SAES® Group is the world leader in the field of getters and it has pioneered this technology for more than 70 years. Non-evaporable getter (NEG) pumps, in particular, are one of the company’s core businesses and SAES has steadily contributed to the growth of NEG technology over the years, by introducing a wide range of getter alloys for different applications, as well as by innovating pumps’ design, manufacturing processes and testing techniques.
Beyond UHV: the new ZAO® alloy for HV applications
While NEG pumps are perfectly suitable for usage in the ultra-high and extremely-high vacuum range (UHV-XHV), one of their main limitations has always been the possibility to effectively use them in the high vacuum (HV) range, corresponding to 10–6–10–9 mbar. The main residual gas in typical UHV systems is H2, for which a NEG pump can provide a high pumping speed and a virtually infinite sorption capacity prior to requiring a reactivation. On the contrary, HV systems are often unbaked and viton-sealed, thus the gas loads and the gas composition are different: H2 is no more the main residual gas and a key role is played by H2O, N2, and O2 also, as well as by CO and CO2. In these conditions, the single-run sorption capacity of a NEG pump is often too limited to allow an efficient employment with no need for frequent reactivation.
The latest step forward made by SAES in NEG technology is the recent development of ZAO® alloy (Zr-V-Ti-Al), which gives the possibility to overcome this intrinsic limitation in the usage of NEG pumps.
As a matter of fact, ZAO pumps can work either at room temperature or in warm conditions (150–200 °C), enabling their adoption not only in the usual UHV-XHV range but also in HV systems thanks to the following features:
• a lower H2 equilibrium pressure;
• lower H2 emission during the activation;
• a larger capacity for all the active gases: by keeping the NEG cartridge at the indicated moderate temperatures, more than 20 sorption cycles in HV conditions are possible;
• better mechanical properties: ZAO disks are intrinsically more robust than St172 ones;
• a higher H2 embrittlement limit;
NEG pumps made of ZAO elements—such as CapaciTorr® HV200—can be continuously operated at pressures up to 10–7 mbar, as they are able to efficiently deal with large air leaks and/or big amounts of carbon contaminants while ensuring a very good mechanical stability over time.
The sorption capacity of a getter can be enhanced by operating it at moderate temperature (e.g., ~200 °C) and moderate power (e.g., 10–50 W, depending on the model), which promotes gas diffusion from the surface to the bulk. However, high-load sorption cycles might be detrimental for traditional St707®-based getter alloys, leading to a progressive efficiency loss in the getter reactivation, as sorbed gases keep accumulating inside the bulk.
Figure 1 is an example of ZAO’s ability to continuously work at moderate temperature in HV conditions. Ten CO2 sorption cycles have been made at 1–10–6 Torr with a CapaciTorr HV200 pump working at 200 °C. Each cycle at such pressure corresponds to 1 year of operation at 5–10–8 Torr. The pumping performances have been substantially the same all along the series, without any substantial performance variation between the first and the tenth cycles. This demonstrates how ZAO is able to withstand several reactivation cycles under high gas loads while keeping its performance close to the nominal one, thanks to its higher carbon, oxygen, and nitrogen diffusivity to the bulk.
In addition, H2 could be partially released from standard getter alloys (such as St172 and St707) working at 200 °C, whereas ZAO—having an intrinsically-lower equilibrium pressure—does not release H2 while operating at 200 °C.
A practical example is given by the Pixel detector of CMS at CERN, where 16 vacuum insulated transfer lines (each ~17 m long) with liquid CO2 are used. The lines are grouped 4×4 in 4 sectors with 4 vacuum manifolds, originally pumped by a turbomolecular pump. While the goal was to keep the overall pressure below 10–4 mbar, the presence of a huge magnetic field in the manifold region did not allow to keep either turbomolecular or sputter-ion pumps permanently running. 4 CapaciTorr HV200 replaced the turbomolecular pumps, succeeded in keeping the Pressure in the 10–7–10–8 mbar range for months with no need for any getter reactivation .
SAES®, ZAO®, St707®, and CapaciTorr® are registered trademarks of SAES Group.
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