3.9 Ancillary Systems

The system described in sections 3.3-3.8 oversees and controls technical aspects of the detector and its readout electronics. However, it does not operate alone. The gas supplies, environmental monitoring for unsafe conditions (GSS), the solenoidal magnet, and the LEP machine have been developed independently of the detector Slow Controls system. In order to allow the Slow Controls operator easy access to the condition of these systems, and to allow automatic actions in serious situations, these systems have been interfaced with the DELPHI Slow Controls at the EMU (see section 3.6) and SMI (section 3.8) levels.

The gas and Solenoid control systems were developed within the DELPHI collaboration and both use a combination of G64 and VAX computers. However, in contrast to the detector control described in section 3.3.2, much more intelligence is vested in the G64s, while the VAX is used only for user interaction, logging, and interfacing with other systems. This has the advantage of allowing each system to operate independently. This was necessary as these systems were required before the rest of the detector controls were needed or implemented, and in any case could be run outside normal datataking periods when the other systems may be subject to frequent downtime. It did, however, lead to comparatively inflexible systems as program development on the G64 is painful, and (even using paged RAM) the program size is limited. Despite the different design philosophies, both these systems use the same G64 system software (FLEX, Pascal, etc.), the G64-ethernet card for communication with the OSI protocols, and (for the Solenoid) the RPC protocols.

The environmental surveillance and LEP monitoring systems were developed by independent groups at CERN and, like the gas and Solenoid monitoring, were interfaced a posteriori with the detector Slow Controls.

3.9.1 Gas Systems

All detector gases are provided by an integrated system of supplies, mixers, distributors, and purifiers [83], the state of which is monitored and controlled by 28 G64 systems. A further 6 G64s, which act as supervisors, are equipped with graphical displays and can control equipment and show the results of measurements throughout the system. The flow rates and compositions are carefully monitored, as anomalies could indicate a gas loss or a dangerous mixture.

Serious conditions are reported to a server on the VAX, which can set an ALARM SMI state for the parts of the detector affected, and injects an EMU message describing the problem for the operator. The ALARM state causes detector high voltages to be ramped down. This provides a backup to the hardwired connection directly from the gas system to the CAEN high voltage units.

An RPC server on the VAX is used to translate requests for information into commands for the gas system G64s. This facility is used to log the main gas parameters (as well as the atmospheric pressure) to the Status Update database. It is also used by certain detector partitions which base their high voltage control on the values of these parameters.

3.9.2 General Surveillance System (GSS)

The safe environment of all four LEP experiments is monitored independently by the General Surveillance System [84]. It monitors the ventilation, cooling water, temperatures, and flammable gas and smoke detectors. If problems are detected, it can alert the operator or the fire brigade; it can switch off gas supplies, high voltages, or mains power; and it can activate fire extinguishers. A graphical interface to GSS is provided.

The DELPHI Slow Controls system is linked to GSS by both hardwired signals and computer messages. Hardwired signals are generated in the case of many serious conditions and are used to switch off high voltage and other potentially hazardous equipment independently of any decision made by the software systems. Conditions detected by GSS that are relevant to DELPHI are sent to a server process on the DELPHI VAXcluster, which translates them into EMU messages and maintains the state of SMI objects for each detector partition and electronics barrack. The SMI state changes can provoke automatic actions such as switching off high voltages, before the condition becomes serious enough to force a hardwired switch-off from GSS.

Hazardous conditions detected within DELPHI, such as a gas loss detected by the gas system, are forwarded from EMU to GSS. This allows GSS to take independent action, such as performing a hardwired switch-off of gas supplies and high voltages before the possibility of a buildup of flammable gas in the environment.

3.9.3 Solenoidal Magnet

The solenoidal magnet [50] produces a field of 1.2 tesla by using a superconducting coil carrying a current of 5000 A, maintained at a temperature of 4.5 K. Monitoring is required for the temperature, pressure in the cryostat, current, mechanical strain, and magnetic field in a number of places round the coil. Detailed computer control of the power supplies is required.

These functions are performed by four G64 systems: for the power supplies, vacuum systems, data logging, and NMR magnetic field measurement. A standalone VAXstation 4000-VLC provides user interfaces and logs the time-variation of monitored values onto an independent database (implemented with CARGO; see section 3.7).

Anomalous conditions detected by the G64 systems are sent to an alarm server on the VAX. A few of these conditions, for example a severe fault in the cooling system, can provoke automatic action, such as running down the magnet currents. All messages are injected into a local EMU system, which can forward the more serious to the main cluster, thus notifying the Slow Controls operator in the usual manner.

3.9.4 LEP Accelerator

A typical LEP fill can last up to 24 hours, though background problems seen in the detectors may require intervention. During filling and high background conditions, when large numbers of stray particles can be thrown into DELPHI, the high voltages must be lowered for the Inner Detector, TPC, Outer Detector, forward tracking chambers (FCA, FCB), Barrel and Forward RICHes (RIB, RIF), barrel electromagnetic calorimeter (HPC), Forward Muon Chambers (MUF), and STIC.

Since 1994, these actions are performed automatically by Big Brother (see section 3.8.3). Prior to this it was necessary for all these interactions to be made by hand, with the Slow Controls operator lowering the high voltages when indicated by LEP conditions or planned actions, and keeping the LEP operators informed of the state of DELPHI's high voltages.

Tim Adye 2002-11-06