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Particle-In-Cell/Monte Carlo Simulation of Ion Back Bombardment in Photoinjectors

Description: In this paper, we report on studies of ion back bombardment in high average current dc and rf photoinjectors using a particle-in-cell/Monte Carlo method. Using H{sub 2} ion as an example, we observed that the ion density and energy deposition on the photocathode in rf guns are order of magnitude lower than that in a dc gun. A higher rf frequency helps mitigate the ion back bombardment of the cathode in rf guns.
Date: March 2, 2009
Creator: Qiang, Ji; Corlett, John & Staples, John
Partner: UNT Libraries Government Documents Department

Surface Dependent Electron and Negative Ion Density in Inductively Coupled Discharges

Description: Electron and negative ion density have been measured in a modfied Applied Materials DPS metal etch chamber using gas mixtures of BCl{sub 3}, Cl{sub 2} and Ar. Measurements were performed for four dflerent substrate types to examine the influence of surface material on the bulk plasma properties; aluminurq alumina, photoresist and 50 percent patterned aluminum / photoresist. Electron densities in the Cl{sub 2} / BCl{sub 3} mixtures varied from 0.25 to 4 x 10{sup 11} cm{sup -3}. Photodetachment measurements of the negative ion density indicate that the negative ion density was smaller than the electron density and that the electron to negative ion density ratio varied between 1 and 6. The presence of photoresist had a dominant intluence on the electron and negative ion density compared to alumina and aluminum surfaces. In most cases, the electron density above wafers covered with photoresist was a factor of two lower while the negative ion density was a factor of two higher than the aluminum or alumina surfaces.
Date: January 18, 1999
Creator: Blain, M.G.; Hamilton, T.W.; Hebner, G.A.; Jarecki, R.L. & Nichols, C.A.
Partner: UNT Libraries Government Documents Department

Pick-up ion energization at the termination shock

Description: One-dimensional hybrid simulations are used to investigate how pickup ions are energized at the perpendicular termination shock. Contrary to previous models based on pickup ion energy gain by repeated crossings of the shock front (shock surfing) or due to a reforming shock front, the present simulations show that pickup ion energy gain involves a gyro-phasedependent interaction with the inhomogeneous motional electric field at the shock. The process operates at all relative concentrations of pickup ion density.
Date: January 1, 2009
Creator: Gary, S Peter; Winske, Dan; Wu, Pin & Schwadron, N A
Partner: UNT Libraries Government Documents Department

Enhanced Electron Attachment to Highly-Excited States of Molecules: Implications for Plasma Processing Discharges

Description: Recent studies show that large negative ion densities exist in plasma processing discharges, including those of weakly electronegative gases such as SiH{sub 4} and CF{sub 4}. Also, there is strong evidence that the negative ions could be the precursors for particulate formation in processing discharges. Even though it is now well established that large concentrations of negative ions exist in processing discharges, and that they play a crucial role in such discharges, the source of such high negative ion densities has not been clarified. In particular, gases like SiH{sub 4} and CH{sub 4}, which are commonly used in processing discharges, attach electrons only weakly in their ground electronic states (see the references). Due to the lack of an alternative mechanism, the origin of large negative ion densities in such weakly electronegative gases has been frequently attributed to electron attachment to radicals (molecular fragments) or other byproducts produced in the discharge. This hypothesis had not been tested in direct electron attachment measurements.
Date: June 29, 1998
Creator: Datskos, P.G.; Ding, W.; McCorkle, D.L. & Pinnaduwage, L.A.
Partner: UNT Libraries Government Documents Department

Ion heating and energy partition at the heliospheric termination shock: hybrid simulations and analytical model

Description: The Los Alamos hybrid simulation code is used to examine heating and the partition of dissipation energy at the perpendicular heliospheric termination shock in the presence of pickup ions. The simulations are one-dimensional in space but three-dimensional in field and velocity components, and are carried out for a range of values of pickup ion relative density. Results from the simulations show that because the solar wind ions are relatively cold upstream, the temperature of these ions is raised by a relatively larger factor than the temperature of the pickup ions. An analytic model for energy partition is developed on the basis of the Rankine-Hugoniot relations and a polytropic energy equation. The polytropic index {gamma} used in the Rankine-Hugoniot relations is varied to improve agreement between the model and the simulations concerning the fraction of downstream heating in the pickup ions as well as the compression ratio at the shock. When the pickup ion density is less than 20%, the polytropic index is about 5/3, whereas for pickup ion densities greater than 20%, the polytropic index tends toward 2.2, suggesting a fundamental change in the character of the shock, as seen in the simulations, when the pickup ion density is large. The model and the simulations both indicate for the upstream parameters chosen for Voyager 2 conditions that the pickup ion density is about 25% and the pickup ions gain the larger share (approximately 90%) of the downstream thermal pressure, consistent with Voyager 2 observations near the shock.
Date: January 1, 2009
Creator: Gary, S Peter; Winske, Dan; Wu, Pin; Schwadron, N A & Lee, M
Partner: UNT Libraries Government Documents Department

