Tandem Accelerator

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Warning Warning :   NEEDS PROOF READING


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

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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.

Tandem Pelletron Interlock Panel

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To prevent dangerous operation of or damage to the accelerator an interlock circuit is employed. The status of the various interlock switches is displayed at the interlock panel. The switches and their status are in a serial configuration therefore should one interlock not be closed all others “down stream” will also indicate an open situation and the LEDs will be extinguished. The panel layout is natural in that all the LEDs to the right of the offending switch will be out; once that interlock is satisfied its LED will illuminate and the rest to its right should also shine if the switches are closed. The interlock circuit, depending upon which switch opens up, will either turn off the charging supplies or both the charging supplies and the chain drive motors. All the interlocks up to and including the tank pressure switch will disable both the chain drives and power supplies, the remaining only affect the charging supplies.


There are nine LEDs on the Tandem Pelletron Interlock Panel, see figure 10. A brief description of each interlock or LED indication in the order that they are appear on the panel follows.

Status Power: This lamp will be lit if the power switch key is turned to on and the 12V dc supply used for the series circuit is present.

Control Power: This lamp indicates AC power is present within the chassis when the switch key is turned to on.

HE Tank Door: A push button switch is mounted on the Tandem High Energy Tank Door to sense when the door is fully rotated closed.

LE Tank Door: A push button switch is mounted on the Tandem Low Energy Tank Door to sense when the door is fully rotated closed.

LE Pendulum: A push button switch is mounted to the LE drive sheave assembly to sense excessive Pelletron chain stretch or failure.

HE Pendulum: A push button switch is mounted to the HE drive sheave assemblyto sense excessive Pelletron chain stretch or failure.

Tank Pressure: A pressure switch is mounted to the Tandem pressure vessel to ensure a minimum pressure of 35 psi is established before the Pelletron chain is started.

Vac Ball Valves: This interlock is closed when the Tandem Beam Line Valve Protection System detects no tripped penning gauge controller or beamline SF6 protection ball valve closure. A separate status panel is present in the Control Room for this beamline protection system and an override/status panel is located in the Tandem Vault to the North of the HE door. A more detailed description of that circuit can be located through the Table of Contents.

TPS OV/UV: The Terminal Potential Stabilizer supplies a relay that relaxes whenever the terminal voltage varies beyond a set percent of the value displayed at the terminal voltage reference potentiometer. This TPS feature must be enabled to trip the interlock panel for an out of range voltage excursion.

Should a condition arise where disabling the Tandem Pelletron Interlock Panel is desired, such as a tank opening, an override switch is located in the back of the panel chassis. It is accessible from behind the electronics racks.

Terminal Potential Stabilizer

The Terminal Potential Stabilizer (TPS) consists of five main components: Controller Unit, Generating Voltmeter (GVM), Analyzing Slits, Capacitive Pick-Off Plates (CPO) and the Corona Probe.

The Controller as is suggested, controls the terminal voltage by comparing signals from the GVM, slits and the CPO to produce a correction signal that modulates the current flowing to the terminal from the corona probe.

The GVM is mounted on the tank wall of the Tandem opposite the terminal. A four bladed rotor spins at 3450 rpm over a surface divided into eight segments each the size of one blade. As the rotor spins it uncovers some segments and exposes them to the terminal voltage which results the segment charging to a voltage proportional to the terminal voltage. When the segment is again shielded by the rotor, charge will drain away. The signal produced by the GVM is conditioned locally by an amplifier and routed to the controller.

The analyzing slits are in the typical configuration, downstream of the 90◦ analyzing magnet, parallel to the pole face plane and opposite each other on the exiting beamline. They are adjustable with micrometer style knobs for each and are electrically isolated. Beam being bent around the 90◦ magnet impinges upon the slits creating a signal proportional to the beam it senses. The signals from the slits are conditioned locally by an amplifier and routed to the controller.

There are two CPO plates in the Tandem that are mounted on the tank wall opposite the terminal. These plates are electrically isolated and sense fluctuations of terminal voltage at frequencies above those detectable by either the slits or the GVM. Two plates are used to cancel out physical movement of the column that would be detected as a voltage fluctuation by a single plate as the separation between the terminal and plate changes. The signal from the CPO plates is conditioned locally by an amplifier and routed to the controller.

