Bulletin of the American Physical Society
57th Annual Meeting of the APS Division of Plasma Physics
Volume 60, Number 19
Monday–Friday, November 16–20, 2015; Savannah, Georgia
Session GO8: Beam, Source and Pinch Physics |
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Chair: Igor Kaganovich, Princeton Plasma Physics Laboratory Room: 103/104 |
Tuesday, November 17, 2015 9:30AM - 9:42AM |
GO8.00001: An Exact Formulation of Laser Assisted Electron Emission on a Biased Metal Surface Peng Zhang, Y.Y. Lau, L.K. Ang, D. Shiffler, K.L. Jensen, R.M. Gilgenbach Laser-driven ultrafast electron emission [1, 2] is important to free electron lasers (FELs), laser acceleration of relativistic electrons, and ultrafast electron diffraction. It would enable exciting technological development on four-dimensional (4D) time-resolved electron microscopy [3]. We constructed an analytic solution for the highly nonlinear electron emission from a metal surface that is exposed to both a dc biased electric field and a single frequency laser field. The solution is valid for arbitrary combinations of dc electric field, laser electric field, laser frequency, metal work function and Fermi level. Various emission mechanisms, such as multiphoton absorption or emission, optical or dc field emission, are all included in this single formulation. The time-dependent emission current reveals that intense current modulation may be possible even with a low intensity laser, by merely increasing the applied dc bias. \\[4PT] [1] P. Hommelhoff et al., Phys. Rev. Lett. 97, 247402 (2006).\\[0pt] [2] C. Ropers, et al. Phys. Rev. Lett. 98, 043907 (2007).\\[0pt] [3] Y. Zhu and H. D\"{u}rr, Phys. Today 68, 32 (2015). [Preview Abstract] |
Tuesday, November 17, 2015 9:42AM - 9:54AM |
GO8.00002: On the Interfacial Tunneling Current in Nanoscale Plasmonic Junctions Y.Y. Lau, Peng Zhang, R.M. Gilgenbach Recently, electron tunneling between plasmonic resonators is found to support quantum plasmon resonances [1], which may introduce new regimes in nano-optoelectronics and nonlinear optics. This is a fundamental problem of electron transport in nano-scale. Here, we present a self-consistent model of electron transport in a nano-scale metal-insulator (vacuum)-metal junction [2], by solving the coupled Schr\"{o}dinger and Poisson equations. The effects of space charge, exchange-correlation, anode emission, and material properties of the electrodes and insulator are examined in detail. It is found that these effects may modify the current density by orders of magnitude from the widely used Simmons' formula [3]. Transition from the direct tunneling regime to the space-charge-limited regime is demonstrated. For a given junction, simply increasing the driving field to field emission or space-charge-limited regime could significantly reduce the damping of the charge transfer plasmon due to quantum tunneling. \\[4pt] [1] S. F. Tan, et al., Science 343, 1496 (2014); R. Esteban, Nat. Commun. 3, 825 (2012).\\[0pt] [2] P. Zhang, Sci. Rep., 5, 9826 (2015).\\[0pt] [3] J. G. Simmons, J. Appl. Phys. 34, 1793 (1963). [Preview Abstract] |
Tuesday, November 17, 2015 9:54AM - 10:06AM |
GO8.00003: How Accurate Is Pierce's Theory of Traveling Wave Tube? D.H. Simon, D. Chernin, P. Wong, P. Zhang, Y.Y. Lau, C.F. Dong, B. Hoff, R.M. Gilgenbach This paper provides a rigorous test of the accuracy of Pierce's classical theory of traveling wave tubes (TWTs). The EXACT dispersion relation for a dielectric TWT is derived, from which the spatial amplification rate, ki, is calculated. This ki is compared with that obtained from Pierce's widely used 3-wave theory and his more general 4-wave theory (which includes the reverse propagating circuit mode [1]). We have used various procedures to extract Pierce's gain parameter C and space charge parameter Q from the exact dispersion relation. We find that, in general, the 3-wave