Sector Trench Temperature Control

NLC Timing System
Trench Temperature Control
The temperature of the trench containing the optical fiber must be maintained within
approximately +/- 2 degrees C to keep the fiber length feedback loops within range. The
power required for this is estimated from typical thermal conductivities of earth and rock.
These are in the range of 1 W/M/degree C for concrete or limestone, somewhat lower for
dry sand.
The power required varies with the depth of the trench, but an estimate can be made from
dimensional arguments. For a 20 degree C temperature rise (to compensate for
summer/winter variations), and a sector length of 300 Meters, we get 6 KW/Sector. For
100 sectors, 2 trenches, this is a total of 1.2 MW. This large heat load can be reduced by
insulating the cables. For reasonable insulation, the power requirement can be reduced by
approximately a factor of ten.
Functional Description
Each sector requires two (2) Temperature Control Systems, one for each double redundant
fiber optic trench segment. A system is comprised of a ten (10) KW Power Controller
(commercial unit), a (commercially available) Temperature Readout for local monitoring
purposes, and a VXI module for status and control information for the host computer.
The Power Controller drives a heating element (possibly the steel support member of the
fiber optic cable bundle) buried in the trench. Temperature sensing elements are
monitored by the Controller, thus allowing closed loop control. Independent temperature
sensors are connected to the host computer via the sector’s temperature control VXI