Davis G.K.

Paper	Title	Page
WE205	Electro-mechanical properties of spoke-loaded superconducting cavities	404
	Z. A. Conway, J. D. Fuerst, M. P. Kelly, K. W. Shepard ANL G. K. Davis, J. Delayen TJNAF
	This paper presents experimental data characterizing the electro-mechanical properties of superconducting spoke-loaded cavities developed for high-intensity ionlinac applications, such as the cw ANL Advanced Exotic Beams Laboratory (AEBL) driver linac and the pulsed FNAL High Intensity Neutrino Source (HINS, now project X) proton driver linac. High-gradient cw operation at 4.2 K can produce violent boiling in the liquid helium coolant causing microphonic frequency noise. A spoke cavity designed to minimize the effects of helium pressure on RF eigenfrequency, the total microphonic induced RF frequency variations, were found to be on the level of the phase noise in the reference oscillator. To determine the pulsed cavity RF performance, the Lorentz transfer function was measured and used to predict the dynamic detuning in pulsed operation. There is good agreement between the predicted fit and the measured data, demonstrating the utility of the Lorentz transfer function, which can completely characterize the dynamics of the coupling between the mechanical cavity structure and the cavity RF field due to the Lorentz force.
	Slides(PDF)
WEP28	Latest Results of ILC High-Gradient R&D 9-cell Cavities at JLAB	525
	R. L. Geng, R. Afanador, A. C. Crawford, G. K. Davis, D. Forehand, C. Dreyfus, C. Grenoble, B. Golden, R. Johnson, P. Kushnick, K. Macha, J. Mammosser, J. Saunders, A. Wu JLab D. Bice, D. A. Sergatskov Fermilab
	It has been over a year since JLAB started processing and testing ILC 9-cell cavities in the frame work of ILC high-gradient cavity R&D, aiming at the goal of a 35 MV/m gradient at a Q0 of 1E10 with a yield of 90%. The necessary cavity processing steps include field flatness tuning, electropolishing (EP), hydrogen out-gassing under vacuum, high-pressure water rinsing, clean room assembly, and low temperature bake. These are followed by RF test at 2 Kelvin. Ultrasonic cleaning with Micro-90, an effective post-EP rinsing recipe discovered at JLAB, is routinely used. Seven industry manufactured 9-cell TESLAshape cavities are processed and tested repeatedly. So far, 33 EP cycles are accumulated, corresponding to more than 65 hours of active EP time. An emphasis put on RF testing is to discern cavity quench characteristics, including its nature and its location. Often times, the cavity performance is limited by thermal-magnetic quench instead of field emission. The quench field in some cavities is lower than 20 MV/m and remains unchanged despite repeated EP, implying material and/or fabrication defects. The quench field in some other cavities is high but changes unpredictably after repeated EP, suggesting processing induced defects. Based on our experience and results, several areas are identified where improvement is needed to improve cavity performance as well as yield.
WEP32	Performance of the CEBAF prototype cryomodule renascence	540
	C. E. Reece, E. F. Daly, G. K. Davis, M. Drury, W. R. Hicks, J. Preble, H. Wang JLab
	The prototype cryomodule Renascence was constructed as an energy building block for securing 6 GeV operation of CEBAF and to validate design elements for future CEBAF upgrade modules. These elements include the new "HG" and "LL" 7-cell cavity designs and a new tuner design.[1,2] Issues were identified during initial testing in 2005. The module has been reworked to address the issues with thermal stability, component breakage, and tuner motion. In addition, opportunity was taken to employ upgraded cleaning and assembly techniques for the cavity string. The HOM coupler heating issue was resolved, and seven of the eight cavities in the cryomodule have run stably at an average of 20 MV/m CW. The cryogenic, rf, and mechanical performance of the cryomodule are presented. Commissioning in CEBAF has just been completed in October 2007.
WEP62	Diagnosis, analysis, and resolution of thermal stability issues with HOM couplers on prototype CEBAF SRF cavities	656
	C. E. Reece, E. F. Daly, G. K. Davis, W. R. Hicks, T. Rothgeb, H. L. Phillips, J. Preble, H. Wang, G. Wu TJNAF
	During initial testing of the prototype cavities incorporated into the developmental cryomodule Renascence severe thermal stability issues were encountered during CW operation. Additional diagnostic instrumentation was added. This enabled identification of an unanticipated thermal impedance between the HOM coupler probe feedthrough assembly and the cavity beamtube. Subsequent detailed FE analysis successfully modeled the situation and indicated the need for alternate cooling path for the couplers on those cavities. HOM damping was measured to be adequate employing only two of the four HOM couplers. The two pickup probes on the couplers at the input power coupler side of each cavity were removed, the remaining HOM probe feedthroughs were heat stationed to two-phase helium supply piping, and a novel heat sink was added to station both the inner and outer conductors of the remaining HOM rf cables. The characterization measurements, analysis, modifications, and resulting performance are presented.
WEP67	Low-level RF control of microphonics in superconducting spoke-loaded cavities	669
	Z. A. Conway, M. P. Kelly, S. I. Sharamentov, K. W. Shepard ANL G. K. Davis, J. Delayen TJNAF L. R. Doolittle LBNL
	This paper presents the results of cw RF frequency control and RF phase-stabilization experiments performed with a piezoelectric fast tuner mechanically coupled to a superconducting, 345 MHz, beta = 0.5 triple-spoke-loaded cavity operating at 4.2K. The piezoelectric fast tuner damped low-frequency microphonic-noise by an order of magnitude. Two methods of RF phase-stabilization were characterized: overcoupling with negative phase feedback, and also fast mechanical tuner feedback. The beta = 0.5 triple-spoke-loaded cavity RF field amplitude and phase errors were controlled to +(-)0.5% and +(-)30 respectively.