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TANDEM ACCELERATOR OPERATION I. ACCELERATOR OPERATING POTENTIAL The accelerator operating voltage is determined by the amount of charge delivered to the terminal by the two charging chains, the amount of corona current being drawn from the dome by the corona needle assembly, the column resistor string currents, and the amount of charge stripped from the accelerated beam. The column current is proportional to the terminal voltage (~5 A/MV) and is not independently adjustable. The chains run from each end of the tandem to the terminal and back. To adjust the charging current, two chain controllers are located at the tandem console. Indication of the exact amount of the charge being applied to each chain is provided by a meter located on each chain controller labeled "CHARGING CURRENT". Both chains must ALWAYS be running whenever voltage is being developed on the terminal. As a rule of thumb, the chains should deliver about 3 A of charging current per kilovolt of inductor voltage. Each chain is rated to deliver 100 A of charge to the terminal - DO NOT exceed this value as damage to the chain could result! Both a COARSE and a FINE inductor voltage control are present on each chain controller to permit adjustment of the charging currents. The amount of corona current drawn from the dome is controlled by several factors including: the position of the needle assembly with respect to the tank wall (as indicated on the needle position meter at the console), the insulating gas pressure, the terminal voltage, and the bias voltage on the corona control tube. These are all externally adjustable factors, the needle assembly itself is adjustable - but only when the accelerator is opened. New needles will 'age' with use in that they gradually become dull and do not corona as easily. As a result, the needles will need to be moved closer to the terminal as they become older in order to obtain the desired corona current (other factors remaining the same). The amount of charge stripped from the accelerated beam is controllable by adjusting the amount of negative beam entering the accelerator and the thickness of the stripping foil. Usually the stripper current is limited to about 20 A maximum. To permit the terminal potential to be regulated, a high voltage vacuum tube is connected in series with the corona needle assembly. By modulating the voltage on the control grid of this tube, the amount of corona current (and hence the terminal potential) may be controlled over a fairly wide range (approximately 100 KV/A). The control signal is derived from three sources: the generating voltmeter, the capacitive pickup assembly, and 6/25/17 Tandem Accelerator Operation 6.1 the control slits. These error signals are fed to the corona control tube by the Terminal Voltage Stabilizer circuit. The corona feedback loop bandwidth is limited to dc to ~10 Hz due to the long transit time for ions traveling between the corona needles and the terminal. II. DESCRIPTION OF THE VOLTAGE STABILIZER CONTROLS The terminal voltage stabilizer control is located in a chassis located in the right hand rack at the tandem console. III. TURN ON PROCEDURE (COLD START) Before attempting to start the accelerator, the operator should first determine that the vacuum in the ion source areas, the accelerator tubes, and the high energy beam lines is normal. Attempting to put voltage on the accelerator tubes with a poor vacuum in them can seriously damage the tubes. The following sequence illustrates a normal startup procedure. A klaxon will sound when the charging supplies come on to warn personnel in the accelerator bay that radiation may be produced in the accelerator area. 1) a) Turn on the control power - key switch on the tandem console. Also turn on the TPS and verify that all preamps are active (LED's all lit for GVM, CPO & Slit). b) Open the high energy and low energy tube gate valves. Also open any others in your beam path - after determining that the vacuum is good in each section. c) Put beam onto the low energy Faraday cup. d) Check to make sure that the slit current preamps are cabled to the correct control slits. Use the Test buttons to determine that the preamp is OK. e) Turn the control slit selector switch to the desired control slit position. f) Turn the coarse charging controls on both chain controllers down (full CCW). 2) Terminal Potential Stabilizer Settings: a) Set the control mode switch to Auto. b) Set the control gain to Zero.(full CCW). c) Set the CPO gain to Zero (full CCW). 6/25/17 Tandem Accelerator Operation 6.2 d) Set the Terminal Voltage potentiometer to the desired terminal voltage (010MV). e) Set the Bias Current pot to Zero. f) Set the Corona Probe position to 0% - or 0". g) Set the Terminal amplifier gain setting to 0% - or full CCW. 3) Turn on the charging system. Push the "START" pushbutton switch on each of the chain control panels. If the "ON" does not come on, it may be due to an open interlock. Contact Richard O'Quinn or John Dunham for assistance before proceeding. The interlocks are: (a) Microswitches on the accelerator tank doors. The switch plungers should be depressed by the closed door flanges. (b) Accelerator tube vacuum is poor (>10-4 Torr). (c) The tank pressure switch at the high-energy end. This prevents the chain motors from starting if the tank is under vacuum. (d) The motor phase monitors in the motor control center. 