IceCube Gen1

One of the biggest undertakings in PSL history, the IceCube project is a neutrino observatory at the South Pole. The project called for the design and construction of over 4,000 Digital Optical Modules(DOM) to be placed at depths of up to 2,400 meters below the surface of the ice, which is about 2,500 meters thick. The optical modules are designed and fabricated to survive the incredibly harsh environment in which they are deployed. They must also have nearly perfect reliability to continue operation in this environment for at least 20 years. The holes were drilled with the Enhanced Hot Water Drill. This complex device is the single largest piece of equipment at the South Pole. It must drill the holes in an accurate and rapid manner, while maintaining a high level of reliability and energy efficiency. PSL was the main design, construction, testing, and staging facility for the IceCube project from the start and continued its involvement during deployment. Many PSL employees traveled to the South Pole and assisted during deployment seasons.

DOM    Return Water Hose Reel    Independent Firn Drill    EHWD Drill    Drilling IceCube-Silvia Bravo 2014

Gen1 Drill Schematic

PSL has built more than 3,500 optical modules for the IceCube project over the past several years. Each 330mm diameter optical module contains a large diameter photomultiplier tube and a printed circuit board base potted in a clear glass hemisphere using an optically clear silicone gel. The upper hemisphere contains a connection to the outside world through which the power source and electronic controls on the surface interact with the module. The sealed modules are frozen in the ice, up to 2,400 meters deep, with tubes pointing toward the center of the Earth. Due to their deployment in such harsh conditions, each module is extensively tested for quality, including being frozen in a specially designed dark freezer for six weeks at -55° C.

The melting of the solid ice is done with a straight, high-pressure stream of hot water but this method doesn’t work in the snow (or “firn”) layer at the surface. The water just seeps away through the snow without melting the desired large hole.

This meant that two drills were required for drilling the 2400m deep holes in the ice. A “firn drill” was used for the first 50m of the hole. Hot water was circulated through copper tubing on the outside of the drill to melt snow by contact. Some water was squirted in a solid stream from the tip of the drill to melt a leading hole and some was leaked from circuits on the side of the drill to help with enlarging the hole. When the firn drill completed its portion of the hole, the hot water drill melted the solid ice with a stream of hot water, which finished the hole.

This firn drill head used the same support tower and water supply that was used by the main drill. This saved on equipment but meant that the firn drill had to be set up, used and then dismantled before main drilling could start.

The Design

It soon became apparent that quite a bit of time could be saved if the firn holes could be drilled independently at the same time as other main holes were being drilled. The two processes could be run in parallel instead of in series.

This required:

• A drill with its own water supply and heater
• A drill head that could work without releasing any water since it takes too much heat to melt that much snow on a self-contained sled. (It takes as much heat to melt ice as it does to heat the resulting water from ice temperature to boiling!)

The key to designing a drill head that melts through snow at a reasonable rate is to pay attention to the rate at which heat can be conducted from the water inside the drill into the copper that makes up the drill surface. Conducting heat through the copper, and from the copper surface into the snow, are faster processes so they don’t limit drilling speed.

Increasing heat transfer from the water into the copper is achieved by keeping the water moving over the copper at a high speed. In the coils this is done by using small diameter tubing so that the water moves through it quickly. In the tip, shown at left, a fast stream of water impinges against the inside of a hollow copper cap. This gives good heat transfer to the cap and keeps it melting through the snow.

The independent firn drill also required a reel to hold the supply and return hose, a shelter to house electronics and people during drill and a catch pan to catch any spills of the drilling fluid. (A mix of non-toxic propylene glycol and water was used instead of plain water so that the liquid would not freeze when the drill was not being used.)

In 1995, PSL designed and constructed the first hot water drill for the Antarctic Muon and Neutrino Detector Array, the predecessor to IceCube. The drill was used to drill holes in the Antarctic ice two kilometers deep in which the detector arrays were placed. The drill had to be able to withstand external pressure of 3400N/sq cm (5000 pounds per square inch), send and receive signals through 3200 meters of cable, and deviate from straight by no more than 1 meter. To overcome these difficulties, PSL created a modular drill head including a power and telemetry module, a navigation pack module, and a sensor module. The drill works by shooting hot water (about 90° C) out of a nozzle at the end. As the water melts the ice ahead, it cools and then is pumped out of the hole and back into the water heaters to be reheated and reused. Each hole took about three days of nonstop drilling.

As part of the preparation for IceCube and its installation, PSL helped to develop, commission, maintain and improve the enhanced hot water drill to be capable of drilling over 2 kilometers deep in about 1.5 days. PSL constructed and periodically refurbished multiple drill heads during the installation of IceCube.