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Plasma Science, 2006. ICOPS 2006. IEEE Conference Record - Abstracts. The 33rd IEEE International Conference on

Date 4-8 June 2006

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Displaying Results 1 - 25 of 477
  • Ball lightning project

    Page(s): 1
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    Summary form only given. In the following up of our initial work on ball lightning, we have developed a special camera for photographing the initial stroke of a lightning discharge. The object is to obtain time resolved photographs of the closed current loops observed in sparkovers in the Holifield 25 MV, DC accelerator at the Oak Ridge National Laboratory. The apparatus comprises a commercial lightning photographic trigger that opens a camera shutter on reception of a light pulse from a lightning discharge. In the commercial configuration, the inherent time delay in the camera's shutter mechanism causes the camera to miss the initial lightning stroke. However, in nature, lightning in general has several strokes, and so the camera captures the second and subsequent strokes. In our modification, the lightning image is focused on a semi-transparent disc coated with a phosphor having a time constant of about a second. This disc provides a temporary image storage mechanism while the camera shutter is being activated. Test photographs of pulsed light sources have been quite successful. We anticipate on having experimental lightning results at the forthcoming ICOPS meeting View full abstract»

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  • Effect of dielectric barriers in RF atmospheric pressure glow discharges

    Page(s): 2
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    Summary form only given. Atmospheric pressure glow discharges (APGD) generated at radio frequencies generally employ naked electrodes, since the rapid oscillation of the applied voltage suppress very effectively a sustained energy supply to an unrestricted current growth. Consequently, RF APGD are typically stable and it is usually unnecessary to insulate electrodes with dielectric barriers - a common and often essential technique for low frequency APGD in the kilohertz range. As more and more material processing procedures favor high-density plasmas for a superior application efficiency, it is highly desirable that RF APGD can be operated at increasingly high current densities thus imposing greater demands on reliable control of plasma stability. Inevitably, the rapid oscillation of RF voltages alone becomes no longer adequate to ensure plasma stability, and new and additional stability control strategies become important. In this contribution, we present a computational study of how electrode insulation may be used to enhance plasma stability in RF APGD. Temporal characteristics of the discharge current, the applied voltage and the gas voltage are computed over a full range of the current density, from pre-breakdown to arcing. Relationships of the current density with both the applied and the gas voltages are studied in details. These are used to demonstrate that RF atmospheric dielectric-barrier discharges (DBD) acquire a negative differential conductivity at high current densities and as such acquire a similar level of plasma stability to that in conventional RF APGD. However with the dielectric barriers, the differential conductivity of the plasma enclosing electrode unit is always positive in the external circuit. As a result, any plasma instability is suppressed easily and conveniently by capping the power output of the RF power source. In other words, RF atmospheric DBD are more stable than conventional RF APGD even though their plasma characteristics are v- ry similar. Results reported here are likely to form a basis of a stability control strategy for RF APGD View full abstract»

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  • Nanosecond pulsed atmospheric glow discharges without dielectric barriers

    Page(s): 3
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    Summary form only given. Pulsed excitation is widely believed to offer an additional option in the quest to achieve superior performance of atmospheric pressure glow discharges (APGD), compared to the mainstream sinusoidally excited APGD. For example, very large applied voltage can be used over very short period of time to produce abundant and highly energetic electrons. Yet inherently pulsed excitation has a voltage-off period and this can reduce the averaged dissipated power of APGD below what is typical with sinusoidal excitation. Hence pulsed excitation allows APGD to deliver highly reactive plasma species with very high electrical energy efficiency. In the 1-300 kHz range, electrode insulation using dielectric barriers has long been considered as an essential condition for generating glow discharges at atmospheric pressure. For APGD to be deployed for material processing on an industrial scale, it is often desirable to remove the electrode-insulating dielectric layers that may become contaminated from continuous and heavy usage. From the standpoint of APGD physics on the other hand, it is also of significant interest to explore whether the removal of the dielectric barriers may bring about new and beneficial spatiotemporal behaviors for more superior surface treatment. In this contribution, we report the experimental observation of a versatile barrier-free APGD achieved with nanosecond excitation voltage pulses at 1 kHz instead of the usual sinusoidal excitation. The barrier-free mode of operation is shown to be viable over a very wide range of system parameters. Temporal behaviors of such barrier-free APGD are shown to be different from all other APGD reported so far, including pulsed atmospheric dielectric-barrier discharges. Pulsed barrier-free atmospheric plasma can achieve six current pulses every voltage pulse, suggesting high electrical energy efficiency. Also interesting is the observation that pulsed barrier-free APGD support both cathode and anode sh- aths, even though the excitation is made of unipolar voltage pulses. In terms of plasma chemistry, they are also capable of producing a high flux of oxygen atoms. These desirable features are possible because of a unique combination of pulsed excitation and barrier-free operation View full abstract»