5) Set the corona needles to the position indicated by the chart or from previous log settings for the desired terminal voltage. The needles are adjusted via NEEDLES IN/NEEDLES switch on the terminal potential stabilizer (TPS). The needle position is read on the dual function Corona Current/Points Position meter on the TPS. Adjust the Bias Current Control to 20 - 30%. This is not an absolute setting, the needle position will vary with bias current, tank gas pressure, moisture content, and wear on the needle tips. 6) Adjust the Voltage Set potentiometer to the desired terminal potential . Increase the charging currents slowly until the desired terminal voltage is reached. DO NOT EXCEED 100A on one chain. It is best to divide the required charge between the two chains. 7) Increase the Control Gain and CPO Gain to ~50%, the stabilizer should hold the terminal voltage. 8) Adjust the "FINE" charge knobs and corona points position to obtain ~ 10 A 20A of corona current with the stabilizer balanced. The bias current pot should be set for approximately 20-40% in order to maintain the grid bias at -6 to -12V. The Pelletron system requires very little stabilization compared to a belt charged machine and so 6/25/17 Tandem Accelerator Operation 6.3 substantially less corona current is required than with a belt charged machine. The corona current should generally NOT exceed the column current as this may overtax the charging system and may lead to overdriving of the terminal and sparking. Remember: a change of only 10 A in corona current will change the terminal voltage by 1 MV!!! Corona Points Position With 10% SF 100% 50" 10% 5" 1% 0" 0 2 4 6 8 Inches Corona Points Position (%) 6 10 Terminal Voltage (MV) Figure 5.1 Corona Points Position versus Terminal Potential 9) Adjust the GVM gain and CPO gain to minimize the terminal ripple pattern (trace 'A' on the overhead oscilloscope). The scope calibration is 1V = 100 volts of terminal ripple (200 eV fluctuations for a single-charged ion beam on target). The terminal ripple should normally be <100 Vpp when in control. 10) Prior to putting beam through the machine (or into any target area) an announcement should be made over the lab public address system using the local microphone. If it is expected that the beam will produce radiation in excess of 2.5 mR/Hr it should be announced that "dangerous beam" is about to be put through. Give personnel sufficient time to clear the area before allowing the beam to enter. (Although the beams are seldom 6/25/17 Tandem Accelerator Operation 6.4 dangerous, the radiation safety regulations prohibit personnel from working in an area that has a radiation level above 2.5 mR/Hr.) Put beam through the machine. This will require adjustments to the low energy magnetic steerers, quadrupole and Einzel lenses to maximize beam on the high-energy Faraday cup. The upcharge may also need to be increased to compensate for the stripper current. Balance the stabilizer as in 8. 11) Put the beam through the analyzing magnet and onto the control slits. Adjust the coarse or fine voltage set pots to center the beam on the control slits. It may be necessary to use the magnet trim supply or the magnet supply fine control pot to center the beam. Use the analyzing magnet's field to determine the analyzed beam energy precisely as the generating voltmeter calibration may have an offset. (If the observed offset is large bring it to the attention of the technical staff so that it may be corrected.) 12) With beam on the slits, and the mode selector in “AUTO”, if the beam is above 10 nA the stabilizer will switch from GVM feedback to SLIT feedback. (Switching to “SLIT” forces only the slit signal to be used - thus providing no dc control of the terminal voltage when there is no slit signal.) Minimize the slit difference signal on the overhead scope with the Control gain control, plus tuning the beam to achieve a stable focus just beyond the control slits (if a waist is formed at the slit position, the machine will not control, the waist should be formed just beyond - at the beam stop or slightly beyond). 13) To further regulate the beam energy (and momentum analyzed spatial stability) one may also employ feedback from the control slits to the terminal stripper amplifier. This is done at the terminal amplifier control panel - below the tandem TPS oscilloscope. The gain is adjusted by a ten-turn potentiometer to minimize the ripple on the slit error signal which is displayed on the lower trace of the oscilloscope (when the selector switch is set to SLIT Error). IV. ROUTINE OPERATION IV. 1 Voltage Limits The FN tandem was originally designed to operate at a maximum of 7.5 MV on the terminal. The design aim of the upgrade was 10 MV. The accelerator tubes are rated to operate at 9 MV, while the charging system is designed to deliver sufficient charge to 6/25/17 Tandem Accelerator Operation 6.5 permit at least 10 MV operation. When operating above 9 MV, the machine must first be