Reducing Ferrite Tuner Power Loss by Bias Field Rotation

It has been suggested that ferrite tuners for rf cavities with the magnetic bias field perpendicular to the rf magnetic field would have greatly reduced rf losses. Recent measurements at Los Alamos National Laboratory appear to confirm this effect. A simple model proposed here allows the calculation of tuning characteristics for a variety of bias schemes. The model shows that the perpendicular bias scheme mentioned above requires very much larger bias levels than does the parallel bias scheme in order to achieve the same tuning range with a particular ferrite tuner. However, further investigation with the model has led to the discovery that the use of perpendicular bias at low frequency and parallel bias at high frequency requires only a modest increase in the bias field. In effect, the ferrite is kept highly magnetized, reducing ferrite losses, and is tuned primarily by rotating the bias field direction with respect to the rf field direction. The resulting reduction in dissipation can significantly reduce the amount of ferrite required per cavity.