Stability improvements in the TRIUMF cyclotron magnetic field to +- 0.7ppm

The TRIUMF cyclotron magnet with a hill field strength of 5.6 kG at an excitation current of 18,000 amperes has recently been stabilized to +- 0.7 ppm with a view to running high resolution beam with separated turns at an energy of 500 MeV. The paper will highlight the methodology involved in improving the performance of the power supply by more than an order of magnitude. In addition to current and voltage feedback loops, active field feedback from both a flux coil and an NMR system were used. Further improvement by about a factor two to beam phase stability was achieved using beam phase signals from the extracted beam