M. Amin 1, M. Borghesi 2, C. A. Cecchetti 2, J. Fuchs 3, M. Kalashnikov 4, P. V. Nickles 4, A. Pipahl 1, G. Priebe 5, E. Risse 4, W. Sandner 4,6, M. Schnürer 4, T. Sokollik 4, S. Ter­ Avetisyan 4, T. Toncian 1, P. A. Wilson 2, and O. Willi 1 1 Institut für Laser- und Plasmaphysik, Heinrich-Heine-Universität, Düsseldorf, Germany 2 School of Mathematics and Physics, The Queen’s University Belfast BT7 1NN, UK 3 Laboratoire pour l’Utilisation des Lasers Intenses, UMR 7605 CNRS-CEA-École Polytechnique-Université Paris VI, Palaiseau, France 4 Max-Born-Institut, Max-Born-Str. 2a, Berlin, Germany 5 CCLRC Daresbury Laboratory, Warrington, Cheshire, WA4 4AD, UK 6 Technische Universität Berlin, Straße des 17. Juni 135, Berlin, Germany Time-resolved proton probing of laser- induced front and rear side plasma expansion phenomena

Munib Amin – ILPP Düsseldorf2 Introduction (1) ●Short pulse (~1 ps) high power (~10 18 W/cm 2 ) laser ●Thin (~10 µm) metal target  Plasma expansion on the target surface

Munib Amin – ILPP Düsseldorf3 Introduction (2) – Example ●50 µm Ta wire, ~ 10 J, 0.4 ps 100 µm 0 ps / 3.0 ps5.9 ps10.1 ps ●Plasma expansion  covers an area of a few square millimeters  is accompanied by strong electric fields  spreads along the surface on the picosecond timescale Experiment performed at the LULI 100TW facility

Munib Amin – ILPP Düsseldorf4 Introduction (3) – Benefits ●Electric fields in an expanding plasma can be used to control charged particle beams. Example: The laser proton lens* ●A better understanding of the field evolution could allow to design new targets for beam focusing, collimation or displacement. Laser pulse Metal foil cylinder Proton beam (polyenergetic, divergent) *T. Toncian, et al., Science 312, (2006). Focusing, energy selection

Munib Amin – ILPP Düsseldorf5 How to accelerate protons ●To accelerate protons using a laser you need  Thin foil target (~10 µm)  A laser with short pulse duration (about 1 ps or less) with a high intensity (about W/cm 2 or more) that will not perturb or even burn the target before the main pulse arrives (high contrast) ●You get  Proton emission from the rear side of the target Up to protons Proton energies of up to about 60 MeV A divergent proton beam of high longitudinal and transversal laminarity Small virtual source size (less than 10 µm) A. Maksimchuk, et al., Phys. Rev. Lett., 84, (2000). R. A. Snavely, et al., Phys. Rev. Lett., 85, (2000). S. P. Hatchett, et al., Phys. Plasmas 7, (2000).

Munib Amin – ILPP Düsseldorf6 Laminarity Virtual source ●Longitudinal laminarity: a class of a certain energy cannot overtake another one ●Transversal laminarity: Trajectories do not cross Fast protons Slow protons Proton generation foil M. Borghesi, et al., Phys. Rev. Lett., 92, (2004). T. E. Cowan, et al., Phys. Rev. Lett., 92, (2004).

Munib Amin – ILPP Düsseldorf7 Probing – principle Object moving downwards ●An object varying in time can be probed by a laser generated proton beam (point projection). M. Borghesi, et al.,Plasma Phys. Control. Fusion 43, A267–A276 (2001).

Munib Amin – ILPP Düsseldorf8 Probing – electric field Proton trajectories Displacement Lower density Higher density ●Each proton is displaced by the electric field depending on its direction of emission and its initial kinetic energy. ●The density distribution of a class of protons having the same energy is influenced in a specific way. ●Thus the temporal evolution of the electric field can be mapped.

Munib Amin – ILPP Düsseldorf9 Probing – deflectometry (1) ●In proton deflectometry a mesh is projected to the detector plane. ●The green arrow identifies a certain grid node.

Munib Amin – ILPP Düsseldorf10 Probing – deflectometry (2) ●The electric fields in the plasma deflect the protons and the projection of the mesh appears distorted. ●The grid nodes can still be identified. Their displacement can be measured.

