Status of RF power couplers for superconducting cavities at CERN

For LEP2 fixed RF power couplers of the open-ended coaxial line type with d.c. bias are used. The nominal power under matched conditions is about 120 kW at 352 MHz. However, to avoid ponderomotive instabilities, the cavities may not be detuned, i.e. the reactive beam loading cannot be compensated. The coupler is therefore exposed to standing waves with an equivalent power (travelling-wave (TW) producing the same field as the peak fields on the coupler line) of more than 200 kW. The final design of these couplers, their conditioning sequence and their actual performance are presented. For LHC a motor-driven mobile coupler is required to change the external cavity Q by a factor of four between beam injection and storage. During injection the forward power levels at 400 MHz are about 120 kW CW (for approximately 20 minutes) and 180 kW peak (for several milliseconds). Since practically all this RF power is reflected the equivalent travelling power is 480 kW and 720 kW, respectively. These couplers will be also provided with d.c. bias to suppress multipacting and ³deconditioning²

Thermocurrents and their Role in high Q Cavity Performance

Over the past years it became evident that the quality factor of a superconducting cavity is not only determined by its surface preparation procedure, but is also influenced by the way the cavity is cooled down. Moreover, different data sets exists, some of them indicate that a slow cool-down through the critical temperature is favourable while other data states the exact opposite. Even so there where speculations and some models about the role of thermo-currents and flux-pinning, the difference in behaviour remained a mystery. In this paper we will for the first time present a consistent theoretical model which we confirmed by data that describes the role of thermo-currents, driven by temperature gradients and material transitions. We will clearly show how they impact the quality factor of a cavity, discuss our findings, relate it to findings at other labs and develop mitigation strategies which especially addresses the issue of achieving high quality factors of so-called nitrogen doped cavities in horizontal test

RF thermal and new cold part design studies on TTF-III input coupler for Project-X

RF power coupler is one of the key components in superconducting (SC) linac. It provides RF power to the SC cavity and interconnects different temperature layers (1.8K, 4.2K, 70K and 300K). TTF-III coupler is one of the most promising candidates for the High Energy (HE) linac of Project X, but it cannot meet the average power requirements because of the relatively high temperature rise on the warm inner conductor, some design modifications will be required. In this paper, we describe our simulation studies on the copper coating thickness on the warm inner conductor with RRR value of 10 and 100. Our purpose is to rebalance the dynamic and static loads, and finally lower the temperature rise along the warm inner conductor. In addition, to get stronger coupling, better power handling and less multipacting probability, one new cold part design was proposed using 60mm coaxial line; the corresponding multipacting simulation studies have also been investigated.Comment: 5 pages, 12 figures. Submitted to Chinese Physics C (Formerly High Energy Physics and Nuclear Physics

RF system of electron injector for the race-track microtron-recuperator and results of its operation with electron beam

The RF system is a part of the 1.6 - 2 MeV injector for the Race-Track Microtron - Recuperator (RTMR) that is under construction at BINP, Novosibirsk, for the Center of Photochemistry. RF system has three 180.4 MHz cavities. Buncher cavity operates at the accelerating voltage of 100 kV and two accelerating cavities operate at the gap voltage up to 800 kV. Cavities are driven by 3 power amplifiers. Maximum output power of amplifier which feeds the accelerating cavity is 130 kW. Low level electronics controls phase and amplitude of RF cavity gap voltages and generates signals for synchronization of the electron gun. Maximum current of injector (45 mA) is realized at 22.5 MHz repetition rate of electron bunches. The effects of beam – RF cavities interaction and the RF system operation results are presented

Running CESR at High Luminosity and Beam Current with Superconducting RF System

Abstract The transition from normal conducting to superconducting RF system has been completed at CESR in 1999. The new system is one of the changes, which allowed the collider to operate at higher currents and reach higher peak luminosity. The commissioning results and operating experience with the SRF system are presented alongside with describing its effect on CESR performance. Future plans are discussed