High Voltage

A schematic of the HV system for a station is show here. The design is derived from what was used for the CDF tracker. Note there are three “ground” points: at the 1^st floor racks, required for safety; at the experiment-wide “detector ground”, to minimize noise carried into the DS; and at the tracker, more specifically the station this set of HV channels connects to.

The “crowbar box” contains a high voltage, high current SCR to remove stored energy from the HV distribution system in <1msec. This is to avoid repeated sparking causing damage to the straws or wires. Since these SCRs are large and expensive, several HV channels share a single such crowbar, merged through high voltage diodes. A resistor limits peak current on any one cable. Inductor L1 slows the rate of rise in current so the SCR has time to turn fully on before the current reaches it’s peak.

The “filter box” has a ground break and low-pass filter to reduce noise. It also provides the transition from coax to ribbon cable. Note the “R” in the RC low pass filter has a diode bypass so as not to limit the discharge through the crowbar. As a safety measure, the diode across the ground break resistor is a Zener diode to limit the maximum voltage the ribbon cable shield can go to.

At the detector, the 12 panels in each a station have a shared ground, but the stations are isolated from each other. The HV ribbon’s shield connects to this shared ground, separately for each station. As seen in this schematic, there is another low-pass filter to attenuate any noise picked up on the ~6m run from the IFB to the station. The capacitor also provides a low impedance path to ground which is required for the HV fuse at the preamp to function.