Bulletin of the American Physical Society
51st Annual Meeting of the APS Division of Plasma Physics
Volume 54, Number 15
Monday–Friday, November 2–6, 2009; Atlanta, Georgia
Session BO6: Microwaves, Compton Scattering and Accelerator Physics |
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Chair: Kevin Felch, CPI Room: Hanover FG |
Monday, November 2, 2009 9:30AM - 9:42AM |
BO6.00001: Physics of Frequency Step-tuning in Terahertz Traveling Wave Tube Regenerative Oscillators John Booske, Peng Gao Efficient generation (higher than 0.3 percent) of terahertz regime radiation was recently demonstrated with a traveling wave tube (TWT) regenerative oscillator using a folded waveguide (FWG) slow wave circuit [1]. By varying the beam voltage between 9.2 and 10 kV, the oscillation frequency was observed between 607-675 GHz. However, step-tuning rather than the expected smooth variation of frequency was observed. Theoretical analyses show that constructive phase interference between the passive backward wave and the recirculated feedback wave leads to step tuning. Results will be presented, including the effects of circuit loss.\\[4pt] [1] J. Tucek, et al, Conf. Digest, 2007 IEEE Intl Vac. Elec. Conf., pp. 219-220 (IEEE Cat. No. 07EX1526). [Preview Abstract] |
Monday, November 2, 2009 9:42AM - 9:54AM |
BO6.00002: Remote sensing of an elevated level of background ionization with a 1 THz, 1 kW Gyro-BWO Victor Granatstein, Gregory Nusinovich One application of the THz gyro- BWO under development at the University of Maryland is remote sensing of an elevated level of background ionization in air at NTP; such an elevated level might be found in the vicinity of radioactive material. The aim of the gyro-BWO development is to produce $\sim $1 kW of THz power in a 100 microsecond pulse. When such radiation is focused to a spot of cross-sectional area $\sim \lambda ^{2}$, the electric field would be sufficient to break down the air if the background ionization level were sufficiently elevated. With the time to breakdown dependent on background electron density n$_{o}$, an elevated value of n$_{o}$ will manifest itself by causing a shortening of the transmitted or reflected THz pulse. The range and sensitivity of such a detection scheme will be discussed. [Preview Abstract] |
Monday, November 2, 2009 9:54AM - 10:06AM |
BO6.00003: Self-excitation of a frequency-tunable THz-range gyro-backward-wave oscillator Gregory Nusinovich, Ruifeng Pu, Jiao Yu, Oleksandr Sinitsyn, Victor Granatstein Gyro-backward-wave oscillators (gyro-BWOs) are known as high-power sources of millimeter-wave radiation which can be continuously tunable in a wide frequency range by varying either magnetic field or the beam voltage. Our group is planning to start experiments with frequency-tunable gyro-BWOs in the THz frequency range. The required magnetic field will be produced by a pulsed magnet delivering up to 40 T magnetic fields. To increase the tunability we plan to utilize slightly tapered waveguides. In the present paper, the self-excitation conditions in gyro-BWOs with such waveguides are analyzed. First, the analysis is performed assuming that the axial structure of the electromagnetic field in such waveguide, which consists of the region of electromagnetic propagation and the region where the wave frequency is below cutoff, can be approximated by the Airy function. The results of simulations performed with the use of the self-consistent code MAGY will be presented and compared with analytical data. These results will be used for estimating (a) parameters of the required electron gun and (b) tolerances on fabrication of the microwave circuit. [Preview Abstract] |
Monday, November 2, 2009 10:06AM - 10:18AM |
BO6.00004: Single Mode Excitation in High Power Gyrotrons Dmytro Kashyn, Gregory Nusinovich, Thomas Antonsen Megawatt-class, millimeter-wave gyrotrons are necessary for plasma heating and current drive in large-scale tokomaks and stellarators. These gyrotrons operate at very high-order modes with rather dense spectrum of eigenfrequencies. Therefore, when the voltage rises from zero to its nominal value, self-excitation conditions can be fulfilled for several modes. Analysis of gyrotrons start-up scenarios was performed by many authors. It was shown that in gyrotrons with diode-type electron guns the excitation of at least one parasitic mode is inevitable during the start up. We propose a method of eliminating this excitation. The method is based on usage of the diode-type electron guns where transition from the space-charge to temperature-limited emission takes place at higher voltages. As a result, in the regions where the self-excitation can be fulfilled for parasitic modes, the current density is lower than the required starting value. Results include analysis of beam current densities for different gun configurations as well as the dependence of starting currents on the beam voltage. [Preview Abstract] |
