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12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

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A STUDY INTO THE LONG-TERM STABILITY OF FRONT END X-RAY
BEAM POSITION MONITOR SUPPORT COLUMNS AT
DIAMOND LIGHT SOURCE
C. Houghton∗ , C. Bloomer, L. Bobb, D. Crivelli, H. Patel, J. Melton
Diamond Light Source, Oxfordshire, United Kingdom
Abstract
Sand-filled steel columns are used at Diamond Light
Source to support front end X-ray beam position monitors.
This approach is chosen due to the relatively large thermal
mass of the sand being considered useful to reduce the rate
at which expansion and contraction of the column occurredas the storage ring tunnel temperature varied, particularly
during machine start-up. With the higher requirements formechanical stability for the upcoming Diamond-II upgrade,
there is now a need to assess and quantify the current system’s impact on X-ray beam movement. A study of thermal
and mechanical stability has been carried out to quantify
the stability performance of the front-end X-ray beam position monitor’s columns and the impact that column motion may have on the X-ray beam position measurement.
Measurements have been made over a range of different
timescales, from 250 Hz up to 2 weeks. The measured stability of the support column is presented, showing that itmeets our Diamond-II stability requirements. A comparisonof the stability of the column with and without a sand fillingis presented.

INTRODUCTION
To monitor and improve the stability of the photon beams,
Diamond Light Source utilises X-ray beam position monitors (XBPMs) on most insertion device (ID) front ends.
Each XBPM is mounted on a steel support column bolted
to the synchrotron floor. Currently, the XBPMs are used asa diagnostics tool, monitoring long-term trends and in somecases for slow (0.2 Hz) beam position feedback [1]. Afterthe Diamond-II upgrade, it is proposed that the front end
XBPMs could be included in more critical orbit feedback
systems. Therefore the mechanical stability of the XBPMs
needs to be assessed. Motion of the steel columns arises
from various sources, the largest contribution coming fromthe vertical thermal expansion of the steel over time, correlated with ambient temperature changes in the tunnel.
Each column is constructed from a hollow square tube
with external dimensions of 200 mm x 200 mm and a wall
thickness of 10 mm. The manufacturer, FMB Berlin
GmbH, specifies that the column is produced using S235JR
steel, with a coefficient of linear thermal expansion of
12 × 10−6 K−1 .
The columns are bolted to the concrete floor using four
50 mm long M12 bolts. One of the open questions thiswork is intended to answer was whether the steel base of∗ claire.houghton@diamond.ac.uk

the XBPM pressed into the floor by the tension of thesefour bolts is sufficient to thermally couple the column to thethermal mass of the synchrotron floor slab, or if the column
temperature is more correlated with ambient air temperature.
Each column is filled with fine sand intended to improve
the thermal stability of the column by increasing its thermalmass. Experimental data was first acquired with the column
in its original state, filled with sand. Then, during a machine
shutdown, the sand was vacuumed out of the column (approximately 30 kg of dry sand), leaving it hollow, and theexperiments were repeated.

EXPERIMENTAL SET-UP
Laser Interferometry
In this experiment a laser-interferometry system1 was
utilised to track variations in the height of the XBPM support
column. The setup involved positioning one of the detector
heads on the top of the XBPM vessel using a mounting
bracket. A plane mirror was positioned on the ground andaligned to the laser beam such that the light is reflected backalong the incident path. Figure 1 presents a sketch of thissystem.
Interferometry, the principle underlying this setup, exploits the wave nature of light. When the laser beam splits
and travels different paths – one directly to the detector andthe other along the length of the column to the mirror andthen to the detector – they recombine, forming an interference pattern. By analysing this pattern, a measurement of
the column height variation over time periods of 1 s to daysis acquired. For background information on the various principles behind modern interferometry measurements, a useful
introduction can be found in [2, 3].

