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ZAO non-evaporable getter (NEG) pumps for high-vacuum (HV) applications

A white paper from SAES Group – the world leader in the field of getters and it has pioneered this technology for more than 70 years

Introduction
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.

NEG characteristics and use in UHV systems
NEG materials are reactive metal alloys able to pump chemically-active gases (H2O, CO, CO2, O2, N2) through the formation of stable chemical bounds on their surface; this reaction generates chemical compounds (carbides, nitrides, oxides) on the getter surface, hence gases are permanently removed from the vacuum system. H2 also is very well pumped but, unlike the others, it cannot react and produce chemical compounds, whereas it can diffuse inside the bulk material and form a solid solution. Noble gases and CH4, called non-getterable gases, cannot instead be pumped by a NEG, as they cannot be dissociated into the surface of the NEG and react with the getter surface. The active surface of a NEG is obtained by thermal diffusion of the surface layer of contaminants into the bulk material. This so-called activation process has to be performed under vacuum, by heating the material at moderate temperature (450-550 °C) for 1 h. Afterwards, the NEG is able to sorb gases at room temperature with no need for electrical power; when the maximum surface capacity is reached, or after a venting, the pump must be reactivated and this activation-saturation cycle can be repeated more than 100 times during the typical working life of a NEG pump.


Many getter alloys for different applications have been released by SAES through the years; concerning NEG pumps, in particular, the most suitable materials have been, up to now, St707® (Zr-V-Fe) and St172™ (Zr-V-Fe + Zr powder). St707 dates back to the 1970s and it has been used since then for NEG pump in the form of compressed powder; although being very effective and still in use at several facilities, these compressed getters have some drawbacks:

‒ the surface area and the internal porosity are rather small;
‒ the getter is liable to release dust and it cannot be completely cleaned;
‒ relatively large amounts of material should be used to ensure adequate pumping performance.

St172 has been put on the market in the 1990s to specifically address these issues. St172 getter elements come in the form of disks, which are not compressed but sintered at high temperature under high vacuum conditions; hence, getter powders are tightly bound together in a single body, showing a dramatic decrease of dust emission while keeping a very large surface area and internal porosity. At the same time, these latter features allow the use of a relatively small amount of getter material, leading to a lesser release of gas during activation and, in general, to faster and cleaner pump-down and activation.

 

The power consumption is in turn much lower, thus power irradiated in the vacuum system is much less and temperature-sensitive devices are also safer.

Beyond UHV: 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. As a matter of fact, 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 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.

H2 is one of the main residual gases in HV and UHV systems, thus it progressively loads a getter during prolonged operation; the H2 equilibrium pressure of the getter will increase accordingly. Therefore, a getter should have an intrinsically low equilibrium pressure to be able to operate also under high gas load and at high pressure.
Figure 1 compares H2 equilibrium isotherms of St707 and ZAO. At 200 °C, H2 equilibrium pressure of ZAO is almost two orders of magnitude lower than that of St707. ZAO is therefore an optimal technical solution for systems working in HV conditions, where the getter must be operated at moderate temperature in order to enhance its sorption capacity; on the other side, traditional getter materials should be ruled out because their equilibrium pressures at 200 °C would be too high. ZAO is also an excellent option for room-temperature applications, where UHV-XHV conditions do not require to keep the pump hot.

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Figure 1 – H2 equilibrium isotherm of ZAO and St707.

The sorption capacity of a getter can be thus 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 2 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.

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Figure 2 – CO2 sorption curves of CapaciTorr HV200 during multiple sorption-reactivation cycles.

Finally, another important feature of ZAO is its ability to undergo multiple sorption cycles followed by venting, as shown in Figure 3. The getters have been repeatedly saturated with CO and vented with dry air. H2 and CO pumping speeds have been measured after each reactivation, in order to monitor any eventual performance loss. After 20 sorption-venting-reactivation cycles, ZAO pumping speed was still more than 90%.

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Figure 3 – Initial H2 and CO pumping speed of ZAO, as a function of the number of activation and air venting cycles.

Use of HV pumps for fast pump-down and load-lock systems
HV pumps have proven to be able to speed up the pump-down of unbaked vacuum systems, even when dealing with high gas loads. Recent tests carried out at the Legnaro National Laboratories, Italy (INFN-LNL) aimed at the achievement of a fast pump-down in a typical HV system, as well as at the capability to keep the vacuum level with no power (e.g., simulating a power cut). A CapaciTorr HV1600 was tested in an unbaked viton-sealed system (cf. Figure 4), equipped with a quadrupole mass spectrometer (QMS), together with a 30 l/s turbomolecular pump (TMP). Following a venting in air and the initial pump-down by primary pump, the valve to HV1600 was opened when pressure was 7-10-4 mbar and then it took 7 min only to reach 9-10-7 mbar. Moreover, after pumping the system for 5 days with the HV1600 at room temperature only (thus simulating a power cut-off), pressure was still 8.8-10-7 mbar. These results are a clear example of two of a HV NEG pump’s main features, namely, the ability to have a short recovery time and reduced dead times in case of maintenance work in a system, as well as to keep the experimental conditions and sensitive instruments safe in case of a long power outage.

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Figure 4 – Test-bench for fast pump-down tests with CapaciTorr HV1600 at INFN-LNL (Courtesy of Carlo Roncolato, INFN-LNL).

Similar noteworthy results have been obtained in the framework of the LCLS project at SLAC National Accelerator Laboratory, US (cf. Figure 5). A big monochromator chamber (3x1.2x0.5 m3), with a large surface area, was pumped by one 500 l/s TMP and two 600 l/s sputter-ion pumps (SIP). After 1 week of exposure to the atmosphere, 1-2 days of pump-down by TMP were required to attain 1-10-6 Torr, the maximum pressure at which SIPs can be switched on. In this case, adding a CapaciTorr HV1600 resulted in a much faster pump-down. The NEG pump was activated for 1 h once the system reached 5-10-4 Torr. Then, with the CapaciTorr HV1600 operating in warm conditions at 190 °C, it took 13 h only to achieve 1-10-6 Torr and to switch on SIPs. It is worth mentioning that, according to the system layout, the NEG pump had to be installed on a CF100 flange inside a 30 cm tube; hence, its pumping speed was severely reduced by the existing conductance to approximately one third to one half of the nominal one, depending on the gas.

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Figure 5 – Fast pump-down by CapaciTorr HV1600 in a large monochromator chamber at SLAC (Courtesy of SLAC hard X-ray department).

A third 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 4x4 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 reactivation1.