High voltage crowbar protection for the large CDF axial drift chamber 2003 IEEE nuclear science symposium

The large cylindrical drift chamber in the CDF detector at the Fermilab Tevatron operates with high stored electrical energy. The chamber has more than 63,000 sense and potential wires at high voltage in a jet cell configuration with 8 superlayers. Each wire has about 1 nf added capacitance to ground to provide filtering, signal termination, and cross-talk reduction. Since the high voltage is provided to the chamber by 200 supplies, large numbers of wires must be daisy chained together with a resultant stored energy of more than 2 joules on the daisy-chains of the outermost sense wires. When a sense wire starts to discharge to ground, it is important to keep the stored energy on the other wires in the daisy-chain from recharging the problem wire. Since the recharge time for a sense wire is less than 100 μsec, a mechanical switch is not fast enough. For case of use and reliability we chose a 7 kV SCR for the switch. A crowbar box was designed that would discharge all 25 daisy-chains supplying a superlayer with an initial current of typically 80 A for a sense wire voltage of 3 kV. The choice of high voltage switch, the prototype performance, and the experience of more than two years of running are summarized.