Effects of deliberately introduced magnetic field perturbations on plasma conditions have been studied in the Advanced Toroidal Facility (ATF) torsatron. Magnetic dipole coils, located above the plasma at two positions 180 degrees apart, created a perturbation Bz0/Bϕ0 ⩽ 1.4%, which produced a large l=1/2 island. Two modes of plasma operation were employed: (1) constant gas feed allowing the electron density to vary and (2) constant density with the gas feedback controlled. Without density control, the density and the stored energy were approximately halved. With density control, the stored energy decreased only slightly and the potential increased in the vicinity of the island. It was found that both with and without density control, the pressure profile narrowed and the particle confinement was lowered substantially when the perturbations were applied. Thus, field perturbations, provide an external control on the pressure profile and the particle confinement time