Bulletin of the American Physical Society
54th Annual Meeting of the APS Division of Plasma Physics
Volume 57, Number 12
Monday–Friday, October 29–November 2 2012; Providence, Rhode Island
Session UI2: High Intensity Interactions and Advanced ICF Ignition |
Hide Abstracts |
Chair: Natalia Krasheninnikova, Los Alamos National Laboratory Room: Ballroom DE |
Thursday, November 1, 2012 2:00PM - 2:30PM |
UI2.00001: Marshall N. Rosenbluth Outstanding Doctoral Thesis Award Talk: The Ultrafast Nonlinear Response of Air Molecules and its Effect on Femtosecond Laser Plasma Filaments in Atmosphere Invited Speaker: Yu-hsin Chen When exceeding the critical power $P_{cr}$, an intense laser pulse propagating in a gas collapses into one or multiple ``filaments,'' which can extend meters in length with weakly ionized plasma and local intensity $\sim $ 10$^{13}$ W/cm$^{2}$ radially confined in a diameter of $<$ 100 $\mu $m [1]. While it has been generally accepted the nonlinear self-focusing of the laser pulse leading to beam collapse is stabilized by plasma generation [2], neither the field-induced nonlinearity nor the plasma generation had been directly measured. This uncertainty has given rise to recent controversy about whether plasma generation does indeed counteract the positive nonlinearity [3, 4]. For even a basic understanding of femtosecond filamentation and for applications, the focusing and defocusing mechanisms---nonlinear self-focusing and ionization---must be understood. By employing a single-shot, time-resolved technique based on spectral interferometry [5] to study the constituents of air, it is found that the rotational responses in O$_{2}$ and N$_{2}$ are the dominant nonlinear effect in filamentary propagation when the laser pulse duration is longer than $\sim $ 100fs. Furthermore, we find that the instantaneous nonlinearity scales linearly up to the ionization threshold [6], eliminating any possibility of an ionization-free negative stabilization [3] of filamentation. This is confirmed by space-resolved electron density measurements in meter-long filaments produced with different pulse durations, using optical interferometry with a grazing-incidence, ps-delayed probe [7].\\[4pt] [1] A. Braun \textit{et al}., Opt. Lett. \textbf{20}, 73 (1995).\\[0pt] [2] A. Couairon and A. Mysyrowicz, Phys. Rep. \textbf{441}, 47 (2007).\\[0pt] [3] V. Loriot \textit{et al}., Opt. Express \textbf{17}, 13429 (2009).\\[0pt] [4] P. B\'{e}jot \textit{et al}., Phys. Rev. Lett. \textbf{104}, 103903 (2010).\\[0pt] [5] Y.-H. Chen \textit{et al}., Opt. Express \textbf{15}, 7458 (2007); Opt. Express \textbf{15}, 11341 (2007).\\[0pt] [6] J. K. Wahlstrand \textit{et al}., Phys. Rev. Lett. \textbf{107}, 103901 (2011).\\[0pt] [7] Y.-H. Chen \textit{et al}., Phys. Rev. Lett. \textbf{105}, 215005 (2010). [Preview Abstract] |
Thursday, November 1, 2012 2:30PM - 3:00PM |
UI2.00002: New wave effects in nonstationary plasma Invited Speaker: Paul Schmit In plasma undergoing compression, embedded waves can have very unusual and possibly useful properties. For example, part of the mechanical energy of compressing plasma can be transferred controllably to hot electrons by seeding the plasma with plasma waves. Under compression, wherein wave action is conserved, the wave energy grows as its frequency and wavenumber change adiabatically, until, suddenly, the wave damps, resulting in switch-like production not only of heat [1], but also voltage and current [2]. These bursts can be controlled precisely in time by prescribing the compression script. Several classic problems in wave physics, including the bump-on-tail instability, exhibit new effects under compression [3]. In addition, the waves undergoing compression or expansion affect fundamental properties of plasma, such as the plasma compressibility; moreover, and rather remarkably, nonlinear waves, such as BGK modes, affect the plasma compressibility differently [4]. Wave-particle interactions mediated by plasma compression also can enhance the performance of plasma-based particle accelerators. To describe numerically all these effects, novel particle-in-cell simulations were developed. These findings point towards potentially beneficial applications, including in inertial confinement fusion and high energy density plasma physics, where extreme compression is exercised on dense plasma, which could be seeded with waves. \\[4pt] [1] P. F. Schmit, I. Y. Dodin, and N. J. Fisch, PRL 105, 175003 (2010).\\[0pt] [2] P. F. Schmit and N. J. Fisch, PRL 108, 215003 (2012).\\[0pt] [3] P. F. Schmit et al., J. Plasma Phys. 77, 629 (2011).\\[0pt] [4] P. F. Schmit, I. Y. Dodin, and N. J. Fisch, Phys. Plasmas 18, 042103 (2011).\\[0pt] [5] P. F. Schmit and N. J. Fisch, Phys. Plasmas 18, 102102 (2011). [Preview Abstract] |