Monday, November 2, 2009 10:18AM - 10:30AM |
BO6.00005: Properties and applications of mono-energetic gamma-rays from laser-Compton scattering Felicie Albert, Scott Anderson, Gerry Anderson, Shawn Betts, David Gibson, Christian Hagmann, Micah Johnson, Mike Messerly, Miroslav Shverdin, Frederic Hartemann, Craig Siders, Dennis McNabb, Christopher Barty Properties of a Mono-Energetic Gamma-Ray (MEGa-Ray) Compton scattering source are presented. It produces 0.1 MeV-0.9 MeV photons. Its experimental key parameters are: its size (0.01 mm$^{2})$, divergence (10x6 mrad$^{2})$, duration (ps), spectrum (15{\%} bandwidth) and intensity (10$^{5}$ photons/shot), which yield an on-axis peak brightness of $\sim $10$^{15}$ photons/mm$^{2}$/mrad$^{2}$/s/0.1{\%} bandwidth at 0.478 MeV. We detected the 0.478 MeV nuclear resonance fluorescence (NRF) line of $^{7}$Li. A LiH sample was in the beam path, and the NRF scattered photons were detected by a germanium detector oriented at 90$^{\circ}$ with respect to the incident beam axis. The resulting spectrum shows the 0.478 MeV line of $^{7}$Li, with several characteristic lines from the interaction. With this experiment, we have shown that MEGa-Ray sources will provide a unique specific isotope detection capability. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
Monday, November 2, 2009 10:30AM - 10:42AM |
BO6.00006: MeV Mono-Energetic Gamma Ray Compton Scattering Source R{\&}D Fred Hartemann, Felicie Albert, Scott Anderson, Sam Chu, Rick Cross, Chris Ebbers, David Gibson, Mike Messerly, Vlad Semenov, Miro Shverdin, Craig Siders, Dennis MCNabb, Chris Barty, Arnold Vlieks, Sami Tantawi A precision, tunable gamma-ray source driven by a compact, high-gradient X-band linac is currently under development at LLNL. High-brightness, relativistic electron bunches produced by the linac interact with a Joule-class, 10 ps laser pulse to generate tunable $\gamma $-rays in the 0.5-2.5 MeV photon energy range via Compton scattering. The source will be used to excite nuclear resonance fluorescence lines in various isotopes; applications include homeland security, stockpile science and surveillance, nuclear fuel assay, and waste imaging and assay. The source design, key parameters, and current status are presented. [Preview Abstract] |
Monday, November 2, 2009 10:42AM - 10:54AM |
BO6.00007: Compact Laser Technology for Compton Scattering Sources M. Shverdin, F. Albert, S.G. Anderson, A. Bayramian, S.M. Betts, C. Ebbers, D. Gibson, M. Messerly, F.V. Hartemann, C.W. Siders, D.P. McNabb, C.P.J. Barty We describe compact laser technology for Mono-Energetic Gamma-Ray (MEGa-Ray) Compton scattering light source at LLNL. The high energy, 120W interaction laser utilizes chirped pulse amplification (CPA) in Nd:YAG to amplify a sub-nanometer bandwidth 20 $\mu $J pulses from a fiber system to 1J. A novel pulse stretcher provides a dispersion of over 7000ps/nm to expand a several picosecond wide seed pulse to 6ns. After amplification, the pulse is recompressed to 10ps with a hyper-dispersive pulse compressor. We also describe a technique for over an order of magnitude increase in the generated gamma-ray flux by recirculation of the interaction laser pulse. This technique, termed Recirculation Injection by Nonlinear Gating (RING), consists of frequency doubling the incident laser pulse inside a dichroic mirror cavity. The resonator mirrors transmit at 1$\omega $ and reflect at 2$\omega $. The 2$^{nd}$ harmonic of the incident pulse then becomes trapped inside the cavity. To date, we demonstrated 14 times cavity enhancement of 180mJ, 10ps, 532nm laser pulses. [Preview Abstract] |
Monday, November 2, 2009 10:54AM - 11:06AM |
