Abstract
A novel method has been developed that allows extracting the loaded quality factor of particle accelerating radio-frequency cavities at any time during beam operation from acquired control system process parameters without the need of additional measurement equipment. Concurrently it assesses the attenuation in a cavities' power transmission line for any rf generator to rf cavity mismatch condition, and thus can correct calibration inaccuracies of measured forward and reflected power levels. The mathematical formalism is described. It is specifically derived for cryogenically cooled superconducting cavities for which the loaded quality factor can be equated with the external quality factor of the input power coupler. The described method has been applied for 418 superconducting cavities in operation at the Continuous Electron Beam Accelerator Facility at the Thomas Jefferson National Accelerator Facility. The validation of the method as well as its beneficial use as a diagnostic tool is demonstrated for a finite number of examples utilizing process variables archived during a period in the past when the linac beam current was ramped up multiple times. The method is in the process of being implemented in the machines' control system for an in situ and in operando evaluation of the power coupler's external quality factor.
Highlights
A method is described that enables evaluating the loaded quality factor, Ql, of particle accelerating radio-frequency cavities at any time during beam operation (: Ql method)
A few simplifications concerning the mathematical derivations of the Ql method—well applicable for superconducting rf (SRF) accelerating cavities—are outlined allowing the formalism to be employed with minor penalty
This relates to the condition that the fundamental power input coupler (FPC) is largely overcoupled to the cavity in absence of a beam, which implies that the Ql is nearly equal to the external quality factor, Qext, of the FPC
Summary
A method is described that enables evaluating the loaded quality factor, Ql, of particle accelerating radio-frequency (rf) cavities at any time during beam operation (: Ql method). A few simplifications concerning the mathematical derivations of the Ql method—well applicable for superconducting rf (SRF) accelerating cavities—are outlined allowing the formalism to be employed with minor penalty This relates to the condition that the fundamental power input coupler (FPC) is largely overcoupled to the cavity in absence of a beam, which implies that the Ql is nearly equal to the external quality factor, Qext, of the FPC. The Ql method simultaneously provides the attenuation loss of any power transmission line from the point where a measurement is taken to the cavity entrance, thereby correcting for the “true” power levels delivered to a cavity and reflected back from a cavity Given this capability it is shown that the Ql method can be used simultaneously as a diagnostic tool, especially when the Ql values are recorded as a function of the beam current. The Appendix summarizes the relevant mathematical derivations to formulate the new Ql method, which are separated from the main sections to provide better readability
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