1 / 28

University of Rochester Fusion Science Center

University of Rochester Fusion Science Center. Review of electron beam divergence for Fast Ignition. LLNL Livermore, Ca. August 4 th to 6 th 2010. Michael Storm The Ohio State University. Outline. Introduction. Principle Diagnostic Techniques. Additional Experimental Concerns Results.

Gabriel
Download Presentation

University of Rochester Fusion Science Center

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. University of Rochester FusionScience Center Review of electron beam divergence for Fast Ignition LLNL Livermore, Ca. August 4th to 6th 2010 Michael Storm The Ohio State University

  2. Outline • Introduction. • Principle Diagnostic Techniques. • Additional Experimental Concerns Results. • Summary.

  3. Optimizing the laser-to-electron conversion efficiency, mean electron energy and electron directionality are essential for the viability of FI • Experiments and calculations indicate e = 10 to 50%. • The energy spectrum is approximately Maxwellian with, at I1019 Wcm-2, <E> ~1 MeV. • The electrons must propagate ~50 to ~100 µm along a density gradient that rises from 1021 to 1026 cm-3 to a region of radius ~20 µm in ~20 ps. • For a fuel density of  = 300 gcm-3 and an exponential electron energy distribution with <E> = 1MeV, a collimated electron beam carrying ~27 kJ1 must deposit all its energy. For e 35%, this implies a PW laser energy of 77 kJ • Atzeni, Phys Plasmas 15 056311 (2008)

  4. The PW laser energy requirements increasesignificantly as the electron beam diverges Assuming an initial 20 µm radius solid-beam of uniformly distributed electrons Propagation distance 77 kJ

  5. In experiments the electron beam divergenceis associated with a cone angle • The angle is obtained from the ratio of the measured transverse spatial distribution of some emission and the emission depth. • The angle can be characterized in numerous ways: • Half angle or full angle. • Containment fraction. • The full, half or some width of a fitted curve. • For a series of shots there are the maximum, mean, rms… angles. • These definitions of divergence are used in numerical calculations. A consensus on how to define and report the divergence is needed.

  6. The laser pulse peak intensity, leading edge and farfield distribution need careful characterization • FWHM, Peak Intensity, energy containment fractions… are commonly used to describe the intensity. • Experiments and numerical calculations suggest a connection between electron directionality/divergence and the laser intensity/leading edge profile. Properly determining the laser pulse parameters and establishing commonality in reporting them at different facilities is desirable.

  7. Principle Diagnostic Techniques • Optical probing inside transparent targets. • Optical probing of the target surface blow-off plasma. • Thermal imaging of the target rear surface. • High energy bremsstrahlung angular distribution. • Kα x-ray imaging of buried layers. • Coherent transition radiation. • Incoherent transition radiation.

  8. Inside probe. Surface probe. Thermal imaging. Bremsstrahlung. Kα imaging. CTR. ITR. Side-on optical probing shows collimated jet-like structuresoriginating from the laser interaction region EL10J, L 350fs1 Ionization channels 100’s µm long with 20 µm diameters indicate electrons (total energy < 0.1% Elaser) propagate along the direction of the laser at a velocity close to c1. Slower electrons with v  0.53c  Ee = (-1)mc2  93 keV expand isotropically1. • Gremillet et al, PRL 83 5015 (1999)

  9. Inside probe. Surface probe. Thermal imaging. Bremsstrahlung. Kα imaging. CTR. ITR. Optical probing reveals the transverse size of therear-surface fast-electron-generated plasma1 • Other studies of the rear surface plasma suggest a 1o focusing of the beam2. 75 µm Cu 25 µm Cu 50 µm Cu EL250J, L 450fs, IL5x1020Wcm-2,½  38o (after 200 ps) • Lancaster et al, PRL 98 125002 (2007) • Tatarakis et al, PRL 81 999 (1998)

  10. Inside probe. Surface probe. Thermal imaging. Bremsstrahlung. Kα imaging. CTR. ITR. Thermal radiation is associated with fast electrons reaching the target rear surface • Based on the size of the individual emission size, the electron beam divergence is ½  25o, 12o, 7o respectively. • Collectively from 40 µm to 500 µm, ½ 5o. • Other experiments show the rear surface emission decreasing with increasing target thickness2. UV images from Al, EL = 20J1 Rear surface emission Laser e- Target 40 µm 200 µm 500 µm 200 µm • Kodama et al, Nature 412 798 (2001) • Lancaster et al, PRL 98 125002 (2007)

