Abstract The outer circumference of a BPM button and the inner circumference of the button housing comprise a transmission line. This transmission line typically presents an impedance of a few tens of ohms to the beam, and couples very weakly to the 50 ohm coaxial transmission line that comprises the signal path out of the button. The modes which are consequently excited and trapped often have quality factors of several hundred, permitting resonant excitation by the beam. The thermal distortion resulting from trapped mode heating is potentially problematic for achieving the high precision beam position measurements needed to provide the submicron beam position stability required by light source users. We present a button design that has been optimized via material selection and component geometry to minimize both the trapped mode heating and the resulting thermal distortion

A Blednykh

B Kosciuk

I Pinayev

O Singh

P Cameron

P Cameron#

V Ravindranath

CiteSeerX

 
 
 
BNL-90422-2009-CP 
 
 
 
 
 
BPM Button Optimization to Minimize Distortion 
Due to Trapped Mode Heating 
 
 
P. Cameron, A. Blednykh, B. Kosciuk, I. Pinayev, V. 
Ravindranath, O. Singh 
 
Brookhaven National Laboratory, Upton, NY 11973-5000, USA 
 
 
Presented at the PAC09 Conference 
Vancouver, Canada 
 
May 4 – 8, 2009 
 
 
 
 
National Synchrotron Light Source II Project 
 
Brookhaven National Laboratory 
P.O. Box 5000 
Upton, NY 11973-5000 
www.bnl.gov 
 
 
 
Notice: This manuscript has been authored by employees of Brookhaven Science Associates, LLC under Contract No. DE-
AC02-98CH10886 with the U.S. Department of Energy. The publisher by accepting the manuscript for publication 
acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to 
publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government 
purposes. 
 
 
This preprint is intended for publication in a journal or proceedings.  Since changes may be made before 
publication, it may not be cited or reproduced without the author’s permission. 
 
 
 
DISCLAIMER 
 
This report was prepared as an account of work sponsored by an agency of the 
United States Government.  Neither the United States Government nor any 
agency thereof, nor any of their employees, nor any of their contractors, 
subcontractors, or their employees, makes any warranty, express or implied, or 
assumes any legal liability or responsibility for the accuracy, completeness, or any 
third party’s use or the results of such use of any information, apparatus, product, 
or process disclosed, or represents that its use would not infringe privately owned 
rights. Reference herein to any specific commercial product, process, or service 
by trade name, trademark, manufacturer, or otherwise, does not necessarily 
constitute or imply its endorsement, recommendation, or favoring by the United 
States Government or any agency thereof or its contractors or subcontractors.  
The views and opinions of authors expressed herein do not necessarily state or 
reflect those of the United States Government or any agency thereof.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
BPM BUTTON OPTIMIZATION TO MINIMIZE DISTORTION DUE 
TO TRAPPED MODE HEATING* 
P. Cameron#, A. Blednykh, B. Kosciuk, I. Pinayev, V. Ravindranath, and O. Singh 
BNL, Upton, NY 11973, USA 
 
Abstract 
The outer circumference of a BPM button and the inner 
circumference of the button housing comprise a 
transmission line. This transmission line typically 
presents an impedance of a few tens of ohms to the beam, 
and couples very weakly to the 50 ohm coaxial 
transmission line that comprises the signal path out of the 
button. The modes which are consequently excited and 
trapped often have quality factors of several hundred, 
permitting resonant excitation by the beam. The thermal 
distortion resulting from trapped mode heating is 
potentially problematic for achieving the high precision 
beam position measurements needed to provide the sub-
micron beam position stability required by light source 
users. We present a button design that has been optimized 
via material selection and component geometry to 
minimize both the trapped mode heating and the resulting 
thermal distortion.  
INTRODUCTION 
Awareness [1] and measurement [2] of BPM button 
circumferential resonances preceded the design of 
synchrotrons in which button heating would first become 
an issue.  The problem was studied in some detail in the 
design of BPM systems for the B-factories at SLAC [3,4] 
and KEK [5]. With the continuing push for greater 
brightness, button heating and the resulting thermal 
distortion has emerged as an issue for the stability of 
position measurement in light sources, and the problem 
has been studied in greater detail [6-9]. An informal mini-
workshop to discuss this problem and possible solutions 
was organized concurrent with the recent EPAC 
conference in Genoa.  
As shown in figure 1, the outer circumference of a 
BPM button and the inner circumference of the button 
housing comprise a transmission line. This transmission 
line typically presents an impedance of a few tens of 
ohms to the beam, and couples very weakly to the 50 ohm 
coaxial transmission line signal path out of the button. 
The resonant mode which is consequently excited and 
trapped can deposit tens of watts of power in the button. 
The resulting thermal distortion is potentially problematic 
for maintaining high precision beam position stability, 
and in the extreme case can result in mechanical damage.  
               