conditioned up by one of the technical staff - Richard O’Quinn or someone else designated by the Director. After conditioning, it should be possible to operate the accelerator at 8-9 MV routinely, and at higher voltages with conditioning. When operating above 8 MV, ALWAYS watch the high-energy vacuum gauge for activity. If excessive vacuum activity in the high energy vacuum meter is noted, or other indication of tube or column sparking is detected, the voltage must be immediately reduced to a point where this activity ceases. Any excessive sparking or unusual high energy end vacuum activity should be brought to the attention of Richard O’Quinn or Chris Westerfeldt or another senior staff member. Excessive sparking is defined as: a) Sparking each time an attempt is made to raise the accelerator voltage to the desired value. Note: It is not very unusual to have one or two sparks while initially setting the accelerator operating conditions, especially at high terminal potentials. These should not be considered excessive unless they occur each time the voltage is raised. b) Sparking at a rate in excess of one per hour. If either of these conditions exist, the accelerator voltage must be lowered to a point where the "excessive" sparking ceases. Unusual variation in the column currents is usually a sign of column, control rod, resistor, or charging system damage. Low insulating gas pressure, an incorrect gas mixture or unusually wet gas can also lead to excessive sparking. When these conditions occur, Richard O’Quinn or Chris Westerfeldt should be contacted before proceeding. Normal high energy column currents are indicated in figure 5.2. 6/25/17 Tandem Accelerator Operation 6.6 High Energy Column Current High Energy Column Current vs Terminal Voltage 60 y = -1.1599 + 5.7251x R= 0.99951 50 40 30 20 10 0 0 2 4 6 8 Terminal Voltage (MV) 10 Figure 5.2 Column Current versus Terminal Voltage IV. 2 The Terminal Potential Stabilizer The stabilizer system uses feedback from the capacitive pickoff plates (ac) and from the control slits (dc and ac) or the generating voltmeter (GVM) to drive the corona control tube. When no slit signal is present, the stabilizer will use the generating voltmeter (GVM) signal to provide dc stability while still using the CPO signal as a "derippler". Refer to figure 1 for a block diagram of the system. II. 3 The Terminal Stripper There are normally two beam (electron) strippers located in the tandem terminal. The most commonly used stripper is the carbon foil stripper. This device contains up to 80 carbon foils in the thickness range of from 0.5 to 10.0 g/cm2 for stripping electrons from beams where the energy spread of the beam is not very important. For minimum energy 6/25/17 Tandem Accelerator Operation 6.7 spread in the stripped beam, the gas stripper is normally used. The stripper is presently not installed in the tandem however. Polarized beams are an exception, they must be stripped in a thin foil to avoid depolarization. A log of the foil strippers is kept on the tandem console and should be consulted when changing foils to determine where "good" foils are. Any foils which break during a run (as evidenced by loss of beam stripping efficiency ≥ 50%) should be marked in the log as "blown" or "poor". It is not recommended that the foil changer be operated with beam being put through the machine unless the beam intensity is low (<< 1 A). Also it is important to remember that the carbon foils are supported by a thin plastic film (Collodian) which must be "burned off" before the foil can be used. The Collodian must first be burned off by injecting beam and watching the transmission through the machine (as read on the high energy Faraday cup). The beam current should increase as the Collodian burns away, and stabilize at a higher value. The second stripper is a gas stripper (not currently installed). This device operates from a rocker switch on the top panel of the tandem console. Caution: there is a time delay of 10 - 15 seconds between changing the setting and a getting a response on the high energy vacuum gauge. This stripper introduces Nitrogen gas into a long tube which the beam passes through. The stripping efficiency and energy straggling are less than for foils but the reduced beam energy spread is important for high resolution experiments. The gas stripper is normally operated without a foil in position. See the foil log for the nearest blank foil position. The high energy vacuum is normally ≤ 5 x 10-7 Torr on the control room high-energy vacuum gauge when this stripper is in use. Always turn the gas stripping off (decrease the gas flow until the high energy vacuum returns to its base reading) before shutting down the tandem. IV. CHANGING ENERGY The beam energy should normally be changed while under GVM control. While in GVM control, readjust the Terminal Voltage Knob toward the new setting while monitoring the corona current. Do not decrease the corona current below ~5 A, nor increase it above ~50 A without readjusting the charging currents and the corona points position. These changes should be done slowly as the accelerator has a long time constant and any attempts to change the voltage more quickly than