The development and characterization of an ultrahigh vacuum ‘‘unbalanced’’ dc magnetron sputter deposition system with a variable external axial magnetic field for controlling the ion‐to‐neutral flux ratio at the substrate during deposition with low negative substrate biases is reported. The target assembly is a planar‐magnetron (PM) with a toroidal magnetic‐field electron trap created using a set of permanent magnets. A pair of Helmholtz coils, located outside the vacuum chamber, produces an additional magnetic field Bext which is uniform along the axis orthogonal to both target and substrate surfaces. The value and sign of Bext has a strong effect on the plasma density near the substrate, and hence on the ion flux Ji incident at the substrate, with only a minor effect on the target‐atom flux. For a Ti target sputtered in pure Ar at 20 mTorr with a target‐substrate separation of 6.5 cm, changing Bext from -50 G (opposing the field of the outer PM pole) to +600 G (reinforcing the field of the outer PM pole) varied the ion‐to‐Ti flux ratio Ji/JTi incident at the substrate by a factor of 60 from 0.1 to 6 with the bias held constant at any desired negative value between ≂-15 V (limited by the difference between the floating Vf and plasma Vp potentials) and the highest negative values examined, -100 V. For reactive sputter deposition in N2 (where the primary ion is N2+) under the same conditions, Ji/JTi varied by a factor of 50 from 0.7 to 35. Vp was negative with Bext set to positive values and ranged from ≂0 (Bext=0) to -13 V (Bext≥+200 G) in Ar and 0 to -20 V in N2.