| Paper |
Title |
Page |
| TUP52 |
Methods for Measuring and Controlling Beam Breakup in High Current ERLs
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387 |
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- C. Tennant, K. Jordan, E. Pozdeyev, R.A. Rimmer, H. Wang
Jefferson Lab, Newport News, Virginia
- S. Simrock
DESY, Hamburg
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It is well known that high current Energy Recovery Linacs (ERL) utilizing superconducting cavities are susceptible to a regenerative type of beam breakup (BBU). The BBU instability is caused by the transverse deflecting higher-order modes (HOMs) of the cavities which can have high impedance. We present MATLab simulation results for the BBU stability using the analysis tools of control theory. In this framework, methods of experimentally determining the threshold current and the means of suppressing the onset of the instability become more transparent. A scheme was developed to determine the threshold current due to a particular HOM by measuring the decay and rise times of the mode's field in response to an amplitude modulated beam as a function of the average electron beam current. To combat the harmful effects of a particularly dangerous mode, two methods of directly damping HOMs through the cavity HOM couplers were demonstrated. In an effort to suppress the BBU in the presence of multiple, dangerous HOMs, a conceptual design for a bunch-by-bunch transverse feedback system has been developed. By implementing beam feedback, the threshold for instability can be increased substantially.
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| THP31 |
A Four-Cell Periodically HOM-Damped RF Cavity for High Current Accelerators
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669 |
| |
- 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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| THP64 |
Waveguide Stub Tuner Analysis for CEBAF Application
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757 |
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- H. Wang
Jefferson Lab, Newport News, Virginia
- M. Tiefenback
TJNAF, Newport News, Virginia
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Three-stub WR650 waveguide tuners have been used on the CEBAF superconducting cavities for two changes on the external Qs: increasing the Q from 6·106 to 8·106 on 5-cell cavities to reduce the klystron power at operation gradients and decreasing the Q from 2·107 to 8·106 on 7-cell cavities to ease the control system handling the Lorenz Force detuning. To understand the reactive tuning effects in the machine operations with beam current and mechanical tuning, a network analysis model was developed. The S parameters of the stub tuner were simulated by MAFIA and measured on the bench. We used this stub tuner model to study tuning range, sensitivity, frequency pulling as well as cold waveguide and window heating problems. Detailed experimental results will be compared against this model. Pros and cons of this stub tuner application will be summarized.
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| THP92 |
Effect of the Tuner on the Field Flatness of SNS Superconducting RF Cavities
|
815 |
| |
- 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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