Sharp Density Transition Research Articles

Beam current from downramp injection in electron-driven plasma wakefields

We study the stability of plasma wake wave and the properties of density-downramp injection in an electron-driven plasma accelerator. In this accelerator type, a short high-current electron bunch (generated by a conventional accelerator or a laser–wakefield acceleration stage) drives a strongly nonlinear plasma wake wave (blowout), and accelerated electrons are injected into it using a sharp density transition which leads to the elongation of the wake. The accelerating structure remains highly stable until the moment some electrons of the driver reach almost zero energy, which corresponds to the best interaction length for optimal driver-to-plasma energy transfer efficiency. For a particular driver, this efficiency can be optimised by choosing appropriate plasma density. Studying the dependence of the current of the injected bunch on driver and plasma parameters, we show that it does not depend on the density downramp length as long as the condition for trapping is satisfied. Most importantly, we find that the current of the injected bunch primarily depends on just one parameter which combines both the properties of the driver (its current and duration) and the plasma density.

MBE growth and donor doping of coherent ultrawide bandgap AlGaN alloy layers on single-crystal AlN substrates

Single-crystal Aluminum Nitride (AlN) crystals enable the epitaxial growth of ultrawide bandgap Al(Ga)N alloys with drastically lower extended defect densities. Here, we report the plasma-MBE growth conditions for high Al-composition AlGaN alloys on single-crystal AlN substrates. An AlGaN growth guideline map is developed, leading to pseudomorphic AlxGa1−xN epitaxial layers with x ∼0.6–1.0 Al contents at a growth rate of ∼0.3 μm/h. These epitaxial layers exhibit atomic steps, indicating step flow epitaxial growth, and room-temperature band edge emission from ∼4.5 to 5.9 eV. Growth conditions are identified in which the background impurity concentrations of O, C, Si, and H in the MBE layers are found to be very near or below detection limits. An interesting Si segregation and gettering behavior is observed at the epitaxial AlGaN/AlN heterojunction with significant implications for the formation and transport of 2D electron or hole gases. Well-controlled intentional Si doping ranging from ∼2 × 1017 to 3 × 1019 atoms/cm3 is obtained, with sharp dopant density transition profiles. In Si-doped Al0.6Ga0.4N epilayers, a room-temperature free electron concentration of ∼3 × 1019/cm3, an electron mobility of ∼27 cm2/V s, and an n-type resistivity of ∼7.5 m Ω cm are obtained. The implications of these findings on electronic and photonic devices on single-crystal AlN substrates are discussed.

Dynamic spherical collapses towards growing black holes in relativistically degenerate or hot host mass reservoirs

ABSTRACT We study the self-similar spherical hydrodynamics of γ = 4/3, with or without shocks, of general polytropic (GP) relativistically degenerate or hot matter in global mass reservoirs under the Paczynski–Wiita (PW) gravity that retains essential general relativistic (GR) effects for dynamic accretions on to growing Schwarzschild black holes (BHs). Various self-similar dynamic solutions are derived and constructed. Among others, one important solution type is the expansion-wave collapse solution (EWCS) with a central mass-growing BH event horizon in expansion. Such GP dynamic collapses are highly efficient for the rapid dynamic growth of supermassive and hypermassive BHs (SMBHs and HMBHs) in the Universe including the early Universe, in contrast to much slower disc mass accretion and BH merger processes. Particularly, the extra degree of freedom for the effective pressure of those relativistically degenerate or hot mass reservoirs makes it possible to construct EWCSs with even higher efficiency for BH growths. Pertinent astrophysical applications of our model also include dynamic core collapses of massive or very massive stars or supermassive stars and compact objects as well as supernova explosions. Under joint actions of effective pressure and the PW gravity, self-similar dynamic solutions with central expanding spherical voids or cavities with either sharp or smooth density transitions across their rims can emerge. Dynamic expansion shocks are also constructed and examined.

Deformation Behavior of Foam Laser Targets Fabricated by Two-Photon Polymerization.

