Super-Heavy Ions Acceleration Driven by Ultrashort Laser Pulses at Ultrahigh Intensity

Abstract

The acceleration of super-heavy ions (SHIs, mass number of about 200) from plasmas driven by ultrashort (tens of femtoseconds) laser pulses is a challenging topic awaiting a breakthrough. Detecting and controlling the ionization process and adopting 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 ultrathin targets and ultrashort laser pulses at an intensity of1022W/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 of51+and extending to61+. The measured charge-state distributions supported by 2D particle-in-cell simulations serve as an additional tool to inspect the ionization dynamics associated with SHI acceleration, revealing that the laser intensity is the crucial parameter over the pulse duration for Au acceleration. Achieving a long acceleration time without sacrificing the strength of the acceleration field by utilizing composite targets can substantially increase the maximum energy of Au ions.

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Plain Language Summary

As a new particle acceleration method undergoing rapid development, laser-driven ion acceleration can economically provide ion beams with ultrahigh density and ultrashort duration, which are highly attractive for applications such as nuclear astrophysics, cancer therapy, and high-energy-density physics. However, the acceleration of superheavy ions (mass number of about 200) by ultrashort laser pulses is quite challenging. In this work, we accelerate superheavy ions (SHIs) to unprecedented energy and unveil the role of ionization dynamics in SHI acceleration by experimental measurements and numerical simulations.

We irradiate ultrathin (150 nm) composite gold foils with ultraintense laser pulses. The ultrahigh electric field in the targets deeply ionizes and accelerates the gold atoms. Using a novel self-calibrated spectrometer, we then measure the charge-state distribution of liberated gold ions with unprecedented energy of up to 1.2 GeV. This measurement provides abundant information to inspect the acceleration process associated with the ionization dynamics and, in turn, can be used as a probe for the ultrahigh field. The results indicate the acceleration fields in the composite targets composed of carbon nanotubes and gold foils have similar maximum strengths to simple foil targets, but they decay much more slowly, which eventually leads to higher gold-ion energies.

This work provides the first experimental results of SHI acceleration at such a high laser intensity of more than1022W/cm2. At higher intensity, we expect the generation of SHIs with tens of mega-electron-volts per nucleon. Also, by measuring the charge-state distribution of SHIs, our methodology provides a unique estimate of on-target laser intensity, which at ultrahigh values cannot be measured directly without damaging existing detectors.