Self-generated Electric Fields Research Articles

Two-dimensional investigation of characteristic parameters and their gradients for the self-generated electric and magnetic fields of laser-induced zirconium plasma

Two-dimensional diagnosis of laser-induced zirconium (Zr) plasma has been experimentally performed using the time-of-flight method by employing Faraday cups in addition to electric and magnetic probes. The characteristic parameters of laser-induced Zr plasma have been evaluated as a function of different laser irradiances ranging from 4.5 to 11.7 GW cm−2 at different axial positions of 1–4 cm with a fixed radial distance of 2 cm. A well-supporting correlation between the plume parameters and the laser-plasma-produced spontaneous electric and magnetic (E and B) fields was established. The measurements of the characteristic parameters and spontaneously induced fields were observed to have an increasing trend with the increasing laser irradiance. However, when increasing the spatial distance in both the axial and radial directions, the plasma parameters (electron/ion number density, temperature and kinetic energy) did not show either continuously increasing or decreasing trends due to various kinetic and dynamic processes during the spatial evolution of the plume. However, the E and B fields were observed to be always diffusing away from the target. The radial component of electron number densities remained higher than the axial number density component, whereas the axial ion number density at all laser irradiances and axial distances remained higher than the radial ion number density. The higher axial self-generated electric field (SGEF) values than radial SGEF values are correlated with the effective charge-separation mechanism of electrons and ions. The generation of a self-generated magnetic field is observed dominantly in the radial direction at increasing laser irradiance as compared to the axial one due to the deflection of fast-moving electrons and the persistence of two-electron temperature on the radial axis.

Hohlraum fields with monoenergetic proton radiography at OMEGA.

A more complete understanding of laser-driven hohlraum plasmas is critical for the continued development and improvement of ICF experiments. In these hohlraums, self-generated electric and magnetic fields can play an important role in modifying plasma properties such as heat transport; however, the strength and distribution of electromagnetic fields in such hohlraums remain largely uncertain. To explore this question, we conducted experiments at the OMEGA laser facility, using monoenergetic proton radiography to probe laser-driven vacuum hohlraums. We then utilized reconstructive methods to recover information about proton deflections. To interpret these reconstructions, a new technique for detangling the contributions of electric and magnetic fields to proton deflections was developed. This work was supported in part by the U.S. Department of Energy, the National Laser Users' Facility, and the Laboratory for Laser Energetics.

Characterization of bright betatron radiation generated by direct laser acceleration of electrons in plasma of near critical density

Directed x-rays produced in the interaction of sub-picosecond laser pulses of moderate relativistic intensity with plasma of near-critical density are investigated. Synchrotron-like (betatron) radiation occurs in the process of direct laser acceleration (DLA) of electrons in a relativistic laser channel when the electrons undergo transverse betatron oscillations in self-generated quasi-static electric and magnetic fields. In an experiment at the PHELIX laser system, high-current directed beams of DLA electrons with a mean energy ten times higher than the ponderomotive potential and maximum energy up to 100 MeV were measured at 1019 W/cm2 laser intensity. The spectrum of directed x-rays in the range of 5–60 keV was evaluated using two sets of Ross filters placed at 0° and 10° to the laser pulse propagation axis. The differential x-ray absorption method allowed for absolute measurements of the angular-dependent photon fluence. We report 1013 photons/sr with energies >5 keV measured at 0° to the laser axis and a brilliance of 1021 photons s−1 mm−2 mrad−2 (0.1%BW)−1. The angular distribution of the emission has an FWHM of 14°–16°. Thanks to the ultra-high photon fluence, point-like radiation source, and ultra-short emission time, DLA-based keV backlighters are promising for various applications in high-energy-density research with kilojoule petawatt-class laser facilities.

