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Accélération d’ions

 

Since the eighties, where the first MeV proton bunches from the interaction between a high-power, nanosecond laser, has been reported [8], spectacular progress has been achieved in the so-called laser-ion acceleration process. These advances are mainly due to improvement of the laser beams characteristics, especially in terms of duration (increasing the peak power), improvement of the time-contrast preventing the formation of uncontrolled pre-plasmas [9], and ability to focus on small surfaces thanks to the development of adaptive optics.

Nowadays, proton beams with high beam quality are produced from thin foils irradiated by UHI short laser pulses [10]. Even still very far from the performances achieved on large-scale accelerators (eg. GANIL), these ion beams are laminar, collimated (about 30°), have duration at the source of the order of a picosecond and can reach energies in the 15 MeV range on typical satellite facilities.  As a consequence, these beams are already being considered or applied in high-resolution radiography, for the production of high-energy-density matter of interest for astrophysics, and could also lead to high-brightness injectors for accelerators or sources for proton therapy or radioisotope production.

Laser light focused to high intensity on a solid target surface can be efficiently absorbed by the electrons, resulting in a current of relativistic electrons streaming from the laser-irradiated surface into the target. These electrons have enough kinetic energy to exit into vacuum on the non-irradiated “rear” target surface, and set up a large electrostatic sheath at the steep solid – vacuum interface. This field ionizes atoms in the vicinity of the surface and accelerates them to energies that can be several times larger than the typical electron kinetic energy. This mechanism, known as ‘Target Normal Sheath Acceleration’ (TNSA) [11], has been demonstrated and studied experimentally and numerically by many groups. It is able to produce a remarkably well-collimated proton beam, with a small emittance and a typical efficiency (laser energy to ion energy) of a few percents. At the moment, the state of the art in term of proton energy ranges from 17 MeV for 100 TW-fs class satellite facility [12] lasers to 60 MeV for PW-ps laser systems [13]. Different theoretical works have predicted that under particular interaction conditions (ultra-high contrast, high intensity and circular polarization) radiation pressure acceleration (RPA), or the light sail regime [14] could replace standard TNSA. We intend to study this mechanism and then possibly increase the proton energies to the GeV range.

As for the electron acceleration program, the ion program will take the opportunity of using for the first time a clean (high contrast, perfectly focused) 10PW laser system to extend the limit of laser-accelerated protons into new domains, putting in evidence some new physical regimes originating from both relativistic character of electrons and ions. According to PIC’s simulations, it is expected to extend the energy limit to GeV’s while perfectly controlling the spatial properties of the beams and the shot-to-shot reproducibility. In addition to proton and ion sources, we will also optimize the generation of secondary neutron using the laser-accelerated ion beams. From the application side, the scientific impact is inherently broad for a highly reproducible and controllable proton beam of several hundred MeV. This new and powerful source will unlock innovative, high-impact applications such as radiography of dense objects with picosecond resolution, spallation with very small source size, study of the physics of ion beam interaction with a plasma (multi-ion interaction with non-degenerate matter). It will allow entering into the regime of high-energy nuclear physics, and help realize medical applications. Compared to accelerators, lasers offer attractive parameters. They are more compact, less expensive, and have smaller beam optics.

Using Apollon10P in parallel with the satellites facilities, we will address several topics of large interest to various communities:

Warm and Dense Matter basic studies

Bundles of short-duration (<50 ps) high-energy proton beams generated by a high-intensity short-pulse laser are ideal candidates for heating solids or plasmas to high temperatures.  In these conditions, the characteristic time of hydrodynamic expansion is effectively very long (nanoseconds) compared to the duration of the proton bundle. Producing and probing matter with a density close to or larger than solid density at temperatures from a few eV to a few 100 eV will give new insight in this much unknown warm dense state of matter [15].

Inertial confinement fusion (ICF) research

Some crucial points relevant to inertial confinement fusion may be addressed on high-energy short-pulse laser facilities. Indeed, such mechanisms as heat and hot electron transport in hot plasmas, ion and alpha particle stopping power in hot and dense plasmas still miss conclusive quantitative experiments. In addition to the interest for fundamental physics, knowledge and understanding of the mechanisms of slowing of ions and charge equilibrium are required either for conventional fusion, or for Fast-Ignition.  Performing such experiment with short-pulse high-energy particle beams would open for the first time the possibility to test models of energy loss in correlated plasmas for which there is no experimental data [16].

 

Proton Radiography

Radiography using both x-rays and protons, with picosecond time resolution, has been very beneficial in elucidating basic phenomena in laser plasma interactions. In particular, protons have added new diagnostic capabilities since they are sensitive to electric and magnetic fields as well as plasma density [17].

The production of higher-energy proton beams will give access to longer and higher-density plasmas relevant for laser-plasma interaction physics and high-energy-density physics.

Time-dependent irradiations in material science and chemistry

We propose to exploit the short-duration ion pulses easely coupled with fast laser probes, to investigate ion-induced electronic transitions and ionization processes in dielectric samples. We expect this program to have a significant impact on the field of water and solid material radiolysis since they will yield the first time-dependent measurements of the mechanisms at play, with important repercussions for e.g. medicine or the nuclear energy industry [18].

Neutron sources

Beside ion sources, we also plan to develop neutron sources by inducing nuclear reactions in hot plasmas. This will give users access to unprecedented short neutron bunches, not available on conventional facilities such as neutron tubes or spallation sources.

 

Maj : 16/05/2012 (15)

 

 

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