Speaker
Description
Summary
The low voltage system for the on-detector electronics of the CMS Experiment comprises 12090 channels of low voltage power supplies, requiring 1182 KVA of power at the entrance to the CMS facility at CERN.
The severe radiation environment inside the CMS experimental cavern combined with an ambient magnetic field reaching up to 1.3 kGauss at the detector periphery severely limit the available choices of low voltage supplies, effectively ruling out the use of commercial off-the-shelf DC power supplies.
The preparation for data-taking at the LHC experiments required a dedicated development effort on the part of two vendors specializing in equipment for high-energy physics, in coordination with the electronics design staff of the individual experiments.
Typical current requirements at the CMS detector front end range from 1A-30A per channel at voltages ranging between 1.25V and 8V. This requires in turn that the final stage of the low voltage power supply be located within ~10m of the front-end electronics, that is, on the detector periphery.
The CMS detector is quite large, having a diameter of 15m and a length of 21.5m. It is built around an iron yoke weighing 12500 tons, part of which supports the cryostat of a 4 Tesla superconductiong solenoid. This yoke functions as the main structural element of CMS and serves as the flux return path for the solenoid.
The detector is segmented into 13 sections. The central section (which supports the solenoid) is stationary, while the others can move up to 10m in the longitudinal direction. This allows the detector to be opened up for access to subdetector assemblies mounted on and inside the yoke. All cables to the 12 movable sections pass through articulated cable chains in trenches beneath the detector. The cable paths between the on-detector electronics and the power distribution area in the service cavern (adjacent to the CMS detector and shielded from it by concrete) are typically 100m-140m in length.
Power to the on-detector power units is supplied via these cables at voltages of 385VDC or 230VAC and 380VAC three phase, depending on the system. No neutral is distributed. The on-detector systems convert between input power at several hundred volts to an output between 1V and 8V in one step or two, depending on system architecture.
Power to the CMS front-end electronics is stabilized by a 2 MVA UPS located in a CMS surface building. This UPS isolates the CMS detector from disturbances on the local power grid and provides for 2 minutes of autonomy following a power failure, allowing for an orderly shutdown of detector electronics and controls.
Following the UPS stage, power is fed to the underground power distribution area, which contains isolation transformers, static compensators, distribution switchgear and banks of rectifier units providing 385VDC, as well as electronics for monitoring and control of the system.
This talk will describe the architecture of the CMS Low Voltage system as well as the considerations that went into its design.
