Vacuum performance of the beam screen

Vacuum performance of the beam screen R. Kersevan, CERN-TE-VSC-VSM https://indico.cern.ch/event/619380/timetable/#all.detailed

Contrary to the LHC, where the SR power is marginal, the FCC-hh is a powerful light source; Its SR flux spectrum at 50 TeV is comparable to that of the 3rd generation light source ANKA; • A 1.8 m long arc of bending magnet trajectory corresponds, on average, to 2 m of BS length in the arc sections of FCC-hh(filling factor), giving the correct 31.5 W/m average SR linear power density • SR photon flux: F (ph/s/mA) = a/2p · g · 4.1289E+14· KF

Left: SR flux spectra for FCC-hh vs beam energy (for 5, 10, 20, 30, 40, 50 TeV) for a 1 m-long arc of dipole trajectory Right: Same for the SR power spectra;

Critical energy Ecritof the SR photon spectrum for FCC-hh vs beam energy: it is shown that up to about 5 TeVEcrit stays below 4 eV LHC: Critical energy Ecrit= 43.8 eV (@7 TeV); Flux = 4.2e+16 ph/s/m (@500 mA) Power = 0.183 W/m “ “

SR photon flux: F (ph/s/mA) = a/2p · g· 4.1289E+14· KF SR photon power: P (W/mA) = a/2p · g4· 6.0344E-12/r · KP Left: Fractions of SR Flux (KF) and Power (KP) generated at photon energies higher than 4 eV Right: a) Ratio of fractions normalised to the 5 TeV value; b) Same multiplied by E(TeV)/5: apparent photon flux increase a) b) Choice of injection energy makes big impact on photon flux and therefore on vacuum conditioning and outgassing load

Beam Screen Design Evolution: from LHC to FCC The linear SR power density for a 7 TeV LHC is 0.183 W/m , while for the 50 TeV FCC-hhit is 35.2 W/m (1 m dipole trajectory, @500 mA). The total SR power to dissipateis 3.33 kW vs 2.32 MW, respectively(~1:700) The corresponding linear SR flux densities are 4.19E+16 and 1.46E+17 ph/s/m, respectively (~1:3.5) These facts have a huge impact on the design requirements of the beam screen (BS) and the cryogenic system of the accelerator In particular the cooling capillaries of the LHC (~ 4 mm ID, 2x) cannot be copied to the FCC-hh, due the practical impossibility to remove that amount of power (other heating effects notwithstanding) For reasons of thermodynamic efficiency and cost, the cryogenic group has also made clear that the working temperature of the BS of the FCC-hhcannot be the same as that of the present day LHC, which is between 5 and 20K A number of considerations (vacuum and cryogenics), push for a FCC-hh BS temperature range of 40~60 K (see V. Baglin presentation at the recent KIT workshop) Another important point in the definition of the BS geometry is the ID of the coils in the SC magnets: the baseline for FCC-hh now is 50 mm (see comment/conclusions below) Based on this, a major conceptual design change of the BS geometry has been proposed during the first FCC-week meeting in Washington D.C., and refined later several times (see C. Garion, this workshop) The presentation will only hint at the issue of e-cloud mitigation, for which there is an EuroCirCol-funded program which will look at possibilities of applying specific surface materials, textures, and coatings to decrease the SEY of the BS to sufficiently low values

Beam Screen Design Evolution: from LHC to FCC 2 rows of pumping slots here too! The 1-slot concept originally presented at the FCC Week in Washington has moved to a more practical version (C. Garion, CERN-TE-VSC, shown here above) During an FCC-hh meeting at CERN, it has been noticed that from the impedance point of view a symmetric double slot would be beneficial We have therefore designed and analysed such a configuration (above right) The second slot would also allow the removal of the 2 rows of pumping slots in the beam area required by the 1-slot version, thus decreasing further the impedance contribution

Beam Screen SR Ray-Tracing Analysis: Energy Ramping • SR ray tracing vs beam energy: • A 14.3m-long FCC-hh dipole followed by a 1.36 m-long straight drift with a bigger diameter have been modelled and simulated: a basic section of an 8 dipole ~200 m-long arc cell lattice (v.12, B. Harer, CERN, May 2015) Notice how going from E=50 TeV 30  10  5 the vertical height of the SR fan increases, and eventually starts hitting the internal part of the BS, above and below the 3.2 mm-high slot

