S1G report
15. Alignment
The cavities are hung from the GRP (gas-return pipe). The GRPs are
suspended from the vacuum vessel of the cryomodule by support posts. There
are two support posts in each module. Upstream support post is fixed,
however, the other downstream post can slide in order to absorb the thermal
contraction of the GRP. The cavity position displacement during cool-down
and cooled-state at 2K are major concern for the accelerator alignment.
From the early stage of the STF cryomodule development, we have tried the
measurement of the GRP and cavity displacement during the cool down by
using WPM (wire position monitor), however, we could not succeeded to
obtain a reliable data. In 2009, we have performed the comparative
measurement of GRP movement in the STF-cryomodule-B by three kinds of
methods, by using laser displacement sensor (LDS), by optical telescope, and
WPM. The agreement of the GRP displacement between LDS and optical
telescope was confirmed, however, WPM still gave unstable results.
In this S1G experiment (2010 and 2011), we have upgraded the WPM
electronics from CW RF drive method to pulse RF drive and sample-hold
detection electronics with 100Hz slow filtering, in order to avoid interference
of drive RF leakage to detection circuit. The measured GRP and Cavity
displacement by using LDS and WPM are discussed in the following section.
15.1 Installed instrumentation
To monitor the GRP displacement during cool-down and at 2K temperature,
the LDS are attached in the module-A. The WPMs are installed into the GRP
in both of module-A and module-C. To monitor the cavity displacement
directly, the WPMs are also installed into the cavities in the module-A. The
cavities in the module-C are not monitored, to save engineering and
preparation time of the jacket modification for WPM installation to DESY
cavities and FNAL cavities. GRP1 to GRP5 are the name of the WPM
pickups attached in the GRP of module-A (Fig. 1). CAV-1U, CAV-1D, and so
on, are the name of the WPM pickups attached in the cavities upstream (U)
and downstream (D) of the module-A (see Fig. 1, also).
Fig. 1 Installed displacement monitors in Module-A. LDS1-4 are the laser
displacement sensors, GRP-1-5 and CAV-1U -4D are the wire position
monitors (WPM).
15.2 Laser position monitor (LDS)
Four laser displacement sensors (LDS1, LDS2, LDS3, LDS4 in Fig.1) are
installed in the module-A top position (Fig. 1). There are 4 glass windows to
allow laser light be reached to the reflection mirror on the GRP and reflect
back to the sensor. Keyence LK-G sensors are used as shown in Fig.2. The
specifications are as follows;
measuring range: 400±100 mm
resolution: 2 µm
linearity: ± 0.05 % of F.S.
laser spot diameter: approx. 290 µm.
Fig. 2 LDS (Laser displacement sensor)
15.3 Wire position monitor (WPM)
The two stretched wires of 70µm diameter gold-plated tungsten are used to
monitor GRP displacement and the cavities displacement. The wires are
stretched about 12m distance through the module-C and module-A. The one
upstream end of wire in the module-C are N-type feed-through connector for
feeding the RF pulse into the wire. The other downstream end of the wires in
the module-A are connected to the SUS weight of 500g to apply it stretch
force. The wires are electrically insulated and no resistor is connected at the
end. The aluminum pipe shields are inter-connected between WPM pickups
and thin SUS pipe for endplates. WPM pickup as shown in Fig. 3 is consist of
4 strip-lines (SUS electrodes) and aluminum body with SMA connectors,
where one end is terminated by 50-ohm and the other is connected to the
signal cable. The drive RF pulse is the burst sine-wave signal of 160MHz
with 100ns pulse width, +23dBm amplitude. The mV-level signal from the
pickup electrodes are multiplexed and amplified by a log-amplifier and
amplitude-detected, then converted to DC voltage by a sample hold circuit.
The detected DC voltage are put into 16bit ADC, and sent the data to the
data logging computer. The time interval of the WPM recording is around 10
seconds. The error of position detection is around less than 5%. The
resolution is around a few 10µm. Fig. 4 shows the WPM installation location
in the cross-sectional view of the cryomodule.
Fig. 3 Drawings and picture of WPM pickup.
