210 Matching Results

Search Results

Advanced search parameters have been applied.

Simultaneous measurement of the average ion-induced electron emission yield and the mean charge for isotachic ions in carbon foils

Description: Article discussing the simultaneous measurement of the average ion-induced electron emission yield and the mean charge for isotachic ions in carbon foils.
Date: February 1997
Creator: Arrale, A. M.; Zhao, Z. Y.; Kirchhoff, J. F.; Weathers, Duncan L.; McDaniel, Floyd Del. (Floyd Delbert), 1942- & Matteson, Samuel E.
Partner: UNT College of Arts and Sciences

Microscopic probabilistic model for the simulation of secondary electron emission

Description: We provide a detailed description of a model and its computational algorithm for the secondary electron emission process. The model is based on a broad phenomenological fit to data for the secondary emission yield (SEY) and the emitted-energy spectrum. We provide two sets of values for the parameters by fitting our model to two particular data sets, one for copper and the other one for stainless steel.
Date: July 29, 2002
Creator: Furman, M.A. & Pivi, M.T.F.
Partner: UNT Libraries Government Documents Department

Ecloud Build-Up Simulations for the FNAL MI for a Mixed Fill Pattern: Dependence on Peak SEY and Pulse Intensity During the Ramp

Description: We present simulation results of the build-up of the electron-cloud density n{sub e} in three regions of the FNAL Main Injector (MI) for a beam fill pattern made up of 5 double booster batches followed by a 6th single batch. We vary the pulse intensity in the range N{sub t} = (2-5) x 10{sup 13}, and the beam kinetic energy in the range E{sub k} = 8-120 GeV. We assume a secondary electron emission model qualitatively corresponding to TiN, except that we let the peak value of the secondary electron yield (SEY) {delta}{sub max} vary as a free parameter in a fairly broad range. Our main conclusions are: (1) At fixed N{sub t} there is a clear threshold behavior of n{sub e} as a function of {delta}{sub max} in the range {approx} 1.1-1.3. (2) At fixed {delta}{sub max}, there is a threshold behavior of n{sub e} as a function of N{sub t} provided {delta}{sub max} is sufficiently high; the threshold value of N{sub t} is a function of the characteristics of the region being simulated. (3) The dependence on E{sub k} is weak except possibly at transition energy. Most of these results were informally presented to the relevant MI personnel in April 2010.
Date: December 11, 2010
Creator: Furman, M. A.
Partner: UNT Libraries Government Documents Department

Photoelectron angular distributions beyond the dipole approximation

Description: This paper reports the measurement of nondipolar asymmetry parameters for the angular distributions of Ar 1s, Kr 2s, and Kr 2p photoemission within 2-3 keV above their respective thresholds. Pronounced asymmetries with respect to direction of photon propagation are present even at low electron kinetic energies. The energy dependence of the asymmetry is Z- and subshell-specific and causes the direction of preferred electron emission to change sign at least once in each case. Towards higher energies, the asymmetry increases and the photoelectron emission pattern is forward-skewed for all 3 cases. Measured asymmetry parameters are in good agreement with recent predictions from nonrelativistic calculations which include interference between electric-dipole and electric-quadrupole transition amplitudes in the photoabsorption process.
Date: August 1996
Creator: Kraessig, B.; Jung, M.; Gemmell, D. S.; Kanter, E. P.; LeBrun, T.; Southworth, S. H. et al.
Partner: UNT Libraries Government Documents Department

Effect of Secondary Electron Emission on Electron Cross-Field Current in E×B Discharges

Description: This paper reviews and discusses recent experimental, theoretical, and numerical studies of plasma-wall interaction in a weakly collisional magnetized plasma bounded with channel walls made from different materials. A lowpressure ExB plasma discharge of the Hall thruster was used to characterize the electron current across the magnetic field and its dependence on the applied voltage and electron-induced secondary electron emission (SEE) from the channel wall. The presence of a depleted, anisotropic electron energy distribution function with beams of secondary electrons was predicted to explain the enhancement of the electron cross-field current observed in experiments. Without the SEE, the electron crossfield transport can be reduced from anomalously high to nearly classical collisional level. The suppression of SEE was achieved using an engineered carbon velvet material for the channel walls. Both theoretically and experimentally, it is shown that the electron emission from the walls can limit the maximum achievable electric field in the magnetized plasma. With nonemitting walls, the maximum electric field in the thruster can approach a fundamental limit for a quasineutral plasma.
Date: February 10, 2011
Creator: Yevgeny Raitses, Igor D. Kaganovich, Alexander Khrabrov, Dmytro Sydorenko, Nathaniel J. Fisch and Andrei Smolyakov
Partner: UNT Libraries Government Documents Department

