Dependence of RF characteristics on ac bias current for large‐sized ferrites

A ferrite‐loaded radio‐frequency (RF) cavity with resonant frequency of several megahertz is commonly used for proton synchrotrons. The resonant frequency of the cavity is shifted upward during beam acceleration by increasing the bias current. Because the bias current is swept sinusoidally at a high repetition rate, the RF characteristics of ferrites deteriorate due to disappearance of the magnetic anisotropy induced by Co²⁺ and Co³⁺ ions. This paper presents test results of the RF characteristics for three ferrites with large diameters of over 500 mm. The tests were carried out on our ferrite test bench. The RF frequency and the ac bias current vary from 2 MHz to 3.3 MHz and from 0 A to 1000 A, respectively, at the maximum frequency of 75 Hz. As the frequency of the ac bias current became high, deterioration of the RF characteristics and an increase in the ac bias current were observed. Consequently, it was verified that an ac bias test with full RF power using the test bench was inevitable for designing an actual RF cavity. © 1999 Scripta Technica, Electr Eng Jpn 127(4): 1–8, 1999