The installation of optical modules (OMs) in the ice is referred to as string deployment. In a wider sense the string deployment includes the preparation of the string hardware at the South Pole and the termination of the string in the IceCube counting house. Once the borehole of approximately 50 cm diameter is drilled (melted) into the ice, the drill is removed from the hole and the site is available for the string deployment. From that moment on, the water column of 2400 m depth begins to refreeze. The refreeze process is complete after about 5 days. However, already after about 24 hrs the diameter of the water column shrinks to a diameter comparable to the size of the OMs. The OMs must reach their final position before the water column becomes too narrow. Figure 76 shows an image taken with a TV camera, which was deployed with an AMANDA string. The glass sphere shown in the figure was deployed for demonstration purposes only. Its diameter was 10 in, which is less than the 13 in spheres used for the OMs. Nonetheless, it can be seen that in this case the water column was much larger than needed, and therefore that more fuel was used to drill the hole than necessary. An engineering project called Smart Drill is underway to optimize the drilling by maintaining an adequate hole diameter while minimizing fuel consumption.
The deployment coordinator must decide how much time is needed for the deployment. The drill team must provide a hole that meets certain requirements. These requirements and the protocol of the hand-over are discussed later. Once the hole is released for deployment the countdown starts. After this moment the time constraints for the string deployments are absolute and unforgiving. In the worst case scenario a string can get stuck in the ice at a shallow depth where it is of little or no scientific value.
AMANDA deployed a total of 23 strings to depths between 1000 and 2350 m. The chronology of deployments is listed in table 13. The importance of the experience and expertise obtained in five AMANDA expeditions with string deployments cannot be overestimated for future IceCube deployments. The first deployments of strings to a depth of 1000 m were of an exploratory nature. Over time, the number of OMs per string increased from 20 to 42. In addition, the complexity of the OMs themselves increased. Optical modules on strings 11 to 19 of AMANDA-II use both electrical cable and optical fibers for signal transmission. For the DOM string (string 18) a more complex connectorization scheme was used. A total of 3 winches were operated simultaneously for all AMANDA-II deployments. During the 99/00 field season the installation time was as little as 8 min per OM, despite the enhanced complexity. In fact, the overall deployment time for the strings remained approximately constant for all strings while the number of OMs per string and OM complexity increased. This was accomplished due to a steady improvement in the deployment procedure.
| Year | Mission | Strings | OMs/string | Depth (m) | Duration (h) |
|---|---|---|---|---|---|
| 93/94 | AMANDA-A | 4 | 20 | 1000 | 13-18 |
| 95/96 | AMANDA-B4 | 4 | 20 | 2000 | 15 |
| 96/97 | AMANDA-B10 | 6 | 36 | 1950 | 12-15 |
| 97/98 | AMANDA-II | 3 | 42 | 2350 | 18-21 |
| 99/00 | AMANDA-II | 6 | 42 | 2020 | 10-15 |
Table 13: Chronology of AMANDA string deployments.
The basic components of a string are the main cable and the OMs. The OMs are attached to the cable for the first time at the South Pole. There are no test installations prior to the shipment of these components to the South Pole, so precise specification and quality assurance procedures of all components are critical for a successful deployment.
Figures 77-79 illustrate the important components of the deployment sequence. Three people work on the actual installation of the OM on the main cable. In total, a minimum of 7 people are needed for a deployment. For each OM the tension on the main cable is temporarily transferred to the tower winch cable. Then the OM is integrated in the main cable and the tension is transferred back to the main cable. The OM is now integrated into the main cable.
Figure 80 shows the progress of the deployment with time. A pressure sensor mounted to the bottom of the string monitors the depth of the string. The drill hole is filled with water up to a level of about 50 m below the snow surface. The pressure begins to increase strongly after the sensor reaches the water level. The pressure sensor is the primary measure of the absolute depth of the string. Other measures are cable marks and possibly a secondary pressure sensor. Each stop corresponds to the installation of on OM. The deployment reached speeds of 8 min per OM. Once all OMs are installed the pressure corresponds to about 350 m water equivalent pressure (the length of the instrumented portion of the string minus 50 m at the top). Now the "drop" begins. The string is lowered to its target depth at a speed of approximately 15 m/min.