Positive and negative chlorine ion kinetics in inductively-coupled Cl{sub 2}BCl{sub 3} plasmas

Description: Discharges in gas mixtures of Cl{sub 2}, BCl{sub 3}, Ar, and N{sub 2} are used by the integrated circuit industry for metal etching, and are as yet not well understood, especially in inductively-coupled plasma (ICP) sources which are rapidly becoming the industry standard for etching tools. An essential parameter that must be measured in these plasmas is the density of ions, both positive and negative, formed in the plasma. In the work presented here, LIF and laser photodetachment were used to measure relative metastable chlorine ion CL{sup +}* density and temperature and absolute Cl{sup {minus}} density as a function of gas mixture.
Date: May 1, 1997
Creator: Fleddermann, C.B. & Hebner, G.A.
Partner: UNT Libraries Government Documents Department

Numerical Tokamak Turbulence Calculations on the CRAY T3E

Description: Full cross section calculations of ion-temperature-gradient-driven turbulence with Landau closure are being carried out as part of the Numerical Tokamak Turbulence Project, one of the U.S. Department of Energy`s Phase II Grand Challenges. To include the full cross section of a magnetic fusion device like the tokamak requires more memory and CPU time than is available on the National Energy Research Scientific Computing Center`s (NERSC`s) shared-memory vector machines such as the CRAY C90 and J90. Calculations of cylindrical multi-helicity ion-temperature-gradient-driven turbulence were completed on NERSC`s 160-processor distributed-memory CRAY T3E parallel computer with 256 Mbytes of memory per processor. This augurs well for yet more memory and CPU intensive calculations on the next-generation T3E at NERSC. This paper presents results on benchmarks with the current T3E at NERSC. Physics results pertaining to plasma confinement at the core of tokamaks subject to ion-temperature-gradient-driven-turbulence are also highlighted. Results at this resolution covering this extent of physical time were previously unattainable. Work is in progress to increase the resolution, improve the performance of the parallel code, and include toroidal geometry in these calculations in anticipation of the imminent arrival of a fully configured,512-processor, T3E-900 model.
Date: December 31, 1997
Creator: Lynch, V.E., Leboeuf, J.N., Carreras, B.A.
Partner: UNT Libraries Government Documents Department

Negative Ion Density Fronts

Description: Negative ions tend to stratify in electronegative plasmas with hot electrons (electron temperature Te much larger than ion temperature Ti, Te > Ti ). The boundary separating a plasma containing negative ions, and a plasma, without negative ions, is usually thin, so that the negative ion density falls rapidly to zero-forming a negative ion density front. We review theoretical, experimental and numerical results giving the spatio-temporal evolution of negative ion density fronts during plasma ignition, the steady state, and extinction (afterglow). During plasma ignition, negative ion fronts are the result of the break of smooth plasma density profiles during nonlinear convection. In a steady-state plasma, the fronts are boundary layers with steepening of ion density profiles due to nonlinear convection also. But during plasma extinction, the ion fronts are of a completely different nature. Negative ions diffuse freely in the plasma core (no convection), whereas the negative ion front propagates towards the chamber walls with a nearly constant velocity. The concept of fronts turns out to be very effective in analysis of plasma density profile evolution in strongly non-isothermal plasmas.
Date: December 18, 2000
Creator: Kaganovich, Igor
Partner: UNT Libraries Government Documents Department

Signal Propagation in Collisional Plasma with Negative Ions

Description: The transport of charged species in collisional currentless plasmas is traditionally thought of as a diffusion-like process. In this paper, it is demonstrated that, in contrast to two-component plasma, containing electrons and positive ions, the transport of additional ions in multi-species plasmas is not governed by diffusion, rather described by nonlinear convection. As a particular example, plasmas with the presence of negative ions have been studied. The velocity of a small perturbation of negative ions was found analytically and validated by numerical simulation. As a result of nonlinear convection, initially smooth ion density profiles break and form strongly inhomogeneous shock-like fronts. These fronts are different from collisionless shocks and shocks in fully ionized plasma. The structure of the fronts has been found analytically and numerically.
Date: December 18, 2000
Creator: Kaganovich, I.; Berezhnoi, S.V. & Shin, C.B.
Partner: UNT Libraries Government Documents Department