The corona probe is also mounted on the tank wall opposite the terminal. The probe is mounted on a shaft that can be driven to different distances to the terminal depending on the terminal operating voltage. The end of the probe consists of a shell with 12 needle points extending slightly beyond the polished shell surface. The voltage on the needle points determines the flow of electrons to the positively charged terminal. Modulating this flow of electrons is done by the Controller based on the information provided by the GVM, slits and the CPO.

The typical operation of the system is described to assist the operator in understanding how the system maintains terminal stability. The description is not intended to be used for bringing up the terminal voltage as it excludes the Charging Controllers and positioning of the corona probe. Feedback or modulation of the probe current will take place only when the TPS Control Gain is turned up. The Controller has three possible modes of operation namely GVM, Slit or Auto. The Auto mode allows the TPS to switch into one of the other two modes depending whether the terminal voltage is approximately 50 KV off of the controller Terminal Voltage Reference Potentiometer setting and/or beam current above 10 nanoamperes is sensed by the analyzing slits. Typically the Controller is in the Auto mode.

With less than 10 nanoamperes of beam on the analyzing slits, the Tandem will be in GVM mode. The correction signal in this mode is derived by comparing the front panel Terminal Voltage Reference Potentiometer setting to the GVM measurement of actual terminal voltage. The CPO signal is added to this to produce the error signal that ultimately modulates the corona probe voltage. The operating voltage of the probe needles is proportional the Grid Bias indicated on the Controller chassis. In this mode the terminal voltage will track the Reference Potentiometer setting and allows one to scan the terminal voltage until beam strikes the analyzing slits.

With beam on the slits and the GVM and Terminal Voltage Reference Potentiometer in relative agreement the TPS Controller will switch to Slit mode. The controller compares the current striking one slit to the other to create an error signal that attempts to keep the measured current balanced. The slit on the inside of the analyzing magnet bend is the Low Energy (LE) slit and the one on the outside is the High Energy (HE) slit. If an excess of beam is detected by the LE slit an error signal is produced by the TPS that will reduce the probe electron current flowing to the terminal causing the terminal voltage to rise which will bring more beam current to bear on the HE slit until a balanced situation is again reached. Like the GVM mode, the CPO signal is added to modulate the probe current. In this control mode adjusting the Terminal Voltage Reference Potentiometer has no effect until the disagreement with the GVM causes the TPS to revert to GVM mode. When the TPS is in Slit mode the Reference Potentiometer should be adjusted to agree with the GVM indication.


Operating Instructions for the Tandem Pelletron

To Turn the Tandem Pelletron On

  1. Set the TERMINAL POTENTIAL STABILIZER CONTROL MODE to AUTO and turn the CONTROL GAIN and CPO GAIN to zero.
  2. Turn on the LOW ENERGY CHARGING CONTROLLER AND THE HIGH ENERGY CHARGING CONTROLLER by turning their key switches CW to the ON position. To start the charging chains lift the momentary CHAIN switch to
  3. the ON position for each CONTROLLER. After approximately ten seconds the lamp below the switch will illuminate indicating the chain is up to speed.
  4. Set the corona probe BIAS CURRENT to 35 microamps on the TPS
  5. Set the corona probe to the approximate position indicated in the table for the desired terminal potential.
  6. Set the GVM REFERENCE VOLTAGE knob to the correct voltage (e.g. 8 MV would be “0800”).
  7. Ensure the TERMINAL CHARGE POTENTIOMETER on the LOW ENERGY CHARGING CONTROLLER is turned down to zero, CCW.
  8. Turn both POWER SUPPLY switches to ON on the CHARGING CONTROLLERS. Their indicator lamps should glow.
  9. While watching the corona probe GRID voltage, carefully increase the charge carried by the chains by turning the TERMINAL CHARGE POTENTIOMETER CW. Turn the TERMINAL CHARGE POTENTIOMETER until the GVM/TV
  10. Balance indicates approximately zero or the GRID voltage has reached -9V. If any time the GRID voltage reading goes beyond -1OV, run the corona probe out to reduce it. Fine tune the corona probe position and the up-charge so that the GRID voltage reads approximately -9V when the desired terminal voltage is reached.
  11. Enable the OVER/UNDER VOLTAGE PROTECTION and set both the CONTROL GAIN and CPO GAIN to 5.
  12. Readjust the corona probe if necessary to attain -9V; do not exceed this voltage on the GRID.
  13. Set the 90◦ magnet to the appropriate field and put beam through the Tandem. If the TPS does not switch to the SLIT CONTROL MODE and lockup on a beam the GVM REFERENCE VOLTAGE knob can be adjusted slightly to scan for the
  14. beam; only a slight adjustment should be necessary.
  15. With beam on the slits, and the TPS in SLIT CONTROL MODE adjust the TERMINAL CHARGE POTENTIOMETER so that the Slit Balance meter reads zero, and retrim the corona probe position, if necessary, to get -9V on the GRID.
  16. Adjust the GVM REFERENCE VOLTAGE knob so that the GVM/TV Balance meter indicates approximately zero.


