High, ultra high and extreme high vacuum: the fundamentals
It is only by examining the differences (at a molecular level) between the various vacuum levels that one can begin to appreciate the challenges associated with achieving and working with high vacuum (HV), ultra-high vacuum (UHV) and extreme high vacuum (XHV).
In rough and medium the major source of gas is from the “bulk” or original gas whereas in HV and UHV the gas load is dominated by outgassing from surface gas desorption; in XHV the main load is from gas permeation from the chamber walls and other materials.
What is the definition of high, ultra and extreme high vacuum?
The pressure range of XHV is usually defined as 10-12 mbar and lower, while UHV is between 10-7 and 10-12 mbar, and HV between 10-7 and 10-3 mbar. XHV is associated with the levels found in outer-space in the form of geo-stationary orbiting satellites, UHV with high-energy physics and nuclear research and HV for industrial and research applications.
As one would expect, the established norms, rules and protocols that define and govern vacuum factors and matters, from how to obtain such vacuum levels, the pump set-up, safeguards, measurement methods, as well as leak detection, must all be thoroughly re-examined and frequently re-engineered.
Key Considerations for Working in HV, UHV and XHV Conditions
Several key considerations for working in HV, UHV and XHV conditions are associated with the system design, including the materials being employed.
In addition, the condition of the system/chamber surface is also important and can be optimized by:
- minimizing the chamber’s internal surface area
- only welding from the inside
- using materials with low desorption/outgassing rates
- suitable pre-treatment of materials (e.g. electro-polishing)
- making sure there are no internal gaps or trapped volumes (e.g. tapped blind holes)
- reducing the number of seals, feed-throughs etc.
- and employing metallic seals
Pre-treatment of the system is important and includes heating to a high-temperature (known as “baking”), careful handling using powder-free latex gloves to prevent fingerprint greasing, and thorough cleaning to remove hydrocarbons, fillings and other contaminants (both chemical and physical).
What pump types can you use to generate HV, UHV and XHV?
Obtaining HV, UHV and XHV levels can only be effectively and efficiently obtained by using a fore pump that charges the main pump. Fore pumps, (sometimes called “backing pumps”) reduce the pressure to a level where HV, UHV and XHV pumps can take over to operate in a safe, efficient and effective manner. However, pairing different types of vacuum pumps for optimum performance is not straight forward. There are no off-the-shelf pumping systems which cover all applications, eventualities and requirements simultaneously, as there are a myriad of critical factors and impacts to be taken into account.
The selection of which pump (both fore and main) depends on a number of factors including noise/vibration, cost (initial and on-going), tolerance to contamination, footprint, maintenance schedules, resilience to shock etc. However, there is no single ideal HV, UHV or XHV pump: each type has its own set of advantages and detractions.
How to measure High, Ultra & Extreme High Vacuum?
In terms of measuring pressures in HV, UHV and XHV, traditional pressure/vacuum gauges are unsuitable due to the previously discussed outgassing effect. As a result, ionization gauges are employed instead: these use the probability of gas ionization to determine the particle number density. There are two types: cold and hot cathode ionization gauges.
Leak Detection in High, Ultra & Extreme High Vacuum
No vacuum device or system can ever be absolutely vacuum-tight and actually, it does not need to be. The simple fact is that the leak rate should be low enough so that the required operating pressure, gas balance and ultimate pressure in the vacuum container, are not unduly influenced. In terms of HVs, XHVs and UHVs, it is small leaks that are the main cause of concern, and the only credible method to detect leaks smaller than 10-7mbar.l/s is with a helium leak detector (HLD).
A leak diameter-equivalent of 10-12 mbar.l/s (which equates to 1Å) is also the diameter of a helium molecule, and is the smallest leak rate that can be detected. This association with helium is one of the reasons why one of the most accurate and rapid leak detection methods employs helium as the tracer gas, and a mass spectrometer for the analyzing/measuring.
High, Ultra-high & Extreme High Vacuum: the fundamentals
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