Download New Section 6

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
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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).
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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
100A 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 20A 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
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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
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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
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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.
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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
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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
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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.
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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.
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6.10
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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
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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
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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.
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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)

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6.15