1. Leak testing of aluminium target vessel
At room temperature, the target vessel was connected with the leak detector, and a
vacuum of ~10-5 torr inside the vessel was obtained. The vessel was covered by plastic
bag filled with helium. A background helium leak rate into the vessel was only 1.5x10-9
atm.cc/sec. Then the vessel (with vacuum inside) was dunked into liquid nitrogen
contained in a plastic bucket. When the boiling of the LN2 stops, the vessel was taken out
of the bucket and kept in the air for 10 minutes. Again the vessel was dunked into LN2
and the procedure was repeated 4 times. Finally, it was kept inside the LN2 container
until all the LN2 boils off and there is no freezing on the outside surface of the vessel.
Leak checking is again repeated by keeping the target vessel inside a plastic bag filled
with helium gas. A background helium leak rate was 3.0x10-9 atm.cc/sec which did not
increase. So, there was no leak through body and weld joints of the vessel above this
level.
2. Pressure tests of Al vessel and H2 fill/vent lines inside cryostat
a) 1st test (October 2nd week, 2004)
At room temperature, the assembly of the aluminium target vessel and the hydrogen
fill/vent lines inside the cryostat were made leak tight (He background ~ 3x10-9
atm.cc/sec). Now the leak detector was connected to the vacuum enclosure surrounding
the target vessel assembly. Both the target vessel and the box vacuum vessel enclosure
were evacuated to a pressure of <10-4 torr. The target vessel assembly was pressurized
with an internal helium pressure of 75 psig. While pressurizing, the helium background in
the vacuum enclosure surrounding the above assembly was continuously monitored by
the leak detector as well as the RGA. The helium leak rate did not increase above the
base level leak of ~ 1x10-8 atm.cc/sec.
The target vessel was evacuated and then cooled down to a temperature of 25 K. The
assembly was again pressurized by helium gas to a pressure of 75 psig. A huge leak >10-6
atm.cc/sec was detected. It was warmed up to the room temperature and again
pressurized. Still there was a leak of similar magnitude. The back cover plate of the box
vacuum vessel was opened. Leak detector was connected to the target vessel assembly
and it was found that there is leak in the joint between aluminum flange of the target
vessel and the SS flange of the fill line.
b) 2nd test (November 1st week, 2004)
The above assembly was again made leak tight (He background ~ 3x10-9 atm.cc/sec) by
tightening the bolts by applying their maximum allowable torques (e.g. ¼” ss bolts: 100
in-lb; ¼” brass bolts: 80 in-lb; 8-32 brass bolts: 20 in-lb). Now the leak detector was
connected to the vacuum enclosure surrounding the target vessel assembly. Both the
target vessel and the box vacuum vessel enclosure were evacuated to a pressure of <10-4
torr. At room temperature, the target vessel was pressurized by internal helium pressure
of 75 psig. While pressurizing, the helium background in the vacuum enclosure
surrounding the above assembly was continuously monitored by the leak detector. There
was no leak found (He background ~ 1x10-8 atm.cc/sec).
The target vessel was evacuated and then cooled down to a temperature of 15 K. The
assembly was again pressurized by helium gas to a pressure of 20 psig. Helium leak rate
increased from 1x10-8 to 1x10-6 atm.cc/sec. Then the target was warmed up to the room
temperature and again pressurized. Still there was a leak but of smaller size. The leak rate
increases from 1x10-8 to 1x10-7 atm.cc/sec. The back cover plate of the box vacuum
vessel was opened. Leak detector was connected to the target vessel assembly and it was
found that there was no leak from outside to inside of the target vessel assembly. So, the
conclusion is that there is still a leak from inside to outside both at room temperature as
well as liquid hydrogen temperature. So, we need to modify the joints to get rid of these
leaks.