Particle-in-cell method for plasmas in the one-dimensional electrostatic limit

We discuss the particle-in-cell (PIC) method, which is one of the most widely used approaches for the kinetic description of plasmas. The positions and velocities of the charged particles take continuous values in phase space, and spatial macroscopic quantities, such as the charge density and self-generated electric fields, are calculated at discrete spatial points of a grid. We discuss the computer implementation of the PIC method for one-dimensional plasmas in the electrostatic regime and discuss a desktop application (PlasmAPP), which includes the implementation of different numerical and interpolation methods and diagnostics in a graphical user interface. To illustrate its functionality, the electron-electron two-stream instability is discussed. Readers can use PlasmAPP to explore advanced numerical methods and simulate different phenomena of interest.

Investigation of charged particles dynamics along with self-generated electric and magnetic fields in axially resolved laser-induced magnesium plasma

Laser ablated Mg plasma was generated by employing Nd-YAG laser (532 nm, 10 ns). Faraday cups with Electric and Magnetic probes have been employedto evaluate axial plasma parameters and Self-Generated Electric and Magnetic Fields (SGEMFs) at various axial distances from target (1 cm to 4 cm) and at various laser irradiances 1.8 GW/cm2 to 4.5 GW/cm2. The evaluated number densities of ions vary from 1.4 × 1013 cm−3 to 6.91 × 1013 cm−3 and electron number densities variation is from 2.38 × 1013 cm−3 to 5.15 × 1013 cm−3. The electron temperature varies from 1.3 eV to 125 eV, whereas, ion kinetic energies vary from 114 eV to 385 eV. Moreover, self-generated electric field variation is from 0.28 V/cm to 20 V/cm and magnetic field is from 978 Gauss to 2628 Gauss. Furthermore, the maxima of ion signal amplitude have been observed at 3 cm distance from target, whereas, electron signals amplitude decreases as Faraday cup to target distance increases. Both evaluated SGEMFs show decreasing trends with increasing target to probe distances and increase with increasing laser irradiance. It is observed that the shape of SGEMFs signals have been significantly affected by increasing laser irradiances and transform from unipolar to dipolar. The charge separation of plasma species, two-electron temperature distribution and bipolar signals of SGEMFs confirm the quadruple distribution of charges in plasma. It uncovers the importance of ion and electron number density gradients in plasma.

Investigation of number density, temperature, and kinetic energy of nanosecond laser-induced Zr plasma species for self-generated electric and magnetic fields in axial expansion of plume

The number density, temperature, and kinetic energy of laser-induced Zr plasma species have been evaluated experimentally under ultra-high vacuum conditions as a function of 1 to 4 cm axial distances of the plume at laser irradiances ranging from 4.5 to 11.7 G W c m − 2 . Zr plasma parameters have been evaluated using time-of-flight (ToF) measurements by using a Faraday cup (FC) as a diagnostic technique. The amplitudes of the signals as well as the number density and temperature are strongly dependent upon laser irradiance and axial distances. With increasing laser irradiance the laser-induced plasma (LIP) parameters show an increasing trend. The increasing trend of the plasma parameters along with increasing laser irradiance is attributed to an enhanced energy deposition per atom, which is responsible for an enhanced ablation rate, surface temperature, and ablation/shock pressures. However, a decreasing trend is obtained with increasing axial distances, which is explainable based on adiabatic expansion, cooling, and recombination losses. The LIP electron and ion number densities vary from 3.13 × 1 0 13 to 6.81 × 1 0 13 c m − 3 and 1.84 × 1 0 13 to 4.41 × 1 0 13 c m − 3 , respectively. The electron temperature varies from 1.54 to 125 eV, whereas the kinetic energy of the ions varies from 1.2 to 34 keV. The maxima of Zr plasma parameters are achieved away from the target surface. In order to correlate the charge particle distribution and their energies with self-generated electric and magnetic fields (SGEMFs), the measurements have been performed by employing electric and magnetic probes, respectively. The signal profiles of the SGEMFs reveal the quadrupolar distribution, which is attributed to two oppositely generated dipoles whose contribution is strongly dependent upon axial distances and laser irradiances. The strength of both SGEMFs shows an increasing trend with increasing laser irradiance, and a decreasing trend is obtained with increasing probe-target distance. For overall plume axial expansion, the LIP electric and magnetic fields vary from 30 to 3140 V/m and 0.2 to 0.4 T, respectively. Similar trends of the SGEMFs and the axial gradients of the electron-ion number density confirm the quadrupolar structure of plasma, which corresponds to the electron-ion charge separation. This charge separation is due to the spatially evolved number density and temperature gradients in the plasma. Their correlation and optimization will be helpful for using LIP as a powerful source of electric and magnetic fields in ion acceleration and inertial confinement fusion.

