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Modern superconducting radio-frequency linear accelerators require the cavity bandwidth and detuning to be within a specified range to maximize the efficiency of the machine. To correctly estimate these states during operation, the measured RF signals should be calibrated. Due to the finite isolation of the waveguide directional couplers, cross-coupling effects in the forward and reflected channels complicate the calibration of the RF signals in Low-Level RF control systems. Past work proposed a compensation method employing least-squares optimization. This method requires the directivity of the directional couplers to be much higher than one. Additionally, the algorithm requires a tuning parameter to optimize the calculated calibration. However, for some accelerating systems, finding an acceptable value for such a parameter is challenging and time-consuming. In this paper, we present a way to overcome these limitations by performing a nonlinear least square optimization constrained by energy conservation laws. The method is tested with L-band superconducting resonators at loaded quality factors of 4. 6 ⋅ 1 0 6 and 2. 8 ⋅ 1 0 7 . [ABSTRACT FROM AUTHOR]

Present superconducting accelerators require a central refrigeration system and piping to distribute liquid helium and nitrogen. Stand-alone cryocooler-based systems are of significant interest both to scientific and industrial customers, as they allow applications at facilities where no cryogenic equipment and expertise exists, where it is undesirable to develop additional cryogenic systems, or where a mobile system that can be transported between beamlines or facilities is required. The technical challenge of this approach is to minimize the heat losses to the liquid helium so that commercially available, self-contained cryocoolers are able to handle the thermal load. This article presents an improved design for a stand-alone quarter-wave resonator (QWR) cryomodule based on commercially available cryocoolers. This cryomodule has a particular application in the Argonne Tandem Linac Accelerator System intensity upgrade project, where it will house a 72.75 MHz QWR for beam energy adjustments to experimental areas, where no helium cryogenic systems exist. The described cryomodule may also be the critical part of a turnkey system with application to material processing, semiconductor manufacturing, food irradiation, and homeland security. The development of low-heat loss peripheral components reduces the cost and widens applicability of the system. We will discuss the cryomodule and RF coupler designs, including numerical thermal and structural analyses. [ABSTRACT FROM AUTHOR]

Field emission currents emitted by micro-emitters are a limiting factor for the operational gradients of accelerating radio frequency (rf) cavities. Within the rf field emission theory the existence of needle like micro field emitters with very high length relative to the radius and corresponding high enhancement factor (β) is assumed. In this article the hypothesis that micro field emitters consists of long chains of conductive micro-particles is considered. Five different forces acting onto the particles in a high rf field are considered and the respective equations are derived. Some experimental observations and their explanation within this hypothesis are discussed. [ABSTRACT FROM AUTHOR]

One factor limiting the accelerating gradients in radiofrequency (rf) cavities are field emission currents emitted by micro-emitters. The value of emitter heating power plays a key role in theories of an rf cavity processing allowing to enhance the accelerating gradient. In this paper, the emitter heating by rf current is studied. This heating mechanism associates with a large heating power (by several orders of magnitude higher than the power of field emission current) and demonstrates explicit dependence on the frequency of the electromagnetic rf field (scales with the square of the rf field frequency). [ABSTRACT FROM AUTHOR]