Work package 2, task 1: C1 - Cryogenic Last Stage Suspension
Task aims in the 3rd year
 Measurements on the cryogenic facility with the payload at low temperature

Major achievements in the third 12 months
The construction and the mechanical characterization of the small scale cryogenic
payload at room and low temperature has been concluded.
The payload is composed by four different elements:
- the reference mass of the marionette hosting 8 coils for steering the marionette
- the marionette, a composite system with an inner core made of silicon
- the reference mass of the mirror hosting 4 coils for controlling its alignment
- the mirror hosting on its back border the 4 magnets for the actuation
In the figure 1 we show the payload coupled to the cold head of the cryocooler. The reaction
masses of the mirror and the marionette are made of gold coated copper.
Figure 1. - The payload assembled for the cooling and ready to be inserted in the cryostat.
In the figure 2 we show the displacement spectrum of the mirror of the payload obtained by
using the PSD readout. The PSD provides two signal outputs for the vertical and horizontal
displacements. The two spectra are compared with that one obtained using the optical fibre
bundle when the payload is cooled at low temperature.
The payload is located in the inner vacuum chamber of the cryostat, tightened by an indium
gasket. We report here the temperature behaviour of the payload once the quasi-steady
status is achieved.
In the figure 3 we show the temperature value of the payload top nearly coincident with both
the mirror temperature curve and that of the aluminium flange of the inner vacuum chamber.
We conclude that we have been able to optimize the path for the transmission of the
refrigeration power from the inner vacuum chamber to the mirror.
PSD (Horizontal displacement)
PSD (Vertical displacement)
Fiber Bundle (Sensor used also at low temp.)
1
10
10-1
V/Hz
2
-3
10
-5
10
-7
10
Hz
-9
10
0
1
2
3
4
5
Figure 2. - Displacement spectrum of the mirror measured with the PSD and fibre bundle
readout in the low frequency range
Figure 3. - Temperature vs. time of the various parts of the payload in a quasi stationary
condition. The pressure in the inner vacuum chamber is 10-8 mbar.
Another important aspect of the cooling process is the temperature fluctuation of the
mechanical elements cooled at low temperature. In particular we notice in the plot of figure
3 that the temperature of the second stage of the cold heat is fluctuation significantly. We
have demonstrated that on the second stage cold head the temperature oscillation amplitude
is ~ 0.7 K with the typical periodicity of 1 Hz related to the main harmonic of the gas
standing wave of pulse tube refrigerator. At the same time we have set an upper limit of
~0.01 K in the temperature fluctuation for all the massive payload components.
Delays and possible alteration of the planning
None expected
Work package 2, task 2: C2 - Cryogenic suspension system for advanced resonant
detectors
Task aims in the 3rd year
 Cool down MiniGRAIL to below 50mK
 Test on the performance with 6 capacitive transducers
Major achievements in the third 12 months
The spherical gravitational wave antenna MiniGRAIL is being prepared for the next cooldown, using the experience acquired it he last cool-down of 2005. The problems found
were:
1- Lack of sensitivity of the transducers when cooled to 60 mK, the resonance peaks
being barely above the thermal noise.
2- Instability of the SQUIDs due to RF noise and too high an intrinsic SQUID noise
3- Low cooling power of the dilution refrigerator limiting the minimum sphere
temperature to 60 mK.
The actions taken consumed most of the year 2006 and are described below:
1- To improve the sensitivity of the transducers we have re-designed them so as to
achieve a higher capacitance by increasing the area but also to facilitate the polishing of
the surfaces of both oscillating mass and electrode. This was obtained by using a central
fixing bolt instead of 8 bolts on the perimeter. Figure1 shows the oscillating mass and the
removable central post to which the electrode is glued. A 10 m plastic foil is placed on
the mass and the electrode is placed freely on it and glued to the central post with low
contraction epoxy glue. The capacitance of the 12 cm diameter transducer is typically 5
nF, a factor 5 better than the old one. Several transducers were made and tested at room
temperature on the sphere. The 6 best were chosen for mounting on the sphere after
tuning.
