When working with high vacuum (HV) and ultra-high vacuum (UHV) systems, there are specific aspects to consider to make sure they remain efficient and safe.
What Are HV and UHV Conditions?
HV pressure ranges are those between 10 x 10-3 and 10 x 10-7 mbar.
UHV pressure ranges are those between 10 x 10-7 and 10 x 10-12 mbar.
As with all vacuum systems, the established standards, rules, and protocols that govern vacuum factors and matters must be frequently reexamined and re-engineered.
Users should routinely:
- Check vacuum levels
- Review the pump setup
- Perform safety checks
- Evaluate measurement methods
- Undergo leak detection processes
HV applications include:
- Metallurgical processes
- Nuclear physics
- Space simulations
- Analytical instruments
UHV applications include:
- Surface analysis
- High-energy physics
- Molecular beam epitaxy (MBE)
Users should carry out a careful assessment of design, materials, and the condition of the vacuum system to identify problem areas. HV and UHV vacuum system efficiency can be improved by following several best practices:
- Minimize the chamber’s internal surface area.
- Weld only from the inside.
- Use low desorption/outgassing rates.
- Pre-treat materials — for example, undergo electro-polishing.
- Check for internal gaps or trapped volumes, like tapped blind holes.
- Use metallic seals.
- Reduce the number of seals and feed-throughs.
Working Condition Considerations
Outgassing is the process of releasing gas that was dissolved, trapped or absorbed in some material. To create and maintain clean HV and UHV vacuum environments, users must carefully monitor outgassing.
Outgassing typically occurs when materials not usually considered absorbent release enough molecules to interfere with industrial or scientific vacuum processes. Common sources of outgassing include:
- Metal impurities
- Glass cracks
Cleaning surfaces, heating individual components, or performing a bake-out can drive off volatiles.
In HV and UHV conditions, outgassing and degassing levels must be kept as low as possible.
Minimizing effective surfaces reduces the impact of outgassing. The higher the surface area, the greater the outgassing and the higher the system pressure.
How do I Know my Operating Conditions?
A residual gas analyzer (RGA) is a small quadrupole mass spectrometer, typically designed for environment analysis, process control, and contamination monitoring in vacuum systems. RGA's can monitor the quality of the vacuum by detecting (and measuring) minute traces of impurities in a low-pressure gaseous environment.
RGA's are usually mounted directly onto, and into, the vacuum chamber. RGAs perform various functions which would be difficult to obtain by other methods, including: the analysis of various gas-phase reactions, monitoring changes that occur in any gas environment, detecting vacuum leaks, and checking mass flow controllers.
Pump Selection Considerations
There are many pump types capable of producing HV and UHV vacuum pressure, and most applications require multiple pumps working together. However, pairing different types of vacuum pumps to optimize performance is not always a simple matter.
Several factors impact the choice of pumps, such as:
- Noise and vibrations
- Initial and ongoing costs
- Contamination tolerance
- Maintenance requirements
- Shock resilience
Fore pump options include:
Main secondary pump options capable of HV and UHV levels include:
- Turbomolecular pumps
- Diffusion pumps
- Cryogenic pumps
- Ion getter pumps
- Titanium sublimation pumps
- Non-evaporable getter pumps
These pumps produce vacuum conditions by rapidly evacuating, capturing, or tying up gas molecules.
Understanding Pump Types
Each pump type has unique advantages and disadvantages, which adds complexity to the selection process.
For example, turbomolecular pumps advantages include:
- Easy, kinetic operation
- Low maintenance
- Hydrocarbon-free operation
- Vibration-generating moving parts
- Reduced pumping speed for light gases
- Sensitivity to mechanical shock
- Particulate contamination
Buyers should undergo a similar comparison process for each pump type to reach an informed decision.
When creating HV and UHV conditions, it is crucial to consider these three factors: efficiency, working condition considerations, and pump selection. An approach that focuses on these key elements will keep you on the path to delivering optimum performance and reliable HV and UHV conditions.