arXiv subjects

Igor D. Kaganovich

Publications and source records attributed to Igor D. Kaganovich.

At least 55 records · Page 3Linked to original sources

arXiv2018

Analytical model of three regimes of cold cathode breakdown in helium

An analytical model has been developed to map out the low-pressure (left-hand) branch of the Paschen curve at very high voltage when electrons are in the runaway regime and charge exchange/ionization avalanche by ions and fast neutral atoms becomes important. The model has been applied to helium gas between parallel-plate electrodes, at potentials ranging in magnitude between 10 and 1000 kilovolt. The respective value of reduced electric field E/n varies in the range of 50-6000kTd, with reduced density nd (where n is the gas density and d is the inter-electrode distance) on the order of 10^20 m^-2. For fast neutral atoms produced via charge exchange, the following interactions prove essential to understanding the breakdown mechanism: contribution to impact ionization, strongly anisotropic scattering in collisions with background atoms,and backscattering (of both atoms and ions, which become neutralized) from the cathode. Three regimes of the breakdown have been identified according to relative share of impact ionization by electrons, by ions, and by fast neutrals. In the fast-neutral regime of particular interest here, the ionization avalanche is directed from anode towards cathode. It is initiated by the fast neutral beam backscattered from the cathode, and charge multiplication occurs through multiple successive cycles of ionization and charge exchange. Further, the double-valued shape of the Paschen curve with a branch point near 200 kV is found to be due to heavy species undergoing a transition to runaway regime. The analytical Paschen curve is compared to those obtained with a detailed Particle-In-Cell/Monte Carlo (PIC/MCC) simulation, and also to experimental measurements. The model provides accurate predictions for E/n up to ~ 1000 kTd, constrained by availability and quality of required input data.

physics.plasm-ph↗

arXiv2017

Amplification due to Two-Stream Instability of Self-Electric and Magnetic Fields of an Ion Beam Propagating in Background Plasma

Propagation of charged particle beams in background plasma as a method of space charge neutralization has been shown to achieve a high degree of charge and current neutralization and therefore enables nearly ballistic propagation and focusing of charged particle beams. Correspondingly, use of plasmas for propagation of charged particle beams has important applications for transport and focusing of intense particle beams in inertial fusion and high energy density laboratory plasma physics. However, the streaming of beam ions through a background plasma can lead to development of the two-stream instability between the beam ions and the plasma electrons. The beam electric and magnetic fields enhanced by the two-stream instability can lead to defocusing of the ion beam. Using particle-in-cell (PIC) simulations, we study the scaling of the instability-driven self-electromagnetic fields and consequent defocusing forces with the background plasma density and beam ion mass. We identify plasma parameters where the defocusing forces can be reduced.

physics.plasm-ph↗

arXiv2017

Optimizing Beam Transport in Rapidly Compressing Beams on the Neutralized Drift Compression Experiment - II

The Neutralized Drift Compression Experiment-II (NDCX-II) is an induction linac that generates intense pulses of 1.2 MeV helium ions for heating matter to extreme conditions. Here, we present recent results on optimizing beam transport. The NDCX-II beamline includes a 1-meter-long drift section downstream of the last transport solenoid, which is filled with charge-neutralizing plasma that enables rapid longitudinal compression of an intense ion beam against space-charge forces. The transport section on NDCX-II consists of 28 solenoids. Finding optimal field settings for a group of solenoids requires knowledge of the envelope parameters of the beam. Imaging the beam on scintillator gives the radius of the beam, but the envelope angle dr/dz is not measured directly. We demonstrate how the parameters of the beam envelope (r, dr/dz, and emittance) can be reconstructed from a series of images taken at varying B-field strengths of a solenoid upstream of the scintillator. We use this technique to evaluate emittance at several points in the NDCX-II beamline and for optimizing the trajectory of the beam at the entry of the plasma-filled drift section.

physics.acc-ph↗

arXiv2017

Modeling of reduced secondary electron emission yield from a foam or fuzz surface

