High-brightness picosecond ion beam source based on BNL TW CO 2 laser:

1. High-brightness picosecond ion beam source based on BNL TW CO2 laser: Proof-of-principle experiments Peter Shkolnikov, Stony Brook University Igor Pogorelsky, Vitaly Yakimenko, Igor Pavlishin, Marcus Babzien, ATF BNL Theoretical support Alexander Pukhov, Heinrich-Heine-Universitat Dusseldorf , Germany Alexei Zhidkov, National Institute of Radiological Sciences, Japan Experimental technical help Karl Kusche, Aron Korostyshevski, Donald Davis, and Todd Corwin, ATF BNL Acknowledgements: Louise Willingale(Imperial Colledge, UK), andKirk Flippo(LANL) for sharing expertise and materials Julien Fuchs(LULI, France), Paul Gibbon(Forschungszentrum Jülich GmbH, Germany), andOleg Semyonov (SBU) for fruitful discussions

2. Background The effect Thin foils irradiated by sub-picosecond laser pulses at relativistic intensities emit ions/protons at MeV energies in bright, well-collimated beams. The current status of the field Ultra-fast solid state lasers, at λ ~ 1 μm ; usually national facilities. Protons and ions at 25-100 MeV at intensities 2 orders of magnitude > those at conventional accelerators. We propose To build and study a high-brightness multi-MeV ion and proton beam source driven by the unique picosecond TWCO2 laser developed at the ATF. Applications: accelerators; radiotherapy; nuclear research

3. Physics of ion acceleration by a laser in a foil target: TNSA Stage 1. On the target surface, the laser beam creates plasma with relativistic electrons moving into the target • Main source of electrons • – area of sub-critical density, ne ~ < ncrit ~ π/(reλ2) ncrit ~ 1019 cm-3 for λ=10μm • Needed: • laser intensity is sufficiently high (~ relativistic ponderomotive energy) ; • pulse is short and rises rapidly (electrons accelerate before the target disintegrates) • prepulse weak (target cold and borders well-defined before the main pulse arrives) The number of electrons accelerated into the target Ne =f W/Thot , f = 1.2 × 10−15 (Iλ2)0.74 , W – energy in the pulse, Temperature of the fast electrons is ~ the laser ponderomotive potential Thot ~ Tp = mec2[(1+ Iλ2 /1.37×1018)1/2 −1]; Iis the laser intensity in W/cm2 , λ is the laser wavelength in μm Tp = 0.5 MeV for I=4x 1018 cm-2

4. Physics of ion acceleration by a laser in a foil target: TNSA cτlaser Stage 2. Relativistic electrons propagate through the target and form a dense cloud behind it Ssheath Ne electrons are accelerated over the laser pulse duration, τlaser, and spread over the surface of the sheath Ssheath; Electron cloud density:ne0 =Ne/(cτlaserSsheath) Ssheath = π(r0 + d ×tanθ)2,  depends on the half-angle divergence θ~25◦of hot electrons, the target thickness dand the initial radius r0 of the accelerated electron “beam. • Longer pulses, if appropriate, are better • Tighter focusing is beneficial • Thinner targets, if appropriate, are preferable

5. Physics of ion acceleration by a laser in a foil target: TNSA - Stage 3. Electron cloud and the positively charged target create electric field, which ionizes atoms of the target back surface and pulls ions (protons) out, accelerating them + The maximum (cutoff) energy of accelerated ions Emax =2Thot [ln(tp+(tp2+1)1/2)]2 , tp ~ ωpi τlaser ωpi = [(4 π Zi × e2 ×ne0) / mi]1/2 For long pulses and high electron density, ion energy can be much higher than electron energy Calculations mostly follow J. Fuchs, et al

6. TNSA with CO2 laser • Electrons are accelerated in laser plasma at the foil front to several MeV • MeV electrons shoot through the foil until delayed by the positive charge of the target • Charge separation field ~ 1012 V/m ionizes rear-surface atoms and pulled out ions TNSA with CO2 laser: potential pros and cons as compared to solid-state lasers Pros: larger wavelength and longer pulses  more electrons; relatively high ponderomotive potential Cons: larger wavelength and longer pulses  larger electron volume (lower density); target overheating • Bottom line: • significant potential advantages of the CO2 laser • need to thoroughly investigate the physics in this new regime

7. Unique potentials of ATFCO2 laser 2. Structured solid targets: H-rich microdots 1. Uniform solid targets Laser of moderate power New physics: very little is known Eprot Proton energy (MeV) vs laser power (TW). Red is for 10 μm, blue is for 1 μm Low critical plasma density: 1019 cm Gas jets as targets

8. Proof-of-principle experiments The physical foundations of the effect not in doubt; specific calculations lacking (some done by our theoretical support in preparation to this work) • Main questions to answer initially, in experiment and theory: • Wavelength scaling of TNSA parameters and output in solid targets • Acceptable, then optimal laser and setup parameters for TNSA with CO2 laser • Next stage: • Proton acceleration with micro-dot targets • Proton acceleration in a gas jet

9. Target chamber for ion acceleration experiments: Where we are now Chamber is normally under 10-3 torr. Parabolic mirror has fine alignment to control aberrations in the focus, which is imaged on IR camera with x40 magnification. Target position is adjusted with a stepper motor. Transverse target motion allows multiple shots without replacing foil. Signals on x-ray and optical detectors are recorded without RCF

11. Components of experimental setup: current and in the near future

12. Conclusion: current plans • Experimental design • 1. Improve the design of the target holder to keep the target surface • in the predetermined area of the laser beam • Increase the output power of the laser and eliminate pre-pulses • Attain tighter focusing • Develop quantitative ion beam diagnostics (Thomson spectrometer) • Introduce another laser for controlled plasma generation Personnel We have invited other groups to take part (a several-month-long visit of a researcher with equipment) A dedicated staff is needed for beam and plasma diagnostics Funding: applied for a DOE grant