If there is a spark

The OVER/UNDER VOLTAGE PROTECTION interlock on the TERMINAL POTENTIAL STABILIZER panel will be tripped and the red LED should be lit. The CHARGE CONTROLLER POWER SUPPLIES are disabled by this protection circuit, however, the chains will continue to run.

  1. Turn the CONTROL GAIN and CPO GAIN to zero
  2. On the LOW ENERGY CHARGING CONTROLLER turn the TERMINAL CHARGE POTENTIOMETER back to zero.
  3. Reset the TPS OVER/UNDER VOLTAGE PROTECTION interlock by depressing the momentary switch.
  4. Turn up the TERMINAL CHARGE POTENTIOMETER until the previous terminal potential is reached; watch for signs of conditioning.
  5. Check that the GRID voltage reads approximately -9V .
  6. Set the CONTROL GAIN and CPO GAIN to 5.
  7. Put beam through the Tandem Pelletron and adjust the TERMINAL CHARGE POTENTIOMETER to center the Slit Balance meter.
  8. Enable the OVER/UNDER VOLTAGE PROTECTION circuit.

To TURN OFF the Tandem Pelletron

  1. Disable the “TPS OVER/UNDER VOLTAGE PROTECTION” and turn the “CONTROL GAIN” and “CPO GAIN” to zero.
  2. On the Charging Controller Turn the “TERMINAL CHARGE POTENTIOMETER” back to zero and turn off the CHARGING POWER SUPPLIES at the toggle switch. Depress the momentary switch for the chains to turn them off and then
  3. turn the panel key switch to off.

To Change the Terminal Potential

  1. Disable the TPS OVER/UNDER VOLTAGE PROTECTION and turn the CONTROL GAIN and CPO GAIN to zero.
  2. If the terminal potential is to be decreased, reduce the CHARGE CONTROLLER TERMINAL CHARGE POTENTIOMETER until the TPS displays the TERMINAL VOLTAGE desired. Typically the terminal voltage will drop as the corona probe is run in requiring increases of chain current. Adjust the corona probe position and the TERMINAL CHARGE POTENTIOMETER to get approximately -9V on the GRID.
  3. If the terminal potential is to be increased, run the corona probe out to the approximate position indicated in the table for the desired terminal potential.
  4. While watching the corona probe GRID voltage, carefully increase the CHARGE CONTROLLER TERMINAL CHARGE POTENTIOMETER. Increase the TERMINAL CHARGE POTENTIOMETER until the TPS displays the TERMINAL VOLTAGE desired or the GRID voltage has reached -9V. If at any time the GRID voltage reading goes beyond -1OV, run the corona probe out to reduce it. Readjust the corona probe position and the TERMINAL CHARGE POTENTIOMETER so that the GRID voltage reads approximately -9V at the desired terminal voltage.
  5. Adjust the GVM REFERENCE VOLTAGE knob so that the GVM/TV Balance Meter indicates zero.
  6. Enable the TPS OVER/UNDER VOLTAGE PROTECTION and turn the CONTROL GAIN and CPO GAIN to five.
  7. Readjust the corona probe if necessary to attain approximately -9V on the GRID; do not to exceed this voltage.
  8. Set the 90◦ magnet to the appropriate field and put beam through the Tandem Pelletron.
  9. If the TPS does not switch to the SLIT CONTROL MODE and lockup on a beam the GVM REFERENCE VOLTAGE knob can be adjusted slightly to scan for the beam; only a slight adjustment should be necessary.
  10. With beam on the slits, and the TPS in SLIT CONTROL MODE adjust the TERMINAL CHARGE POTENTIOMETER so that the Slit Balance meter reads zero; retrim the corona probe position, if necessary, to get approximately -9V on
  11. the GRID.
  12. Adjust the GVM REFERENCE VOLTAGE knob so that the GVM/TV Balance meter indicates approximately zero while the TPS is in SLIT CONTROL MODE.