Figure 1: Illustration of the trapped modes 
DESIGN RULES 
A collection of simple design rules has emerged from 
the various publications, workshops, and discussions. 
These rules fall into one of two categories: 
1. Geometric design rules 
a. Make the buttons as small as possible consistent 
with signal power and position measurement 
resolution requirements – the frequency of the 
trapped mode is high relative to the position 
signal frequency, and power at the mode 
frequency drops off very quickly as it moves into 
the tail of the coherent beam spectrum. 
b. Minimize the trapped mode impedance and the 
resulting power deposited in this mode by the 
beam – the transmission line impedance is 
minimized by minimizing its inductance and 
maximizing its capacitance, resulting in a button 
which is thick and has a small gap to the outer 
housing. 
c. Maximize the power re-radiated back into the 
beampipe – this is accomplished by making the 
button thin with a large gap to the outer housing, 
requirements that are in conflict with minimizing 
the mode impedance. 
d. Adjust the button diameter to keep the trapped 
mode resonance away from strong lines in the 
beam spectrum to avoid resonant excitation – the 
strongest lines are at multiples of the bunching 
frequency, which in the case of bunching at the 
RF frequency are separated by 500MHz, 
permitting them to be avoided. 
 ___________________________________________  
*Work supported by DOE contract DE-AC02-98CH10886 
# cameron@bnl.gov 
e. Minimize the distance from the button to the 
thermal and mechanical anchors – while 
previous rules seek to minimize power deposited 
in the button, this rule seeks to minimize 
temperature rise and resulting deformation. 
2. Materials design rules 
a. Electrical conductivity – maximize conductivity 
of the outer circumference of the button and 
minimize conductivity of the inner 
circumference of the shell in the vicinity of the 
button [6], the difference in resistances resulting 
in the majority of the trapped mode power being 
dumped into the well thermally anchored shell, 
rather than the comparatively isolated button. 
b. Thermal conductivity – maximize thermal 
conductivity of all materials between the button 
surface and the thermal anchor. 
c. Thermal expansion – minimize the coefficient of 
expansion of all materials between the button 
surface and the thermal anchor. 
Prior to optimization of specific internal geometric 
and materials details of the pickup, attention was given to 
the tradeoff between horizontal and vertical sensitivities 
and to button dimensions, as indicated by items 1a thru 1d 
of the design rules. The result of those preliminary studies 
is shown in figure 1.  
The initial studies made it clear that reducing button 
diameter from the 10mm baseline to 7mm would provide 
substantial relief from the trapped mode heating problem, 
without unduly compromising the available power at the 
500MHz signal frequency, satisfying design rule 1a. With 
the 7mm button diameter, a horizontal button separation 
of 16mm was needed to achieve the desired vertical 
sensitivity in the 25mm aperture. The mechanical 
constraints of this comparatively close horizontal spacing 
resulted in the two-in-one design shown in the figure.  
With the overall geometry defined, button dimensions 
were then optimized via GdfideL simulations to satisfy 
rules 1b thru 1d. This resulted in a button diameter of 
7mm, thickness of 2mm, and spacing to the outer housing 
of 0.25mm, as shown in the figure. The trapped mode 
resonance frequency was calculated to be ~13.3GHz, 
approximately midway between the strong 500MHz RF 
lines as desired. 
Figure 1:  Two-in-one button assembly showing button details (dimensions in inches) 
FINAL DESIGN DETAILS 
With the overall feedthu and button geometry defined, 
a design competition was initiated. Qualified vendors 
were provided with an outline drawing and specifications, 
and invited to submit designs for details of the internal 
construction. Given that the vendors are the experts in the 
art of this detailed design, and that such information is 
proprietary and closely guarded, this was seen to be the 
best approach to arriving at an optimized design. The 
proposals were evaluated, and the best design was 
selected to provide five first article assemblies, which are 
presently in production. Details of the winning proposal 
are proprietary, however it can be said that they conform 
well to the design rules.  
GdfideL simulation indicated that, with 10psec bunch 
length and 500mA beam current uniformly distributed in 
the 1320 NSLS-II 500MHz buckets, the resulting trapped  
Figure 2: The temperature distribution 
mode power in the button would be about 4 watts. This 
power was applied to the design in an Ansys simulation, 
and the resulting temperature distribution and thermal  
 