theory is a poor representation to the exact dispersion relation if C \textgreater 0.05. However, the 4-wave theory gives excellent agreement even for C as high as 0.12 and over more than 20 percent bandwidth, if the quantity (k$^2$ $\times$ C$^3$) is evaluated accurately as a function of frequency, and if Q is expanded to first order in the wavenumber k [2], where Q is the difference between the exact dispersion relation and its 4-wave representation in which Q is set to zero [3]. Similar tests will be performed on the disk-on-rod slow wave TWT, for which the hot tube dispersion relation including all space harmonics has been obtained. \\[4pt] [1] J.R. Pierce, Traveling Wave Tubes, p. 113 (1950).\\[0pt] [2] D. Dialetis, et al, IEEE Trans. ED 54, 888 (2007).\\[0pt] [3] Y. Y. Lau and D. Chernin, Phys. Fl. B4, 3473 (1992). [Preview Abstract] |
Tuesday, November 17, 2015 10:06AM - 10:18AM |
GO8.00004: Theory of Harmonic Generation on a Traveling Wave Tube C.F. Dong, P. Zhang, D. Chernin, Y.Y. Lau, D.H. Simon, P. Wong, G. Greening, R.M. Gilgenbach In a klystron, charge overtaking of electrons leads to an infinity of AC current. The harmonic content therein has been calculated accurately, with or without space charge effects [1]. This paper extends the klystron theory [1, 2] to a traveling wave tube (TWT). We calculate the harmonic content on the beam current on a TWT that results from an input signal of a single frequency. We assume that the electron motion is described by linear theory, which is generally accurate over 85 percent of the tube length. These linear orbits may lead to charge overtaking and therefore harmonic generation, as in a klystron. We calculate the buildup of harmonic content as a function of distance from the input, and compare these analytic results with the CHRISTINE code [3]. Reasonable agreement was found. A dimensionless ``bunching parameter'' for TWT, X $=$ sqrt[(Pi/Pb)/C], is identified, which characterizes the harmonic content in the AC current, where Pi is the input power of the signal, Pb is the DC beam power, and C is Pierce's gain parameter. \\[4pt] [1] C. B. Wilsen, et al., IEEE Trans. Plasma Sci. 30, 1176 (2002).\\[0pt] [2] Y. Y. Lau, et al., IEEE Trans. Plasma Sci. 28, 959 (2000).\\[0pt] [3] T. M. Antonsen and B. Levush, IEEE Trans. Plasma Sci. 26,774 (1998). [Preview Abstract] |
Tuesday, November 17, 2015 10:18AM - 10:30AM |
GO8.00005: 3-D Printed High Power Microwave Magnetrons Nicholas Jordan, Geoffrey Greening, Steven Exelby, Ronald Gilgenbach, Y.Y. Lau, Brad Hoff The size, weight, and power requirements of HPM systems are critical constraints on their viability, and can potentially be improved through the use of additive manufacturing techniques, which are rapidly increasing in capability and affordability. Recent experiments on the UM Recirculating Planar Magnetron (RPM)[1], have explored the use of 3-D printed components in a HPM system. The system was driven by MELBA-C, a Marx-Abramyan system which delivers a -300 kV voltage pulse for 0.3-1.0 us, with a 0.15-0.3 T axial magnetic field applied by a pair of electromagnets. Anode blocks were printed from Water Shed XC 11122 photopolymer using a stereolithography process, and prepared with either a spray-coated or electroplated finish. Both manufacturing processes were compared against baseline data for a machined aluminum anode [2], noting any differences in power output, oscillation frequency, and mode stability. Evolution and durability of the 3-D printed structures were noted both visually and by tracking vacuum inventories via a residual gas analyzer. \\[4pt] [1] R. M. Gilgenbach, Y. Y. Lau, D. M. French, B. W. Hoff, J. Luginsland, and M. Franzi, U.S. Patent US 8 841 867B2, Sep. 23, 2014.\\[0pt] [2] M.A. Franzi, G.B. Greening, N.M. Jordan, R.M. Gilgenbach, D.H. Simon, Y.Y. Lau, B.W. Hoff, J. Luginsland, Plasma Science, IEEE Transactions on, vol.43, no.5, pp.1675,1682, May 2015. [Preview Abstract] |