Fig-1a: new transducer mass
Fig-1b: simulation with finite elements
2- To decrease the external perturbations on the SQUID the 600 mK and 60 mK shields
as well as the corresponding flanges were lead-tin plated to make concentric
superconducting shields. Each shield and flange was covered with a layer of glasmet tape
that should strongly reduce the trapped magnetic field seen by the transducers. Silodamp
foam will be placed under the concrete support of the sphere to decrease the low
frequency vibrations. Six new transformer boxes were made, with smaller diameter that
the previous ones so that they all fit easily on the last damping copper mass. A new
SQUID multi-channel electronics was bought from Star-Cryoelectronics together with 6
SQUID array amplifiers that will be placed after the input DC SQUID. Experiments are
being done to find out which of the available Dc SQUIDS has the smallest noise.
3- A new dilution refrigerator unit with large surface mixing chamber and heat
exchangers was made and installed. We expect a cooling power of at least 50 W at 30
mK that should allow a lower sphere temperature to be obtained.
In November we will do a quick cool-down of the system without the sphere, but with
complete transducers (with different SQUIDs) mounted on the last mass so as to test 3
complete measuring chains, the effect of the shielding and the performance of the dilution
refrigerator. If all is well we will then mount the sphere with the best SQUIDs and 6
transducers. Figure 2 shows the present experimental set-up.
Delays and possible alteration of the planning
None expected
Work package 2, task 3: C3 - Cryogenic suspension system for interferometers
Task aims in the third year
 Design of the cryogenic facility for the Superattenuator
 Production and test of high-conductivity high-compliance links
 Production and assembly of a cryo-compatible suspension prototype
Major achievements in the third 12 months
In December 2005 the cryostat was delivered to EGO, and its installation in the EGO
cryogenic facility began in January 2006. The facility infrastructure needed to be upgraded:
a new cryostat lifting and positioning system has been installed, and a dust protective
enclosure has been built over the cryostat experimental area (see figure1).
Figure 1: Installation of the cryostat at the EGO facility in Cascina.
In June 2006 most of the cryostat installation works have been completed: the cryostat
has been assembled and tested in vacuum at room temperature, attaining a residual
pressure of 10-6 mbar.
The data acquisition system and the temperature probes have been also installed. A
flexible modular and re-configurable architecture has been adopted to satisfy evolving
requirements of ADC, DAC, real time controller and data storage. It is based on hardware
and Lab View real-time software (National Instruments).
300
300
two
twostage
stagepulse
pulsetube
tubePT410
PT410
cooldown
cooldownwith
withreduced
reducedload
load
1st
1ststage
stageheat
heatexchanger
exchanger
2nd
2ndstage
stageheat
heatexchanger
exchanger
Temperature
(K)
Temperature
(K)
250
250
200
200
150
150
4K
4Kafter
after11hr
hr
100
100
50
50
00
00
30
30
60
60
Time
Time(min)
(min)
Figure 2: Pulse tube test chamber (left) and cooling performance (right).
90
90
The two pulse tube (PT) refrigerators have been tested into a small-dedicated vacuum
tank, and their cooling power has been characterized (see figure 2). The results confirmed
the manufacturer specifications: 1st and 2nd stage minimum temperatures respectively at
30 K and 2.8 K.
The installation of the PT refrigerators into the cryostat started in June 2006.
Different types of soft copper thermal links (see figure 3) have been tested and their
optimization is still progressing.
Figure 3: Pulse tube thermal links prototypes.
Part of the activity has been also devoted to the upgrade of the INFN-Pisa laboratory,
to prepare the assembly and the installation of the small-scale, cryo-compatible,
Superattenuator prototype. In September 2006 a new crane has been installed and the
assembly of the prototype in air has begun (see fig. 4). While progressing with the
installation some new components such as blades and suspension wires have been
produced and tested. Since January 2007 the room temperature characterization of the
Superattenuator prototype is in operation.
Figure 4: The new crane and the Superattenuator prototype installation at the Pisa INFN
laboratory.
Delays and possible alteration of the planning
None expected