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  • Optimization of /sub 2/ (/sup 1//spl Delta/) yields in pulsed RF flowing plasmas for chemical oxygen iodine lasers

    Page(s): 4
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    Summary form only given. Chemical oxygen-iodine lasers (COILs) achieve oscillation on the 2P1/2rarr2P3/2 transition of atomic iodine at 1.315 mum by a series of excitation transfers from O2(1Delta). In conventional COILs, O2(1Delta) is produced by liquid phase chemistry. In electrically excited COILs, (eCOILs) the O2(1Delta) is produced in a flowing plasma, typically He/O2, at a few to 10s torr. One method to improve the efficiency of producing O2(1Delta) in eCOILS is by lowering the average value of electron temperature, Te, using spiker-sustainer (S-S) excitation. In the S-S technique a high power pulse (spiker) is followed by a lower power period (sustainer). Excess ionization produced by the spiker enables the sustainer to operate with a lower Te. Previous investigations have suggested that S-S techniques can significantly raise yields of O2 (1Delta). In this paper, we report on results from a 2-dimensional computational investigation of radio frequency excited flowing He/O2 plasmas with emphasis on optimization of the S-S method. The model is a 2-dimensional plasma hydrodynamics simulation encompassing a solution of Navier Stokes equations for neutral flow dynamics. We found that the efficiency of S-S excitation, as measured by the yield of O2(1Delta), depends on a variety of parameters. These parameters include the details of the pulse shape, the carrier frequency, duty cycle (fraction of the S-S cycle for the spiker), S-S frequency (time between spiker pulses), spiker pulse shape and the ratio of spiker to sustainer voltage as well as on pressure. For a given Te, the yield of O2(1Delta) largely depends on the energy deposition per O2 molecule. As a consequence, the yield depends somewhat linearly on O2 mole fraction while the total O2(1Delta) production is less sensitive to O2 mole fraction View full abstract»

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  • Electrical discharges in bubbled water

    Page(s): 5
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    Summary form only given. Electrical discharges in gas bubbles in water are investigated by applying microsecond long rectangular pulses of 6-20 kV to needle-to-plane electrodes submerged in water. Ar or O2 bubbles; 1-5 mm in diameter, are introduced through the Pt needle that serves as the negative electrode. A bubble remains on the needle through numerous discharge processes. The voltage across the electrodes and the current to the ground are measured. Electrical measurements suggest that a corona-type atmospheric-pressure discharge is ignited in the gas bubble (composed of Ar or O2 gas and water vapor) without the electrical breakdown of the entire water filled electrode gap. The discharge characteristics are investigated as a function of the applied voltage, the distance between the electrodes, the bubbled gas (Ar or O2), the size of the gas bubble, and the pH and conductivity of the water. Optical and electrical measurements are used to explore the properties of the discharge. The size of the bubble as compared to the electrode distance affects the electrical characteristics of the discharge. The liquid water-metal interface and the liquid water-gas interface both play an important role in the initiation and the development of the electrical discharge in the gas bubbles. Evidence from environmental application studies supports the strong dependence of the discharge properties, active species production, and cleaning efficiency on all the parameters mentioned above including the applied voltage, the electrode distance, bubble size, and the initial characteristics of the solution View full abstract»

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  • Influence of plasmas on the surface of electret films

    Page(s): 6
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    Summary form only given. In the corona charging of electret films, the high voltage results in plasma in the air. The plasmas near the surface of the charged film have the double influences on the sample. On the one hand, the charged ions deposit on the surface of the polymer electret film and injected into it, which make the sample to become good electrets. On the other hand, the plasmas may injure the surface of the sample, which is harmful to the deposition and permanent storage of charges near the surface of the film. The theoretical analysis and experiments show the control of exerted voltage during the corona charging is very important to the result. The critical voltage that is best for the storage of charges, is found to depend on the shape of the electrode of discharging and the distance to the sample. It is interesting that the negative voltage of corona charging has less negative influence on the surface of Teflon samples than the positive one. We think that the kind of negative ions in the plasmas result in the phenomena View full abstract»

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  • Characterization of time-modulated inductively coupled and capacitively coupled plasmas

