1. 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)

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

3. 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

4. Properties of guiding in capillary tubes suitable for the linear regime
Intensity in the range 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

5. 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

6. 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

7. 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

8. 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

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

10. Active stabilization system reduces pointing fluctuations
AAC2010, B. Cros

11. 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

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

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

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

15. 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

16. 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, (2009)
AAC2010, B. Cros

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

18. 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

19. 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

20. 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

21. 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

22. 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