arXiv subjects

I. D. Kaganovich

Publications and source records attributed to I. D. Kaganovich.

17 recordsLinked to original sources

arXiv2026

Laser ion acceleration from concave targets by subpicosecond pulses

Laser-driven proton acceleration provides a powerful route for generating ultrashort, high-charge proton beams. Many applications, including secondary neutron sources and inertial fusion, benefit from tight proton beam focusing. Concave targets offer a robust solution, yet the scaling of proton focusing with laser and target parameters remains poorly understood. Here, we present a numerical study of laser-driven proton acceleration and focusing from hemispherical targets using the fully kinetic, relativistic Particle-In-Cell code EPOCH. We focus on the sub-picosecond laser-pulse regime (duration $\lesssim 10^2$ fs), centering on the laser parameters of our recent experiment at the CSU ALEPH laser facility. We investigate the proton acceleration mechanisms, characterize proton focusing, and assess how focal spot parameters scale with laser and target parameters. We identify Target Normal Sheath Acceleration as the dominant mechanism, supplemented by a secondary post-acceleration stage near the geometrical center of the hemisphere. We demonstrate that both the proton focal spot size and focal plane position scale approximately linearly with the hemisphere radius, with the focal plane consistently located downstream of the geometrical center. The opening angle of the concave target mainly affects the proton beam waist. Energy-dependent proton focusing is interpreted as a consequence of the evolving curvature of the accelerating structure, which departs from the target curvature. Evidence for self-similar proton focusing is found in the regime of nearly uniform target irradiation.

physics.plasm-ph↗

arXiv2024

Numerical Method for Modeling Nucleation and Growth of Particles that Prevents Numerical Diffusion

State-of-the-art models for aerosol particle nucleation and growth from a cooling vapor primarily use a nodal method to numerically solve particle growth kinetics. In this method, particles that are smaller than the critical size are omitted from consideration, because they are thermodynamically unfavorable. This omission is based on the assumption that most of the newly formed particles are above the critical size and that the subcritical-size particles are not important to take into account. Due to the nature of the nodal method, it suffers from the numerical diffusion, which can cause an artificial broadening of the cluster size distribution leading to significant overestimation of the number of large-size particles. To address these issues, we propose a more accurate numerical method that explicitly models particles of all sizes, and uses a special numerical scheme that eliminates the numerical diffusion. We extensively compare this novel method to the commonly used nodal solver of the General Dynamics Equation (GDE) for particle growth and demonstrate that it offers GDE solutions with higher accuracy without generating numerical diffusion. Incorporating small subcritical clusters into the solution is crucial for: 1) more precise determination of the entire shape of the particle size distribution function and 2) wider applicability of the model to experimental studies with non-monotonic temperature variations leading to particle evaporation. The computational code implementing this numerical method in Python is available upon request.

physics.comp-ph↗

arXiv2024

Boron adatom adsorption on graphene: A case study in computational chemistry methods for surface interactions

Though weak surface interactions and adsorption can play an important role in plasma processing and materials science, they are not necessarily simple to model. A boron adatom adsorbed on a graphene sheet serves as a case study for how carefully one must select the correct technique from a toolbox of computational chemistry methods. Using a variety of molecular dynamics potentials and density functional theory functionals, we evaluate the adsorption energy, investigate barriers to adsorption and migration, calculate corresponding reaction rates, and show that a surprisingly high level of theory may be necessary to verify that the system is described correctly.