Confinement of Pure Ion Plasma in a Cylindrical Current Sheet

Description: A novel method for containing a pure ion plasma at thermonuclear densities and temperatures has been modeled. The method combines the confinement properties of a Penning-Malmberg trap and some aspects of the magnetic field geometry of a pulsed theta-pinch. A conventional Penning trap can confine a uniform-density plasma of about 5x1011 cm-3 with a 30-Tesla magnetic field. However, if the axial field is ramped, a much higher local ion density can be obtained. Starting with a 107 cm-3 trapped deuterium plasma in a conventional Penning-Malmberg trap at the Brillouin limit (B = 0.6 Tesla), the field is ramped to 30 Tesla. Because the plasma is comprised of particles of only one sign of charge, transport losses are very low, i.e., the conductivity is high. As a result, the ramped field does not penetrate the plasma and a diamagnetic surface current is generated, with the ions being accelerated to relativistic velocities. To counteract the inward j x B forces from this induced current, additional ions are injected into the plasma along the axis to increase the density (and mutual electrostatic repulsion) of the target plasma. In the absence of the higher magnetic field in the center, the injected ions drift outward until a balance is established between the outward driving forces (centrifugal, electrostatic, pressure gradient) and the inward j x B force. An equilibrium calculation using a relativistic, 1-D, cold-fluid model shows that a plasma can be trapped in a hollow, 49-cm diameter, 0.2-cm thick cylinder with a density exceeding 4 x 1014 cm-3.
Date: December 10, 1999
Creator: Phillips, C.K.; Chao, E.H.; Davidson, R.C. & Paul, S.F.
Partner: UNT Libraries Government Documents Department

Electron and Negative Ion Densities in C(2)F(6) and CHF(3) Containing Inductively Coupled Discharges

Description: Electron and negative ion densities have been measured in inductively coupled discharges containing C{sub 2}F{sub 6} and CHF{sub 3}. Line integrated electron density was determined using a microwave interferometer, negative ion densities were inferred using laser photodetachment spectroscopy, and electron temperature was determined using a Langmuir probe. For the range of induction powers, pressures and bias power investigated, the electron density peaked at 9 x 10{sup 12} cm{sup -2} (line-integrated) or approximately 9 x 10{sup 11} cm{sup -3}. The negative ion density peaked at approximately 1.3 x 10{sup 11} cm{sup -3}. A maximum in the negative ion density as a function of induction coil power was observed. The maximum is attributed to a power dependent change in the density of one or more of the potential negative ion precursor species since the electron temperature did not depend strongly on power. The variation of photodetachment with laser wavelength indicated that the dominant negative ion was F{sup -}. Measurement of the decay of the negative ion density in the afterglow of a pulse modulated discharge was used to determine the ion-ion recombination rate for CF{sub 4}, C{sub 2}F{sub 6} and CHF{sub 3} discharges.
Date: December 7, 1999
Partner: UNT Libraries Government Documents Department

Design and operation of the pellet charge exchange diagnostic for measurement of energetic confined alphas and tritons on TFTR

Description: Radially-resolved energy and density distributions of the energetic confined alpha particles in D-T experiments on TFTR are being measured by active neutral particle analysis using low-Z impurity pellet injection. When injected into a high temperature plasma, an impurity pellet (e.g. Lithium or Boron) rapidly ablates forming an elongated cloud which is aligned with the magnetic field and moves with the pellet. This ablation cloud provides a dense target with which the alpha particles produced in D-T fusion reactions can charge exchange. A small fraction of the alpha particles incident on the pellet ablation cloud will be converted to helium neutrals whose energy is essentially unchanged by the charge transfer process. By measuring the resultant helium neutrals escaping from the plasma using a mass and energy resolving charge exchange analyzer, this technique offers a direct measurement of the energy distribution of the incident high-energy alpha particles. Other energetic ion species can be detected as well, such as tritons generated in D-D plasmas and H or He{sup 3} RF-driven minority ion tails. The diagnostic technique and its application on TFTR are described in detail.
Date: May 1, 1996
Creator: Medley, S.S.; Duong, H.H. & Fisher, R.K.
Partner: UNT Libraries Government Documents Department

