Elettra 2.0: The Vacuum System Design for a New Generation Storage Ring

Next-generation synchrotron facilities currently under development and construction are promising and delivering great advances in terms of machine design and research physics. At the same time, they are pushing the limits of technological solutions for various subsystems: to achieve the desired ultra-low electron beam emittance required in the next generation machines, the magnetic lattice has to adopt a compact design with reduced magnet apertures and other constraints like high heat load from synchrotron radiation, dynamic pressure inside vacuum chambers and chamber wall impedance are presenting unique challenges for the vacuum and engineering teams. At Elettra-Sincrotrone Trieste (Italy), the Elettra 2.0 project aims to develop a new-generation storage ring. Taking into consideration the above-mentioned constraints, we decided to adopt a new design of a vacuum chamber, while utilizing novel pumping solutions to overcome hugely reduced conductance compared to the current machine. Large sputter ion pumps (SIP) will be in the majority replaced by distributed non-evaporable getter (NEG) coatings and small NEG cartridges and SIP pumps. For the synchrotron radiation handling, due to the tight space constraints imposed by the compact lattice, the photon absorption will be managed jointly with discrete and distributed solutions: photon absorbers have been carefully studied for combining compact form and high power density loads, while key sections of the new storage ring will be water-cooled. Throughout the whole development phase, we were using Monte-Carlo simulation codes like SynRad and MolFlow+ as effective tools supporting the design of new vacuum chambers and photon absorbers. The current state of development for the Elettra 2.0 vacuum system, the challenges we faced and the solutions we adopted are presented here.


THE NEW 2.4 STORAGE RING
The new storage ring under Elettra 2.0 project is based on a multi-bend achromat (MBA) lattice design [1] with reverse bends and longitudinal gradient dipoles [2,3].The storage ring has a 12-fold symmetry, i.e. 12 equal achromats.Each achromat consists of 6 bending magnets divided into two arcs separated by one short 1.26 section while the achromats are separated by a long straight section of 5.1.Four of the short straight sections will be populated with an RF cavity (Fig. 1) while the rest will be used for short insertion devices or machine physics diagnostics.The "first" long straight section is dedicated to the injection.
1.1.Vacuum design of Elettra 2.0 Different types of vacuum chambers will form a new storage ring where each component will have to comply with strict requirements.Magnets define the maximum dimensions of the vacuum chambers and their relative magnetic permeability µ r , which should be lower than 1.01 for chambers, tubes and flanges.The vacuum system will be made out of different materials such as stainless steel (∼ 30%), aluminium (∼ 20%) and copper (∼ 50%) with more than 90% of the ring being coated with NEG film.As a result of optimization, a rhomboidal external cross-section of 30 x 20 is used inside multipole magnets, whereas the maximum height inside the bending magnets is 35.The majority of Cu vacuum components, i.e. all the rhomboidal vacuum chambers between bending magnets, will mostly be made from silver-bearing oxygenfree copper (Cu-OFS), due to its higher temperature of recrystallization compared to the more widely used oxygen-free high thermal conductivity copper (Cu-OFHC), while reducing electrical conductivity only by 1% [?].Copper Chrome Zirconium (CuCrZr) alloy will be used to fabricate effective photon absorbers (PAs) and CF flanges machined from only one body, without welding or brazing, as already proved in other similar accelerators (EBS-ESRF, SLS2-PSI) [4,5].For the long and narrow vacuum chambers of insertion devices, aluminium will be used for its ease of manufacturability and extrusion, while it is also completely non-magnetic.According to the Elettra experience with low gap vacuum chambers [6] about reducing initial outgassing after pump down / venting cycles, the best compromise is to have aluminium chambers, internally fully coated with NEG to reduce the photon-stimulated desorption (PSD) and the gas bremsstrahlung, and, hence, the photon doses to condition them.The bending magnet vacuum chamber (BMVC) and light exits can safely be made out of stainless steel (AISI 316L) if sufficient care is taken to shield the internal walls from excessive photon radiation and thus not subject them to high thermal loads.With the low cost of the material and ease of manufacturing compared to other vacuum materials, also the budget impact is reduced.In the e-beam direction, next is a BMVC with a PA (f) and a set of SIP55, Penning and pumping port.In the middle of the achromat, we will have either a fully reused RF cavity (g) from the current storage ring or a short ID chamber with an RF sectioning valve (not in the figure).The rest of the achromat is the repeated sequence of just described first half.
1.2.Pumping system of Elettra 2.0 NEG coating technology is now widely used in particle accelerators where very low vacuum conductance chambers are needed [7,8].Ti-V-Zr sputtering on metal surfaces allows for to reduce significantly the pressure under normal machine operation and, thus, reduces any vacuumrelated issues: beam lifetime is no more limited by electron scattering due to residual gases.The extensive use of NEG coating increases the machine impedance.To limit this effect, it is necessary to reduce the NEG thickness to make it more "transparent" to electromagnetic waves.For this reason, in Elettra 2.0 the average coating thickness is set to 0.5µm along the beam path up to 1.5µm elsewhere.
Due to the reduced inter-magnet space, conventional vacuum pumps are installed only where space permits or where they are strictly required.A 55 l/s SIP and a 400 l/s NEG pump are installed at each BMVC to increase the local pumping efficiency close to the PAs.A combined 500 l/s NEG + 20 l/s SIP pump substitutes the old 120 l/s SIP on each RF cavity, kept from the present Elettra storage ring.The straight sections, connecting BMVCs will be a simple, rhomboidal copper pipe internally coated with a non-evaporable getter acting as an effective, distributed pumping system and as a barrier for gases from the material below (stainless steel, copper and aluminium).This effect significantly reduces the conditioning time of the vacuum system.

