LEA Source Stability and Revisions
Problem
The source stability can be difficult to maintain at times. Often times there is discharging between the stages. While some discharging - especially in the days following a short revision are normal - it usually calms down after some time. When the sparking frequency increases too much, it can cause elevated blank levels and eventually total source shut down. Various factors affect the longevity of being able to run the ion source before needing to do a source revision. By running gas samples routinely, this time is shortened considerably due to the higher amounts of cesium and sputtered metal involved in gas sample analysis.
Common symptoms
When stages are shorted and no current is emitted from the source, the source cathode detection at the 12C-LE Faraday cup will fail. During an HTI Measurement, ACS will retry on 3 positions before throwing an error and stopping the measurement task.
Shorting potentials
When the stages of the ion source get shorted, the beam can no longer be extracted into the accelerator reliably if at all. The following section gives clues on how to more precisely diagnose shorted potentials:
Helpful for diagnosing problems of shorts in the ion source is to know the arrangement of the electric potentials. The ionizer potential is referenced relative to ground. Both the cathode and extractor potentials float relative to the ionizer potential but independent from each other.
Knowing this can help with diagnosing short circuits in the source. Depending on which stages are in current limitation, you can better locate the cause according to the following table:
Cathode Current | Ionizer Current | Extractor Current | What to look for |
|---|---|---|---|
|
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| cathode stage shorted to ionizer stage |
|
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| ionizer stage shorted to ground |
|
|
| extractor stage shorted to ionizer stage |
|
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| cathode stage shorted to ground |
|
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| extractor stage shorted to ground |
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|
| Check out our video on source revisions! |
= current limitation
Example
After a source revision, it was found that the cathode and ionizer currents were stuck in current limitation (around -7.5 mA) and their voltages were both stuck close to 0 kV
By the above table, this would indicate that the cathode stage was shorted to ground and took the ionizer potential with it. The extractor stage, being independent from this short, remained at potential with minimal current load. Thus, the search for a cathode potential element short to ground ensued.
It was found that the metal capillary for gas samples was in contact with the grounded metal plate that shields the cathode potential feedthrough (and particularly its insulator) from cesium.
Underlying causes of source instability
Cs vapor and sputtering debris covering insulators and creating tips on surfaces. Typically, due to prolonged use, accumulation of dirt and coating of surfaces.
Excess Cs vapor leading to lowering of breakdown voltage in vacuum.
Sputtering of pure titanium without CO2 going into the source exacerbates breakdown problems. Mechanism unknown.
Flakes and dirt/dust may come loose and create short-circuits.
Scratches and burrs from build-up create stronger local fields, leading to more sparks.
Potential counter measures
During operation
Run the source less hard (meaning lower Cs reservoir temperature and thus lower C mass flow and lower currents).
Run ion source less lean (meaning don’t grossly underfeed with CO2).
Run samples for longer (increasing the ratio of time spent pre-sputtering pure titanium vs. sputtering with CO2 present).
If you notice that pre-sputtering takes longer than it used to: heat out gas cathodes as per our note.
Avoid cooling down the ionizer other than for a source maintenance.
Put the system to sleep (cooling down Cs to standby-temperature) when system is not in use.
Keep a solid cathode in the source whenever there is no measurement ongoing.
During source revision
Test exposed surfaces for burrs and defects (visually, with fingertips). Gently polish scratches, build-ups and burrs with fine emery paper. Screws sticking out are also candidates for problems.
Make sure to remove any dust or fine flakes or debris with pressurized air at the end of the cleaning.
Wires should be kept at equal distance to other wires and surfaces. Don’t kink wires. Replace damaged, kinked wires.
Having a second source lens assembly at hand can help with shorter down-time and decoupling cleaning from the operation.
After a source revision, especially when refilling/swapping out Cs, adjust the Cs reservoir temperature.
Usually, a fuller reservoir requires a reduced temperature.
Common pitfalls during source revisions
Loose connections
Not bracing target cooling lines while tightening
O-ring falls out of target cooling lines and leaks cooling gas
Stage separation measurement inconsistency
Check that the cesium beam is well centred using a burn-in dummy cathode