Two-photon polymerization (2PP), which is a three-dimensional micro/nano-scale additive manufacturing process, is used to fabricate component for small custom experimental packages (“targets”) to support laser-driven, high-energy-density physics research. Of particular interest is the use of 2PP to deterministically print millimeter-scale, low-density, and low atomic number (CHO) polymer matrices (“foams”). Deformation during development and drying of the foam structures remains a challenge when using certain commercial acrylic photo-resins. Acrylic resins were chosen in order to meet the low atomic number requirement for the foam; that requirement precludes the use of low-shrinkage organic/inorganic hybrid resins. Here, we compare the use of acrylic resins IP-S and IP-Dip. Infrared and Raman spectroscopy are used to quantify the extent of the polymerization during 2PP vs. UV curing. The mechanical strength of beam and foam structures is examined, particularly the degree of deformation that occurs during the development and drying processes. The magnitude of the shrinkage is quantified, and finite element analysis is used in order to simulate the resulting deformation. Capillary drying forces during development are shown to be small and are likely below the elastic limit of the foam log-pile structures. In contrast, the substantial shrinkage in IP-Dip (~5–10%) causes large shear stresses and associated plastic deformation, particularly near constrained boundaries and locations with sharp density transitions. Use of IP-S with an improved writing procedure results in a marked reduction in deformation with a minor loss of resolution.

Numerical studies of density transition injection in laser wakefield acceleration

The quality of laser wakefield accelerated electrons beams is strongly determined by the physical mechanism exploited to inject electrons in the wakefield. One of the techniques used to improve the beam quality is the density transition injection, where the electron trapping occurs as the laser pulse passes a sharp density transition created in the plasma. Although this technique has been widely demonstrated experimentally, the literature lacks theoretical and numerical studies on the effects of all the transition parameters. We thus report and discuss the results of a series of particle in cell (PIC) simulations where the density transition height and downramp length are systematically varied, to show how the electron beam parameters and the injection mechanism are affected by the density transition parameters.

Dosimetric properties of plasma density effects on laser-accelerated VHEE beams using a sharp density-transition scheme

In this paper, the effects of the plasma density on laser-accelerated electron beams for radiation therapy with a sharp density transition are investigated. In the sharp density-transition scheme for electron injection, the crucial issue is finding the optimum density conditions under which electrons injected only during the first period of the laser wake wave are accelerated further. In this paper, we report particle-in-cell simulation results for the effects of both the scale length and the densitytransition ratio on the generation of a quasi-mono-energetic electron bunch. The effects of both the transverse parabolic channel and the plasma length on the electron-beam’s quality are investigated. Also, we show the experimental results for the feasibility of a sharp density-transition structure. The dosimetric properties of these very high-energy electron beams are calculated using Monte Carlo simulations.

Energy boost in laser wakefield accelerators using sharp density transitions

The energy gain in laser wakefield accelerators is limited by dephasing between the driving laser pulse and the highly relativistic electrons in its wake. Since this phase depends on both the driver and the cavity length, the effects of dephasing can be mitigated with appropriate tailoring of the plasma density along propagation. Preceding studies have discussed the prospects of continuous phase-locking in the linear wakefield regime. However, most experiments are performed in the highly non-linear regime and rely on self-guiding of the laser pulse. Due to the complexity of the driver evolution in this regime, it is much more difficult to achieve phase locking. As an alternative, we study the scenario of rapid rephasing in sharp density transitions, as was recently demonstrated experimentally. Starting from a phenomenological model, we deduce expressions for the electron energy gain in such density profiles. The results are in accordance with particle-in-cell simulations, and we present gain estimations for single and multiple stages of rephasing.

Elementary stratified flows with stability at low Richardson number

We revisit the stability analysis for three classical configurations of multiple fluid layers proposed by Goldstein [“On the stability of superposed streams of fluids of different densities,” Proc. R. Soc. A. 132, 524 (1931)], Taylor [“Effect of variation in density on the stability of superposed streams of fluid,” Proc. R. Soc. A 132, 499 (1931)], and Holmboe [“On the behaviour of symmetric waves in stratified shear layers,” Geophys. Publ. 24, 67 (1962)] as simple prototypes to understand stability characteristics of stratified shear flows with sharp density transitions. When such flows are confined in a finite domain, it is shown that a large shear across the layers that is often considered a source of instability plays a stabilizing role. Presented are simple analytical criteria for stability of these low Richardson number flows.

