650 MHz, Beta .61 DRESSED CAVITY, FRS, ED0001834, Rev. D
Fermi National Accelerator Laboratory
P.O. Box 500 - Batavia, Illinois - 60510
Functional Requirement Specification
650 MHz, Beta .61 DRESSED CAVITY, FRS
ED0001834, Rev. D
Prepared by:T. Khabiboulline, RF Engineer / Fermilab
TD/SRF / Date
03/31/2016
Approved by:
T. Nicol, Lead Engineer / Fermilab
TD/SRF / Date: as approved in TC
Approved by:
V. Yakovlev, SRF Department Head / Fermilab
TD/SRF / Date: as approved in TC
Approved by:
A. Rowe, PIP-II Project Engineer / Fermilab
TD/SRF / Date: as approved in TC
Approved by:
V. Lebedev, PIP-II Project Scientist / Fermilab
AD/PIP-II / Date: as approved in TC
Approved by:
S. Holmes, PIP-II Project Manager / Fermilab
PIP-II Office / Date: as approved in TC
Revision control is managed via Fermilab Teamcenter Workflows.
Date
/ Description / Originated By / Section No. / Approved By1 / 09 APR 2013 / Version 1 from Docdb / T. Khabiboulline / ALL
2 / 06 AUG 2014 / Uploaded into Teamcenter from Docdb / Ralph Pasquinelli / ALL
- / 21 AUG 2015 / Initial release / T. Khabiboulline / ALL / As signed
A / 31 MAR 2016 / End groups modified / T. Khabiboulline / I / as approved in TC
B / 7 NOV 2016 / Lorentz Force Detuning coefficient updated to 1.2 Hz/(MV/m)2 / V. Lebedev / p.6 / As approved in TC and by Change Review Board
C / 11-AUG-2017 / The FRS was made consistent with the PIP-II CDR and other SRF FRSs. The names in the signature table were corrected in accordance with the present organizational chart. / V. Lebedev / ALL / As approved in TC and by Change Review Board
D / 01-NOV-2017 / Tables 1 and 2 revised and few typos corrected over the text. / V. Yakovlev / ALL
Table of contents
Date
I.SCOPE
Introduction
LB650 Cavity Design
Low beta Cavity RF design
Cavity Mechanical Design
Helium Vessel Design
Tuning System
Functional Specification Compliance
Cavity Inspection
Cavity Test
II.Project Interfaces
III.Preliminary Safety Requirements
IV.Quality Assurance Requirements
V.Reviews
VI.References
I.SCOPE
The 650 MHz 5-cell elliptical cavities will be designed, manufactured, processed, tested, and assembled into cryomodules forthe PIP-II linac. This document covers the performance and test requirements forLB650 (low beta βG = 0.61) cavity which consists of the following parts:
-Niobium Superconducting cavity
-Liquid Helium containment vessel
-Active frequency-adjustment system (the cavity must fit the HB650 tuning system)
-High power RF coupler (the cavity must fit the HB650 RF coupler)
Introduction
PIP-II is a multi-MW proton accelerator facility based on an H- linear accelerator using superconducting RF technology [1] [2] . The PIP-II800 MeV CW linac employs 650 MHz elliptical 5-cell cavities to accelerate up to 2 mA of average beam current of H- in the energy range 185 – 800 MeV. The cavity design should not prevent operation with 5 mA beam current, presently considered for a possible linac upgrade. The LB650section of the linac should accelerateH-particles from185 to 500 MeV. There are 11 LB650 cryomodules, each housing three650 MHz low beta cavities, required for the project.
We describe the functional requirements of the 650 MHz cavities and facilities required to ensure the functional requirements are met.
LB650 Cavity Design
The final cavity design shall be determined by a review process based on the criteria given in this document, and the performance of prototype cavities. The cavity RF and mechanical design parameters are summarized in Table 1; the cavity operational and test requirements are summarized in Table 2.
