J. Kirchgessner

Parallel Coupled Cavity Structure

A parallel coupled RF cavity structure which provides favorable solutions to all of the requirements for use in an e+ e-storage ring is described. Properties of this structure have been determined mathematically and through measurements on S-band models. An L-band prototype is being constructed and will be tested at high power.

Simulation Measurement of Bunch Excited Fields and Energy Loss in Vacuum Chamber Components and Cavities

Following the method of Sands and Rees, we have developed a two arm comparison apparatus of high sensitivity and good temporal resolution for the measurement of bunch engendered fields. With a relative timing uncertainty of .5 picoseconds the apparatus has demonstrated a sensitivity of .05¿ for the resistive impedance in the presence of a reactive impedance of 4.2¿ for a gaussian shaped bunch of standard deviation 2.1 centimeters. The apparatus is described and results for a typical component are presented. Both energy loss and bunch generated fields are discussed.

Fabrication of Superconducting Niobium Radio Frequency Structures

During the last several years a variety of superconducting radio frequency structures have been designed, fabricated and tested. The diverse structures and fabrication techniques will be described, For much of the work described, no claim is made as to the original conception having taken place in our laboratory. This paper is a description of our experiences in this field.

Development of Superconducting Cavities of Cylindrical Symmetry at Cornell

The fabrication, processing and measurement of five single cell and one 5-cell L-band niobium cavities of elliptical shape are discussed. Q-values of 2 to 5×109 have typically been obtained without high temperature firing. In single cells magnetic surface fields as high as 56 mT corresponding to an accelerating field of 7 MV/m have been measured. In the 5-cell structure a gradient of 4.7 MV/m was achieved.

Superconducting Cavities for a Large e+ e- Colliding Beam Accelerator

An L-band cavity accelerator assembly suitable for large storage ring service is being developed. Design criteria are presented and development status is detailed.

Design Studies for a 1500 MHz Superconducting Accelerator Cavity for use in an e+e− Storage Ring

The cost of a high-energy e+ e− machine can be substantially reduced if superconducting cavities with accelerating gradients of several MV/m can be used1,2). From an RF performance as well as economic point of view, higher operating frequencies are preferred. The upper limit, however, is set by requirements of storage ring operation such as useful aperture, beam bunch length, stored energy, and synchrotron oscillation frequency. Based on these considerations, design studies on a 1500 MHz structure have been carried out with a view towards installing 2m of superconducting cavity in CESR. These studies include measurements on higher-mode properties of the multi-cell structure, tests of devices that provide the coupling necessary for higher-mode power extraction, and damping of instability driving modes, multipactor control techniques on superconducting sections, and tests of 1500 MHz multi-cell superconducting sections.

Azimuthal Shaping of Cylindrical Accelerating Cavities for Improved Higher Order Mode Extraction

In cylindrically symmetric accelerator cavities equipped with higher order mode (HOM) couplers, the two polarizations of each deflecting mode are often fixed by the couplers themselves, leaving one polarization inadequately damped. This problem can be overcome by deliberate breaking of the cells cylindrical symmetry. We describe a quasi-cylindrical cavity profile in which the polarization of the dipole, quadrupole, and sextupole HOMs are fixed at ±¿/4, ±¿/8, ±¿/12 with respect to the intended coupler orientation, providing a maximally favorable situation for damping by a single coupler. A single cell S-band cavity was built to test the practicality of our design. The experimental data, in the range 3-7 GHz, conclusively shows that the mode polarizations can be controlled with only minor perturbations to the cells cylindrical symmetry.

A Single Beam Multibunch Instability at CESR

A transverse coupled bunch instability has been observed in the Cornell Electron Storage Ring CESR for both positrons and electrons. This instability shows strong horizontal or vertical coherent signals but very weak longitudinal signals. Positron injection, which requires sixty-one uniformly spaced bunches in CESR, was originally restricted by the enlarged effective horizontal beam size resulting from this instability. Although several cures have been discovered, only external octupoles provide a sufficient increase in the threshold and are compatible with injection. The theory for coupled bunch motion correctly predicts the instability threshold assuming that the driving mechanism is a 1140 MHz parasitic resonance in the RF cavity. Changes in the threshold with tune and octupole field strength are also correctly predicted.

Details of the Cryogenic System for the Cornell Superconducting R.f. Beam Tests

During the course of the most recent and past Superconducting R.F. beam tests in CESR, various techniques and concepts were developed in the implementation of the cryogenic system1,2,3,4. Some of these items will be described and will include, the long liquid helium transfer line, the liquid phase separation, the liquid nitrogen cooling system, feeding the wavequide heat exchangers, the JT valve, the temperature regulation and pumping system, the overall control and monitoring system, large and many indium seals, and mechanical details of the cryostat vessel.