Ion Sources

Sputter Source

Sputter source Operation schematic

Sputter source Volatge Schematic

The Sputter Source used at the FSU accelerator is also called SNICS.

Theory of Operation

The Source of Negative Ions by Cesium Sputtering (SNICS) produces a negative ion beam. A reservoir of cesium metal is heated so that cesium vapor is formed. This cesium vapor comes from the cesium oven into an enclosed area between the cooled cathode and the heated ionizing surface. Some cesium condenses onto the cool surface of the cathode and some of the cesium is ionized by the hot surface. The positively charged ionized cesium accelerates towards the cathode, sputtering material from the cathode at impact. Some materials will preferentially sputter neutral or positive particles which pick up electrons as they pass through the condensed cesium layer on the surface of the cathode, producing the negatively charged beam.

Ion source states

The ion source and pre-accelerator can be in put into different states, depending on the phases of operation. The following procedures will make reference to transitions from on state to another. A) "Secured, Cold" state, B) "Cold Idle" state, C) "Warm Idle" state, D) "Beam" state.

Only qualified personnel Brian Schmidt, Dr. Lagy Baby and Dr. I. Wiedenhoever place the ion source into "Secured, Cold" state or to return it to initiate operation from this state.

Temporary Turn Down: Bringing the source from "Operation" to "Warm Idle" state

Qualified Personnel: All trained accelerator operators

When a break or interruption in the experiment occurs and the negative ion source will not be required by the experimental group for four or more hours, it is desirable to turn down the source to prolong its operational life time.

1. In the control room, turn the preaccelerator high voltage supply to zero Volts and switch the power supply off.
2. In the Tandem Vault, close the source exit valve.
3. Reduce the Cesium Boiler Heater to 35 on the variac scale by turning the control rod located at the high voltage cage.
4. Reduce the ionizer heater current in four ampere steps separated by two minute intervals to fifteen amperes by turning the control rod attached to the ionizer power supply located at the high voltage cage.
5. Note in the Tandem Log Book, located in the Control Room, the time the source was turned down to the "Warm Idle" mode.

Turn Source Down from "Warm Idle" to "Cold Idle" state

Qualified Personnel: All trained accelerator operators

When the use of the negative ion source is no longer required by the experiment, it is desirable to leave the source in a state to prolong its operational life time: idle mode.

1. Apply all steps listed for the Temporary Turn-Down above.
2. Turn down and off the source deflectors in the Control Room.
3. Turn the inflector magnet supply completely down.
4. Turn the Lens, Extractor and Cathode H.V. supplies to zero with the control rod extending from the H.V. cage.
5. Reduce the einzel lens voltage to zero by depressing the labeled rocker switch at the H.V. cage control panel.
6. Note in the Tandem Log Book, located in the Control Room, the time the source was turned down to the "Cold Idle" mode.

Bringing the Preaccelerator and SNICS Source to "Secured, Cold" state.

This procedure will only be performed by Brian Schmidt, Dr. L. Baby and Dr. I. Wiedenhoever

1. This state is required for access to the SNICS Source or the SNICS Power Supply cage, for service or repair.
2. Bring the source to "Cold Idle" Mode, following the steps described above.
3. In the Tandem vault, turn off the preaccelerator power supply, which is located in a rack below the RF Buncher assembly and unplug the power cord. Lock the power plug using a Lock-out device, following procedures available in the Tandem control room.
4. In the SNICS source power cage, verify that the Extraction, Cathode and Lens High-Voltage supplies read "zero" Volts.
5. Turn the key switch at the panel to turn off the isolation transformer providing platform power to the H.V. cage.
6. Remove the key from the control panel and keep it on your person or in the Key Vault below the Tandem console.
7. Open the cage door in front of the SNICS source.
8. A shorting rod is provided attached to the cage door. Hold the shorting rod by its insulated handle, with the tip of the rod touch the source on each side of the endmost ceramic insulator. Repeat touching the parts of the source multiple times, verify that no sparking to the tip occurs.
9. Hang the shorting rod between the air lock VAT valve and the HPS vacuum fitting clamp.
10. The source and preaccelerator are in "Secured Cold" state, Access to the external source body and the power supply enclosure is now possible.