    Page(s): 7
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    Summary form only given. Time-resolved probe measurements are carried out using the boxcar measurement method in pulsed and low- pressure inductively coupled plasma. The characteristics of the transient behavior of electron energy distribution function (EEDF) and plasma parameters such as electron density, electron temperature, and plasma and floating potentials are presented and analyzed with global model of pulsed discharge. It is found from the relaxation behavior of the EEDF and the analysis of the characteristic relaxation times of the electron density and temperature that the initial fast relaxation of high-energy electrons just after the power is turned off is dominated by electron-atom inelastic collisions rather than the diffusive cooling effect at low pressure. A revised global model in which the effect of electron-atom inelastic collisions is included is presented. After the power is turned on, an initial very sharp rise in electron temperature followed by a decay of electron temperature is observed and the peak electron temperature is found to increase with reducing the duty cycle of the power pulse. The comparison of the measured EEDFs shows that the increase of the peak electron temperature under shorter duty cycle pulse is caused by the depopulation of low-energy electrons, not by the overpopulation of high-energy electrons View full abstract»

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  • Plasma parameters of dense gas discharge with runaway electrons

    Page(s): 8
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    Summary form only given. The discharge with runaway electrons in dense gas (for short - runaway discharge, RD) has plasma characteristics essentially different in comparison with plasma characteristics of all other discharges in dense gas. The basic particularity of RD is connected with generation of sub-relativistic (20-400 keV) electrons in discharge volume during sub-nanosecond period. Last property complicates essentially the study of RD plasma and demands a development of new methods for some basic parameters measurement at RD. In particular, for measurement of RD runaway electron current and average energy we had developed a technique on the basis of voltage measurement on pure inductive collector of current: Uthetas = Lsd(i D + thetasiB)/dt, where Ls is collector inductance, iD is the displacement current in the collector circuit, iB is the current of fast electron beam through the collector butt, thetas is the fast electron transmission factor of the screen or foil placed between the anode and collector. On the fig. 1 the typical example of such measurements is resulted on which the physics of processes in RD plasma is discussed View full abstract»

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  • B-field diagnostics in transient plasmas

    Page(s): 9
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    Summary form only given. Measurements of magnetic fields are a key issue in many studies of equilibrium and transient laboratory plasmas. However, the traditional techniques are not useful in situations where the magnetic field has no definite direction during the time of observation, or for magnetic fields with amplitudes that vary in time or space, e.g. in laser-produced or pinch plasmas. To overcome these difficulties, a new spectroscopic approach was proposed, applicable to measurements of such "isotropic" magnetic fields. The technique is based on the spectroscopic analysis of line-shapes of different fine-structure components of the same atomic multiplet that undergo different splitting under the B-field. Another advantage of this method is that, when coupled with detailed line-shape calculations, it allows for an accurate B-field determination even when the line-shapes are strongly influenced by Stark and Doppler broadenings in the plasma. Here we report on an experimental implementation of the proposed method to measure the magnetic field in a laser-produced plasma injected into a coaxial vacuum transmission line carrying a 200-kA current. The pulsed magnetic and electric fields, generated in the transmission line, are of spatially varying orientations at the plasma boundary. This causes a complex motion of the plasma particles, that is further complicated due to the 3-D geometry of the experimental setup. This results in a twisting and a depolarization of the magnetic field lines in the plasma. The profiles of spectral lines emitted by the plasma demonstrate complex shapes due to the combined inhomogeneous Stark, Zeeman, and Doppler broadenings. Nevertheless, using the method presented, the magnetic field is determined unambiguously. Magnetic-field penetration into the plasma that is faster than the diffusing rate expected from the Spitzer resistivity is observed. The results here presented demonstrate the powerfulness of the method as a tool for single-sh- t measurements of magnetic fields with complex geometries View full abstract»

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  • Laser-induced fluorescence measurement of argon in argon-xenon plasma sheath boundary with tunable diode laser

    Page(s): 10
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    Summary form only given. The Bohm sheath criterion in single and two-species plasma is studied with laser-induced fluorescence (LIF) using a diode laser. Xenon is added to a low pressure argon unmagnetized DC hot filament discharge confined by surface multidipole magnetic fields. The argon II transition sequence at 668.614 nm is adopted for optical pumping to detect the fluorescence from the plasma. The laser diagnostic system is first calibrated with an iodine cell and wavelength meter because of its small mode-hop free regions. The regions of the plasma sheath and presheath are determined from the plasma potential profile measured by an emissive probe. The ion concentrations of the two-species in the bulk plasma are calculated from measured ion acoustic wave phase velocity. The measured phase velocity combined with the argon data from LIF is used to determine the xenon ion velocity. The spatial profiles of ion drift velocities are compared to predictions of mobility-limited flow velocities and to the generalized Bohm sheath criterion View full abstract»