A diagram of the full IceCube deployment setup, depicting the counting house, the surface cable, an IceTop detector module and the down-hole cable, is shown in fig. 81.
The mechanical installation of the OM is illustrated in fig. 82. The OMs are attached to the main cable below and above as shown. A chain with a shortening clutch allows adjusting the length to a precision of 1 in. This design is identical to that used on AMANDA strings 14 to 19 and is well understood.
Each OM is connected electrically to the cable via twisted quad cable breakouts at every fourth OM position as shown in fig. 83. Each twisted quad cable consists of 2 twisted pairs. Each twisted pair provides 2 OMs with power and communication from the surface. Figure 84 illustrates how 4 OMs are connected to the main cable. The OMs are spaced by 17 m. Thus, there will be a breakout of a quad cable every 68 m. A wiring harness will be used as an interface between the twisted quad breakout on the main cable and the OM.
The requirement to deploy 16 strings with 60 OMs each in a single season, particularly in view of South Pole population constraints, requires a further improvement of the deployment operation. During the AMANDA deployments, the shortest time between the start of two subsequent deployments was 120 hr. For IceCube the average deployment cycle time will be 84 hr for a period of about 60 days, excluding 5 days of down time. The increase in the efficiency is possible by optimizing every single element of the deployment procedures, in particular in the pre- and post-deployment activities.
The digital signal transmission based on the design of the DOM allows a significant change and simplification of the deployment procedure of IceCube strings. No optical fiber cables will be required. Thirty twisted pair cables will provide power and signal transmission for 60 OMs. The absence of optical fibers reduces the cargo by about 9000 lb per string, and simplifies test procedures prior to and during the deployment. (During deployments of AMANDA strings 11-13, problems were encountered when the breakouts of fiber cables and electrical cables did not match up. These phasing problems were one of the reasons why the deployment times were particularly high for strings 11-13. The phasing problems were solved in the 99/00 campaign. The deployment with a single cable, of course, means such phasing problems cannot occur.)
The phases of string deployment are as follows:
Several pressure sensors on the string are used to monitor the correct motion of the string. Communications and power to the pressure sensors and to the newly connected OM is provided through a slip ring mounted at the cable winch. The actual payout of cable will be measured at the surface. The measured payout at the surface and the pressure increase at the bottom of the string will be monitored for consistency. The correct electrical connection of each OM is verified at time of installation. A short electronic test sequence of no more than a few minutes is performed at this time. Additional communication tests to the recently installed OMs can continue for a few more minutes.
The shelter at the deployment site is very important for high quality OM installation at a rate of 2 strings per week. In the past, these shelters consisted of simple wind shields or a tent structure. Figures 85 and 86 show the dome structure that was used for AMANDA string 14-19 deployments.
For IceCube a heated building (see figs. 87–90) coupled to the drilling apparatus will be used as a deployment facility. This approach will improve the work conditions for the deployers substantially. All deployers work indoors. The electronics control room is inside this building but separated by a wall with large windows. The building is large enough to bring all or at least most of the OMs inside prior to the deployment. A furnace will replace noisy bullet heaters and eliminate exhaust near the work site. The ceiling may be partially transparent to take advantage of solar power and light. This building will also serve as a drilling shelter during the drill operation.
| Diameter | 43.0 cm |
| Depth | 2410 m |
| Lifespan of 1st hole per season | 32:00 hr |
| Lifespan of subsequent holes | 29:30 hr |
Table 14: Requirements which the drill hole must meet at the time of handover to the deployment team.