Bringing the source up from "Cold Secured" state to "Beam" state

These instructions assume the correct cathode is installed and the Ion source vacuum is correct and sufficient to connect to the accelerator

This procedure will only be performed by the qualified personnel B. Schmidt, Dr. L. Baby or Dr. I. Wiedenhoever

1. At the Ion source: Remove the grounding rod from the Ion source and hang it from the cage door in front of the SNICS source
2. Close the cage doors in front of the SNICS source
3. Verify that the power supply cage is closed
4. Insert the key into the Power supply front panel and turn it to "on".
5. If no parameters have been recorded or cannot be found set the SNICS Cathode and Extractor supplies to 7.5 KV.
6. Open the source exit valve
7. Remove the lock on the pre-accelerator power supply module power cord and plug it in, noting the "unlock" in the "log-out Tag-out" log located in the control room.
8. Turn the pre-accelerator power supply on and push the green "High Voltage button".
9. In the Control room: Turn on the pre-accelerator panel and raise the pre-accelerator voltage to the desired Voltage, typically 120 kV.
10. Insert the L.E. faraday cup and depress the corresponding push button on the integrator panel. Select the most sensitive scale on the integrator initially and then adjust the scale if and when the beam current pegs the meter.
11. Turn the ion source deflectors to zero.
12. Adjust the inflector magnet supply to indicate the predicted setting for the negative ion that is to be injected into the Tandem. A table is provided on the inflector control panel.
13. If the beam has been run before and the source parameters are known, adjust the SNICS Cathode and Extractor supplies to those values at the High Voltage Cage out in the Tandem Vault. If the Ionizer heater is to be adjusted, do it in 4 ampere steps separated by 4 minutes intervals.
14. Beam should now be present on the L.E. cup; if not, adjust the inflector magnet slightly and attempt to find and maximize beam current. The inflector magnet should not be more than 0.15 units off of the predicted setting.
15. An iterative approach is used to maximize beam on the L.E. cup. After the magnet is fine tuned, adjust the SNICS Cathode and Extractor supplies in the Tandem Vault to maximize beam current. When satisfied, again adjust the inflector magnet in the Control Room and then the source H.V. supplies again. After this second round of tuning use the source deflectors in the Control Room to increase beam current on the L.E. cup. More rounds of adjustments should be attempted; however, maximum beam transmission through the Tandem does not occur at the same settings as those for maximum L.E. faraday cup beam current. Mostly the source deflectors and the inflector magnet require slight readjustment when tuning through the Tandem.
16. Assuming all the focusing and deflector elements are maximized for beam current, one can only adjust the Ionizer heater and the boiler temperature to attain more beam form the source. First, increase the Ionizer one ampere steps until the desired current is reached. The beam is ready for injection.

Should one not have success with tuning the desired beam, reducing the ion source deflectors to 50% of the optimum for a couple of iterations of source voltages and inflector magnet adjustments might be a useful approach. Ultimately, the deflectors should be adjusted to give the maximum beam on target.

Bringing the source up from "Warm Idle" to "Beam" state

Qualified Personnel: All trained accelerator operators

1. Insert the LE Faraday cup and select the LE cup current on the beam integrator.
2. In the Tandem Vault, open the source exit valve after verifying the source vacuum status.
3. Verify that the source cage is closed and that the source is ready to start beam production
4. In the control room, turn on the preaccelerator power supply and increase the Voltage to the value required for the beam, which is typically 120 kV.
5. Increase the Ionizer heater current in four-Ampere steps, separated by two-minute intervals while observing the beam current measured on the LE CUP
6. Note in the Tandem Log Book, located in the Control Room, the time the source was reactivated and make not of the beam currents.

Description of Typical Cathode Behavior

Most often the material packed into the SNICS cathodes is either a solid or powder. In general new, unused cathodes are often found to require more time to reach the output level of cathodes that have seen previous use. The nature of the material and how it was stored, particularly if it is a hygroscopic or deliquescent, can be of paramount importance when investigating a low negative ion output. Most powdered cathodes are pressed here in the lab and are inherently gassy due to voids in the pressed material. The solid material cathodes are the preferred cathode whenever possible. To reduce damage to the cathodes while they are not in use it is imperative that the cathodes be wrapped tightly in aluminum foil and placed in a tightly capped bottle that has been back filled with argon.

Solid cathodes when initially inserted into the source, assuming it is suitable material (ice cubes don’t work well), will outgas in a very short period of time. Beam from the ion source can typically be had in sufficient quantity in the time it takes the ionizer and boiler temperatures to be brought to the typical operating values.

Powder cathodes work best when they are pressed with dry powder and stored in a dry environment. These cathodes will take longer to outgas than their solid counterparts and the ion source extractor and cathode voltages will often suffer stability problems. Previously used cathodes, especially depending on the nature of the material, can sometimes take up to half a day to behave in a stable fashion and produce the desired beam current.