Tuesday, November 17, 2015 10:30AM - 10:42AM |
GO8.00006: Short Intense Ion Pulses for Materials and Warm Dense Matter Research Peter Seidl, Q. Ji, S.M. Lidia, A. Persaud, M. Stettler, J.H. Takakuwa, W.L. Waldron, T. Schenkel, J.J. Barnard, A. Friedman, D.P. Grote, R.C. Davidson, E.P. Gilson, I.D. Kaganovich We have commenced experiments with intense short pulses of ion beams on the Neutralized Drift Compression Experiment-II at Lawrence Berkeley National Laboratory, by generating beam spots size with radius r \textless 1 mm within 2 ns FWHM and approximately 10$^{10}$ ions/pulse. To enable the short pulse durations and mm-scale focal spot radii, the 1.2 MeV Li$+$ ion beam is neutralized in a 1.6-meter drift compression section located after the last accelerator magnet. An 8-Tesla short focal length solenoid compresses the beam in the presence of the large volume plasma near the end of this section before the target. The scientific topics to be explored are warm dense matter, the dynamics of radiation damage in materials, and intense beam and beam-plasma physics including selected topics of relevance to the development of heavy-ion drivers for inertial fusion energy. We will describe the accelerator commissioning and time-resolved ionoluminescence measurements of yttrium aluminium perovskite using the fully integrated accelerator and neutralized drift compression components (arXiv:1506.05839). [Preview Abstract] |
Tuesday, November 17, 2015 10:42AM - 10:54AM |
GO8.00007: Theoretical studies of defect formation and target heating by intense pulsed ion beams J.J. Barnard, T. Schenkel, A. Persaud, P.A. Seidl, A. Friedman, D.P. Grote, R.C. Davidson, E.P. Gilson, I. Kaganovich We present results of three studies related to experiments on NDCX-II, the Neutralized Drift Compression Experiment, a short-pulse ($\sim$ 1ns), high-current ($\sim$ 70A) linear accelerator for 1.2 MeV ions at LBNL. These include: (a) Coupled transverse and longitudinal envelope calculations of the final non-neutral ion beam transport, followed by neutralized drift and final focus, for a number of focus and drift lengths and with a series of ion species (Z$=$1-19). Predicted target fluences were obtained and target temperatures in the 1 eV range estimated. (b) HYDRA simulations of the target response for Li and He ions and for Al and Au targets at various ion fluences (up to 10$^{12}$ ions/pulse/mm$^{2})$ and pulse durations, benchmarking temperature estimates from the envelope calculations. (c) Crystal-Trim simulations of ion channeling through single-crystal lattices, with comparisons to ion transmission data as a function of orientation angle of the crystal foil and for different ion intensities and ion species. \\[4pt] *This work was performed under the auspices of the U.S. DOE under contracts DE-AC52-07NA27344 (LLNL), DE-AC02-05CH11231 (LBNL) and DE-AC02-76CH0307 (PPPL) and was supported by the US DOE Office of Science, Fusion Energy Sciences. LLNL-ABS-67521 [Preview Abstract] |
Tuesday, November 17, 2015 10:54AM - 11:06AM |
GO8.00008: Total Kinetic Energy Of Non-Thermal Electron-Beams In Z-Pinch Experiments Ben Hammel, Erik McKee, Matt Wallace, Radu Presura, Aaron Covington, Tim Darling An approach to infer the total energy of energetic electron-beams generated in pulsed-power driven pinch experiments is discussed. Using x-pinch wire arrays, we measured the dynamic response of a target anode material as a result of ablative shock loading following the rapid deposition of energy from the incident electron-beam. The time-profile of the drive is obtained through measurement of bremsstrahlung emission with scintillator-PMT diagnostics. MCNP is then used to correlate electron-beam spectrum to the detected hard x-ray signal, and compared with experiments fielding a timer-resolved electron-energy analyzer. Shock strength is inferred by using a line-imaging Velocity Interferometer System for Any Reflector, which recorded the target's free-surface velocity at shock breakout. Lastly, hydrodynamic simulations in HYDRA allow us to infer the total energy of the drive under the boundary conditions of the measured drive profile and shock strength. Information on the total beam-energy provides a better understanding of plasma pinch dynamics that contribute to the observation of non-thermal bremsstrahlung and detection of cold-characteristic x-ray emission from ``hot-spots.'' [Preview Abstract] |