Electron emission and defect formation in the interaction of slow,highly charged ions with diamond surfaces

Description: We report on electron emission and defect formation in theinteraction between slow (v~;0.3 vBohr) highly charged ions (SHCI) withinsulating (type IIa) and semiconducting (type IIb) diamonds. Electronemission induced by 31Pq+ (q=5 to 13), and 136Xeq+ (q=34 to 44) withkinetic energies of 9 kVxq increase linearly with the ion charge states,reaching over 100 electrons per ion for high xenon charge states withoutsurface passivation of the diamond with hydrogen. Yields from bothdiamond types are up to a factor of two higher then from reference metalsurfaces. Crater like defects with diameters of 25 to 40 nm are formed bythe impact of single Xe44+ ions. High secondary electron yields andsingle ion induced defects enable the formation of single dopant arrayson diamond surfaces.
Date: May 31, 2006
Creator: Sideras-Haddad, E.; Shrivastava, S.; Rebuli, D.B.; Persaud, A.; Schneider, D.H. & Schenkel, T.
Partner: UNT Libraries Government Documents Department

Simulation of secondary electron emission based on a phenomenological probabilistic model

Description: We provide a detailed description of a model and its computational algorithm for the secondary electron emission process. The model is based on a broad phenomenological fit to data for the secondary emission yield (SEY) and the emitted-energy spectrum. We provide two sets of values for the parameters by fitting our model to two particular data sets, one for copper and the other one for stainless steel. We also present details of the electron-cloud simulation code POSINST that are relevant to the secondary emission process. This note expands on our previously published article.
Date: June 2, 2003
Creator: Furman, M.A. & Pivi, M.T.F.
Partner: UNT Libraries Government Documents Department

Benchmarking Multipacting Simulations in VORPAL

Description: We will present the results of benchmarking simulations run to test the ability of VORPAL to model multipacting processes in Superconducting Radio Frequency structures. VORPAL is an electromagnetic (FDTD) particle-in-cell simulation code originally developed for applications in plasma and beam physics. The addition of conformal boundaries and algorithms for secondary electron emission allow VORPAL to be applied to multipacting processes. We start with simulations of multipacting between parallel plates where there are well understood theoretical predictions for the frequency bands where multipacting is expected to occur. We reproduce the predicted multipacting bands and demonstrate departures from the theoretical predictions when a more sophisticated model of secondary emission is used. Simulations of existing cavity structures developed at Jefferson National Laboratories will also be presented where we compare results from VORPAL to experimental data.
Date: May 1, 2009
Creator: C. Nieter, C. Roark, P. Stoltz, K. Tian
Partner: UNT Libraries Government Documents Department

Electrons in a positive-ion beam with solenoid or quadrupole magnetic transport

Description: The High Current Experiment (HCX) is used to study beam transport and accumulation of electrons in quadrupole magnets and the Neutralized Drift-Compression Experiment (NDCX) to study beam transport through and accumulation of electrons in magnetic solenoids. We find that both clearing and suppressor electrodes perform as intended, enabling electron cloud densities to be minimized. Then, the measured beam envelopes in both quadrupoles and solenoids agree with simulations, indicating that theoretical beam current transport limits are reliable, in the absence of electrons. At the other extreme, reversing electrode biases with the solenoid transport effectively traps electrons; or, in quadrupole magnets, grounding the suppressor electrode allows electron emission from the end wall to flood the beam, in both cases producing significant degradation in the beam.
Date: June 4, 2007
Creator: Molvik, A.W.; Kireeff Covo, M.; Cohen, R.; Coleman, J.; Sharp, W.; Bieniosek, F. et al.
Partner: UNT Libraries Government Documents Department

