Bulletin of the American Physical Society
55th Annual Meeting of the APS Division of Plasma Physics
Volume 58, Number 16
Monday–Friday, November 11–15, 2013; Denver, Colorado
Session YO6: Radiation Sources and Cathodes |
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Chair: John Verboncoeur, Michigan State University Room: Governor's Square 11 |
Friday, November 15, 2013 9:30AM - 9:42AM |
YO6.00001: Reduced Conductivity of Copper Between 400 and 850 GHz Matthew Kirley, John Booske Effective design of sources and low-loss transmission line components for sub-millimeter wavelength radiation requires the ability to predict the amount of dissipation loss caused by conducting surfaces. However, the theoretical understanding of the conductivity of metals is incomplete beyond 100 gigahertz (GHz). Electromagnetic simulation tools, used in THz device design, would be enhanced by improved models for metal conductivity as a function of frequency and surface characteristics. Confirming such a theory is difficult because experimental measurements in this regime are unavailable, unrepeated or even contradictory. We measured the conductivity of smooth (surface roughness $\le $ 5 nm) copper surfaces between 400 and 850 GHz using a high quality factor quasi-optical (QO) hemispherical resonator. These measurements are compared to the predictions of the Sommerfeld theory of electronic conduction and the classical relaxation effect model. Our results show that copper is significantly less conductive than expected in this frequency range. We are investigating the hypothesis that the microstructure of the copper metal is responsible for the discrepancy. [Preview Abstract] |
Friday, November 15, 2013 9:42AM - 9:54AM |
YO6.00002: Effects of Random Circuit Fabrication Errors on the Mean and Standard Deviation of Small Signal Gain and Phase of a Traveling Wave Tube I.M. Rittersdorf, T.M. Antonsen, Jr., D. Chernin, Y.Y. Lau, D.M.H. Hung Random fabrication errors in the slow wave circuits may have detrimental effects on the performance of traveling-wave tubes (TWTs) of all types. Pengvanich \emph{et. al.} [1] considered the evolution of the three forward waves in a TWT in which the Pierce parameters vary randomly along the tube axis. A peculiar feature of the results in Ref. [1] is that, in the statistical evaluation of a large number of samples with random errors in the circuit phase velocity, a significant number of these samples show an output gain that is higher than the corresponding error-free tube. It was recently proved that the deviation from the mean (which is a second order effect in random errors), is much less than the standard deviation (which is a first order effect in random errors). A significant number of the samples in a statistical analysis would naturally show an output gain that is higher than the corresponding error-free tube, as observed in [1]. This paper summarizes our recent study, together with an optimization of the random error profile to maximize the small signal gain of a TWT.\\[4pt] [1] P. Pengvanich, D. Chernin, Y. Y. Lau, J. W. Luginsland, and R. M. Gilgenbach, IEEE Trans. Electron Devices 55, 916 (2008). [Preview Abstract] |
Friday, November 15, 2013 9:54AM - 10:06AM |
YO6.00003: Simulation of Terahertz Generation in Corrugated Plasma Waveguides Chenlong Miao, Andrew Pearson, John Palastro, Thomas Antonsen Intense, short laser pulses passing through corrugated plasma channels offer an efficient means to generate THz radiation [1]. Corrugated channels support EM modes that have a Floquet-type dispersion relation. These modes consist of a sum of spatial harmonics with subluminal phase velocities, and thus allow the possibility of phase matching between the ponderomotive potential of the laser pulse and the EM modes. Here, we simulate the response of a modulated plasma channel to an ultrashrot laser pulse with the goal of increasing the efficiency of energy conversion to THz modes. A range of channel and modulation parameters are considered. For a fixed drive pulse with 15 micron spot size in a modulated channel with on axis average density of 10$^{18}$ cm$^{-3}$, 6{\%} of the energy extracted from the laser pulse is converted to THz with the remainder going to plasma waves. This result increases the previous simulation result [2] by a factor of 5. To realize high THz energies, the laser pulse energy must be largely depleted in a short distance. Full format PIC simulations are conducted to investigate this regime. \\[4pt] [1] T. M. Antonsen et. al., Phys. Plasmas 14, 033107 (2007).\\[0pt] [2] A. J. Pearson et. al., Phys. Review E 83, 056403 (2011). [Preview Abstract] |
