Four basic rules for working under HV and UHV conditions
When working with high vacuum (HV) and ultra-high vacuum (UHV), there are specific aspects to consider to ensure an efficient and safe system.
To clarify, the pressure range of UHV conditions are defined as between 10-7 and 10-12 mbar, whereas HV conditions are defined as between 10-3 and 10-7 mbar. Some of the main applications of HV include metallurgical processes, nuclear physics, space simulation and analytical instruments. On the other hand, UHVs are used for surface analysis, in high-energy physics and Molecular Beam Epitaxy (MBE).
In this blog, we discuss the four main considerations you need to bear in mind when working under HV or UHV conditions.
1. Vacuum systems design, materials and surfaces
As with all vacuum systems, the established standards, rules and protocols that define and govern vacuum factors and matters (such as how to obtain such vacuum levels, the pump set-up, safeguards, measurement methods and leak detection), must all be thoroughly re-examined and frequently re-engineered.
In addition, the design, materials used, and the condition of the vacuum system must be assessed – and the following can help improve their efficiency:
minimising 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.
employing metallic seals
2. Managing working conditions and cleanliness
One of the main challenges in creating and maintaining clean, high and ultra-high vacuum environments is managing outgassing. Outgassing refers to the release of gas that was dissolved, trapped, adsorbed or absorbed in some material.
This phenomenon happens when materials not customarily considered absorbent release enough molecules to interfere with industrial or scientific vacuum processes. Moisture, sealants, lubricants, and adhesives are the most common sources, but even metals and glasses can release gases from cracks or impurities. Cleaning of surfaces or heating of individual components or the entire assembly (a process called bake-out) can drive off volatiles.
To achieve extremely high vacuum levels, the total gas load must be as low as possible. Thus, any outgassing or degassing from materials needs to be as low as possible in HV/UHV conditions. In addition, the effective surface areas need to be minimised to reduce the impact of outgassing as the higher the surface area, the greater the outgassing and hence the higher the pressure will be in the system.
3. Choosing the right pump technology
The accuracy of a vacuum gauge will depend on many factors but in general, a gauge will arrive from a manufacturer with only a “rough” calibration (which with no correction factor applied) and can have between 20 and 50% uncertainty within the stated range. Using a constant gas correction factor can improve this to between 10 and 20%. However, where higher accuracy is required, gauges should be calibrated over the whole pressure range. For a high-quality gauge, individually calibrated for each gas type over the full range against a primary standard, accuracy can be improved to between 2 and 5%.
To efficiently and effectively achieve either HV or UHV vacuum levels, a fore pump that charges the main pump is required. Of course, while fore pumps reduce the pressure to a level where HV and UHV pumps can safely take over, pairing different types of vacuum pumps for optimum performance is not always a simple matter.
Each use case and system requirement needs to be considered as unique. A careful evaluation of critical factors and impacts needs to be undertaken to select the most effective pump combination. Several factors impact the choice of pumps, including noise/vibration, cost (initial and on-going), tolerance to contamination, footprint, maintenance schedules, and resilience to shock. But even when armed with this information, the fact is that there is no single solution as each pump type has its own advantages and disadvantages.
Understanding pump types (advantages and disadvantages)
Each of these pumps has its strengths and weaknesses. TMPs are kinetic-type pumps and are easy to operate. They are low maintenance; provide a hydrocarbon-free operation; require no regeneration; and operate at high pumping speeds in the HV and UHV range. That said, they also come with drawbacks such as moving parts that can generate vibrations. Other problems include reduced pumping speeds for light gases and sensitivity to mechanical shock and to particulate contamination.
The next category of pumps is the capture-type, such as IGPs, but these too have their own advantages and disadvantages. For instance, they are better than TMPs when it comes to vibration as they have no moving parts and require almost no maintenance. Furthermore, they are built with radiation tolerant materials above 1e8 Gray, and by removing the magnets, they can be baked up to 450°C, which is essential for high-pressure systems.
The primary drawbacks of IGPs are their low pumping efficiency when handling noble gases, and the decrease in pumping speed in HV and UHV applications. Also, on a practical level, they need high voltage and magnetic fields, and they are heavy.
5. Conductance influence
Conductance is critical when working under HV and UHV conditions, precisely because it defines just how readily a vacuum system or component can allow the flow of gas through it.
In many respects, conductance can be thought of as the “inverse” of flow resistance. By definition, conductance refers to the flow of gas between two points divided by the pressure drop that’s driving the flow. Therefore, the dimensions that describe conductance are the same as pumping speed (volume per unit time), but conductance is a phenomenon that’s usually used to describe pipings and openings — not the vacuum pump.
To sum up, the four main considerations for working under HV and UHV conditions are to:
Create an efficient vacuum system
Regularly check your working conditions and cleanliness
Choose the right vacuum pump for your application
Consider conductance influence
All of these considerations must be borne in mind when operating a HV or UHV system for any application. It is a crucial step on the path to delivering the optimum performance of a vacuum system and achieving the desired HV and UHV conditions.
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