Extended Tandem Pelletron Shutdown

If the current experiment has concluded and machine downtime is expected, the following list should be followed. Ensure a log sheet has been filled out that includes all the beamline devices (figure 4) settings, Faraday cup currents, Tandem parameters,...etc.

  1. Return beam to the L.E. Faraday cup.
  2. Turn the Terminal Potential Stabilizer Control and CPO Gain to zero, Disable the over/under voltage protection.
  3. Turn the Terminal Charge Potentiometer on the Low Energy Charging Controller down to zero, CCW.
  4. Turn both power supply switches to off on the Charging controllers.
  5. Stop the charging chains by depressing the momentary chain switch to the off position for each Controller.
  6. Turn off the Low Energy Charging Controller and the High Energy Charging Controllers by turning their key switches CW to the off position.
    1. Turn down the supplies for these devices:
    2. switching magnet,
    3. 90◦ magnet
    4. Target room quadrupoles and deflectors
    5. accelerator quadrupoles and deflectors
    6. L.E. beamline lens 2.
  7. Close the hand valve before the Tandem 90◦ magnet.
  8. Follow the source turn down instructions (Sec. ??).
  9. Remove control power key if a tank opening is required.

Terminal Stripper Foil Changer

A change in terminal stripper foil is indicated by a decrease in beam transmission through the Tandem. To change the foil one of the two rocker switches on the foil changer panel in the Control Room should be depressed. The direction of movement, either forward (increment) or reverse (decrement) should be that which would bring a fresh foil into the beam path, consult the logbook for used foil information. Motion of the foil band will continue while a switch is pressed. When the direction of the foil change is reversed a considerable amount of backlash, inherent with magnetic couplings, will result in a longer period than for the initial foil change. Use the rocker switches to maximize beam transmission through the Tandem.

The foil changer can be placed in an automatic change mode that allows the foil to be incremented or decremented to a preset number. On the foil changer panel is a thumb wheel numeric display that should be adjusted to indicate the desired foil number. A small toggle switch is located at the left corner of the display should be switched to the “ena.” (enable) mode. The preset push button should be depressed, the labeled LED below the display should light. Now depress the rocker switch that will allow the foil changer to either increment or decrement to the chosen value. Fine tuning of the foil will be required after the preset is reached. When finished with the preset function put the toggle switch back to the “dis.” (disable) position.

The equipment employed in the changing of the foils consist of the control panel in the Control Room, a slosyn motor, a shaft encoder, control rod with coupling hardware to span the L.E. column and a magnetic coupling to transfer motion to the stripper foil band which resides in vacuum. Within the stripper foil housing are two bands of foils, 300 foils per band for a total complement of 600 foils. A description of the band changer can be found in Sec. 25. The foils are rotated into the beam path by the slosyn motor via a control rod spanning the L.E. column and terminating in the terminal. The 110 Vac slosyn motor is bidirectional and receives power from the Control Room via the L.E. bulkhead feedthrough BU 9, pins No. 5,6 and 7 (Increment, decrement and AC neutral respectively). The 3000 count shaft encoder is used for record keeping of the foil inventory. The encoder is coupled to the motor by a timing chain and a 10 to 1 reducing gear. The encoder signal is transmitted to a four digit counter LED display on the foil control panel via the L.E. bulkhead feedthrough BU 7, pins No. 4,5,6,7,8 and 9.