| Deployment phase | Allocated Time (hr) | Total | |
|---|---|---|---|
| Nominal | Contingency | ||
| Preparation | 1:00 | 1:00 | 2:00 |
| Weights, pressure sensor | 0:40 | 0:20 | 1:00 |
| OM installation | 14:00 | 6:00 | 20:00 |
| Drop | 2:30 | 2:00 | 4:30 |
| Finish and secure string | 1:00 | 1:00 | 2:00 |
| Total | 19:10 | 10:20 | 29:30 |
Table 15: Breakdown of deployment phases and allocated times
The drill hole needs to be of a minimum diameter and depth for at least the time required for the string deployment. The minimal diameter is determined by the size of the OM (36 cm, including the harness) plus the cable (4 cm). The tolerances on the OM size are 0.8 cm. The tolerance on the cable diameter is 0.7 cm. We add an additional 1.5 cm for tapes and cable ties. The maximum diameter of the string is 43.0 cm. The drill hole requirements are listed in table 14. The specified duration contains contingency time for the deployment operation. The nominal total deployment time is estimated to 19 hr. This allows for more than 10 hr of contingency. The nominal depth is 2403 m. We allocate 7 m of contingency.
The deployment operation begins when the drill hole is accessible for the deployers. At this time the drill and the downhole pump have been removed from the hole. In addition it is required that the area behind the tower be accessible to position the winch with the IceCube cable. Once these conditions are met the hole is handed over from the drilling to the deployment command. A breakdown of the different phases of the string deployment operation and the estimated time for each phase is given in table 15. Contingency times are based on experience and possible failure mechanisms. Possible failure scenarios include power, winch control, winch drive, tower winch, computer and unexpected situations.
Figure 91 gives an overview of the major operations during a single drilling and deployment cycle of 84 hr, and fig. 92 shows a schematic view of arrangement of drilling and deployment infrastructure in the course of drilling five holes.
Optical modules and associated components are all tested at various stages leading up to and including deployment. Tests which are done at McMurdo and the South Pole are described below.
McMurdo Test Stand All OMs undergo a detailed test procedure and burn-in operation before they are shipped to the South Pole. OM tests concern the integrity of the hardware after the transportation. Most OMs will be shipped by vessel. They arrive in January in McMurdo and stay there until about October of the same year. From here they are sent by air to the South Pole. The infrastructure at McMurdo permits extensive testing of OMs there. Therefore, only a very brief test sequence will be required at the Pole. (The details of the tests will be described elsewhere.) It is a fundamental concept of the test procedures that they do not require unpacking the OMs from the shipping crates. The crates are therefore light-tight. It is planned to wire the OMs inside the crate to a patch panel, which is accessible from the outside.
Only in the first season (FY04) will all OMs be shipped by air to McMurdo. In the following seasons an increasing fraction of OMs will be shipped by vessel. This requires that up to 1000 OMs be stored in McMurdo, and hence a similar number need to be tested there each season. This is done with a DAQ system with 500 channels to test the all the OMs in two batches. The duration of the tests is 2–3 months. The DAQ hardware can be shipped to the pole with the last batch of OMs in the last year of string deployments and used for the main DAQ.
South Pole Test Stand The deployment infrastructure at the pole can be divided in three categories:
All OMs are delivered to the central deployment lab. Here they undergo a final test sequence of not more than 60 min per OM. Again, the OMs are tested in the crates. The crates must be light-tight such that the OM can be powered up and noise pulses and cosmic-rays observed.
A test DAQ is available to operate up to 100 OMs at a time. The test DAQ is very similar to systems used at various locations during the OM production and shipping. The tests are short in duration, ranging from 10 min to not more than 60 min. The tests are pass/fail and executed in software. OMs that fail the Q/A procedure are rejected for deployment and sent back. Only minor mechanical problems are fixed at the South Pole. OMs that show electronic problems are not opened for repair at the Pole, instead they are sent back to McMurdo.
About 60 OMs will be tested at a time. Fifteen twisted quad cables (or 30 twisted pairs) need to be connected to the crates waiting for tests. The time to wire up all the crates is estimated to be 1–2 hr. While the test runs no longer than one hour, the time for debugging and evaluation may take several hours. In total we allocate 24 hr of labor per string at the central deployment lab.
