Ion Sources

Sputter Source

Sputter source Operation schematic

Sputter source Volatge Schematic

Sputter Source is also called SNICS.

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.

Tuning Beam out of the SNICS Source

These instructions assume the correct cathode is installed and the necessary hardware/cable changes for selecting an ion source has been made. Also assumed is that the source has been turned down after a previous run but not necessarily to the idle mode. Do not inject more than 10 microamperes of beam into the accelerator under any circumstances.

1. Open the source exit valve in the Tandem Vault.
2. 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.
3. In the Control Room, turn the ion source deflectors to zero.
4. heater current to increase the beam current; do not exceed 290 watts or premature failure of the heater will occur. If there is still insufficient beam current, increase the cesium boiler temperature in 100◦ steps separated by 45 minute intervals. Do not exceed 490◦ or the cesium vapor will cause internal arcing in the source. If there is too much beam, reduce the boiler temperature and monitor the beam current. If less beam is still desired reduce the Ionizer heater current in Turn on and bring up the Preaccelerator High Voltage Supply to 120 KV.
5. 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.
6. 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 minute intervals.
7. If no parameters have been recorded or cannot be found set the SNICS Cathode and Extractor supplies to 7.5 KV.
8. Beam should 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.
9. 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.
10. 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.

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.

Temporary Turn Down

When a break or interruption occurs during the process of data collection and the use of 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. Turn down and off the preaccelerator high voltage supply in the Control Room.
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 idle mode.

Turn Down to Idle Mode

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

1. Turn down and off the preaccelerator high voltage supply in the Control Room.
2. Turn down and off the source deflectors in the Control Room.
3. Turn the inflecter magnet supply completely down.
4. In the Tandem Vault, close the source exit valve.
5. Reduce the Cesium Boiler Heater to 35 on the variac scale by turning the control rod located at the high voltage cage.
6. 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.
7. Turn the Lens, Extractor and Cathode H.V. supplies to zero with the control rod extending from the H.V. cage.
8. Note in the Tandem Log Book, located in the Control Room, the time the source was turned down to the idle mode.

Cathode Change Instructions

Before beginning the cathode change procedure ensure the current settings for the source parameters are recorded in the Cathode Utilization Log hanging on the Source H.V. Cage. This procedure assumes the SNICS (Source of Negative Ions by Cesium Sputtering) is in operation. The cage surrounding the SNICS source is referred to as the “Source H.V. cage” and the cage surrounding the power supplies as simply the “H.V. cage”.

1. In the Control Room, turn down the Preaccelerator H.V. supply and then turn the power off.
2. In the Tandem Vault source area, close the source exit valve; observe that the
3. H.V. warning beacon is not rotating.
4. Reduce the ionizer heater current in four ampere steps separated by intervals of two minutes until fifteen amperes is reached. This can be accomplished by turning the supply control rod sticking through the H.V. cage door; heater current is indicated by the supply. Also, turn down the extractor voltage and cathode voltage supplies by turning the appropriate control rods at the cage door; output voltages are indicated by the supplies.
5. 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.
6. Open the source H.V. cage by releasing the two latches located at the right edge of the door: pull on the latch handle until it rotates 90 degrees towards you and then turn the latch handle until it clears the fixed cage frame. The cage door is a bifold type and should be opened by pushing it to the left.
7. Investigate the H.V. cage to ensure the ionizer heater supply and the Cesium feed line, indicated by a lit power switch, have not been interrupted.
8. Using the shorting rod that is hanging on the cage at the stainless steel U-bolt, touch the source on each side of the endmost ceramic insulator to reference the stainless steel (stainless steel) elements to ground. Hang the shorting rod between the air lock VAT valve and the HPS vacuum fitting clamp. * BE WARY OF THE WHITE IONIZER HEATER LEADS AND THE BOILER FEED TUBE AS THEY ARE LIVE *
9. Remove the H.V. cathode lead from the stop clamp (aluminum bar) attached to the cathode stainless steel coolant tube.
10. 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.
11. The green knob on the Nupro valve can now be opened to let the cathode air lock region up to atmospheric pressure.
12. 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.
13. 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.
14. 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.
15. At the source vacuum control panel, shut the backing valve by putting its toggle switch in the manual close position.
16. 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 roughed out. 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.
17. 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.
18. Remove the nylon roughing line from the Nupro valve and place it back at the bottom cage grating.
19. 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.)
20. 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.
21. Reconnect the clear lead for the cathode H.V. supply back to the stop clamp.
22. Remove the shorting rod from the source and hang it back on the cage at the stainless steel U-bolt. Close the cage door and remake the latches.
23. Inspect the H.V. cage to ensure the extractor and lens supplies are again energized. The cathode supply must be reset by inserting a nonconducting rod (PVC or nylon) through the cage and depressing the green square button near the left edge of the supply front panel.
24. Increase the Ionizer current in four ampere steps separated by intervals of five minutes until the desired operating current is achieved (normally 23 amps).
25. The cathode change is now completed. The source exit valve can be reopened and the preaccelerator brought back to voltage after the source vacuum has recovered to ∼1 x 10−6 torr. The inflecter 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.

