IMPROVEMENT OF THE CURRENT STABILITY FROM THE TRIUMF CYCLOTRON

The νr = 3/2 resonance, driven by the third harmonic of the magnetic gradient errors, causes modulation of the radial beam density in the TRIUMF cyclotron. Since extraction is by H− stripping, this modulation induces unwanted fluctuations of the current split between the two high-energy beam lines. To compensate field imperfections, the cyclotron has sets of harmonic correction coils at different radii, each set constituted of 6 pairs of coils placed in a 6-fold symmetrical manner. The 6-fold symmetry of this layout cannot create a third harmonic of arbitrary phase, and so a single set of harmonic coils cannot provide a full correction of third harmonic errors driving the νr = 3/2 resonance. However, the outermost two sets of harmonic correction coils are azimuthally displaced. We took advantage of this fact to achieve a full correction of the resonance. This greatly reduces rapid fluctuations of the beam current in the high-energy beam lines. An active feedback system has also been implemented to compensate for the slow fluctuations (∼ 1minute range and above). This feedback acts on the amplitude of the first harmonic Bz correction produced by the outtermost set of harmonic coils. A proper choice of the phase of this first harmonic correction allows us to affect the split ratio, without changing the energy of the extracted beams.