After this test the OMs are powered off and the crates are opened for the first time since shipment from the northern hemisphere. The content of the crates is checked for completeness of all mechanical components. At this time the pressure inside the OMs is checked to make sure there are no leaks. Once the OMs are certified the crates will be marked and sent the next deployment site.
Once the OMs are certified for deployment they can be moved to the string deployment site. Sixty OMs plus about eight spares are loaded into the deployment room and partially unpacked. During the deployment a final continuity and communications test is performed. This test requires at most a few minutes and can be performed in parallel with the mechanical installation. The OMs are connected in groups of four. Once connected the deployment DAQ verifies the continuity and runs some basic electronic and communications tests.
Experience has shown that the first string deployment in each season is particularly challenging due to lack of experience of most of the team members. Two strategies are planned to optimize the performance during the first deployment. A pre-deployment workshop of about two weeks is planned and budgeted for all deployers. This is an opportunity to familiarize the team with the procedure and the instruments used. Unexpected situations can be discussed. Particular responsibilities can be defined. A test stand will be available to practice the actual OM installation. The full deployment team must be at the South Pole two weeks prior to the first deployment to begin with the preparations, test procedures, and the setup of the instruments and labs. A practice hole of about 50 m depth together with the full deployment infrastructure (tower, string deployment shelter, etc.) will allow deployers to practice the deployment on the site. A subset of the deployment team needs to arrive at least three weeks prior to the first IceCube deployment to prepare the new infrastructure.
| Year | FY04 | FY05 | FY06 | FY07 | FY08 | FY09 | |
|---|---|---|---|---|---|---|---|
| baseline schedule | 6 | 12 | 16 | 16 | 16 | 16 | 14 |
| possible range | 3-6 | 8-14 | 15-17 | 15-18 | 15-18 | 14-18 |
Table 16: Schedule for string deployments. The numbers indicate the how many strings are to be deployed in each season in the baseline schedule and the possible range in the number of strings deployed each year, as described in the text.
Deployment Sequence and Cable Plan Table 16 shows the deployment sequence during the IceCube construction period, and fig. 93 illustrates this sequence. The baseline schedule calls for six strings to be installed in FY04, followed by 12 in FY05, then 16 for FY06–FY08 and finally 14 strings in FY09. In FY04, the chief goal is to achieve an end-to-end test of all detector systems, a goal which is best achieved with a minimum of three strings, as indicated by the first number in the "possible range" row in Table 16. Similarly, in subsequent years the first number represents the minimum number of strings to be deployed, and the second number the maximum, subject to the constraint that the sum over all years equals 80. Experience gained in earlier years may permit even larger numbers of strings to be deployed in later years than indicated in the table. For any year in which the number of strings deployed exceeds the baseline schedule, there are production, logistics, and schedule implications which have not yet been addressed. It is possible that an accelerated schedule, using contingency funds in earlier years, will result in an overall savings to the project.
The addition of 12 strings in FY05 will embed the AMANDA array in IceCube strings. The SPASE-2 array will be embedded only in FY09. A few SPASE-2 stations will have to be relocated then, assuming that it is still in full operation. Trenches are used to bury the cables at depths which will keep them secure for ∼ 15 yr.
Cable Trenches The separation of the main cable in a surface cable and an in-ice cable allows for a very flexible cable management. The surface cables will be laid and trenched independent of the exact timing of the drilling and deployment. In this design, the surface cable will be trenched before the drilling starts. Ideally the surface cable is already wired up in the counting house by the time the string is deployed. As soon as the string is deployed it will be connected to the surface cable and the string will be operated from the counting house hours after the deployment is completed. Another advantage is that the valuable time during November will be used to do the cable trenching. There is no need to wait for the strings to be deployed first. The cables will be trenched starting from the counting house. This avoids the pile-up of excess cable in its vicinity. The small amount of excess cable, a few tens of meters, will be buried together with the IceTop tank in the snow. This is where the connection between the in-ice cable and the surface cable will be located. Cable trenching is done by the NSF contractor.