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  • Optical emission spectroscopy characterization of low temperature plasma created in water

    Page(s): 11
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    Summary form only given. Optical emission spectroscopy has been applied to study characteristics of low-pressure plasma created in water vapour. Plasma was generated by inductively coupled RF generator (RIZ SW-amp.) with the frequency of 13.56 MHz and adjustable output power up to 300 W in a glass discharge tube with the inner diameter of 36 mm. The discharge tube was pumped with a two-stage rotary pump. The water vapour pressure was about 3 Pa. Plasma radiation was measured with a 60 cm optical spectrometer MDR23-Lomo. Emission from highly excited species was found in the ultra-violet, visible and infrared part of the spectra from 200 to 1000 nm. The highest energy of excited radicals was about 15 eV. The rich emission spectra were dominated by atomic transition lines from hydrogen and oxygen: Halpha Hbeta, Hgamma, O(3p5P>3s5S), and O(4p3P>3s3S). Fulcher band molecular hydrogen lines as well as OH (A>X) band were also found. No emission from O2 and H2O was detected indicating that the concentration of both molecules in water plasma was low. The observed spectra were explained by collision phenomena in gas phase and on the walls of the discharge tube View full abstract»

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  • Vibrational population distribution and the gas temperature in the compact helicon nitrogen plasma source chewie

    Page(s): 12
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    Summary form only given. We report preliminary results on the development of a helicon plasma source with variable activated nitrogen composition for plasma assisted molecular beam epitaxy of III/V-nitrides. The main idea is to alter the population of specific reactive nitrogen species in a helicon plasma source by modifying the electron energy distribution function through the resonant wave-particle interaction arising from electrons traveling at same velocity as the phase velocity of the helicon wave. The high plasma density and high ion exit flow speed (ne = 1013 cm-3 and v i = 8,000 m/s for argon) should yield significantly higher fluxes at the substrate surface and consequently an improved deposition rate over existing MBE plasma sources. Epilayer quality could also be improved by lowering kinetic energy of reactive species. The active nitrogen source is a steady state, high density, helicon plasma source CHEWIE (Compact HElicon Waves and Instabilities Experiment). The helicon vacuum chamber is a 12 cm long, Pyrex tube, 6 cm in diameter, connected to a stainless steel diffusion chamber, 30 cm long, 15 cm in diameter. Three magnetic field coils surround the source and are capable of generating an axial magnetic field up to 1200 G in the source and about 100 G at the end of the expansion chamber. A 7 cm long, water cooled, Boswell saddle type antenna couples the RF energy into the plasma. RF power of up to 600 W over a frequency range of 3-28 MHz is used to create the steady state plasma in the source which expands away into a region of decreasing magnetic field. Optical emission spectroscopy investigations in the plasma source show that under certain working conditions, the N2 first positive system (B3Pi g rarr A3 Sigmau +) are the dominant transitions in nitrogen, helicon-generated plasma. From band head intensities a Boltzmann relative vibrational pop- lation distribution is obtained. From the fit of the Deltav=+1 (at 891.24 nm), Deltav=+2 (at 891.24 nm) and Deltav=+3 (at 687.50 nm) bands, a gas temperature of ~350 K for an input power of 300 W, a magnetic field of 800 G and N2 gas pressure of 20 mtorr is inferred View full abstract»

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  • Optimization of a micro-retarding potential analyzer for high-density flowing plasmas

    Page(s): 13
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    Summary form only given. This research addresses the limitations of existing retarding potential analyzers (RPAs) which can operate in plasmas with electron densities in excess of 1times1018 m -3, but are susceptible to space charge limitations. RPAs of a traditional gridded design have been implemented both experimentally and as in situ diagnostics to measure ion energy distributions, neutral particle flux, and species concentrations. The single-channel micro-retarding potential analyzer (SC-muRPA) developed has a channel diameter and electrode spacing on the sub-millimeter scale and can accurately operate in plasmas with densities of up to 1times1017 m-3. To eliminate space charge effects and to increase the operating range of the SC-muRPA to densities above 1times1018 m-3, a low transparency microchannel plate (MCP) has been incorporated as the floating electrode to produce the multi-channel micro-retarding potential analyzer (MC-muRPA) design. Improvements to the current collection theory for both the SC-muRPA and the MC-muRPA are also derived. Current-voltage curves are obtained for incoming flowing plasmas that range from near-stationary to hypersonic, with temperatures in the range of 0.1 to 10 eV, and with densities in the range of 1times1015 m-3 to 1times1021 m-3. The SC-muRPA current collection theory is validated by comparisons with the classical RPA theory as well as with 3D particle-in-cell simulations performed on an unstructured tetrahedral mesh. Determination of unknown plasma properties is based on a fuzzy-logic approach using generated look-up tables of current-voltage curves to converge towards the experimental curve. Thermal and sizing analyses were performed to confirm the elimination of melting and density gradients during experiments. RPA sizing recommendations are listed for a broad range of plasma properties, as well as the next it- ration of the muRPA design for high-density plume characterization View full abstract»

