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Self-generated and external magnetic fields in plasmas

FSC. Self-generated and external magnetic fields in plasmas. HEDSA Symposia on High Energy Density Plasmas Atlanta, GA 1 November 2009. J. P. Knauer Laboratory for Laser Energetics University of Rochester. Summary. FSC.

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Self-generated and external magnetic fields in plasmas

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  1. FSC Self-generated and external magnetic fields in plasmas HEDSA Symposia on High Energy Density Plasmas Atlanta, GA 1 November 2009 J. P. Knauer Laboratory for Laser Energetics University of Rochester

  2. Summary FSC Self-generated and externally-generated magnetic fields are measured in OMEGA experiments • Magnetic reconstruction has been measured laser-generated fields • Magnetic fields have been observed in spherical implosions • DRACO/MHD simulations show that the moderate external magnetic field of <10 Tesla can be compressed to hundreds of Mega-Gauss at the implosion stagnation • Cylindrical targets embedded in a seed magnetic field of 10 - 60 kG have been imploded with 14 kJ of laser energy creating amplified fields of 10 – 40 MG • Magnetic fields in HED plasmas open up new fields of investigation

  3. Outline FSC Self-generated and external magnetic fields in plasmas Reconnection of Laser-Generated Magnetic Fields Self-Generated Magnetic Fields External Magnetic Fields

  4. FSC Collaborators O. Gotchev, P. Chang, N. W. Jang , O. Polomarov, R. Betti, D. D. Meyerhofer J. A. Frenjie, C. K. Li, M. Manuel, R. D. Petrasso, F. H. Seguin Laboratory for Laser Energetics Departments of Physics and Mechanical Engineering University of Rochester Plasma Science and Fusion Center Massachusetts Institute of Technology

  5. Reconnection of Laser-Generated* Magnetic Fields * C. K. Li et al., Phys. Rev. Lett. 99 055001 (2007)

  6. 5mm 0.31 ns 0.51 ns 0 .69 ns 0.97 ns 1.24 ns 1.72 ns 2.35 ns FSC Magnetic reconnection has been observed and quantified 5 mm Bdℓ (MG-µm) Bdℓ (MG-µm) > 95% field strength was reduced in the region where bubbles overlap 0.04 ns 0.67 ns 1.42 ns C. K. Li et al., Phys. Rev. Lett. 99 055001 (2007)

  7. Since hydro dominated, characteristic times of this reconnection differ from “standard” experiments Reconnect ~ expansion ~ L / Cs ~ 0.2 ns SP ~ (resist Alfven )1/2 ~ 5 ns (Sweet-Parker) Where: Alfven ~ L/ vA ~ 1 ns resist ~ L2 /DB ~ 30 ns As a consequence that β ~ 100, reconnection energy ~ 0.01 nkT, currently immeasurable FSC The topology is dominated by hydrodynamics and isn’t strongly affected by fields, even though MG fields are present.

  8. FSC Reconnection energy has little impact on the dynamics of the interacting bubbles for such high- plasma • Field energy  plasma internal energy in the reconnection region • ER = (8LB2)-1∫  Bdℓ2 dV ~ 2.5102 J cm-3 • Where LB= B/B • Taking ne around the bubble edge to be ~ 1-10% of the (nc ~ 1022 cm-3),  Te  1-10 eV A small and presently immeasurable fraction ( 1%) of Te (~ 1 keV).

  9. Self-Generated Magnetic Fields* * J. R. Rygg et al, Science (2008)

  10. FSC The MIT proton radiography experiments measure EM fields generation during ICF implosions J. R. Rygg et al, Science (2008)

  11. FSC Self-generated magnetic fields for non-uniformly irradiated laser implosions in MHD framework Main mechanisms • Grad N x Grad T as a source. • Hot spot amplification (non-linear) due to • Tidman instability (linear) due to • RT instability. • Converging shock front instability or corrugation.