Terminal Foil Band Changer

The terminal band changer is actuated in the Control Room at the foil changer panel. A momentary three position switch is used along with a keyed enable switch in hopes of removing the uncertainty of knowing which band is in use and which band, if any, is exhausted. To adjust or change bands insert the key into the “Band Change Enable” slot and turn the key clockwise. The key is kept in the red box below the console.

Ensure you are aware of what band is being used before making a change. Press and hold the appropriate toggle switch until beam on the HE cup drops off and then returns. Continue holding the switch and note the maximum beam current observed before the beam begins to fade again. Release the switch when the beam current begins to drop. Now fine tune the stripper foil by pressing the switch to move back towards the previous band position; release the switch when the previously observed maximum beam current returns to the HE cup. When satisfied with the band position, ensure the correct LED band indication is lit for the correct band by a short flip of the band changer switch. Turn the key counter clockwise to disable the band changer mechanism.

This paragraph describes the details of the band changer mechanism. The Foil/Band changer electrical leads run out to the L.E. Tandem base plate and then internally via the bulk head feedthrough BU 9 on pins #2, 3 and 4. Inside the machine, behind the L.E. base plate aluminum cover, is the slosyn motor that drives the control rod whenever the enable key and momentary switch in the Control Room is closed. The control rod spans the length of the L.E. column and terminates in the terminal; the control rod is on the West side of the column and is above the accelerator tube. The control rod rotates a cam/microswitch assembly in the terminal that is capable of sensing the direction of rotation of the control rod. The microswitches energize a timer/relay once per revolution of the control rod (∼ 3 seconds) which in turn powers the motor that either lifts or lowers the end to the stripper foil housing for a preset interval. Movement of the stripper foils is achieved by a gimbaled pivot point on the housing allowing positioning of the foils at the beam end of the housing. The travel or pivot arc of the housing is limited by another set of microswitches that interrupts the band changer motor power when at the end of the adjustment range.


Letting the Accelerator Tubes Up to Dry Nitrogen

These instructions are specifically written for letting up the minimum beamline volume to gain access to the accelerator tubes and anything between the HE fast valve and the LE Gas Security Ball Valve – typically for a foil change. Puffs of gas or excessive gas flows must be avoided to protect the fragile stripper foils in the terminal of the Tandem. If access to additional beamline length is necessary, more pumps may need to be isolated and different valves closed.

  1. In the Control Room turn down the lens and deflectors on the L.E. beam line.
  2. Turn off the L.E. Buncher amplifier in the vault.
  3. Turn power off to the L.E. and Cup 1 controllers before letting up the beam line to dry Nitrogen. Remove the lead from the L.E. cup that connects to the 300 V dc suppression battery.
  4. Turn off the Low Energy and High Energy Penning gauge controllers.
  5. Pump the beamline through the sputter source. Put the SNICS source in the Idle mode (consult Sec. 9 and place the exit, baffle and backing valves in the unprotected mode. Ensure the source exit valve and the others are open.
  6. Close the Low Energy cryopump gate valve.
  7. Close the Low Energy Gas Security Ball Valve. With the key kept under the console in the Control Room, switch the valves at the vault control panel to the unprotected position to prevent the High Energy Ball Valve from automatically closing when letting up the tubes. Additional information on the Ball Valves can be located in Sec. ??.
  8. Close the H.E. fast valve to maintain a vacuum in the down stream beamline. Ensure the hand valve before the 90◦ magnet is open.
  9. Close the High Energy cryopump valve.
  10. Close the LINAC entrance valve to the North of the Tandem 90◦ magnet.
  11. The valves and gauges necessary for letting up the tubes are at the H.E. end where the beamline exits the Tandem. A manifold with copper tubing is on the East side of the beam line. Slowly open the 1/4” ball valve exiting the manifold and in series with the 0-50 L/min Air flow gauge so that the tubes can be let up to Nitrogen. Ensure the needle valve at the Nitrogen flow gauge is closed. The compound gauge should indicate vacuum when the valve is opened.
  12. Adjust the flow gauge needle valve so that the gauge at the inlet is at 29 in. of Hg and note the flow rate; it should be about 25 L/min. Periodically adjust the flow rate to maintain that which was previously noted. The gas will need to flow for ∼ 1 hour to fill the void.
  13. When the tubes are at atmosphere or slightly above, close the flow gauge valve and, if necessary, slowly bleed off the excess pressure by slowly opening the pump out port 1 1/4” ball valve that is attached to the copper manifold.
  14. Depending on the circumstances it may be preferable to allow a small flow of Nitrogen to reduce the quantity of air that will migrate into the tubes while they are open. Open the flow gauge slightly after the beamline vacuum seal is broken.

