Large Number Of Energetic Electrons Research Articles

Influence of nitrogen and oxygen admixture on the development of helium atmospheric-pressure plasma jet

This paper presents the experimental and numerical results about the influence of nitrogen (N2) and oxygen (O2) admixture on the development of a helium (He) atmospheric-pressure plasma jet (APPJ) in a long dielectric tube. The results revealed that the jet length and the propagation velocity are strongly affected by introducing N2 or O2 into the He flow. Specifically, it was observed that a higher N2/O2 admixture led to the decrease in the density of both energetic and relative low-energy electrons outside the grounded electrode, which corresponds to the shortening of the jet length. In the He/O2 mixture, the electrons are easily captured by O2/O in the region of the plasma bulk. In the He/N2 mixture, the jet propagation characteristics will change since N2 has many low-level excitation states that consumed a large number of energetic electrons. The simulation shows that the magnitude of the axial electric field in the jet head depends strongly on the amount of N2 and/or O2 in the gas flow. In both cases, the peak electric field is on the order of 5 kV/cm, which is significantly higher than that in pure helium of 3 kV/cm even if the admixture is low (less than 4% N2 or 2% O2 in this work). Positive charges of higher density in the jet head are needed to induce a stronger electric field for the jet propagating in N2(x%)/He and O2(x%)/He mixtures compared with that in pure He.

(Keynote) Generation of Hot Electrons in Plasmonic and Hybrid Metastructures for Photochemical Applications

Generation of energetic (hot) electrons is an intrinsic property of any plasmonic nanostructure under illumination. Simultaneously, a striking advantage of metal nanocrystals over semiconductors lies in their very large absorption cross sections. Therefore, metal nanostructures with strong and tailored plasmonic resonances are very attractive for photocatalytic applications in which excited electrons play an important role [1,2]. However, the central questions in the problem of plasmonic hot electrons are the number of optically excited energetic electrons in a nanocrystal and how to extract such electrons. Here we develop a theory describing the generation rates and the energy distributions of hot electrons in nanocrystals with various geometries [3-5]. In particular, we show that nanostructures with strong and extended plasmonic hot spots generate unusually large numbers of energetic electrons, which can be observed using ultra-fast transient spectroscopy [4,6] as well as photochemistry. Hot-electron generation together with non-radiative plasmonic transfer [7] represent efficient methods to convert optical energy into other forms, such as electricity or chemical energy. V. Hartland, L. Besteiro, P. Johns, A. O. Govorov, What’s so Hot about Electrons in Metal Nanoparticles? (perspective paper), ACS Energy Letters, 2, 1641–1653 (2017).Sousa-Castillo, M. Comesaña-Hermo, B. Rodríguez-González, M. Pérez-Lorenzo, Z. Wang, X.-T. Kong, A. O. Govorov, and M. A. Correa-Duarte, J. Phys. Chem. C, 120, 11690–11699 (2016).O. Govorov, H. Zhang, V. Demir, and Y. K. Gun’ko, Nano Today, 9, 85 (2014).Harutyunyan, A. B. F. Martinson, D. Rosenmann, L.K. Khorashad, L.V.Besteiro, A.O.Govorov, and G.P. Wiederrecht, Nature Nanotechnology 10, 770–774 (2015).V. Besteiro, X.-T. Kong, Z. Wang, G. V. Hartland, A.O. Govorov, ACS Photonics, DOI: 10.1021/acsphotonics.7b00751 (2017). E. Sykes, J. W. Stewart, G. M. Akselrod, X.-T. Kong, Z. Wang, D. J. Gosztola, A.B.F. Martinson, D. Rosenmann, M. H. Mikkelsen, A. O. Govorov, G. P. Wiederrecht, Nature Communications, 8, 986 (2017). -M. Roller, L.V. Besteiro, C. Pupp, L. Khosravi Khorashad, A. O. Govorov, T.Liedl, Nature Physics, 13, 761-765 (2017).

