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Register a new userSuper-Heavy Ions Acceleration Driven by Ultrashort Laser Pulses at Ultrahigh Intensity
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The acceleration of super-heavy ions (SHIs) from plasmas driven by ultrashort (tens of femtoseconds) laser pulses is a challenging topic waiting for breakthrough. The detecting and controlling of the ionization process, and the adoption of the optimal acceleration scheme are crucial for the generation of highly energetic SHIs. Here, we report the experimental results on the generation of deeply ionized super-heavy ions (Au) with unprecedented energy of 1.2 GeV utilizing ultrashort laser pulses (22 fs) at the intensity of 10^22 W/cm2. A novel self-calibrated diagnostic method was developed to acquire the absolute energy spectra and charge state distributions of Au ions abundant at the charge state of 51+ and reaching up to 61+. The measured charge state distributions supported by 2D particle-in-cell simulations serves as an additional tool to inspect the ionization dynamics associated with SHI acceleration, revealing that the laser intensity is the crucial parameter for the acceleration of Au ions over the pulse duration. The use of double-layer targets results in a prolongation of the acceleration time without sacrificing the strength of acceleration field, which is highly favorable for the generation of high-energy super heavy ions.
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A cascaded ion acceleration scheme is proposed by use of ultrashort laser-irradiated microtubes. When the electrons of a microtube are blown away by intense laser pulses, strong charge-separation electric fields are formed in the microtube both along the axial and along the radial directions. By controlling the time delay between the laser pulses and a pre-accelerated proton beam injected along the microtube axis, we demonstrate that this proton beam can be further accelerated by the transient axial electric field in the laser-irradiated microtube. Moreover, the collimation of the injected proton beam can be enhanced by the inward radial electric field. Numerical simulations show that this cascaded ion acceleration scheme works efficiently even at non-relativistic laser intensities, and it can be applied to injected proton beams in the energy range from 1 to 100 MeV. Therefore, it is particularly suitable for cascading acceleration of protons to higher energy.
The future applications of the short-duration, multi-MeV ion beams produced in the interaction of high-intensity laser pulses with solid targets will require improvements in the conversion efficiency, peak ion energy, beam monochromaticity, and collimation. Regimes based on Radiation Pressure Acceleration (RPA) might be the dominant ones at ultrahigh intensities and be most suitable for specific applications. This regime may be reached already with present-day intensities using circularly polarized (CP) pulses thanks to the suppression of fast electron generation, so that RPA dominates over sheath acceleration at any intensity. We present a brief review of previous work on RPA with CP pulses and a few recent results. Parametric studies in one dimension were performed to identify the optimal thickness of foil targets for RPA and to study the effect of a short-scalelength preplasma. Three-dimensional simulations showed the importance of ``flat-top radial intensity profiles to minimise the rarefaction of thin targets and to address the issue of angular momentum conservation and absorption.
We study and discuss electron acceleration in vacuum interacting with fundamental Gaussian pulses using specific parameters relevant for the multi-PW femtosecond lasers at ELI-NP. Taking into account the characteristic properties of both linearly and circularly polarized Gaussian beams near focus we have calculated the optimal values of beam waist leading to the most energetic electrons for given laser power. The optimal beam waist at full width at half maximum correspond to few tens of wavelengths, $Delta w_0=left{13,23,41right}lambda_0$, for increasing laser power $P_0 = left{0.1,1,10right}$ PW. Using these optimal values we found an average energy gain of a few MeV and highest-energy electrons of about $160$ MeV in full-pulse interactions and in the GeV range in case of half-pulse interaction.
A new diagnosis method for high energy ions utilizing a single CR-39 detector mounted on plastic plates is demonstrated to identify the presence of the high energy component beyond the CR-39s detection threshold limit. On irradiation of the CR-39 detector unit with a 25 MeV per nucleon He ion beam from conventional rf-accelerators, a large number of etch pits having elliptical opening shapes are observed on the rear surface of the CR-39. Detailed investigations reveal that these etch pits are created by heavy ions inelastically backscattered from the plastic plates. This ion detection method is applied to laser-driven ion acceleration experiments using cluster-gas targets, and ion signals with energies up to 50 MeV per nucleon are identified.
After the introduction of the ionization-injection scheme in Laser Wake Field Acceleration and of related high-quality electron beam generation methods as two-color or the Resonant Multi Pulse Ionization injection, the theory of thermal emittance by C. Schroeder et al, has been used to predict the beam normalised emittance obtainable with those schemes. In this manuscript we recast and extend such a theory, including both higher order terms in the polinomial laser field expansion and non polinomial corrections due to the onset of saturation effects in a single cycle. Also, a very accurate model for predicting the cycle-averaged $3D$ momentum distribution of the extracted electrons, including saturation and multi-process events, is proposed and tested. We show that our theory is very accurate for the selected processes of Kr$^{8^+rightarrow10^+}$ and Ar$^{8^+rightarrow10^+}$, resulting in a a maximum error below $1%$ even in deep saturation regime. This highly accurate prediction of the beam phase-space can be implemented e.g., in laser-envelope Particle in Cell (PIC) or hybrid PIC-fluid codes, to correctly mimic the cycle-averaged momentum distribution without the need of resolving the intra-cycle dynamics. Finally, we introduce further spatial averaging with Gaussian longitudinal and transverse laser profiles, obtaining expressions for the whole-beam emittance that fits with Monte Carlo simulations in a saturated regime, too.
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