Low beta Cavity RF design
The 650 MHz 5-cell elliptical cavities with geometric velocity factors βG = 0.61 have been selected to optimize acceleration efficiency. The cavities are required to operate in pulsed and CW modes in superfluid helium at a temperature2.0K, with nominalenergy gain of 11.9 MeV at optimal betaand unloaded quality factors, Q02.15 x 1010. The cell shape shall be designed to minimize the peak surface magnetic and electric fields, Hpeak and Epeak, to minimize field emission and multipacting. The EM design parameters of LB650 cavity are summarized in Table 1.The cavity beam line aperture shall be optimized within the constraints on field stability, surface fields and RF load. The cavity design shall include end groups with ports for fundamental RF power couplerand pickup coupler, and interface for frequency tuner.The RF coupler design should support a future upgrade path with average currents as high as 5 mA. The EM design parameters are summarized in Table 1.
Table 1.LB650 Cavity EM parameters
Parameter / ValueFrequency / 650 MHz
Shape, number of cells / Elliptical, 5 cells
Iris aperture / 88 mm
Effective length Leff = 5∙(βgλ/2) / 703 mm
Geometrical/Optimal beta βg/βopt / 0.61/0.65
Optimal shunt impedance (R/Q)opt / 341 Ω
Energy gain at optimal betaVopt / 11.9 MeV
Surface RF electric fieldEpeak / 40MV/m
Surface RF magnetic fieldBpeak / < 75 mT
Cavity Mechanical Design
The beam line aperture and cell shape shall be optimized to maintain mechanical stability and a high probability of effective surface processing. The cavity wall thickness and stiffening ring location shall be designed to satisfy FNAL engineering safety standards, acceptable response to microphonics and Lorentz-force detuning, and overall tunability. The presence and type of fast and/or slow tuners shall be determined before the cavity design is considered complete. The cavity mechanical design shall be consistent with suitable mounting and alignment schemes for cryomodule assembly. The end groups shall incorporate a suitable interface between the cavity and its helium vessel as well as with tuner.
In order to meet the requirements of the Fermilab ES&H Manual[5] [6] [7] several coupled thermal/structural analyses must be performed to assure a safe operation. These may include, but should not be limited to the following: elastic, elastic-plastic, collapse, buckling and ratcheting. The cavity mechanical design shall be consistent with suitable mounting and alignment schemes for cryomodule assembly.The cavity operational and test requirements are summarized in Table 2.
Table 2.Beta 0.61 Cavity operational/test requirements
Parameter / ValueOperating mode / Pulsed with CW capability
Maximum BeamCurrent / 5 mA
Max Leak Rate (room temp) / < 10-10 atm-cc/sec
Operating cavity gradient Gacc = Vopt/Leff / 16.9 MV/m
Maximum gradientin VTS / ≤ 20 MV/m
Operating temperature / 2.0 K
Unloaded quality factor Q0 / 2.15∙1010
Dynamic RF power dissipation / 20 W
Operating LHe Pressure / 30±5 mbar
Operating cavity Q-loaded/bandwidth / 1.04∙107 / 63 Hz
Sensitivity to LHe pressure fluctuations / 25Hz/mbar (dressed cavity)
Lorentz Force Detuning coefficient / <1.4 Hz/(MV/m)2
Longitudinal stiffness / < 5 kN/mm
Operating frequency tuning sensitivity / > 150 kHz/mm
Field Flatness in dressed cavity / > 90%
MAWP / 2 bar (RT), 4 bar (2K)
Operating input RF power CW / ≤70 kW
Operating field probe RF Power CW / 100 ̤500mW
Multipacting / none within ±10% of operating gradient
Helium Vessel Design
The Helium vessel shall be fabricated from titanium designed to house a 2 K helium bath sufficient to remove up to 33 W average dissipated power, with appropriately sized supply and return piping. It must meet the requirements of the Fermilab ES&H Manual for cryogenic pressure vessels and be rated at an MAWP (Maximum Allowable Working Pressure) of no less than 2 bars at room temperature and 4 barat 2 K. Every effort should be made to minimize the weight and physical size of the helium vessel in all dimensions. The Helium vessel design has to support effective magnetic field repulsion in the course of fast cavity cooling.
Tuning System
The requirements to the tuning system are set in a separate Functional Requirement specification (ED0004407). They are shortly summarized in this section. The tuning system has to be the same as for the HB650 cavity.The coarse tuner is predominantly used to achieve consistently the resonant frequency during cool-down operations and for preloading the piezo-electric actuator. The range necessary to compensate for the cool-down and detuning uncertainties is estimated to be ±75 kHz. If a cavity needs to be detuned as a result of a malfunction, the coarse tuning system must be able to shift the frequency away from resonance by at least 1000 bandwidths which equal to ≈ 64 kHz, so that the beam is not disturbed.For preloading of the piezo-electric actuator additional deformation of the cavity needed corresponding to 50 kHz frequency shift. Total frequency tuning range is200 kHz.
