Studies on proton beam generation for fast ignition-related applications

1. Studies on proton beam generation for fast ignition-related applications J. Badziak, S. Jabłoński, M. Kubkowska, P. Parys, M. Rosiński, J. Wołowski,Institute of Plasma Physics and Laser Microfusion, Warsaw, Poland A. Szydłowski The Andrzej Soltan Institute for Nuclear Studies, Świerk , Poland, P. Antici, J. Fuchs, A. Mancic and the LULI laser team LULI, Ecole Polytechnique, CNRS, CEA, UPMC; Palaiseau, France Outline • Introduction • Results of the LULI 100 TW experiment on high-current proton beam generation and PIC simulations • Conclusions

2. High-ion-energy applications • Particle and nuclear physics • Cancer therapy • Accelerator technology • Requirements for an ion beam: • high or very high ion energy • (10MeV – 10GeV or higher) • narrow ion energy spectrum • (∆Ei/Ei ~ 0.1 – 10%) • low transverse emmittance • moderate ion current and current density of an ion pulse (ii < 1kA, ji <1MA/cm2) • A recogmized method for production of • high-ion-energy beamsis Target Normal • Sheath Acceleration (TNSA) • High-ion-current applications • Fast Ignition of ICF targets • High energy-density physics • Radioisotope production for PET • Ion implantation • Requirements for an ion beam: • high or very high ion current (beam power) of an ion pulse(1kA – 1GA) • high or very high ion current density (beam intensity)of anion pulse (1MA/cm2 – 1TA/cm2 or higher) • moderate ion energy (0.1 – 10MeV) • broad ion energy spectrum is acceptable • low or moderate transverse emmittance A promising method for production of high-ion-current • beams is Skin-Layer Ponderomotive Acceleration (SLPA) • (alsoreferred to as Radiation Pressure Acceleration – RPA)

3. Comparison of SLPA and TNSA Requirements and characteristic features SLPA TNSA (forward acceleration) • a metal target, typically of LT 5 - 50mm • moderate preplasma scalelength Ln 1 - 10lL • the laser beam diameter dL– no limitation • ionsare produced and accelerated at the rear target surface, mostly along the surface normal • focusing of the ion-beam by curving therear surface is possible; • the ion density at the source is moderate: ni ~ 1019cm-3 • the ion source area Ss >>SL • an insulator target, typically of LT0.5 - 5mm • small preplasma scalelength Ln ≤2ln • dL>> Ln, lL, LT • ions are produced and accelerated in front of the target,mostly along the front surface normal • focusing of the ion beam by curving the front target surfaceis possible • the ion density at the source ishigh: • ni  1021- 1022cm-3 • the ion source area Ss»SL

4. Proton Fast Ignition PFI with SLPA proton beam • Requirements for a proton beam: • (at  300 g /cm3) • beam energy 15 – 20 kJ • p+ mean energy 3 – 5 MeV • beam intensity (7 – 10)  1019W/cm2 • pulse duration 10 – 20 ps • beam size 40 m • p+production efficiency  15% proton number Np31016 proton density np 41022 cm-3 • focusing of the proton beam by curving the front target surface • the proton source area is small (Ss~ 0.01 mm2); • only slight focusing of the proton beam is needed; • a broad proton energy spectrumis acceptable

5. The 100TW LULI Experiment Experimental set-up and basic parameters • Laser beam • pulse duration L = 0,35 ps • energy EL = 0.5 – 15 J • intensity IL = 5 × 1016 – 2 × 1019 W/cm2 • focal spot diameter dL = 10 – 50 m • prepulse intensity Ipre≤1010 – 2 × 1012 W/cm2 • wavelength L = 1.05 m or 0.53 m • Target • polystyrene (PS) of the thickness LT = 0.6 – 5 m • double-layer Au/PS of LT = 1 – 3 m • (PS covered by 0.05 – 0.2 m Au front layer) The laser-target interaction conditions approach the SLPA requirements

6. The proton beam intensity at the source and the laser-protons energy conversion efficiency as a function of the 1w laser beam intensity for moderate-energy (0.1 – 3 MeV) protons For MeV protons, the beam intensity approaches 1018 W/cm2 and the conversion efficiency is nearly 4% at 2 x 1019 W/cm2. These parameters are remarkably higher when sub-MeV protons are also taken into account

7. The proton beam intensity at the source as a function of the preplasma density gradient scale length experiment: Both 1D PIC simulations and the momentum-conservation model result in the proton beam intensity value ~1018W/cm2, in agreement (within a factor 2) with the value estimated from the measurements (assuming the SLPA model of proton acceleration)

8. A comparison of the proton beam intensity at the source and the laser-protons energy conversion efficiency for Au/PS and PS targets for protons of energy Ep< 3MeV IL= 2 x 1018 W/cm2 , lL= 1.05 mm The proton beam intensity and the conversion efficiency for Au/PS target are twice as high as those for PS target

9. A comparison of the proton energy spectrum in the high-energyrange for PS and Au/PS targets at different laser intensities of the 1w beam Au/PS target produces a proton beam of higher intensity and temperature for both moderate and high laser intensity. A high-temperature component can be seen for both laser intensities

10. A comparison of parameters of moderate-energy (1 – 3MeV) and high-energy (>3 MeV) proton beams generated by the 1w and 2w laser beam The intensity of a proton beam generated by the 2w laser beam is significantly higher than that produced by the 1w beam with the same value of ILl2, while these intensities are comparable when

11. The proton beam intensity at the source as a function of the preplasma density gradient scale length – a comparison of results of 1D PIC simulations for the 1w and 2w laser beam. ILl2 = 5 x 1018W/cm2. The intensity of a proton beam generated by the 2w laser beam is significantly higher than that produced by the 1w beam with the same value of ILl2– in agreement with the results of measurements

12. Prospect for Proton Fast Ignition with SLPA proton beams The use of AuPS target allowed a significant increase in the production efficiency (up to ~ 8%) and intensity (up~ 2x1018 W/cm2) of MeV protons at moderate laser intensity and short (subps) laser pulse duration Achieving the efficiency ~ 15 – 20% and the proton beam intensity above 1019 W/cm2, required for PFI, seems to be feasible when using longer laser pulses (5 – 10 ps) and an optimized targetand, possibly, circularly polarized laser beam

13. Conclusions • SLPA makes it possible to produce collimated dense proton beams of extremely high intensities which are significantly higher than those produced by TNSAatsimilar laser intensities. In particular, the proton beams of intensity at the source~ 1018W/cm2 can be produced at laser energy only ~ 10J and laser intensity~ 1019 W/cm2. • Parameters of SLPA-driven proton beams (intensity, mean energy etc.) aswell as the laser-protons energy conversion efficiency can be significantly improvedby using structured (eg. double-layer) targets. • Short-wavelength (2w) laser beams can produce remarkably higher proton beam intensities than long-wavelength (1w) laser beams provided that • The production of SLPA-driven MeV proton beams of intensities and powers approaching those required for fast ignition of ICF targets (i.e. Ii ~1020W/cm2 , Pi ~1PW), seems to be feasible when high-contrast multi-PW laser pulses will be available.