Tuesday, November 17, 2015 11:06AM - 11:18AM |
GO8.00009: Development and Characterization of Pulsed Neutron Sources at NTF Erik McKee, Ben Hammel, Danny Lowe, Radu Presura, Vladimir Ivanov, Showera Haque, Aaron Covington, Jeremy Iratcabal, Zephyr McCormick, Tim Darling Short duration, high-intensity pulsed neutron sources are being developed on the Zebra 1-MA/100ns pulsed-power generator. Ion beam collisions above threshold energies in a Z-pinch containing deuterium are the primary production mechanism of the 2.45 MeV neutrons. Deuterium treated palladium wire-arrays have been successfully used to produce neutrons on Zebra, but the deuterium content of the Pd wire storage diminishes rapidly. More traditional single-shell gas puffs have also been designed and implemented and allow for much higher repetition rates and ability to control the load composition; both pure deuterium and binary mixtures of krypton and deuterium gases were used. Both sources are capable of producing ~1e10 neutrons per pulse. The yield and spectrum of the neutron pulse was measured by a combination of Ag and Y activation detectors and time-of-flight scintillator-PMT detectors. A model of the experimental area was used in the MCNP code to determine the scattering contribution and assist in calibration of the neutron detectors. Support for this work is provided by DOE/NNSA grant DE-NA0002075. [Preview Abstract] |
Tuesday, November 17, 2015 11:18AM - 11:30AM |
GO8.00010: The Role of the Driver Circuit in the Neutron Yield of the Plasma Focus Jason Sears, Andrea Schmidt, Anthony Link, Dale Welch Emperical observations have suggested that dense plasma focus (DPF) neutron yield increases with driver impedance. Using the particle-in-cell code LSP [1], we reproduce this trend in a kJ DPF [2], and demonstrate in detail how driver impedance is coupled to neutron output. We implement a 2-D model of the plasma focus including self-consistent circuit-driven boundary conditions. We show that m=0 growth is central to beam formation and is a chaotic, non-deterministic process. Neutrons are produced when high, short-lived electric fields in the low-density cavity of an m=0 mode accelerate a beam of ions into the dense downstream pinch region. Neutron yield is highest when the ion beam is generated within 50 ns of the pinch formation on axis, because at that time the pinch (target) density is highest. High driver impedance contributes to prompt beam formation in two ways. First, the high impedance driver, losing less energy to run-down, has a faster run-in velocity and hence larger Rayleigh-Taylor features that more readily seed the m=0 instability. Second, the shorter anode of the high-impedance driver retains less trailing mass in the run-down region and thus exhibits fewer and less parasitic restrikes.\\[4pt] [1] A. Schmidt, V. Tang, D. Welch, PRL, 2012\\[0pt] [2] J. Ellsworth, RSI, 2014 [Preview Abstract] |
Tuesday, November 17, 2015 11:30AM - 11:42AM |