Thomson scattering diagnostic analyses to determine the energetic particle distributions in TFTR. Final report

Description: Lodestar has been an active participant in the low power Collective Thomson Scattering (CTS) diagnostic at TFTR in collaboration with MIT. Extensive studies were conducted regarding the use of gyrotron scattering as a low cost diagnostic for both energetic ions and alpha particles on TFTR. The numerical scattering code has been improved and compared with similar code developed at JET. The authors have participated and assisted in the CTS experiments through onsite visits and have successfully performed most of the data analysis tasks remotely. Through their analysis on the initial data base accumulated, they are able to understand qualitatively the general features of the anomalous large scattered signal, have proposed an explanation for its generation mechanism, and have suggested a potential new use of CTS as an edge diagnostic.
Date: February 16, 1995
Creator: Aamodt, R.E.; Cheung, P.Y. & Russell, D.A.
Partner: UNT Libraries Government Documents Department

Feasibility of measuring density and temperature of laser produced plasmas using spectroscopic techniques.

Description: A wide variety of experiments on the Z-Beamlet laser involve the creation of laser produced plasmas. Having a direct measurement of the density and temperature of these plasma would an extremely useful tool, as understanding how these quantities evolve in space and time gives insight into the causes of changes in other physical processes, such as x-ray generation and opacity. We propose to investigate the possibility of diagnosing the density and temperature of laser-produced plasma using temporally and spatially resolved spectroscopic techniques that are similar to ones that have been successfully fielded on other systems. Various researchers have measured the density and temperature of laboratory plasmas by looking at the width and intensity ratio of various characteristic lines in gases such as nitrogen and hydrogen, as well as in plasmas produced off of solid targets such as zinc. The plasma conditions produce two major measurable effects on the characteristic spectral lines of that plasma. The 1st is the Stark broadening of an individual line, which depends on the electron density of the plasma, with higher densities leading to broader lines. The second effect is a change in the ratio of various lines in the plasma corresponding to different ionization states. By looking at the ratio of these lines, we can gain some understanding of the plasma ionization state and consequently its temperature (and ion density when coupled with the broadening measurement). The hotter a plasma is, the higher greater the intensity of lines corresponding to higher ionization states. We would like to investigate fielding a system on the Z-Beamlet laser chamber to spectroscopically study laser produced plasmas from different material targets.
Date: September 1, 2008
Creator: Edens, Aaron D.
Partner: UNT Libraries Government Documents Department

Suppression of Beam-Ion Instability in Electron Rings with Multi-Bunch Train Beam Fillings

Description: The ion-caused beam instability in the future light sources and electron damping rings can be serious due to the high beam current and ultra-small emittance of picometer level. One simple and effective mitigation of the instability is a multi-bunch train beam filling pattern which can significantly reduce the ion density near the beam, and therefore reduce the instability growth rate up to two orders of magnitude. The suppression is more effective for high intensity beams with low emittance. The distribution and the field of trapped ions are benchmarked to validate the model used in the paper. The wake field of ion-cloud and the beam-ion instability is investigated both analytically and numerically. We derived a simple formula for the build-up of ion-cloud and instability growth rate with the multi-bunch-train filling pattern. The ion instabilities in ILC damping ring, SuperKEKB and SPEAR3 are used to compare with our analyses. The analyses in this paper agree well with simulations.
Date: August 18, 2011
Creator: Wang, L.; Cai, Y.; Raubenheimer, T.O.; /SLAC; Fukuma, H. & /KEK, Tsukuba
Partner: UNT Libraries Government Documents Department

Confinement of laser plasma by solenoidal field for laser ion source

Description: A laser ion source can provide high current, highly charged ions with a simple structure. However, it was not easy to control the ion pulse width. To provide a longer ion beam pulse, the plasma drift length, which is the distance between laser target and extraction point, has to be extended and as a result the plasma is diluted severely. Previously, we applied a solenoid field to prevent reduction of ion density at the extraction point. Although a current enhancement by a solenoid field was observed, plasma behavior after a solenoid magnet was unclear because plasma behavior can be different from usual ion beam dynamics. We measured a transverse ion distribution along the beam axis to understand plasma motion in the presence of a solenoid field.
Date: May 23, 2010
Creator: Okamura, M.; Kanesue,T.; Kondo, K. & Dabrowski, R.
Partner: UNT Libraries Government Documents Department