When an intense laser pulse interacts with a thin film target it instantaneously turns a thin front layer of the film into a dense plasma. In this layer, where laser's and plasma's frequencies are equal, the laser pulse is strongly absorbed, terminating its propagation into denser plasma region. This layer is called the plasma's critical surface. Interaction of the intense laser light with plasma at this surface produces highly energetic electrons with multi-MeV energies which are emitted forward and propagate through the foil. Several processes can be responsible for generation of high-energy electrons such as resonant absorption, ponderomotive acceleration due to v x B force, stochastic heating, "vacuum heating", etc. Hot electrons propagating all the way through a thin film can establish a very strong electrostatic field at the back of the target which can accelerate protons with maximum energy of tens of MeV and ion beams with energy of hundreds MeV [1-4]. University of Michigan researchers were among the first to observe this effect. Figure 1 shows a typical experimental setup for ion acceleration.

Beams of highly energetic ions have a wide array of applications such as a compact sources of radioactive isotopes [5-7] and ion injector sources [8]. Beams of energetic proton can be used to generate quasi-homogeneous warm dense matter by isochoric heating of solid density foils [9] or can be used as a radiography source to detect and study the evolution of electric fields in laser-matter interaction [10]. The possibilities have been discussed for using beams of laser driven ions for hadron radiation therapy [11], fast ignitor research [12,13] and even for production of elementary particles [14,7].

Fig. 1. This picture shows the experimental apparatus, placed inside a steel vacuum chamber. The laser beam, shown in green, is focused by a parabolic mirror, in the foreground, onto aluminum foil (which is stretched across a screen mesh). The ions are accelerated in the plasma formed at the laser focus and detected by track detector, shown in the background.

Most proton beam applications require controllable quasi-monoenergetic beams that are orders of magnitude above current laser accelerated proton energies. For example, proton radiation therapy, which is superior to electron or x-ray radiation therapies, is currently performed with large accelerators and requires proton energies of about 200 MeV. At present, protons from laser-plasma interaction are produced from targets orders of magnitude thicker than the plasma skin depth in the target normal sheath acceleration (TNSA) regime [15]. This generates proton beams with energy up to several to tens of MeV but with an energy spectrum exhibiting an exponential decay of the number of the accelerated protons followed by a cutoff.

With HERCULES laser we will be able to explore a new regime of ion acceleration using interaction of ultra-relativistic and ultra-clean laser pulses with ultra-thin membranes to achieve controllable quasi-monoenergetic beams of protons with energy above 100 MeV. We anticipate that this research will result in the development of the efficient high-repetition rate table-top proton source, which can be of interest for proton radiation therapy.

Directed Coulomb Explosion Regime of Ion Acceleration

High laser contrast enables the direct interaction of an intense laser pulse with solid targets, as opposed to the lower density interaction that occurs when the target is pre-ionized by Amplified Spontaneous Emission. When a high contrast, intense laser pulse (~10^22 W/cm^2) interacts with a sub-micron thick foil, it ionizes the target within a few femtoseconds maintaining the integrity of the overdense plasma. For very thin targets, the ponderomotive force of the laser expels electrons from a region having a transverse dimension on the order of the laser spot size - resulting in a region of net positive charge. If the energy of the accelerated electrons is large enough to overcome the Coulomb attraction of the positively charged ion region, then the electrons will not return, resulting in ion acceleration in the "Coulomb explosion" regime [16,17].

For double-layer (high Z/low Z) targets having thickness up to several hundred nanometers, these interactions not only expel electrons from the irradiated area but also accelerate the remaining heavy ion core, which begins to move in the direction of laser propagation. The moving heavy ion layer experiences a Coulomb Explosion due to the excess of positive charge and expands predominantly in the direction of the laser pulse propagation. This expanding cloud generates a moving longitudinal electric field due to charge separation, which consequently provides very directional acceleration for low Z ions to high energies. We refer to this mode of ion acceleration as the Directed Coulomb Explosion (DCE) regime. Simulations predict that in this regime much higher energy and more efficient proton beams are possible than any observed to date [18].

We have simulated the direct interaction of a 500 TW (peak intensity of 3x10^22 W/cm^2) 30 fs Gaussian laser pulse with a double layer aluminum-hydrogen foil in the DCE regime. Fig. 2 shows ion density distribution. The spectrum of protons (Fig. 3) has quasi-monoenergetic features and is peaked at 140 MeV, with a FWHM of 10 MeV. The width of the peak is mainly due to the Coulomb repulsion of protons and can be varied by changing the amount of hydrogen, giving additional control.

Fig. 2. 2D Particle In-Cell simulation results - ion density distribution.

Fig. 2. 2D Particle In-Cell simulation results - Spectrum of protons accelerated by a focused (f/D=1.5) 500 TW Gaussian laser pulse interacting with a 0.1 micron thick aluminum foil with 0.05 micron thick hydrogen second layer.

The experimental results on ion acceleration with Hercules laser are presented in the poster "Proton Acceleration From thin Foils via Ultra-Intense Ultra-Clean Laser Pulses". (poster pdf file)

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