High brightness electron beams from density transition laser wakefield acceleration for short-wavelength free-electron lasers

We use three-dimensional simulations to study injection and electron beam quality in laser wakefield acceleration (LWFA) using the density transition technique. We vary the density transition length scale, covering both the sharp and gradual density transition regimes. We find that the injected charge decreases monotonically as the density transition scale length increases, and as a consequence the energy-spread and emittance improve monotonically. Therefore, there is no optimal transition length that gives the best quality beam, contrary to earlier suggestions. However, the density transition technique does give high brightness electron beams with kA current, energy-spread of around 1% and normalized rms emittance of around 1π mm-mrad. We study the application of these LWFA beams as drivers for a short-wavelength free-electron laser (FEL), using analytic formulae as well as three-dimensional simulations. Because higher current favours a shorter transition length, while smaller energy-spread and emittance favour a longer transition length, there is now an optimal density transition scale length (for our parameters, 50 µm) that gives the best FEL performance: lasing at 270 nm, with a saturated power of around 360 MW, over an undulator length of only 6 m. Further improvements, like lower plasma density and laser guiding, could result in GeV-class beams of sufficient brightness to drive a soft x-ray FEL.

Delayed settling of marine snow at sharp density transitions driven by fluid entrainment and diffusion-limited retention

Marine snow is central to the marine carbon cycle, and quantifying its small-scale settling dynamics in different physical environments is essential to understanding its role in bio- geochemical cycles. Previous field observations of marine aggregate thin layers associated with sharp density gradients have led to the hypothesis that these layers may be caused by a decrease in aggregate settling speed at density interfaces. Here, we present experimental data on aggre- gate settling behavior, showing that these particles can dramatically decrease their settling veloc- ity when passing through sharp density transitions. This delayed settling can be caused by 2 potential mechanisms: (1) entrainment of lighter fluid from above as the particle passes through the density gradient, and (2) retention at the transition driven by changes in the density of the par- ticle due to its porosity. The aggregates observed in this study exhibited 2 distinct settling behav- iors when passing through the density transition. Quantitatively comparing these different behav- iors with predictions from 2 models allow us to infer that the delayed settling of the first group of aggregates was primarily driven by diffusion-limited retention, whereas entrainment of lighter fluid was the dominant mechanism for the second group. Coupled with theory, our experimental results demonstrate that both entrainment and diffusion-limited retention can play an important role in determining particle settling dynamics through density transitions. This study thus pro- vides insight into ways that delayed settling can lead to the formation of aggregate thin layers, important biological hotspots that affect trophic dynamics, and biogeochemical cycling in the ocean.

Delayed settling of marine snow at sharp density transitions driven by fluid entrainment and diffusion-limited retention

Delayed settling of marine snow at sharp density transitions driven by fluid entrainment and diffusion-limited retention

Quasi-monoenergetic electron beams production in a sharp density transition

Using a laser plasma accelerator, experiments with a 80 TW and 30 fs laser pulse demonstrated quasi-monoenergetic electron spectra with maximum energy over 0.4 GeV. This is achieved using a supersonic He gas jet and a sharp density ramp generated by a high intensity laser crossing pre-pulse focused 3 ns before the main laser pulse. By adjusting this crossing pre-pulse position inside the gas jet, among the laser shots with electron injection, more than 40% can produce quasi-monoenergetic spectra. This could become a relatively straight forward technique to control laser wakefield electron beams parameters.

