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.

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:

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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:

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The different stages of the MICADAS are colour coded according to their electric potentials. It is important that elements at different potentials are kept away each other and from ground as well as possible.

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 from gas cathodes 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

• 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.

• Check that the cesium beam is well centred using a burn-in dummy cathode