The requirement on the resolution of the coarse tuning system was set to a value that would allow operation in the event of a failure of the fine-tuning system. Based on other applications, it is believed that such resolution can be achieved with a coarse tuning system.
It is conservatively assumed that the coarse system cannot be operated during beam acceleration, it is thought that the vibration of a stepper motor may induce vibrations in the cavity severe enough to disrupt the operation.
Fine tuners shall be designed to compensate the frequency shifts of the cavity induced by the Lorentz Force Detuning (LFD) and fluctuations of the helium bath pressure. The use of fine tuners is mandatory for pulsed operation and will reduce considerably the hysteresis of the system by limiting the elements in motion during the tracking of the frequency for pulsed and CW operations. An operation of the fine (piezo) tuners will be controlled by LLRF hardware. The control algorithm should prevent cavity detuning due to microphonics and the Lorentz Force Detuning. The latter is critical for cavity operation in the pulsed regime.
A particular design effort shall be dedicated to facilitate the access to all actuating devices of the tuning system from access ports on the vacuum vessel. All actuating devices must be replaceable from the ports, either individually or as a whole cartridge.
Table 3.Tuning system requirements
Parameter / ValueCoarse tuner frequency range / 200 kHz
Coarse tuner frequency resolution / ≤3 Hz
Coarse tuner hysteresis / ≤100 Hz
Fine tuner frequency range / 1000 Hz
Finetuner frequency resolution/stiction / ≤ 0.5 Hz
Functional Specification Compliance
Features and availability at several facilities shall be required to ensure compliance with the cavity functional specification.
Cavity Inspection
Acceptance tolerances are: for the cavity length, ± 3 mm, for the fundamental mode frequency, ± 0.5 MHz.
Cavity Test
The vertical test shall be used for initial qualification of the manufacturing and processing efficacy. Cavity performance shall reach at least 20% above the operational gradient and 20% above the operational Q0 requirements to be considered qualified in the vertical test.
The cavity will need to be protected from mechanical deformation due to vacuum pressure differential.
The horizontal test shall be used as a test of the coupler, tuning system and dressed cavity assembly. Performance consistent with operational requirements shall be required for horizontal qualification of the cavity and peripherals.
II.Project Interfaces
The cavity shall interface to the cryomodule, RF input and output coupler ports, and instrumentation feedthroughs. The cavities shall also include fiducial features that will aid in alignment.
III.Preliminary Safety Requirements
All designs shall be built to applicable FNAL engineering safety standards, and all cavity handling, processing and testing shall be performed according to applicable FNAL environmental safety and health requirements. All cavity and peripherals handling, processing and testing shall be subject to additional training and safety requirements specific to the relevant facilities.
IV.Quality Assurance Requirements
Electronic cavity travelers shall be developed documenting all stages of cavity fabrication, inspection, processing and tests. Each cavity will be identified individually by a serial number appearing on the cavity (e.g. on one of the cavity flanges). A document summarizing the location, status and test results of all cavities shall be publicly accessible and continuously updated.
V.Reviews
Following the acceptable performance of prototype cavities, all elements that will be utilized on the production cavities (e.g. helium vessel, tuning system) will undergo design reviews prior to release for fabrication. The PIP-II/SRF management team will convene an appropriate review committee consisting of experts.
VI.References
[1] The PIP-II Reference Design Report (April 2015)search for document #1370
[2] Project X retreat (November 2, 2010) summary
[3] 650 MHz,β.61, Cryomodule Functional Requirements Specification, in preparation.
FNAL Teamcenter Document # ED0001830
[4] Project X Optics v.3.7.4 (November 19, 2010)
[5] Fermilab ES&H Manual Chapter 5031.6: Dressed Niobium SRF Cavity Pressure Safety
[6] Fermilab ES&H Manual Chapter 5031: Pressure Vessels
[7] Fermilab ES&H Manual Chapter 5031: Pressure Vessels
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