Complex structures on a material surface can significantly reduce the total secondary electron emission yield from that surface. A foam or fuzz is a solid surface above which is placed a layer of isotropically aligned whiskers. Primary electrons that penetrate into this layer produce secondary electrons that become trapped and not escape into the bulk plasma. In this manner the secondary electron yield (SEY) may be reduced. We developed an analytic model and conducted numerical simulations of secondary electron emission from a foam to determine the extent of SEY reduction. We find that the relevant condition for SEY minimization is $\bar{u} \equiv AD/2 >>1 $, where $D$ is the volume fill fraction and $A$ is the aspect ratio of the whisker layer, the ratio of the thickness of the layer to the radius of the fibers. We find that foam can not reduce the SEY from a surface to less than 0.3 of its flat value.

physics.plasm-ph↗

arXiv2017

Generation of forerunner electron beam during interaction of ion beam pulse with plasma

The long-time evolution of the two-stream instability of a cold ion beam pulse propagating though the background plasma is investigated using a large-scale one-dimensional electrostatic kinetic simulation. The three stages of the instability are identified and investigated in detail. After the initial linear growth and saturation by the electron trapping, a portion of the initially trapped electrons becomes detrapped and moves ahead of the ion beam pulse forming a {forerunner} electron beam, which causes a secondary two-stream instability that preheats the upstream plasma electrons. Consequently, the self-consistent nonlinear-driven turbulent state is set up at the head of the ion beam pulse with the saturated plasma wave sustained by the influx of the cold electrons from the upstream of the beam that lasts until the final stage when the beam ions become trapped by the plasma wave. The beam ion trapping leads to the nonlinear heating of the beam ions that eventually extinguishes the instability.

physics.plasm-ph↗

arXiv2017

Simulations of Ion Velocity Distribution Functions Taking into Account Both Elastic and Charge Exchange Collisions

Based on accurate representation of the He+-He angular differential scattering cross sections consisting of both elastic and charge exchange collisions, we performed detailed numerical simulations of the ion velocity distribution functions (IVDF) by Monte Carlo collision method (MCC). The results of simulations are validated by comparison with the experimental data of the ion mobility and the transverse diffusion. The IVDF simulation study shows that due to significant effect of scattering in elastic collisions IVDF cannot be separated into product of two independent IVDFs in the transverse and parallel to the electric field directions.

physics.plasm-ph↗

arXiv2016

Modeling of Reduced Effective Secondary Electron Emission Yield from a Velvet Surface

Complex structures on a material surface can significantly reduce total secondary electron emission from that surface. A velvet is a surface that consists of an array of vertically standing whiskers. The reduction occurs due to the capture of low-energy, true secondary electrons emitted at the bottom of the structure and on the sides of the velvet whiskers. We performed numerical simulations and developed an approximate analytical model that calculates the net secondary electron emission yield from a velvet surface as a function of the velvet whisker length and packing density, and the angle of incidence of primary electrons. We found that to suppress secondary electrons, the following condition on dimensionless parameters must be met: π/2 DA tan θ >> 1 where θ is the angle of incidence of the primary electron from the normal, D is the fraction of surface area taken up by the velvet whisker bases, and A is the aspect ratio, A {\equiv} h/r, the ratio of height to radius of the velvet whiskers. We find that velvets available today can reduce the secondary electron yield by 90% from the value of a flat surface. The values of optimal velvet whisker packing density that maximally suppresses secondary electron emission yield are determined as a function of velvet aspect ratio and electron angle of incidence.

physics.plasm-ph↗

arXiv2016

Effect of collisions on the two-stream instability in a finite length plasm

The instability of a monoenergetic electron beam in a collisional one-dimensional plasma bounded between grounded walls is considered both analytically and numerically. Collisions between electrons and neutrals are accounted for the plasma electrons only. Solution of a dispersion equation shows that the temporal growth rate of the instability is a decreasing linear function of the collision frequency which becomes zero when the collision frequency is two times the collisionless growth rate. This result is confirmed by fluid simulations. Practical formulas are given for the estimate of the threshold beam current which is required for the two-stream instability to develop for a given system length, neutral gas pressure, plasma density, and beam energy. Particle-in-cell simulations carried out with different neutral densities and beam currents demonstrate good agreement with the fluid theory predictions for both the growth rate and the threshold beam current.

