4.3   Drift Chamber Operation

Drift chambers require low voltage (6 V) power for the on-board pre-amplifiers, high voltage (~2000 V) for biasing the sense and field wires, and a source of high-purity gas. Here we discuss the safety aspects of these three utilities.

4.3.1   Low Voltage

There are eighteen low voltage supplies, one for each chamber. The supplies are located in Hall B in racks on each level of the "space frame". Below each supply is a distribution box which contains the power bus and two fuses for each pair of power cables (outgoing and return).

The system is set up such that there are FEW SAFETY ISSUES (to persons or equipment) during operation. The supplies cannot exceed 7.5 V by design; thus, they cannot harm the on-chamber voltage regulators. Each output and return line is separately fused with a maximum 2.5 A fuse, where the nominal current draw is about 1.5 A. Any unaccounted-for power draw must be less than about 10 Watts (7.5 V x 1 A) thus posing a negligible fire risk. Furthermore, software limits are set about 2 A above the nominal current draw for each supply as a whole. If the total current draw exceeds the limit, the supply is shut off. The only plausible hazard arises from improper placement of fuses. The following are the RULES of SAFE OPERATION:

  1. Do not change the voltage.
  2. Reset the supply if necessary and record the current.
  3. If necessary, replace fuses ONLY with ones with the SAME RATING .
  4. Notify Mac Mestayer or Amrit Yegneswaran if fuses repeatedly trip.

4.3.2   High Voltage

The high voltage power is supplied by CAEN modules which are arranged as 10 boards of 24 channels each per crate. Three crates power all of the CLAS chambers. The CLAS shift personnel will be required to turn channels ON and OFF, as well as monitoring the system for tripped channels which will require RESETTING .

Although there is NO DANGER OF ELECTROCUTION from these supplies (the maximum output current on a channel is 40 micro-amps) damage to the drift chambers can occur if they are not used properly. Details of how to run the drift chamber control program, DCHV, are contained in the Drift Chamber section of the Operations Manual. The following are the RULES of SAFE OPERATION:

  1. In normal operation, it will not be necessary to change the voltage setting. If you must initialize the settings, do not exceed the standard voltage setting as described in the Drift Chamber section of the Operations Manual unless authorized by Mac Mestayer or Amrit Yegneswaran.
  2. Do not increase the current trip level setting beyond the nominal value for the target and beam luminosity in question as indicated in the Operations Manual.
  3. During extended periods without beam, set the current trip level to the beam-off value as indicated in the Operations Manual.
  4. Do not reset a channel more than three times in an hour without consulting Mac Mestayer or Amrit Yegneswaran.

4.3.3   Gas System

The gas used in the drift chambers is a mixture of Argon and Carbon Dioxide. The gas is mixed and filtered and the flow controlled by apparatus located in the gas shed and in Hall B at the gas panel (on deck 2 of the "space frame"). The system is set up such that there are FEW SAFETY ISSUES (to persons or equipment) during operation. The gas mixture is non-flammable, and there is no high-pressure (< 1 bar PSIG) gas within the Hall. The gas pressure in the Hall is high enough to harm the chambers' gas bags, however.

To guard against harmful pressure excursions, the gas system is designed with two levels of protection: a hardware protection system which will automatically bypass flow to the chambers in the event of a number of error signals (e.g. pressure at the chamber too high or too low) and a software system with looser limits which reports error conditions through an EPICS alarm handler.

During normal operation the shift personnel ONLY need to respond to the ALARM HANDLER. These are the corresponding RULES of SAFE OPERATION :

  1. For a minor YELLOW ALARM, acknowledge the alarm handler and note the occurrence in the logbook as described in the Operations Manual.
  2. For a major RED ALARM, acknowledge the alarm handler and call or page George Jacobs immediately. Although the system is set up to avoid damage to the equipment, a major alarm may indicate a condition in which the data is corrupted.