Thermal Monitoring
The temperature of the column, the air, and the concrete
floor upon which the column was bolted were measured
in order to correlate any variation in column height to thetemperature of the column and its surroundings.
A total of five temperature sensors, with a thermal resolution of 0.025 °C, were attached to the support column, on
different faces of the column and at different heights. The
intent of this was to determine if there were any temperature gradients across the column, or if non-uniformity of
the column temperature could lead to bending or twistingof the column as different sides of it expanded or contracted
at different rates. One temperature sensor was bolted into a
1 Renishaw RLE10.

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Figure 1: A drawing of the XBPM and its support column,
with the locations of the different devices. Piezoelectric
seismic accelerometers are shown by blue squares. Circles
highlight the locations of the thermal sensors; red for column
thermal sensors, purple for air thermal sensors and green
for a sensor bolted to the floor. Two thermal sensors whichwere installed on the back and right side of the column are
not shown in the schematic.
25 mm deep hole drilled in the concrete floor at the foot of the
XBPM, at an equidistant location between two of the floor
bolts of the support column which are separated by 200 mm.
This sensor had a low-density packing-foam cover placed
over it to insulate it from the ambient air temperature. Thisway, the sensor measured the long-term temperature stabilityof the floor slab, without detecting short-term fluctuationsin ambient air temperature.

Accelerometers
Piezoelectric seismic accelerometers2 were used to measure the horizontal and vertical stability of the support column over the frequency range of 10 Hz to 250 Hz with abroadband resolution of 9 × 10−4 m/s2 rms. Piezoelectric
accelerometers use the piezoelectric effect to measure forcesinduced due to the acceleration of the material. A seismic
mass is attached to a sensing piezo-crystal. When under
acceleration the seismic mass causes stress on the sensing
piezo-crystal which outputs a proportional electrical signal [4]. Four piezoelectric accelerometers were used tomeasure the support column and the surrounding area, measurements were taken simultaneously. The accelerometers
were placed on the floor next to the support column, on thebase of the support column, on the top of the support column, and on the stepper motor to which the XBPM vessel
is mounted referred to as the XBPM mounting plate, seeFig. 1.

RESULTS

Temperature Stability
Figure 2 shows the variation in the thermal sensor measurements installed on and around the support column over a
50 h period. The air temperature is seen to oscillate by about
0.4 °C peak-to-peak. However, the column has a signifi2 PCB Piezontronics model 393B31.

Figure 2: Graph showing the change in air, column andfloor temperature over a 50-hour period with sand (Top) andwithout sand (Bottom).
cantly larger thermal mass, and is to some extent thermallybonded to the concrete floor slab. Thus, the temperature
variations of the column are damped relative to the air, andvary over much longer timescales. The column temperature
variation is broadly seen to follow the pattern of air temperature variation, albeit low-pass filtered. In addition, thelocation of the different sensors placed on the column does
not make a significant impact on the temperature variation.
This suggests that the thermal conductance of the support
column is uniform across all faces. This relationship does
not change when the sand is removed, see the bottom graphof Fig. 2. This is to be expected, given the thermal conductivity of steel and the timescales over which measurements
were made. Over the full 11 day data collection period thestandard deviation of the average column temperature was
0.064 °C with sand increasing to 0.071 °C when the sand isremoved.

Short Term Stability
The short-term stability was measured using accelerometers. There are two useful metrics by which the overall
stability of the support column and the effect of the internalsand, can be compared: the transmissibility ratio of displacement transmitted through the column, and the power spectral
density (PSD) of the motion of the support column.
The transmissibility ratio is the measure of the physicaldisplacement transmitted through a system (i.e. the support
column). This was obtained by measuring the motion using
the accelerometer mounted at the bottom of the column andcomparing the results to the XBPM support column stability
using the accelerometer mounted at the top of the column.
In the results presented here the ratio was taken from thebase plate at the bottom of the support column to the XBPM
mounting plate.
Typically, at low frequencies the whole system will moveas one body, as all motion is transmitted through the column
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Transmissibility ratio

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With Sand
Without Sand

100
10
1
0.1

10

20

40

60

80 100150

200 250

20

40

60

80 100150

200 25040

60

80 100150

200 250

Phase (deg)5000Coherence

-500
10

0.8
0.6
0.4
0.2
10

20

Frequency (Hz)Phase (deg)Transmissibility ratio

Figure 3: The horizontal transmissibility ratio of the XBPM
support column bottom plate to the XBPM mounting plate.
With Sand
Without Sand
10

