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The Renascence cryomodule [1] installed in CEBAF in 2007 consists of 8 cavities as shown in Figure 1. The first three cavities (No.1-No.3) in the upstream end are of the Low Loss (LL) shape design, and the remaining 5 cavities (No.4-No.8) on the beam downstream end are the High Gradient (HG) shape design. The fundamental power couplers (FPCs) are the rectangular waveguides, and the little cylindrical structures are the HOM couplers. The locations of the FPC in the last four cavities are mirrored about the beam z axis. Cavities No.4 and No.5 form a back-to-back cavity pair. Among the HG cavities installed in the Renascence cryomodule, the only identifiable difference from their fabrication documentation is that cavity No.5 received an extra EBW pass on one equator weld, specifically cell 5. The non-uniform mechanical tuning required to compensate the fundamental mode tune and flatness for the extra shrinkage of this cell is believed to contribute the most significant differences from the other HG cavities. Beam based instability studies on this cryomodule in CEBAF have shown a significant beam breakup (BBU) threshold current reduction, well below design value. Frequency spectrum peaked by the off-sided beam power indicated the cause is due to abnormal high Q modes in the cavity No.5. Measured beam off-axis position at the cavity No.5 does not correspond to the shunt impedances calculated for an ideal cavity. Low power RF measurements have identified that the problematic modes are in the second dipole band (TM110 like). Three of the modes have external Qs two orders magnitude higher than the others, while the rest of modes in the first two dipole bands are normal in terms of the design values. The cause of this abnormality and the future impact on the BBU was not able to be resolved due to the limitations of information that can be obtained from the measurements. It is important to understand the cause of this abnormality so that effective QA/QC measures can be implemented to avoid such problem in the final upgrade design and manufacture. The goal of this work is to utilize advanced simulation tools to understand the high external Q (Q{sub ext}) problem observed in the Renascence cryomodule. In the past years, SLAC has built a set of state-of-the-art advanced simulation tools based on finite-element unstructured meshes and parallel computation implementations on supercomputers [2, 3]. The codes are capable of simulating large complex RF systems with unprecedented resolution and turnaround time. They have been successfully applied to many existing and future accelerator R&amp;D projects to improve the machine performance and to optimize the designs. These tools are essential to perform accurate full system analyses such as the JLab's SRF cavities. We will use the simulation results and the data from the RF measurements to gain a better understanding of the cavity performance and tolerance issues and provide a solid foundation to do the BBU simulation and prediction for the 12GeV Upgrade project by using JLab's BBU codes. In this report, we will focus on the following two main tasks: (1) Ideal cavity simulation--to evaluate the effectiveness of the damping by the higher-order-mode (HOM) couplers, and search for possible trapped modes in a back-to-back cavity pair (e.g. cavity No.4 & No.5). (2) Abnormal cavity study--to understand the cause of the high Q{sub ext} modes in cavity No.5 using an advanced Shape Determination Tool

Analysis of the Cause of High External Q Modes in the JLab High Gradient Prototype Cryomodule Renascence