  11. Inside probe. Surface probe. Thermal imaging. Bremsstrahlung. Kα imaging. CTR. ITR. The distribution of thermal radiation is influenced by refluxing, penetration depth, surface propagation effects and temperature • The transverse size decreases in thicker targets because the electrons fail to penetrate. • ExB drift along the rear surface contributes to the size of the thermal emission. • Higher frequency emission terminates sooner. 2mm x 2.5mm x 40µm Cu/Al1 400 x 360 x 40µm3 Cu/Al1 Refluxing1 Thermal emission cannot reliably determine the electron beam divergence. • Nakatsutsumi et al, IFSA 2007 112 022063 (2008) • Lancaster et al, PRL 98 125002 (2007) • Forslund et al, PRL 48 1614 (1984)

  12. Inside probe. Surface probe. Thermal imaging. Bremsstrahlung. Kα imaging. CTR. ITR. Nuclear activation by high-energy bremsstrahlung photonsdiagnoses the divergence of the most energetic electrons • Activation typically requires photon energies ≥ 10 MeV. • The bremsstrahlung opening angle is ½~1/ so forEe = 10 MeV, ½ ~2.7o. • Magnetic fields broaden the bremsstrahlung distribution by perturbing the electron trajectories (Calculation : 20 MeV collimated electrons reproduce ½ =19o distribution)1. Activated atom fraction X-ray > 200 keV (TLD) EL 600J, IL 6x1020 Wcm-2 indicates ½ 50o[2] TLDs are sensitive to photons > 200 keV2 The beam directionality was seen to vary by ± 35o[2]. • Zepf et al, Phys Plasmas 8 2323 (2001) • Hatchett et al, Phys Plasmas 7 2076 (2000)

  13. Inside probe. Surface probe. Thermal imaging. Bremsstrahlung. Kα imaging. CTR. ITR. Kα radiation imaging measures the electrondivergence using buried fluorescent layers Fluor 20µm Cu in 130µm Al1 Cu Kα Ti Kα (inset) Slowing down Linear fit Monte Carlo Laser e- x Propagation layer The Kα spot size remains constant over the first 100 µm after which it diverges as ½  20o[1] • Stephens et al, PRE 69 066414 (2004)

  14. Inside probe. Surface probe. Thermal imaging. Bremsstrahlung. Kα imaging. CTR. ITR. K α imaging is a leading candidate for correctlydetermining the electron divergence • The Kα emission indicates the location of electrons whose energy is above the threshold for the process. • In thin targets refluxing smears the desired image. • Numerical calculations are needed to extract the spatial distribution of first-pass electrons from the spatial distribution of Kα. • Higher energy Kα is desirable (Ag). • The effect of the impedance mismatch needs to be quantified experimentally1. Resistive interface between materials leads to magnetic field generation: dB = ∫(xJ) dt Laser e- 1 2 1 3 Electron slowing down region Propagation layer Electrons with diverging trajectories are perturbed or trapped at the interface. • Davies et al, PRE 58 2471 (1998)

  15. Inside probe. Surface probe. Thermal imaging. Bremsstrahlung. Kα imaging. CTR. ITR. Coherent transition radiation (CTR) diagnoses thedivergence in the absence of refluxing • Refluxing reduces the correlation between propagating electrons so that electrons that return to the rear surface no longer generate CTR. x104 ½  16o CTR 50 2 Laser e- 25 x (m) 1 0 0 50 25 0 y (m) CTR from 30 µm Au foil irradiated with EL5J, IL2x1019Wcm-2 • Storm et al, PRL 102 235004 (2009)

  16. Inside probe. Surface probe. Thermal imaging. Bremsstrahlung. Kα imaging. CTR. ITR. CTR generating electrons account for only a fraction(0.1%)1 of the electrons that were accelerated by the laser • The CTR emission duration is 50 fs for a 400 fs long laser pulse1. • The CTR signal strength has a dependence on target material, suggesting scattering is important, but in the divergence which should be influenced by scattering is independent of target material2. • The CTR signal is brighter than competing emission processes. • Due to velocity dispersion, the CTR generating electron cutoff energy is ~ 1MeV. The reliability of the CTR technique to identify divergence should be determined • Baton et al, PRL 91 105001 (2003) • Storm et al, PRL 102 235004 (2009)

  17. Inside probe. Surface probe. Thermal imaging. Bremsstrahlung. Kα imaging. CTR. ITR. Incoherent transition radiation (ITR) diagnoses thedivergence of all electrons that reach the rear surface • Experiments using Al foils with EL10J, IL1x1019Wcm-2. ITR Time resolved images of the rear surface emission Laser e- ½  17o • Santos et al, PRL 89 025001 (2002)

