A STUDY OF THE WEIBEL INSTABILITY

Plasma (an electrified gas with atoms dissociated into positive ions and negative electrons) is often said to be the most abundant form of matter in the universe. The density of a plasma can vary over 28 orders of magnitude – lower density plasmas behaving like alternating gradient synchrotrons (where single particle trajectories need to be considered) while higher density plasmas tend to behave like fluids (motions of individual particles are unimportant) – thus encouraging us to think of plasmas as a ‘fourth state of matter’. In this report, we analyse the basic parameters of a plasma, briefly looking at the equations governing its behaviour to start with. We then proceed to study the Weibel Instability, and explore its evolution with time using numerical simulations

Intense ү-Ray bursts following the interaction of laser pulse with steep density gradients

We investigate the production of intense ү-rays following the interaction of ultraintense laser pulse with a hybrid combination of under-dense plasma associated with a thin foil of fully ionized Al or Cu or Au at the rear side. Relativistic electrons are accelerated following the interaction of high intensity laser pulses with an under-dense plasma. These electrons are then stopped by the thin foils attached to the rear side of the under-dense plasma. This results in the production of intense-ray bursts. So, the enhancement of photon generation is due to the under-dense plasma electrons interacting with different over-dense plasma. Using open-source PIC code EPOCH, we study the effect of different electron densities in the under-plasma on photon emission. Photon emission enhancement is observed by increasing the target Z in the hybrid structure. Hybrid structure can enhance photon emission; it can increase the photon energy and yield and improve photon beam divergence. Simulations were also performed to find the optimal under-dense plasma density for ү-ray production

Micrometer-sized negative-ion accelerator based on ultrashort laser pulse interaction with transparent solids

We report here energetic (>100keV) negative hydrogen ions (H−) generated in the interaction of moderately intense (1018Wcm−2) ultrashort laser pulses (45 fs) with transparent hydrogen containing solid targets. An unambiguous and consistent detection of negative hydrogen ions, with a flux of 8×1011H− ions/sr, has been observed in every single laser shot, using a Thomson parabola ion spectrograph. Simple estimates based on charge transfer cross sections match well with experimental observations. Our method offers the implementation of an intense, ultrashort laser based negative-ion source at a higher repetition rate, which can be important for various applications

High-intensity laser-accelerated ion beam produced from cryogenic micro-jet target

We report on the successful operation of a newly developed cryogenic jet target at high intensity laser-irradiation. Using the frequency-doubled Titan short pulse laser system at Jupiter Laser Fa- cility, Lawrence Livermore National Laboratory, we demonstrate the generation of a pure proton beam a with maximum energy of 2 MeV. Furthermore, we record a quasi-monoenergetic peak at 1.1 MeV in the proton spectrum emitted in the laser forward direction suggesting an alternative acceleration mechanism. Using a solid-density mixed hydrogen-deuterium target, we are also able to produce pure proton-deuteron ion beams. With its high purity, limited size, near-critical density, and high-repetition rate capability, this target is promising for future applications

Laser-Driven Ultrafast Field Propagation on Solid Surfaces

The interaction of a 3×1019  W/cm2 laser pulse with a metallic wire has been investigated using proton radiography. The pulse is observed to drive the propagation of a highly transient field along the wire at the speed of light. Within a temporal window of 20 ps, the current driven by this field rises to its peak magnitude ∼104  A before decaying to below measurable levels. Supported by particle-in-cell simulation results and simple theoretical reasoning, the transient field measured is interpreted as a charge-neutralizing disturbance propagated away from the interaction region as a result of the permanent loss of a small fraction of the laser-accelerated hot electron population to vacuum

Dynamics of the Electromagnetic Fields Induced by Fast Electron Propagation in Near-Solid-Density Media

The propagation of fast electron currents in near solid-density media was investigated via proton probing. Fast currents were generated inside dielectric foams via irradiation with a short ( ∼ 0.6     ps ) laser pulse focused at relativistic intensities ( I λ 2 ∼ 4 × 10 19     W   cm − 2   μ m 2 ). Proton probing provided a spatially and temporally resolved characterization of the evolution of the electromagnetic fields and of the associated net currents directly inside the target. The progressive growth of beam filamentation was temporally resolved and information on the divergence of the fast electron beam was obtained. Hybrid simulations of electron propagation in dense media indicate that resistive effects provide a major contribution to field generation and explain well the topology, magnitude, and temporal growth of the fields observed in the experiment. Estimations of the growth rates for different types of instabilities pinpoints the resistive instability as the most likely dominant mechanism of beam filamentation

Probing bulk electron temperature via x-ray emission in a solid density plasma

Bulk electron temperatures are calculated for thin Cu targets irradiated by the petawatt class Vulcan laser, from the Kα yield obtained using highly oriented pyrolytic graphite crystals. Cu-Kα emission studies have been used to probe the bulk electron temperature. A 30-80 eV core temperature extends homogeneously over distances up to ten times the laser focal spot size. Energy shifting has been observed due to different ionization states produced for different temperatures in the plasma. Polarization dependencies of plasma temperature are observed through the production of x-rays in different targets. 2D PIC simulations were performed to measure the polarization dependency of bulk electron temperature, which supports our experimental results. This paper could be of importance in understanding the different behavior of laser coupling at different polarizations and their role in x-ray production

Plasma instabilities and Ultrafast collisional ion heating by Electrostatic shocks

High-intensity lasers can be used to generate shockwaves, which have found applications in nuclear fusion, proton imaging, cancer therapies and materials science. Collision less electrostatic shocks are one type of shockwave widely studied for applications involving ion acceleration. Here we show a novel mechanism for collision less electrostatic shocks to heat small amounts of solid density matter to temperatures of ∼keV in tens of femto-seconds. Unusually, electrons play no direct role in the heating and it is the ions that determine the heating rate. Ions are heated due to an interplay between the electric field of the shock, the local density increase during the passage of the shock and collisions between different species of ion. In simulations, these factors combine to produce rapid, localized heating of the lighter ion species. Although the heated volume is modest, this would be one of the fastest heating mechanisms discovered if demonstrated in the laboratory

Proton Energy Spectra Calculation from Radiochromic Films

In this article, we report the various kinds of Radiochromic films and their char- acteristics as well as their applications in the medical field. We also developed an algorithm which gives the proton distribution at particular amount of energy for the Radiochromic film GafChromic HD-810 and compared the obtained result with al- ready exiting results of Breschi et al., [6] and G. S. Hicks et al., [4] with some error analysis