Munib Amin – ILPP Düsseldorf11 2D proton imaging – with MCP ●magnification: ~10 ●observed area: ~3 mm ●TiSa:  40 fs,  ~ W/cm 2 ●Nd:glass  1.5 ps  ~10 18 W/cm 2 Experiment performed at the Max-Born-Institut Berlin Graphics by Th. Sokollik ●proton energy: MeV ●exposure time: ≤ 400 ps

Munib Amin – ILPP Düsseldorf12 Results Shadow of the target Increase of proton density along horizontal line Direction of the incoming laser Displaced target edge Cross section seen by the spectrometer Region of widened mesh No interaction laser5J shot to the target Image editing by Th. Sokollik

Munib Amin – ILPP Düsseldorf13 Proton Streak Camera Experiment performed at the Max-Born-Institut Berlin A slit cuts out a one dimensional cross section of the obtained proton density distribution. This cross section is dispersed into a second dimension concerning the kinetic energy of the protons. deflection Graphics by Th. Sokollik

Munib Amin – ILPP Düsseldorf14 Image obtained using the spectrometer Proton energy Slow decrease Region of high field strength Protons hitting the front Region of increased intensity and mesh line density No widening Glass laser shot: 0.3 J in focus, 6 x W/cm 2, 1.5 ps

Munib Amin – ILPP Düsseldorf15 Temporal and spatial field evolution on the target surface ●A proton beam is used to probe the electric field on the surface of a laser irradiated metal foil cylinder. ●The density distribution of the electron beam is recorded by a stack of radiochromic films. Laser pulse 1 Laser pulse 2 RCF stack (detector) Metal foil cylinder Proton generation foil Proton beam Experiment performed at the Vulcan Laser of the Rutherford Appleton Laboratory

Munib Amin – ILPP Düsseldorf ps38 ps- 18 ps 7.32 MeV - 72 ps49 ps- 11 ps 6.48 MeV - 67 ps63 ps- 2.3 ps 5.55 MeV - 61 ps84 ps12 ps 4.48 MeV - 48 ps120 ps38 ps 3.14 MeV 6.8 ps300 ps160 ps 1.06 MeV Images obtained using the radiochromic film stack RCF calibration by Toma Toncian Glass laser shot: 20 J, 1.7 x W/cm 2, 1.2 ps

Munib Amin – ILPP Düsseldorf17 Reconstruction of the electric field – an iterative method Experiment 1: StreakingExperiment 2: Imaging Imaging Experimental result Simulation Parameter fit Streaking Experimental result Simulation Parameter fit Modelling Parameter transfer

Munib Amin – ILPP Düsseldorf18 Comparison – simulation and experiment (1) ●Target charge up: C/m 2 ●Decay time: 600 ps ●Velocity of front propagation: 1.4 x 10 6 m/s ●Maximum field strength: 3.75 x 10 8 V/m ●Decay time: 3 ps[decay ~ (1 + t / τ) -1 ] Image editing by Th. Sokollik Time of flight/ns Deflection on MCP/mm Energy/MeV Time of flight/ns Energy/MeV

Munib Amin – ILPP Düsseldorf19 Comparison – simulation and experiment (2) 7.32 MeV6.48 MeV5.55 MeV4.48 MeV3.14 MeV1.06 MeV ●Maximum field strength: 1.1 x V/m ●Propagation velocity over target surface: 0.3 x speed of light

Munib Amin – ILPP Düsseldorf20 Outlook ●Future work will concentrate on conceiving a model that bases on simulations reconstructing the plasma processes of this special phenomenon. ●Such a model might also apply to the plasma expansion inside a hollow target that is used to manipulate charged particle beams.

