Tandem Accelerator

Tandem Beamline SF6-Gas Security Ball Valves

Each beam line exiting the Tandem has a large diameter, pneumatically actuated Worcester Controls ball valve that will automatically close under normal operating conditions if a pressure rise on the order of in. of Hg is detected on the Tandem side of the valve. Because the Tandem is filled with SF6 at 85 psi and the accelerator tubes and beamline are under vacuum a failure of an O-ring seal, accelerator tube metal to glass bond or any major compromise of vacuum integrity within the machine could have catastrophic results. The intention of the valves are two fold. One is to contain the gas to prevent its loss, a $70,000 investment. The other is to prevent damage to the equipment on the beam line that is designed to withstand the implosive forces of vacuum and not the explosive 85 psi tank pressure.

An Edwards High Vacuum Ltd. pressure switch is located on the L.E. end of the Tandem beam line to sense the fault condition along with a 30 in. Hg to 60 psi compound gauge to monitor the tube vacuum/pressure in the range where penning gauges and thermocouples are not usable. The control panel is in the vault L.E. electronics rack below the beamline, and the key switch for disabling control of the ball valves is kept in the red case underneath the Control Room console. When the vault control panel is in the unprotected key switch position the ball valves can be opened and closed at will. In the protect position the ball valves can only be opened when the beamline pressure switch is closed. An LED on the control panel, labeled sensor contact, will illuminate when the switch is closed.

The Gas Security Ball valve controller is integrated with the Tandem Pelletron interlock circuit. A closure of the ball valve either manually or due to a failure of the vacuum integrity is sensed by the Ball valve controller and results in the Pelletron charging supplies being interrupted via the interlock panel.

Because they are pneumatically controlled, a loss in compressed air service will result in the valves closing. There will be no electric circuit indication such as the sensor contact LED being off or the Pelletron H.V. supplies being tripped off. If beam transmission through the machine is nil, investigating the ball valves themselves will allow ascertaining the valves’ state. On top of the pneumatic actuators are square posts that turn with the ball valve, the valves are open if the small silver stud on one of the flats is facing West.

Tandem Beamline Vacuum Protection System

The logic and status chassis for the Tandem Vault beam lines is above the switching magnet power supply, North of the H.E. vault shield door. There is a status panel also in the Control Room above the beamline devices panel (figure 4). The L.E., H.E., Sputter Source and Polarized Source vacuum indications are displayed in the Control Room also.

Each Tandem beamline fast valve has a dedicated control box and penning gauge except for the 90◦ magnet which has a penning gauge but no fast valve to control. Each controller has a “good vacuum” output signal that is “anded” together in the logic chassis with the other controllers. If any of the five penning gauge controllers detect a vacuum incident, all four of the fast valves will close and the Tandem Pelletron charging H.V. supplies will be disabled.

The Tank Gas Security Ball Valve Controller is also “anded” with the beamline penning gauge controllers in the logic chassis and an LED indication for the ball valves status is provided. If either the L.E. or H.E. Gas Security ball valve is closed, the Pelletron H.V. charging supplies are disabled. If the ball valves have closed on their own something could be severely wrong and a staff member familiar with this system should be consulted before they are reopened. More information about the Gas Security ball valves can be found in Sec. 18.

The source penning gauge is not included in the “anded” logic to close the beamline valves. The source penning gauge will close the source exit valve and turn off the preaccelerator H.V. supply if the ion source vacuum sufficiently degrades. The valve control boxes for the two sources are different, so, arbitrary interchanging of the boxes cannot be done.

The fast valve control box/penning gauge controllers themselves do have many common features. The valve control boxes can be used to open and close a valve at will in the unprotected mode (no vacuum consideration). In the protected mode, the valve can only be opened if the vacuum at the associated gauge head is better than the chosen set point, approximately 5 x 10−6. A controller that has sensed a vacuum excursion will have a flashing LED; to rearm the protection circuit and open the valve(s) that has tripped close, depress the push button reset switch on the front panel of that controller, the LED should stop flashing. Be cautious with the beamline valves: if all the controllers sense a good vacuum and all valve switches are in the open position, all four fast valves will open simultaneously. If this is not desired, place the valve control box switches to close, reset the tripped penning gauge controller, and then open the valves individually. If a penning gauge controller is out of service or it is desirable to prevent a controller from closing the beam line valves, it can be overridden at the logic panel by placing the switch indicated for that controller in the override position. The Ball valves should only be overridden after a staff member familiar with this system has been consulted.

Power Failure Procedures

When the mains fail the emergency generator should start automatically. If the emergency lights come on, the generator is running correctly. The changeover breaker should switch automatically and restore power to all pump circuits. If this does not occur, the change over breaker, located on the West wall of the accelerator vault between the L.E. door and the sink, should be assisted with a 2-by-4. Note: In the emergency-power position, the central arm should be down.

When emergency power is available at the pumping stations a number of things should be checked

1. Check that the recirculating pump, located just to the South of the L.E. door in the accelerator vault, is running. Restart if necessary. See Water Cooling System.
2. Check that source pumping stations are operating normally. Restart pumps and reopen valves as necessary.
3. Check the turbo pump at the H.E. end, it may have tripped and be coasting down. It should be restarted.
4. Check Cryopump. If the supply was only interrupted for a few seconds the compressors will restart automatically, but the input and output pressures will remain the same for approximately one minute; then the unit will switch and normal pressures will be reestablished. When this occurs the coldhead will restart and the vibration of this motion will be felt at the cold head. If the power is off long enough for the pump to start to warn and outgas, it will have to be started again. See the section 4 for cryopump head regeneration.
5. The pumps in the target room should be checked to ensure that they are running normally. Restart or close off as necessary.
6. When mains voltage is restored the changeover breaker will wait a short time, to ensure that the supply will probably remain on, and then will automatically change over and the emergency generator will shutdown. If the breaker hangs up it can be assisted up with the 2-by-4. When the mains supply has been restored to the pumping stations the previous checks should be repeated. When all vacuums are normal the beamline valves should be checked and reopened as necessary. Make sure that the vacuum is good on both sides before opening any valve.