INVESTIGATION OF NON-EVAPORABLE GETTER FILMS

1. INVESTIGATION OF NON-EVAPORABLE GETTER FILMS O. B. Malyshev, K.J. Middleman, A. Hannah and S. Patel ASTeC Vacuum Science Group, STFC Daresbury Laboratory, UK J.S. Colligon, R. Valizadeh and V. Vishnyakov Department of Chemistry, Manchester Metropolitan University, UK Joint DL-RAL Accelerator Workshop

2. What are usual considerations for vacuum Required pressure P is defined by gas desorption Q in the vessel and effective pumping speed Seff. In a simple case it is P Q U (l/s) Pump, S (l/s) Thermal, photon, electron and ion stimulated desorption Joint DL-RAL Accelerator Workshop Oleg Malyshev

3. Usual accelerator vacuum chamber • Average pressure depends on vacuum conductance u of the beam vacuum chamber, which depends on the cross section and the length L Joint DL-RAL Accelerator Workshop Oleg Malyshev

4. Vacuum chamber with a distributed pump B • SIP in dipole and quadrupole magnetic field • Does not pump when magnets off • Requires HV supply • Getter strip in LEP at CERN • Does not pump Noble gases and CxHy • Requires activation Joint DL-RAL Accelerator Workshop Oleg Malyshev

5. NEG coated vacuum chamber • Non-Evaporable Getter (NEG) coating magnetron sputtered onto the inner walls recent innovation technique developed at CERN and is an attractive solution for many UHV applications. • One such application is in the vacuum systems of particle accelerators that have to be designed so as to provide sufficiently low pressure in the beam pipe during machine operation: • NEG film have to be optimised to exhibit low photon, electron and ion stimulated desorption yields and reduce secondary electron emission. • Pumping speed and capacity are important parameters for design. • There are a number of issues which are still not yet fully understood to engineer, to optimise and to use such coatings. Joint DL-RAL Accelerator Workshop Oleg Malyshev

6. Why Do We Want to Coat the Chambers with NEG? • Accelerator chambers have limited conductance of a few l/(sm). Especially in the insertion device chambers with gaps of ≤ 10mm. • Need 10-10 mbar to reduce Bremsstrahlung radiation  Linear pumping • Photons and charged particles will desorb electrons and molecules and impact the lifetime and stability of the beam Joint DL-RAL Accelerator Workshop Oleg Malyshev

7. Source of Gas in a Vacuum System Vacuum Subsurface Bulk layers Thermal ,photon, electron or ion stimulated desorption: • Molecules diffusing through the bulk material (mainly subsurface layers) of the vacuum chamber, entering the surface and desorbing from it • Molecules adsorbed on the surface (initially or after the air venting) and desorbing when vacuum chamber is pumped Outgassing rate depends on many factors: choice of material, cleaning procedure, pumping time, bombardment (irradiation) dose, etc... Joint DL-RAL Accelerator Workshop Oleg Malyshev

8. What NEG coating does Vacuum NEG Subsurface Bulk Coating Layers • A pure metal film ~1m thick without contaminants. • A barrier for molecules from the bulk of vacuum chamber. • A sorbing surface of entire vacuum chamber surface Joint DL-RAL Accelerator Workshop Oleg Malyshev

9. Stainless steel vs NEG coated vacuum chamber under SR Joint DL-RAL Accelerator Workshop Oleg Malyshev

10. Study and optimising the NEG coatings Collaboration between ASTeC and MMU was set-up • Surface science: • NEG film deposition (existing and new technologies) • NEG film surface analysis with SEM, XPS, RBS, etc. • Vacuum science: • Pumping properties evaluation • Gas dynamics modelling • Photon, electron, ion stimulated desorption • PEY and SEY • Application to accelerator design (coating geometry, pumping scheme, activation procedure, etc.) • Gas dynamic model in accelerator beam chamber Joint DL-RAL Accelerator Workshop Oleg Malyshev

11. Why Do We Want to Coat the Chambers with NEG? •   e-, M  ΔP , BS • e-  e- (SEY), M  ΔP  , and cause multipacting and e-cloud in e+& hadron machine • M+ ΔP  , stability • NEG coated surface will • reduce the surface desorption yields induced by photons , electrons e- and ions M+ • provide pumping which in turn minimizing the desorption • provide low SEY to suppress multipacting (which reduces electron stimulated desorption flux) and e-cloud Joint DL-RAL Accelerator Workshop Oleg Malyshev

12. NEG coating is a technology for UHV and XHV If pressure during activation is 10-9 mbar, then the amount of molecules hitting the wall is an equivalent of If pressure of NEG-sorbing gases (CO, CO2, H2O) during activation P > ~10-10 mbar => • the NEG film is continuously poisoning by these gases => • the activation is not full The CO capacity of the NEG coating is about 1 monolayer for CO and CO2 Joint DL-RAL Accelerator Workshop Oleg Malyshev

