Proton generation
Proton generation

Caracteristic picture from Thomson parabola. Ions from laser-matter interaction are separated from the mass/charge ratio.

Although well known since long-time [1], the ions (and particularly protons) acceleration through laser-matter interaction rouses nowadays the attention of a range of communities, not exclusively scientific, thanks to the potential applications of such a particle beam. The research in this field is stimulated to a large extent by the hope of using laser produced protons for medical cancer treatment (proton therapy) [2] despite the large number of hard stages still to overcome. Nonetheless, protons beams are quite interesting even for a large spread of (maybe less evocative) applications such as high-resolution radiography [3], high-density matter production for astrophysics applications [4], fast ignitor physics [5] or radio-isotopes production [6].

Thanks to the advances in high energy lasers technology, and CPA [7] technique in particular, T3 (Table Top Térawatt) lasers are nowadays able to reach, and sometimes get over, intensities around 1020 W/cm2 . Focusing such an ultra-high intensity laser beam on thin foils (~ µm) allows to get proton bunches with several dozens MeV maximum energy. It is a shared opinion that the main acceleration mechanism is the following. Target electrons are pushed by the laser ponderomotive force through the target: once they come out on the target back side they are drawn back by the space charge effect this building up a very intense electrostatic field (~ 1012 V/m) on the back target surface. The water and hydrocarbon thin layer which lies on the target surface due to the environmental unavoidable pollution represent an optimal ions reservoir for such a field. Among these ions, protons are more easily accelerated by this mechanism (TNSA : target normal sheat acceleration [8]), due to their favourable mass on charge ratio. Ususally, proton bunches last more or less as long as the laser pulse and show low emittance (~ 0.01 π mm mrad), around 20° angular spread and high laminarity. All these features make of proton bunches a promising tool for all the applications cited before. Of course, target and laser characteristics play a role in produced proton number and maximum energy. In particular, the laser beam contrast (pic intensity over pedestal intensity ratio) influence the laser-matter coupling and, as a consequence, the electron acceleration and number.

The acceleration of charged particles is one of the domains of interest of the PHI group. We participate to experimental runs concerning electron acceleration through laser / gas jet interaction in collaboration with the Laboratoire pour l’Utilisation des Lasers Intenses (LULI, Palaiseau, France) and the Istituto per i Processi Chimico Fisici (IPCF – CNR, Pisa, Italia). More specifically, we turn our attention to the generation and characterization of proton beams and their application to the study of material damaging. To this aim we have realised some diagnostics (Thomson parabola, RCF based spectrometer) allowing us to know the space and energy distribution of produced proton bunches (Fig.1).

First results (T. Ceccotti et al. to be submitted) allow to put in evidence the effects of laser contrast and target thickness on protons maximum energy. In the close future, we plan to play an experimental campaign mainly devoted to clear up the influence of pulse duration and polarization, impinging angle and target nature, under such high contrast conditions, on proton energies.


[1]Gitomer et al. Phys. Fluids 29, 2679 (1986)

[2] S.V. Bulanov et al., Phys. Lett. A 299, 240 (2002) ; E. Fourkal et al., Med. Phys. 29, 2788 (2002) ; V. Malka et al., Med. Phys. 31, 1587 (2004)  

[3] M. Borghesi et al., Phys. Plasmas 9, 2214 (2002)

[4] P. Patel et al., Phys. Rev. Lett. 91, 125004, (2003)

[5] T. Cowan et al., Phys. Rev. Lett. 92, 204801, (2004)

[6] K. Ledingham et al., Science 300, 1107 (2003)

[7]D. Strickland and G. Mourou, Opt. Commun. 56, 219 (1985)

[8] Wilks et al., Phys. Plasmas 8, 542 (2001)

Proton generation

Each fo the 4 radiochromic films ahs been exposed to protons with different energy (a = 0.45 MeV, b = 0.85 MeV, c = 1.2 MeV et d = 1.45 MeV). The analysis of the picture allows inferring the number of incident particles as well as their spatial distributions.

#719 - Màj : 04/10/2006


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