the case of a circular focal spot. We performed three-dimensional particle-in-cell simulations which reproduced the experimental results and revealed dynamics of an asymmetric self-injection process. This simple scheme offers a control method on laser wakefield acceleration to produce tailored electron beams and x rays for various applications.

Laser wakefield accelerators rely on relativistically moving micron-sized plasma cavities that provide extremely high electric field >100GV/m. Here, we demonstrate transverse shaping of the plasma cavity to produce controlled sub-GeV electron beams, adopting laser pulses with an axially rotatable ellipse-shaped focal spot. We showed the control capability on electron self-injection, charge, and transverse profile of the electron beam by rotating the focal spot. We observed that the effect of the elliptical focal spot was imprinted in the profiles of the electron beams and the electron energy increased, as compared to the case of a circular focal spot. We performed 3D particle-in-cell (PIC) simulations which reproduced the experimental results and revealed dynamics of a new asymmetric self-injection process. This simple scheme offers a novel control method on laser wakefield acceleration to produce tailored electron beams and x-rays for various applications.

The plasma produced by frequency mixing of an ultrafast pulsed laser and its second-harmonic field in air is a promising, table-top source of terahertz radiation. To maximize the generation efficiency of such a source, multi-parameter optimization is essential. A parametric study involving various external controls is presented here. Starting from the optimization of phase and polarization of the second-harmonic field, the effect of laser intensity on the terahertz flux is reported. Moreover, the effect of external focusing conditions, such as focal length of the lens and effect of laser beam aperture, is explored in detail. Contrary to the previous reports, we find that with the increase of laser intensity, terahertz (THz) flux increases with more conversion efficiency; this is attributed to the multiple ionization of the air, leading to stacked release of contributory electrons. Exploration of beam convergence reveals that relatively weaker focusing leads to increased terahertz production compared to sharp focusing conditions. Moreover, temporal characterization of the emitted terahertz pulse conducted using electro-optic sampling provides pulse duration of 368 fs with corresponding frequency spectra up to 3.6 THz. The maximum electric field of the terahertz pulse is estimated to be 1.12 MV cm ⁻¹ in our experimental conditions. Results of 2D particle-in-cell simulations are also presented with the objective of validating the role of transient photocurrent mechanism in THz generation from two-color laser-induced gas plasma. We believe the broadband, bright THz source presented here will be useful in the imaging of bigger samples as well as nonlinear spectroscopy.

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.

We report a study on the generation of relativistic fast electrons during the interaction of an ultrashort (45 fs) laser pulse with a variety of targets (Al, Cu, Mo, Si, polyvinyl chloride, and Perspex), at a nearly grazing angle of incidence (∼70°) at an intensity of ∼1.2 × 10¹⁸ W/cm². The fast electron emission direction was found to be dependent on the laser pulse amplified spontaneous emission (ASE) contrast, its extent, and laser pulse duration. For a 45 fs laser pulse and an ASE extent of 1 ns, the electron emission direction changes from a nearly target surface (∼7° ± 2°, divergence: ∼30°) to a specular direction (∼20° ± 2°, divergence: ∼20°) when the contrast was decreased from ∼10⁻⁷ to ∼10⁻⁶. It further shifts toward target normal with an increase in the ASE duration from 1 ns to 5 ns. For an ASE contrast of ∼10⁻⁷ and ∼10⁻⁶ with a 1 ns duration, preplasma density scale lengths of ∼30 μm and ∼45 μm were estimated, respectively, from the HELIOS hydrodynamic code. For a longer ASE duration in the range of 1–5 ns, the density scale length was in the range of ∼45–140 μm. Further, for a 45 fs pulse and a 1 ns ASE level, the electron beam charge was found to be higher in the case of Mo (∼52 pC) and Si (∼122 pC) targets compared to the other targets (∼23–34 pC). A further increase in the charge of the electron beam of ∼1.45× (∼75 pC) and ∼2.7× (∼141 pC) was also observed with Mo for a longer pulse duration of ∼170 fs (ASE 1 ns) and a larger ASE duration of 5 ns at (45 fs pulse), respectively. The electron beam properties were found to be independent of laser polarization (p, s, and circular) except a slightly higher charge (∼1.5×) was observed in the case of p-polarized laser light. The fast electron spectrum showed a maximum energy of 1 MeV with electron temperatures of ∼390 keV and ∼320 keV for the target surface and specular direction, respectively. Electron acceleration and their emission are explained in terms of applicable mechanisms in preformed plasma on the target surface considering the role of surface fields which is also established by the 2D particle in cell simulation.

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.