1

0.1
10

20

40

60

80 100150

200 250

200

-200
10

20

40

60

80 100150

200 25040

60

80 100150

200 250

0.8
0.6
0.4
0.2
10

tion in this range of frequencies. There is a relatively large
damping trough at 178 Hz in the without sand data which is
not present in the column when there is sand. The vibrationof the bottom of the column and top of the XBPM mounting
plate go out of phase at 84 Hz, likely indicating a resonanceof the system. The coherence of the system also has similarities between the two datasets, with slight changes in thepeak frequencies. Removing the sand does not make thecolumn more incoherent.
Vertically, the removal of sand has a larger impact on
the transmissibility ratio, see Fig. 4. When sand fills thecolumn the transmissibility ratio is close to one across thefrequency range, except for a peak at 50 Hz - 70 Hz. New
relatively large peaks at 167 Hz and 193 Hz were present
once the sand was removed this suggests that the sand works
to damp these resonances. Without sand the drop out ofphase is shifted from 215 Hz to 144 Hz.
Overall, it is found that there are not significant changes tothe short-term stability of the column as a result of the sand.
The changes that are present could be explained simply as
an effect of small differences in the bolt tightness following
the unbolting and re-bolting of securing screws during thesand removal process.
The second metric by which the support column stability
can be compared is the power spectral density for the horizontal and vertical motion of the XBPM mounting plate,seen in Fig. 5 and Fig. 6 respectively. The results here aresimilar to the transmissibility figures, in that they show very
similar frequency responses both with and without sand.

Long Term Stability0

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20

Frequency (Hz)Figure 4: The vertical transmissibility ratio of the XBPM
support column bottom plate to the XBPM mounting plate.
(i.e. transmissibility ratio = 1). A transmissibility ratio equal
to one means a perfectly rigid system [5]. At higher frequencies the ground motion may no longer be transmitted throughthe column and is damped (i.e. transmissibility ratio < 1)or ground motion can even be amplified by the column atresonant frequencies (i.e. transmissibility ratio > 1). This
enables the amplification, resonances and damping from thecolumn to be seen.
Horizontally, the transmissibility ratio was similar bothwith and without sand up to ∼ 120 Hz, see Fig. 3. Thereis an amplification of ground motion above 30 Hz in bothdatasets, with and without sand. The transmissibility ratio without sand shows fewer damping ‘troughs’ between
140 Hz and 166 Hz suggesting an overall larger amplifica-

During a typical week at Diamond Light Source, the electron beam is turned off to allow for machine development
and maintenance. During this time the ambient air temperature in the tunnel typically drops as the thermal heating fromthe accelerator is removed. This temperature drop causes
a contraction in the height of the XBPM support column.
This can be seen clearly in a relatively large change in thecolumn height in Fig. 7, highlighted by the black dashedline. The beam is returned to users at 9 AM the following
day. The maximum change in height of the column during
this beam-off period was 6.5 µm for the column with sandand 8.4 µm without sand. The standard deviation of the support column height variation over the 11 day measurement
increases from 3.26 µm to 6.3 µm when the sand is removed.
This variation is still below the 9.6 µm long-term stability
needed for the planned upgrade to Diamond-II.
The correlation between the support column height andthe temperature of the column can be seen in Fig. 8. At thestart of both datasets the storage ring was not running. For
the data set taken with sand, the electron beam was injected
up to the user current, 300 mA, within 4 h. However, for thedata taken without sand, the current did not reach 300 mA
for close to 24 h. Therefore, the time for the column to reachan equilibrium temperature was longer. User current was
returned to the machine at ∼ 200 h in both runs. The column
temperature takes 5 h longer to return to the level prior to
the beam dump when it was filled with sand.

