Effects of laser prepulse on proton generation D.Batani Diartimento di Fisica G.Occhialini Università di Milano Bicocca
Co-authors M. Veltcheva, R.Dezulian, R.Jafer, R.Redaelli Dipartimento di Fisica G.Occhialini, Università di Milano Bicocca, Milan, Italy O.Lundh, F. Lindau, A.Persson, K.Osvay, C.-G.Wahlström Department of Physics, Lund University, Sweden D.C.Carroll, P.McKenna SUPA, Department of Physics, University of Strathclyde, Glasgow G4 0NG, UK A.Flacco, V.Malka LOA, Ecole Polytechnique, Palaiseau, France
Effect of laser pre-pulse
Effect of laser pre-pulse The laser prepulse has two kinds of effects: 1) On front side: It creates a preplasma which may either improve or deteriorate laser absorption 2) On rear side: it may create a smooth density jump which may negatively affect acceleration by TSNA
Effect of laser pre-pulse
The effect depends on
Pressure - intensity relation On breakout, such shock unloads to virtually zero pressure as the material relaxes
High pressure phase diagram of Al
The case of large shock pressures Unloading curve obtained by Sesame tables (Los Alamos)
The case of smaller shock pressures
Rear side density profiles
Hydrodynamical codes A number of 1D and 2D hydro codes are well adapted to describing the hydrodynamics and the ablation of ns-laser irradiated targets. Eg. MULTI developed by R.Ramis et al. at UPM laser Interaction of a 6 µm foil Al target with laser radiation with τ = 3 ns at 800 nm and 10 12 W/cm 2
1D hydro simulations (MULTI) laser After shock breaks out on target rear side, the whole target is accelerated by the laser beam Interaction of a 6 µm foil Al target with laser with τ = 3 ns at 800 nm and 5 10 12 W/cm 2 (no plasma creation). Temporal flat-top laser profile Notice target motion: it can go out of focus!!
Effect of target displacement On a long time scale, the effect is a parabolic motion, ie., uniformly accelerated under the effect of laser ablation pressure P s x = 1 2 P S ρd τ 2 30 µm This must be compared with the depth of focus of the focusing system: For λ 1 µm and F 3, we get l 20 µm
Effect of laser pre-pulse More recent measurements by A.Flacco et al. at LOA at I 5 10 19 W/cm 2. Correlation between proton cutoff energy and target thickness Experimental data (red) compared with points (green) from Kaluza for I 10 19 W/cm 2 and τ ASE 0.5ns
Effect of laser pre-pulse A different way of looking at the problem Proton cut-off energy for diffrent focus position on Aluminum targets of 2 µm and 400 nm. The plot for the thinner target exibits a minimum in the focus position where a thicker target has a maximum (dashed lines are added to help visualization).
Effect of laser pre-pulse Theorical pedestal flux at different contrast ratios
2D simulations Interaction of a 6 µm foil Al target with laser radiation with τ = 3 ns at 800 nm and 5 10 12 W/cm 2 Shock front Ablation front Unperturbed Material Relaxation front laser Compressed region Plasma corona
2D simulations
Effects on proton beam direction Due to target deformation, the angular spread of the proton beam may be increased, the proton beam may be deviated, it may fall outside the collection angle of small-angle diagnostic devices
Experimental Results: CR39 data F.Lindau, O.Lundh, A.Persson, P.McKenna, K.Osvay, D.Batani, C.G.Wahlstrom Laser-Accelerated Protons with Energy Dependent Beam Direction Physical Review Letters, 95, 175002 (2005)
Deviation from target normal
Effect of target material
Effect of target material
Calculated beam deviation Proton beam deflection from laser axis vs ASE duration. The target is 6 µm Al and Cu and the inferred ASE intensity is 2 10 11 W/cm 2. The proton beam stays on the target normal for the shortest ASE pedestal (bottom right panel). After shock breakout, the beam is steered toward the laser axis (top right panel). The inset shows the calculated shape of the foil for a 2.0 ns long ASE pedestal. The modelled emission direction (solid lines) is the calculated target normal direction at the point where the laser axis intersects the rear surface O.Lundh, F.Lindau, A.Persson, C.G.Wahlstrom, P.McKenna, D.Batani Influence of shock waves on laser-driven proton acceleration Physical Review E, 76, 026404 (2007)
Separate plasma formation beam Illustration of proton beam steering by shock deformation of target. (b)-(d) Measured angular deflection of proton beam as a function of: (b) relative position of the line focus (intensity of 2.5 10 10 Wcm 2 and t = 7.5 ns); (c) t (intensity of 2.5 10 10 Wcm 2 and relative position of -15 µm); and (d) intensity of the line focus ( t = 7.5 ns and relative position of +10 µm). The angular shift predicted by the rear surface shock deformation model is shown (black line). The relative position of the line and spot foci is shown schematically for (c) and (d). The proton energies are: blue squares 0.9, green triangles 1.4 MeV and red circles 2.8 MeV.
Effect of laser pre-pulse The laser prepulse has two kinds of effects: 1) On front side: It creates a preplasma which may either improve or deteriorate laser absorption 2) On rear side: it may create a smooth density jump which may negatively affect acceleration by TSNA
Effect of laser pre-pulse on front side Recent techniques (Plasma mirrors, XPW) allow improving the cotrast up to10 10. However a very sharp interface at the vacuum / solid interface may deteriorate laser absorption. PIC simulations show that there is an optinal plasma scalelength
Interferometric expt: 2D density evolution A.Flacco, A Guemnie-Tafo, R Nuter, M.Veltcheva, D.Batani, E.Lefebvre, and V.Malka Characterization of a controlled plasma expansion in vacuum for laser driven ion acceleration Journal of Applied Physics, vol. 104, 103304 (2008)
Interferometric expt: 2D density evolution Experimental results allow to determine the time needed to create a given plasma scale-length on target front. Comparison with simulations allow to determine the position of the shock wave inside the material D 16 km/s Shock breakout from reflectivity measurement
Conclusions from interferometric expts
Experimental effect of prepulse delay Measuremens by A.Flacco et al. Some effect but smaller than expected, not well reproducible, and NOT at the expected scale length!!
Experimental effect of prepulse delay Even less convincing results obtained by Yannick et al. In Lund
Experimental effect of prepulse delay D.C.Carroll et al. Dynamic control and enhancement of laser-accelerated protons using multiple laser pulses Compt. Rend. Phys (2009)
Experimental effect of prepulse delay
Conclusions The physics of laser generated protons is complex The state of the target material influences many aspect of the generation. Only strong shocks which vaporize the material are detrimental to proton acceleration The presence of prepulse is often negative. However special prepulses can be used to manipulate the proton beam (beam steering) On front side the presence of prepulse is essential for laser absorbtion. This can be calculated consistently with prepulse effects on rear side Recent experimental results also correlate the proton spectra to the presence of prepulse
Thank you!!