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 YI3: Radiation Based Sources and Diagnostics, Postdeadline Invited Talks |
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Chair: Dmitri Ryutov, Lawrence Livermore National Laboratory Room: Plaza F |
Friday, November 15, 2013 9:30AM - 10:00AM |
YI3.00001: Absolute measurements of high field generation of plasmas in gases with femtosecond time- and micron space-resolution Invited Speaker: Jared Wahlstrand Laser-driven ionization of gases and gaseous density plasmas is a fundamental aspect of light-matter interactions affecting everything from propagation of high power laser pulses to coherent and incoherent laser-driven light sources. Using ultra-broadband 2D spectral interferometry we have performed absolute time- resolved and 2D in space-resolved measurements of the ionization of the noble gases He through Xe and several diatomic molecules, including O$_2$ and N$_2$. The refractive index is measured with a time resolution of 5 femtoseconds and spatial resolution of 3 microns, with absolute results ranging from the multiphoton through tunneling ionization regimes. With these results, combined with our previous measurements at nonionizing intensities [1,2], we have characterized the nonlinear response of the noble gases from the weak field limit up to full ionization of the first electron. For the molecular gases, we use prepulses to prepare an ensemble of partially aligned molecules and examine alignment-dependent ionization. Finally, we assess whether the nonequilibrium electron gas generated immediately after ionization has birefringent optical properties. Our measurements will lead to more accurate models of the propagation of intense optical pulses in gases and may serve as an experimental benchmark of calculations of strong field ionization. \\[4pt] [1] J. K. Wahlstrand, Y.-H. Cheng, and H. M. Milchberg, Phys. Rev. A 85, 043820 (2012).\\[0pt] [2] J. K. Wahlstrand, Y.-H. Cheng, and H. M. Milchberg, Phys. Rev. Lett 109, 113904 (2012). [Preview Abstract] |
Friday, November 15, 2013 10:00AM - 10:30AM |
YI3.00002: A High Power Wide-band Gyro-BWO for Terahertz Operation Invited Speaker: Wenlong He Terahertz waves have many exciting applications. If the frequency can be tuned these applications can be greatly enhanced with higher range resolution and improved sensitivity or selectivity. The potential of gyrotron backward wave oscillators (gyro-BWOs) as a high power (kW) coherent powerful microwave source with wide-frequency tunability, has hitherto been achieved only at frequencies well below the terahertz range. This abstract presents the first successful operation of a high power gyro-BWO with a wide frequency tuning capability in the low terahertz frequency range. A novel helically corrugated interaction region (HCIR) and 1.5 A, 40 kV thermionic cusp electron gun were used in the gyro-BWO. An ``ideal'' eigenwave, achieved from the resonant coupling of the modes in the HCIR, allows for broadband microwave amplification or wide frequency tuning. Stable single mode output was achieved in a wide frequency tuning band of 88-102.5 GHz by adjusting the cavity magnetic field with a maximum power of 12 kW and an electronic efficiency of 20{\%}. The performance of the gyro-BWO is consistent with 3D particle-in-cell (PIC) numerical simulations. This method can, in principle, be extended to the higher terahertz range. [Preview Abstract] |
Friday, November 15, 2013 10:30AM - 11:00AM |
YI3.00003: Application of Nonresonant Optical Lattices for Advanced Plasma and Gas Remote Nonintrusive Diagnostics Invited Speaker: Mikhail Shneider In this talk recent theoretical and experimental results on the interaction of optical lattices with neutral gases will be reviewed. A novel concept for remote laser diagnostics of weakly-ionized plasma, gas and gas mixtures with nanoparticles will be presented It is based on creation of optical lattice produced by interference pattern of two non-resonant laser pulses. Small ($\sim 10^{-7} - 10^{-5}$) gas density perturbations are produced by interaction of interference laser field with induced dipoles of polarized atoms and molecules and form periodic perturbation of gas density and index of refraction -- a Bragg lattice. A probing laser scatters coherently off the Bragg lattice and the scattered signal can be used for variety of diagnostic. For sufficiently large laser beam intensities, the optical potential can trap a large fraction of the gas (up to 30{\%}). In this case, trapped fraction of gas can be accelerated or decelerated in a moving optical lattice. The process of transfer of energy and momentum from an optical potential to gas particles is analogous to collisionless nonlinear Landau damping of electrostatic potentials in plasmas. At elevated laser intensities the optical lattice has also led to the observation of collisionless Spectral Narrowing in Coherent Rayleigh Scattering of a room temperature gas. [Preview Abstract] |