physics.comp-ph↗

arXiv2020

Validated two-dimensional modeling of short carbon arcs: anode and cathode spots

In order to study properties of short carbon arcs, a self-consistent model was implemented into a CFD code ANSYS-CFX. The model treats transport of heat and electric current in the plasma and the electrodes in a coupled manner and accounts for gas convection in the chamber. Multiple surface processes at the electrodes are modeled, including the formation of space-charge limited sheaths, ablation and deposition of carbon, emission and absorption of radiation and electrons. The simulations show that the arc is constricted near the cathode and the anode front surfaces leading to the formation of electrode spots. The cathode spot is a well-known phenomenon and mechanisms of its formation were reported elsewhere. However, the anode spot formation mechanism discovered in this work was not reported before. We conclude that the spot formation is not related to plasma instability, as commonly believed in case of constricted discharge columns, but rather occurs due to the highly nonlinear nature of heat balance in the anode. We additionally demonstrate this property with a reduced anode heat transfer model. We also show that the spot size increases with the arc current. This anode spot behavior was also confirmed in our experiments. Due to the anode spot formation, a large gradient of carbon gas density occurs near the anode, which drives a portion of the ablated carbon back to the anode at its periphery. This can consequently reduce the total ablation rate. Simulation results also show that the arc can reach local chemical equilibrium (LCE) state in the column region while the local thermal equilibrium (LTE) state is not typically achieved for experimental conditions. It shows that it is important to account for different electron and gas temperatures in the modeling of short carbon arcs.

physics.plasm-ph↗

arXiv2020

Numerical benchmark of transient pressure-driven metallic melt flows

Fluid dynamics simulations of melting and crater formation at the surface of a copper cathode exposed to high plasma heat fluxes and pressure gradients are presented. The predicted deformations of the free surface and the temperature evolution inside the metal are benchmarked against previously published simulations. Despite the physical model being entirely hydrodynamic and ignoring a variety of plasma-surface interaction processes, the results are also shown to be remarkably consistent with the predictions of more advanced models, as well as experimental data. This provides a sound basis for future applications of similar models to studies of transient surface melting and droplet ejection from metallic plasma-facing components after disruptions.

physics.comp-ph↗

arXiv2020

Model of a transition from low to high ablation regime in a carbon arc

Graphite ablation in a presence of inert background gas is widely used in different methods for the synthesis of carbon nanotubes, including electric arc and laser/solar ablation. The ablation rate is an important characteristic of the synthesis process. It is known from multiple arc experiments that there are two distinguishable ablation regimes, so-called "low ablation" and "high ablation" regimes in which the ablation rate behaves rather differently with variation of the arc parameters. We developed a model that explains low and high ablation regimes by taking into account the presence of a background gas and its effects on the ablation rate. We derive analytical relations for these regimes and verify them by comparing them with full numerical solutions in a wide arc parameter range. We comprehensively validate the model by comparing to multiple experimental data on the ablation rate in carbon arcs, where various arc parameters were varied. Good qualitative and quantitative agreement between full numerical solutions, analytical solutions, and experimental data was obtained.

physics.plasm-ph↗

arXiv2018

Design and Implementation of a Thomson Parabola for Fluence Dependent Energy-Loss Measurements at the Neutralized Drift Compression eXperiment

The interaction of ion beams with matter includes the investigation of the basic principles of ion stopping in heated materials. An unsolved question is the effect of different, especially higher, ion beam fluences on ion stopping in solid targets. This is relevant in applications such as in fusion sciences. To address this question, a Thomson parabola was built for the Neutralized Drift Compression eXperiment (NDCX-II) for ion energy-loss measurements at different ion beam fluences. The linear induction accelerator NDCX-II delivers 2 ns short, intense ion pulses, up to several tens of nC/pulse, or 10$^{10}$-10$^{11}$ ions, with a peak kinetic energy of ~1.1 MeV and a minimal spot size of 2 mm FWHM. For this particular accelerator the energy determination with conventional beam diagnostics, for example, time of flight measurements, is imprecise due to the non-trivial longitudinal phase space of the beam. In contrast, a Thomson parabola is well suited to reliably determine the beam energy distribution. The Thomson parabola differentiates charged particles by energy and charge-to-mass ratio, through deflection of charged particles by electric and magnetic fields. During first proof-of-principle experiments, we achieved to reproduce the average initial helium beam energy as predicted by computer simulations with a deviation of only 1.4 %. Successful energy-loss measurements with 1 μm thick Silicon Nitride foils show the suitability of the accelerator for such experiments. The initial ion energy was determined during a primary measurement without a target, while a second measurement, incorporating the target, was used to determine the transmitted energy. The energy-loss was then determined as the difference between the two energies.

