Ion trapping around small positively charged objects in magnetized plasma

We present a novel trapping mechanism for ions around small positively charged objects in magnetized plasma. Here, the Larmor radius is larger than the object, but approximately equal to the characteristic radial length of the object's electrostatic structure. The resulting steady state solution from state-of-the-art particle-in-cell simulations shows a bipolar electric field around the object, with a density enhancement in the quasi-neutral region. Test particle simulations explore the mechanism behind the ion trapping by focusing on single-particle-motion, and transforming the particles to gyro-center, Larmor radius space. We also observe a kinetic energy gradient in the quasi-neutral region, where the ions organize themselves with respect to distance from the object according to their velocity, where the gyro-centers of particles are nearly coincident with the object's position. This leads to a shift in the velocity distribution. Analysis of test particles' temporal evolution in the gyro-center, Larmor radius space reveals regions of instability and stability, which correspondwell with the density enhancement and ion depletion. The results are similar for spherical and cylindrical objects, suggesting that the trapping mechanism is fundamental. These findings have implications for understanding plasma interactions with small objects, with potential applications in space physics, fusion devices, and plasma diagnostics.