Electron Cloud Effects Research Articles

Halogens for negative ion beams and ion–ion plasmas

Negative ions have attractive features as drivers for inertial confinement fusion, because they will avoid electron cloud effects, and could be efficiently photodetached to neutrals after the final focus, which could also be beneficial in heating warm dense matter targets. The halogens have large electron affinities, and thus should be able to produce high current densities of relatively robust negative ions. Recent experiments comparing chlorine beams to argon beams using the same source, extraction optics, and diagnostics have demonstrated that Cl − beams can be produced with similar emittance to Ar + beams, and with about 3 4 the current density from the same configuration. The observed effective beam temperature of about 1 3 eV , and the similarity of current densities show that negative halogen beams can meet the current density and emittance requirements of heavy ion fusion. The near equivalence of the Cl − and Cl ++Cl 2 + current densities reaching the Faraday cup after passage through a substantial line density of effluent gas demonstrates that beam losses in the higher vacuum of a heavy ion fusion accelerator should be acceptable. A number of lines of evidence show that negative ion–positive ion plasmas (hereafter ion–ion plasmas), composed primarily of negative and positive ions with a small population of electrons, were produced in the sources near the extractor plane. Since Cl, F, I, and Br should all show similar chemistry, any of these halogens should be suitable as fusion driver beams, and heating thin iodine or bromine foils may produce ion–ion plasmas in the warm dense matter regime.

Simulating electron clouds in high-current ion accelerators with solenoid focusing

Contamination from electrons is a concern for the solenoid-focused ion accelerators being developed for experiments in high-energy density physics (HEDP). These electrons are produced directly by beam ions hitting lattice elements and intercepting diagnostics, or indirectly by ionization of desorbed neutral gas, and they are believed responsible for time dependence of the beam radius, emittance, and focal distance seen on the solenoid transport experiment (STX) at Lawrence Berkeley National Laboratory. The electrostatic particle-in-cell code WARP has been upgraded to include the physics needed to simulate electron-cloud phenomena. We present preliminary self-consistent simulations of STX experiments suggesting that the observed time dependence of the beam stems from a complicated interaction of beam ions, desorbed neutrals, and electrons.

Absolute differential, elastic integral and moment transfer cross sections for electron scattering by SO2 at intermediate and high energies

Taking into consideration the overlapping effect of electron clouds between two bonded atoms in a molecule, a modificed potential method is presented which can be used to accurately calculate various cross sections for electron scattering by molecules at intermediate and high energies. The modified complex optical potential is directly employed to calculate the absolute differential, elastic integral and moment transfer cross sections for electron scattering by SO2 in the energy range 100—1000 eV by using the additivity rule at Hartree-Fork level. The quantitative results are compared with those obtained by measurement and other theories wherever available. It is shown that the additivity rule model together with the modified complex optical potential can give the encouraging results, which are much closer to the measurements than the unmodified ones.

A preliminary assessment of the electron-cloud effect for the FNAL main injector upgrade

We present results from a preliminary assessment, via computer simulations, of the electron-cloud (EC) density for the FNAL Main Injector upgrade at injection energy. Assuming a peak value for secondary emission yield δmax = 1.3, we find a threshold value of the bunch population, Nb,th≃ 1.25 ×1011, beyond which the EC density ρe reaches a steady-state level that is ∼104 times larger than for Nb<Nb,th, essentially neutralizing the beam, and leading to a tune shift ∼0.05. Our investigation is limited to a field-free region and to a dipole magnet region, both of which yield similar results for both Nb,th and the steady-state value of ρe. Possible dynamical effects from the EC on the beam, such as emittance growth and instabilities, remain to be investigated separately.

A modification potential method of calculating total cross sections of electrons scattering from complex molecules C 2 H 6 , C 2 F 6 , C 6 H 6 and C 6 F 6 at 100 eV–5000 eV

A complex optical model potential modified by incorporating the concept of bonded atom, which takes into consideration the overlapping effect of electron clouds between two atoms in a molecule, is first employed to calculate the total cross sections for electrons scattering from such complex molecules as C2H6, C2F6, C6H6 and C6F6 using the additivity rule model at Hartree–Fock level over the energy range from 100 eV to 5000 eV. The total cross sections are quantitatively compared with those obtained by experiments wherever available and they are in good agreement with each other over a wide energy range. It is shown that the modified potential together with the additivity rule model is completely suitable for the calculation of total cross sections of electrons scattering from such complex molecules as C2H6, C2F6, C6H6 and C6F6 above 200 eV–300 eV.