3. Test of thermometry
There are 10 temperature sensors (T1, T2, ……,T10) located at different locations of the
target and h2 fill/vent lines inside the cryostat to monitor the temperature distribution. In
addition there are 3 heaters (H1, H3 and H4), out of which two (H1 & H3) are on the
colder heads of both the refrigerators and H4 is on the fill/vent line to supply heat to
increase temperature when required. All the sensors and the heaters are connected
through an instrumentation feed-through to 4 controllers kept in the control panel. While
cooling down the target the complete thermometry was tested and found to be working
satisfactorily. When a temperature was set at a particular point of the target (e.g. sensors
T2 & T3 of refrigerator cold head and sensor T7 of vent line), the sensor at that point
acquired the set temperature (20 K and 75 K respectively) by putting the associated
heater (H1, H3 or H4) on, and the set temperature was maintained for as long as the
temperature controller was in control mode. The test of setting the temperatures of T2, T3
and T7 were also verified at a temperature (305 K) slightly higher than room temperature.
4. Test of pressure gauges
The transducer type pressure gauges of the gas handling system (i.e., PT101, PT102,
PT103, PT104, PT105, PT106, PT201, PT202, PT203) have been calibrated by
measuring the voltages produced by them at atmospheric pressure (777 mbar) and at high
vacuum (~0 mbar). Other pressure gauges are tested by applying pressure in the GHS.
Their readings are consistent with the values read by already calibrated pressure gauges.
5. Helium Channels
There are seven helium channels around the joints of the vacuum enclosure (the box
vacuum vessel) which are meant for fast detection of leak from outside air to inside of
vacuum enclosure. They are connected in series having one inlet and one outlet which are
connected to the helium supply manifold. Each of these channels has two seals: Indium
(inner seal) and Viton O-ring (outer seal). Helium flows in between these two seals. Each
helium channel has been leak tested separately and made leak tight ( with helium leak
rate < 5.0x10-9 atm.cc/sec.
6. Operation of valves and Interlocks
a) Manual valves
There are about 25 manually operated valves. Their open/close status are monitored
in the panel-view which is connected through the PLC. Each of these valves are
operated and checked that the status is read correctly. Initially, the status of some of
the valves (like V108, V121, V126 and V304) were not reading correctly. It was
found that their micro switch positions were not correct. So they were adjusted and
made the status readings correct.
b) Solenoid valves and interlocks
There are seven solenoid-operated valves (V114, V129, V201, V204, V205, V207
and V303) in the present GHS. These valves are operated by pressing their
corresponding buttons on the panel-view (as programmed through the PLC). First,
their operation were tested by running the PLC in test mode. Operation of valve V204
is not very smooth. It needs several open/close operation before it becomes smooth.
To open V114 the solenoid needs an air pressure of about 110 psig. However all other
valves need only about 60 psig. After the valve operations were tested successfully
with PLC in test mode, the PLC was switched into interlock mode. It was checked
that each of the above valves opens/closes only when all the interlock conditions get
satisfied.
7. Test of Residual Gas Analyzer (RGA)
During target cool down tests, the RGA, which is connected to the main vacuum
enclosure by opening V207 and V307, was kept ‘on’ to monitor the partial pressure of
different gases including helium. The partial pressure of helium was compared with the
reading of the helium leak detector (which was also connected to the main vacuum). The
total pressure read by the RGA was compared to the reading shown by the vacuum
gauges PT303 and PT302 connected to the vacuum system. It was observed that the
readings for both the total pressure and the partial pressure are consistent.
8. Leak testing of gas handling systems
a) H2 gas handling system
Leak detector was connected to inlet (near V107 & V108) of GHS.
(i)
To start with, all the valves of the GHS were closed. There was no leak in the
connection line (flexible hose) between the leak detector and the H2 inlet port.
Base level helium background was 0.3x10-9 atm.cc/sec.
(ii)
V108, V109 and V110 were opened. Leaks found at the joints around V113,
P102, V111 and PT102. By tightening the joints, most of the leaks were
corrected. But the leak between FM101 and V111 could not be corrected. So,
the joints of the flow-meter were opened and reconnected with a new SS VCR
gaskets. Now, the helium leak rate did not increase above the base level he
leak of 0.5x10-9 atm.cc/sec.
V113 was opened. A leak was found near V116 (~1x10-7 atm.cc/sec). The
joint was tightened and made leak tight (background he leak rate ~0.3x10-9
atm.cc/sec).
