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.
(iii) 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.
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:
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2nd stage 1st stage
CRYOMECH regrigerator 8.2 K 45.1 K
CVI refrigerator 12.1 K 42.2 K
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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