Science & Applications: Secondary Sources

Introduction

Particles and radiation have many applications in several fields of activity. Today, particles are produced by linacs, synchrotrons, X-ray tubes or radioactive nuclei. They are used in, for example, medicine (radiotherapy, imaging by PET), chemistry, agro-alimentary, material science, nuclear physics and many other domains. It has been demonstrated in early 2000 - both experimentally and theoretically - that particles and radiation sources can be produced by intense lasers.

Extremely high electric fields, with values in excess of 1 TV/m (Malka 2002), have been produced by focusing ultrashort high-intensity laser pulses onto targets. Depending on the target (gas jets, thin foils, clusters, etc…), it is possible  to generate radiation (from X-ray to γ-ray) and energetic particles (ions and electrons). The pulse duration and brightness of these beams make them unique and different from conventional accelerator sources. Since the value of the electric field is more than 10,000 times greater than electric fields generated in RF cavities, the length required to generate these sources (less than one millimetre) is orders of magnitude smaller than that required by conventional    means. Ultra-intense laser systems thus allow the realization of new physics with potentially important applications in the near future. ELI will serve as a research centre for development and application of laser-based particle acceleration in both the intense and ultra-intense regimes.



Electron beam produced by laser

State-of-the-art

The state of the art of laser plasma accelerators is the bubble regime, in which quasi monoenergetic electron beams with high charge and low emittance can be achieved. Laser Wake Field Acceleration (LWFA) works for laser pulses shorter than the plasma wavelength. When driven into the highly non-linear wave-breaking regime, the wake field takes the form of a solitary bubble (Pukhov 2002). In the bubble regime, ultra-short bunches of electrons with superior properties are produced. Large amounts of electrons are self-trapped and accelerated to    relativistic energies (γ -factors of 100 - 1000) with high efficiency. Very dense low emittance pulses are created and, most importantly, quasi-monoenergetic  electron spectra are obtained.




Fig. 1: 3D particle-in-cell simulation. Solitary laser-plasma cavity produced by 12-J, 33-fs laser pulse. (a) ct/ λ=500, (b) ct/ λ=700, (c) electron trajectories in the frame moving together with the laser pulse. Trapped electrons are coloured red.

 
In the very first experiments conducted in the bubble regime, monoenergetic electro beams in the range from 70 to 170 MeV were measured [Mangles 2004; Geddes 2004;    Faure 2004]. Currently, the parameters of the electron beam are as follows: energy up to 1 GeV, total charge of 0.5 nC, divergence of a few mrad, and ~10 fs duration.

Future developments

Several ideas have been proposed to improve the parameters of the electron beam: an ultra-short and mono-energetic electron beam can be produced by using two laser beams with moderate intensity in combination with a plasma channel. Another solution is to use a more powerful laser system and/or to use a multi-stage acceleration scheme based on a succession of laser-plasma accelerators (Euroleap.eu). Successful workshops dedicated to the application of laser-based accelerators, funded by the European Science Foundation, offer possibilities of considering different approaches and generating new collaborations between the different European groups Since mono-energetic electron beams can be used for the future generation of photoinjectors, this research is also of importance to the accelerator physics community.

Projection from the actual experimental and theoretical data achieved with 100 TW laser to ELI indicates that hundreds of GeV electron bunch could be generated. Indeed, the central result of the theory is that the bubble regime of electron acceleration is stable, and scalable, and that the scaling for the maximum energy Emono of the monoenergetic peak in the electron spectrum is

Emono ≈ 0.65 mec2[P/Prel]1/2ct/λ

Here, P is the laser pulse power, Prel =me2 c5/e2≈8.5 GW is the natural relativistic power unit and λ=2πc/ω0is the laser wavelength. This scaling assumes that the laser pulse duration satisfies the condition ct < R. The scaling for the number of accelerated electrons Nmono in the monoenergetic peak is

Nmono ≈ 1.8/k0re [P/Prel]1/2,

where re=e2/mec2 is the classical electron radius, and k0=2p/λ. The acceleration length Lacc scales as

Lacc ≈ 0.7 ct/λZR,

where ZR= pR2/λ is the Rayleigh length.

The parametric dependences in these scalings follow from the analytical theory. The numerical pre-factors are taken from 3D PIC simulations. These pre-factors may change, depending on the particular shape of the pulse envelope. However, as soon as the envelope of the incident laser pulse is defined, the pre-factors are fixed. The parametric dependences are universal and do not depend on the particular pulse shape.


Table 1: Numerical examples of the electron beam energy scalings (W. Mori et al, UCLA).

Efficient electron acceleration in the 10-100 GeV range is expected to be reached in the bubble regime (Gordienko and Pukhov 2005). The same range of energy can also be reached by using a multi-stage approach. Both approaches will be considered.

Applications

New science has already been performed with these sources, which extend the well known parameter regimes of electron beams to ultra-short durations. It has been shown that ultra-short electron beams generated with a laser are perfectly synchronized with the laser and can be used for pump-probe experiments. Such experiments can provide a fundamental understanding of the first events occurring in radiolysis with particles (Brozek-Pluska 2005).

Electron beam line for applications of the quasi-monoenergetic electron beam at 200-300 MeV for fast chemistry, radiotherapy (Glinec 2005) and material science.

The availability of relatively inexpensive, compact accelerators for a wide range of electron energies (multi- GeV) or proton energies of (10-200 MeV) should make them affordable to universities and industries. A spin-off of the ELI project will be to demonstrate significant progress towards the achievement of compact laser plasma accelerators in the GeV range.