Self-Consistent 3D Modeling of Electron Cloud Dynamics and Beam Response

Description: We present recent advances in the modeling of beam electron-cloud dynamics, including surface effects such as secondary electron emission, gas desorption, etc, and volumetric effects such as ionization of residual gas and charge-exchange reactions. Simulations for the HCX facility with the code WARP/POSINST will be described and their validity demonstrated by benchmarks against measurements. The code models a wide range of physical processes and uses a number of novel techniques, including a large-timestep electron mover that smoothly interpolates between direct orbit calculation and guiding-center drift equations, and a new computational technique, based on a Lorentz transformation to a moving frame, that allows the cost of a fully 3D simulation to be reduced to that of a quasi-static approximation.
Date: April 2, 2007
Creator: Furman, Miguel; Furman, M.A.; Celata, C.M.; Kireeff-Covo, M.; Sonnad, K.G.; Vay, J.-L. et al.
Partner: UNT Libraries Government Documents Department

A Masked Photocathode in Photoinjector

Description: In this paper, we propose a masked photocathode inside the photoinjector for generating high brightness election beam. Instead of mounting the photocathode onto an electrode, an electrode with small hole is used as a mask to shield the photocathode from the accelerating vacuum chamber. Using such a masked photocathode will make the replacement of photocathode material easy by rotating the photocathode behind the electrode into the hole. Furthermore, this helps reduce the dark current or secondary electron emission from the photocathode material. The masked photocathode also provides transverse cut-off to a Gaussian laser beam that reduces electron beam emittance growth from nonlinear space-charge effects.
Date: December 14, 2010
Creator: Qiang, Ji
Partner: UNT Libraries Government Documents Department

Very High Power THz Radiation Sources

Description: We report the production of high power (20 watts average, {approx}1 Megawatt peak) broadband THz light based on coherent emission from relativistic electrons. Such sources are ideal for imaging, for high power damage studies and for studies of non-linear phenomena in this spectral range. We describe the source, presenting theoretical calculations and their experimental verification. For clarity, we compare this sources with one based on ultrafast laser techniques.
Date: October 1, 2002
Creator: Carr, G.L.; Martin, Michael C.; McKinney, Wayne R.; Jordan, Kevin; Neil, George R. & Williams, Gwyn P.
Partner: UNT Libraries Government Documents Department

A new approach to nuclear microscopy: The ion-electron emission microscope

Description: A new multidimensional high lateral resolution ion beam analysis technique, Ion-Electron Emission Microscopy or IEEM is described. Using MeV energy ions, IEEM is shown to be capable of Ion Beam Induced Charge Collection (IBICC) measurements in semiconductors. IEEM should also be capable of microscopically and multidimensionally mapping the surface and bulk composition of solids. As such, IIEM has nearly identical capabilities as traditional nuclear microprobe analysis, with the advantage that the ion beam does not have to be focused. The technique is based on determining the position where an individual ion enters the surface of the sample by projection secondary electron emission microscopy. The x-y origination point of a secondary electron, and hence the impact coordinates of the corresponding incident ion, is recorded with a position sensitive detector connected to a standard photoemission electron microscope (PEEM). These signals are then used to establish coincidence with IBICC, atomic, or nuclear reaction induced ion beam analysis signals simultaneously caused by the incident ion.
Date: November 1, 1998
Creator: Doyle, B.L.; Vizkelethy, G.; Walsh, D.S.; Senftinger, B. & Mellon, M.
Partner: UNT Libraries Government Documents Department

Emission of secondary particles from metals and insulators at impact of slow highly charged ions

Description: Emission of secondary electrons and ions from clean Au, CxHy-Au, and SiO{sub 2} surfaces at impact of slow (v{approx}0.3 v{sub Bohr}) ions were measured as function of incident ion charge for 1+{le}q{le}75+. Electron yields from thermal SiO{sub 2} films (150 mm on Si) were found to be lower than those from the other two for q>3+. Yields of negative secondary ions from SiO{sub 2} and CxHy-Au were recorded in parallel with electron emission data and exhibit a q{sub 4} dependency on incident ion charge. Direct comparison of collisional and electronic contributions to secondary ion production from SiO{sub 2} films using a beam of charge state equilibrated Xe (at 2.75 keV/u) shows positive and negative secondary ion yield increases with incident ion charge of >400. Results are discussed in relation to key signatures of electronic sputtering by Coulomb explosions.
Date: October 31, 1996
Creator: Schenkel, T.
Partner: UNT Libraries Government Documents Department