Understanding the electromagnetic topology during the ohmic breakdown in tokamaks considering self-generated electric fields

The physical mechanisms of the ohmic breakdown in a tokamak have been understood based on the classical Townsend avalanche theory. However, a new systematic theory (Yoo et al 2018 Nat. Commun. 9 3523) recently demonstrated that electron avalanches during the ohmic breakdown are completely different from the Townsend avalanche due to strong self-generated electric fields. In this study, we elucidate the multi-dimensional effects of the self-generated electric field on plasma dynamics during the ohmic breakdown. We also propose a novel electromagnetic topology analysis method that can easily predict the overall plasma behavior and where the main plasma is generated. The topology analysis method is validated by a state-of-art particle simulation for various magnetic configurations. New physical insights into the complex electromagnetic topology would facilitate designing more reliable and optimized ohmic breakdown scenarios in future tokamaks, such as ITER and beyond.

Evaluation and measurement of laser induced Zr-plasma parameters along with self-generated electric and magnetic fields under various pressures of Ar environment

Nd:YAG (532 nm, 6 ns) laser generated Zr plasma parameters under various pressure of Ar at irradiance of 17 GWcm−2 have been investigated and their correlation with the estimated Self-Generated Electric and Magnetic fields (SGEMF’s) has been established. Faraday Cup is used for measurements of Kinetic Energy (K.E) and fluence, whereas, electric and magnetic probes have been employed for the measurements of SGEMFs of Zr plasma. Under various Ar pressures ranging from 1 Torr to 200 Torr, an increasing trend of K.E and decreasing trend for the fluence of Zr plasma ions have been observed. The K.E of Zr ions increases from 15.6 keV to 140.8 keV, whereas, fluence decreases from 1.2×1012 cm−2 to 6.8×1011 cm−2 by increasing the Ar pressures. The electron fluence increases up to 4.25×1013 cm−2 and then decreases from 3.4×1013 cm−2 to 9.1×1012 cm−2 by increasing Ar gas pressure. Similarly, the self-generated electric field (SGEF) of Zr plasma decreases form 95 Vm−1 to 0 and self-generated -magnetic-field (SGMF) initially increases from 100 G to 340 G and then decreases to 145 G. The monotonic decrease in electric field with increasing the Ar gas pressure is attributed to the monotonic decrease of ion fluence due to ambipolar diffusion and decrease in charge separation by recombination losses. Similarly, the initial increase and then a decrease of magnetic field along with reversal is related to initial increase and then decrease of electron fluence and non-parallel density and temperature gradients (∇ne×∇Te≠ 0) of Zr plasma.

Measurement of characteristic parameters and self-generated electric and magnetic fields (SGEMFs) of laser-induced aluminum plasma

The characteristic parameters as well as self-generated electric and magnetic fields of Nd:YAG laser (532 nm, 10 ns) ablated Al plasma have been measured by employing Langmuir, electric and magnetic probes, respectively. Plasma parameters such as electron temperature ( $${T}_{e}$$ ) and electron number density ( $${n}_{e}$$ ) as well as SGEMFs of Al plasma are measured as a function of laser irradiance ranging from 5.2 to 9.4 GW/cm2 at fixed probes to target distance of 10 mm. Both $${T}_{e}$$ and $${n}_{e}$$ of Al plasma show an increasing trend with the increase of laser irradiance and vary from 8 to 11 eV and 3.9 × 1014 to 7.07 × 1014/cm3, respectively. The SGEMFs show an overall increasing trend with the increase of laser irradiance and a decreasing trend with increasing probe to plume distances. The evaluated values of SGEMFs vary from 50 to 380 V/m and 89 to 165 G. The growth of SGEMFs has been explained on the basis of quadrupole distribution of charges at leading and trailing fronts of expanding plasma. By controlling plasma parameters such as electron temperature and electron density, laser generated plasmas can be efficiently used as sources of pulsed electric and magnetic fields in wake field accelerators.

