Gas Target

Revision history

Version	Author	Description	Date
1.0	D. Hutcheon	Initial Document	27 September 2002
1.1	D. Hutcheon	Hidden valves,shack	4 April 2007
1.2	D. Hutcheon	Leak checking	27 April 2007
1.3	D. Hutcheon	Decadal update	22 January 2019
1.4	A. Lennarz	Transferred to wiki	June 25th, 2025

Safety with hydrogen as the target gas

The following is a brief outline of safety considerations. For more details, consult the DRAGON Safety Report.

The strategy for safe operation with hydrogen gas is:

• avoid formation of an explosive mixture of air and hydrogen
• avoid ignition sources at pressures where combustion may take place

Normal operation

When the zeolite cleaning trap is in operation, there is a large inventory of "invisible" hydrogen adsorbed on the zeolite molecular sieve. For a trap inlet pressure (CMTRIN) of 45 Torr the adsorbed hydrogen amounts to 5 litres at STP, while the "free" recirculating hydrogen may be only 15% of this amount. The heat of combustion of this total amount of hydrogen is approximately 50 kJ - enough to raise 5 tonnes a height of 1 m.

IT IS ESSENTIAL THAT THE TARGET OPERATOR KNOWS WHETHER THE TRAP IS COLD AND WHETHER IT IS "LOADED" WITH HYDROGEN.

The correct response to the sonalert alarm from the LN2 automatic filler or from the EPICS system depends upon this knowledge. Therefore:

ANY SIGNIFICANT CHANGE IN THE INVENTORY OF HYDROGEN IN THE CLEANING TRAP MUST BE ACCOMPANIED BY AN ENTRY IN THE DRAGON “Equipment Status” elog (https://elog.triumf.ca/Dragon/}.

The entry should indicate trap status (warm/cold), estimated hydrogen inventory.. The 6-litre Buffer Tank should be used to control the amount of gas loaded into the trap. The number of 50-Torr "loads" from the Inlet Buffer Tank should be tallied during an initial fill of the trap. Any large deviation from the expected 15-20 "loads" to reach approx. 45 Torr pressure must be investigated and the problem fixed.

(During tuning for a new beam energy it is common for ISAC Operations to request that gas be removed from the DRAGON target. Each time that this is done, some of the "free" circulating hydrogen is lost. It is convenient to replenish the supply by adding 1 or 2 "loads" of gas through the Inlet Buffer Tank when the target cell is again filled with recirculating gas. This is considered a routine operation which need not be reported in the Equipment Status elog)

The purpose of the cleaning trap is to remove air and other impurities from the recirculating hydrogen gas. Thus, when the trap does its job it is almost impossible to detect any small leak of air into the target system. The implications for target operation with a trap are:

• A cold trap which has been loaded with hydrogen should be treated as though it contains air also, in a potentially explosive mixture. An exhaust fan must be running whenever a cold trap contains hydrogen, to allow safe dilution of hydrogen in the event of an unanticipated release from the trap through the pressure relief valve. Normal procedure when the trap is being warmed up is to pump on it with the roughing pump, which vents to the high-air-flow exhaust line.
• Before cooling the trap, it must be checked that any air leaks are less than could amount to 1.5 atm-litre over a 2-week period (assuming a trap will be kept cold no longer than 2 weeks at a time). Possible sources include leaks in the differential pumping stages, in the trap itself or in the buffer tank and gas supply lines. The leak rates should be measured by pumping down the system, then isolating it with the Roots blowers continuing to run; the gauge CMTRIN connects to an effective volume of 12 litres, from which the quantity of gas (leaking or outgassing) may be calculated using an observed rate of increase in CMTRIN. Other sources are HEBT or the DRAGON separator when isolation valves HEBT2:IV8 or DRA:IV11 are open. At an estimated 1000 litre/sec conductance in the beam pipes, the sum of partial pressures of air upstream and downstream of the target should be less than 1E-6 Torr when there is no gas added to the trap.
• Failure of the automatic LN2 filling system or failure of the primary exhaust fan must trigger immediate action, either to fix the fault or to begin controlled warmup of the trap.

Unattended gas target, trap containing absorbed hydrogen

At times it may be desirable for the DRAGON gas target operator to leave the target unattended for a period of many hours. This may be done, provided it is arranged that someone else (e.g. an ISAC Operator) checks the Trap status via Epics at least once every two hours. Of greatest concern is failure to keep the trap cold, for example if an LN2 dewar goes empty or the automatic filling system fails.