Wire chambers require special nonair gas mixtures. The FPP gas mixture is based on Argon, a noble gas that ionizes relatively easily, and either CO2 or ethane as a quencher. If there are too many strongly electronegative components in a gas, all electrons will recombine before they can drift to the wire. If the gas has insufficient electronegative components, discharges can be self sustaining. Components such as ethane, CO2, and alcohol act to prevent the discharge from self-sustaining.
We have measured gain for both of the gases mentioned in test straws, and have also run with Ar-isobutane bubbled through alcohol.For the two rear FPP chambers, the gas system has been designed so that the straws are supplied and exhausted in parallel. This allows for maxmium gas throughput with minimal pressure drop. Operation of the chamber at lower pressures should minimize the leak rate. Also, by using 1/4" and 1/2" OD tubing on the exhaust lines, we keep the exhaust manifolds to no more than about 1 torr over atmospheric pressure.
Each of these chambers is furnished with 6 input manifolds and 6 output manifolds, located above (Upper) and below (Lower) the straws, along the edges of the carbon fiber. The chamber manifolds are independently supplied by six 1/4" OD nylon 11 gas lines, each with a separate flow meter. The chamber exhaust manifolds are each exhausted through short 1/4" lines, to match output impedance, followed by 1/2" OD nylon 11 lines. These lines will be tied internally in the chamber frame to a single 1/2" exhaust line to a bubbler, for use at CEBAF. Since the manifold pressure is less than 1 torr, the oil depth must be kept to << 1 mm.
The chamber 3 system has been tested at Rutgers at high flow rates, about 30 liters/minute. (Yes, this is about 30 times the planned CEBAF purge rate!) About 70% of the supplied gas was measured to have exhausted through the 1/2" exhaust lines. Because of the low exhaust manifold pressures, it was difficult to make a precise measurement at lower flow rates; however one expects for lower flow rates that the chamber pressure decreases, the magnitude of any leaks decrease, and a greater fraction of the supplied gas will exhaust through the exhaust lines.
The manifolds are numbered counterclockwise, looking from above, starting from the +U corner. For chamber 3, the X straws are intermixed with the U and V straws, and lead to input manifolds being connected to multiple output manifolds, and vice versa. For chamber 4, the differing lengths of the manifolds lead to cross coupling, but the U and V straw systems are totally independent within the chamber. The chamber 3 manifolds and the straws they supply or exhaust are described in the following table.
| Manifold # | Upper Supply/Exhaust planes | Lower Supply/Exhaust planes |
|---|---|---|
| M1 | S V1 V2 X2 | E U1 U2 X1 |
| M2 | S V1 V2 X2 | E U1 U2 X1 |
| M3 | E V1 V2 | E U1 U2 |
| M4 | E V1V2X2 | S U1 U2 X1 |
| M5 | E V1V2X2 | S U1 U2 X1 |
| M6 | S V1 V2 | S U1 U2 |
The volume of the gas system is about 170 liters for chamber 3 and 213 liters for chamber 4. At standard flow rates of 5 liters per hour per chamber, the gas is replaced every ~42 hours for chamber 4. Flushing the chamber, done with 10 times the flow rate, requires about 4 hours per volume change and thus about 1 day before the chamber can be turned on.
To try to ensure gas system tightness, straws are individually pressure tested after ferrules are glued in, and later after being strung and gas capped. In the post gas capping test, straws are typically pressurized to about 200 torr over atmospheric pressure (about 1.3 atmospheres), and observed for about 15 s for indications of a drop in pressure. No drop seen indicates a drop of less than about 5 - 10 torr, indicating less than about 1% of the gas has escaped.
For a more or less typical straw length of 117 cm, corresponding to a volume of 100 cm3, this implies less than about 1 cm3 of gas has escaped in 15 s. Under normal operating conditions, the pressure is about 1 torr over atmospheric pressure. Neglecting shape changes in the gas cap that lead to larger leakage, this implies each straw will leak less than about 1/200/15 cm3/s, or 3*10-4 cm3/s. Each chamber has about 1600 straws, leading to a chamber leakage rate of about 0.5 cm3/s. Of course, the tubes that connect the straws to the manifolds allow additional leakage points.
Revised May 6, 1996 Norma Lucero