Tuner Effect on the Field Flatness of SNS Superconducting RF Cavity PDF Download
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Languages : en
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The field flatness (FF) in a multi-cell superconducting cavity affects not only net accelerating voltage, but also peak surface field [1] and Lorentz detuning coefficient [2]. Our measurements indicate that the tuner's motion changes not only the cavity frequency but also its FF. This field amplitude change is a linear tilt and proportional to the distance between each cell center and the cavity's geometric center. This tilt also changes the coupling of the Fundamental Power Coupler (FPC) and Field Probe (FP) on either side of the cavity. The tilt has been measured and simulated, and is ≈20%/MHz on the Spallation Neutron Source (SNS) medium [beta] cavities. The FF change is not only due to the uneven volume change within the cell, but also due to the cell-to-cell coupling.
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Languages : en
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Book Description
The field flatness (FF) in a multi-cell superconducting cavity affects not only net accelerating voltage, but also peak surface field [1] and Lorentz detuning coefficient [2]. Our measurements indicate that the tuner's motion changes not only the cavity frequency but also its FF. This field amplitude change is a linear tilt and proportional to the distance between each cell center and the cavity's geometric center. This tilt also changes the coupling of the Fundamental Power Coupler (FPC) and Field Probe (FP) on either side of the cavity. The tilt has been measured and simulated, and is ≈20%/MHz on the Spallation Neutron Source (SNS) medium [beta] cavities. The FF change is not only due to the uneven volume change within the cell, but also due to the cell-to-cell coupling.
Author: Pei Zhang
Publisher: Springer Science & Business Media
ISBN: 3319007599
Category : Science
Languages : en
Pages : 128
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An energetic charged particle beam introduced to an rf cavity excites a wakefield therein. This wakefield can be decomposed into a series of higher order modes and multipoles, which for sufficiently small beam offsets are dominated by the dipole component. This work focuses on using these dipole modes to detect the beam position in third harmonic superconducting S-band cavities for light source applications. A rigorous examination of several means of analysing the beam position based on signals radiated to higher order modes ports is presented. Experimental results indicate a position resolution, based on this technique, of 20 microns over a complete module of 4 cavities. Methods are also indicated for improving the resolution and for applying this method to other cavity configurations. This work is distinguished by its clarity and potential for application to several other international facilities. The material is presented in a didactic style and is recommended both for students new to the field, and for scientists well-versed in the field of rf diagnostics.
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Languages : en
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Superconducting cavities for the CKM RF separated kaon beamline at Fermilab have modes that are closely spaced compared to the resonance bandwidths when warm, and this complicates the field flatness (warm) tuning process. Additionally, it is necessary to maintain the azimuthal orientation of the mode during the tuning deformations. the authors present two analytic techniques to warm-tune cavities with overlapping modes, a finite-element analysis of the tuning process, the design of a warm tuner which maintains mode polarization, and the results of tuning a cavity in which initial manufacturing variations caused the desired[pi] and nearby[pi]-1 modes to be indistinguishable before field flatness tuning.
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Languages : en
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During vertical testing of SNS superconducting RF cavities, a lot of electron activity was observed. This could be evidence of in-cell multipacting, cell to cell electron mitigation or near-axis dark current. To study the potential for these electron activities in SRF cavities, a multi-purpose electron tracking code (FishPact) was developed based on the Poisson/Superfish Field solvers. Electron trajectories between the impacts in both medium-[beta] and high-[beta] cavities are tracked under varying accelerating gradient (E{sub acc}), RF phases, and emission locations. It is found that the high beta cavities show a greater propensity for potential sources of dark current than their medium beta counterparts. It is further proposed that field emission induced multipacting is a potential, though rare, source of further Q0 degradation.
