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Magnetic Collimation of Fast Electrons using Structured Targets

Magnetic Collimation of Fast Electrons using Structured Targets. A.P.L.Robinson, M.Sherlock, P.A.Norreys (Central Laser Facility, STFC RAL) M.Zepf, and S.Kar. (Queen’s University, Belfast). Presentation at 35 th EPS Plasma Physics Conference. Why Collimate Fast Electron Beams?. > 200keV.

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Magnetic Collimation of Fast Electrons using Structured Targets

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  1. Magnetic Collimation of Fast Electrons using Structured Targets A.P.L.Robinson, M.Sherlock, P.A.Norreys (Central Laser Facility, STFC RAL) M.Zepf, and S.Kar. (Queen’s University, Belfast) Presentation at 35th EPS Plasma Physics Conference

  2. Why Collimate Fast Electron Beams? > 200keV I > 1018 Wcm-2 • Put as much flux/energy as possible into a small area/volume. • Fast Ignition Inertial Confinement Fusion • Proton/Ion Acceleration • X-ray backlighter • Heating solids to high temperatures.

  3. Magnetic Collimation Current Balance • Collimation can result from resistive generation of magnetic field. B-field r LASER jf + jc ≈ 0 z Electric Field E = -ηjf + Magnetic Diffusion + other terms

  4. Problems with ‘Natural’ Collimation Field too weak to bend electrons around. Electrons too divergent. Low resistivity • Collimation does not necessarily occur. • See Bell & Kingham, Phys.Rev.Lett., 035003 (2003) • Many experiments indicate that fast electron flows are not strongly collimated. • e.g. Lancaster et al., Phys.Rev.Lett., 98, 125002 (2007) TOO HOT!

  5. Strucured Collimator Concept • Enhance generation of collimating magnetic field by structuring the target resistivity, i.e. by using different Z materials. Electric Field E = -ηjf Published in Phys.Plasmas, 14, 083105

  6. Structured Collimator Concept E = -ηjf dB/dt = -curl E B-fields initiate collimation Fast electron spray Net Curl of E-field

  7. Analytic Model • Use a “Rigid Beam” model. • Resistivity gradient builds field. • Sufficient to deflect fast • electrons. “Rigid Beam” = Static, Specified jf

  8. LEDA simulations LEDA is a 2D hybrid Vlasov-Fokker-Planck code. Fast Electrons (VFP, KALOS) Background (hybrid) Milchberg resitivity, Thomas-Fermi Model for s.h.c. Fields (hybrid)

  9. Target Set-up Use Al fibre with Li cladding. One laser pulse (5 x 1019Wcm-2;1ps). This shows the Target Z, i.e the ion charge. Z=13 regions are Al, and Z=3 regions are Li.

  10. A Typical Run I = 5 x 1019Wcm-2 (1 micron wavelength). Fast electron Divergence half-angle of 340. Al target initially at 200eV.

  11. Comparison Same laser conditions. Fast electron density profiles at 1ps. Homogeneous Al target Structured Collimator

  12. B-field in this run

  13. Magnetic Field Growth Greatly helped by positive feedback

  14. Fibre Width

  15. Cold Target Effects Examined Simulations carried out for 1eV start.

  16. Works in Reverse Too…

  17. 3D Struc.Coll.s 1. Slab Geometry Al Sn No enhancement to confinement parallel to slab. 2. Cylindrical Geometry Simulate using a 3D particle-hybrid code.

  18. 3D simulation of slab confinement Al-Sn-Al target Pure Al target x-y midplane plot of fast electron density

  19. 3D Transport Patterns 3x1026m-3 fast electron density isosurface(s) Al-Sn-Al Fan Pattern Magnetic Guiding Pure Al Conical Spray Pattern Ballistic Transport

  20. Bz field in 3D Bz grows at material interface creating a “magnetic wall” to confine fast electrons.

  21. Zepf-Kar Experiment Sn (~10 um wide) Al Al 532 nm 700 nm Total signal is ~ twice the reference Total signal is ~ twice the reference slide courtesy of S.Kar

  22. 3D Wire Confinement Al with 40 micron Fe wire Al only 1019 Wcm-2 500fs pulses. Fast electron Density plots at 1.5ps.

  23. Summary • Structured Collimator: Simple concept that exploits the induction equation at a basic level. • Positive Feedback: Once collimation is initiated it helps itself. • Cold Target Effects: May not be a significant problem. • Geometry and Materials: These are important considerations, but there is flexibility. • Experimental Realization: Results from Zepf, Kar, and co-workers suggest that this has worked in the slab geometry.

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