BO6.00008: Three-Dimensional Space-Charge Modeling of Intense Beams in a Circular Conducting Pipe Mark Hess, Chong Shik Park We demonstrate a novel 3-D method for modeling the space-charge fields of intense beams in a circular conducting pipe. In this method, we can solve the electromagnetic space-charge fields using two time-dependent Green's functions. Since these Green's functions satisfy the Helmholtz wave equation, the method is completely free of numerical dispersion making this method an ideal alternative to the traditional FDTD methods which are typically used for fully electromagnetic simulations. In addition, we show how this method can be implemented numerically to compute the space-charge fields of intense beams undergoing non-trivial motion, such as breathing mode oscillations. [Preview Abstract] |
Monday, November 2, 2009 11:06AM - 11:18AM |
BO6.00009: Adiabatic thermal beam equilibrium in an alternating-gradient focusing channel Chiping Chen, Ksenia Samokhvalova, Jing Zhou An adiabatic warm-fluid equilibrium theory for a thermal charged-particle beam in an alternating-gradient~(AG) focusing field is presented. Warm-fluid equilibrium equations are solved in the paraxial approximation. The theory [K.R. Samokhvalova, J. Zhou and C. Chen, Phys. Plasmas \textbf{16}, 043115 (2009)] predicts that the 4D rms thermal emittance of the beam is conserved, but the 2D rms thermal emittances are not constant. The rms beam envelope equations and the self-consistent Poisson equation, governing the beam density and potential distributions, are derived. Although the presented rms beam envelope equations have the same form as the previously known rms beam envelope equations, the evolution of the rms emittances in the present theory is given by analytical expressions. The density does not have the simplest elliptical symmetry, but the constant-density contours are ellipses, and the aspect ratio of the elliptical constant-density contours decreases as the density decreases along the transverse displacement from the beam axis. [Preview Abstract] |
Monday, November 2, 2009 11:18AM - 11:30AM |
BO6.00010: Characterization of Plasmas in Negative Polarity Rod Pinch Diodes* Mark Johnston, Bryan Oliver, Joshua Leckbee, Dale Welch, Darryl Droemer, Marlon Crain Experiments at SNL are underway to investigate plasma formation in rod pinch diodes fielded in negative polarity on the RITS-6 accelerator (7.5MV and 180kA at 40 Ohms). The rod pinch diode consists of a small diameter metal rod which extends through a larger diameter metal cathode plate. Electrons formed at the cathode accelerate across the gap, become self-insulated, and are focused at the rod tip, generating x-rays used for flash radiography. Most of the previous rod pinch work has been performed in positive polarity; however, there is an interest in operating this diode in negative polarity at higher voltages, allowing more flexibility and incorporation into a wider variety of pulsed power devices. In an effort to better understand the basic physics, and the role ions play in the impedance behavior, a series of shots were taken looking at plasma formation in the diode. Diagnostics include optical imaging and spectroscopy using nanosecond gated ICCD cameras, streak cameras, and photodiode arrays. *Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Monday, November 2, 2009 11:30AM - 11:42AM |
BO6.00011: Effects of external magnetic fields on the operation of rf accelerating structures Diktys Stratakis, Juan Gallardo, Robert Palmer Recent experiments on rf cavity breakdown have shown severe surface damage and a reduction of the maximum accelerating gradient when an external magnetic field is applied. This finding implies serious problems for cooling lattices in which rf cavities and external magnetic fields coexist, such as those of the proposed neutrino factory and muon collider. While existing data suggest that such operational problems were associated with the unwanted emission of electrons from locally enhanced field regions, the mechanism that drives the breakdown is poorly understood. We show that field-emitted electrons from one wall of a cavity are accelerated by the rf fields, and in the presence of a external magnetic field are focused into small spots at another location in the cavity where they heat the surface. We construct a simple model to analyze the dependence of wall temperature on emission current and external magnetic fields. We show that if the magnetic field strength is on the order of 1 T, the surface can suffer significant thermally-induced deformation that eventually limits the cavity's accelerating gradient. We compare the results from our model to existing experimental data from an 805-MHz cavity. Possible solutions and suggestions for future experiments to study those problems are offered. [Preview Abstract] |