Shock-wave-based density down ramp for electron injection

We demonstrate a sharp density transition for electron injection in laser wakefield acceleration through numerical study. This density transition is generated by a detached shock wave induced by a cylinder inserted into a supersonic helium gas flow. In a Mach 1.5 flow, the scale length of the density transition ${L}_{\mathrm{grad}}$ can approximately equal to plasma wavelength ${\ensuremath{\lambda}}_{p}$ at the shock front, and can be further reduced with an increase of the flow Mach number. A density down ramp with ${L}_{\mathrm{grad}}\ensuremath{\ge}{\ensuremath{\lambda}}_{p}$ can reduce the phase velocity of the wakefield and lower the energy threshold for the electrons to be trapped. Moreover, the quality of the accelerated beam may be greatly improved by precisely controlling of ${L}_{\mathrm{grad}}$ to be one ${\ensuremath{\lambda}}_{p}$. For an even sharper density down ramp with ${L}_{\mathrm{grad}}\ensuremath{\ll}{\ensuremath{\lambda}}_{p}$, the oscillating electrons in the plasma wave will up shift their phase when crossing the ramp, therefore a fraction of the electrons are injected into the accelerating field. For this injection mechanism, there is no threshold requirement for the pump laser intensity to reach wave breaking, which is a big advantage as compared with other injection mechanisms.

Parameter Study on Radiation Therapy with Laser-Accelerated Electrons Using a Sharp Density Transition Scheme

A parameter study of radiation therapy with laser-accelerated electrons using a sharp density transition scheme was performed via computer simulations. Through particle-in-cell (PIC) simulations, a study of the optimum conditions for the generation of a monoenergetic electron beam was conducted. The beam quality can be controlled by adjusting the laser focal spot size. The charge of the electron bunch is related to the area of the phase mixing region. An electron bunch with the maximum charge was produced in the maximum area of the mixing region in a specific focal spot case. The energy spread of the electron bunch increases with increasing focal spot size owing to the nonlocalized acceleration phase of the wakefield in the larger focal spot case. The transverse bunch size decreases with increasing focal spot size from the narrower transverse size of the ion cavity in the larger focal spot. The dosimetric properties of these very-high-energy electron beams were calculated using Monte Carlo simulations.

SU‐E‐T‐423: Feasibility and Parameter Study on the Radiation Therapy with Laser‐Accelerated Electrons Using a Sharp Density Transition Scheme

Purpose: In the cancer therapy using laser‐accelerated VHEE (very high energy electron) beams, stability of the beam generation is a very important issue. Generation of stable mono‐energetic electron beam is affected by the conditions of electron injection. We used the sharp density transition scheme for the stable generation of the VHEE beams and investigate the characteristics of the mono‐energetic electron beams with a laser focal spot size. Methods: To generate sharp density transition structure experimentally, a study on the generation of plasma shock structure was performed at Korea Electrotechnology Research Institute. The characteristics of VHEE beams generated from laser‐plasma interaction via linearly polarized intense laser pulses are investigated using two‐dimensional particle‐in‐cell simulations. To investigate the feasibility of the cancer therapy with VHEE beams with a sharp density transition scheme, Monte‐Carlo simulations are performed for calculating the dosimetric properties. Results: The sharp density transition structure can be generated using a pre‐pulse propagating orthogonal to the main laser pulse. 200 ps and 400 mJ pre‐pulse can generate the plasma shock structure with a sharp density transition under 10㎛. The charge of the electron bunch is related to the area of the phase‐mixing region. The electron bunch with the maximum charge was produced in the maximum area of the mixing region for the specific laser focal size. Energy spread of electron bunch increases with the increasing focal spot size and transverse bunch size decrease with increasing focal spot size. Dose distributions have also been calculated with the GEANT4 code and we present the feasibility of the particle therapy with sharp density transition scheme based VHEE beams for the deep seated tumor. Conclusions: In this paper, we present the superior feasibility of the cancer therapy using laser‐accelerated VHEE beams with a sharp density transition scheme.

DIRECT OBSERVATION OF A SHARP TRANSITION TO COHERENCE IN DENSE CORES

We present NH3 observations of the B5 region in Perseus obtained with the Green Bank Telescope (GBT). The map covers a region large enough (~11'x14') that it contains the entire dense core observed in previous dust continuum surveys. The dense gas traced by NH3(1,1) covers a much larger area than the dust continuum features found in bolometer observations. The velocity dispersion in the central region of the core is small, presenting subsonic non-thermal motions which are independent of scale. However, it is thanks to the coverage and high sensitivity of the observations that we present the detection, for the first time, of the transition between the coherent core and the dense but more turbulent gas surrounding it. This transition is sharp, increasing the velocity dispersion by a factor of 2 in less than 0.04 pc (the 31" beam size at the distance of Perseus, ~250 pc). The change in velocity dispersion at the transition is ~3 km/s/pc. The existence of the transition provides a natural definition of dense core: the region with nearly-constant subsonic non-thermal velocity dispersion. From the analysis presented here we can not confirm nor rule out a corresponding sharp density transition.