Author: T. Powers
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Languages : en
Pages :
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All eighty-one fundamental power couplers for the 805 MHz superconducting cavities of the SNS linac have been RF conditioned and installed in the cryomodules successfully. The couplers were RF processed at JLAB or at the SNS in ORNL: more than forty couplers have been RF conditioned in the SNS RF Test Facility (RFTF) after the first forty couplers were conditioned at JLAB. The couplers were conditioned up to 650 kW forward power at 8% duty cycle in traveling and standing waves. They were installed on the cavities in the cryomodules and then assembled with the airside waveguide transitions. The couplers have been high power RF tested with satisfactory accelerating field gradients in the cooled cavities.
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Languages : en
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Category :
Languages : en
Pages : 4
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In contrast to a normal conducting RF cavity, a superconducting RF cavity is very susceptible to shifts in its resonance frequency. The main sources of the shift are Lorentz force detuning and microphonics. In SNS, to compensate for the frequency shift, a feedforward control is to be applied. In this paper, as an initiative step, a frequency shift observer is proposed which is simple enough to be implemented with a digital signal processor in real time. Simulation results of the proposed frequency shift observer show reliable performance and acceptable computation time for the real time implementation.
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Languages : en
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The Spallation Neutron Source (SNS) makes use of 33 medium beta (0.61) and 48 high beta (0.81) superconducting cavities. Each cavity is equipped with a fundamental power coupler, which should withstand the full klystron power of 550 kW in full reflection for the duration of an RF pulse of 1.3 msec at 60 Hz repetition rate. Before assembly to a superconducting cavity, the vacuum components of the coupler are submitted to acceptance procedures consisting of preliminary quality assessments, cleaning and clean room assembly, vacuum leak checks and baking under vacuum, followed by conditioning and RF high power testing. Similar acceptance procedures (except clean room assembly and baking) were applied for the airside components of the coupler. All 81 fundamental power couplers for SNS superconducting cavity production have been RF power tested at JLAB Newport News and, beginning in April 2004 at SNS Oak Ridge. This paper gives details of coupler processing and RF high power-assessed performances.
Author: Kiyomitsu Asano
Publisher:
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Category :
Languages : en
Pages : 6
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Author: Ramona Leewe
Publisher:
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Languages : en
Pages : 102
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RF cavities are resonators and the key structures in particle accelerators. An electromagnetic field within the cavities provides the acceleration field. Within linear particle accelerators, particle bunches travel along a beam pipe and through the aligned cavities. A positive electric field along the beam line results in an acceleration kick of the particle bunch. The speed of the traveling particle bunches is synchronized with the RF field withinthe cavities. Hence, tuning of the natural resonance frequency of an RF cavity is essential for accelerator structures to achieve efficient beam acceleration and to reduce power requirements. Operational cavities are typically tuned using phase comparison techniques. Phase measurement is subject to temperature drifts which renders this technique labor and time intensive. In this thesis, we developed two novel resonance frequency tuning schemes solely depending on the reflected power component of a cavity. The base for control scheme development is a mathematical model of the cavity in terms of the steady state signals. The first control scheme was derived through a Lyapunov analysis and incorporates a gradient estimator of the performance function. The second control scheme is based on sliding mode extremum seeking. Both systems are analyzed in terms of the stability; conditions are provided which guarantee stable system behavior up to twice the cavity bandwidth. A simulation study verifies the derived stability conditions. An experimental test bench, including a room temperature quarter wave cavity, was built to test the control schemes under various conditions. Although both control schemes show similar tuning results, a higher tuning accuracy is obtained by the sliding mode based control scheme. Hence, the latter was chosen to be implemented on two resonators, DTL tanks, of TRIUMF's ISAC I facility. The system was fully commissioned on both DTL tanks and has been in operation since April 2016. Reflected power, forward power, and tuning position are monitored and analyzed. Long term measurements showed the influence of environmental temperature variations. As the influence of environmental temperaturevariations can be neglected for reflected power measurements, the reflected power based tuning system provides a higher tuning accuracy compared to the traditional phase based tuning system. The start-up time and the need for human oversight are reduced significantly.