Monday, November 2, 2009 11:42AM - 11:54AM |
BO6.00012: Experimental Measurements of a High-Energy Pseudospark-Produced Electron Beam Jing Hu, Joshua L. Rovey, Scott Kovaleski This paper presents the progress thus far in the investigation of pseudospark-produced electron beams for medical applications. A twenty-six-gap pseudospark device is fabricated with different insulators and tested as the electron beam source. The discharge capacitor between the anode and cathode to store electrical energy is incorporated into the stacked ring design to minimize inductance. The system is operated at 100kV. Two Rogowski coils are positioned in the hole of anode flange and external capacitors to measure the total beam and discharge currents. Further, a small Langmuir probe and a Faraday cup are used to determine the energy of the beam through the comparison of the time that the e-beam passes them. Results of the scaling study for the electron beam current with breakdown voltage, gas pressure, external capacitance, and the effect of insulator material on pseudospark operating characteristics are presented. [Preview Abstract] |
Monday, November 2, 2009 11:54AM - 12:06PM |
BO6.00013: Numerical studies of multipactor in dielectric-loaded accelerator structures Oleksandr Sinitsyn, Gregory Nusinovich, Thomas Antonsen Multipactor (MP) is known as the avalanche growth of the number of secondary electrons emitted from a solid surface exposed to an rf electric field under vacuum conditions. MP may occur in various microwave and rf systems such as microwave tubes, rf windows and launchers, accelerating structures, and rf satellite payloads. In this work we present results of MP analysis in dielectric-loaded accelerator (DLA) structures. The starting point of our work was experimental and theoretical studies of DLA structures jointly done by Argonne National Laboratory and Naval Research Laboratory (J. G. Power et al., Phys. Rev. Lett. 92, 164801 (2004); J. G. Power et al., AIP Conf. Proc. 877, 362 (2006)). In the theoretical model developed during those studies the space-charge field due to the total number of particles is taken into account as a parameter. We perform our studies using a self-consistent approach with the help of time-dependent two-dimensional code developed at the University of Maryland (O. V. Sinitsyn et al., Phys. Plasmas 16, 073102 (2009)). Results include analysis of MP evolution at an early stage, detailed studies of individual electron trajectories, analysis of MP onset time under various conditions and comparison of some results with the experimental data. [Preview Abstract] |
Monday, November 2, 2009 12:06PM - 12:18PM |
BO6.00014: Lithium alumino-silicate ion source development Prabir Kumar Roy, Peter A. Seidl, Joe W. Kwan, Wayne G. Greenway, William L. Waldron, James K. Wu, Kavous Mazaheri We report experimental progress on Li+ source development in preparation for warm dense matter heating experiments. To uniformly heat targets to electron-volt temperatures for the study of warm dense matter, we are pursuing the use of a low (E $<$ 5 MeV) kinetic energy singly ionized lithium beam and a thin target. Two kinds of lithium (Li+) alumino-silicate ion sources, $\beta $-spodumene and $\beta $-eucryptite, each of area 0.31 cm2, have been fabricated for ion emission measurements. These surface ionization sources are heated to 1200 to 1300\r{ }C where they preferentially emit singly ionized alkali ions. Tight process controls were necessary in preparing and sintering the alumino-silicate to the porous tungsten substrate to produce an emitter that gives uniform ion emission, sufficient current density and low beam emittance. Current density limit of the two kinds have been measured, and ion species identification of possible contaminants has been verified with a Wien (E x B) filter. [Preview Abstract] |
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