Reduction of the Q-Loss-Effect in Ferrite-Loaded Cavities

The performance of cavities loaded with Ni-Zn ferrites under parallel DC-biasing is often limited by the highloss-effect (HLE) or quality-loss-effect (QLE) [1]. After some milliseconds at fixed frequencies, a sudden drop of the resonator's voltage occurs above a specific threshold level. The mechanism of this loss has not been fully understood yet. At GSI, a simple method has been found to work against this effect with the aid of mechanical damping of acoustical surface waves [2]. We are able to show that similar methods applied to full size rings lead to a significant increase of the onset voltage of the QLE. Most of the existing ferrite loaded accelerating cavities with QLE limitations can be improved in terms their accelerating voltage by this modification. Investigations related to the QLE Modal structure of the QLE The sound fields within the ferrite core were analysed numerically by 2D-calculations. Some results are presented in figure 1. At higher frequencies there are mainly pure surface waves. The mode density is dominated by the largest physical dimension – that means by the outer circumference of the ferrite cores. Our results are summarised in table 1 where the mode density is roughly given in 'number of lines/10kHz'. The geometric influence is evident. The mode density could be measured as a corresponding subharmonic magnetic RF-response as shown in figure 2. In large rings, discrete modes seem to vanish at higher RF-levels as they can hardly be measured because of their overlapping. Table 1: Acoustical Mode Densities of our Samples Material Ni-Zn Ferrites MAGNETON400 N PHILIPS 8C12 std. CMI CMD 5005 PHILIPS 8C12 mod. Size: D x d x h 32x20x5 36x28x6 100x50x25 498x270x25 1st Resonance 57 kHz 54 kHz 26.4 kHz 4.5 kHz calc. Density 2.4/10kHz 1.6/10kHz 28/10kHz 70/10kHz meas. Density 3.4/10kHz 1.2/10kHz 28/10kHz 35/10kHz Q@QLF ~ 500 ~ 500 ~ 2500 ~ 5000 Figure 1: examples of circumferentialand shell-modes Figure 2: corresponding 1⁄2fRF QLE signals Suppressing the QLE with coatings Two ring cores made of modified FXC8C12 by FERROXCUBE treated with a thin bitumen coating were measured at 1MHz with a DC-bias of 300A/m. The coating has a thickness of approx. 1mm. The QLE threshold is shifted from 180Vp to > 370Vp per ring (figure 3). Figure 3 (l): QLE-suppression with bitumen coating Figure 4 (r): Prototype increase of the gap voltage In order to verify the effect of mechanical damping, the SIS18 prototype cavity was covered with ordinary insulation tape on its accessible surface (less than 50%). A considerable increase by approx.10% of the final gap voltage was achieved – see figure 4. Conclusion and Future PlansCurrently, the gap voltages reached in the SIS18 cavi-ties are still limited by the QLE. But according to the de-scribed experimental results, one can expect a consider-able increase just by coating the ferrites’ surfaces with anadequate damping material. The Budker Institute of Nu-clear Physics (BINP) developed a cavity, based on en-tirely encapsulated ferrite stacks [3]. On the basis of thisassembly, we expect a sufficient suppression of the QLE. References[1] J. E. Griffin, G. Nicholls: 'A Review of Some Dy-namic Loss Properties of Ni-Zn Accelerator RF Sys-tem Ferrite';IEEE Trans.o.N.S., Vol. NS-26, 1979[2] H. G. König, S. Schäfer: "Reduction of Q-loss-Effects in Ferrite-loaded Cavities", GSI, EPAC 2008[3] V. S. Arbuzov et al.: 'Accelerating RF Station forHIRFL-CSR, Lanzhou, China ', BINP Novosibirsk,Proceedings of RuPAC XIX, Dubna 2004163kHz157kHz127kHzGSI SCIENTIFIC REPORT 2008GSI-ACCELERATORS-12