(iv)
V122 was opened. No leak was found (background he leak rate ~ 0.3x10-9
atm.cc/sec)
(v)
V125 was opened. No leak found (background he leak rate ~ 0.3x10-9
atm.cc/sec)
(vi)
V111 and V107 were opened. No leak found (background he leak rate ~
0.3x10-9 atm.cc/sec)
(vii) V117, V119A and V120 were opened which open to LN2 trap. It took long
time to get vacuum for leak checking. So, it was kept for ovenight pumping.
No leak found (background he leak rate ~ 0.75x10-9 atm.cc/sec)
So finally, the whole GHS was made leak tight with base level helium leak rate not
increasing beyond 0.75x10-9 atm.cc/sec.
(iii)
b) H2 supply manifold
The hydrogen supply manifold (without H2 gas cylinders and pressure regulators) was
leak tested independently. Leak detector was connected to V106.
i)
The points at the H2 outlet and near V104 and V102 were blanked off, and all
other valves were closed. V106 was opened. Leak checking was done and no
leak was found (with base level he background ~1.0x10-9 atm.cc/sec).
ii)
V131 was opened. No leak was found (background he leak rate ~ 1.0x10-9
atm.cc/sec).
iii)
V130 was opened. No leak was found (background he leak rate ~ 1.0x10-9
atm.cc/sec).
c) Helium and Argon supply manifold
Helium and Argon gas supply manifolds (without the pressure regulators and cylinders)
were leak checked and found to be leak tight (with back ground he leak rate ~5.0x10-8
atm.cc/sec). The base level leak rate was little higher which is understandable because
some of the helium and argon lines are made of poly tubes.
9. Target cool down without hydrogen
a) Cool down during October 2nd week
During ten days starting from October 4th, we had a cool down test of the target
without hydrogen, in the target shed. The tests and observations are as follows.
1. Temperature versus time graphs for all the temperature sensors placed at different
locations inside the cryostat were obtained. Most of the cool down characteristics were as
expected from our earlier cool down test in Indiana.
2. The minimum temperatures of the two stages of the refrigerators achieved were as
follows:
---------------------------------------------------------------2nd stage
1st stage
CRYOMECH regrigerator 8.2 K
45.1 K
CVI refrigerator
12.1 K
42.2 K
---------------------------------------------------------------The minimum temperature of the target vessel could reach up to 24.4 K. The reason, why
the target temperature did not go lower, is due to the radiation load from the front
aluminum window to the front end of the target vessel. There was only one thin window
between these two surfaces. So, we need to put some extra radiation shield to reduce the
heat load.
3. When the temperatures of the 2nd stage of the cryo-refrigerators were set at a
particular temperature (20 K) in the temperature controller, the set temperatures were
maintained for as long as the controllers were in controlled mode operation.
4. Helium leak testing of the main vacuum was done by supplying helium through the
helium channels around the cryostat joints/welds. A leak (1x10^-6 atm.cc/sec) from a
helium channel (around the back cover plate+upper cryo+vent stack ) flange into the
main vacuum was found. No leak from target vessel to the main vacuum was detected.
5. Residual Gas Analyzer was tested and found to be working fine. The total pressure and
partial pressure of helium read by the RGA was compared with the pressure readings
shown by the vacuum gauges PT302 & PT303 and the leak detector. The pressure values
of the RGA compare well with the others. So we can use the RGA to detect any leak of
hydrogen or helium into the main vacuum during the target operation, without connecting
a separate leak detector to the system. And, the leak detected by RGA can give
appropriate WARN or ALARM signal for the operator to take correct actions.
6. We filled the target vessel with helium to see the heat load on the refrigerators. The
temperature of the target goes up from 24.9 K to 38 K at a helium pressure of 494 mbar.
The helium was allowed to cool down, and after 18 hours, the temperature went down to
24.7 K, and the helium pressure becomes 306 mbar.
7. We put one of the refrigerators (CVI) off, and kept only CRYOMECH refrigerator on
to see how much temperature it can hold. It was found that the target temperature of 32 K
could be maintained (instead of 24.7 K when both refrigerators are on).