GO8.00011: Characterisation of the Current Switch Mechanism in Two-stage Wire Array Z-pinches Guy Burdiak, S. Lebedev, A. Harvey-Thompson, G. Hall, G. Swadling, F. Suzuki-Vidal, S. Bland, L. Pickworth, P. de Grouchy, L. Suttle, E. Waisman We describe the operation of a two-stage wire array z-pinch driven by the 1.4 MA, 240 ns rise-time Magpie generator at Imperial College London. In this setup an inverse wire array acts as a fast current switch, delivering a current pre-pulse into a cylindrical load wire array, before rapidly switching the majority of the generator current into the load after a 100-150 ns dwell time. Preconditioning of the load array dramatically alters the ensuing implosion dynamics; the ablation phase is eliminated and no trailing mass remains at the initial array radius during the final implosion. The main current switch occurs as the inverse array begins to explode and plasma expands into the load region. Electrical and imaging diagnostics indicate that the main current switch may evolve as a plasma flow switch, driven by the expansion of a magnetic cavity and plasma bubble along the length of the load array. Analysis of implosion trajectories suggests that approximately 1 MA switches into the load in 100 ns. Attempts to measure the current profile throughout the current switch will be presented. Potential scaling of the device to higher current machines is discussed. [Preview Abstract] |
Tuesday, November 17, 2015 11:42AM - 11:54AM |
GO8.00012: Experiments and Simulations on Magnetically Driven Implosions in High Repetition Rate Dense Plasma Focus Luis Caballero Bendixsen, Simon Bott-Suzuki, Samuel Cordaro, Mahadevan Krishnan, Stephen Chapman, Phil Coleman, Jeremy Chittenden Results will be shown on coordinated experiments and MHD simulations on magnetically driven implosions, with an emphasis on current diffusion and heat transport. Experiments are run at a Mather-type dense plasma focus (DPF-3, Vc: 20 kV, Ip: 480 kA, E: 5.8 kJ). Typical experiments are run at 300 kA and 0.33 Hz repetition rate with different gas loads (Ar, Ne, and He) at pressures of $\sim$ 1-3 Torr, usually gathering 1000 shots per day. Simulations are run at a 96-core HP blade server cluster using 3GHz processors with 4GB RAM per node.Preliminary results show axial and radial phase plasma sheath velocity of $\sim$ 1x105 m/s. These are in agreement with the snow-plough model of DPFs. Peak magnetic field of $\sim$ 1 Tesla in the radial compression phase are measured. Electron densities on the order of 1018 cm$^{-3}$ anticipated. Comparison between 2D and 3D models with empirical results show a good agreement in the axial and radial phase. [Preview Abstract] |
Tuesday, November 17, 2015 11:54AM - 12:06PM |
GO8.00013: Azimuthal Current Density Distribution Resulting from a Power Feed Vacuum Gap in Metallic Liner Experiments at 1 MA Simon Bott-Suzuki, S.W. Cordaro, L.S. Caballero Bendixsen, L. Atoyan, T. Byvank, W. Potter, B.R. Kusse, J.B. Greenly, D.A. Hammer, J.P. Chittenden, C.A. Jennings We present a study investigating the initiation of plasma in solid, metallic liners where the liner thickness is large compared to the collisionless skin depth. A vacuum gap is introduced in the power feed and we investigate the effect of this on the azimuthal initiation of plasma in the liner. We present optical emission data from aluminum liners on the 1 MA, 100ns COBRA generator. We use radial and axial gated imaging and streak photography, which show a dependence of onset of emission with the size of a small power-feed vacuum gap. The evolution of ``hot-spots'' generated from breakdown vacuum gap evolves relatively slowly and azimuthal uniformity is not observed on the experimental time-scale. We also show measurements of the B-field both outside and inside the liner, using miniature Bdot probes, which show a dependence on the liner diameter and thickness, and a correlation to the details of the breakdown. These data will be compared to magneto-hydrodynamic simulations to infer how such non-uniformities may affect full liner implosion experiments. [Preview Abstract] |
Tuesday, November 17, 2015 12:06PM - 12:18PM |