Shutdown and Disassembly

1. In the Control Room, ensure the L.E. Faraday Cup is in and the preaccelerator is switched off. Turn down and off the Source Deflectors.
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 to zero amperes in four ampere steps separated by two minute intervals by turning the control rod attached to the Ionizer Heater power supply located at the High Voltage Cage.
5. Leave the source to cool for an hour and thirty minutes then turn the cooling water off.
6. Turn off the Cesium Feed Line Heater by depressing the illuminated power button with an insulated rod.
7. Turn down the Extractor and Cathode H.V. supplies by turning the control rods attached to each power supply at the High Voltage Cage.
8. Reduce the einzel lens voltage to zero by depressing the labeled rocker switch at the H.V. cage control panel. Turn the key switch at the panel to turn off the isolation transformer providing platform power to the H.V. cage. Both the Source and H.V. Cages may now be opened.
9. Using the shorting rod that is hanging on the cage at the stainless steel U-bolt, touch the source on each side of the endmost ceramic insulator to reference the stainless steel elements to ground. Hang the shorting rod between the air lock VAT valve and the HPS vacuum fitting clamp.
10. Access to the external source body is now possible. If breaking vacuum is necessary for work to proceed then continue with this list. If the source is to be let up to Argon, proceed.
11. Turn off the cold cathode vacuum gauge. Verify that the source exit valve is closed.
12. Close the diffusion pump gate valve. The source may now be back filled to atmospheric pressure with argon: open the cylinder valve and adjust the regulator for 10 psi, open the flow gauge valve several turns, open the valve attached to the source vacuum box, adjust the flow to 10 L/min. Allow the gas to flow into the source box until the pressure relief valve on top of the source vacuum box indicates excess flow. Close the valves associated with letting up the source.
13. After five minutes feel the end flange of the source to see if it is warm. If it is warm allow the source to sit until it is cool to the touch.
14. The cesium boiler assembly may now be dismantled so that the cesium reservoir can be removed. Be very careful when removing the reservoir because some Cesium vapor may have condensed around the VCR gaskets. The NEC “Cesium Saver” may be used to protect the cesium from contamination; if needed, its use is described in the NEC Instruction manual.
15. Remove the H.V. cathode lead from the stop clamp aluminum bar attached to the cathode assembly.
16. Ensure the compression fitting sealing the cathode assembly is only finger tight and then withdraw the assembly.
17. Access to the source inner components is obtained by removal of the electrical connections, the cathode air lock valves and quick connect seals and the end most 12 hole flange. Be careful on removing the flange so that damage is not done to the parts mounted on the inside surface.
18. When the source work is finished and it has been reassembled, the source can be roughed out by turning on the roughing pump at the fuse box mounted beneath the H.V. cage by the mechanical pump which connects to the source vacuum box by PVC piping. Open the ball valve between the source box and the pump. Close it at a source vacuum of 300 millitorr. Shut down the roughing pump and let the roughing line up to air with the valve mounted on the PVC near the pump.
19. Place the backing and baffle valves in the “unprot” mode and open the baffle valve. When the vacuums are below the set points for the respective controllers, place the backing and baffle valves back to the “protect” mode.
20. Reconnect the electrical leads removed from the source, install a cathode and close the source cage.
21. Vacuum permitting, the source is ready to be brought back to operating voltage, current and temperature.

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 Operaation

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.