Ion Cyclotron Resonance (ICR) is a method that uses cyclotron waves at the resonant frequency of target ions to heat the plasma. Owing to its low energy consumption, high separation factor, and ability to effectively circumvent Coulomb repulsion limitations caused by space charge effects in conventional electromagnetic methods, it has become a highly promising isotope separation technology. Magnetic field is a key parameter affecting the performance of isotope separation using the ICR method. Existing theoretical studies on ICR-based isotope separation generally assume a uniform background magnetic field and an ideal circularly polarized electric field (neglecting the RF magnetic field). However, under experimental conditions, plasma collective effects and the actual structure of the antenna can disrupt the ideal circular polarization state of the electric field, in which case the influence of the RF alternating magnetic field on ion motion may no longer be negligible.
Based on the parameters of the Russian SIRENA device, a three-dimensional physical model for lithium isotope separation by ion cyclotron resonance heating with a four-arm helical antenna was built in the HybridCircuit/3DLHDAP simulation software. Under the single-particle approximation, the finite element method, together with a collision-inclusive cold plasma dielectric tensor, was used to analyze how the RF antenna's alternating magnetic field affects the transverse energy and separation distance of lithium ions at the collector plate.
The simulation results indicate: 1) Under the influence of the RF alternating magnetic field, which makes the magnetic field gradient drift effect no longer negligible, both the peak transverse energy of the resonant ion ⁶Li⁺ and the peak separation distance between ⁶Li⁺ and ⁷Li⁺ at the collector plate shift toward lower frequencies. Furthermore, both parameters show nonlinear dependences on the feeding voltage and initial transverse velocity, with optimal thresholds of 6 kV and 30 km/s, respectively. 2) When the RF alternating magnetic field effect is taken into account, the optimal threshold of the initial axial velocity becomes higher than that without this effect. 3) Under the conditions of an RF frequency of 774 kHz, a feeding voltage of 6 kV, an ion transverse velocity of 30 km/s, and an axial velocity of 40-60 km/s, the RF alternating magnetic field is beneficial for enhancing the transverse energy of ⁶Li⁺ and the separation distance between ⁶Li⁺ and ⁷Li⁺ at the collector plate.
This study reveals the dual-action mechanism of the RF alternating magnetic field in ICR lithium isotope separation, corrects the theoretical deviation of the conventional uniform magnetic field model, and provides a more accurate theoretical basis for the experimental design and parameter optimization of ICR isotope separation devices. Future work will further investigate the effects of plasma temperature, antenna structure, and other factors on the isotope separation process.