several. seconds may induce a tank spark. The machine energy may also be changed in steps that are greater than the 6/25/17 Tandem Accelerator Operation 6.8 range of the corona control circuit however you must realize that there is no control of the terminal voltage fluctuations under these conditions and it is therefore more risky. V. TERMINAL STEERER There is a set of vertical electrostatic steerers located in the tandem terminal to aid in transporting the beam through the accelerator. Indication of the amount of steering being applied is provided on the console. The steering is controlled by a rocker switch on an upper panel at the tandem console to the right of the foil stripper controls. There are stops at both ends of the range (UP/DOWN), but do not attempt to steer beyond ±100% (±5KV) or the meter reading may become uncalibrated. VI. TURN OFF PROCEDURE 1) 2) 3) Put the low energy and high energy Faraday cups in. Decrease both charging chain current controls to zero. Turn the chain motors off. 4) If you have been using gas stripping, turn it off by observing the high energy vacuum reading while decreasing the gas flow. The high energy tube vacuum should return to about 5 x 10-7 Torr on the control room high-energy vacuum gauge. If someone else is ready to use the accelerator, stop here. Shut down your ion source, magnet supplies etc., and close appropriate gate valves. Turn the chain charging controllers off, and the tandem control power off. Remove control power, ion source, and tandem control keys and lock them in the key safe. 5) 6) 7). VII. LABORATORY SHUTDOWN PROCEDURE If the accelerator is to be shut down and left unattended, overnight, several steps must be taken prior to leaving. 1) 2) The accelerator(s) must be shut down as detailed previously. The keys to the charging chain controllers and the control power switches must be removed and locked up in the key safe. 6/25/17 Tandem Accelerator Operation 6.9 3) 4) 5) 6) 7) 8) 9) The ion source must be shut down as detailed in the appropriate operating instructions for the source being used. The analyzing magnet current and associated lens currents should be run down to zero either individually or by using the master reference control - if it is being used. Turn off the analyzing magnet controls in the control room (DC OFF, POWER OFF), and the various other magnet power supplies in the power supply racks out in the bay. Close the low energy, high energy, input 90˚-90˚, and input 70˚-70˚ beam line gate valves. The first two must be closed from the low energy and high energy pumping stations respectively. The latter two are closed from the control room Giesler panel below the main vacuum metering station in the extreme right hand console cabinet. This isolates the accelerator and other vacuum sections - reducing the potential for damage should there be a vacuum system failure during the night. Turn off most of the accelerator bay lighting but leave enough on for people to move about safely - i.e. leave every fourth light on. This is accomplished by flipping the circuit breakers in the panel just outside the control room - accelerator bay door. Make sure that the back door (in mechanical room) is closed and locked. Also check the door in the hallway which exits onto the back parking lot. Shut off the control room lights - except for the row nearest the stairway. The circuit breakers are located in the service closet near the stairway. Make sure that the front door is latched and locked on your way out. Description of NEC Terminal Potential Stabilizer Circuits 1. General The voltage stabilizer system consists of several precise energy control networks, combinations of which may be used to regulate the terminal potential. Variations in terminal voltage are electronically sensed and used through feedback loops to correct these voltage fluctuations by regulating corona current from the terminal to ground. Corona stabilization always uses a fast feedback signal from a capacitive pickup to reduce short term ripple in combination with a slow feedback signal from either the generating voltmeter or the image slit to reduce long term drift. The automatic switching system will switch to generating voltmeter control in the event of loss of slit signals or large terminal fluctuations, such as terminal discharges. Lockout circuitry prevents return to slit control until the terminal has returned close to the preset potential. A block diagram of the Voltage Stabilizer System is shown below. 6/25/17 Tandem Accelerator Operation 6.10 6/25/17 Tandem Accelerator Operation 6.11 A. FRONT PANEL CONTROLS AND INDICATORS The front panel of the stabilizer control assembly is shown below and contains the following: 1. Power Section a. POWER SWITCH The switch controls AC power to the TPS Controller and also controls status (ON/OFF) control power to the remotely controlled auxiliary units (GVM Amplfier, CPO Amplifier, Slit Log Amplifier and Corona Probe Controller). b. Power ON LED. The indicator is "On" when the TPS unit is energized. c. Remote POWER LED’s. SLITThe indicator is "On" when the Slit Log Amplifier is energized. GVMThe indicator is "On" when the GVM Amplifier is energized. CPOThe indicator is "On" when the CPO Amplifier is energized. PROBE- The indicator is "On" when the Corona Probe Controller is energized. 