To meet the demanding UHV requirements of accelerators and beamlines, the vacuum team utilizes a series of techniques and tools which allow to achieve, keep and monitor the extremely low pressures needed inside our vacuum systems.
Membrane and multi-stage roots pumps (roughing pumps) are usually used to reduce the pressure from atmospheric values down to 10-1 – 10-3 mbar (low-mid vacuum); below this value, turbomolecular pumps (TMPs) are deployed to further bring the pressure down to the high vacuum (HV) regime (10-3 – 10-8 mbar) and till the upper region of ultra-high vacuum (10-9 mbar and below).
For practical reasons, roughing pumps and TMPs are often combined together into mobile carts (pumping stations) that allow the vacuum team to start a pump-down procedure whenever and wherever it is required and quickly bring a system from atmospheric pressure to the HV or high UHV pressure regimes.
Starting from pressures in the 10-7 mbar range, turbomolecular pumps are typically paired with, or substituted by, other types of pumps which are more effective than mechanical pumps at very low pressures, usually classified as capture pumps.
At MAX IV, the most widely used pumps of this type are ion pumps, which use electric and magnetic fields to ionize residual gas molecules and then trap them inside the pumps’ internal surfaces, usually made of a highly reactive metal such as titanium. Other capture pumps used in our accelerators are the titanium sublimation pumps (TSPs), which, as the name suggests, can deposits a layer of titanium onto the inner walls of a vacuum chamber by sublimating this material from a titanium filament heated to high temperature by an electrical current. Owning to the high reactivity of the metal, the deposited film can capture molecules from the vacuum volume, thus transforming the walls of the chamber itself into a pumping device.
Other very effective types of capture pumps are the so called non-evaporable getters (NEGs), which are used inside our storage rings and some beamlines. NEGs exploit the high chemical reactivity of special metal alloys that, once activated via a thermal process, can bond with most gaseous molecules that happen to hit the alloy’s surface. The captured molecules then slowly dissolve into the bulk of the material, thus allowing the surface to capture more molecules, until saturation of the material is achieved after some time (usually years).
NEGs can be used in the form of cartridges, strips, or even as a coating layer covering the inner surface of a vacuum chamber. They allow to achieve extremely low pressures, up to 10-11, 10-12 mbar, without the use of any energy source during operation after being activated. Upon saturation, the pumping capabilities of NEGs can be regenerated by the means of thermal reactivation of the surface.
MAX IV’s 3 GeV storage ring has vacuum chambers that are coated with NEG thin film nearly all along the electron path (~528 m lengthwise).
Pressure reading and residual gas analysis
The vacuum in all sections of our vacuum systems is constantly monitored so that the laboratory’s automated machine protection system and the vacuum team can readily react to anomalies or problems such as pressure rises or the presence of unwanted gas species in our vacuum chambers. For this purpose, more than 60 pressure gauges, typically of the Penning (cold cathode) type, are scattered across the length of our accelerators, and over 100 gauges of the Penning or full-range type (usually made of a Pirani + Penning combination) are additionally installed on the beamlines. These pressure gauges are supported by additional readings provided by the controllers of the ion pumps. For the reading of very low pressures in specific points, extractor gauges (hot cathode type) are also employed.
Residual gas analyzers (RGAs) are installed in strategic sections of the accelerators and the beamlines to help monitoring the gas species present inside the vacuum system after reaching a certain target pressure. These devices can provide mass spectra that allow to identify the type and relative abundance of the residual gas molecules and, upon calibration, give an estimate of the partial pressure of each species.