Lecture 9. Radiation pressure acceleration in. Dr. Ashutosh Sharma Zoltán Tibai

Transcription

1 Preparation of the concerned sectors for educational and R&D activities related to the Hungarian ELI project Ion acceleration in plasmas Lecture 9. Radiation pressure acceleration in plasmas (contd.) Dr. Ashutosh Sharma Zoltán Tibai 1

2 Contents 1. Radiation Pressure Acceleration Light Sail Regime 2. Theoretical Model 3. Results and Experiment 2

3 Light Sail Regime: Introduction In the previous lecture, hole boring RPA was applied to a thick target. Thick target i.e. much thicker than the skin layer. If the target is thin enough and has low mass, it may be accelerated as a whole up to high velocity, this is named as the Light Sail (LS) regime. (a) 3

4 Theoretical Model with 4

5 Theoretical Model (9.1) G.Marx, Nature 211, 22 (1966) J. F. L. Simmons and C.R.McInnes, Am. J. Phys. 61, 205 (1993) 5

6 Theoretical Model By using the integration variables We may obtain Using the integration variable w the Eq. (9.1) can be written as (9.2) 6

7 Theoretical Model Integration the Eq. (9.2) for limits 0 and w yields, (9.3) In Eq. (9.3), F(w) shows the fluence of the EM wave i.e. total t energy per unit surface. We may write the mirror velocity from Eq. (9.3) as, (9.4) 7

8 Theoretical Model The energy per nucleon can be then written as (9.5) Efficiency (ratio between sail energy and incident EM field energy at retarted time) can be written as, (9.6) 8

9 Theoretical Model The efficiency of the acceleration process can also be obtained by a simple argument of conservation of number of photons plus the Doppler shift of the reflected light: (b) 9

10 10 Theoretical Model

11 Theoretical Model Efficiency can be written as, Notice that 100% Efficiency in the relativistic limit. 11

12 Results: Simulation and Experiments Light Sail Target: 5 nm DLC foils Henig et al., Phys. Rev. Lett. 103, (2009) (JanUSP -Ti:Sa- LLNL) 12

13 Results: Simulation and Experiments Hole Boring Target: H gas jet Palmer et al., Phys. Rev. Lett. 106, (2011) (ATF- CO 2 -BNL) 13

14 Results: Simulation and Experiments PIC investigations of Radiation Pressure Acceleration (Light Sail regime) Condition for stability of Light Sail identified (smooth transition between hole boring and light-sail phase, GeV protons at > W/cm 2 ) Qiao et al, Phys Rev Lett 102, (2009). 14

15 Coulomb explosion in small clusters PIC investigations of Radiation Pressure Acceleration (Light Sail regime) 15 Qiao et al, Phys Rev Lett 102, (2009).

16 Results: Simulation and Experiments PIC investigations of Radiation Pressure Acceleration (Light Sail regime) Generally unstable at lower intensities, but 2-species targets lead to stabilized acceleration of lighter species already at ~10 20 W/cm 2. Qiao et al, Phys Rev Lett 105, (2010). 16

17 Results: Simulation and Experiments Qiao et al, Phys Rev Lett 105, (2010). 17

18 Results: Simulation and Experiments Recent Experiments Kar et. al., Phys Rev. Lett. 109, (2012) Possible indications of the onset of the LS regime have recently presented in an experiment performed using 800 fs, W/cm 2 high contrast (10 9 ) pulses from the VULCAN laser and very thin (1 μm) metallic targets. 18

19 Problems Q9.1. A9.1. True or False. Light Sail regime applies to the thin target, i.e. the thickness of target is comparable to the laser wavelength. False. Q9.2. Why thin target t regime of RPA is called as light sail? A9.2. Light sail regime is appropriate to refer to a thin target of finite inertia which is having low mass and large surface. Q9.3. In a non-relativistic estimate of Light Sail regime, the change in momentum of entire foil balances the laser momentum. Write the expression of momentum balance which shows that acceleration becomes more effective with reduced mass density (σ). A9.3. dp/dt = 2I/σ. Q9.4. A9.4. How the ion energy scales with laser intensity in Light Sail regime? Quadratically. 19

20 Problems Q9.5. A9.5. Write down the favorable experimental requirements of Light Sail regime. Matching the laser field amplitude and target density, super Gaussian intensity profile to avoid strong deformation, ultrahigh contrast and avoid of prepulse. Q9.6. What happens when laser-piston is applied to a thin foil? A9.6. Target acts as a relativistic mirror. As ion speed approaches c, Doppler shift of reflected radiation results in advantageous laser-target energy coupling. Q9.7. A9.7. What effects can be suppressed in Light Sail regime by using circularly polarised light? No J B acceleration, No TNSA, No target heating. 20

21 Problems Q9.8. A9.8. Mention the features of light sail regime of RPA. Cyclical re-acceleration of ions, Narrow-band spectrum (whole-foil acceleration), Fast scaling with intensity. Q What will be the optimum thickness of thin target t in Light Sail model? A9.9. a 0 ~ π (n e /n c ) (l/λ). Q9.10. A9.10. Light Sail regime of RPA acceleration is not front side acceleration? True or False. True. 21

22 References 1. F. F. Chen, Introduction to Plasma Physics,,(Plenum Press, New York, 1974). 2. S. Eliezer, The Interaction of High-Power Lasers with Plasmas, (IOP Publishing, Bristol, 2002). 3. W. L. Kruer, The Physics of Laser Plasma Interactions, (Addison-Wesley Publishing Company, California, 1988). 4. P. Gibbon, Short Pulse Laser Interaction with Matter, (Imperial College Press, 2005). 5. Macchi et.al., Review of Modern Physics 85, 751 (2013). 22