Magnetic Centrifuge Effects in Ultrafast Laser Ablation Plasmas

TL;DR

This work develops and tests a self-consistent magnetic centrifuge model to explain anomalous isotope enrichment in ultrafast laser ablation plumes. It identifies cyclotron rotation in self-generated longitudinal fields as the primary driver of mass separation, with Ion Bernstein Waves providing resonant and broad-spectrum enhancements that amplify enrichment and inflate inferred field values. The analysis combines a cylindrical plasma centrifuge with a Gaussian radial field, Ampere-based current derivations, and a rigid-rotor benchmark to separate true magnetic effects from electrostatic wave contributions. A two-component field picture, B_eff = B_z + B_IBW, captures both static and wave-mediated influences on ion transport, with resonance conditions clarifying charge-state–selective enrichment. The results have implications for isotope harvesting and advance our understanding of laser-plasma physics under extreme fields.

Abstract

A self-consistent model is developed to explain the anomalously large enrichment of nickel isotopes observed in ablation plumes from ultrafast laser irradiation of solid surfaces. The model is based on the spontaneous creation of a magnetic centrifuge in the ablation plume and the associated cyclotron rotation of plasma ions with effective rotation rates on the order of $10^9$ radians per second. Mass separation occurs around the radial coordinate of cylindrical symmetry with longitudinal axis normal to the ablating surface. A Gaussian shaped radial magnetic field $B_{eff}$ is extracted for Ni isotopes which is shown to be a combination of an axial $B_z$ component and a second contribution $B_{ibw}$ that represents the equivalent of an effective magnetic field contributing to the isotopic separation due to broad spectrum Ion Bernstein Waves providing electrostatic acceleration to the cyclotron orbits. These IBWs are also responsible for a profound resonance of enrichment observed for certain specific charge states. In addition to cyclotron rotation of ions, a rigid rotor model is also presented that is associated with the hydrodynamic rotation of the entire plasma and is shown to be of little consequence for the isotope enrichment. Cyclotron rotation and IBWs dominate the process.

Figures (10)

• Figure 1: Ablation Model for Enrichment Data as a Function of Observation Angle $\theta$. In the present work $r_m$ is taken as 125 microns which is 1.5 times the 50 $\mu\text{m}$ radius of the $(1/e^2)$ laser intensity profile
• Figure 2: Ni/Cu Enrichment Data as a Function of Angle
• Figure 3: Ni Isotope Enrichment Data as a Function of Angle
• Figure 4: Ion Energy Distributions of $Ni^{58}$ as Function Charge State
• Figure 5: Best Fit for Normalized Separation Factor using Hybrid of Ion and Isotope Masses