Pumping Down the Accelerator Tubes

After all the broken seals are remade the tubes are ready to be pumped out. An oil free vacuum pumping system must be employed when pumping down the tubes. Do not use an oil sealed mechanical pump as they can allow oil mist to back flow into the accelerator tubes. A Carbon Vane pump followed by a Sorption pump is sufficient. To protect the fragile foils, puffs of gas or excessive gas flows must be avoided so use care. Open valves slowly to begin the pumping.

  1. The port for pumping down the tubes is in series with the 1 1/4” ball valve attached to the manifold at the HE beamline. Attach the pumping station to the end of the 1” copper pipe port and evacuate the pipe up to the closed 1 1/4” ball valve. Slowly begin opening this ball valve and stop as soon as either the pumping station vacuum gauge indicates a pressure rise or the vacuum pump audibly responds to a gas load. Slowly open the ball valve more as the vacuum continues to improve in the tubes. The valve should be completely open at ∼ 25 in. of Hg.
  2. Experience has shown that the vacuum in the trapped volume of the closed L.E. ball valve will degrade during the period the valve is closed. When the tube/beamline vacuum is at 100 millitorr open the ball valve and allow the gas load to be pumped by the Sputter Source diffusion pump.
  3. Switch the gas security ball valves at the vault control panel to the protected position with the key switch. Put the key back in the red box under the console in the Control Room.
  4. After the tube vacuum has dropped to 50 millitorr close the 1 1/4” manifold ball valve and open the both cryopump gate valves. Remove the pumping station and return it to where it was found.
  5. Turn on the Low Energy and High Energy Penning gauge controllers.
  6. Turn the L.E. and Cup 1 controllers back on and reconnect the 300 V dc suppression battery coax cable to the L.E. cup.
  7. Open the HE fast valve and close the hand valve before the 90◦ magnet.
  8. Place the SNICS source exit, baffle and backing valves back in the protected mode and close the exit valve.

Venting the Tandem to Air

See Venting the Tandem to Air


Tandem Pressure Vessel Sulfur Hex Alarm Circuit Description

The alarm has been set to trip at 75 PSIG and is intended to safeguard the gas inventory while in the Tandem. Should a substantial (not just cold gas) change in the pressure be noted or if the panel alarm begins to annunciate immediately call a member of the staff. Not only is the SF6 inventory expensive ($70,000) but also is very heavy and will displace air which could possibly lead to asphyxiation. Do not linger in enclosed areas common to the gas leak. Sulfur Hexaflouride, SF6, is, however, nontoxic.

The alarm can be muted at the front panel, but the visual LED will continue to flash as long as the pressure is below the set point value. For extended tank openings the power switch can be turned off.

The pressure and temperature transducers are mounted to the flange on the southernmost top tank port. The circuit box adjacent to the flange should be turned on and plugged into a source of 110Vac. The transducers’ output is directed to the Control Room panel for display. Pressures below atmospheric are indicated by a negative sign with -14.8 psi being the lower limit of the transducers’ operating range.

The tank pressure is also indicated by two Bourdon tube, mechanical pressure gauges and a Varian 501 Thermocouple gauge located at the H.E., West side of the pressure vessel. These gauges are independent of the alarm circuit. The error between the pressure indications in the Control Room with some of the others can be as high as 11%.