Thursday, November 1, 2012 3:00PM - 3:30PM |
UI2.00003: Investigations of High Intensity, High Contrast Laser Solid with Short Pulses Invited Speaker: Franklin Dollar Experimental discoveries related to laser-based ion acceleration from thin foils and the production of high brightness x-rays from high order harmonic generation from short pulses are presented. High power femtosecond lasers are ideally suited for use as tabletop particle accelerators since their short pulse duration enables very high intensities to be generated at high repetition rates from a compact laser. However, if laser pulse energy arrives before the main short pulse, it can interact with the target to cause ablation making high intensity investigations of laser-solid interactions difficult. In the following experiments, the laser pulse-to-pedestal contrast was improved to 15 orders of magnitude out to nanosecond timescales, allowing for excellent control over the interaction of a short pulse with solid density material. A sharply-rising laser pulse with 50 TW of power was focused to a 1.2 micron focal spot, achieving intensities over $10^{21} \rm{Wcm}^{-2}$. Protons accelerated due to sheath acceleration were studied in ultrathin targets. By sculpting the plasma density using shaped ultrafast pulses, control over the proton and ion spectra was also demonstrated. Finite spot effects from circular polarized laser pulses produced efficient acceleration for ultrathin foils, which resulted from the efficient conversion of laser light into high energy electrons. Finally, as the laser pulse drives the critical electron density relativistically, harmonics of the driving laser are produced. Harmonics up to order 60th were observed. It was observed that for a plasma scale length beyond a threshold value, parametric instabilities strongly modulated the harmonic spectra. Numeric simulations were performed to support the physical interpretation. [Preview Abstract] |
Thursday, November 1, 2012 3:30PM - 4:00PM |
UI2.00004: Spectra of laser generated relativistic electrons using cone-wire targets Invited Speaker: Hiroshi Sawada We report on the characterization of the in situ energy spectrum of fast electrons generated by ultra-intense (I$\sim $10$^{19}$ W cm$^{-2})$ short pulse ($\tau \sim $0.7 and 10 ps) laser-plasma interactions using the TITAN and OMEGA EP lasers. That in situ spectrum is a key component of ignition efficiency for the Fast Ignition (FI) Inertial Confinement Fusion (ICF) concept. It is challenging to model and, until now, has resisted direct experimental characterization; other techniques have very large error bars or measure the modified spectrum of escaped electrons. This technique also gives an indication of the forward coupling efficiency of the laser to fast electrons. This information is derived from the measurement of Cu K$\alpha $ x-rays emitted from a 1.5 mm long Cu wire attached to the tip of Au or Al cone targets. Fast electrons, generated in the cone, transport through the cone tip with a fraction of coupling to the wire. Electrons in the wire excite fluorescence measured by a monochromatic imager and an absolutely calibrated HOPG spectrometer. An implicit hybrid-PIC code, LSP, is applied to deduce electron parameters from the K$\alpha $ measurements. Experiments on the TITAN laser with Au cones attached to wires show an increase in pre-pulse energy from 17 to 1000 mJ, decreases the fast electron fraction entering the wire from 8.4{\%} to 2.5{\%}. On OMEGA EP with Al cones attached to wires, total K$\alpha $ yield, normalized to laser energy, drops $\sim $30{\%} for laser pulse length increasing from 1 to 10 ps, indicative of a saturation mechanism. For Au cones, K$\alpha $ yields were 50{\%} of that measured for Al cones indicating a strong material dependence. In all cases, the spatial distribution can only be fit with a two-temperature electron energy distribution, the relative fractions depending on prepulse level. These results are being used to develop an optimum cone design for integrated FI experiments. This work was performed under the auspices of the USDOE by LLNL under Contract DE-AC52-07NA27344 and DE-FG-02-05ER54834. [Preview Abstract] |
