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UNIVERSITY OF MARYLAND AT COLLEGE PARK

UNIVERSITY OF MARYLAND AT COLLEGE PARK. High-intensity optical slow-wave structure for direct laser electron acceleration. H.M. Milchberg, B.D. Layer, A. York, J. Palastro, T, Antonsen University of Maryland, College Park. HEDSA 2009. Conventional accelerators. high energy physics.

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UNIVERSITY OF MARYLAND AT COLLEGE PARK

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  1. UNIVERSITY OF MARYLAND AT COLLEGE PARK High-intensity optical slow-wave structure for direct laser electron acceleration H.M. Milchberg, B.D. Layer, A. York, J. Palastro, T, Antonsen University of Maryland, College Park HEDSA 2009

  2. Conventional accelerators high energy physics 27 km circumference LEP (CERN) (100 GeV) SLAC (50 GeV) 3 km R > Rminsynchrotron radiation loss Eaccel<106-7 V/m structure breakdown constraints:

  3. accelerator waveguide structure internal breakdown (lightening!) and self-destruction if wave fields are greater than ~ 107 Volts/m ‘slow-wave’ structure wave phase velocity < c Etransverse Solution: use ‘milder’ fields over longer distance EM propagation & particle accel. Ez view from space Btransverse 50 GeV in 3.2 km 50 GeV/(1.7x107 V/m) ~ 2 miles The SLAC structure is periodically modulated

  4. ‘conventional’ laser-plasma wakefields: intense laser pulse enters gas jet and relativistic electron beam emerges relativistic electron beam relativistic electron spectrometer 150 m Plasma oscillation: “wake-field” pulse speed is vg < c - + - + - + Laser pond. force for >1018 W/cm2 pushes electrons out of the way E E E E E

  5. (e) (e) Axially modulated plasma waveguide Radially modulated 100ps Nd:YAG laser pulse 13 µm 13µm (a) 35 fs Ti:Sapphire laser pulse 200µm (b) Axicon 200 50 fs transverse interferometer probe 300 µm r (µm) (c) 50µm (d) -200 50µm ~35 μm 0 1000 z (µm) 35µm 35 µm But can we imitate SLAC using a plasma? YES! 

  6. experimental electron density profiles after pulse: Plasma cross-section during and immediately after pulse: 104 bar pressure blast wave expansion “hollows” the Neprofile 0 25 radius (m) A hollow electron density profile acts as a focusing element Ne(r) lower in middle results in index n larger there plasma index of refraction focusing Principle of plasma waveguide: example of hydrodynamic shock generation

  7. Charged particle dephasing Phase mismatch vparticle < vwave phase vpump ≠ vgenerated electron Ez Epump z-vphaset z-vpumpt vphase>c kLcoherence= Lcoherence ‘Slow wave’ structure quasi-phase matching Particle acceleration EM wave generation

  8. r z d Bloch-Floquet condition: where where Wave number of mth axial harmonic mth harmonic is ‘slow’ if Slow wave picture

  9. Electron energy gain For the ‘matched’ case get Electron acceleration: slow wave picture

  10. Example: density modulation Mod periodd=L1+L2 n1 > n2 Ld1 Ld2 Accelerating region: low plasma density (high index) Decelerating region: high plasma density (low index) Energy gain per period: where Quasi-phase matching picture The driving wave speeds up and slows down in successive portions of the modulation so that the acceleration in the first part is not completely cancelled by deceleration in the second part.

  11. Outline • reminder about clusters • -heating and plasma formation with femtosecond pulses (PRLs <2005) • -heating and plasma formation with long (many picosecond) pulses • formation of axially modulated (corrugated) plasma fibres using long pulses • - axially modulated heating pulse • - tailored cluster flow • direct laser acceleration

  12. EUV spectrum* Clusters are essential! Clusters few Å ~ 500 Å ~10-107 atoms—explode in < 1 ps TOF mass spectrum† X-ray signal* >90% laser absorption Energetic electrons/ions Neutrons X-rays Cluster jet Laser pulse Scattering EUV X-rays: A. McPherson et al., PRL 72, 1810 (1994). EUV and x-rays: *E. Parra et al., PRE 62, R5931 (2000). Optical properties: Kim, Alexeev, Milchberg, PRLs 2003, 2005 Fast electrons and ions: Y. L. Shao et al., PRL 77, 3343 (1996); † V. Kumarappan et al., PRL 87, 085005 (2001). Nuclear fusion: T. Ditmire et al., Nature (London) 398, 489 (1999).

