Accelerated electrons often steal the spotlight when it comes to electron beam (they even are listed as the titular character!), but behind the curtain (or better yet, the titanium foil) and responsible for the whole production is vacuum. Good vacuum makes EB possible; the better the vacuum, the better the operation of the beam, so understanding why vacuum is needed, how it works and best practices when it comes to vacuum maintenance will help ensure the beam is belting out electrons for a long time to come.
Why vacuum?
To keep things clean, of course! All jokes aside, cleanliness is an important component of what vacuum provides. Low-energy electron beams (≤ 300 kV) use a high-voltage potential to accelerate electrons. 1 To prevent high-voltage arcing or sparking in the vacuum chamber, vacuum is pulled to ‘clean up’ or remove molecules of air, dust or any other material that can become ionized by the voltage potential and create an unintended path for electricity. While the high-voltage power supply is designed to quench sparks within milliseconds during operation, a large-enough spark event can cause the high voltage to turn off, stopping production. Establishing good vacuum helps prevent these events. Note: High-voltage sparks are internal to the beam and pose no safety risks under normal operation; the high-voltage enclosure, cable and power supply are insulated and bonded, and safety interlocks prevent operation of the system if enclosures are left open.
Removing these molecules also enables the accelerated electrons to reserve their energy. Accelerated electrons lose energy when they collide with matter – even individual molecules of nitrogen and oxygen in the air – so traveling in a vacuum prevents unnecessary energy loss until the electrons must exit the vacuum through a foil to interact with whatever material is being processed by the beam. Minimizing energy loss along the path of the electron means more of that energy can be deposited in the intended material.
Last, but certainly not least, filament life is tied to how well the vacuum has cleaned up the oxygen molecules in the vacuum chamber. Tungsten filaments are used to supply the beam with electrons; once electronically heated to ≥ 2,300˚ C, they give off free electrons through a process called thermionic emission. At these high temperatures, however, the filaments are very reactive to oxygen. In air, the filaments would oxidize almost instantaneously and be useless as an electron source.
What makes a vacuum?
The need for vacuum is a major factor in the design, material choice and build of an electron beam. Conduction of heat in a vacuum is extremely poor; therefore, the materials inside the vacuum chamber must have adequate heat resistance, and other methods of heat dissipation must be incorporated into the machine design. Materials that outgas in low-pressure, high-temperature environments should be avoided. Specific welding techniques are used to produce a chamber that can pass a helium leak test, and surfaces are electropolished to avoid any roughness that high-voltage sparks may be drawn to. These and other preparations lay the foundation of a good vacuum system during the manufacture of beam.
Low-energy EBs fall into two vacuum categories – sealed and continuously pumped. In a sealed system, the vacuum is pulled and sealed during the manufacture of the EB emitter. Similar to a light bulb, once the vacuum is breached on a sealed emitter, the whole thing is removed and replaced. Sealed systems are a great option for tight spaces since they are not encumbered with vacuum equipment, but they also tend to be quite narrow beams because the wider the beam, the larger the vacuum chamber, and the harder it is to achieve adequate vacuum for a reasonable system lifetime. Continuously pumped systems are manufactured with vacuum equipment, which pulls vacuum during operation of the beam and even when the beam is idle. The vacuum typically is only let go for maintenance purposes or if a beam is powered down for an extended period of time.
Vacuum equipment can vary on continuously pumped, low-energy EB systems, but a typical set-up combines a roughing pump and a cryopump and compressor. The roughing pump first reduces the pressure in the vacuum chamber to 10-3 Torr and then the cryopump and compressor take over, further reducing the pressure to ~10-7 Torr. Broadly speaking, a cryopump, or cryogenic pump, works by freezing and trapping any molecules that come into contact with its core of charcoal-lined baffles, which are kept at a temperature of 10 to 20 K. 2 These frigid temperatures are achieved by compressing helium and circulating it through the cryopump.
Vacuum Maintenance.
To achieve a high vacuum, it is extremely helpful to start off with a clean system – the more dirt, oil and grime to start with in the vacuum chamber, the more molecules there are to get rid of, and the harder the vacuum equipment has to work. And while most EBs are installed in industrial settings, many of which where ‘clean’ is a dirty word, there are a few simple things that can be done to save hours of downtime. Anytime the vacuum chamber isn’t sealed (e.g., a hole in the foil or open for maintenance purposes), the opening should be covered or nitrogen should be flowed to create positive pressure and prevent contamination from dust and adsorption of moisture. Personnel must wear lint-free or latex gloves when handling any vacuum system hardware. Tools used in vacuum system maintenance must be cleaned with isopropanol or acetone prior to use. Clean parts should be stored in plastic bags when not in use.
After maintenance, the vacuum must be regenerated, and the EB system conditioned – that is, accelerating voltage and current are brought up slowly, in small increments, until operational power is reached. During conditioning, it is common for internal sparking to occur as the high voltage finds contaminants and vaporizes them. Conditioning has a cleansing effect on the system (improving the vacuum level), but it is time-intensive to rely on it alone and, over time, damage left by extensive sparking can require additional maintenance. Figure 1 shows a close up of a cathode structure someone installed without wearing gloves. Body oils left on the structure caused electrical sparking at that location, and the fingerprints became etched into metal structure.
The health of a beam’s vacuum system routinely can be monitored through the vacuum pressure level, the cryo temperature and periodically checking the helium pressure in the cryo compressor. Keeping a log of these values also can establish trends that are helpful for troubleshooting issues. If the cryo temperature is remaining steady in its normal range but the vacuum level is poor, it may indicate a pinhole in the foil. Watch the vacuum level as the foil is sprayed with isopropanol; it will momentarily spike when the hole is located. If the cryo temperature steadily is increasing over a period of weeks or months, the charcoal-lined baffles of the cryopump might be getting full of captured molecules.
Defrosting the cryopump by bringing it up to room temperature cleans the baffles (similar to defrosting a freezer), and the vacuum system can work more efficiently after regeneration. If the cryo temperature still is high after regeneration and the helium pressure in the compressor is in normal range, it may be time for the cryopump and compressor to be rebuilt. Replacing or rebuilding the cryopump/compressor typically is required every three to five years.
In summary, the vacuum system is a crucial component in the operation of an electron beam. To call it the heart of the beam isn’t even hyperbole, since the cryopump quite literally makes a steady heartbeat sound during operation. Treat it well, keep it happy and for goodness’ sake don’t even look at a vacuum chamber without gloves on!
References
- Schissel, S. A Long-lived EB: Machine Function and Maintenance Part 1. UV+EB Tech. (1) 2022.
- Day, Christian, Basics and Applications of Cryopumps, CAS 2006 – CERN Accelerator School, Vacuum in Accelerators, Proceedings, 2006.
Sage Schissel, Ph.D.
Applications Specialist
PCT Ebeam and Integration LLC
sage.schissel@pctebi.com