Cathode Change Instructions

This procedure will only be performed by the qualified personnel B. Schmidt, Dr. L. Baby or Dr. I. Wiedenhoever Bring the source to the "Cold Secured" State, locking out the High-Voltage potentials and power to the source.

1. At the source vacuum control panel are two toggle switches that must be placed in the “unprot” position; this is to prevent the isolation transformer from shutting off supplies due to a vacuum excursion in the source. Note the current vacuum indication at the backing line thermocouple controller on the source vacuum panel. Depress the red push button next to the baffle switch while making the change.
2. Open the source H.V. cage.
3. Hang the shorting rod between the air lock VAT valve and the HPS vacuum fitting clamp.
4. Remove the H.V. cathode lead from the stop clamp (aluminum bar) attached to the cathode stainless steel coolant tube.
5. Ensure the compression fitting sealing the cathode assembly is only finger tight and then withdraw the assembly slowly and carefully. At 5 3/4” of travel a scribed line on the stainless steel tubing will emerge from the compression fitting. At 1/4” later a second scribed line will emerge indicating that clearance has been afforded the VAT air lock valve and it can be closed. To close the valve rotate the black handle clockwise (up and to the right) until it comes to a stop.
6. The green knob on the Nupro valve can now be opened to let the cathode air lock region up to atmospheric pressure.
7. Use one hand to withdraw the probe assembly (cooling lines still attached) until it is free from the source. With a glove on the other hand remove (unscrew) the old cathode and replace it with the new one.
8. Install the cathode assembly back into the compression fitting; loosen the fitting if necessary to get the probe started and then tighten it finger tight again. Slide the assembly in until the first scribed line is even with the compression fitting.
9. A nylon roughing line with a stainless steel tube fitted to its end should be at the source bottom cage grating, ensure its free of debris and insert it into the Nupro valves compression fitting. Snug the fitting on the tube and ensure the valve is still open.
10. At the source vacuum control panel, shut the backing valve by putting its toggle switch in the manual close position.
11. On the backing line of the source diffusion pump is a right-angle, 1/4” brass valve with a black toggle handle, open this valve to allow the cathode air lock region to be pumped out with the roughing pump. The vacuum can be monitored at the backing line thermocouple controller on the source vacuum panel. When the vacuum has reached the reading previously noted (about 10 microns) close the green handled Nupro valve and then the brass valve on the backing line.
12. Vacuum permitting, open the backing line valve with the toggle switch at the source vacuum control panel and place the two toggle switches in step 4 back to the “prot” position. Depress the red push button next to the baffle switch while making the change.
13. Remove the nylon roughing line from the Nupro valve and place it back at the bottom cage grating.
14. While watching the sputter source penning gauge, open the VAT air lock valve by rotating the black handle counter clockwise (up and to the left) until it comes to a stop. A temporary degradation of the vacuum should be indicated by the penning gauge. If the vacuum does not show signs of recovering in 10 seconds close the air lock valve and determine the source of the gas load. Outgassing of the cathode is not uncommon and is indicated by a vacuum that quickly recovers but not to the level previous to the air lock valve being opened. The vacuum will slowly improve and attain the expected base vacuum of about 1 x 10−7 torr.)
15. Carefully slide the cathode assembly into the source until the aluminum stop clamp on the assembly is flush with the compression fitting. There may be an premature stop encountered before the stop clamp is flush with the compression fitting, if so, slightly wiggle the cathode assembly until it is free to be fully installed. Ensure the compression fitting is snug when the installation is complete.
16. Reconnect the clear lead for the cathode H.V. supply back to the stop clamp.

1. The cathode change is now completed.
2. Bring the source back to operation after the source vacuum has recovered to ∼1 x 10−6 torr. The inflector magnet and source supplies can now be tuned for the desired ion and beam current on the L.E. cup.

Outgassing a New Ionizer

DO NOT RUN THE IONIZER OVER 290 WATTS

1. Since the source has just been rebuilt it will need to be pumped on for at least three hours. After this time the vacuum should be in the low 10−6 torr range if there is no leak. Note in the logbook the source vacuum indication previous to outgassing the ionizer filament. If the vacuum has not reached this level in five hours a leak may be present and it will have to be corrected prior to heating the ion source filament.
2. The ionizer is typically left with a small current flowing through the ionizer after the rebuild just to ensure continuity. Assuming the vacuum has recovered, increase the ionizer current in three ampere steps every thirty minutes. If the vacuum has degraded to over 5 x 10−6 torr due to a previous current increase, hold off on subsequent increases. When the vacuum has returned to 5 x 10−6 torr or less begin the three ampere increases again.
3. When the Ionizer filament power is up to 290 watts for 30 minutes and the vacuum has improved to better than 5 x 10−6, begin looking for beam on the L.E. cup. Go to Sec. 7 for instructions for tuning beam out of the source. Instabilities in the Cathode and Extractor high voltage elements will cause the beam current to be erratic and can be an indication of poor vacuum in the ionizer region. The voltage instability also appears on the high voltage supply meter indications in the source high voltage cage. Patience or a reduction in the filament power level, if beam current requirements allow, should correct this problem. Regardless, reduce the ionizer temperature if possible to prolong its operational lifetime.