physics.ins-det↗

arXiv2017

Investigation of the Short Argon Arc with Hot Anode, Part II: Analytical Model

Short atmospheric pressure argon arc is studied numerically and analytically. In a short arc with inter-electrode gap of several millimeters non-equilibrium effects in plasma play important role in operation of the arc. High anode temperature leads to electron emission and intensive radiation from its surface. Complete self-consistent analytical model of the whole arc comprising of models for near-electrode regions, arc column and a model of heat transfer in cylindrical electrodes was developed. The model predicts width of non-equilibrium layers and arc column, voltages and plasma profiles in these regions, heat and ion fluxes to the electrodes. Parametric studies of the arc have been performed for a range of the arc current densities, inter-electrode gap widths and gas pressures. The model was validated against experimental data and verified by comparison with numerical solution. Good agreement between the analytical model and simulations and reasonable agreement with experimental data were obtained.

physics.plasm-ph↗

arXiv2017

Excitation of a global plasma mode by an intense electron beam in a dc discharge

Interaction of an intense electron beam with a finite-length, inhomogeneous plasma is investigated numerically. The plasma density profile is maximal in the middle and decays towards the plasma edges. Two regimes of the two-stream instability are observed. In one regime, the frequency of the instability is the plasma frequency at the density maximum and plasma waves are excited in the middle of the plasma. In the other regime, the frequency of the instability matches the local plasma frequency near the edges of the plasma and the intense plasma oscillations occur near plasma boundaries. The latter regime appears sporadically and only for strong electron beam currents. This instability generates copious amount of suprathermal electrons. The energy transfer to suprathermal electrons is the saturation mechanism of the instability.

physics.plasm-ph↗

arXiv2017

Irradiation of Materials with Short, Intense Ion pulses at NDCX-II

We present an overview of the performance of the Neutralized Drift Compression Experiment-II (NDCX-II) accelerator at Berkeley Lab, and report on recent target experiments on beam driven melting and transmission ion energy loss measurements with nanosecond and millimeter-scale ion beam pulses and thin tin foils. Bunches with around 10^11 ions, 1-mm radius, and 2-30 ns FWHM duration have been created with corresponding fluences in the range of 0.1 to 0.7 J/cm^2. To achieve these short pulse durations and mm-scale focal spot radii, the 1.1 MeV He+ ion beam is neutralized in a drift compression section, which removes the space charge defocusing effect during final compression and focusing. The beam space charge and drift compression techniques resemble necessary beam conditions and manipulations in heavy ion inertial fusion accelerators. Quantitative comparison of detailed particle-in-cell simulations with the experiment play an important role in optimizing accelerator performance.

physics.acc-ph↗

arXiv2016

Recent Experiments At Ndcx-II: Irradiation Of Materials Using Short, Intense Ion Beams

We present an overview of the performance of the Neutralized Drift Compression Experiment-II (NDCX-II) accelerator at Berkeley Lab, and summarize recent studies of material properties created with nanosecond and millimeter-scale ion beam pulses. The scientific topics being explored include the dynamics of ion induced damage in materials, materials synthesis far from equilibrium, warm dense matter and intense beam-plasma physics. We summarize the improved accelerator performance, diagnostics and results of beam-induced irradiation of thin samples of, e.g., tin and silicon. Bunches with over 3x10^10 ions, 1- mm radius, and 2-30 ns FWHM duration have been created. To achieve these short pulse durations and mm-scale focal spot radii, the 1.2 MeV He+ ion beam is neutralized in a drift compression section which removes the space charge defocusing effect during final compression and focusing. Quantitative comparison of detailed particle-in-cell simulations with the experiment play an important role in optimizing accelerator performance; these keep pace with the accelerator repetition rate of ~1/minute.

physics.acc-ph↗

arXiv2016

Band Structure of the Growth Rate of the Two-Stream Instability of an Electron Beam Propagating in a Bounded Plasma