  13.                                                                                                                                                                                                         Critical density layer                                                                                                                                                                       Super-critical plasma                         a                                                                                                                                                                                                                                            50 Å ~ 600 Å Single Ar cluster            Sub-critical plasma High Z, cool, under-dense plasma Why do 100ps pulses efficiently heat clusters? H. Sheng et al, Phys. Rev. E 72, 036411 (2005) • The far leading edge of the 100ps beam disassembles / ionizes the clusters, leaving a cool high Z plasma that the remainder of the pulse heats. • Much more efficient than heating an unclustered gas (for same average Z in a plasma, up to 10x less pump energy required) -40-50% absorption

  14. Cryogenic cluster jet • enhanced absorption, even for very long (100ps) pulses • because absorption is local to a cluster, can ultimately form plasma channels with Ne ~ 1018 cm3 electron density* and lower • efficiently makes plasma channels in anything that decently clusters • Typically 10X more efficient than for equivalent vol. average pressures of unclustered gas 2 cm Controlled cryogenic cooling of the jet enhances clustering

  15. First modulation method- modulated Bessel beam and uniform cluster flow 100-300mj 100ps Nd:YAG pulse, axially modulated with diffractive optics, incident on unmodulated cluster jet flows Ex. ~2mm corrugation period 1.5 cm 1.5cm Breakdown in atmosphere Breakdown in Argon clusters

  16. 15µm (i) (b) (ii) (iii) 1017 W/cm2 500 µm 700 µm 200 mJ 300 mJ 500 mJ + misalign. Waveguide generation pulse energy and alignment controls modulation features Guiding in corrugated hydrogen plasma channels • H2 jet cryogenically cooled to enhance clustering • Electron densities of~1.5*1018 cm-3on axisand ~3*1018 cm-3 at channel wall for a delay of 1ns

  17. 1 mm 700µm Extended high intensity guiding beads continuous No injection No injection injection injection Pump scattering Pump scattering Abel inversion Abel inversion

  18. 1018cm-3 8 6 4 2 Extended high intensity guiding 660µm without injection injection, 2x1017 W/cm2 at exit laser 3 mm

  19. 1.0 1018 W/cm2 0.2 Propagation simulation using the code WAKE* Simulation using experimental density profiles Attenuation from leakage at gaps * P. Mora and T. M. Antonsen Jr., Phys. Plasmas 4, 217 (1997).

  20. Second method: wire-tailored cluster flow, unmodulated laser pulse uniform 500mj 100ps Nd:YAG pulse incident on axially modulated Argon cluster target 1mm corrugation period 1.5 cm

  21. Features persist for the full life of the waveguide B.D. Layer et. al, Opt Express 17, 4263 (2009) Nitrogen cluster target @ -150 deg C, 25 m wires Argon cluster target @ 22 deg C, 25 m wires 0.5 ns 0.5 ns 1.0 ns 1.0 ns 2.0 ns 2.0 ns 320 μm 160 μm 6.0 ns 6.0 ns 600 μm 600 μm (200 consecutive shot averages)

  22. Direct laser acceleration- inverse Cherenkov acceleration (ICA) 580-MW peak power  31 MeV/m. 10 TW peak powers are now routine, but the need for neutral-gas phase matching strongly limits peak intensities.

  23. axicon Nd:YAG laser pulse Diffractive optic Radially polarized fs laser pulse Clustered H2 jet Relativistic electron bunch Corrugated plasma waveguide

  24. This is a linear process with no threshold. 1 mJ regenerative amplifier alone P = 20GW  Effective accel. gradient: 11 MV/cm Corrugated guide: simple estimates of dephasing lengths and acceleration gradients Estimate acceleration gradients using index modulation: One full dephasing cycle n1 > n2 Decelerating-phase region: high index Accelerating-phase region: low index } λ = 800nm Ne1 = 3*1018 cm-3 Ne2 = 6*1018 cm-3 wch = 12μm p = 1, m = 0 For P = 1 TW, Ez =0.55 GV/cm, giving an effective gradient of 77 MV/cm Ld1= ~260 μm Ld2=~165 μm Wakefield comparison: Malka et al. used a 30 TW laser at λ = 0.8 μm to produce an acceleration gradient of ~0.66 GV/cm

  25. Direct laser acceleration- energy gain • electrons distributed uniformly on axis 1to 11 m behind pulse peak • no transverse momentum m=1 phase velocity matched to initial electron velocity m=1 phase velocity set to c 400 400 o=1000 o=1000 Ideal scaling Ideal scaling   o=100 o=100 o=30 o=30 0 30 60 0 30 60 time (ps) time (ps) it is better when electrons catch up with a faster wave than to start them phase matched to a slower wave

  26. Comparing direct accel to other schemes for direct accel we have: parameters used for comparison: =800 nm wch=15 m ao=.25 no=7x1018 cm-3 =.9 m=.035 cm o=100 z=300 fs*c  = 1000 vacuum beat wave acceleration:  = 8.3 (1=22) semi-infinite vacuum acceleration: laser wakefield acceleration: (best case scenario)  = 12.5  = 14.3

  27. Electron Beam Density final electron density 1 81m • density peaks off axis; beam has • acquired sizeable transverse spread xf num. (a.u.) • off-axis peaks mostly composed of low • energy electrons -81m 0 number averaged final momentum 300 81m • high energy electrons remain • confined to center of beam xf pz (mec) only the ponderomotive transverse force is significant for these electrons -81m 0 zf -1 m -11 m

  28. Summary • Can make modulated plasma waveguides with two distinct methods- modulating either the laser heating profile or the clustered target flow • Can control nearly every aspect of the waveguide by varying cluster parameters and pump laser intensity • Gas cluster channels can be more than 10X less dense than unclustered gas channels (1017’s-1018 ’s vs. 1019 ’s) and use 10X less laser energy for generation- • Cluster-generated plasma waveguides are extremely stable (longitudinal AND transverse) and can support finely engineered structures. • One application: • Direct laser accelerator optical-frequency LINAC with no damage threshold

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