Electrical Installation

Hall A contains a lot of electrical equipment and almost all of it could serve as an ignition source in the presence of an explosive oxygen and hydrogen mixture. We have made an effort to minimize the dangers from the equipment that is most likely to come into contact with hydrogen gas.

There are a number of electrically powered devices associated with the target gas handling system. All the pressure transducers in the system are approved for use in a hydrogen atmosphere. The solenoid valves on the gas panels are explosion-proof. The AC power for the solenoids is carried by wires which are contained in either hard or flexible conduit. There are also LEDs on the gas panels that provide an indication as to the status of the valve solenoids. These are powered by a 24 $V$ DC supply. The readouts for the pressure transducers are mounted on the gas panels and the AC power for these readout units is in conduit. All the pressure transducers have 4-20 mA outputs.

In addition to the electrical devices in the gas handling system, there are a number of devices inside of or mounted on the scattering chamber.

All the devices which are in the scattering chamber must have their power delivered to them by wires in vacuum. The insulation of these wires should be radiation resistant, so Kapton has been used where available.

The following electrical items are in close proximity to or are actually in the hydrogen system.

Axial Circulation Fan

The fans which circulate the hydrogen in the target are AC induction motors and therefore contain no brushes and are practically immune to sparking. The three phase power for these fans is delivered to them by 18 gauge stranded copper wire with Kapton insulation. The maximum current that the fans draw is 5 $A$ for a maximum power consumption of 200 $W$ when pumping liquid hydrogen/deuterium. The current and voltage drawn by the fans is monitored by the control system.

Fan Motor Tachometer

The fans have a tachometer which consist of a coil that views the flux change caused by a permanent magnet attached to the motor rotor. The tachometer signals are carried on 22 gauge stranded wire with Kapton insulation. This is a low power signal. The control system monitors the frequency of the fans.

Low Power Heater

This is a ``hair dryer'' style heater ( it resembles the heater elements found in hair dryers and heat guns) that is immersed in the hydrogen. The heater is made of 0.0179 $in$ diameter Nichrome wire with a resistance of 1.993 $\Omega$ per foot wrapped on a G10 carrier board. The maximum power available to this heater is 80 $W$. The power for the low power heater is supplied by a Oxford ITC-502 temperature controller. The heater lead wire is 18 gauge Kapton insulated copper stranded wire. The heaters have a DC resistance of 20 $\Omega$ and hence will draw a maximum of 2 $A$. The power supplied to this heater is monitored by the control system.

High Power Heater

There are two kapton enclosed incoloy heater foils wrapped on the inside wall of each heat exchanger. The maximum power available to each heater is 500 $W$. The heater has a DC resistance of 26 $\Omega$ and two heaters in parallel are driven by a 150 $V$, 7 $A$ power supply. The current and voltage supplied to this heater are monitored by the control system and there is a software power maximum enforced on the power setting of this heater. The heater is connected to the outside world by 18 gauge stranded wire with Kapton insulation.

Resistors

There are two Allen Bradley and four Cernox resistors immersed in each target loop. These resistors provide temperature measurements of the target fluid. The temperature controllers that read them use a current of less than 30 $\mu$A to excite them ( they are excited with a constant voltage which for our resistors is on the order of 30 mV). The Cernox resistors are connected to the outside world with quad strand 36 gauge phosphor bronze wire with Formvar insulation. The Allen Bradley resistors are wired with 30 gauge Kapton insulated copper stranded wire.

Target Lifter

There are two AC servo motors which provide the power to lift the target ladder. These motors are powered by three phase 208 $V$ power and are equipped with fail safe brakes (the brakes are released by a 24 $V$ DC control voltage) and 50 to 1 gear reducers. On power up, there is a delay relay that insures that the motors are always energized before the brakes are released.

Vacuum Pumps

The scattering chamber is evacuated by two Leybold 1000 $l/s$ turbo pumps that are backed by a Leybold 65 $cfm$ mechanical pump. The turbo pumps are powered by 120 $V$ AC power while the backing pump requires three phase 208 $V$ AC power. The motor on the backing pump is explosion proof and approved for use in NEC Class 1, Division 1, Group D (hydrocarbons but not hydrogen) environments. An identical mechanical pump is used in the pump and purge system of the gas panels. Both the scattering chamber backing pump and the pump and purge system's mechanical pump exhaust to the vent line.

Vacuum Gauges

The chamber vacuum is monitored by an HP cold cathode gauge. This gauge has a maximum operating voltage of 4000 $V$ and a maximum current of 133 $\mu$A. The pressure at the entrance to the roughing pump is measured by a convectron gauge.