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  • A simple model for microchannel plate output with applications to x-ray diagnostics

    Page(s): 14
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    Summary form only given. Microchannel plates (MCPs) are an essential component in an imaging diagnostic known as an X-ray framing camera, which is used extensively to image the plasma produced by radiation imploded targets at facilities such as NIF, Nike, Omega, and Z. An MCP is used to convert incident X-ray photons into electrons with gains of the order 102 to 104. These electrons are then accelerated into a phosphor screen that produces a visible light image, which can then be captured by film or CCD. A variety of parameters, such as MCP photocathode material type (e.g., Au, Ni, CsI), photocathode coating depth, and MCP bias angle, affect the gain and gain variations in the MCP electron output. We present initial results of a simple 3D MCP output model along with an experimental comparison; in addition to exploring several techniques for increasing MCP gain and reducing gain variations View full abstract»

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  • Optogalvanic spectroscopy of the zeeman effect in singly-ionized xenon

    Page(s): 15
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    Summary form only given. We present optogalvanic measurements of Zeeman effects on two xenon lines: the 5d 2F7/2 rarr 6p 2D0 5/2 transition of singly-ionized xenon at 834.724 nm and the 6p [3/2]1 rarr 6d [5/2]0 1 transition of neutral xenon at 834.745 nm. Hamamatsu L2783 xenon-neon galvatron is placed in the gap of a C-coil electromagnet with the laser beam path perpendicular to the magnetic field. Excitation is provided by a narrow-linewidth TUI Optics TA-100/830 tapered-amplifier diode laser; lock-in amplification isolates the optogalvanic signal from noise in the galvatron discharge. A half-wave plate controls the polarization of the beam, permitting separation of the components. By comparing the observed spectra with computational models of the Zeeman effect on fine and hyperfine energy levels, we explore the utility of Zeeman splitting of xenon laser-induced fluorescence spectra as a magnetic component intensity diagnostic in electrostatic thruster plumes View full abstract»

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  • Multipactor discharge in a dielectricloaded accelerating structure

    Page(s): 16
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    Summary form only given. Based on the previous observation of multipactor discharge during high-power testing of an alumina-based dielectric loaded accelerating (DLA) structure, where strong normal and tangential RF electric fields are present, and RF power flow is parallel to the surface, we extend the model of multipactor discharge (on a simple RF window) to describe the electron multiplication and its power deposited in the DLA structure. Using Monte Carlo simulation with random distributions in the emission velocities and emission angles of the secondary electron, taking realistic secondary electron yield curves as input, our dynamic calculation takes into account the evolution of DC electric field and beam loading of the external RF electric field. It is found that the fraction of power absorbed by multipactor at saturation is much larger than the case for a simple RF window and it is sensitive to the incident power, which confirms the prior experimental observation View full abstract»

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  • Applications of the michelle 2d/3d electron gun and collector code

    Page(s): 17
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    Summary form only given. Three-dimensional (3D) electron guns, focusing channels, and collectors are the trend to improved device performance, and accurate modeling of these devices leads to reductions in development time and costs. This includes gridded guns, multibeam guns, sheet beam guns, quadrupole focusing, multibeam collectors, and asymmetrical collectors. The new MICHELLE1 2D/3D steady-state and time-domain particle-in-cell (PIC) code that employs electrostatic and now magnetostatic finite-element field solvers. In recent years the code has been employed successfully to design and analyze a wide variety of devices that include the above and also ion thrusters. In this presentation we will present how MICHELLE has been used to model a variety of devices. Two designs in particular are a MM/SMM wave sheet beam gun and a Ka band gun with quadrupole (strong) focusing. The design of a sheet beam electron gun is underway at NRL for application to a MM/SMM wave orotron. The gun design parameters are for a beam of 3 mm by 40 mum (aspect ratio of 75:1), and a total current of 50-100 mA. The smaller of these dimensions is set by the requirement that the beam thickness be smaller than the transverse scale length of Lperp = gammabetalambda/2pi. The preliminary gun design utilizes an immersed flow design with a flat rectangular cathode. Further results from this design effort will be presented. At mm-wavelengths, a key limitation to the peak and average power of vacuum electronics devices stems from the difficulty of control and confinement of high-current density beams. In this paper we summarize an ongoing design study of a strong-focusing lattice employing permanent magnet quadrupoles (PMQs) capable of transporting a sub-mm radius electron beam for vacuum electronics applications View full abstract»

