Density profiles and electron fraction Y_e (dashed) from the three-fluid 2D MHD simulation. The left panel shows the initial conditions, with equal proton (red) and neutron (blue) densities in the jet core (r < R_j) and alpha particles (green) in the stellar envelope and developing cocoon (r > R_j). The three right panels show the evolved state for weak, moderate, and strong magnetic sieve regimes (increasing field strength from top to bottom). Each snapshot on the right panel is at different times in the range of 10⁻⁴–10⁻² s.