Drift-Chamber Gas System Controls Development for the CEBAF Large Acceptance Spectrometer

The CEBAF Large Acceptance Spectrometer (CLAS) is a superconducting toroidal magnet with a large volume of drift chambers for charged particle tracking. The performance of these chambers depends on accurate monitoring and control of the mixture, flow rate, pressure, temperature, and contaminant levels of the gas. To meet these requirements, a control system is being developed with EPICS. The interface hardware consists of VME ADCs and three RS-232 low-level hardware controllers. The RS-232 instruments include MKS 647A mass flow controllers to control and monitor the gas mixture and flow, MKS 146B pressure gauge controllers to measure pressures, and a Panametrics hygrometer to monitor temperatures and the concentrations of oxygen, water vapor, and ethane. Many of the parameters are available as analog signals which will be monitored with XYCOM VME analog input cards and configured for alarms and data logging. The RS-232 interfaces will be used for remote control of the hardware and verification of the analog readings. Information will be passed quickly and efficiently to and from the user through a graphical user interface. A discussion of the requirements and design of the system is presented. INTRODUCTION The primary instrument in Hall B at the Continuous Electron Beam Accelerator Facility (CEBAF) is the CEBAF Large Acceptance Spectrometer (CLAS) shown in FIG. 1. This device is a toroidal multi-gap magnetic spectrometer and is described in detail in the Conceptual Design Report on CEBAF Basic Experimental Equipment [1]. The magnetic field is generated by six iron-free superconducting coils. The particle detection system consists of drift chambers to determine the trajectories of charged particles, Cerenkov detectors for the identification of electrons, scintillation counters for time-of-flight measurements, and electromagnetic calorimeters to identify electrons and to detect photons and neutrons. The six segments will be instrumented individually to form six independent spectrometers. Commissioning of the CLAS will begin in the fall of 1996. Tracking of charged particles is accomplished by three regions of drift chambers that are located at different radial positions from the target. The inner chambers, called “Region I”, surround the target in a region of low magnetic field. The “Region II” chambers are somewhat larger and are situated between the toroidal magnet coils in a region of high field, while “Region III” chambers are large devices located radially outward of the magnet. Achieving the design goals of better than 0.5% momentum measurement and angle resolutions of ≈ 1 mrad requires constraints placed on the gas mixture within the chambers. These constraints are discussed in detail in the User Manual and Operation and Safety Procedures (OSP) for the Hall B Drift Chamber Gas System [2] and require control and/or monitoring of the flow rates, mixture, pressures, temperatures, and contaminant levels of the gas. There will be two levels of control in the gas system. The basic flow control loops and alarm system will be hardwired at one level. At a higher level, the system described in this paper will control and monitor the gas system in such a way as to allow: • Safe operation of the gas system. • Remote monitoring of signals. • Remote control of system elements. • Ease of use via a graphical user interface (GUI). • Detection of anomalous operational modes, alerting the users of such situations and, where possible, executing automatic procedures to ensure personnel and hardware protection. • Archiving operational parameters for future restoration and analysis. • Automation to handle the processing of very large numbers of signals.