Beam Screen Cross-Section Optimization Proposal Latest one, courtesy of J. Fernandez Topham, CIEMAT/CERN, March 2017 • Stiffeners (“ribs”): 1.5 mm thick; • Spacing: 70.25 mm; • Placed under external cold-sprayed Cu rings ;

Beam Screen Cross-Section Optimization Proposal Propagation of reflected SR photon with no ribs vs with ribs cases Power to Cold-Bore: ~ 0.9 W/dipole Power to Cold-Bore: 0.05 W/dipole @50 TeV, perfectly aligned beam (He cooling pipes are NOT modelled)

Beam Screen Vacuum Analysis • Assigning a properly scaled outgassing rate to each photon flux texture element allows a simulation of the gas density/pressure: Gas density specification: nuclear gas scattering (NS) lifetime > 100 hrs, which corresponds to ~ 1/5 the LHC spec of 1015 H2-equiv./m3. We aim therefore at a density lower than 2x1014H2-equiv./m3, after scaling for the energy-dependent cross-section sNSand the residual gas composition. Comparison of double-slot BS vs LHC-like one: It has been assumed that 8x rows of 8x1 mm2 pumping slots are present on the top/bottom face of the BS (~480/m) H2 pumping speed for 1 dipole at 50 K = 10600 l/s

https://indico.cern.ch/event/625274/

The foreseen ramp-up time from injection energy to 50 TeV is expected to be 20’, vs 120’ of this calculation; • The consequence could be a rapid rise (~E4) of the SR power which needs to be accomodated by the cryo-system: if the latter cannot cope timely with it then vacuum instabilities may ensue (e.g. rise of equilibrium vapour density with Temp.);

Conclusions, Part 1: Design History and Early Results Compared to the LHC, FCC-hh at 50 TeV is going to be a much more powerful source of SR For injection energies lower than ~ 5 TeV, the critical energy of the SR spectrum is lower than what is commonly accepted as a threshold for the generation of photoelectrons and/or photodesorption, i.e. 4 eV This could have an important implication for the injection phase of FCC-hh, as it would reduce the gas load and also the seeding of e-cloud effect; on the other hand an injection at less than 5 TeV would mean that no vacuum conditioning would take place during the (probably) long time devoted to the injection procedure. Only during energy ramping would the SR start becoming suddenly a source of gas and seed of e-cloud, and related conditioning/scrubbing could follow 3D models and extensive MC ray-tracing for both SR and vacuum show that a slotted BS is more effective at reducing photodesorption and improving pumping (optimization to be done) Mechanically and thermally a slotted solution for the BS seems to be feasible. A realistic discharge curve during a dipole quench at 50 TeV (15.9 T) has been used as an input to a self-consistent FE calculation of the induced eddy currents and Lorentz forces: all calculated deformations and stresses are within the limits known for the materials considered (co-laminated Cu with P506 stainless steel, like for LHC) In order to ease the impedance budget contribution created by the asymmetry of a single-slot BS, a double-slot version has been proposed and designed; in the framework of a Eurocircol collaboration, we are designing now a ~ 2m-long prototype to be tested at the ANKA light source, simulating the FCC spectrum, power and flux All models and calculations made so far assume a 50 mm coil ID for the SC magnets: according to an analysis made by E. Todesco (CERN), the additional capital cost of the SC coils scales with the square root of the ID opening, and therefore it is envisaged to increase this ID in case a bigger BS is required by injection-energy beam size considerations

Case 2: Nominal 3.2 mm slots: Effect of a vertical offsets of 1 or 2 mm From presentation at meeting of 26/11/2015 A misalignment of 2 mm increases the number of photons reflected towards the opposite side of the beam screen

Case 2: Nominal 3.2 mm slots: Effect of a vertical offsets of 1 or 2 mm From presentation at meeting of 26/11/2015 The average density more than doubles when the BS is vertically misaligned by 2 mm (Black Line: reference profile when photon scattering is not included)

Design Updates • Symmetrical design • Better impedance • Pumping holes hidden by the screen • Thermal copper coating on the outer side • Bigger pumping holes – no constraint for the distribution • Polygonal shape of the screen