Ion focusing effect of electron cloud produced by laser-plasma interaction

We propose a focusing mechanism of high-energy ions by an electron cloud produced by a laser interaction with slab plasma. In our 2.5-dimensional (2.5D) particle-in-cell simulations, the laser intensity is 2 × 1020 W/cm2, the laser wavelength λ is 1.053 μm, and the laser spot size is 2.5λ. When the high intensity laser irradiates slab plasma, electrons are accelerated, oscillate around the plasma and produce the electron cloud locally at the sides of the plasma. Because the electrons are localized transversely, a static electric potential is formed to focus ions and at the same time the ions are accelerated longitudinally. Though the longitudinal ion acceleration has been studied well, the ion focusing effect is reported for the first time in this paper. In our calculations, the maximum energy and intensity of the protons are 8.61 MeV and 1.89 × 1017 W/cm2, and the diameter of the proton bunch accelerated are focused to 71.2% of its initial size.

A modification potential method for total cross section calculation of electron scattering by molecules at medium and high energies

Taking into consideration the overlapping effect of electron clouds between two atoms in a molecule, a modification method is presented which can be used to calculate the total cross sections for electron scattering from numerous molecules at medium and high energies. The modified complex optical potential composed of static, exchange, polarization plus absorption is directly employed to calculate the total cross sections for electron scattering from isoelectronic molecules (HCl, H2S, PH3 and SiH4) over the energy range 30—5000eV using the additivity rule model at Hartree-Fock level. The obtained quantitative molecular total cross sections are compared with those obtained by experiments and other theories wherever available, and good agreement is obtained. It is shown that the additivity rule model together with the modified complex optical potential can give much better results than the unmodified one. So, the modification potential method presented here can improve the accuracy of the total cross section calculations of electron scattering from molecules.

Total cross sections for scattering of positron from CO, HCl, NH3 and CH4 at intermediate and high energies

A complex optical model potential modified by incorporating the concept of bonded atom, which takes into consideration the overlapping effect of electron clouds between two atoms in a molecule, is firstly employed to calculate the total cross sections for positron scattering from CO, HCl, NH3, SiH4 in the incident energy range 30—3000eV by using additivity rule model at Hartree-Fock level. In the study, the complex optical model potential composed of static, correlation polarization plus absorption contributions firstly uses the bonded-atom concept. The results of quantitative molecular total cross sections are compared with those obtained by experiments and other theories wherever available, and good agreement is obtained. It is shown that the additivity rule model together with the complex optical potential model modified by incorporating the concept of bonded atom can give much better calculation results than the unmodified one.

Measurement of dynamic beam-beam effects

A series of measurements were carried out to investigate the dynamic beam-beam effects at KEKB. The beam-beam kick and the coherent beam-beam tune shift were measured by comparing the beam parameters between a colliding bunch and a noncolliding one. We found that collisions occur with a horizontal orbit offset to obtain the maximum luminosity, and that this reproducible offset appears to be attributed to an electron cloud effect, which might play an important role in the beam-beam interaction. The horizontal beam size estimated from the beam-beam kick slightly decreased as the beam-beam parameter increased. The horizontal emittance estimated from the beam-beam tune shift was higher than an unperturbed emittance, and agreed with the calculated emittance using a higher-mode tune. The ratio of the square-root betatron function between two locations in the ring was measured as a function of the beam-beam parameter. These experimental results would provide useful information on dynamic beam-beam effects.

Total cross sections for electrons scattering from simple molecules containing the larger atom sulfur at 30–5000eV

A complex optical model potential modified by the concept of bonded atom, which takes into consideration the overlapping effect of electron clouds, is employed to calculate the total cross sections for electrons scattering from simple molecules (SO2, H2S, OCS, CS2 and SO3) containing the larger atom, sulfur, at 30–5000eV by using the additivity rule model at Hartree–Fock level. The quantitative molecular total cross section results are compared with those obtained in experiments and other calculations wherever available and good agreement is obtained. It is shown that the additivity rule model together with the complex optical model potential modified by the concept of bonded atom can give the results closer to the experiments than the one unmodified by it. So, the introduction of bonded-atom concept in complex optical model potential betters the accuracy of the total cross section calculations of electrons from the molecules containing the larger atom, sulfur.

