Anand Moorti

Generation of a highly collimated, mono-energetic electron beam from laser-driven plasma-based acceleration

An electron acceleration experiment in a self-modulated laser wake- field acceleration regime was carried out using a 45fs duration Ti:sapphire laser focused to an intensity of 1.8◊10 18 Wcm 2 on helium gas-jet plasma. A highly collimated and mono-energetic electron beam was observed at a plasma electron density of 8.5◊10 19 cm 3 . The electron beam was produced with a minimum full cone divergence angle of 4mrad, an energy spread of ±4% and a peak energy up to 21MeV. The experimental results were explained on the basis of self-modulation of the laser beam and acceleration in the bubble regime.

Energy distributions of highly charged ions escaping from a plasma via a low-voltage laser-induced discharge

The results of experimental studies of ion beams escaping from both a laser-produced plasma and a plasma of laser-induced vacuum discharge are presented. The parameters of the discharge with an Al cathode are as follows: current amplitude is about 1.7?kA and current rise rate is about 7.5 ? 109?A?s?1 as the capacitor voltage is 2.3?kV. The discharge is initiated with a laser pulse of 30?ps length, energy less than 10?mJ and power density IL = 5 ? 1011?W?cm?2. It is shown that the discharge is a source of accelerated and highly charged ions of the cathode material. The production of Al ions is observed at the instant when the discharge current rise rate attains a peak value and just at that same instant a pinching occurs in front of the cathode jet expanding into the interelectrode gap. Ion energy distributions are characterized by the presence of a significant non-Maxwellian tail of the accelerated ions. The maximum ion charge state and energy per charge unit are +8 and 13?keV/Z, respectively. The energy is comparable to the value observed under similar experimental conditions for the laser-produced plasma at a pulse energy of 400?mJ and power density IL = 2 ? 1013?W?cm?2.

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.

Flux of multiple charged metal ions of high energy from plasma produced by a moderate energy laser pulse

Studies of the energy spectrum and charge state distribution of ions produced from an Al target irradiated with a laser pulse of moderate energy (90 mJ) and light intensity (4.2 × 1012 W cm−2) and of 27 ps pulse duration are presented. It is established that highly charged ions of the target material up to Al11+ are emitted from the laser produced plasma and evidence of the presence of Al12+ ions is also established. The maximum of the bulk of ion energy distributions approaches 7 keV/Z, which exceeds the values typically observed earlier in plasma produced by laser radiation of the intensity mentioned above. Theoretical examination of the experimental results reveals that the relatively high ion energies can be explained by the specific conditions of the experiments, namely the relatively large diameter of focal spot of the laser beam, appropriate length of the laser pulse, and so on.

High-current-density electron emission from lead zirconium titanate ferroelectric disc under application of short-duration high-voltage pulses

High-current-density pulsed-electron emission is observed from a poled lead zirconium titanate (PZT, Zr/Ti:53/47) ferroelectric ceramic disc on application of short-duration (/spl sim/6 ns) negative high-voltage (/spl sim/4 kV) pulses. Electron-emission pulses with a peak current density /spl sim/400--450 A/cm/sup 2/ and a full-width at half-maximum (FWHM) duration of /spl sim/200--250 ns were recorded in the presence of a dc extraction field. These are comparable to the various earlier results obtained using excitation pulses of duration comparable to the polarization switching time (/spl ges/100 ns). Self-emission of electrons with a current density of /spl sim/20--30 A/cm/sup 2/ was also observed. The experimental observations indicate occurrence of partial polarization reversal in the ferroelectric sample followed by plasma formation on the surface.

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

Generation of multi-keV monochromatic twin x-ray point sources based on laser-driven vacuum diode

A simple method of generating multi-keV, monochromatic, twin x-ray point sources based on laser-driven vacuum diode is presented. Electrons from a laser-produced aluminum plasma cathode were accelerated towards two point-tip titanium anodes placed symmetrically with respect to the cathode. Temporal and spatial characterizations of the K-shell x-ray emission from the twin anodes are carried out. Each source (∼300μm diameter) emits ∼109TiKα (hν≈4.51keV) photons in a pulse of ∼20ns duration with a peak brightness of ∼4×1018photons∕cm2ssr. Single-shot twin x-ray shadowgrams of physical objects were recorded at a distance of several centimeters from the two sources. Such a system may be useful for single-shot stereoradiography for differential imaging in the multi-keV energy region.

Formation of single pinched plasma point in the cathode plasma jet of a multipicosecond laser-triggered vacuum discharge

Characteristics of cathode plasma jet pinching and x-ray emission from a multipicosecond laser-triggered vacuum discharge are presented. Discharge was created in between a planar Al cathode and a conical point-tip Ti anode (separation: 2-15 mm, circuit inductance of approximately 0.53 microH, peak discharge current of approximately 3 kA, and rise time of approximately 400 ns). For anode-cathode separation of approximately 13.5 mm, only a single pinched plasma point was formed in the cathode plasma jet at a distance of approximately 9.5 mm from the cathode. Quantitative analysis of the x-ray signals recorded using a pin diode with different filters and viewing different regions of the discharge, shows soft ( approximately keV photon energy) x-ray emission from the plasma point with a flux of approximately (3-5)x10(10) photons/sr, and multi-keV x-ray emission from the Ti anode with Kalpha ( approximately 4.51 keV) photon flux of approximately 10(10) photons/sr.

Fast ion generation in the cathode plasma jet of a multipicosecond laser-triggered vacuum discharge

Ion generation in the cathode plasma jet of a moderate-current (approximately 2.3 kA), low-energy (< or = 20 J) vacuum spark discharge triggered by approximately 27 ps, 10 mJ laser pulses is studied using time of flight technique. Fastest ion velocity and velocity corresponding to the peak of the time of flight signals for Al cathode were measured to be approximately 5.25x10(8) cm/s (energy of approximately 143 keV/u) and approximately 8.1x10(7) cm/s (energy of approximately 3.4 keV/u), respectively. Corresponding velocities in the case of ions generated from laser-produced Al plasma (energy of approximately 550 mJ, intensity of approximately 10(14) W/cm(2)) were found to be much smaller, viz., approximately 1.05x10(8) cm/s (energy of approximately 5.75 keV/u) and approximately 2.63x10(7) cm/s (energy of approximately 0.36 keV/u), respectively. Study shows efficient acceleration of ions in a current-carrying cathode plasma jet of a small-energy multipicosecond laser-triggered spark discharge as compared with that in a high-energy multipicosecond laser-produced plasma plume.