  12. FSC Magnetic fields are calculated to be in the corona Isotropic TT B~0.02MG Anisotropic TT B~0.5MG Tele Tele Corona Edge Shell

  13. FSC Magnetic field persist into the compressed target Isotropic TT B~0.2MG Anisotropic TT B~5MG Tele Tele Shell Shock front

  14. External Magnetic Fields* * O. Gotchev et al, to be published in Physical Review Letters

  15. FSC The performance of ICF targets can be improved by MG magnetic fields Yn ~ h2 <sv> <sv> ~ 1/T ½ e-a/T for constant Phs h ~ 1/T NIF 1.5 MJ,direct-drive point design ρhs 30g/cc, Ths 7keV (before ignition), rhs 50µm k^ / k|| ~ 0.2 for B = 10 MG rL=27m ~1/2 rhs for B = 100 MG Bhs rhs

  16. FSC MIFEDS provides in-target seed fields between 10 and 150 kG depending on coil geometry and energy settings Faraday rotation measurements of seed field MIFEDS TIM 6 MIFEDS Laser MIFEDS is a compact, self-contained system, that stores less than 100 J and is powered by 24 VDC. It delivers ~110kA peak current in a 350 ns pulse

  17. FSC Coil geometry and placement of the cylindrical target have been optimized for OMEGA implosions Coil geometry Radius = 2 mm Separation = 5.25 mm Cylindrical target Radius = 430 mm Length = 1.5 mm Wall thickness = 20 mm Fill = 9 atm D2 B B Cylindrical tube Coil Coil Contours of |B| Cylindrical implosion target is positioned in a uniform field region between the coils

  18. D2 FSC High magnetic fields are generated through laser compression of a seed field1 In a cylindrical target, an axial field can be generated using Helmholtz like coils. The target is imploded by a laser to compress the field =BzR2const

  19. FSC Reversing the polarity of the seed field reverses the deflection of the proton probe Standard polarity seed field Reversed polarity seed field B0~ -6.2T The minimum, average magnetic field matching this deflection is 40 MG The minimum, average magnetic field matching this deflection is 30 MG

  20. FSC 1D-MHD simulations show a Tion with magnetic field ~ 2X Tion without magnetic field Density and Temperature at stagnation B-field and plasma beta 100 B = 60 kG B = 0 kG  80 10 60 B (MG) B 40  20 1 0 10 20 0 5 15 r (mm) B-field compressed to ~100 MG at the hot spot center The plasma beta is ~ 1 where the magnetic field peaks

  21. FSC I0 B0 Spherical implosions will be used to probe the effect of magnetic fields > 10 MG on fusion yield Spherical target inserted into a two coil axial magnetic field Spherical target with an inserted with for an azitmuthal magnetic field

  22. FSC Magnetic fields may play a significant role in the collimation of astrophysical jets OMEGA jet Hubble Space Telescope images OMEGA laboratory jets have cocoon pressures of the order of 30 kBar equal to the magnetic pressure of a 0.8 MG field

  23. FSC B The applications of laser driven flux compression go beyond ICF B=0 • Guiding fields for hot electrons in fast ignition. • Generation of positron-electron plasma in the laboratory1. • Propagation of plasma jets in large scale magnetic field. OMEGA EP beam B=10 MG Compressed field Wire target e- e+ 500 μm 1500 μm OMEGA EP beam OMEGA beams 1J. Myatt et al., Bull. Am. Phys. Soc. 51 (7), 25 (2006)

  24. Summary/conclusions FSC Self-generated and externally-generated magnetic fields are measured in OMEGA experiments • Magnetic reconstruction has been measured laser-generated fields • Magnetic fields have been observed in spherical implosions • DRACO/MHD simulations show that the moderate external magnetic field of <10 Tesla can be compressed to hundreds of Mega-Gauss at the implosion stagnation • Cylindrical targets embedded in a seed magnetic field of 10 - 60 kG have been imploded with 14 kJ of laser energy creating amplified fields of 10 – 40 MG • Magnetic fields in HED plasmas open up new fields of investigation

  25. FSC Proton deflectometry is used to measure the magnetic field in the compressed core p Proton backlighter D B Hot spot D + 3He → 4He + p (14.7 MeV) p L  Cylindrical target Initial seed field of B < 90 kG CR-39 Detector GEANT4 simulations are used for an accurate interpretation of the data

  26. FSC The protons with the largest deflection probethe highest B-field region in the target hot spot p p p SIMULATION Dense shell B Hot spot Detector plate Protons that travel through the hot spot loose less energy that the protons that only travel through the dense shell

  27. FSC 2-D simulations of spherical implosions show higher ion temperatures with a magnetic field

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