(Invited) The Mechanisms of Hot-Electron Generation in Plasmonic and Hybrid Nanomaterials: Comparing Different Material Systems and Geometries

Generation of energetic (hot) electrons is an intrinsic property of any plasmonic nanostructure under illumination. Simultaneously, a striking advantage of metal nanocrystals over semiconductors lies in their very large absorption cross sections. Therefore, metal nanostructures with strong and tailored plasmonic resonances are very attractive for photocatalytic applications in which excited electrons play an important role. However, the central questions in the problem of plasmonic hot electrons are the number of optically excited energetic electrons in a nanocrystal and how to extract such electrons. Here we develop a theory describing the generation rates and the energy distributions of hot electrons in nanocrystals with various geometries [1-4]. In particular, we show that nanostructures with strong and extended plasmonic hot spots generate unusually large numbers of energetic electrons, which can be observed using the ultra-fast transient spectroscopy [2,4,5] as well as photochemistry. Hot-electron generation together with non-radiative plasmonic transfer [6] represent efficient methods to transfer and localize optical and photo-chemical energies. A.O. Govorov, H. Zhang, V. Demir, and Y. K. Gun’ko, Nano Today, 9, 85 (2014).H. Harutyunyan, A. B. F. Martinson, D. Rosenmann, L.K. Khorashad, L.V.Besteiro, A.O.Govorov, and G.P. Wiederrecht, Nature Nanotechnology 10, 770–774 (2015).L.V. Besteiro, X.-T. Kong, Z. Wang, G. V. Hartland, A.O. Govorov, ACS Photonics, DOI: 10.1021/acsphotonics.7b00751 (2017).M.E. Sykes, J. W. Stewart, G. M. Akselrod, X.-T. Kong, Z. Wang, D. J. Gosztola, A.B.F. Martinson, D. Rosenmann, M. H. Mikkelsen, A. O. Govorov, G. P. Wiederrecht, Nature Communications, 8, 986 (2017). G.V. Hartland, L. Besteiro, P. Johns, A. O. Govorov, What’s so Hot about Electrons in Metal Nanoparticles? (perspective paper), ACS Energy Letters, 2, 1641–1653 (2017).E.-M. Roller, L.V. Besteiro, C. Pupp, L. Khosravi Khorashad, A. O. Govorov, T.Liedl, Nature Physics, 13, 761-765 (2017).

Enhanced laser proton acceleration by target ablation on a femtosecond laser system

Proton acceleration during the interaction of an ultraintense (6 × 1019 W/cm2) femtosecond (fs) laser pulse with a thin (2.5 μm) foil target pre-ablated by a picosecond (ps) pulse is experimentally and numerically investigated. Enhancements in both proton cut-off energy and charge are observed with the target ablation due to a large number of energetic electrons generated from the preformed preplasma in front of the target. The enhanced proton beams are successfully collected at 4–9 MeV with ±4% energy spread and then transported to the irradiating platform. The results show that for the interaction between fs laser pulse and μm-thickness target, proton energy and charge can be enhanced by target ablation using a ps laser pulse, which is valuable for application like cancer radiotherapy.

(Invited) Hot-Electron Generation and Energy Transfer in Plasmonic Nanostructures with Hot Spots: Quantum and Classical Mechanisms

Generation of energetic (hot) electrons is an intrinsic property of any plasmonic nanostructure under illumination. Simultaneously, a striking advantage of metal nanocrystals over semiconductors lies in their very large absorption cross sections. Therefore, metal nanostructures with strong and tailored plasmonic resonances are very attractive for photocatalytic applications in which excited electrons play an important role. However, the central questions in the problem of plasmonic hot electrons are the number of optically excited energetic electrons in a nanocrystal and how to extract such electrons. Here we develop a theory describing the generation rates and the energy distributions of hot electrons in nanocrystals with various geometries [1-4]. In particular, we show that nanostructures with strong and extended plasmonic hot spots generate unusually large numbers of energetic electrons, which can be observed using the ultra-fast transient spectroscopy [2,4] as well as photochemistry. Hot-electron generation together with non-radiative plasmonic transfer [5] represent efficient methods to transfer and localize optical and photo-chemical energies. A.O. Govorov, H. Zhang, V. Demi, and Y. K. Gun’ko, Nano Today, 9, 85 (2014).H. Harutyunyan, A. B. F. Martinson, D. Rosenmann, L.K. Khorashad, L.V.Besteiro, A.O.Govorov, and G.P. Wiederrecht, Nature Nanotechnology 10, 770–774 (2015).L.V. Besteiro, X.-T. Kong, Z. Wang, G. V. Hartland, A.O. Govorov, ACS Photonics, DOI: 10.1021/acsphotonics.7b00751 (2017).M.E. Sykes, J. W. Stewart, G. M. Akselrod, X.-T. Kong, Z. Wang, D. J. Gosztola, A.B.F. Martinson, D. Rosenmann, M. H. Mikkelsen, A. O. Govorov, G. P. Wiederrecht, Nature Communications, 8, 986 (2017).E.-M. Roller, L.V. Besteiro, C. Pupp, L. Khosravi Khorashad, A. O. Govorov, T.Liedl, Nature Physics, 13, 761-765 (2017).