Bright betatron x-rays generation from picosecond laser interactions with long-scale near critical density plasmas

Our previous experimental and three-dimensional (3D) particle-in-cell (PIC) simulation results demonstrated that a well-directed electron beam with space charge of about μC and maximum energy of 100 MeV can be generated via a sub-petawatt, picosecond laser pulse interacting with a long-scale near-critical-density plasma. Effective laser energy coupling into hot electrons occurs in the presence of strong self-generated quasistatic electric and magnetic fields that confine fast electrons in relativistic ion channels. Here, we report results of 3D PIC simulations, which show that this direct laser accelerated electron beam can work as a compact high-brightness source of x rays. The relativistic electrons make betatron oscillations in the transverse fields of the ion channel and emit a bright broadband x-ray radiation with the critical energy of about 5 keV. Due to the huge number of accelerated electrons, our simulation shows that with a picosecond, 20 J laser pulse, an x-ray spectrum with a photon number of 7×1011 (>1 keV) can be generated, resulting into a peak flux of 2 ×108 photons/eV and a brilliance of 3.3×1020 photons/s/mm2/mrad2/0.1%BW.

Exclusion of Cosmic Rays from Molecular Clouds by Self-generated Electric Fields

Abstract It was recently discovered that in some regions of the Galaxy, the cosmic-ray (CR) abundance is several orders of magnitude higher than previously thought. Additionally, there is evidence that in molecular cloud envelopes, the CR ionization may be dominated by electrons. We show that for regions with high, electron-dominated ionization, the penetration of CR electrons into molecular clouds is modulated by the electric field that develops as a result of the charge they deposit. We evaluate the significance of this novel mechanism of self-modulation and show that the CR penetration can be reduced by a factor of a few to a few hundred in high-ionization environments, such as those found near the Galactic center.

Self-Generated Magnetic and Electric Fields at a Mach-6 Shock Front in a Low Density Helium Gas by Dual-Angle Proton Radiography.

Shocks are abundant both in astrophysical and laboratory systems. While the electric fields generated at shock fronts have recently attracted great attention, the associated self-generated magnetic field is rarely studied, despite its ability to significantly affect the shock profile in the nonideal geometry where density and temperature gradients are not parallel. We report here the observation of a magnetic field at the front of a Mach ∼6 shock propagating in a low-density helium gas system. Proton radiography from different projection angles not only confirms the magnetic field's existence, but also provides a quantitative measurement of the field strength in the range ∼5 to 7T. X-ray spectrometry allowed inference of the density and temperature at the shock front, constraining the plasma conditions under which the magnetic and electric fields are generated. Simulations with the particle-in-cell code lsp attribute the self-generation of the magnetic field to the Biermann battery effect (∇n_{e}×∇T_{e}).

Characterization of laser-driven proton acceleration from water microdroplets

We report on a proton acceleration experiment in which high-intensity laser pulses with a wavelength of 0.4 μm and with varying temporal intensity contrast have been used to irradiate water droplets of 20 μm diameter. Such droplets are a reliable and easy-to-implement type of target for proton acceleration experiments with the potential to be used at very high repetition rates. We have investigated the influence of the laser’s angle of incidence by moving the droplet along the laser polarization axis. This position, which is coupled with the angle of incidence, has a crucial impact on the maximum proton energy. Central irradiation leads to an inefficient coupling of the laser energy into hot electrons, resulting in a low maximum proton energy. The introduction of a controlled pre-pulse produces an enhancement of hot electron generation in this geometry and therefore higher proton energies. However, two-dimensional particle-in-cell simulations support our experimental results confirming, that even slightly higher proton energies are achieved under grazing laser incidence when no additional pre-plasma is present. Illuminating a droplet under grazing incidence generates a stream of hot electrons that flows along the droplet’s surface due to self-generated electric and magnetic fields and ultimately generates a strong electric field responsible for proton acceleration. The interaction conditions were monitored with the help of an ultra-short optical probe laser, with which the plasma expansion could be observed.

