In order to enhance the proton fraction in hydrogen ion beams, a high magnetic field, large‐volume magnetic multipole ion source has been designed and tested. The plasma source is made of a water‐cooled rectangular copper chamber, which is surrounded by a set of mild steel strips and samarium–cobalt magnets arranged in a continuous line‐cusp geometry. The magnetic field at the inner wall surface is 2.7 kG. This strong magnetic field enables us to enlarge the chamber volume without increasing the ion loss area. Large plasma volume and small ion loss area increase the ion confinement time and enhance the proton yield. The proton fraction in the beams, as measured by both a magnetic momentum mass analyzer and a optical spectrometer, is found to be more than 90% at an ion beam current of 28 A ( j=250 mA/cm2). This source is also operated with a weak magnetic field (∼0.6 kG) by replacing the samarium–cobalt magnets with AlNiCo magnet, in which case the proton fraction decreases to 80%. These experimental values are in good agreement with the values predicted by a simple scaling equation of the proton ratio on the plasma volume and the ion loss area.