High Voltage System for the CMS Forward Calorimeter

The Compact Muon Solenoid (CMS) is a general purpose detector for precision detection of gammas,leptons, hadrons and jets at the Large Hadron Collider (LHC) at CERN [1]. The hadronic calorimeters[2] consist of three subsystems: Hadronic Barrel (HB), Hadronic Endcap (HE), and Hadronic Forward(HF), as shown in Figure 1. There are two identical forward calorimeters (HF+ and HF−) locatedat ±11 meters from the interaction point. The absorber structure is composed of 5-mm-thick stackedgrooved iron plates. The active medium consists of fused-silica core optical fibers inserted into thesegrooves. The signal is generated when charged shower particles above the Cherenkov threshold (E ≥ 190keV for electrons) generate Cherenkov light, thereby rendering the calorimeter mostly sensitive to theelectromagnetic core of hadronic showers. The absorber structure is azimuthally subdivided into 20-degree modular wedges (18 on either side of the interaction point). The fibers run parallel to the beamline and are bundled to form 0.175×0.175 in ∆η×∆φ towers. Each calorimeter is functionally subdividedinto two longitudinal depth segments. Half of the fibers run over the full depth of the absorber (165 cm)while the other half starts at a depth of 22 cm from the front of the detector. These two sets of fibers areread out separately by Hamamatsu R7525 photomultipliers (PMTs). This arrangement makes it possibleto distinguish showers generated by electrons and photons, which deposit a large fraction of their energyin the first 22 cm, from those generated by hadrons, which produce signals in both segments.Each calorimeter is housed in a hermetic radiation shielding consisting of layers of 40 cm thick steel, 40cm of concrete, and 5 cm of polyethylene. A large plug structure in the back of the detector providesadditional shielding (Figure 2).In this paper, we describe our design for supplying high voltage to 1728 PMTs, maintaining economyand ease of use without sacrificing performance. In Section 2, we elaborate on the design considerations.In Sections 3 and 4, we concentrate on the design of the power supply and high voltage units. Thedescription of the controller and monitoring is in Section 5. The software system is discussed in thefollowing section, Section 6. After a brief presentation of critical system parameters in Section 7, wesummarize in Section 8.