Particles sources

 The generation of energetic particles beams (electrons, protons, ions, neutrons) has been and still is extensively developed by large particle accelerators based on accelerating radiofrequency (RF) modules. These accelerators are used in many fields for medical applications, particle physics, radio-biology and physics of condensed matter. This technology, developed for about a half-century, is robust: it can produce beams of electrons, positrons or ions with high stability and with beams of very high quality. The accelerating field in these RF cavities is limited to a few tens of million volts per meter. Getting higher particle energies requires higher accelerator length which can lead to large infrastructures. For example, the successor to the Large Hadron Collider at CERN will be the International Linear Collider to accelerate electrons and positrons to 250 GeV over 31 km. In this context, it is important to seek alternatives that could be more compact.

 

 

         

 

A breakthrough has occurred a few years ago in the field of laser-produced plasma using intense femtosecond lasers. The peak intensities obtained on target (greater than 1018 W/cm2) were used to efficiently generate beams of energetic particles. Since the plasmas are ionized environments, the amplitude of the accelerating fields are no longer limited by electrical breakdown like in RF cavities. We are able to produce electric fields of the order of several hundred gigavolts per meter, more than 10,000 times higher compared to the fields used in these conventional accelerators. The hope, using plasma, is then reduce the length of acceleration of several orders of magnitude.

 

The plasma waves produced in the wake of an intense femtosecond laser pulse can be simulated by numerical codes. The laser (orange) disrupts the distribution of free electrons of the plasma during its propagation in the gas. This produces ion cavities in which electrons are expelled. Following the electrostatic forces generated by the space charge separation, electrons trapped in the cavity are accelerated to high energies.

 

The LOA is a world leader in this field. In particular, the SPL group demonstrated the feasibility of this technique as well as optical injection to generate stable beams, which is essential for a practical use.

Currently, the laser-based accelerators generated from a gas can accelerate electrons to energies of several hundreds of MeV to 1 GeV over a distance of few centimeters. Although we are still far from the energy required for Particle Physics, these new sources of electrons have unique properties: they produce ultra-short electron bunches (100 fs or less) from an extremely small point source (only a few micrometers).

The short bunch of electrons allows applications that require a temporal resolution in the hundred femtosecond time scale such as the development of X-ray free electron laser, femtosecond radiolysis or biochemistry.

 

 

Optical injection can control the energy and the dynamics of electrons trapped in the ion cavity. At LOA, this is realized with a second laser beam, that counter-propagates with respects to the first beam used to create the ion cavity. The shot to shot stability of the accelerated electron beam is greatly improved. This is an important step towards the use of laser-produced plasma electron beams. 

 

 

 

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