Controlled injection and acceleration of electrons in plasma wakefields by colliding laser pulses


In laser-plasma-based accelerators1, an intense laser pulse drives a large electric field (the wakefield) which accelerates particles to high energies in distances much shorter than in conventional accelerators. These high acceleration gradients, of a few hundreds of gigavolts per metre, hold the promise of compact high-energy particle accelerators. Recently, several experiments have shown that laser-plasma accelerators can produce high-quality electron beams, with quasi-monoenergetic energy distributions at the 100 MeV level2,3,4. However, these beams do not have the stability and reproducibility that are required for applications. This is because the mechanism responsible for injecting electrons into the wakefield is based on highly nonlinear phenomena5, and is therefore hard to control. Here we demonstrate that the injection and subsequent acceleration of electrons can be controlled by using a second laser pulse6. The collision of the two laser pulses provides a pre-acceleration stage which provokes the injection of electrons into the wakefield. The experimental results show that the electron beams obtained in this manner are collimated (5 mrad divergence), monoenergetic (with energy spread <10 per cent), tuneable (between 15 and 250 MeV) and, most importantly, stable. In addition, the experimental observations are compatible with electron bunch durations shorter than 10 fs. We anticipate that this stable and compact electron source will have a strong impact on applications requiring short bunches, such as the femtolysis of water7, or high stability, such as radiotherapy with high-energy electrons8,9 or radiography10 for materials science.

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Figure 1: Raw images of the electron beam obtained with the electron spectrometer.
Figure 2: A typical quasi-monoenergetic electron spectrum obtained by colliding pulse injection.
Figure 3: Evolution of the electron beam peak energy and its energy spread.


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We thank J.-P. Rousseau, F. Burgy, B. Mercier, A. Tafzi, D. Douillet, T. Lefrou, F. Alahyane and J.-L. Charles. J.F. thanks S. Sebban for his patient help with the experiment. J.F. acknowledges discussions with G. Fubiani. We also acknowledge the support of the European Community Research Infrastructure Activity under the FP6 “Structuring the European Research Area” programme (CARE).

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Correspondence to V. Malka.

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Supplementary Information

This file contains figures and methods. Figure S1 shows a schematics of the experimental set-up. Figure S3 shows a graph showing the electron peak energy and plasma density versus longitudinal position. Figure S3 shows results of the simulations of colliding pulse injection. Supplementary Methods discuss a description of the model which was used for simulating the injection and acceleration of electrons in the colliding pulse experiment. (PDF 747 kb)

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Faure, J., Rechatin, C., Norlin, A. et al. Controlled injection and acceleration of electrons in plasma wakefields by colliding laser pulses. Nature 444, 737–739 (2006).

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