Munib Amin – ILPP Düsseldorf21 Thank you. Projects were funded by DFG TR18, GRK 1203 and Laserlab Europe

Munib Amin – ILPP Düsseldorf22 Modelling ●Setting up a one dimensional field configuration from simulations or previously published models or experimental results ●Setting reasonable starting parameters for the analysis of the experimental results ●Generalizing to three dimensions according to the experimental geometry

Munib Amin – ILPP Düsseldorf23 The electric field configuration ●The fraction of laser energy absorbed by hot electrons and the hot electron temperature are estimated depending on laser intensity and wave length according to Fuchs[2006]. ●The electric field is supposed to build up in a plasma expanding into vacuum as described by Mora[2003]. ●Spatial dependence in one dimension and temporal evolution are given by PIC and MHD-simulations conducted by Romagnani[2005]. J. Fuchs, et al., Nature Physics 2, (2006). P. Mora, Phys. Rev. Lett. 90, (2003). L. Romagnani, et al., Phys. Rev. Lett. 95, (2005).

Munib Amin – ILPP Düsseldorf24 Electric field evolution – Mora field Electric field strength/(V/m) Distance from target surface/mm 0 ps 5 ps 10 ps 250 ps 500 ps 750 ps

Munib Amin – ILPP Düsseldorf25 Electric field evolution – charge up Electric field strength/(V/m) Distance from target surface/mm 250 ps 500 ps 750 ps

Munib Amin – ILPP Düsseldorf26 Total electric field Electric field strength/(V/m) Distance from target surface/mm 0 ps 5 ps 10 ps 250 ps 500 ps 750 ps

Munib Amin – ILPP Düsseldorf27 One dimensional electric field Field strength / (V/m) Time / s Position / m The field distribution according to Mora[2003] and Romagnani[2006] is modelled in one dimension.

Munib Amin – ILPP Düsseldorf28 Generalizing to more dimensions Front E x E x y Weaker and retarded The field distribution can be generalized to two or three dimensions by assuming the expansion to start later and the electron density to be lower at larger distances from the centre of the interaction.

Munib Amin – ILPP Düsseldorf29 The target geometry t3t3 t2t2 t1t1 x y The one dimensional field distribution is applied along the dashed lines Plain targetCurved or cylindrical target

Munib Amin – ILPP Düsseldorf30 Overview ●Quasi monoenergetic particles can be generated by  A special treatment of the foil target (thin layer or dots containing the particles to be accelerated on the rear surface)  A second target that selects one velocity class of protons: a laser irradiated hollow metal foil cylinder ●The proton beam can be focused by using  A hemispherical proton generation foil  A second target that focuses the divergent proton beam: a laser irradiated hollow metal foil cylinder B. M. Hegelich, et al., Nature, 439, (2006). H. Schwoerer, et al., Nature, 439, (2006). P. K. Patel, et al., Phys. Rev. Lett. 91, (2003). T. Toncian, et al., Science 312, (2006).

Munib Amin – ILPP Düsseldorf31 Overview – applications ●Medical applications  Tumour therapy  Production of radioisotopes ●Fast ignition ●Ion source for conventional particle accelerators ●Isochoric heating: creation of warm dense matter ●Diagnostics for dense plasmas: proton probing Maybe one day Routine F. Pegoraro, et al., Laser and Particle Beams, 22, 19–24 (2004). M Roth, et al., Plasma Phys. Control. Fusion, 47, B841–B850 (2005). P. K. Patel, et al., Phys. Rev. Lett. 91, (2003). K. W. D. Ledingham, et al., Science 300, 1107 (2003).

Munib Amin – ILPP Düsseldorf32 How to accelerate protons – hot electrons Hatchett, et al., Phys. Plasmas 7, 2076 (2000). A. Pukhov, Phys. Rev. Lett., 86, (2001).

Munib Amin – ILPP Düsseldorf33 How to accelerate protons – TNSA (Target Normal Sheath Acceleration)

Munib Amin – ILPP Düsseldorf34 Detection – radiochromic film stack ●The density distribution of the proton beam is recorded by a stack of radiochromic films. ●Since protons deposit most of their energy in the Bragg peak, one film shows the distribution corresponding to only one specific energy. RCF stack

Munib Amin – ILPP Düsseldorf35 Detection – proton streak camera (1) Top viewSide view Back side – detector Permanent magnet Slit Detector (MCP) T. Sokollik, et al., to be published

Munib Amin – ILPP Düsseldorf36 Detection – proton streak camera (2) Side view Back side – detector Top view ●The spectrometer setup can be used map the time evolution of an electric field in one dimension. ●In contrast to a film stack this setup provides a high energy resolution for lower energetic protons (up to ca. 5 MeV).