Vacuum system simulations
Vacuum simulations are necessary to identify critical issues in the vacuum system design and to optimise individual components to achieve desired global results.A Monte Carlo simulator like "Molflow+", developed at CERN and freely distributed, is a great tool to complete all indicated tasks.Its sister code "SynRad+" is used to calculate flux and power distribution on internal surfaces caused by synchrotron radiation [9,10].The results are easily transferred to "Molflow+" to consider not only the thermal outgassing, the main contribution to the total pressure when the machine is at rest but also the PSD when the machine is operating.Results from Synrad simulations are used by Engineering Group to investigate more accurately the heat load on vacuum chambers and absorbers and calculate the equilibrium temperature of different components when a proper cooling system is applied.This study provides feedback to Vacuum Group to apply corrections to thermal outgassing calculations.In Fig. 2 we present the partial pressure PSD simulation results for the carbon monoxide (CO) and hydrogen (H 2 ) gases after 5Ah, 50Ah and 100Ah conditioning beam dose.

Photon absorbers
A great amount of attention has been focused on PAs in BMVCs with and without light exits.These components made out of copper chrome zirconium (CuCrZr), have to deal with high thermal load from synchrotron radiation (SR) irradiation.The way how SR is absorbed/reflected by PA greatly affects the vacuum conditions in the surrounding vacuum chamber.The initial PA design in a BMVC without a light exit was based on a saw-tooth absorbing edge at a grazing angle which performed reasonably well from a thermal management point of view.Its drawback was a high amount of reflected SR which gets absorbed by BMVC.This creates local hotspots on internal walls where higher PSD yield locally increases the pressure level.To prevent this we proposed the PA design modifications which included the shield that significantly reduced the amount of reflected SR being absorbed by vacuum chamber walls but rather PA itself (see Fig. 3) [11].With such design optimization and simulations, we were able to reduce the amount of locally absorbed SR on vacuum chamber walls which resulted in reduced PSD yield in those areas and optimise the effective pumping speed in the vicinity of the PA which eventually resulted in lower global pressure levels.

Sectioning valve mask
Every short straight section has a sectioning valve to divide the achromat into two shorter sections.Those valves are custom-made devices with delicate RF shielding and numerous internal components which all together contribute to the quite significant outgassing and consequently local pressure increase.Even though the upstream and downstream vacuum chambers will be NEG coated, low conductance limits the effective pumping speed across the valve area.From SynRad+ simulations results, we also observed an area of increased SR absorption on the internal part of the exit flange and RF shielding fingers.This not only poses a problem in terms of thermal management but also in terms of elevated outgassing from numerous components in a small volume.To mitigate this problem a short mask (see Fig. 4) has been proposed in front of the valve and the bellows with the intent of introducing a gradual transition from a defined rhomboidal shape to a reduced one and back rhomboidal since also bellows and valves utilize the same cross-section geometry to maintain continuity.This mask shields the valve components from reflected SR and virtually eliminates the previously observed

Conclusion
The vacuum system of the new storage ring under the Elettra 2.0 project is challenging, considering the small aperture of the vacuum chambers and tight space constraints.Using the Monte-Carlo simulation codes, we were able to study the pressure profile inside the storage ring vacuum chambers, along the electron beam trajectory, improving the vacuum performance of individual components.The NEG coating greatly contributes to lowering the average partial pressure across each achromat but uncoated, mainly stainless steel components like valves and bellows contribute locally to higher pressure peaks.The NEG coating technology, applied wherever possible, and the use of small NEG and ion pumps where high PSD occurs, look like the correct recipe to meet successfully the vacuum system requirements.

Figure 1 .
Figure1.The presented achromat starts with a long ID chamber (a) which has an RF sectioning valve together with SIP20 and Penning gauge (b).Following is the BMVC with light exit and PA (c) hosting SIP55, Penning gauge and pumping port.In the light exit axis, we have a photon mask with SIP75 (d) before the beam shutter and stopper configuration (e) with two SIP500 pumps.In the e-beam direction, next is a BMVC with a PA (f) and a set of SIP55, Penning and pumping port.In the middle of the achromat, we will have either a fully reused RF cavity (g) from the current storage ring or a short ID chamber with an RF sectioning valve (not in the figure).The rest of the achromat is the repeated sequence of just described first half.

Figure 2 .
Figure 2. PSD contribution of CO and H2 to the total pressure at 5Ah, 50Ah and 100Ah of machine conditioning.

Figure 3 .
Figure 3.The difference in the absorbed SR on BMVC wall (red oval) between one of the early PA prototypes (a) and the last proposed design (b).

14th
International Particle Accelerator Conference Journal of Physics: Conference Series 2687 (2024) 082030hotspot which, together with the addition of a 20 l/s SIP pump on the valve body, reduces the pressure levels.The mask doesn't significantly affect the RF impedance.

Figure 4 .
Figure 4. Simulation model of proposed symmetrical SR mask (red arrow) that protects RF bellows and section valve.Only one side of the restriction is visible in the model due to software rendering settings.