Friday, November 15, 2013 10:06AM - 10:18AM |
YO6.00004: THz Generation by Optical Cherenkov Emission from Ionizing Two-Color Laser Pulses Luke Johnson, John Palastro, Ki-Yong Kim, Thomas Antonsen Coherent mixing of an ultrashort laser pulse (800nm, 40fs) and its harmonic (400nm) in a nitrogen gas cell produces broadband, THz radiation emitted in a narrow cone around the optical axis. Asymmetry in the time dependence around the peaks of the two-color electric field results in a post-ionization plasma current that varies slowly in time, driving the THz radiation. We have simulated the THz and optical pulse propagation using a 2D scalar unidirectional propagation equation for the electric field spectral components [2]. We find that the emission angle can be explained as an optical Cherenkov effect: specifically, the front of the low frequency photocurrent moves faster than the phase and group velocity of the THz, resulting in conical emission. The optical Cherenkov model can be unified with the ``off-axis phase matching model'' [3], so that both mechanisms for generating off-axis THz can be considered as limits of a single model. Furthermore, we will discuss the application of tilted intensity front pulses to improving THz generation efficiency and directionality.\\[4pt] [1] K. Y. Kim, Phys. Plasmas 16, 056706 (2009). \newline [2] M. Kolesik and J. Moloney, Phys. Rev. E 70, 036604 (2004). \newline [3] Y. S. You, T. I. Oh, K. Y. Kim, Phys. Rev. Lett. 109, 183902 (2012) [Preview Abstract] |
Friday, November 15, 2013 10:18AM - 10:30AM |
YO6.00005: Mode Control and Extraction on the Recirculating Planar Magnetron Matt Franzi, Ronald Gilgenbach, Brad Hoff, Geoff Greening, David Simon, Nicholas Jordan, Y.Y. Lau Experiments on a 12 cavity, 1 GHz, Recirculating Planar Magnetron (RPM-12a) [1] are underway using MELBA accelerator at -300 kV, 1-10 kA and pulse lengths of 0.3-1 microsecond. A mode control cathode (MCC) has been designed and constructed to address RPM mode competition and cross-oscillator coupling. The MCC is a periodically spaced conducting network designed to act as both an electron source and a resonant electromagnetic coupler between the two planar RPM oscillators. MCC simulations (MAGIC and ICEPIC) have verified such mechanisms, resulting in faster mode development and phase locking in the RPM. Manipulation of the cathode's geometry analytically enhances mode separation of the cold slow wave structure. Experimental frequency and phase measurements using the MCC on the RPM-12a will be discussed. Design and simulated results will also be presented for a new 12-cavity, 1.9 GHz RPM prototype, RPM-12c. This prototype will employ both the MCC and an axially oriented coaxial extraction line, which has been simulated to yield $\sim$450 MW at 60{\%} efficiency when operated under approximately the same conditions as the RPM-12a. *Research supported by AFOSR grant{\#}: FA9550-10-1-0104, AFRL, and L-3 Communications Electron Devices. \\[4pt] [1] Gilgenbach et al., IEEE Trans PS 39, 980 (2011); also, patent pending. [Preview Abstract] |
Friday, November 15, 2013 10:30AM - 10:42AM |
YO6.00006: Modeling a Gyrotron Cavity Using a 3D CFDTD PIC Method M.C. Lin, D.N. Smithe Modeling a gyrotron cavity in 3D and time-domain is very challenging due to the open-end features of cavity structure and higher-order mode excitation employed to achieve high efficiency. In this work, a conformal finite-difference time-domain (CFDTD) method is proposed to simulate a gyrotron cavity and a particle-in-cell (PIC) algorithm is used to describe a gyro electron beam. Our preliminary result shows that the 3D CFDTD PIC method could provide an alternative modeling tool for gyrotron research and design. [Preview Abstract] |
Friday, November 15, 2013 10:42AM - 10:54AM |