GO8.00014: In-Situ Anode Heating and Its Effects on Atomic Constituents in the A-K Gap in Self-Magnetic Pinch (SMP) Experiments* Sean Simpson, Timothy Renk, Mark Johnston, Mike Mazarakis, Sonal Patel The RITS-6 inductive voltage adder (IVA) accelerator (3.5-8.5 MeV) at Sandia National Laboratories produces high-power (TW) focused electron beams (\textless 3mm diameter) for flash x-ray radiography applications. The Self-Magnetic Pinch (SMP) diode utilizes a hollowed metal cathode to produce a pinched focus onto a high-Z metal anode converter. There is not a clear understanding as to the effects various contaminants such as C, CO, H, H$_{2}$O, H$_{\mathrm{m}}$C$_{\mathrm{n}}$, O$_{\mathrm{2}}$, and N$_{2}$, on the anode surface or in the bulk may have on impedance dynamics, beam stability, beam spot size, and reproducibility. Heating pure Ta anodes with and without a thin Al coating have been investigated using temperatures ranging from 400 $^{\circ}$C to 1000 $^{\circ}$C. Initial experiments indicate a significant reduction in H and C as seen in high-speed spectral analysis of plasmas at the converter and a reduction in the back-streaming proton current. Experiments are ongoing, and latest results will be reported. \\[4pt] *Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Tuesday, November 17, 2015 12:18PM - 12:30PM |
GO8.00015: Anode heating/cleaning and its effects on diode impedance in Self-Magnetic Pinch (SMP) Experiments* Timothy Renk, Sean Simpson, Jacob Zier, Bruce Weber The SMP diode is fielded on both the RITS-6 (3.5-8.5 MV) and Mercury (5.5 MV) accelerators, located at Sandia and the Naval Research Laboratory, respectively. This diode utilizes a hollowed metal cathode to produce focused electron beams ($<$3 mm diameter) onto a high-Z converter for flash x-ray applications. We observe on some shots unexplained impedance collapse beyond what may be attributed to normal A-K gap closure. This could be caused by gas evolution off the as-provided hardware making up the anode and cathode. The goal of heating the anode is to remove gases trapped within the bulk of the Ta anode, and so reduce the volume of evolving gases near the A-K gap. Two heating techniques have been investigated, a short-pulse ($\sim$1 sec) resulting in high Ta temperature ($\sim$3000 $^{\circ}$C), and a longer ($\sim$100 sec) heating of the Ta to lower peak temperature ($\sim$1000 $^{\circ}$C). Initial experiments indicate a modest improvement to diode performance. Additional experiments are ongoing, and latest results will be reported.\\[4pt] *Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Tuesday, November 17, 2015 12:30PM - 12:42PM |
GO8.00016: Dependence of the Back-Streaming Ion Current on the Self-Magnetic Pinch (SMP) Electron Diode Parameters Michael Mazarakis, Mark Johnston, Mark Kiefer, Josh Leckbee, Dan ielsen, Timothy Renk, Timothy Webb, Derek Ziska, Nichelle Bennett The Sandia National Laboratories RITS accelerator presently drives a self-magnetic pinch diode (SMP) that generates small electron beam spots. The diode's anode, 5 cm in diameter, is made of high Z metal in order to produce copious and energetic flash x-rays for radiographic imaging of high areal density objects. A number of cathode sizes, A-K gap lengths, anode target x-ray converter designs, and cleaning techniques are being implemented. The focusing of the electron beam on the target is accomplished by the space charge neutralization of the electron beam. The larger diameter target compared to the A-K length and cathode diameter, the plasma formed near the surface of the anode, and the counter-streaming ions emitted by the anode plasma and accelerated into the A-K gap, all contribute to that effect. We are currently measuring the back-streaming ion currents emitted from the various design anodes propagating through the hollow cathode tip, and we evaluate the A-K gap voltage by energy filtering techniques. [Preview Abstract] |
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