(Invited) Generation of Hot Plasmonic Electrons and Heat in Nanocrystals with Hot Spots

The efficiency of generation of energetic plasmonic carriers in metal nanostructures strongly depends on the optical design and material composition. This study demonstrates the ability to generate large numbers of hot plasmonic carriers in specially-designed hybrid nanostructures [1,2]. The hot-electron generation becomes especially efficient in plasmonic nanostructures with electromagnetic hot spots [1]. The energy distribution of optically-excited plasmonic carriers is very different in metal nanocrystals with large and small sizes [2]. For metal nanocrystals with smaller sizes (less than 10nm) or in nanostructures with hot spots, the excited state gets a large number of carriers with high energies. Nanostructures with a strong plasmonic enhancement generate unusually large numbers of energetic electrons, which can be observed using ultra-fast spectroscopy, photo-currents and photo-chemistry [1,3,4]. Along with the hot-electron generation, which is a quantum effect, the electromagnetic radiation typically causes efficient heating in metal nanostructures, which may also be concentrated in thermal hot spots [5]. The results obtained in this study can be used to design plasmonic hot-electron nanodevices for photo-catalysis, photo-detectors and solar-energy applications. [1] H. Harutyunyan, A. B. F. Martinson, D. Rosenmann, L.K. Khorashad, L.V. Besteiro, A.O.Govorov, and G.P. Wiederrecht, Nature Nanotechnology 10, 770–774 (2015). [2] A.O. Govorov, H. Zhang, V. Demir, and Y. K. Gun’ko, NanoToday 9, 85 (2014). [3] ] W. Li, Z. J. Coppens, L. Vázquez, W. Wang, A. O. Govorov, and J. Valentine, Nature Communications 6, 8379 (2015). [4] L. Weng, H. Zhang, A. O. Govorov, and M. Ouyang, Nature Communications 5, 4792 (2014). [5] A. Govorov and H.H. Richardson, NanoToday 2, 20 (2007).

The hollow cathode effect in a radio-frequency driven microhollow cathode discharge in nitrogen

A two-dimensional particle-in-cell Monte-Carlo code has been developed to study the physical mechanism of the hollow cathode effect (HCE) in an rf microhollow cathode discharge (rf-MHCD). Under the simulated conditions, the HCE in the rf-MHCD is the result of sheath-superposition, and both α ionization and γ ionization play a role. However, α ionization mode is predominant. Electrons undergo a pendular motion during the negative portion of the rf cycle. When the rf hollow electrode has a positive voltage, the majority of the electrons move toward the rf electrode, and the mean electron energy near the rf electrode is higher than that in a dc hollow cathode discharge, resulting in a large number of energetic electrons bombarding the hollow cathode wall, an important characteristic of the rf-MHCD. When the hollow cathode aperture is sufficiently small, many electrons strike the hollow electrode and are removed from the discharge space, so that the plasma density decreases. The average energy of the ions in the quasi-neutral plasma region near the axis is greater than the thermal energy in a molecular gas. Therefore, high density, high energy, and high chemical activity are characteristic advantages of rf-MHCD plasma sources.

The effect of the driving frequency on the confinement of beam electrons and plasma density in low-pressure capacitive discharges

The effect of changing the driving frequency on the plasma density and the electron dynamics in a capacitive radio-frequency argon plasma operated at low pressures of a few Pa is investigated by particle-in-cell/Monte-Carlo collision simulations and analytical modeling. In contrast to previous assumptions, the plasma density does not follow a quadratic dependence on the driving frequency in this non-local collisionless regime. Instead, a step-like increase at a distinct driving frequency is observed. Based on an analytical power balance model, in combination with a detailed analysis of the electron kinetics, the density jump is found to be caused by an electron heating mode transition from the classical α-mode into a low-density resonant heating mode characterized by the generation of two energetic electron beams at each electrode per sheath expansion phase. These electron beams propagate through the bulk without collisions and interact with the opposing sheath. In the low-density mode, the second beam is found to hit the opposing sheath during its collapse. Consequently, a large number of energetic electrons is lost at the electrodes resulting in a poor confinement of beam electrons in contrast to the classical α-mode observed at higher driving frequencies. Based on the analytical model this modulated confinement quality and the related modulation of the energy lost per electron lost at the electrodes is demonstrated to cause the step-like change of the plasma density. The effects of a variation of the electrode gap, the neutral gas pressure, the electron sticking and secondary electron emission coefficients of the electrodes on this step-like increase of the plasma density are analyzed based on the simulation results.