YO6.00007: Cathode Plasma Formation in High Intensity Electron Beam Diodes$^{1}$ Mark Johnston, Mark Kiefer, Bryan Oliver, Nichelle Bennett, Darryl Droemer, V. Bernshtam, R. Doron, Yitzhak Maron This talk will detail the experimental results and conclusions obtained for cathode plasma formation on the Self-Magnetic Pinch (SMP) diode fielded on the RITS-6 accelerator (4-7.5 MeV) at Sandia National Laboratories. The SMP diode utilizes a hollowed metal cathode to produce high power (TW), focused electron beams (\textless~3mm diameter) which are used for flash x-ray radiography applications. Optical diagnostics include high speed (\textless~10ns) framing cameras, optical streak cameras, and spectroscopy. The cathode plasma in this high electric (MV/cm) and magnetic (\textgreater~10 Tesla) field environment forms well-defined striations. These striations have been examined for a number of different cathode sizes, vacuum gap spacings, and diode voltages. Optical streak images have been taken to determine the time evolution of the plasma, and optical spectroscopy has been employed to determine its constituents as well as their densities and temperatures inferred from detailed time-dependent, collisional-radiative (CR) and radiation transport modelings. Comments will be made as to the overall effect of the cathode plasma in regards to the diode impedance and electron beam focusing. \\[4pt] $^{1}$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] |
Friday, November 15, 2013 10:54AM - 11:06AM |
YO6.00008: Nonlinear dephasing and brightness optimization in Compton scattering Sheldon Wu, Frederic Hartemann In Compton scattering light sources, a laser pulse is scattered by relativistic electrons to generate tunable x and gamma rays. Due to the inhomogeneous nature of the incident radiation, the relativistic Lorentz boost of the electrons is modulated by the ponderomotive force during the interaction, leading to intrinsic spectral broadening and brightness limitations. These effects are discussed, along with an optimization strategy to account for the laser bandwidth, diffraction, and nonlinear ponderomotive force. It is found that the maximum brightness is obtained when pulse duration and diffraction effects are balanced, and one operates near the onset of the weakly nonlinear ponderomotive dephasing. [Preview Abstract] |
Friday, November 15, 2013 11:06AM - 11:18AM |
YO6.00009: LLNL X-band Test Station Status David Gibson, Scott Anderson, Frederic Hartemann, Roark Marsh, Sheldon Wu, C.P.J. Barty An X-band test station is being built at LLNL to support inverse Compton-scattering x-ray and gamma-ray source development. The major components for the X-band test station have been designed, fabricated, installed, and aligned. Commissioning of the high power RF and accelerator is complete, and the current status of commissioning and first beam will be presented and discussed. Future upgrade paths and configuration for a variety of x-ray and gamma-ray applications will be discussed along with schedule for planned experiments. [Preview Abstract] |
Friday, November 15, 2013 11:18AM - 11:30AM |
YO6.00010: 100 Hz repetition rate, high average power, plasma-based soft x-ray lasers Brendan Reagan, Keith Wernsing, Cory Baumgarten, Mark Berrill, Leon Durivage, Federico Furch, Alden Curtis, Bradley Luther, Dinesh Patel, Carmen Menoni, Vyacheslav Shlyaptsev, Jorge Rocca Numerous applications demand high average power / high repetition rate compact sources of coherent soft x-ray radiation. We report the demonstration table-top soft x-ray lasers at wavelengths ranging from 10.9nm to 18.9nm from plasmas created at 100Hz repetition rate. Results includes a record average power of 0.15mW at $\lambda =$18.9nm from a laser-produced Mo plasma and 0.1mW average power at $\lambda =$13.9nm from a Ag plasma. These soft x-ray lasers are driven by collisional electron impact excitation in elongated line focus plasmas a few mm in length heated by a compact, directly diode-pumped, chirped pulse amplification Yb:YAG laser that produces 1J pulses of ps duration at 100Hz repetition rate. Pulses from this laser irradiate the surface of polished metal targets producing transient population inversions on the 4d$^{1}$S$_{0}\to $4p$^{1}$P$_{1}$ transition of Ni-like ions. Tailoring of the temporal profile of the driver laser pulse is observed to significantly increase soft x-ray laser output power as well as allow the generation of shorter wavelength lasers with reduced pump energy. [Preview Abstract] |