physics.plasm-ph↗

arXiv2015

Short intense ion pulses for materials and warm dense matter research

We have commenced experiments with intense short pulses of ion beams on the Neutralized Drift Compression Experiment-II at Lawrence Berkeley National Laboratory, by generating beam spots size with radius r < 1 mm within 2 ns FWHM and approximately 10^10 ions/pulse. To enable the short pulse durations and mm-scale focal spot radii, the 1.2 MeV Li+ ion beam is neutralized in a 1.6-meter drift compression section located after the last accelerator magnet. An 8-Tesla short focal length solenoid compresses the beam in the presence of the large volume plasma near the end of this section before the target. The scientific topics to be explored are warm dense matter, the dynamics of radiation damage in materials, and intense beam and beam-plasma physics including selected topics of relevance to the development of heavy-ion drivers for inertial fusion energy. Here we describe the accelerator commissioning and time-resolved ionoluminescence measurements of yttrium aluminium perovskite using the fully integrated accelerator and neutralized drift compression components.

physics.acc-ph↗

arXiv2015

Structure of Velocity Distribution of Sheath-Accelerated Secondary Electrons in Asymmetric RF-DC Discharge

The ballistic population is thought to be responsible for alleviating the electron shading effect and the notching of the photoresist layer. We have performed test-particle simulations where the features in the EVDF of electrons impacting the RF electrode are fully resolved at all energies. An analytic model has been developed to predict existence of peaked and step-like structures in the EVDF.

physics.plasm-ph↗

arXiv2005

Revisiting the anomalous rf field penetration into a warm plasma

Radio frequency waves do not penetrate into a plasma and are damped within it. The electric field of the wave and plasma current are concentrated near the plasma boundary in a skin layer. Electrons can transport the plasma current away from the skin layer due to their thermal motion. As a result, the width of the skin layer increases when electron temperature effects are taken into account. This phenomenon is called anomalous skin effect. The anomalous penetration of the rf electric field occurs not only for transversely propagating to the plasma boundary wave (inductively coupled plasmas) but also for the wave propagating along the plasma boundary (capacitively coupled plasmas). Such anomalous penetration of the rf field modifies the structure of the capacitive sheath. Recent advances in the nonlinear, nonlocal theory of the capacitive sheath are reported. It is shown that separating the electric field profile into exponential and non-exponential parts yields an efficient qualitative and quantitative description of the anomalous skin effect in both inductively and capacitively coupled plasma.

physics.plasm-ph↗

arXiv2004

Scaling and Formulary cross sections for ion-atom impact ionization

The values of ion-atom ionization cross sections are frequently needed for many applications that utilize the propagation of fast ions through matter. When experimental data and theoretical calculations are not available, approximate formulas are frequently used. This paper briefly summarizes the most important theoretical results and approaches to cross section calculations in order to place the discussion in historical perspective and offer a concise introduction to the topic. Based on experimental data and theoretical predictions, a new fit for ionization cross sections is proposed. The range of validity and accuracy of several frequently used approximations (classical trajectory, the Born approximation, and so forth) are discussed using, as examples, the ionization cross sections of hydrogen and helium atoms by various fully stripped ions.

physics.atom-ph↗

arXiv2004

Influence of ambipolar potential on the properties of inductively coupled discharges at the bounce resonance condition

The importance of accounting for an ambipolar electrostatic potential or a non-uniform density profile for modelling of inductively coupled discharges is demonstrated. A drastic enhancement is observed of the power transfer into plasma for low-collisional, low-pressure, non-local discharges with non-uniform electron density profiles under the condition of bounce resonance. This enhanced plasma heating is attributed to the increase of the number of resonant electrons, for which the bounce frequency inside the potential well is equal to the rf field frequency.

physics.plasm-ph↗

arXiv2003

Comparison of quantum mechanical and classical trajectory calculations of cross sections for ion-atom impact ionization of negative - and positive -ions for heavy ion fusion applications

