Microwave Quality Assurance for Single Cells of the Next Linear Collider

摘要

The Next Linear Collider comprises about 5,000 traveling wave sections, each manufactured from about 200 precision machined cells. Each cell in a section has a different shape. Measuring the shape of a cell with sufficient accuracy takes approximately of 1/2 hour using conventional CMM techniques. This is because many points on the interior of the cell must be measured to assure the cell will be resonant at the proper monopole (accelerating) frequency and the proper dipole frequency. On the other hand these frequencies can be measured to the required accuracy of a fraction of a megahertz in a few seconds using either a modern microwave network analyzer or a specialized system designed just for measuring these cell resonances. The microwave measurement in effect does an integral over the whole surface contour to find the net error in desired frequency. In order to avoid damaging cells it is ideal to use a non-contacting RF test apparatus with non-contacting shorts and choke joints on either side of the cell. The complex geometry of the proposed damped detuned cell structures makes it difficult to obtain high Q with non-contacting shorting plates[1]. It is still possible to get sub-megahertz resolution by measuring phase rather than amplitude at resonance and using reference cavities for calibration. It is assumed that the system will be highly automated with automatic cell handling and data recording.