Efficient proton beam generation from a foam-carbon foil target using an intense circularly polarized laser

Energetic proton acceleration from interaction of intense circularly polarized laser pulse with a foam-carbon foil target is investigated using two-dimensional particle-in-cell simulation. It is found that protons from this target are accelerated to much higher energy in comparison with double-layer targets and simple flat targets. This result can be attributed to an enhancement of energy conversion efficiency from laser to electrons inside foam region. When a large number of energetic electrons are generated from the nearcritical plasma, they transport through the foil and form a strong backside sheath field which accelerates protons more efficiently.

High-quality proton bunch from laser interaction with a gas-filled cone target

Generation of high-energy proton bunch from interaction of an intense short circularly polarized (CP) laser pulse with a gas-filled cone target (GCT) is investigated using two-dimensional particle-in-cell simulation. The GCT target consists of a hollow cone filled with near-critical gas-plasma and a thin foil attached to the tip of the cone. It is observed that as the laser pulse propagates in the gas-plasma, the nonlinear focusing will result in an enhancement of the laser pulse intensity. It is shown that a large number of energetic electrons are generated from the gas-plasma and accelerated by the self-focused laser pulse. The energetic electrons then transports through the foil, forming a backside sheath field which is stronger than that produced by a simple planar target. A quasi-monoenergetic proton beam with maximum energy of 181 MeV is produced from this GCT target irradiated by a CP laser pulse at an intensity of 2.6 × 1020 W/cm2, which is nearly three times higher compared to simple planar target (67 MeV).

An Extreme Solar Event of 20 January 2005: Properties of the Flare and the Origin of Energetic Particles

The famous extreme solar and particle event of 20 January 2005 is analyzed from two perspectives. Firstly, using multi-spectral data, we study temporal, spectral, and spatial features of the main phase of the flare, when the strongest emissions from microwaves up to 200 MeV gamma-rays were observed. Secondly, we relate our results to a long-standing controversy on the origin of solar energetic particles (SEP) arriving at Earth, i.e., acceleration in flares, or shocks ahead of coronal mass ejections (CMEs). Our analysis shows that all electromagnetic emissions from microwaves up to 2.22 MeV line gamma-rays during the main flare phase originated within a compact structure located just above sunspot umbrae. In particular, a huge (≈ 105 sfu) radio burst with a high frequency maximum at 30 GHz was observed, indicating the presence of a large number of energetic electrons in very strong magnetic fields. Thus, protons and electrons responsible for various flare emissions during its main phase were accelerated within the magnetic field of the active region. The leading, impulsive parts of the ground-level enhancement (GLE), and highest-energy gamma-rays identified with π 0-decay emission, are similar and closely correspond in time. The origin of the π 0-decay gamma-rays is argued to be the same as that of lower-energy emissions, although this is not proven. On the other hand, we estimate the sky-plane speed of the CME to be 2 000 – 2 600 km s−1, i.e., high, but of the same order as preceding non-GLE-related CMEs from the same active region. Hence, the flare itself rather than the CME appears to determine the extreme nature of this event. We therefore conclude that the acceleration, at least, to sub-relativistic energies, of electrons and protons, responsible for both the major flare emissions and the leading spike of SEP/GLE by 07 UT, are likely to have occurred nearly simultaneously within the flare region. However, our analysis does not rule out a probable contribution from particles accelerated in the CME-driven shock for the leading GLE spike, which seemed to dominate at later stages of the SEP event.

Operation of target diagnostics in a petawatt laser environment (invited)

The operation of target diagnostics in a high-energy petawatt laser environment is made challenging by the large number of energetic electrons, hard x rays, and energetic particles produced in laser-target interactions. The charged particles and x rays from the target create secondary radiation and a large electromagnetic pulse (EMP) when they hit structures inside the target chamber. The primary particles create secondary particles and radiation that can create excessive background in sensitive detectors. The large EMP can impair or damage electronic equipment and detectors, especially inside the target chamber. Shielding and EMP mitigation strategies developed during experiments at the Rutherford Appleton Vulcan petawatt laser facility will be presented for a variety of detection systems, such as single-photon-counting x-ray charge-coupled device cameras, multiple diamond x-ray detectors, and scintillator-photomultiplier detectors. These strategies will be applied to the development of diagnostic systems for the OMEGA EP, high-energy petawatt laser facility, currently under construction at the Laboratory for Laser Energetics.