Laser-driven acceleration of quasi-monoenergetic, near-collimated titanium ions via a transparency-enhanced acceleration scheme

Laser-driven ion acceleration has been an active research area in the past two decades with the prospects of designing novel and compact ion accelerators. Many potential applications in science and industry require high-quality, energetic ion beams with low divergence and narrow energy spread. Intense laser ion acceleration research strives to meet these challenges and may provide high charge state beams, with some successes for carbon and lighter ions. Here we demonstrate the generation of well collimated, quasi-monoenergetic titanium ions with energies ∼145 and 180 MeV in experiments using the high-contrast (<10−9) and high-intensity () Trident laser and ultra-thin (∼100 nm) titanium foil targets. Numerical simulations show that the foils become transparent to the laser pulses, undergoing relativistically induced transparency (RIT), resulting in a two-stage acceleration process which lasts until ∼2 ps after the onset of RIT. Such long acceleration time in the self-generated electric fields in the expanding plasma enables the formation of the quasi-monoenergetic peaks. This work contributes to the better understanding of the acceleration of heavier ions in the RIT regime, towards the development of next generation laser-based ion accelerators for various applications.

High Energy electron and proton acceleration by circularly polarized laser pulse from near critical density hydrogen gas target

Relativistic electron rings hold the possibility of very high accelerating rates, and hopefully a relatively cheap and compact accelerator/collimator for ultrahigh energy proton source. In this work, we investigate the generation of helical shaped quasi-monoenergetic relativistic electron beam and high-energy proton beam from near critical density plasmas driven by petawatt-circularly polarized-short laser pulses. We numerically observe the efficient proton acceleration from magnetic vortex acceleration mechanism by using the three dimensional particle-in-cell simulations; proton beam with peak energy 350 MeV, charge ~10nC and conversion efficiency more than 6% (which implies 2.4 J proton beam out of the 40 J incident laser energy) is reported. We detailed the microphysics involved in the ion acceleration mechanism, which requires investigating the role of self-generated plasma electric and magnetic fields. The concept of efficient generation of quasi-monoenergetic electron and proton beam from near critical density gas targets may be verified experimentally at advanced high power – high repetition rate laser facilities e.g. ELI-ALPS. Such study should be an important step towards the development of high quality electron and proton beam.

Study of self-generated fields in strongly-shocked, low-density systems using broadband proton radiography

We report results from experiments on the study of field generation at the shock front in low-density gas configured in quasi-planar geometry using broad-energy proton probing. Experiments were conducted using three long pulse laser beams with a total energy of 6.4 kJ in 2 ns for shock generation and an 850 J, 10 ps short pulse laser to produce broadband protons for radiography. Observations of the deflection pattern of probe protons show the existence of self-generated electric fields at the shock front with the electric potential on the order of 300 V. Analytical and particle tracking methods support this conclusion.

Energetics of Sensing and Communication in Electric Fish: A Blessing and a Curse in the Anthropocene?

