Control of laser wakefield amplitude in capillary tubes B. Cros, K. Cassou, F. Wojda Laboratoire Physique Gaz et Plasmas –CNRS- Université Paris sud 11 (France) G. Genoud, M. Burza, O. Lundh, A. Persson, C.-G. Wahlström Atom Fysik department - Lund University- LLC (Sweden) N. Andreev, M. Veysman IHED, Moscow (Russia)

Outline Active control of laser properties to improve guiding Plasma wave in a weakly non linear regime over several centimeters AAC2010, B. Cros

Motivation Create a controlled accelerating structure Element of a multi-stages accelerator scalable to high energy Determine the conditions for creating Plasma wave at the limit of the linear regime Over the dephasing length AAC2010, B. Cros

Properties of guiding in capillary tubes suitable for the linear regime Intensity in the range 1017-1018 W/cm² can be achieved over lengths in the range 1-100cm Plasma density can be arbitrarily low Laser wakefield near resonance achieved by matching plasma density and pulse duration Capillary tubes lifetime increases with laser beam stability and quality AAC2010, B. Cros

Theoretical studies to quantify the effects of pointing and symmetry variations 3D calculations in vacuum Solving Maxwell equations with boundary condition for dielectric capillary tube Describes asymmetry in position and angle M. Veysman et al., 27, 1400 (2010) AAC2010, B. Cros

Transmission > 80% for angle < 5mrad and dR/R <0.3 Lcap = 4.92cm, R= 50µm, wx=wy= 32µm Transmission of a symmetrical Gaussian beam dR: displacement from axis Calculations agree with measurements AAC2010, B. Cros

An angle > 5 mrad produces asymmetrical transverse patterns Capillary: Lcap = 1.5 cm, R= 50µm, wx=wy= 32µm, dR= 0 Symmetrical Gaussian beam as input dR: displacement from axis = 0 AAC2010, B. Cros

The asymmetry of the pulse shape lowers the guided intensity Capillary: R= 50µm, wx= 14.9µm, wy= 29.5µm, dR= 0, angle = 0 Asymmetrical Gaussian beam as input dR: displacement from axis = 0 AAC2010, B. Cros

Experiments using the Lund Laser Centre 20 TW Laser Improvement of beam coupling Measurement of plasma wave AAC2010, B. Cros

Active stabilization system reduces pointing fluctuations AAC2010, B. Cros

Fluctuations in position can be reduced to dR/R = 0.1 or 3 µrad 10 µm / 37 µm 7.9 µm / 20.5 µm Fluctuations in position (µm): Rms / maximum Initial situation After improving mechanical stability After inhibiting shots during the most unstable part of the 10 Hz cycle With piezo-electric mirror 5.9 µm / 19.0 µm 5.2 µm / 12.4 µm AAC2010, B. Cros

Beam quality improved by adaptive optics Control of the focal spot shape with deformable mirror 080215_117cap81p7 080215_280foc AAC2010, B. Cros

Experimental set-up Energy in the capillary 120 mJ Pulse duration 45+/-5fs Capillary length:2-8cm, diameter 100µm AAC2010, B. Cros

Good quality guiding achieved in gas filled capillaries Input focal spot Imax 2x1017W/cm² Output after 7 cm AAC2010, B. Cros

Plasma wave amplitude deduced from the laser spectrum modification The pulse propagates in a rapidily varying medium Analytical Frequency shift in a capillary (linear, input Gaussian pulse) AAC2010, B. Cros

Dependence as a function of pressure and tube length 12mm 50mm 71mm 81mm Energy in the capillary 120 mJ When pressure is increased beyond the resonant value, nonlinear effects appear for long enough distance F. Wojda et al. PRE 80, 066403 (2009) AAC2010, B. Cros

Experimental results agree with simulations N. Andreev et al. New J. Phys. 12 (2010) 045024. AAC2010, B. Cros

Pulse evolution over a long distance leads to higher plasma wave amplitude Ionisation leads to a steepening of the front of the pulse Self-phase modulation and NL GVD creates a steepening after Imax 70 mm Ionisation 120mJ, 40 mbar 70 mm Pre-ionised AAC2010, B. Cros

In spite of the NL evolution of the pulse the PW remains linear Lcap = 71mm Pressure 60 mbar 120 mJ Pulse and wakefield potential on axis Transverse structure of the potential AAC2010, B. Cros

Experimental accelerating field 1-7 GV/m deduced from comparison to simulations Capillary: D = 100 µm, L 7 cm, filled with hydrogen Pump pulse: l = 0.8 µm, tFWHM= 51 fs, IL= 1017 W/cm2 AAC2010, B. Cros

Summary Controlled energy distribution and stability in the focal plane improve coupling efficiency and reproducibility Validation of the optical diagnostic by the agreement between experiment and simulation results Plasma wave measured over 8 cm with accelerationg field in the range 1-10 GV/m AAC2010, B. Cros

Outlook Test longer capillaries / longer pulse duration/ higher laser energy to increase the gradient-length product Inject electrons Dephasing length Depletion Capillary damping AAC2010, B. Cros