B. S. Rao

Laser wake-field acceleration in pre-formed plasma channel created by pre-pulse pedestal of terawatt laser pulse

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 (IL = 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.

Study of fast electron jet produced from interaction of intense laser beam with solid target at oblique incidence

Fast electrons generated along target normal direction from the interaction of intense ultrashort Ti:sapphire laser pulses (λ0 = 800 nm) with planar copper target at 45° incidence angle have been experimentally studied under different interaction conditions. Angular spread and energy spectrum of the fast electrons was measured for both p- and s-polarized laser irradiation at intensities in the range 4 × 1016 – 4 × 1017 W/cm2 (for a fixed pulse duration of 45 fs) and for pulse duration in the range 45 fs–1.2 ps (for a fixed laser fluence of 1.8 × 104 J/cm2). The fast electrons were observed consistently along the target normal direction over the entire range of interaction conditions in the form of a collimated jet, within a half cone angle of 20°. The fast electrons have continuous energy spectrum with effective temperature 290 keV and 160 keV, respectively, for p- and s-polarized 45 fs laser pulse irradiation at intensity 4 × 1017 W/cm2. Scaling laws for temperature of fast electrons with laser intensity...

High-quality electron beam from laser wake-field acceleration in laser produced plasma plumes

Generation of highly collimated (θdiv ∼10 mrad), quasi-monoenergetic electron beam with peak energy 12 MeV and charge ∼50 pC has been experimentally demonstrated from self-guided laser wake-field acceleration (LWFA) in a plasma plume produced by laser ablation of solid nylon (C12H22N2O2)n target. A 7 TW, 45 fs Ti:sapphire laser system was used for LWFA, and the plasma plume forming pulse was derived from the Nd:YAG pump laser of the same system. The results show that a reproducible, high quality electron beam could be produced from this scheme which is simple, low cost and has the capability for high repetition rate operation.

Angular distribution and dose measurements of hard x-ray emission from intense laser-plasma interaction

An experimental study on hard x-ray Bremsstrahlung radiation due to fast electron generation from a planar solid copper target irradiated by 150 mJ, 45 fs Ti:sapphire laser pulses at I∼1018 W∕cm2 is reported. Angular distribution of the hard x-ray dose rate (hν≥40 keV) is observed to be strongly forward peaked in the direction of target normal with a measured peak value of 40 μSv∕h, at a distance of 500 mm from the target. Two sources of this radiation, one at the laser irradiated target and the other at the chamber wall facing the target, are inferred from the measurements of angular distribution and distance dependence of the x-ray dose rate.

Cathode Plasma Jet Pinching and Intense X-Ray Emission in a Moderate-Current Laser-Triggered Vacuum Discharge

Occurrence of cathode plasma jet pinching and its dependence on anode-cathode separation (d=2-10 mm) in a moderate-current (~10 kA) low-energy (les 20 J) vacuum discharge is presented. Discharge was created between a planar titanium cathode and a conical point-tip titanium anode by producing plasma on the cathode using 27-ps-duration laser pulses. For d=2-3 mm, constricted flow of cathode plasma jet up to anode was observed along with multiple feeble local pinchings. A neck formation was observed for d=5-6 mm, whereas multiple pinching occurred for larger separations. Temporal profiles of the X-ray emission from the anode due to bombardment by electrons extracted from the expanding cathode plasma, for different anode-cathode separations, were consistent with the characteristics of cathode plasma jet pinching

A comparative study of the inner-shell and the ionic line radiation from ultra-short laser-produced magnesium plasma

The inner-shell radiation (K-α) and the ionic line radiation (He-α) from magnesium plasma, generated by the interaction of a 3 TW, 45 fs laser, have been studied simultaneously using an x-ray crystal spectrograph. The effect of the variation of the laser intensity and its offset from the best focus position has been studied. He-α and K-α x-ray yields are found to scale with the laser intensity as IL1.5 and IL0.6 respectively. The K-α x-ray conversion shows a maximum at the best focus and reduces symmetrically on either side of the best focus position, whereas the He-α conversion peaks when the target is placed before the focused laser beam. The angular distribution for the He-α as well as the K-α emissions shows a maximum in the forward direction and the intensity reduces with the increase in angle θ with respect to the target normal as cosα θ. The value of α is 0.7 and 3 for He-α and K-α respectively. The experimentally observed variation of the He-α line conversion for different laser parameters has been explained by considering the change in preformed plasma conditions, and the variation in the K-α emission has been explained by considering hot electron generation and their propagation in the bulk solid target. The plasma conditions prevalent during the emission of the x-ray spectrum were identified by comparing the experimental spectra with the synthetic spectra generated using the spectroscopic analysis code PrismSPECT. The results will be useful in designing laser-produced plasma x-ray line radiation sources of photon energy in the range of 1–2 keV, for its potential use as a probe pulse in x-ray backlighting, or time-resolved x-ray diffraction studies.

Enhanced water window x-ray emission from in situ formed carbon clusters irradiated by intense ultra-short laser pulses

Enhanced water window x-ray emission (23–44 A) from carbon clusters, formed in situ using a pre-pulse, irradiated by intense (I > 1017 W/cm2) ultra-short laser pulse, is demonstrated. An order of magnitude x-ray enhancement over planar graphite target is observed in carbon clusters, formed by a sub-ns pre-pulse, interacting with intense main pulse after a delay. The effect of the delay and the duration of the main pulse is studied for optimizing the x-ray emission in the water window region. This x-ray source has added advantages of being an efficient, high repetition rate, and low debris x-ray source.

Betatron resonance electron acceleration and generation of relativistic electron beams using 200 fs Ti:sapphire laser pulses

Generation of collimated, quasi-monoenergetic electron beams (peak energy ~17-22MeV, divergence ~10mrad, energy spread ~20%) by interaction of Ti:sapphire laser pulse of 200fs duration, focussed to an intensity of ~ 2.1x10^18 W/cm^2,with an under-dense (density~3.6x10^19 to ~1.1x10^20 cm-3) He gas-jet plasma was observed. Two stages of self-focusing of the laser pulse in the plasma were observed. Two groups of accelerated electrons were also observed associated with these stages of the channeling and is attributed to the betatron resonance acceleration mechanism. This is supported by 2D PIC simulations performed using code EPOCH and a detailed theoretical analysis which shows that present experimental conditions are more favorable for betatron resonance acceleration and generation of collimated, quasi-thermal/quasi-monoenergetic electron beams.

Quasi-monoenergetic electron beams from a few-terawatt laser driven plasma acceleration using a nitrogen gas jet

An experimental investigation on laser plasma acceleration of electrons has been carried out using 3 TW, 45 fs duration titanium sapphire laser pulse interaction with nitrogen gas jet at intensity of 2x10^18 W/cm^2. We have observed stable generation of well collimated electron beam with divergence and pointing variation ~ 10 mrad from the nitrogen gas jet plasma at an optimum plasma density around 3x10^19 cm^-3. The energy spectrum of the electron beam was quasi-monoenergetic with average peak energy and charge around 25 MeV and 30 pC respectively. The results will be useful for better understanding and control of ionization injection and laser wakefield acceleration of electrons in high-Z gases and also to develop practical laser wakefield accelerators for various applications including injectors for high energy accelerators.

Ultra-high intensity laser-matter interaction studies at RRCAT, India

This article presents some of the recent experimental studies in ultra-high intensity laser-matter interaction, carried out at Raja Ramanna Centre for Advanced Technology, India, at laser intensities of ~3×10¹⁸ and ~5×10¹⁹ W/cm², using 10TW and 150 TW Ti:sapphire laser systems.