Ferrite loaded radio-frequency (rf) cavity with resonant frequency of several MHz is commonly used for proton synchrotron. Resonant frequency of the cavity is shifted upward during the beam acceleration by increasing the bias current. Because the bias current is swept sinusoidally at high repetition rate, rf characteristics of ferrites deteriorate considerably due to the disappearance of induced magnetic anisotropy by Co²⁺ and Co³⁺ ions.
This paper presents test results of rf characteristics for three ferrites with large diameter over 500mm using a ferrite test bench. The rf frequency and ac bias current vary from 2MHz to 3.3MHz and from OA to 1000A at maximum frequency of 75Hz, respectively. As the frequency of ac bias current becomes high, deterioration of rf characteristics and increase in ac bias current were observed. Consequently, it was verified that the ac bias test with full rf power using the test bench is inevitable for designing actual rf cavity.