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  • Modeling of mode competition in gyrotrons with coaxial cavities by using magy

    Page(s): 18
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    Summary form only given. Gyrotrons with coaxial cavities are very promising sources of high power short millimeter wavelength radiation for plasma heating and current drive in control fusion applications. Recent experimental studies of coaxial gyrotrons have shown that up to two Megawatt of output power in region of 165-170 GHz can be achieved. However, the coaxial gyrotrons are very complex devices for design because of presence of a tapered azimuthally corrugated inner conductor in order to provide mode selection. The design of high power gyrotrons with coaxial cavities requires a lot of computational efforts. The major challenge for coaxial gyrotrons design is to predict accurately mode competition in complex coaxial cavities and avoid spurious mode excitation. MAGY (MAryland GYrotron) is very effective design code used in the past by the US vacuum electronic industry to design high power gyrotrons for fusion applications. Recently MAGY has been developed to be suitable for accurate and efficient simulation of gyrotrons with coaxial cavities. The MAGY model includes a self-consistent, nonlinear solution of the three-dimensional equations of motion of electrons and the solution of the time-dependent field equations. The effect of azimuthally corrugations at inner conductor on electromagnetic fields is described by equivalent surface impedances. The extended MAGY model allows predicting properties of modes of complex coaxial structures. The modified code has been validated by comparison of simulations results with the results of experimental measurements presented. The calculated dependence of the output power on beam voltage agrees with the results of measurements View full abstract»

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  • Effects of periodic DC and AC electric fields on spoke formation in a crossedfield diode

    Page(s): 19
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    Summary form only given. The time-dependent behavior of electron sheaths in a magnetically insulated (B>BHull) anode-cathode gap with crossed electric and magnetic fields is studied. The crossed field, space-charge limited diode is modeled for various magnetic fields by means of multidimensional (1d and 2d), self consistent, electromagnetic, particle-in-cell (PIC) simulations in both cylindrical and planar geometries. It is shown in 1d planar geometry that the cycloidal flows collapse into a steady, near-Brillouin flow. Our 2d electromagnetic PIC simulations (both planar and cylindrical) show that cycloidal flow also collapses into a perturbed flow that is dominated by the E cross B drift, but is neither steady nor stable. A slow wave structure (SWS) is added to the anode that matches the wavelength and frequency of the fastest growing fluid instability in the smooth-bore case. The SWS is added by three different methods to separate the RF effects from the DC electric field effects created by the SWS. The first method to add the SWS is to add a thin material with a large permeability to the anode that does not affect the DC electric fields; the second is to add a thin dielectric (with and unphysical large dielectric constant) which does affect the DC electric fields, and last is to add the geometric SWS. The SWS is then perturbed so that wavelength and/or frequency does not match the smooth bore diode growth rate and the region of `lock-in' to the SWS is found. The same set of simulations are performed again, this time changing the cathode to limit the emission area (representing a PAL cathode), adding perturbations to the cathode (representing a `shaped' cathode), and adding an emitting rod above the surface (representing a transparent cathode). These simulations focus on explaining the qualitative change in flow for these different cathodes under current development View full abstract»

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  • 3-d modeling of broadband multi-cavity circuits

    Page(s): 20
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    Summary form only given. We investigate a 3-D generalized modal-expansion technique to model the fields and resonance characteristics of low-Q cavities and multi-cavity circuits found in broadband klystron and multiple-beam klystron amplifiers. Such cavities are often complex, multi-gap structures and full 3-D analysis is essential to their design. For broadband input and output circuits, that have low Qexternal, a traditional cavity eigenmode representation is more difficult to define due to the strong coupling of the cavity to an external waveguide. Usually, numerical solution requires addition of an absorbing boundary condition inside the waveguide port to determine a leaky eigenmode of the open system, or one must resort to computing many driven-frequency solutions across the frequency band and perform subsequent field analysis to determine the cavity stored energy dependence on frequency and hence extract the resonance frequency and Q. The analysis becomes more problematic when multiple overlapping resonance peaks occur inside the operating bandwidth, as is often the case. In our approach, we avoid the numerical solution of the open cavity-waveguide system, and introduce a new technique using only modes of a finite closed cavity, terminated at the waveguide aperture by short-circuiting either the tangential electric or magnetic field. By representing fields inside the cavity using a dual set of eigenmodes we determine the coupling to an external waveguide to determine the actual frequency and Q of the open cavity-waveguide system. We present the theory behind the analysis, and compare to existing methodology View full abstract»