  18. Inside probe. Surface probe. Thermal imaging. Bremsstrahlung. Kα imaging. CTR. ITR. ITR imaging requires a high resolution temporal gate • ITR radiation is characterized by a prompt bright emission of duration a few times longer than the laser pulse duration. • In thin targets the ITR signal will be smeared by refluxing. • Refluxed electrons are less energetic and more diffuse than the electrons during their first pass through the target. • High-resolution, time-resolved imaging of the ITR could be used to benchmark the CTR emission. ITR 35 µm Aluminum target1 • Santos et al, PRL 89 025001 (2002)

  19. Divergence versus Diagnostic Diagnostic 20o  280 kJ and 611 kJ PW for 50 and 100 µm propagation respectively ½ (degrees)

  20. Additional concerns and experimental results • The laser pulse leading: • Displacement and shocks • Double pulse • Pre-plasma • Compressed matter • Resistive Channels

  21. Pre-pulse -Shocks. -Double pulse. -Pre-plasma. Compression. Resistive Channel The laser pulse leading edge leads to targetexpansion, heating and pre-plasma generation • The laser pulse peak interacts with a non-zero scale-length plasma • Self focusing and filamentation modify the laser intensity and focal spot distribution. • Shocks heat, compress and displace the bulk target material1. Density Map Density profiles z = 0 g/cc 8 5.24 7 4.19 Rear surface stable at t0 for thicker targets. 6 CHIVAS 1D Hydro1 5 3.14 time (ns) 4 40 µm Al 2.09 3 t0 LASER 2 1.05 1 Front surface initially at z = 0. 0 0 -50 0 50 z (µm) • Santos et al, Phys. Plasmas 14 103107 (2007)

  22. Pre-pulse -Shocks. -Double pulse. -Pre-plasma. Compression. Resistive Channel Accounting for target overdense thicknesschanges the inferred value of divergence Thermal radiation Thermal radiation CTR CTR Inferred angle is larger when considering the calculated over dense thickness. • Santos et al, Phys. Plasmas 14 103107 (2007)

  23. Pre-pulse -Shocks. -Double pulse. -Pre-plasma. Compression. Resistive Channel Pre-plasma effects on the divergencewere diagnosed using activation • Ta targets were irradiated at 45o. • The pre-plasma scale-length was varied. JxB 18o Large plasma Small plasma 28o Laser filamentation/hosing… Two e- beams overlaps Vacuum Two e- beams • The scale length determines the dominant laser absorption mechanism. • Rippling of the critical surface2 or self-generated fields can seed the directionality3. • Santala et al, PRL 84 1459 (2000) • Lasinski et al, Phys. Plasma 6 2041 (1999) • Ren et al, PRL 93 185004 (2004)

  24. Pre-pulse -Shocks. -Double pulse. -Pre-plasma. Compression. Resistive Channel Electron beam divergence in cylindricallycompressed matter has been measured1 • Cu doped foam or CH filled cylinders are imploded. • Divergence increases or decreases with compression evolution depending on the initial density. 200 µm Penetration decreases with increasing delay Cu plate Ni plate Delay • Reduced penetration, resistive confinement and shell truncation may explain the decreasing emission size with delay for densities that are initially low • Perez et al, Plasma Physics and Controlled Fusion 51 124035 (2009)

  25. Pre-pulse -Shocks. -Double pulse. -Pre-plasma. Compression. Resistive Channel Double pulse experiments attempt to demonstrate divergence control • The lower-intensity, pulse preheats the target to form a resistive magnetic channel. No clear reduction in the rear surface spot size was observed with Ti K • Scott et al, CLF annual report 65 (2007/2008)

  26. Pre-pulse -Shocks. -Double pulse. -Pre-plasma. Compression. Resistive Channel. Recent studies using resistive channels to seedmagnetic guiding show promising results1,2 • The targets use a high resistivity core and a low resistivity cladding. • The sign of the resistive gradient should be maintained during heating. CTR1 25 µm or 50 µm Fe 250 µm Al X-ray pinhole HOPG X-ray imager Guiding = full symbols Foil = open symbols • Kar et al, PRL 102 055001 (2009) • Ramakrishma (to be published) (2010)

  27. Summary Analysis of the previous work suggestsappreciable electron divergence • No specific angle or “narrow” range of angles is evident. • Access to previous raw data and shot sheets would allow for a comprehensive and consistent assessment of the previous work. • A common way to characterize the laser pulse is needed. • A common way to characterize divergence is necessary. • It necessary to determine which diagnostics are reliable. • Conduct concentrated experimental campaigns.

  28. Acknowledgements Dimitri Batani Tony Bell Claudio Bellei Riccardo Betti Jonathon Davies Roger Evans Richard Freeman Laurent Gremillet David Meyerhofer Christopher RidgersMark Sherlock Andrey Solodov Richard Stephens Douglas Wertepny Linn Van Woerkom Sentoku Yasuhiko

More Related