2. Control Mode Section. The TPS Controller has two basic control modes, GVM and Slit. In both cases, the controller compares two signals and generates an error signal. This forms the low6/25/17 Tandem Accelerator Operation 6.12 frequency error correction signal which is combined with the high-frequency CPO error correction signal. The composite signal is then summed with the Corona Probe "DC" bias current and the resulting error correction signal controls the Corona Probe. a. CONTROL MODE switch. GVMIn GVM control mode (switch up), the TPS compares the GVM voltage to the Terminal Voltage knob setting and regulates terminal voltage by keeping the two voltages equal. SLITIn slit control mode (switch down), the TPS compares the high energy slit current to the low energy slit current and regulates terminal voltage by AUTO- b. c. keeping the two currents equal. In Auto control mode (switch in center), the TPS internally chooses between Slit control or GVM control. If there is at least +10 nADC of current on both slits, and the GVM voltage is within +/- 10 kV of the Terminal Voltage knob setting, then the TPS switches to Slit control. If these conditions are not met, then the TPS reverts to GVM mode. GVM mode LED The indicator is "On" when the TPS Controller is in GVM control mode. SLIT mode LED. The indicator is "On" when the TPS Controller is in Slit control mode. 3. a. Slit and GVM Control Section. Control GAIN knob. The knob sets the TPS closed loop control system gain in both the GVM and Slit control modes. Specifically, it control the amplitude of the error correction signal that is combined with the corona probe "DC" bias level to form the Corona Probe current control signal. This is an overall system gain knob that adjusts the gain of both low frequency signals (GVM - TV) and H.E. - L.E. Slit) as well as the gain of the high frequency CPO signal. When the knob is set to zero, the TPS does not regulate terminal voltage. If the knob is set too high, the control system may oscillate (under certain conditions). b. CPO GAIN knob. The knob sets the CPO signal gain in both GVM and Slit control modes. It controls the amplitude of the high frequency CPO error correction signal that is 6/25/17 Tandem Accelerator Operation 6.13 combined with the low frequency GVM or Slit error correction signals. When the knob is set to zero, the CPO signal is not used to help regulate terminal voltage. However, this does not prevent the GVM - TV or the HE - LE Slit signals from regulating terminal voltage. If the knob is set too high, the control system may oscillate (under certain conditions). c. TERMINAL VOLTAGE knob. The knob sets the desired terminal voltage. That is, the control is the reference voltage to which the GVM voltage is compared in GVM control mode. The 10turn counting dial has a scale factor of 1 turn per MV. d. TERMINAL VOLTAGE meter. The digital panel meter displays terminal voltage in DC Megavolts. The 4-1/2 digit display has +/- 1 KV resolution. e. Slit current meters. H.E. SLIT - The meter displays high energy slit current over a five decade range from 0.1 nA to 10 A. The scale is logarithmically compressed with 2 x 10-9 and 5 x 10-4 subdivisions. L.E. SLIT - The meter displays low energy slit current in the same manner as the high energy slit meter. 4. Corona Probe Section. a. BIAS CURRENT knob. The knob sets the "DC" (quiescent) Corona Probe current level. The control range is 0 to +50 ADC. The knob operates independently from the Control Gain knob setting and is active whenever the TPS unit is "ON". b. POSITION IN/UT switch. The switch changes the corona probe position. Normally, it is in the center off position. Momentarily pushing the switch up moves the probe in (towards the terminal) and pushing the switch down moves the probe out. c. GRID VOLTAGE meter. 6/25/17 Tandem Accelerator Operation 6.14 The meter displays the corona triode (tube) grid voltage. The display range is 0 to -25VDC. The meter is used to verify that the triode is operating within its active region. d. CURRENT/POSITION meter and switch. The meter displays both Corona Probe current and Corona Probe position. When the meter select switch is up, the meter displays probe current over a 0 to +100 ADC range. The display indicates actual probe current and will give an upscale reading only when the corona triode is biased in its active region, by properly adjusting the probe position. With the switch down, the meter displays probe position in relative position units. 0 corresponds to maximum "Out" position and 100 corresponds to maximum "In" position. – with respect to the tank wall. The range of the TUNL corona probe is 0 to 44 inches corresponding to 0 to 88% of full scale. A plot of the probe position versus terminal voltage is provided below. As the position is sensitive to insulating gas composition, pressure, and needle conditions, it is best to obtain setting from recent machine logs if available. Corona Points Position With 10% SF 6 50" 10% 5" Inches Corona Points Position (%) 100% 0" 1% 0 2 4 6 8 10 Terminal Voltage (MV) 6/25/17 Tandem Accelerator Operation 6.15