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  • Large signal klystron simulations using a curnow circuit model

    Page(s): 21
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    Summary form only given. Simulation codes used for large signal analysis of coupled cavity traveling wave tubes commonly use a lumped element model of the traveling wave circuit due to Curnow. When the circuit unit cells are not connected to one another, we show that for suitable choices of lumped element circuit parameters, a Curnow cell can be used to model klystron input, buncher, or output cavities, thereby enabling a coupled-cavity simulation code based on the Curnow circuit to be used to simulate klystrons. In the absence of loss, the Curnow cell is defined by the specification of 6 lumped element values, basically 4 inductances and 2 capacitances. The addition of loss adds a seventh value, for a resistor. To model a klystron input cavity, for example, a designed might specify the resonant frequency of the cavity, R/Q, Qinternal, and Qexternal. In addition, we assume that the impedance of the transmission line from the driver to the input cavity is specified. We show that specification of these physical quantities permits specification of all of the lumped element values of the Curnow circuit. An approximate analysis, assuming large values of Q and frequencies near the cold cavity resonant frequency, has produced analytic forms for the Curnow parameters in terms of the physical parameters characterizing the klystron cavities. The klystron model is being implemented in the coupled-cavity version of the CHRISTINE code, which will be used to study klystron designs for frequency multiplication (harmonic generation) at very high drive levels. The analysis leading to the extraction of the Curnow parameters from the physical parameters that specify the klystron cavities, and sample results from the CHRISTINE code will be discussed View full abstract»

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  • Parallel modeling of multiple beam klystons with tesla

    Page(s): 22
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    Summary form only given. We present simulation results for a multiple-beam klystron using a new parallel algorithm in the TESLA code, where individual beams are modeled separately but coupled self-consistently through the resonators. Multiple beam klystrons (MBKs) are high power and high efficiency amplifiers that use separate beamlets, each propagating in its own beam tunnel, but interacting with common fields in gaps near cavities. In this way the perveance of the individual beamlets can be kept low, reducing space-charge effects, while the total beam current can be high, facilitating high-power operation. Mainly the beamlets are isolated from each other, except for their interaction with the common cavity fields. In the lowest approximation, the beamlets can be considered identical and the response of a single beamlet can be multiplied by the number of beamlets in the numerical simulation of a device. However, beamlets are not identical, having different R/Q values for example. Nevertheless, the high degree of isolation of beamlets allows for efficient parallel simulation of multiple beamlets. We have modified the large signal code TESLA by assigning each beamlet to a separate process and introducing MPI-calls to handle the communication between them through cavity fields. As the amount of communication is small, the running time of the code increases only slightly with beamlet number. Results for the effects of R/Q variations for the NRL MBK will be presented View full abstract»

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  • On the performance improvement of a parallel 3-D PIC-FEM code

    Page(s): 23
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    Summary form only given. In the previous ICOPS meeting, we have presented a parallel 3-D PIC code using the finite-element method with an unstructured tetrahedral mesh for the flexibility of modeling objects with complex geometry. In addition, the dynamic domain decomposition using the graph-partitioning technique is employed for a better load balancing among the processors. Parallel efficiency of this code, implemented on HP clusters could be as high as 82% with 32 processors (40 particles per cell, ~30,000 nodes). However, one of the major drawbacks of this code is the relatively poor runtime performance as compared to the previous PIC codes using the finite-difference method with a structured mesh. In this paper, we will present some improvements, including the Poisson's equation solver and the particle tracing technique, to greatly enhance the code performance. First, we have replaced the original parallel conjugate gradient method by either a sparse direct matrix solver (MUMPS) for fewer processors (<10) or a preconditioned (geometrical additive-Schwartz method) for more processors (>10). With the MUMPS for fewer processors, the assembled coefficient matrix is factorized into the L and U matrices once initially and they are stored for further use at each time step. At each time, only the source term (charge density) changes while the L and U matrices remain unchanged, which makes solving the matrix equation very fast. Second, a tetrahedral mesh is replaced by a multi-block hybrid structured-unstructured mesh to both maintain the flexibility of dealing with complicated geometry and the maximal efficiency of particle tracing. In the structured-mesh block with the pure hexahedral cells, the particle tracing takes advantage of the simple relation between mesh coordinate and mesh index, which is very fast. While in the unstructured-mesh block with the mixed tetrahedral and pyramid cells, the similar technique is adopted. A RF capacitive discharge between t- o circular electrodes in a hexahedral metal chamber is used to demonstrate the performance improvement and preliminary results show reduction of the runtime up to five times can be achieved in the test example View full abstract»

