Recently, Professor Mamiko Nishiuchi’s team from the National Institute of Quantum Science and Technology of the Kansai Institute of Photonic Science, Japan, presented a report entitled “Enhanced ion acceleration from transparency-driven foils demonstrated at two ultraintense” in Light: Science & Applications Laser Facilities”. Dr. Nicholas P. Dover of the Kansai Institute of Photonic Sciences, Japan and Dr. Tim Ziegler of the Technical University of Dresden (HZDR), Germany, are co-first authors, and Professor Mamiko Nishiuchi is the corresponding author. In addition, the John Adams Institute for Accelerator Science at Blackett Laboratories at Imperial College and Kyushu University in Japan contributed to this paper.
Research background
The emergence of reliable high-quality, high-power lasers has led to rapid development in particle acceleration. Further, with the improvement of ultra-strong laser pulse transmission performance and the breakthrough of technology to control the interaction between extreme electromagnetic fields and matter, laser-driven ion source technology has also been greatly developed. Based on the unique characteristics of laser-accelerated ion beams, this technology can be widely used in radiography, ultrafast material response research, material processing and high-dose-rate radiobiology and other fields of high-energy density physics experiments.
The traditional way to drive an ion source with a laser is to target normal sheath acceleration (TNSA). When a laser irradiates a thin foil, electrons absorb the energy of the laser on the front surface of the thin foil and leave the focus. As they leave the target, charge injection drives a quasi-static electric field that accelerates surface ions with maximum proton energies exceeding 70 MeV. However, the presence of the right amount of intensity scaling for this mechanism makes further increasing ion beam energy challenging. In order to increase the energy of the ion beam, ultra-strong pulses are required for acceleration, and laser pulses with ultra-high temporal contrast are also required because the thin foil is very fragile. In addition, experiments and simulations using real laser parameters show that it is difficult to achieve coherent acceleration over the entire duration of the laser pulse. Conversely, the instability of the target and the heating of the target electrons cause rapid expansion, reducing the core density of the plasma and weakening the acceleration effect. Eventually, the electron density drops below the critical density of the relativistic correction. Existing studies have shown that other acceleration mechanisms can also generate high ionic energies for targets driven in a relativity-induced transparent (RIT) state, including volume-enhanced sheath acceleration, magnetic eddy current acceleration (MVA), etc. Experiments in the RIT system have shown that ion beams with carbon ions exceeding 1 GeV and proton beams close to 100 MeV can be generated using a high-energy, picosecond laser system, demonstrating the importance of RIT over a wide range of laser intensities.
In addition, many applications for laser-driven ion sources require a transition from a single proof-of-principle study to a continuous repetitive operation. Currently, the implementation of high repetition rates requires the use of femtosecond high-power laser drivers, which can provide high-intensity but moderate laser energy compared to typical picosecond systems. Therefore, it is of great significance to use femtosecond lasers to optimize ion generation, especially to study RIT mechanisms to maximize ion energy.
Innovative research
Laser-driven ion source technology is used to generate high-energy, high-peak ion beams, and has very important applications in materials science, ultrafast laser science and other fields. Because this technology places demanding requirements on laser energy, time contrast, stability and controllability of the light source, it is impossible to replicate the ion acceleration performance on a stand-alone laser system with other similar parameters.
Figure 1. Experimental structure diagram. (a) A laser is irradiated at 45° onto a thin polyvinyl acetal foil to detect accelerated particles spatially by stacking two radiochromic films (RCFs). When RCF is not in use, frosted glass screens scatter transmitted laser light for detection by CCD cameras. (b) Time intensity contrast of laser pulses measured by third-order autocorrelation and spectral interferometry at sub-70 ps and sub-1 ps scales (inset), respectively. (c) Illustration of the prepulse induction extension of the main pulse reaching the target before it arrives.
In response to this problem, Professor Mamiko Nishiuchi’s team proposed a new ion acceleration device based on relatively transparent foil drive and enhancement, and carried out a series of experiments. In experiments, when the researchers irradiated a submicron-thick polyvinyl formaldehyde thin foil with a laser intensity greater than 1021 W/cm2, the system was able to produce high-energy protons in excess of 60 MeV and fully ionized carbon ions in excess of 30 MeV/u.
Figure 2. Experimental observation of accelerated ions. (a) The maximum ionic energy per nucleon recorded on the TPS detector for different target thicknesses d. (b) Proton spectra from RCF, combined with signals on two RCF stacks, for three sample target thicknesses. (c) For d=250 nm, spatial distribution of proton beams at different energies.
Figure 3. (a) Simulation comparison plot with laser prepulse (dashed/circled) and absent (dashed/triangle) 50 fs before the pulse peak, showing the maximum carbon and proton energies and the transmitted laser energy as a function of target thickness. (b) For different target thicknesses, an electron density line of 35 fs before the pulse peak arrives. For d=60 nm, the target is already transparent, resulting in electron beaming.
Specifically, the researchers avoided the need for a plasma mirror system by carefully selecting the foil thickness that matched the contrast of the laser, thereby preparing the target density for relative transparency. The ions are accelerated by an extreme local space charge field at a speed 1 million times faster than conventional accelerators. This field is formed by the rapid expulsion of electrons from the target by relativistic induced transparency, where the relativistic correction of the refractive index allows the laser to be transported through the normally opaque plasma.
Figure 4. Kinetic study of laser plasma interaction at optimal target thickness. (a) Two-dimensional plot (z=0) of Ey, Ex and carbon/proton density of the 250 nm thickness target at t = 0 when the pulse peak arrives. The solid (dashed) profile gives the relativistic (classical) critical electron density. (b) When the main pulse interaction ends, i.e. at t = 70 fs, the ion acceleration separated in the hot sheath field is demonstrated.
The researchers verified the experimental results by fluid dynamics and 3D particle intracellular (PIC) simulations, showing that when the foil becomes transparent, the extreme space charge-induced electric field due to electron discharge accelerates most of the energy particles, and then further accelerates in the diffusion sheath. The researchers replicated this mechanism on two different laser systems, highlighting the robustness of the mechanism. By actively controlling the laser contrast, it can be demonstrated that the optimal target thickness decreases when the amount of laser prepulse is reduced. These results show that high-energy ions can be accelerated by this mechanism at different contrasts, thereby relaxing the laser requirements of laser-driven ion source technology, and indicating the parameters for achieving application-specific beam transmission, paving the way for the development of high-repetition laser-driven ion source technology using a relatively induced transparency mechanism.
The study was published in Light: Science & Applications under the title “Enhanced ion acceleration from transparency-driven foils demonstrated at two ultraintense laser facilities.” (Source: LightScience Applications WeChat public account)
Related paper information:https://www.nature.com/articles/s41377-023-01083-9
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