This paper presents a study of the two-stream instability of an electron beam propagating in a finite-size plasma placed between two electrodes. It is shown that the growth rate in such a system is much smaller than that of an infinite plasma or a finite size plasma with periodic boundary conditions. Even if the width of the plasma matches the resonance condition for a standing wave, a spatially growing wave is excited instead with the growth rate small compared to that of the standing wave in a periodic system. The approximate expression for this growth rate is $γ\approx (1/13)ω_{pe}(n_{b}/n_{p})(Lω_{pe}/v_{b})\ln (Lω_{pe}/v_{b})[ 1-0.18\cos ( Lω_{pe}/v_{b}+{π}/{2}) ]$, where $ω_{pe}$ is the electron plasma frequency, $n_{b}$ and $n_{p}$ are the beam and the plasma densities, respectively, $v_{b}$ is the beam velocity, and $L$ is the plasma width. The frequency, wave number and the spatial and temporal growth rates as functions of the plasma size exhibit band structure. The amplitude of saturation of the instability depends on the system length, not on the beam current. For short systems, the amplitude may exceed values predicted for infinite plasmas by more than an order of magnitude.

physics.plasm-ph↗

arXiv2015

Ion sound instability driven by ion beam

Ion sound instabilities driven by the ion flow in a system of a finite length are considered by analytical and numerical methods. The ion sound waves are modified by the presence of stationary ion flow resulting in negative and positive energy modes. The instability develops due to coupling of negative and positive energy modes mediated by reflections from the boundary. It is shown that the wave dispersion due to deviation from quasineutrality is crucial for the stability. In finite length system, the dispersion is characterized by the length of the system measured in units of the Debye length. The instability is studied analytically and the results are compared with direct, initial value numerical simulations.

physics.plasm-ph↗

arXiv2015

Generation of anomalously energetic suprathermal electrons by an electron beam interacting with a nonuniform plasma

Generation of anomalously energetic suprathermal electrons was observed in simulation of a high- voltage dc discharge with electron emission from the cathode. An electron beam produced by the emission interacts with the nonuniform plasma in the discharge via a two-stream instability. Efficient energy transfer from the beam to the plasma electrons is ensured by the plasma nonuniformity. The electron beam excites plasma waves whose wavelength and phase speed gradually decrease towards anode. The short waves near the anode accelerate plasma bulk electrons to suprathermal energies. The sheath near the anode reflects some of the accelerated electrons back into the plasma. These electrons travel through the plasma, reflect near the cathode, and enter the accelerating area again but with a higher energy than before. Such particles are accelerated to energies much higher than after the first acceleration. This mechanism plays a role in explaining earlier experimental observations of energetic suprathermal electrons in similar discharges.

physics.plasm-ph↗

arXiv2013

Secondary electron emission yield in the limit of low electron energy

Secondary electron emission (SEE) from solids plays an important role in many areas of science and technology.1 In recent years, there has been renewed interest in the experimental and theoretical studies of SEE. A recent study proposed that the reflectivity of very low energy electrons from solid surface approaches unity in the limit of zero electron energy2,3,4, If this was indeed the case, this effect would have profound implications on the formation of electron clouds in particle accelerators,2-4 plasma measurements with electrostatic Langmuir probes, and operation of Hall plasma thrusters for spacecraft propulsion5,6. It appears that, the proposed high electron reflectivity at low electron energies contradicts to numerous previous experimental studies of the secondary electron emission7. The goal of this note is to discuss possible causes of these contradictions.

physics.acc-ph↗

arXiv2012

Sheath instabilities in Hall plasmas devices

New class instabilities is identified in Hall plasmas in configurations with open magnetic field lines. It is shown that sheath resistivity results in a robust instability driven by the equilibrium electric field. It is conjectured that these instabilities play a crucial role in anomalous transport in Hall plasmas devices.

physics.plasm-ph↗

arXiv2012

Absence of Debye Sheaths Due to Secondary Electron Emission

A bounded plasma where the electrons impacting the walls produce more than one secondary on average is studied via particle-in-cell simulation. It is found that no classical Debye sheath or space-charge limited sheath exists. Ions are not drawn to the walls and electrons are not repelled. Hence the plasma electrons travel unobstructed to the walls, causing extreme particle and energy fluxes. Each wall has a positive charge, forming a small potential barrier or "inverse sheath" that pulls some secondaries back to the wall to maintain the zero current condition.

physics.plasm-ph↗