Friday, November 15, 2013 11:30AM - 11:42AM |
YO6.00011: Gain dynamics measurement in injection-seeded soft x-ray laser plasma amplifiers Yong Wang, S. Wang, L. Li, E. Oliva, T.T. Thuy Le, D. Ros, M. Berrill, J. Dunn, Ph. Zeitoun, L. Yin., B. Luther, J.J. Rocca Herein we report the first measurement of the gain dynamics in a soft x-ray plasma amplifier seeded by high harmonic pulses. A sequence of two time-delayed spatially-overlapping high harmonic pulses was injected into a $\lambda =$18.9 nm Ni-like Mo plasma amplifier to measure the regeneration of the population inversion that follows the gain depletion caused by the amplification of the first seed pulse. Collisional excitation is observed to re-establish population inversion depleted during the amplification of the seed pulse in about $\sim$ 1.75 ps. The measured gain-recovery time is compared to model simulations to gain insight on the population inversion mechanisms that create the transient gain in these amplifiers. This result supports the concept of a soft x-ray laser amplification scheme based on the continuous extraction of energy from a soft x-ray plasma-based amplifier by an stretched seed pulse has the potential to generate ultra-intense fully phase-coherent soft x-ray laser pulses. [Preview Abstract] |
Friday, November 15, 2013 11:42AM - 11:54AM |
YO6.00012: Single-Shot Wavefront Measurement of an Injection-seeded Plasma-based Soft X-Ray Laser S. Wang, L. Li, Y. Wang, E. Oliva, L. Yin, B. Luther, G. Maynard, D. Ros, J.J. Rocca, Ph. Zeitoun The wavefront of a $\lambda$ = 18.9 nm soft x-ray beam from an injection-seeded plasma amplifier created by irradiation of a solid target was measured using a Hartmann wavefront sensor with an accuracy of $\lambda $/32 in a single shot. A significant improvement in wavefront aberrations from 0.51 $\pm$ 0.06 $\lambda $ rms of high harmonic seed to 0.23 $\pm$ 0.01 $\lambda $ rms for the amplified seeded beam was observed. The variation of wavefront characteristic as a function of time delay between the injection of the seed and peak of soft x-ray amplifier pump was studied. The wavefront sensor allows for the independent measurement of the different aberrations. The strongest improvement of the wavefront as it exits the amplifier is observed for coma, with values improve by more than a factor of 2, from 0.4l $\lambda $ to 0.18 $\lambda $ rms. The measurements were used to reconstruct the soft x-ray source and confirm its high peak brightness of about 1$\times$10$^{\mathrm{26}}$ photons/(s.mm$^{2}$.mrad$^{2}$. 0.01$\%$ bandwidth). [Preview Abstract] |
Friday, November 15, 2013 11:54AM - 12:06PM |
YO6.00013: Diffraction Grating Interferometer for Single-shot Soft-x-ray Laser Linewidth Measurement Liang Yin, Yong Wang, Shoujun Wang, Mark Berrill, Mario Marconi, Oscar Martinez, James Dunn, Jorge Rocca The linewidth of soft x-ray laser beams generated from highly charged ions in dense plasma amplifiers is an important parameter that ultimately limits their minimum pulsewidth. An innovative interferometer design was used to measure the linewidth of a plasma-based table-top soft x-ray laser operating in the $\lambda =$14.7 nm 4d$^{1}$S$_{0}$-4p$^{1}$P$_{1}$ line of Ni-like Pd in a single shot. The technique uses diffraction gratings as beam splitter in a Mach-Zehnder configuration that introduces a time delay across the detector plane. Measurement of the temporal coherence length, and hence the linewidth, were obtained for different grazing incidence angles of the optical pump laser that heats the plasma. The temporal coherence length for plasmas generated by irradiation at grazing incident angle of 23$^{\circ}$, 30$^{\circ}$, 37$^{\circ}$ was measured, yielding values that vary from 221$\pm $10$\mu $m for the smaller angle to 191$\pm $8 $\mu $m, corresponding to FWHM spectral linewidths of $\Delta \lambda $/$\lambda =$ 2.94$\times $10$^{-5}$ and 3.42$\times $10$^{-5}$ respectively. Results were also obtained for an injection-seeded configuration in which a high harmonic pulse is use to seed the amplifier. [Preview Abstract] |
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