| Paper |
Title |
Page |
| MOP84 |
First Cryogenic Tests with JLab's new Upgrade Cavities*
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216 |
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- P. Kneisel, G. Ciovati, G. Myneni, G. Wu
Jefferson Lab, Newport News, Virginia
- J. Halbritter
FZ Karlsruhe, Karlsruhe
- J. Sekutowicz
DESY, Hamburg
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For the upgrade of CEBAF to 12 GeV two types of 7-cell cavities have been developed: the High Gradient type (HG) has been optimized with respect to the ratio of Epeak/Eacc and for the Low Loss (LL) type the shunt impedance has been maximized. Each cavity type features four DESY type coaxial HOM couplers and a waveguide input coupler. Design goals for these cavities have been set to Eacc = 20 MV/m with a Q-value at 2.05 K of Q0 = 8·109. A niobium prototype of each cavity has been fabricated at JLab and in a first test the HG cavity has been evaluated at cryogenic temperatures after appropriate buffered chemical polishing. Data for Q(E) were taken at several temperatures after R(T) was measured during initial pump down. In addition the pressure sensitivity as well as the Lorentz force detuning were evaluated. The damping of approximately 20 High Order Modes was measured to verify the room temperature data. Measurements on the LL prototype are in progress. We present in this contribution a summary of measured results of tests we performed on the new proposed shapes of the upgrade cavities.
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| THP31 |
A Four-Cell Periodically HOM-Damped RF Cavity for High Current Accelerators
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669 |
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- G. Wu, R.A. Rimmer, H. Wang
Jefferson Lab, Newport News, Virginia
- J. Sekutowicz
DESY, Hamburg
- A. Sun
ORNL/SNS, Oak Ridge, Tennessee
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A periodically Higher Order Mode (HOM) damped RF cavity is a weakly coupled multi-cell RF cavity with HOM couplers periodically mounted between the cells. It was studied as an alternative RF structure between the single cell cavity and superstructure cavity in high beam current application requiring strong damping of the HOMs. The acceleration mode in this design is the lowest frequency mode (Zero Mode) in the pass band, in contrast to the traditional “π” acceleration mode. The acceleration mode of a four-cell Zero Mode cavity has been studied along with the monopole and dipole HOMs. Some HOMs have been modeled in HFSS with waveguide HOM couplers, which were subsequently verified by MAFIA time domain analysis. To understand the tuning challenge for the weakly coupled cavity, ANSYS and SUPERFISH codes were used to simulate the cavity frequency sensitivity and field flatness change within proper tuning range, which will influence the design of the tuner structure. This paper presents this novel accelerating structure that may be used for variety of accelerator applications.
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| THP92 |
Effect of the Tuner on the Field Flatness of SNS Superconducting RF Cavities
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815 |
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- A. Sun
ORNL/SNS, Oak Ridge, Tennessee
- H. Wang, G. Wu
Jefferson Lab, Newport News, Virginia
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Field flatness in a multi-cell superconducting cavity affects not only the net accelerating voltage, but also the peak surface field and the Lorenz Force detuning coefficient. Our measurement indicates that the field flatness changes both external Q of the Fundamental Power Coupler (FPC) and external Q of the Field Probe (FP). The field amplitude tilts linearly to the distance between the cell center and the cavity’s geometry center (pivot point). The tilt rate has been measured in a cryomodule cold (2 K) test, being about 2%/100 kHz, relative the field flatness at the cavity’s center frequency of 805 MHz. Bead-pull measurements confirmed that the field flatness change is 2.0%/100 kHz for a medium β cavity with helium vessel, and 1.72%/100 kHz without helium vessel. These results matched the predictions of simulations using ANSYS and SUPERFISH. A detailed analysis reveals that longitudinal capacitive gap deformation is the main cause of the frequency change. Field flatness change was not only due to the uneven stored energy change within the cell, but also due to cell-to-cell coupling.
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