Positron total cross sections for collisions with O 2 , H 2 O and CH 4 in energy range from 30 to 3000eV

Total (elastic plus inelastic) cross sections (TCSs) for the scattering of positrons by molecules O2, H2O and CH4 in the energy range from 30 to 3000eV are calculated using the additivity rule model at Hartree–Fock level. A complex optical model potential modified by incorporating the concept of bonded atom which takes into consideration the overlapping effect of electron clouds between two atoms in a molecule is firstly employed for the TCS calculation of positron–molecule scattering. TCSs are quantitatively compared with those obtained in experiments and other theories wherever available and good agreement is obtained in the energy range from 30 to 3000eV. It is shown that the additivity rule model together with the modified complex optical model potential is completely suitable for the TCS calculations of the positron–molecule scattering.

Energy loss, range, and electron yield comparisons of the CRANGE ion–material interaction code

We present comparisons of the CRANGE code to other well-known codes, SRIM and ASTAR, and to experimental results for ion–material interactions such as energy loss per unit length, ion range, and ion-induced electron yield. These ion–material interaction simulations are relevant to the electron cloud effect in heavy ions accelerators for fusion energy and high energy density physics. Presently, the CRANGE algorithms are most accurate at energies above 1.0 MeV/amu. For calculation of energy loss per unit length of a potassium ion in stainless steel, results of CRANGE and SRIM agree to within 10% above 1.0 MeV/amu. For calculations of the range of a helium ion in aluminum, results of CRANGE and ASTAR agree to within 2% above 1.0 MeV/amu. Finally, for calculations of ion-induced electron yield for hydrogen ions striking gold, results of CRANGE agree to within 10% with measured electron yields above 1.0 MeV/amu.

High current transport experiment for heavy ion inertial fusion

The High Current Experiment (HCX) at Lawrence Berkeley National Laboratory is part of the US program to explore heavy-ion beam transport at a scale representative of the low-energy end of an induction linac driver for fusion energy production. The primary mission of this experiment is to investigate aperture fill factors acceptable for the transport of space-charge-dominated heavy-ion beams at high space-charge intensity (line-charge density /spl sim/ 0.2 /spl mu/C/m) over long pulse durations (>4 /spl mu/s) in alternating gradient electrostatic and magnetic quadrupoles. This experiment is testing - at driver-relevant scale - transport issues resulting from nonlinear space-charge effects and collective modes, beam centroid alignment and beam steering, matching, image charges, halo, electron cloud effects, and longitudinal bunch control. We present the results for a coasting 1 MeV K/sup +/ ion beam transported through the first ten electrostatic transport quadrupoles, measured with beam-imaging and phase-space diagnostics. The latest additions to the experiment include measurements of the secondary ion, electron and atom coefficients due to halo ions scraping the wall, and four magnetic quadrupoles to explore similar issues in magnetic channels.

Experimental study of the transport limits of intense heavy ion beams in the HCX

The High Current Experiment (HCX) at Lawrence Berkeley National Laboratory is part of the US program to explore heavy-ion beam transport at a scale representative of the low-energy end of an induction linac driver for fusion energy production. The primary mission of this experiment is to investigate aperture fill factors acceptable for the transport of space-charge-dominated heavy-ion beams at high space-charge intensity (line charge density up to ∼0.2 μC/m) over long pulse durations (4 μs) in alternating gradient focusing lattices of electrostatic or magnetic quadrupoles. The experiment also contributes to the practical baseline knowledge of intense beam manipulations necessary for the design, construction and operation of a heavy ion driver for inertial fusion. This experiment tests transport issues resulting from nonlinear space-charge effects and collective modes, beam centroid alignment and beam steering, matching, image charges, halo, electron cloud effects, and longitudinal bunch control. We first present the results for a coasting 1 MeV K + ion beam transported through the first ten electrostatic transport quadrupoles, measured with optical beam-imaging and double-slit phase-space diagnostics. This includes studies at two different radial fill factors (60% and 80%), for which the beam transverse distribution was characterized in detail. Additionally, beam energy measurements will be shown. We then discuss the first results of beam transport through four pulsed room-temperature magnetic quadrupoles (located downstream of the electrostatic quadrupoles), where the beam dynamics become more sensitive to the presence of secondary electrons.

A Correlation Potential Method for Electron Scattering Total CrossSection Calculations on Several Diatomic and Polyatomic Molecules over Energy Range 10∼5000 eV

A complex optical model potential correlated by the concept of bonded atom, which considers the overlapping effect of electron clouds between two atoms in a molecule, is firstly employed to calculate the total cross sections for electron scattering on several molecules (NH3, H2O, CH4, CO, N2, O2, andC2H4) over the energy range 10∼5000 eV using the additivity rule model at Hartree–Fock level. The difference between the bonded atom and the free one in states is that the overlapping effect of electron clouds of bonded atoms in a molecule is considered. The quantitative total cross sections are compared with the experimental data and with the other calculations wherever available and good agreement is obtained over the energyrange 10∼5000 eV. It is shown that the correlated calculations are much closer to the available experimental data than the uncorrelated ones at lower energies, especially below 500 eV. Therefore, considering the overlapping effect of electron clouds in the complex optical model potential could be helpful for the better accuracy of the total cross section calculations of electron scattering from molecules.