Friday, November 15, 2013 11:00AM - 11:30AM |
YI3.00004: Multi-level Monte Carlo Methods for Efficient Simulation of Coulomb Collisions Invited Speaker: Lee Ricketson We discuss the use of multi-level Monte Carlo (MLMC) schemes - originally introduced by Giles for financial applications - for the efficient simulation of Coulomb collisions in the Fokker-Planck limit. The scheme is based on a Langevin treatment of collisions, and reduces the computational cost of achieving a RMS error scaling as $\varepsilon$ from $O\left(\varepsilon^{-3}\right)$ - for standard Langevin methods and binary collision algorithms - to the theoretically optimal scaling $O\left(\varepsilon^{-2}\right)$ for the Milstein discretization, and to $O\left(\varepsilon^{-2} (\log \varepsilon)^2\right)$ with the simpler Euler-Maruyama discretization. In practice, this speeds up simulation by factors up to 100. We summarize standard MLMC schemes, describe some tricks for achieving the optimal scaling, present results from a test problem, and discuss the method's range of applicability. [Preview Abstract] |
Friday, November 15, 2013 11:30AM - 12:00PM |
YI3.00005: Performance of high-density-carbon (HDC) ablator implosion experiments on the National Ignition Facility (NIF) Invited Speaker: Andy Mackinnon A series of experiments on the National Ignition Facility (NIF) have been performed to measure high-density carbon (HDC) ablator performance for indirect drive inertial confinement fusion (ICF). HDC is a very promising ablator material; being 3x denser than plastic, it absorbs more hohlraum x-rays, leading to higher implosion efficiency . For the HDC experiments the NIF laser generated shaped laser pulses with peak power up to 410 TW and total energy of 1.3 MJ. Pulse shapes were designed to drive 2, 3 or 4 shocks in cryogenic layered implosions. The 2-shock pulse, with a designed fuel adiabat of $\sim$ 3 is 6-7ns in duration, allowing use of near vacuum hohlraums, which greatly increases the coupling efficiency due to low backscatter losses. Excellent results were obtained for 2,3 and 4 shock pulses. In particular a deuterium-tritium gas filled HDC capsule driven by a 4-shock pulse in a gas-filled hohlraum produced a neutron yield of 1.6 x 10$^{15}$, a record for a non-cryogenically layered capsule driven by a gas-filled hohlraum. The first 2-shock experiment used a vacuum hohlraum to drive a DD gas filled HDC capsule with a 6.5ns, laser pulse. This hohlraum was 40{\%} more efficient than the gas-filled counterpart used for 3 and 4 shock experiments, producing near 1D performance at 11x convergence ratio, peak radiation temperature of 317eV, 98{\%} laser-hohlraum coupling, and DD neutron yield of 2.2e13, a record for a laser driven DD implosion. The HDC campaigns will be presented, including options for pushing towards the alpha dominated regime. [Preview Abstract] |
Friday, November 15, 2013 12:00PM - 12:30PM |
YI3.00006: Suppression of Type-I ELMs with a Reduced I-coil Set in DIII-D Invited Speaker: D.M. Orlov Recent experiments in DIII-D have demonstrated that Edge Localized Modes (ELMs) in a tokamak can be controlled with a reduced number of magnetic perturbation coils, demonstrating an important role of spectral sidebands, and showing promise of the technique for future fusion devices, where ELMs risk potentially damaging heat loads. The ELMs can be controlled with external magnetic perturbation used to regulate pressure gradients and maintain stability. The new results show that the coil currents required for ELM suppression with the reduced coil sets are comparable to what is typically required for the full set. This counterintuitive result provides an important validation of recent modeling [1,2] of the physical mechanisms involved. This modeling shows that the spectral sidebands introduced by deactivating individual coils can often increase the magnetic stochasticity within the plasma, thereby increasing transport and facilitating ELM suppression. Deactivating individual coils results not only in the reduction of the dominant n=3 component of the perturbation field, but also in a significant increase in the amplitudes of n=1 and n=2 sidebands. These sidebands may also be amplified by the plasma response. Application to ITER finds that the ITER ELM coils may be able to tolerate a loss of up to five of its 27 coils, while leaving a sufficient margin of current in the remaining coils to still meet the DIII-D ELM suppression criterion. Further, the new experiments show that the presence of the spectral sidebands does not adversely affect the plasma rotation or confinement. Both vacuum and two-fluid modeling are used to interpret and understand these results.\par \vskip6pt \noindent [1] D.M.\ Orlov, et al., Fusion Eng.\ Design {\bf 87}, 1536 (2012).\par \noindent [2] T.E.\ Evans, et al., Nucl.\ Fusion {\bf 53} (2013) accepted for publication. [Preview Abstract] |
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