Weakly electric freshwater fish use self-generated electric fields to image their worlds and communicate in the darkness of night and turbid waters. This active sensory/communication modality evolved independently in the freshwaters of South America and Africa, where hundreds of electric fish species are broadly and abundantly distributed. The adaptive advantages of the sensory capacity to forage and communicate in visually-unfavorable environments and outside the detection of visually-guided predators likely contributed to the broad success of these clades across a variety of Afrotropical and neotropical habitats. Here we consider the potentially high and limiting metabolic costs of the active sensory and communication signals that define the gymnotiform weakly electric fish of South America. Recent evidence from two well-studied species suggests that the metabolic costs of electrogenesis can be quite high, sometimes exceeding one-fourth of these fishes' daily energy budget. Supporting such an energetically expensive system has shaped a number of cellular, endocrine, and behavioral adaptations to restrain the metabolic costs of electrogenesis in general or in response to metabolic stress. Despite a suite of adaptations supporting electrogenesis, these weakly electric fish are vulnerable to metabolic stresses such as hypoxia and food restriction. In these conditions, fish reduce signal amplitude presumably as a function of absolute energy shortfall or as a proactive means to conserve energy. In either case, reducing signal amplitude compromises both sensory and communication performance. Such outcomes suggest that the higher metabolic cost of active sensing and communication in weakly electric fish compared with the sensory and communication systems in other neotropical fish might mean that weakly electric fish are disproportionately susceptible to harm from anthropogenic disturbances of neotropical aquatic habitats. Fully evaluating this possibility, however, will require broad comparative studies of metabolic energetics across the diverse clades of gymnotiform electric fish and in comparison to other nonelectric neotropical fishes.

Electrolocation of objects in fluids by means of active sensor movements based on discrete EEVs

Weakly electric fish use self-generated electric fields for communication and for active electrolocation. The sensor part of the biological system consists of a vast amount of electroreceptors which are distributed across the skin of the electric fish. Fish utilise changes of their position and body geometry to aid in the extraction of sensory information. Inspired by the biological model, this study looks for a fixed, minimal scanning strategy compiled of active receptor-system movements that allows unique identification of the positions of objects in the vicinity. The localisation method is based on the superposition of numerical extracted contour-rings of rotated and/or linearly shifted EEVs (Solberg et al 2008 Int. J. Rob. Res. 27 529–48), simulated by means of FEM. For the evaluation of a movement sequence, matrices of unique intersection points and respective contrast functions are introduced. The resultant optimal scanning strategy consists of a combination of a linear shift and a rotation of the original EEV.

Efficient quasi-monoenergetic ion beams from laser-driven relativistic plasmas.

Table-top laser–plasma ion accelerators have many exciting applications, many of which require ion beams with simultaneous narrow energy spread and high conversion efficiency. However, achieving these requirements has been elusive. Here we report the experimental demonstration of laser-driven ion beams with narrow energy spread and energies up to 18 MeV per nucleon and ∼5% conversion efficiency (that is 4 J out of 80-J laser). Using computer simulations we identify a self-organizing scheme that reduces the ion energy spread after the laser exits the plasma through persisting self-generated plasma electric (∼1012 V m−1) and magnetic (∼104 T) fields. These results contribute to the development of next generation compact accelerators suitable for many applications such as isochoric heating for ion-fast ignition and producing warm dense matter for basic science.

Quadrupole distribution generated by a laser induced plasma (LIP) in air in earliest instants using pulses of 532 or 355 nm

The self-generated electric and magnetic fields in laser induced plasmas (LIPs) in air during the first 40 ns are experimentally investigated using different electric, magnetic and optical techniques. To produce LIPs we used the second and third harmonics (532 and 355 nm) of a Nd:YAG nanosecond pulsed laser with a range of irradiance from to W . The variation in time of the electric field was detected using the tip of a coaxial cable, and the spontaneous magnetic field (SMF) was measured using a probe. The spatial and temporal evolution of the plasma was studied using shadowgraphy and fast photography. It was observed that produced LIPs using pulses of 532 and 355 nm, generate plasmas of double core over the laser axis, while we observed that produced LIPs by pulses of 1064 nm are composed of a single core plasma. We found that the double-core plasmas have a quadrupole distribution of the charge, consisting of two oppositely directed dipoles which in turn correspond to each plasma core. The magnetic diagnostic showed an oscillating magnetic field azimuthal to the main axis of the double-plasma.