Experience with the ZEUS central tracking detector

The central tracking detector of the ZEUS experiment has been reliably operated since 1992. It is a cylindrical drift chamber approximately 2 m in length and with a radius of 82 cm, and operates in a magnetic field of 1.43 T with a gas mixture of 82:5:13% argon:CO₂:ethane with a trace (∼0.5%) of ethanol. During the course of the 2000 running period, operational problems attributed to the Malter effect were experienced. These were remedied by adding water to the gas mixture in 2000. Searches for long term loss of gain have been inconclusive. The effects on the chamber performance due to the addition of the water are detailed, and since this time, these problems have not reoccurred. During the commissioning runs for HERA II luminosity operation, very high beam-related background conditions have been experienced, raising fears of the potential reoccurrence of the operational problems observed in 2000. So far, none have been observed despite backgrounds during regular luminosity operation being several times that during HERA I operation. Data from the CTD has been central in unraveling the nature of these larger than expected backgrounds.