RF Source

RF Source Operation Schematic

Operation Principle

Helium is admitted into a quartz tube at a few mtorr where it is ionized to He+ in an RF discharge. The ions are extracted through a tantalum exit canal by applying a 6 kV positive bias to the probe at the other end of the tube. Most of the probe bias potential is dropped across a small region near the exit canal and the field lines have the effect of focusing and accelerating the ions through the canal. The ions emerge with an energy close to the probe potential. A large coil around the quartz tube produces a magnetic field which improves the coupling of RF power into the discharge. The He+ ion beam from the exit canal then passes through the Rubidium vapor charge exchange cell. About 1% of the He+ entering the REC should get converted to He− in the 1s2s2p 4P5 2 metastable state. The negative ion lifetime of approximately 0.5ms is sufficient to reach the Tandem terminal stripper foil. The He- ions are then focused by passing through a gap lens with a potential difference of 7kv. The emerging beam is injected into a pre-accelerator tube.

RF Discharge Contaminants

Oxygen and other species form negative ions with efficiencies much higher than helium. The admitted helium and desorbed gases from the RF source bottle and boron nitride exit canal insulator can be sources of contaminant beams. For optimum performance it is essential that the RF source bottle assembly be leak tight, that the helium used be greater than 99.99% pure, and that all He tubing and fittings be clean. In addition, it is also important to operate the RF discharge with the helium flowing for 1 or more days prior to the accelerator beam time to allow some of the contaminants to dissipate.

Summary of Operation

RF-Source Einzel Lens Schematic

1. Check that the setup procedures have been properly carried out. In particular, ensure that the anode HV supply (OPPLIS PMT supply) has been disconnected from the PMT and has been set to the positive polarity. Turn on the ENI blue RF amplifier and power supply for the 114 MHz oscillator. Set the attenuator between the oscillator and amplifier to 10db; the output will be more than 30W.
2. Open the valve on the red helium bottle. Open the helium leak until the discharge starts. Close it until the discharge is no longer pink and has become blue. The spherical deflector denoted Penning gauge should only read a few 10−6 torr.
3. Turn on the anode high voltage and set to +4 kV. (To use the program set the remote/manual switch at the back to remote).
4. Turn on the einzel lens supply. Set to 4.5 kV using the program.
5. Insert cup-1 em (use the control program) and optimize the positive current by varying the gas pressure.
6. Turn on the magnet supply (HP power supply). Optimize the positive current with respect to magnet current. Note: There are strange “hysteresis” type effects in the discharge. The magnet current which was previously “optimum” is often not the optimum on a subsequent optimization attempt.
7. Focus the positive beam onto cup-2 using the einzel lens “focus”, Q1 and the deflectors.
8. Analyze for He+ using the Wien filter:
   1. Set WF electrostatic to 160V.
   2. Step the WF magnet current up from zero and monitor cup-2 current. He+ should occur at about 265 mA. If the source has not been run recently 300 nA of analyzed He+ on cup-2 is typical.
   3. Continue scanning to 1000 mA and look for other strong peaks.

It has been found that better transmission can be obtained by operating at reduced Wien voltage, and correspondingly reduced Wien magnetic field.

