1. • The 12 GeV upgrade requires the production of approximately eighty SRF
cavities in the next two years.1 Jefferson Lab manufactures SRF cavities for
other projects and R&D as well.
• For each cavity, bead-pull measurements are usually taken more than ten
times.5
• Bead-pull measurements will continue to be used for large-scale, cavity
production for future projects.1,5
• Manual measurements for the upgrade cavities take between half an hour to
four hours. Manual measurements for a newly designed cavity can take up to a
week.5
• Consequently, an automated bead-pull system could make Jefferson Lab’s SRF
cavity production and R&D more efficient.1
Software for an Automated Bead-Pull System for the Production of SRF Cavities at Jefferson Lab
Stephen Lowery: Harvey Mudd College, Claremont, CA 91711, USA
Haipeng Wang: Thomas Jefferson National Accelerator Facility, Newport News, VA 23606, USA
The passage of a small dielectric through a cavity that contains resonating EM fields, such as the superconducting radiofrequency (SRF) cavities used at Jefferson Lab, alters the relative phases of the EM
fields in the cavity. These phase changes can be used to find the field profiles for resonant modes in the cavity, among other data critical to the operation of the CEBAF accelerator. This measurement is
called bead-pulling and is performed several times on each cavity during production at Jefferson Lab. The purpose of this project was to write data acquisition software for an automated bead-pull system.
The apparatus used to test the software consisted of a network analyzer for generating radio waves in the cavity and measuring their phase changes, and a pre-existing pulley system controlled by an
electric motor for moving the dielectric through the cavity. The software was developed in the Microsoft .NET framework, runs from a desktop PC and controls the network analyzer. This project shows
that a cost-effective automated bead-pull system is not only feasible and has the additional benefit of reducing errors in measurement caused by thermal drift. The next step in this project will be to build a
better apparatus to improve mechanical stability and to integrate mechanical control into the software.
Bead-pulling is a well-understood, routine measurement performed
several times on each niobium SRF cavity during production at Jefferson Lab.2,5
Bead-pulling can be done by suspending a small object, such as a metal cylinder
or dielectric “bead”, on a thin string and then pulling the object through the
central axis of a cavity from one end to the other. While the object is moving
through the cavity, RF signals at resonant frequencies are generated at one end
of the cavity and received at the other end, usually by a network analyzer. By
recording the change in the phase of the S21 scattering parameter as a function
of the position of the bead, many important characteristics of that cavity can be
determined.2
One such characteristic is the field profile for a given mode. This can be
found, albeit normalized with respect to the electromagnetic energy stored in
the cavity, with the relationship
where E is the electric field at the location of the bead, z is the position of the
bead along the length of the cavity, U is the electromagnetic energy stored in
the entire cavity, QL is the loaded quality factor and C is a constant factor that
depends on the object pulled through the cavity. The derivation of this
relationship uses perturbation theory and classical electromagnetism.7 The
significance of this result is that it can be used to find the field flatness of
resonant modes. A flat field is desirable because it improves the efficiency of
the accelerating modes and mitigates the threat of BBU posed by certain
HOMs.1-4
INTRODUCTION METHOD
[1] H. Wang, “Bead-Pulling System Automation Work Proposal for Cavity Production and
R&D”, internal memo, July 2010.
[2] H. Wang “TM010 Pass Band Modes of TESLA 9-cell Cavity”, internal memo,
September 4, 2007.
[3] F. Marhauser, K. Tian, H. Wang, “Critical Higher Order Modes in CEBAF Upgrade
Cavities”, August 2009.
[4] H. Padamsee et al. “RF Superconductivity for Accelerators”, Wiley Series in Beam
Physics and Accelerator Technology, page 133, 1
[5] Private communication with Frank Marhauser, July 2010.
[7] L. B. Mullett, Perturbation of a Resonator, UK Atomic Energy Research Establishment,
Harwell, Berkshire, 1957, Unclassified A.E.R.E. G/R 853.
REFERENCES
SOFTWARE
CURRENT APPARATUS
MOTIVATION
Cables connected to the network
analyzer for sending and
receiving RF signals.
Pulley system for maintaining tension
on the string as it is unwound.
Copper cavity being measured
Dielectric sphere
(the eponymous
“bead”) on a string
LQ
zS
C
U
zE ))(arg()( 21
2


• Allows the user to load measurement settings from a formatted .txt file
• Requires the user to input the approximate resonant frequencies of the cavity
• Finds the exact resonant frequencies corresponding to the input
• Waits for the user to start the motor that pulls the “bead”
• Measures the phase of the S21 scattering parameter while sweeping out the
resonant frequencies as the “bead” passes through the cavity
• One “pull” takes data for all of the modes entered in approximately two
minutes, reducing errors caused by thermal drift
• Plots the results
• Calculates the square of the E-field normalized to the stored energy in the
cavity, and plots this
• Allows the user to export the data to an Excel file.
• Used a pre-existing mechanical system for moving the “bead”
• Used an Agilent E5071C network analyzer for generating and analyzing RF signals
• Wrote software in the Microsoft.NET framework to provide a user interface, control
the network analyzer and record and process the data
• Prepared to build an improved mechanical system that could be controlled by the
software
• Documented the project to allow others to continue to work on it
• Software for a cost-effective automated bead-pull system has been produced
and successfully tested with a pre-existing mechanical system.
• The production facility at Jefferson Lab could save hundreds of hours of labor
by implementing an automated bead-pull system.
These are closer pictures of the pre-
existing mechanical system used to test
the software. A better mechanical system
that can be controlled by the program
has been designed and will be
implemented shortly.
GOAL
The goal of this project was to write and test data acquisition software to
automate bead-pull measurements.
CONCLUSION
ACKNOWLEDGEMENTS
I would like to express my gratitude to the National Science Foundation, Old Dominion
University and Jefferson Lab for sponsoring my experience. I would also like to thank Haipeng
Wang, Frank Marhauser and Tom Goodman for their patience and help with this project, as well
as Hari Areti, Gail Dodge and Lisa Surles-Law for organizing the REU program.
Motor for pulling
the string on which
the bead rests