Figure 3: The thermal deformation 
deformation was calculated. These results are shown in 
figures 2 and 3, with internal proprietary details obscured. 
Button temperature with this beam condition was 
calculated to be ~70oC, and the resulting thermal 
displacement of the button relative to the reference 
mounting surface is ~4.4 microns. 
The item of interest is not the beam current dependent 
displacement of the individual buttons, but rather the 
current dependent error in the position measurement that 
results from these displacements. If the beam is centered 
in the pickup there will be no differential displacement, 
and no error in the position measurement. Knowing the 
position dependence of the power deposited in the 
individual buttons, the measurement error as a function of 
position offset and beam current can be calculated. This is 
plotted in figure 4.  
The vertical axis in the figure is the beam offset in the 
pickup. The horizontal axis is the average trapped mode 
power in the buttons with no beam offset, with 4W 
corresponding to previously mentioned 10psec bunch 
length and 500mA beam current uniformly distributed in 
the 1320 NSLS-II 500MHz buckets. The z-axis 
displacements are in microns. 
 
         
Figure 4:  Position measurement error as a function of beam power and offset 
CONCLUSION 
A reasonable expectation is that beam will be 
vertically centered in the pickups within a few hundred 
microns. The red line in figure 4 is drawn at 200 microns 
beam offset. The resulting current dependent 
measurement error in the extreme circumstance of going 
from no beam to the full design current is about 260nm, 
as indicated by the red dot.  The design manual 
requirement is that vertical thermal stability shall be less 
than 200nm rms on the time scale from minutes to 8 
hours. The button design clearly meets this requirement 
for any reasonable operations scenario. 
ACKNOWLEDGEMENTS 
The authors thank Julien Bergoz, Fritz Caspers, Jean-
Claude Denard, Robert Hettel, Eric Plouviez, Guenther 
Rehm, Kai Wittenberg, and the participants in the 
EPAC08 mini-workshop on button heating for many 
helpful comments and discussions. 
REFERENCES 
[1] R.E. Shafer, “Beam Position Monitoring”, BIW89, 
Brookhaven, p.40. 
[2] A. Jacob and G.R. Lambertson, “Impedance 
Measurements on Button Electrodes”, PAC89, 
Chicago. 
[3] N. Kurita et al, “Design of the Button BPM for PEP-
II”, PAC96, Dallas. 
[4] C.K. Ng et al, “Simulation of the PEP-II BPMs”, 
ibid. 
[5] T. Obina et al, “Damped Button Electrode for the B-
Factory BPM System”, ibid. 
[6] R. Nagaoka et al, “Recent Studies of Geometric and 
Resistive Wall Impedance at Soleil”, EPAC06, 
Edinburgh. 
[7] A. Olmos et al, “BPM Design for ALBA 
Synchrotron”, ibid. 
[8] A. Bandyopadhyay et al, “Computations of 
Wakefields for BPMs of PETRA III”, ibid. 
[9] P. Cameron et al, “Comparative Study of Button 
BPM Trapped Mode Heating”, these proceedings. 


BPM Button Optimization to Minimize Distortion Due to Trapped Mode Heating

http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=963EF0B3FC7D9152253E538569B79E57?doi=10.1.1.1068.2886&rep=rep1&type=pdf

BPM Button Optimization to Minimize Distortion Due to Trapped Mode Heating

Abstract

Similar works

Full text

Available Versions

CiteSeerX