1. To allow some of the contaminants to dissipate operate the RF discharge with the helium flowing for 1 or more days prior to the accelerator beam time. After this purge period the analyzed He+ beam on cup-2, after optimizing the focusing, should be more than +5 microamperes. If it is not, try adjusting the gas leak and magnet current. Adjustment of the gas, however, may be frustrating: increasing the flow usually increases He+ output temporarily, followed by a decrease and vice versa.
2. Increase the cesium charge exchange canal middle tube section temperature setting to 150 C.
3. Monitor the temperature rise. At about 100 C the positive current reading on cup-2 should start to fall; at about 135 C the current should pass through zero. The beam collected will consist of contaminant species and ions rendered neutral by the charge exchange canal some of which will pass undeterred by the Wein filter to cup-2. The beam current indication therefore is not due to helium alone and can be misleading as demonstrated by the reading becoming positive when the source platform is raised to high voltage. At 150 C you should see on the order of
4. 50nA at cup-2.
5. Close the source cage, and follow the instructions for Changing Ion Sources in Sec. 5 of this lab guide. Check for isolation of the platform from ground and nearby objects.
6. Raise the preaccelerator voltage to 8kV.
7. Look for beam on the LE-cup and the source scanner. There should be a beam on the scanner which does not respond to the inflector magnet or the deflectors, and superimposed, a beam which does. The non-responsive beam is the neutral helium and contaminants, the responsive beam is the negative He and contaminants.
8. Inject the beam into the tandem and analyze on BS-1 or BS-2. Carefully check the NMR reading and terminal voltage (allowing for the offset in the GVM reading) to ensure that it is He++ which is being analyzed. 16O 6+ can be transmitted with the same NMR setting for He if the terminal potential is roughly 200kV over the required setting.
9. Using the program and other controls, optimize the He current on the beam stop. A current of 50 to 100 nA on the beam stop should be expected.

RF Source Tuning Instructions with the PC

Focusing and Steering Control

Use the arrow keys to move the highlighted rectangle to the “device” column and then to the specific element in that column you wish to adjust. After the particular lens or deflector is highlighted use the right arrow key to move to the “setnow” column and press the “F” key. Tuning of the chosen device is done by pressing the up and down arrow keys to arrive at the optimum setting. Using the left arrow to return the highlighted rectangle to the “device” column will set the devices operating parameters.

Optimizing on Source Cup-2

Press the “N” key to allow beam current to be measured at cup-2; the current is displayed at the lower center of the control screen. If the output is less than 100 nanoampers of negative beam follow the “A” set of instructions below. If the current is 100 nanoamperes of negative beam or greater the source is operating well and one should follow the “B” instructions.

A) Press the “P” key and observe the beam current on cup-1. It should be 40 to 80 microamperes of positive beam. If the reading is less, adjustment of the discharge gas pressure is necessary. Due to the location for gas adjustment it is not physically possible to monitor the effect of the pressure change at the computer screen so a digital volt meter must be connected to the coax cable labeled “cup-1” near the flow control valve to measure changes in beam current. Open the Helium gas flow control valve at the RF discharge tube inlet in steps no greater than a 12 degree turn of the control rod; the effect should be noticeable within seconds. If the beam output doesn’t reach 40 microamperes seek assistance from Brian Schmidt.

When the cup-1 beam current has reached 40 to 80 microampers remove the cup from the beam path and adjust the Wein Filter electrostatic voltage while monitoring current at source cup-2. If the current still is not -100 nanoamperes check the status of the light blue vacuum protection panel located in the source cage: if the red light is on simultaneously press the two reset buttons on the panel. If the vacuum protection panel has not tripped and beam output hasn’t reach 100 nanoamperes of negative beam at cup-2 seek assistance from Brian Schmidt. When adequate beam is attained at cup-2 follow instructions “B”.

B) The current on the LE cup should be 50-100% of source cup-2. If it is less than this, the source needs some additional tuning. Press the “E” key and then try to small adjustments of the Wein filter electrostatic voltage while observing beam either at the LE cup or the HE tandem cup-1. Increases in beam transmission may also be found by fine tuning the source exit deflectors (rack mounted in the control room above the PC screen), quadrupole-2 (adjusted with the computer software) and the inflector zero supply (located in the control room above rack “G”).

Typical Beam Currents:

• Source Cup-1 = 40-80 microamperes positive
• Source Cup-2, With the Preaccelerator Off = 100-150 nanoampers negative
• Tandem L.E. Cup = 100 nanoamperes negative
• Tandem H.E. Cup = 50-100 nanoamperes positive

1. Close the LINAC beam line pneumatic valve. Ensure the vacuum in the Tandem 90◦ magnet, H.E. beamline and switching magnet are below 1 x 10−5 torr. If this is so, open the hand valve before the 90◦ magnet and the switching magnet pneumatic valve.
2. In the Control Room, exchange the input connector for the gauss meter from that for the LINAC to that labeled for the Tandem. Ensure the Tandem 90◦ magnet is in the correct deflection mode. If not turn supply down before making the switch.
3. Control of deflectors D-1, D-2, QIII-A and QIII-B is obtained by raising the switch at the bottom of panel F to the position labeled Tandem. Ensure the appropriate supplies are off.
4. Close the gate between the two Target Rooms. Rope off the passage between the Tandem Vault and the LINAC Hall.