Overview of US heavy-ion fusion progress and plans

Significant experimental and theoretical progress has been made in the US heavy-ion fusion program on high-current sources, injectors, transport, final focusing, chambers and targets for high-energy density physics (HEDP) and inertial fusion energy (IFE) driven by induction linac accelerators. One focus of present research is the beam physics associated with quadrupole focusing of intense, space–charge dominated heavy-ion beams, including gas and electron cloud effects at high currents, and the study of long-distance-propagation effects such as emittance growth due to field errors in scaled experiments. A second area of emphasis in present research is the introduction of background plasma to neutralize the space charge of intense heavy-ion beams and assist in focusing the beams to a small spot size. In the near future, research will continue in the above areas, and a new area of emphasis will be to explore the physics of neutralized beam compression and focusing to high intensities required to heat targets to high-energy density conditions as well as for inertial fusion energy.

Maps for electron clouds

The electron cloud effect has been studied by means of detailed simulation codes that typically track the particles' evolution under the influence of the corresponding electromagnetic forces and fields. In this paper we show that, for the RHIC case, the electron cloud can be treated from an abstract point of view as a bunch to bunch evolution using simple maps. Secondly, we show how this treatment yields a useful conclusion, which is otherwise difficult to obtain: for a fixed number of bunches and total beam current in RHIC, it is possible to determine the best way to distribute the bunch pattern around the ring to minimize the electron cloud formation. This application is an example of how maps become a useful tool for exploring the electron cloud evolution in parameter space.

Simulating electron cloud effects in heavy-ion beams

Stray electrons can be introduced in heavy-ion fusion accelerators as a result of ionization of ambient gas or gas released from walls due to halo-ion impact, or as a result of secondary-electron emission. We summarize here results from several studies of electron-cloud accumulation and effects: (1) We calculate the electron cloud produced by electron desorption from computed beam-ion loss and the importance of ion scattering is shown. (2) We simulate the effect of specified electron cloud distributions on ion beam dynamics. We find electron cloud variations that are resonant with the breathing mode of the beam have the biggest impact on the beam (larger than other resonant and random variations), and that the ion beam is surprisingly robust, with an electron density several percent of the beam density required to produce significant beam degradation in a 200-quadrupole system. We identify a possible instability associated with desorption and resonance with the breathing mode. (3) We carry out preliminary investigations of a long-timestep algorithm for electron dynamics in arbitrary magnetic fields.

Overview of US heavy ion fusion research

Significant experimental and theoretical progress has been made in the US heavy ion fusion programme on high-current sources, injectors, transport, final focusing, chambers and targets for high-energy density physics and inertial fusion energy (IFE) driven by induction linac accelerators. One focus of the present research is the beam physics associated with quadrupole focusing of intense, space-charge-dominated heavy ion beams, including gas and electron cloud effects at high currents, and the study of long-distance-propagation effects such as emittance growth due to field errors in scaled experiments. A second area of emphasis in the present research is the introduction of background plasma to neutralize the space-charge of intense heavy ion beams and assist in focusing the beams to a small spot size. In the near future, research will continue in the above areas, and a new area of emphasis will be to explore the physics of neutralized beam compression and focusing to high intensities required to heat targets to high-energy density conditions as well as for IFE.

Differential, elastic integral and moment transfer cross sections for electron scattering from N 2 at intermediate- and high-energies

A complex optical model potential modified by incorporating the concept of bonded atom, with the overlapping effect of electron clouds between two atoms in a molecule taken into consideration, is firstly employed to calculate the differential cross sections, elastic integral cross sections and moment transfer cross sections for electron scattering from molecular nitrogen over the energy range 300–1000eV by using additivity rule model at Hartree–Fock level. The bonded-atom concept is used in the study of the complex optical model potential composed of static, exchange, correlation polarization and absorption contributions. The calculated quantitative molecular differential cross sections, elastic integral cross sections and moment transfer cross sections are compared with the experimental and theoretical ones wherever available and they are found to be in good agreement with each other. It is shown that the additivity rule model together with the complex optical model potential modified by incorporating the concept of bonded atom is completely suitable for the calculations of differential cross section, elastic integral cross section and moment transfer cross section over the intermediate- and high-energy ranges.