Development of higher-order-mode (HOM) absorbers for KEKB superconducting cavities

KEKB, B-Factory at KEK, is a double ring collider of 3.5 GeV positrons and 8 GeV electrons. The two rings were named LER (Low Energy Ring) and HER (High Energy Ring) after their energies. They were constructed in the tunnel that was used for TRISTAN. One of the main physics objectives of this facility is the detection of so-called CP-violation, which may give an answer to the question why the amount of matter and anti-matter is not the same. With regard to the accelerator, KEKB is a very challenging accelerator in the sense that it has to deal with ampere-class current (2.6 A for LER and 1.1 A for HER), which is about two orders of magnitude higher than that of TRISTAN and has never been used with any machine before. To achieve a high current without suffering from single bunch instabilities, the beam will consist of some 5000 bunches, i.e., every RF bucket filled with a bunch of several 1010 positrons/electrons. In this case, HOM (Higher Order Mode) fields that are excited by a bunch at accelerator components, such as RF cavities, are likely to affect the following bunches because the decay times of the fields are sometimes longer than the bunch interval, e.g., 2 ns for KEKB. This situation makes the beams susceptible to coupled bunch instabilities, i.e. intolerable growth of the oscillatory coupled-bunch motions. For the success of KEKB, it is essential to lower the HOM impedances so that the threshold currents are higher than the designed currents or they are manageable with a bunch-by-bunch feedback system. The most effective way to achieve this is to lower Q's (quality factors) and R/Q's (effective shunt impedances) of most HOM's in RF cavities since RF cavities are normally the main source of HOM's. This can be embodied by so-called HOM-damped structures. At KEK, we use two types of such structures, one is called ARES (Accelerator REsonantly coupled Structure) and the other is SCC (Superconducting Cavity). ARES is the invention of creative people at KEK. Using a three-cell structure with a large energy-storing cell, it can lower the R/Q of fundamental mode to 15. Whereas, SCC is inherently suited for high intensity machines, such as KEKB, since the aperture of the beam pipe can be larger with enough accelerating gradients. The damped structure for SCC has large beam pipes with absorbing material. Most of the HOM's go through these pipes and are absorbed. This scheme, however, that puts microwave absorbing material, such as ferrite, in the same SCC structure and vacuum was a new approach and its feasibility had to be proved. Therefore, one of the key components for the success of this scheme was the HOM absorber, which is the subject of this thesis. The HOM absorber for SCC requires sufficient HOM damping, power handling capability (5 W/cm2<), UHV (Ultra High Vacuum) compatibility (∼1x10-9 Torr >), free of particulates that enter and degrade the cavity, and some surface electric conductivity to prevent charge-up and sparks. Due to the favorable results of preliminary tests on HOM damping, outgassing rate, high availability and short delivery time, we chose to use a microwave-absorbing ferrite IB-004 from TDK, Inc. However, the most difficult part of the development was to bond this material with high integrity to meet the above-mentioned requirements. Neither brazing nor soldering of commercial tiles (60x60x4 mm3) were successful due to cracking during the cooling down from brazing/soldering temperatures. Also, we thought of making a cylindrical-shaped ferrite, but experts at TDK commented that making a thin (∼4 mm) cylinder of large diameter (220/300 mm) ferrite would be very difficult and probably crack. Forming a thin layer with, for example, plasma spraying, would probably result in a very porous surface and be incompatible with UHV and free-of-particulate conditions. The idea, that we demonstrated its usefulness and high potential for more applications, is sinter-bonding of pre-sintered ferrite powder directly onto the inner surface of a copper beam pipe with HIP (Hot Isostatic Press). Having developed the proper tooling and optimizing the conditions, we succeeded in establishing a technique to make full-size absorbers that met all the requirements. We installed a full-size (300-mm in diameter) absorber at the Nikko section of the ring with tapers on both sides and tested its performance. There was no spark, discharge, damage or degradation up to an available maximum single bunch current of 4.4 mA or 2.8x1011 electrons per bunch, which is 20 times that of KEKB-HER. A fully equipped superconducting cavity of KEKB shape was assembled with HOM absorbers attached on the upstream and downstream sides, installed in the east tunnel and tested for three periods of time. SCC could store 0.57 A, about half of the design current at KEKB-HER, without any instability due to cavity HOM's. No degradation of cavity performance was observed and up to a total power of 4.2 kW was recorded at the absorbers without any problem, which is about 80% of the power expected at KEKB-HER. Finally, a total of 8 absorbers were assembled with four KEKB-HER superconducting cavity modules and installed in the Nikko D11 tunnel in the summer of 1998. These modules have shown gap voltages of 2.5-3 MV without beams, which is much higher than the design value of 1.5 MV, assuring a significant margin for stable operation. Since the commissioning started in December of 1998, the system has been operating very smoothly. As of January 31, 2000, the maximum current achieved so far is 0.51 A, about half of the designed value. All HOM absorbers have been showing similar behavior and the maximum power absorbed so far is about 2.5 kW per module, which is consistent with calculations. (J.P.N.)