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102

100

10-2

With Sand
Without Sand

20
15
10
5
0
-5
Beam Off

10-4
10

-10
20

40

60

80 100

150

200

50

100

150

200

250

Frequency (Hz)Time [Hrs]Figure 5: The power spectral density of the horizontal motion taken on the mounting plate of the XBPM.

Figure 7: The change in the support column height measured
with and without sand filling the column.

10-2

10

-4

10

-6

20

0.2

10

0
With Sand0

Beam Off
-0.2

-10

-0.4
0

10

Support Column
Temperauture [°C]

100

0.4
Support Column Height
Column Temperature

50

100

150

200

250

Hours since midnight
20

40

60

80 100150

200 250

Frequency (Hz)Taking into account the expansion of steel, 12 × 10−6 K−1 ,the column height change is larger than expected given the
average column temperature variation. For example, in the
top plot of Fig. 8, when the beam is turned off the column
temperature drops by ∼ 0.1 °C, which should give a change
in height of ∼ 2.9 µm. The actual measured change in heightis around double this. This difference could be due to the
XBPM being twisted by neighbouring vacuum vessels also
warping or changing as the temperature changes. Whilethe column height is correlated with temperature, it is acomplex system that cannot be simplified down to just a steel
support column existing in isolation. The column cannot beassumed to be completely vertically rising and falling withthe temperature but could be ‘bending’ due to the tension in
the bellows. It is a reasonable assumption that the column
itself is not responsible for all the motion that is seen.

CONCLUSION
The mechanical stability of an XBPM support column has
been assessed and results show that sand-filled steel columns
effectively maintain the required thermal and mechanical stability for supporting front-end X-ray beam position monitors
at Diamond Light Source. Through laser interferometry andaccelerometer measurements, both short-term and long-term
stability were examined, highlighting the negligible impactof sand on high-frequency mechanical stability and its beneficial role in reducing vertical column variations over extended
periods. The standard deviation of the column height at a
rate of 1 Hz over a period of 11 days was 3.26 µm with sandand increased to 6.3 µm when the sand was removed. Thesefindings verify the stability of the column for the upcoming

Support Column Height[ m]

Figure 6: The power spectral density of the vertical motion
taken on the mounting plate of the XBPM

30

0.4
Support Column Height
Column Temperature

20

0.2

10

0

0

Without Sand

Beam Off

-0.2

-10

Support Column
Temperauture [°C]

Displacement PSD (nm 2 /Hz)With Sand
Without Sand

Support Column Height
[ m]

30

102

-0.4
0

50

100

150

200

250

Hours since midnight

Figure 8: The change in the support column height andthe average column temperature measured with (top) andwithout (bottom) sand filling the column.
Diamond-II upgrade, ensuring accurate and reliable X-ray
beam position measurements. Removing the sand improves
low-frequency vibrations slightly, but the impact of this is
not significant. The current sand-filled steel columns are
sufficiently stable to meet the Diamond-II requirements andtherefore will remain during the upgrade.

REFERENCES[1] C. Bloomer, G. Rehm, and C. A. Thomas, “Observation and
Improvement of the Long Term Beam Stability using X-ray
Beam Position Monitors at DLS”, in Proc. IPAC’10, Kyoto,
Japan, May 2010, paper WEPEB047, pp. 2797–2799.
[2] H. Büchner and G. Jäger, ”A novel plane mirror interferometer
without using corner cube reflectors”, Meas. Sci. Technol., vol17, pp. 746, 2006. doi:10.1088/0957-0233/17/4/021[3] S. Yang and Z. Guofeng, ”A review on interferometry for geometric measurement”, Meas. Sci. Technol., vol. 29, 2018.
doi:10.1088/1361-6501/aad732[4] F. Levinzon, ”Ultra‐Low‐Noise Seismic Accelerometers for
Earthquake Prediction and Monitoring”, Earthquakes, 2017.
doi:10.5772/65925[5] A. Piersol and T. Paez, Harris’ Shock and Vibration Handbook,
McGraw Hill LLC, 2009. ISBN: 0-07-137081-1

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With Sand
Without Sand

Support Column Height m

Displacement PSD (nm 2 /Hz)104

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