Stripping cross sections in nitrogen have been calculated using the classical trajectory approximation and the Born approximation of quantum mechanics for the outer shell electrons of 3.2GeV I$^{-}$ and Cs$^{+}$ ions. A large difference in cross section, up to a factor of six, calculated in quantum mechanics and classical mechanics, has been obtained. Because at such high velocities the Born approximation is well validated, the classical trajectory approach fails to correctly predict the stripping cross sections at high energies for electron orbitals with low ionization potential.

physics.atom-ph↗

arXiv2002

Self-consistent system of equations for a kinetic description of the low-pressure discharges accounting for the nonlocal and collisionless electron dynamics

For low-pressure discharges, when the electron mean free path is larger or comparable with the discharge length, the electron dynamics is essentially nonlocal. Moreover, the electron energy distribution function (EEDF) deviates considerably from a Maxwellian. Therefore, an accurate kinetic description of the low-pressure discharges requires knowledge of the nonlocal conductivity operator and calculation of the nonMaxwellian EEDF. The previous treatments made use of simplifying assumptions: a uniform density profile and a Maxwellian EEDF. In the present study a self-consistent system of equations for the kinetic description of nonlocal, nonuniform, nearly collisionless plasmas of low-pressure discharges is derived. It consists of the nonlocal conductivity operator and the averaged kinetic equation for calculation of the nonMaxwellian EEDF. The importance of accounting for the nonuniform plasma density profile on both the current density profile and the EEDF is demonstrated.

physics.plasm-ph↗

arXiv2002

Effect of Electron Energy Distribution Function on Power Deposition and Plasma Density in an Inductively Coupled Discharge at Very Low Pressures

A self-consistent 1-D model was developed to study the effect of the electron energy distribution function (EEDF) on power deposition and plasma density profiles in a planar inductively coupled plasma (ICP) in the non-local regime (pressure < 10 mTorr). The model consisted of three modules: (1) an electron energy distribution function (EEDF) module to compute the non-Maxwellian EEDF, (2) a non-local electron kinetics module to predict the non-local electron conductivity, RF current, electric field and power deposition profiles in the non-uniform plasma, and (3) a heavy species transport module to solve for the ion density and velocity profiles as well as the metastable density. Results using the non-Maxwellian EEDF model were compared with predictions using a Maxwellian EEDF, under otherwise identical conditions. The RF electric field, current, and power deposition profiles were different, especially at 1mTorr, for which the electron effective mean free path was larger than the skin depth. The plasma density predicted by the Maxwellian EEDF was up to 93% larger for the conditions examined. Thus, the non-Maxwellian EEDF must be accounted for in modeling ICPs at very low pressures.

physics.plasm-ph↗

arXiv2002

Effect of non-local electron conductivity on power absorption and plasma density profiles in low pressure inductively coupled discharges

A self-consistent 1-D model was developed to study the effects of non-local electron conductivity on power absorption and plasma density profiles in a planar inductively coupled argon discharge at low pressures (< 10 mTorr). The model consisted of three modules: (1) an electron energy distribution function (EEDF) module to compute the non-Maxwellian EEDF, (2) a non-local electron kinetics module to predict the non-local electron conductivity, RF current, electric field and power deposition profiles in the non-uniform plasma, and (3) a heavy species transport module to solve for the ion density and velocity profiles as well as the metastable density. Results using the non-local electron conductivity model were compared with predictions of a local theory (Ohm's law), under otherwise identical conditions. The RF current, electric field, and power deposition profiles were very different, especially at 1 mTorr for which the effective electron mean free path was larger than the skin depth. However, the plasma density profiles were almost identical (within 10%) for the same total power deposition in the plasma. This result suggests that, for computing plasma density profiles, a local conductivity model (Ohm's law), with much reduced computational expense, may be employed even in the non-local regime.

physics.plasm-ph↗

arXiv2002

How to Patch Active Plasma and Collisionless Sheath: Practical Guide

Most plasmas have a very thin sheath compared with the plasma dimension. This necessitates separate calculations of the plasma and sheath. The Bohm criterion provides the boundary condition for calculation of plasma profiles. To calculate sheath properties a value of electric field at the plasma-sheath interface has to be specified in addition to the Bohm criterion. The value of the boundary electric field and robust procedure to approximately patch plasma and collisionless sheath with a very good accuracy are reported.

physics.plasm-ph↗