Optimum Condition of Sharp-Density-Transition Scheme for the Generation of a High-Quality Electron Bunch in the Laser Wakefield Acceleration

In this paper, the effects of scale length and density transition ratio on the generation of a high-quality electron bunch with a sharp density transition in the laser wakefield accelerator were analyzed using 2-D particle-in-cell simulations. The simulation results show the effect of the density transition on the quality of the beam, i.e., the total charge of the electron bunch depends on the scale length and the density transition ratio and the size of the electron bunch only depends on the density transition ratio. The simulations also show that the energy spread of the bunch in the first period can be reduced with a sharp scale length. For this paper, the optimum condition for the generation of a high-quality electron bunch was found in the parameter space of both the scale length and the density transition ratio.

Effects of Scale Length and Transition Density Ratio on the Electron Trapping and Acceleration with a Sharp Density Transition in Laser Wakefield Accelerator

In this paper, the effects of scale length and transition density ratio on the electron trapping and acceleration with a sharp density transition in laser wakefield accelerator (LWFA) have been studied using one-dimensional particle-in-cell (PIC) simulations. Simulation results show that the number of accelerated electrons depends on the plasma density in the high density region and the scale length of the density transition region. For the detailed investigation of the dynamical features of the electron trapping and acceleration, spatiotemporal patterns of the laser wake wave are also used. Throughout this work, the optimum condition for the electron acceleration could be found in the relevant parameter space of the scale length and the transition density ratio.

Modifications to the resistive MHD spectrum due to changes in the equilibrium

The normal-mode spectrum of the linearized resistive magnetohydrodynamics (MHD) operator is determined numerically for an inhomogeneous plasma slab. The spectrum consists of fast and slow magnetosonic and Alfvén sub-spectra. The obtained Alfvén sub-spectrum consists of discrete resistive Alfvén modes and an ‘ideal quasi-mode’ that originates from a surface mode due to a sharp density transition layer in the plasma. The quasi-mode interacts with the resistive Alfvén modes when the width of the transition layer is modified. Eventually, when the transition layer spreads over a substantial part of the plasma, the quasi-mode even loses its global or ‘collective’ behaviour and gradually transforms into an ordinary resistive Alfvén mode. In other words, it joins the resistive Alfvén sub-spectrum. Avoided crossings occur during the interactions of the ideal quasi-mode and the resistive spectrum.

Lamellar Structured Nanoparticles Formed by Complexes of a Cationic Block Copolymer and Perfluorodecanoic Acid

A double hydrophilic block copolymer was synthesized by controlled free radical polymerization, namely by the RAFT method, from a poly(ethylene oxide) macromonomer and a cationic acrylate. Stoichiometric complexes with sodium decanoate and with sodium perfluorodecanoate were prepared by self-assembly. Both complexes are soluble in water forming core−shell nanoparticles with hydrodynamic radii of 54 and 22 nm, respectively, and neutral ζ potentials. The complex containing decanoate decomposed upon dilution, while the complex with perfluorodecanoate is stable even against dialysis and in salt solution. Only the nanoparticles containing perfluorodecanoate show a strong contrast in small-angle X-ray scattering (SAXS). It is produced from their cores that have an elliptical shape (short axes = 8.5 nm, long axis = 23.3 nm) and a sharp density transition to the shell. The cores exhibit an internal lamellar structure with a long period of 3.65 nm consisting of fluorocarbon rich lamellae (d1 = 3.00 nm) and lamellae enriched in ionic groups (d2 = 0.65 nm). These fluorinated nanoparticles coexist in physiological buffer with human serum albumin and fibrinogen without inducing aggregation. Moreover, the content of α-helix of these serum proteins increased when in solution with the fluorinated nanoparticles.