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. Fig. 1 shows a typical experimental setup for ion acceleration.

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.

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].

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 (~10²² Wcm^–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 electronsfrom 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 (highZ/low Z) targets having thickness up to several hundred nanometers, these interactions not only expel electrons from the irradiated area but alsoaccelerate the remaining heavy ion core, which begins to move in the directionof 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 amoving 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 andmore efficient proton beams are possible than any observed to date [18].

We have simulated the direct interaction of a 500 TW (peak intensity of 3x10²² Wcm^-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.

Plasma Mirrors and Contrast

What many of these ion acceleration schemes, and also High Harmonic Generation rely upon is a high contrast laser pulse. The TNSA mechanism is severally degraded if the pre-pulse intensity is high enough to heat and shock the target so that pre-plasma on the rear surface of the targetis generated, which acts to screen ions from the accelerating electric field [19]. Any scheme reliant upon ultra-thin targets, i.e.laser-piston regime, radiation pressure regime, directed-Coulomb explosion regime, are highly dependent upon a low pre-pulse level. A pre-pulse that generates plasma will preheat the target and destroy the sharp plasma boundaries crucial to the acceleration mechanisms. Unfortunately, the pre-pulse levels are increasingly becoming more problematic as ever-greater intensities are reached.

Currently, the most successful way to eliminate pre-pulses from a high-intensity laser pulse, thus creating an improved contrast ratio, is to use plasma mirrors [20]. Plasma mirrors are typically anti-reflection coated optics, which only become highly reflective once the laser intensity exceeds some threshold above which thesurface becomes ionized to plasma. A high plasma reflectivity is not the only parameter of consideration for a plasma mirror. For there to be negligible phase front distortion in the reflected beam (important for the focusability ofthe beam), the inequality c_st<l_laser must be met (c_s is the sound speed, t is the time from plasma formation to the peak of the pulse) [21]. Typical contrast enhancements (ratio of the plasma reflectivity to the anti-reflection coated optic before ionized) have been measured to be about 2 orders of magnitude with reflectivity of up to 70% [22]. Plasma mirror contrast enhancement at relativistic intensities has been demonstrated by using pulses in the high contrast plasma mirror (HCPM) regime. Pulses with contrast ratio >10¹⁰ were used to form the plasma mirror surface at >10¹⁹ Wcm^–2 producing reflectivities of ~60% with excellent spatial filtering and reflected near field beam profile allowing good focusability [23]. 

Ion Production using the Lambda-cubed Laser

Using high-contrast, 3 mJ pulses from the femtosecond 0.5 kHz lamda-cubed laser interacting with a bulk glass target, ions are accelerated backward (toward vacuum) along the normal direction. Protons with energies > 265 keV are recorded in a well-collimated beam with an opening angle of about 16° FWHM. Spectral measurements indicate that the central flux for protons exceeding 90 keV is 8.5×10¹¹s^-1sr^-1and for those exceeding 0.5 MeV, the peak is collimated to 3.2×10⁹s^–1sr^-1.Such a proton beam may be suitable for injection into external accelerators, for example. The same process could be used to produce energetic deuterons through laser interaction with a deuterated target, and subsequently generate significant fluxes of neutrons via fusion reactions [24].

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