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  • 3-d simulations for radar cross-section reduction using plasma absorbers

    Page(s): 24
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    Summary form only given. Radar cross section (RCS) is the measure of a target's ability to reflect radar signals in the direction of the radar receiver. A collisional unmagnetized plasma, surrounding the target, acts as a good absorber of electromagnetic waves over a wide frequency range, reducing its RCS. This has given rise to world wide interest in plasma stealth technology. We have performed 3-D finite difference time domain (FDTD) simulations for calculating electro-magnetic wave scattering and absorption due to plasma-shielded objects. We have earlier validated our 3-D calculations against experimental results for wave scattering from a plasma-shielded metallic plate. Those simulations yielded a reasonable match with experimental measurements. That study also showed that bending of waves inside plasma due to density gradients plays as important a role as absorption. Those results have major implications for plasma stealth applications, which have heretofore assumed that plasma absorption is the main mechanism. We have also compared two techniques for studying the bending/refraction of electromagnetic energy flow through a plasma with spatial density gradients. These are the accurate FDTD method and the much faster, albeit more approximate, ray-tracing method. Our earlier work focused on the near-field region. The RCS refers to far-field measurements. In this paper, we present actual far-field (RCS) results for objects with generic shapes, such as flat plates, cylinders and spheres, both with and without plasma shielding. In this paper, we also report on the dependence of bistatic RCS on plasma parameters, such as the peak electron density, the spatial profile of density and the collision frequency. Finally, we provide a physical interpretation for the results. Such an interpretation is only possible using the detailed spatio-temporal evolution of electromagnetic fields that is provided by the FDTD method. To our knowledge, this is the first detailed three di- ensional calculation of plasma-based RCS reduction for real-life objects View full abstract»

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  • Atmospheric pressure pecvd coating and plasma chemical etching for continuous processing

    Page(s): 25
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    Summary form only given. In recent years there has been increasing interest in APPlasma surface processing, however the potential of the technology to coat or etch surfaces offers further significant future potential. Plasma processing at atmospheric pressure (APPlasmas) has attractions for both economic and technological reasons. Potential cost saving factors are linked to on-line processing capability, which substantially reduce substrate handling cost, and increase throughput due to high deposition rates. Capital cost savings for both equipment and line space (foot print), and relative ease of integration, are further benefits in comparison to low pressure technology approaches. Considering coating technologies compatible with industrial requirements three key challenging aspects will be treated: (i) availability of scalable wide area plasma sources having sufficient "robustness" for long-term continuous operation, (ii) APPlasma reactors being applicable for continuous air-to-air processing, and (iii) coating/etching processes being characterized by both high dynamic rates being compatible with the throughput requirements of the whole production line and high surface performance being compatible to specifications of homogeneity, structure, etc. At Fraunhofer IWS, currently two methods for atmospheric pressure processing are under development, microwave CVD and DC arcjet-CVD based on a linearly extended plasma source. All kinds of AP-PECVD reactors are designed for continuous air-to-air processing on flat or slightly shaped substrates. They comprise purged curtains for control of deposition atmosphere which allows deposition of non-oxide films. The reactors operate in a remote plasma mode being imperative for long term stability of the coater head. Supported by extensive fluid dynamic modeling a gas flow system has been designed which effectively balance out the three main factors being on influence on process performance: (i) throughput/high deposition rate, (ii- avoid/control powder formation, and (iii) avoid stray deposition or etch attack on reactor walls/plasma source. Typical rates for PECVD are in the range of 5-100 nm/s (static) and up to 2 nm*m/s (dynamic). The rates for plasmachemical etching are typically 2-3 times higher. Developments are underway to explore the use the innovative coating technology for e.g. scratch resistant coatings on metals, barrier layers, self-clean functional surfaces and, for antireflective coatings. Coating materials range comprise: silica, titanium, carbon and silicon nitride. Layer characterization, still underway; demonstrates that both composition/structure and optical/mechanical properties are close to data being well known from low pressure PECVD View full abstract»

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