To reset the trip point to some other value, follow this procedure:

  1. At the back of the Pressure Vessel panel is a circuit board with a toggle switch position labeled calset; move the switch to that position. While observing the front panel pressure indication, adjust the cal potentiometer until the desired pressure at which to trip is displayed.
  2. Also on the circuit board is a potentiometer labeled trip; adjust this potentiometer until alarm sounds and the front panel LED is flashing.
  3. Place the toggle switch in “1.” back to the normal position. The panel pressure display should again indicate the current tank pressure.

Voltage-Conditioning the Tandem

After the machine has been opened for an extended period it is often noted that the machine will not return to the previously reached voltages and run stability. Letting up the accelerator tubes to atmospheric pressure for installation of new equipment, replacement of the stripper foil supply or other work also contributes to the degradation of the voltage holding capability of the machine. Preferably, the required terminal voltage for the experiment subsequent to a tank opening will be somewhat less than those reached previous to the opening. Regardless of the voltage requested, it should be assumed that some conditioning will take place and some care should be taken when first bringing up the terminal voltage. Also, the conditioning should be carried out until a voltage of at least 250KV over the experimenter’s requested voltage is attained. As the machine conditions, the required charging current to attain a specific voltage will typically decrease as corona sites on the column and other high voltage surfaces begin to dissipate allowing more current to flow through the resistance grading.

The machine should be set up to condition by first withdrawing the corona probe to the tank wall. The H.E. faraday cup should be inserted and the beam current integrator set to measure picoamperes of cup current. The integrator upper limit alarm should be adjusted to mid scale so that meter excursions will sound the alarm. The Capacitiv Pick Off (CPO) trace should be displayed on an oscilloscope as well as recorded by the chart recorder. Since the machine is being conditioned the Terminal Potential Stabilizer (TPS) feedback is not necessary when charging the terminal so the Control and CPO Gains should be kept at zero. The TPS is, however, used for monitoring the Terminal Voltage and the chain charging trip feature it allows should be employed. By zeroing out the GVM error with the Terminal Reference Potentiometer, the increase in terminal potential for a fixed upcharge setting can be displayed as the “error” grows, but keep in mind the under/over protection circuit set point may be reached.

Evidence of conditioning below 6MV is not common and voltage stability problems at this potential (sparking) are indicative of greater problems and a tank opening may be necessary. In general, any voltage instabilities associated with the terminal can be considered signs of conditioning; whether or not the voltage stability improves or persist, short of a mechanical/electrical failure, determines the length of the conditioning period and its success. At higher voltages the instabilities associated with conditioning become more evident. Since the capacitive energy residing in the charged terminal and column is strongly dependent on the voltage (CV2/2), more caution should be used when conditioning at higher voltages as the discharges become more destructive. When heavy conditioning begins to occur, maintain the current voltage until the conditioning events begin to subside. Voltage increases in steps of 200KV is suggested. If concern that damage is being done to the accelerator while attempting to condition (excessive sparking for example), please err on the side of caution and consult the staff.

INDICATIONS OF CONDITIONING

  1. Voltage instabilities indicated by the CPO either at the oscilloscope or the chart recorder.
  2. Current indications by the beam current integrator.
  3. Fluctuations of the terminal voltage indication by the TPS that are fast and greater than 20KV.
  4. Sparking of the machine, may or may not be audible, but will usually trip the under/over protection circuit. There are in general three types of sparks. Tank and Column sparks both occur in the SF6 gas. The first is a discharge between the column and the tank wall and the other is along the column structure. Tube sparks occur within the evacuated space of the accelerator tubes and across tube electrode/insulators gaps.
  5. Vacuum fluctuations at either the H.E. or L.E. end of the machine as indicated by the control room penning gauge meters.
  6. Radiation emissions from the machine as indicated by the control room gamma/xray meter. Should these be in short bursts separated by even longer intervals and appear to be subsiding, then maintaining that voltage and waiting for improved stability is the suggested course of action. However, should the radiation persist at levels greater than 10 mREM/h and be steady, the machine should be turned off and the staff notified.

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.

Contact

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