13. The conditions for NEG film activation To allow NEG film to be activated and not to be poisoned by residual gas molecules for the duration of the experiment: • The background pressure due to thermal desorption from uncoated part should be better than 10-11 mbar for CO, CO2, H2O, O2 and N2 • NEG film activation must be performed only after the bakeout of the uncoated parts of vacuum chamber, when desorption from uncoated parts of the test system is low • the temperature of the test chamber and the NEG coated sample should be maintained independently (separate heaters and air or water cooling). • The area and capacity of uncoated parts should be much smaller than NEG coated one to avoid NEG saturation during and after (re-)activation for the duration of time until the gas injection experiment started. • No ‘short pressure increase’ can be tolerated after NEG coating activation. • ex.: to switching on the gauge and the RGA, by opening or closing a valve, etc. Joint DL-RAL Accelerator Workshop Oleg Malyshev

14. Sample deposition Solenoid magnetron deposition Planar magnetron deposition Joint DL-RAL Accelerator Workshop Oleg Malyshev

15. Set-up for NEG pumping evaluation Sticking probability  is calculated from pressure measurements during gas injection using the results of TPMC: Joint DL-RAL Accelerator Workshop Oleg Malyshev

16. Usual activation procedure Joint DL-RAL Accelerator Workshop Oleg Malyshev

17. Reducing of CO, CO2 and H2O pressure in the open geometry set-up: • There is an area where temperature changes from the temperature of the NEG coated sample TNEG to the temperature of the rest of vacuum chamber TVC: • During the set-up bake-out this are is under-baked • During the NEG activation this area temperature is higher than TVC and outgases. It might be the main source of gas. Area with transitional temperature Cooling channel TNEG TVC Test sample with NEG coating Joint DL-RAL Accelerator Workshop Oleg Malyshev

18. ASTeC activation procedure Joint DL-RAL Accelerator Workshop Oleg Malyshev

19. NEG film density • Four TiZrV coated cup sample were prepared: • Cup 1: thin and columnar • Cup 2: two times thicker and columnar • Cup 3 & 4: dense and thick as Cup 2 Joint DL-RAL Accelerator Workshop Oleg Malyshev

20. Cap 3 Joint DL-RAL Accelerator Workshop Oleg Malyshev

21. Comparison of three samples - Thin - Thick - Dense Joint DL-RAL Accelerator Workshop Oleg Malyshev

22. SEM images of films Cup 1 & 2:Cup 3 and 4: columnar dense Joint DL-RAL Accelerator Workshop Oleg Malyshev

23. NEG film composition • Different combination of Ti, Zr, V and Hf • Same deposition parameters • Binary, ternary and quadruple alloys Joint DL-RAL Accelerator Workshop Oleg Malyshev

24. TiV binary alloy coating Joint DL-RAL Accelerator Workshop Oleg Malyshev

25. Triple alloy coating TiZrV TiHfV Joint DL-RAL Accelerator Workshop Oleg Malyshev

26. TiZrHfV quadruple alloy coating Joint DL-RAL Accelerator Workshop Oleg Malyshev

27. Binary alloy coatings: TiV, TiZr, ZrV… Joint DL-RAL Accelerator Workshop Oleg Malyshev

28. Ternary alloy coatings Joint DL-RAL Accelerator Workshop Oleg Malyshev

29. TiZrHfV quadruple alloy coating Joint DL-RAL Accelerator Workshop Oleg Malyshev

30. Application to ILC • Pressure along the arc: • inside an aluminium tube: • Bakeout at 220C • A pump with 200 l/s every 5 m • H2, CO and CO2 • Inside NEG coated tube • Activation at 160-180 C • A pump with 20 l/s every 30-40 m • H2 and CH4 Joint DL-RAL Accelerator Workshop Oleg Malyshev

31. Conclusions • New bakeout/activation procedure developed at ASTeC to minimise the NEG film poisoning from uncoated parts. • Measured results don’t depend on injected gas flow rate. • Columnar NEG structure, required for higher pumping speed and sorption capacity, is formed at higher pressure. • Reduced pumping speed and sorption capacity measured for dense films deposited either at lower pressures or by pulsed sputtering. • Larger number of element in the target allows reducing the grain size of the film which, in turn, increase the molecule diffusion along grain boundaries and led to a lower activation temperature. • All together allows engineering the films with different properties Joint DL-RAL Accelerator Workshop Oleg Malyshev

32. Future investigations • New understanding of physics and chemistry of the NEG coating allows to the next stage: • engineering of NEG coating with necessary properties • Study dynamic properties of new coatings such as: • NEG as low dynamic outgassing • NEG as low SEY coating • Combination of both • Accumulated experience allows • Designing vacuum systems of new accelerators considering NEG coating and applying it where it is beneficial (DLS, ILC, NLS, FAIR, CLIC…) • Using the NEG coating in the appropriate way. Joint DL-RAL Accelerator Workshop Oleg Malyshev