(a) GRP-WPM installation position
(b) Cavity-WPM installation position
Fig. 4 WPM position in the cryomodule cross-section
15.4 Position displacement measurement for GRP and Cavities
Cooling down from room temperature to ~150K was done by He gas (~80K),
with mass flow rate of ~1.5 g/sec. Cooling down from 150K to 2K was done by
using liquid He with mass flow rate of ~0.35 g/s. The liquid He consumption
at cold state of 2K was about 0.4 g/s. The cryogenics system was operated
only in the daytime of week days. No operation in night time and week end
was took place (intermittent cooling). The displacement data for the
alignment discussion was selected for the term; from January 19, 2011 to
March 9, 2011. In this term, WPM electronics was the most upgraded state
and seemed more stable than others. The temperature variation of GRP and
helium vessel in this experiment term are shown in Fig. 5 (a), where
cool-down in the beginning and warm-up at end are shown together with 4 of
week days 2K period and 4 of week end temperature rise. Overall data of
WPM in GRP are shown in Fig. 5 (b) and (c). They show X position shift and
Y position shift of GRP during cool-down and at 2K state.
(a) Temperature change during Jan. 2011 to Mar. 2011 experiment
(b) Y position change of GRP by WPM during Jan. 2011 to Mar. 2011 exp.
(c) X position change of GRP by WPM during Jan. 2011 to Mar. 2011 exp.
Fig. 5 GRP-WPM signal overview during Jan. 2011 to Mar. 2011 exp.
During the cool down period, large displacement of GRP Y positions (max
2.7 mm) are observed as shown in Fig. 6 (a), (b), (c), and (d). But the amount
of displacement at 2 K obtained from WPM-GRP Y positions are not
consistent with the Y positions of LDS and with the estimated displacement
( dY = - 0.22 mm). Displacements of WPM-GRP Y positions at 2K are hard to
The behavior of WPM-GRP X position during whole period seems to be
reasonable, however, the amount of displacement at 2 K, around +0.4mm, is
much smaller than the estimated value (+0.62 mm). The behavior of
WPM-GRP-1 X position is different from other WPM-GRP X positions.
The amount of displacements of Cavity-GRP Y positions at 2K are
separated to two groups. The one group is around -0.5mm, the other is -1mm.
The estimated value (-1.07 mm) is consistent to the one group, however, the
other one is hard to understand. The behavior of Cavity-GRP X positions are
roughly consistent with the estimated value, +0.75mm, however, there are
0.4 mm variation.
WPM-Cavity-2 UY and WPM-Cavity-4 UY were omitted because of
unstable data, and WPM-Cavity-1 DY and WPM-Cavity-3 UX were
interchanged because of miss-connection.
(a) GRP Y position change during cool-down
(b) GRP X position change during cool-down
(c) Cavity Y position change during cool-down
(d) Cavity X position change during cool-down
Fig. 6 Displacement measured by WPM and LDS during cool-down period
During the periods of 7 Feb to 11 Feb. and 15 Feb. to 19 Feb., the module-A
was kept at 2 K in the daytime. Fig. 7 shows WPM and LDS variation in the
2K temperature. The fluctuation widths of the LDS signals during the
periods are about 0.2 mm, however, those of the WPM-GRP Y position are
smaller than 0.1 mm. The fluctuation widths of the WPM-Cavity are also
smaller than 0.1 mm. The reason of this difference between LDS stability
and WPM stability is not yet understand.
(a) GRP Y position change during cooled-state
(b) GRP X position change during cooled-state
(c) Cavity Y position change during cooled-state
(d) Cavity X position change during cooled-state
(e) GRP temperature change during cooled-state
Fig. 7 Displacement measured by WPM and LD during cooled-state
As a summary of these measurement;
(1) During the cool down period, rather large vertical displacement (max 2.7
mm) at the end parts of GRP was observed in the signals of LDS and WPM.
(2) The amount of the GRP displacement measured by WPM is roughly 1.4
times larger than that of LDS.
(3) After the cavities are cooled down to 2 K, the fluctuation of GRP position
measured by WPM is less than 0.1 mm, however, the fluctuation of LDS is
about 0.2 mm, bigger than WPM.
The reason of this LDS-WPM discrepancy is not clear yet. However, it seems
that WPM pickups need to modify to have more stable thermal contraction
by this big thermal excursion. Also we found that some of the SMA connector
of signal cables had unstable connection by this big thermal excursion. They
also need to upgrade by the next installation.