Thursday, November 1, 2012 4:00PM - 4:30PM |
UI2.00005: NIF Target Designs and OMEGA Experiments for Shock-Ignition Inertial Confinement Fusion Invited Speaker: K.S. Anderson Shock ignition (SI)\footnote{R. Betti\textit{ et al.}, Phys. Rev. Lett. \textbf{98}, 155001 (2007).} is being pursued as a viable option to achieve ignition on the National Ignition Facility (NIF). Shock-ignition target designs require the addition of a high-intensity ($\sim $5 $\times $ 10$^{15}$ W/cm$^{2}$) laser spike at the end of a low-adiabat assembly pulse to launch a spherically convergent strong shock to ignite the imploding capsule. Achieving ignition with SI requires the laser spike to generate an ignitor shock with a launching pressure typically in excess of $\sim $300 Mbar. At the high laser intensities required during the spike pulse, stimulated Raman (SRS) and Brillouin scattering (SBS) could reflect a significant fraction of the incident light. In addition, SRS and the two-plasmon-decay instability can accelerate hot electrons into the shell and preheat the fuel. Since the high-power spike occurs at the end of the pulse when the areal density of the shell is several tens of mg/cm$^{2}$, shock-ignition fuel layers are shielded against hot electrons with energies below 150 keV. This paper will present data for a set of OMEGA experiments that were designed to study laser--plasma interactions during the spike pulse. In addition, these experiments were used to demonstrate that high-pressure shocks can be produced in long-scale-length plasmas with SI-relevant intensities. Within the constraints imposed by the hydrodynamics of strong shock generation and the laser--plasma instabilities, target designs for SI experiments on the NIF will be presented. Two-dimensional radiation--hydrodynamic simulations of SI target designs for the NIF predict ignition in the polar-drive beam configuration at sub-MJ laser energies. Design robustness to various 1-D effects and 2-D nonuniformities has been characterized. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302. [Preview Abstract] |
Thursday, November 1, 2012 4:30PM - 5:00PM |
UI2.00006: The X-Target: A novel high gain target with single-sided heavy-ion beam illumination Invited Speaker: Enrique Henestroza A new inertial-fusion target configuration, the X-target, using one-sided heavy ion axial illumination has been explored [1]. It takes advantage of the unique energy deposition properties of heavy ion beams that have a classical, long penetration range. This class of target uses heavy ion beams to compress and ignite deuterium-tritium (DT) fuel that fills the interior of metal cases that have side-view cross sections in the shape of an ``X''. X-targets that incorporate inside the case a propellant (plastic) and a pusher (aluminum) surrounding the DT are capable of assembling fuel areal densities $\sim $2 g/cm$^{2}$ using two MJ-scale annular beams to implode quasi-spherically the target to peak DT densities $\sim $100 g/cm$^{3}$. A 3MJ fast-ignition solid ion beam heats the fuel to thermonuclear temperatures in $\sim $200 ps to start the burn propagation, obtaining gains of $\sim $300. The main concern for the X-target is the amount of high-Z atomic mixing at the ignition zone produced by hydro-instabilities, which, if large enough, could cool the fuel during the ignition process and prevent the propagation of the fusion burn. Analytic estimates and implosion calculations using the radiation hydrodynamics code HYDRA in 2D (RZ), at typical Eulerian mesh resolutions of a few microns, have shown that for the relatively low implosion velocities, low stagnation fuel densities, and low quasi-spherical fuel convergence ratios of the X-target, these hydro-instabilities do not have a large effect on the burning process. These preliminary studies need to be extended by further hydrodynamic calculations using finer resolution, complemented with turbulent mix modeling and validated by experiments, to ascertain the stability of the X-target design. We will present the current status of the X-target. \\[4pt] [1] E. Henestroza and B. G. Logan, Phys. Plasmas \textbf{19}, 072706 (2012) [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700