A halo generation mechanism in the non‐periodic lattices such as the SNS linac MEBT (Medium‐Energy Beam‐Transport between RFQ and DTL) is reported. We find that the nonlinear space charge force resulting from large transverse beam eccentricity ∼2:1 in the ∼1.6‐m‐long MEBT chopper section is responsible for halo formation. As a result, the beam distribution, based on the Front End (FE) emittance measurements and multiparticle simulation studies, develops halo that leads to beam loss and radio activation of the SNS linac. Designing lattices with transverse beam eccentricity close to 1:1 suppresses this kind of halo generation. Modifying the MEBT optics and introducing adjustable collimators in the MEBT significantly reduced beam losses in the CCL, which is a preferred scheme for mitigating halo. © 2003 American Institute of Physics