Formation of electron holes and particle energization during magnetic reconnection.

Three-dimensional particle simulations of magnetic reconnection reveal the development of turbulence driven by intense electron beams that form near the magnetic x-line and separatrices. The turbulence collapses into localized three-dimensional nonlinear structures in which the electron density is depleted. The predicted structure of these electron holes compares favorably with satellite observations at Earth's magnetopause. The birth and death of these electron holes and their associated intense electric fields lead to strong electron scattering and energization, whose understanding is critical to explaining why magnetic explosions in space release energy so quickly and produce such a large number of energetic electrons.

Magnetic field in short-pulse high-intensity laser-solid experiments

High-intensity short laser pulses focused on the surface of a solid target produce large numbers of energetic electrons which in turn produce strong electric and magnetic fields as they penetrate the target. Layers of x-ray-emitting materials buried inside the target are often used to diagnose fast electron transport. We show that the magnetic field grows at the surface of the buried layers, and that this field may in some cases possibly inhibit fast electron penetration.

Fast-electron transport in high-intensity short-pulse laser - solid experiments

The interaction of short-pulse high-intensity lasers with solid targets generates large numbers of energetic electrons. The energetic electrons can only penetrate into the solid if the solid can supply an equivalent charge-neutralizing return current. We develop a simple model which shows that in many cases the solid cannot support the required return current and the fast electrons are confined by electric fields to the surface of the target. The target response to laser irradiation depends strongly on the electrical conductivity of the solid.

Theory of electron degradation and yields of initial molecular species produced by ionizing radiation

Ionizing radiations generate in matter a large number of energetic electrons, which in turn collide with molecules in matter, produce ions and excited states, and thereby degrade in energy. The description of the consequences of many collision processes to the electrons and to matter is the goal of the electron degradation theory. We summarize the current understanding of this topic, which is important as a basis of radiation chemistry and biology. In addition, we present an initial report of our new work, namely, a generalization of the Spencer–Fano theory to time-dependent cases.

COLLISION CROSS SECTIONS AND TRANSPORT COEFFICIENTS IN AN INDUCTIVELY COUPLED ARGON PLASMA

Collision cross sections as well as electrical conductivity, thermal conductivity, diffusion coefficients and viscosity coefficients of an inductively coupled argon plasma are calculated. The results indicate that thermal conduction process plays an important role in energy transfer, while ambipolar diffusion leads to a much higher electron density in the cool plasma regions as compared with the value predicted by local thermal equilibrium. More significantly, both three body recombination and superelastic collision give rise to the creation of a large number of energetic electrons in the cool plasma regions. Such a deviation of electron velocity distribution from a Maxwellian may have a special significance for excitation mechanism of the plasma used as an emission spectroscopic source.

Biological detection of reduction in dose at the surface of volume sources containing beta emitters

Dosimetry theory related to volume sources containing uniformly distributed beta-emitting radionuclides predicts a rapid drop in dose at the outer surface to approximately one half of the maximum dose within the volume. Rats were injected with radiocolloids of either 198Au, 113Inm or 99Tcm, to produce liver irradiation from their decay, or their livers were treated with external X-irradiation. After irradiation, a portion of the liver was surgically removed to stimulate cell division in the liver remnant revealing radiation damage in the form of chromosomal aberrations. The percentages of dividing cells with bridges were scored in the outer edges (0.45 mm) of the tissue sections and were compared to levels obtained from the central portion of the liver lobe. No significant difference was observed with X-irradiation or from 99Tcm irradiation. However, irradiation with 198Au and 113Inm (which emit large numbers of energetic electrons) produced significantly fewer aberrations in the outer layer when compared with the central portion.

Long term storage of relativistic particles in the solar corona

A model is presented which shows that large numbers of energetic electrons (0.3-> 10 MeV) and protons (1–30 MeV) can be stored in the solar corona at altitudes around 3 × 105 km for periods in excess of 5 days. Specific reference is made to the time period July 6–16 1968 as an excellent example of energetic solar particle storage. Time histories of interplanetary charged particle intensities observed by the IMP-4 and Pioneer 8 satellites are used to substantiate this contention. Detailed reference is also made to solar X-ray, optical and radio data obtained during the period in question, in addition to interplanetary magnetometer data. This model provides a unique solution to many hitherto unexplained solar particle events, and can also account for the lack of prompt particle emission from certain large solar flares recorded in the past.