Calculation of electron emission from a tantalum foil irradiated by 100-kV and 50-kV x-rays

Description: Two Monte Carlo programs, XITRAN and XMTRAN, were developed for calculating the emission of electrons from high-Z foils irradiated with x rays. XITRAN follows all individual elastic collisions of electrons with atoms, whereas XMTRAN uses the condensed-random-walk model. Both codes take into account photo-electrons, fluorescence radiation, and Auger electrons. Comparisons are made with an experiment by Dolan at Sandia Laboratories involving the backward and forward emission of electrons from a tantalum foil irradiated by 100-kV and 50-kV x-ray beams. There is good agreement between results from the XITRAN and XMTRAN codes. There emitted per incident x-ray photon, and in regard to the angular distribution of the emerging electrons. In regard to the electron energy spectra, there is fair agreement down to a spectral energy of 20 keV, whereas below 20 keV the calculated spectra lie considerably below the measurements.
Date: March 1, 1998
Creator: Berger, M.J.
Partner: UNT Libraries Government Documents Department

Electron emission following the interaction of slow highly charged ions with solids

Description: The interaction of highly-charged ions with surfaces involves many excitation processes of the surface atoms and the bulk material. One such process, the emission of electrons from surfaces due to the potential energy of the incident ions has been studied. The experimental results presented here confirm that the majority of electrons emitted as a result of highly-charged ions interacting with a solid surface have energies of about 20 eV. Auger processes contribute a smaller fraction of the total emitted electrons with increasing Z of the projectile. This contribution to the total electron emission yield is found to be less than 5% for Ne{sup 9+} and less than 1% for Ar{sup 18+}. For Z{>=} 54, no Auger electrons were detected. The early indications that the total number of emitted low energy electrons increases linearly with charge have been demonstrated not to hold for q {>=} 18.
Date: January 1, 1998
Creator: McDonald, J.W., LLNL
Partner: UNT Libraries Government Documents Department

Field emission and growth of fullerene nanotubes

Description: Efforts to control the growth of individual carbon nanotubes from nanotube seed crystals have led to a characterization of their field-induced electron emission behavior. The application of a bias voltage in the growth apparatus was motivated by the prolific formation of nanotubes in the carbon are growth method, in which the electric field appears to play a central role. The authors report here the ability to achieve various tube tip configurations by the controlled application of voltage, heat and chemicals to an individual nanotube, and that these states are well characterized by the emission currents they induce.
Date: November 1994
Creator: Rinzler, A. G.; Hafner, J. H.; Nilolaev, P.; Colbert, D. T. & Smalley, R. E.
Partner: UNT Libraries Government Documents Department

A diffusion model for picosecond electron bunches from negative electron affinity GaAs photo cathodes

Description: Even though theoretical estimates predict response times for the photo emission process of electrons from a negative electron affinity GaAs photo emitter in excess of hundreds of picoseconds, recent measurements found electron bunch durations of 40 ps or less. This work presents precise measurements of picosecond electron bunches from a negative affinity bulk GaAs photo cathode and develops a model which explains the measured bunch durations as well as the observed bunch shapes. The bunch shape turns out to be independent from the quantum efficiency of the photo emitter.
Date: October 27, 1998
Creator: Hartmann, P.; Bermuth, J.; Harrach, D. v.; Hoffmann, J.; Kobis, S.; Reichert, E. et al.
Partner: UNT Libraries Government Documents Department

A long pulse high-power diode based on a microelectronic emitter

Description: Microelectronic cathode emitter technology being developed at Sandia for supplying continuous low current for flat panel displays appears to be a promising technology for providing high currents when operated in a pulsed, higher voltage mode. If currents in excess of one amp per square centimeter could be produced for tens of microseconds at several kilohertz repetition rate, important applications in such as large volume food or waste sterilization in situ detection, and high power microwave production could be achieved. A testbed was built to perform the experiments. The desired current densities have been demonstrated using small emitter arrays.
Date: November 1, 1995
Creator: Marder, B.; Clark, C.; Walko, R. & Fleming, J.
Partner: UNT Libraries Government Documents Department