Abstract
We have investigated proton acceleration in the forward direction from a near-critical density hydrogen gas jet target irradiated by a high intensity (1018 W/cm2), short-pulse (5 ps) laser with wavelength of 1.054 μm. We observed the signature of the Collisionless Shock Acceleration mechanism, namely quasi-monoenergetic proton beams with small divergence in addition to the more commonly observed electron-sheath driven proton acceleration. The proton energies we obtained were modest (~MeV), but prospects for improvement are offered through further tailoring the gas jet density profile. Also, we observed that this mechanism is very robust in producing those beams and thus can be considered as a future candidate in laser-driven ion sources driven by the upcoming next generation of multi-PW near-infrared lasers.
Highlights
Over the past decade, laser-accelerated ion beams[1,2,3,4] have attracted considerable interest due to their unique characteristics and have already enabled many applications
Before presenting the proton acceleration results, we should note that the laser pulse that we used to drive the ion acceleration had a pedestal before the main pulse arrives
Since the prepulse intensity (ILλL2= 1013 W.μm2/cm2) is above the ionization threshold, it modified significantly the gas jet density profile ahead of the main pulse irradiation. This was on one hand beneficial, since it reduced the thickness of the gas target, which increases the efficiency for Collisional Shock Acceleration (CSA), but on the other hand, it had the detrimental effect to push the critical density interface away, i.e. this effectively defocuses the high-intensity laser pulse arriving on the target interface and reduces its ability to drive a strong shock
Summary
Laser-accelerated ion beams[1,2,3,4] have attracted considerable interest due to their unique characteristics and have already enabled many applications. A second scheme relies on the laser radiation pressure, but this time in thicker targets where it directly puts in motion the ions at the critical density interface at which the laser is stopped. This is the so-called hole-boring (HB) mechanism[29] that accelerates these front-surface ions[30]. It is based on the fact that the laser pulse can induce a collisionless shock wave in a near-critical density target, and the propagating shock can reflect ions in the target and accelerate them to high energies Such shock wave is generated by the laser-accelerated fast electrons injected into the target. Due to their high energy, the collisional dissipation into these electrons is negligible[38], collisionless processes (i.e. mediated by instabilities and plasma waves) can provide enough energy dissipation[39]
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