8. The heater (H4) which is connected at the fill/vent line to keep the target in
superheating mode, in order to suppress the bubble formation, was also tested by setting
its nearby temperature sensor (T7) at 75K (normal temperature was 65.5K when target
was at 24.7K). It was observed that due to this heater the target temperature changes from
24.7K to 25.0K. Since the change was very negligible, there should be no problem to
get the objective of the above heater fulfilled.
9. Finally, we pressurized the target vessel with helium gas at a pressure of 80 psig, at
room temperature. It was found that helium has leaked to the main vacuum, which we do
not want. The target was again cooled down and the pressure test was repeated when
target was cold. It was found that helium leaks even more from target vessel to the main
vacuum.
b) Cool down during 1st week of November
The objective of this cool down test was to see i) if we can reach the target temperature to
that of a liquid hydrogen, and ii) if we can make the target vessel and associated h2
fill/vent lines leak tight at high internal pressure (~75 psig).
1. The target cryostat was closed again after few modifications as given below.
i) a thin aluminum window was put at 20K stage of target vessel towards the beam side,
ii) super insulation put around several exposed surfaces of the target fill/vent lines, cryo
heads, etc.
iii) a copper wire connected between the target vessel and the 2nd stage of CVI
refrigerator,
iv) a copper wire connected between 1st stage of CRYOMECH refrigerator and a nearest
point on the 1.5" SS H2 fill/vent line.
v) new brass bolts in the flange joints of the target vessel and the fill/vent lines
vi) all bolts of 6 flange joints tightened upto their maximum limit on torque,
vii) both the bellows of the h2 fill/vent line were guarded by thin SS plates and tied
tightly to stop them geting deformed while pressurizing the target vessel.
2. After the leak test of both the target vessel and the main vacuum enclosure, we did the
pressure test by pressurizing the target vessel at room temperature up to 75 psig by
helium gas. There was no leak of helium from the target vessel to the vacuum enclosure.
3. Then the target was cooled down. The temperature distribution inside cryostat was
following:
T1 (1st stage of CVI)=42.9 K
T2 (2nd stage of CVI)=12.2 K
T3 (1st stage of CRYOMECH)=6.3 K
T4 (2nd stage of CRYOMECH)=43.8 K
T5 (OPC)=13.1 K
T6 (H2 entry point of the target vessel)=15.5 K
T7 (on the h2 fill/vent line, above the liquid h2 level)=83.5 K
T8 (beam entry side of target vessel)=15.7 K
T9 (beam entry side of target vessel, opposite to T8)=15.8 K
T10 (beam entry side of 80K radiation shield)=164.4 K
---------------This implies that we can reach to the liquid hydrogen temperature.
Although the temperatures of the 2nd stages of the cryo heads (which are 6.3 K and 12.2
K) are below freezing temperature of liquid hydrogen, these temperatures can be
maintained at liquid hydrogen temperature (between 14K and 20K) by puting on the
heaters associated with them, but without increasing the vessel temperature much. This
was confirmed by applying heat to one of the cryo heads. Secondly, the temperature
distribution will be very different when the vessel is filled with hydrogen. This was
confirmed when the vessel was filled with helium gas. For example the temperature of T7
reduced from 83.5K to 50K, and the target temperature went up fom 16K to ~45K.
4. When the target vessel was cold, the target vessel was filled by helium gas with
internal pressure of 20 psig. It was found that, helium starts leaking into the vacuum
enclosure (leak rate increases from 1x10^-8 to 1x10^-6 atm.cc/sec. The helium gas was
pumped out.
5. Target was allowed to warm up. Pressure test was repeated when the target reached
room temperature. Helium started leaking into the main vacuum. The background helium
leak ate increases from 3x10^-9 to ~6x10^-8. So, the conclusion is that the h2 fill/vent
line along with the target vessel leaks at high internal pressure.
6. The cryostat was opened again and the leak detector was connected to the target vessel.
We tried to find the leak in the fill/vent line but could not succeed. This means that
although there was small leak from inside to outside, there was no leak from outside to
inside at room temperature.