How it works – The AIM Gauge
How do AIM gauges work?
AIM gauges, or ‘Inverted Magnetron’ gauges work by measuring the discharge current generated by a plasma confined between a rod shaped anode and an outer, cylindrical, cathode electrode.
It is a common misconception that the AIM gauge is also called a ‘Penning gauge’. In fact, the anode and cathode are a completely different shape in the Penning configuration. The important thing to realize is that AIM gauges and Penning gauges work on exactly the same principles.
In an AIM gauge, the anode is typically held at around 3000V and the cathode is normally at 0V. Around the outside of the electrodes, magnets are mounted to generate a magnetic field along the axis of the electrodes.
Due to the orientation of the electric and magnetic fields present, electrons are able to perform long orbits around the anode often resulting in an ionizing collision with a gas molecule.
After the first ionizing collision, very quickly an equilibrium is reached where the number of ions generated through collisions balances the number of charged particles that reach the electrodes. This ‘discharge current’ is detected by the gauge electronics and is related to the pressure inside the gauge.
At high pressures, the discharge current is greater than at lower pressures.
AIM Gauge Striking
AIM gauges need to ‘strike’ before they read pressure correctly. Saying that a gauge ‘has struck’ is simply a way of saying that the plasma has reached equilibrium and the gauge is now reading pressure. It is the process by which the first ionizing collision is generated.
Striking is caused by charged particles and many sources of striking come from random processes.
Examples Include:
Charged particles from field emission from within the gauge.
Other charged particle sources within the chamber (for example hot cathode ionization gauges or filaments on mass spectrometers). Note: even after switching an ion gauge off, residual charged particles can be present for several minutes. This can enhance the chances of AIM gauges striking.
Cosmic rays or charged particles from radioactive decay.
Photons with sufficient energy to ionize gas molecules.
Generating charged gas molecules requires an ionizing collision. If the pressure is higher, there will be many more gas molecules present so the likelihood of a charged particle hitting a gas molecule will be much greater. Ionization probabilities are therefore proportional to the number density of a gas and hence inversely proportional to pressure.
Put another way, the probability of striking in a certain time interval gets less and less at lower and lower pressures. Edwards state that a gauge that is clean will strike within 10 seconds at pressures greater than or equal to 1E-5mbar (1E-3Pa).
It is a well known fact that in certain extremely low pressure situations it can sometimes take over a day for an ‘inverted magnetron’ type gauge to strike.
If the internal surfaces of a gauge become contaminated with an insulating film, they will charge up and there is no longer 3000V between the anode and cathode electrodes. As a result, the chances of generating charged particles from field emission reduce and the gauge will be harder to strike. This is why we specify that the gauge will only strike within 10 seconds at pressures above 1E-5mbar (1E-3Pa) if it is clean.
AIM Gauge Striking Sequence
As soon as pin 7 on the FCC connector is pulled low, the high voltage to the gauge is switched on (typically 3000V).
There will be a delay before the gauge strikes. During this time, the gauge output will be typically 1.6 to 1.8V
AIM Gauge Contamination
If oil or process contaminants are able to build up inside a gauge, the insulating properties of the contamination affect the measuring electrodes ability to measure the discharge current.
The insulating films can charge up and the electric field that normally confines the plasma is no longer effective. The plasma becomes unstable and the output of the gauge becomes unpredictable. Charging up of the surfaces reduces the ability of the gauge to strike.
Eventually, bombardment of oil by charged particles from the discharge can ‘crack’ the oil which results in an insulating plastic like film forming on the electrode surfaces.
The plastic film eventually peels off the surface of the electrodes as flakes of contamination. Flakes of contamination result in gauges with very noisy outputs as the plasma becomes unstable.
Contaminated gauges can jump from one reading to another. This is because the discharge can jump from one discharge mode to another. In the case of the mode being unstable, it cannot be sustained within the electric and magnetic fields and the plasma goes out. The result is that the gauge will indicate a very low pressure.
The only solution for a contaminated gauge is to remove the contamination. Fortunately the AIM gauge is designed for easy disassembly and cleaning. If cleaning is not a preferred solution, electrode service kits or even complete replacement tube assemblies are available.
Contact AVT for more details about these or any other vacuum product.