Short prototype manufacturing Assembly and welding Copper coating

The new BS cross-section has a flat polygonal shape, which allows a better tolerance on the machining and positioning of the slot during assembly/welding of the system, which is becoming critical for the design of the 2 m-long test prototype scheduled to be installed on the ANKA light source ring in early 2017; • During an EuroCirColWP4 collaboration meeting it has been suggested to look at the possibility of having a wider (vertically) slot size, in order to better accommodate and intercept the SR photon fan in case of BS misalignment, or intentional orbit bumps (originator: D. Schulte intervention during the meeting, to allow switching from H to V crossing and reduce radiation damage to IT SC quadrupole coils); • The new BS geometry has been considered, assuming that the beam is offset vertically and deposits all of its SR power on the internal side of the BS:

Source: C. Garion, personal communication; Calculated temperature distribution for a vertically displaced beam, orbit parallel to the slot; ~ all of the SR power impinges on the BS internal wall; Total power is 504 W, ~31 W/m average; The cooling tubes’ temperature is kept constant at 40 K; The 300 mm-thick internal copper layer manages to efficiently transfer the impinging power towards the area in contact with the cooling tube; The temperature distribution is a-symmetric, but still within acceptable values

The new BS geometry has also been modelled with SYNRAD+ and Molflow+, to calculate the SR fan distribution and the related density profiles, as follows:

BPM and/or bellows conical tapers Geometry implemented in the simulations: 14.3 m-long dipole followed by ~1.4 m-long drift with bigger ID to install BPM and bellows, with two conical tapers;

Nominal Slot: (2.28 mm) • 3.0 mm slot: • 4.0 mm slot: • 5.0 mm slot: • “Power 2 CB”: power absorbed by cold-bore (leaking through the pumping slots); Units: Watts; • “Flux 2 BS”: photon flux absorbed (directly or scattered) onto the internal part of the beam screen; Units: ph/s • “Power @Taper”: power absorbed by taper at end of drift section; Units: Watts H2 density profiles vs slot height: fan as the slot height increases, the density diminishes thanks to a higher capture probability of the SR inside the slot; this reduces also the amount of radiation scattered off the BS internal wall (next to the slot); A qualitatively similar behaviour is expected for lower beam energies (to be checked);

H2 density profiles vs vertical beam orbit offset (or vertical angle): The hydrogen density increases quite substantially as the beam orbit is displaced vertically, or a 210 mrad vertical angle (corresponding to 3 mm along the 14.3 m-long dipole) is simulated on the orbit; Note that this refers to the older BS cross-section (rounded walls, not polygonal);

Conclusions Part 2: Sensitivity to Ramp-Up and Misalignents • A re-design of the double-slot beam screen has been carried out: it now has a polygonal shape (octagonal with rounded corners), and optimized pumping slots and reinforcing ribs positions; the profile of the tip of the “deflector” has also been modified (not discussed here); An even simpler cross-section, with exagonal shape, is under study, see slide 9; • A vertically-displaced beam at full power sending its SR fan directly onto the internal wall of the BS does not create an anomalous temperature distribution (see C. Garion); • Increasing the vertical height of the two symmetric longitudinal slots results in a better trapping of the SR fan and the related SR-induced desorption (~20% improvement between 2.28 mm (nominal) and 5.0 mm); (“nominal”== as designed so far); • A vertically-displaced beam (for the 2.28 mm slot, old BS • cross-section) results in higher SR-induced gas densities, • ~ 2.8x higher for a 5 mm V offset with respect to a • perfectly aligned orbit; • The effect on the impedance due to an increased slot size • must be calculated/estimated;

Synchrotron light distribution, including absorber: Power SYNRAD+ ray-tracing simulation code W/cm2 % of Total BM Power, 502W Ribs 32.9% Reflectors 29.3% Cold bore 0.007% Inner copper 6.56% End absorber 12.6% • 68% of the incident power is reflected on the reflector Drift space 8.1% Credit: I. Bellafont, ALBA, EuroCirCol Meeting, 7/11/2016, Barcelona

Synchrotron light distribution, including absorber: Powera END-RADIATION ABSORBER Present LHC Interconnect S. steel, welded to the BS and brazed to the Cu Possible absorber location 22mm long, 12.8o slope Beam direction Credit: I. Bellafont, ALBA, EuroCirCol Meeting, 7/11/2016, Barcelona  QUESTION: IS THIS DESIGN ACCEPTABLE FOR IMPEDANCE REASONS??? 

Synchrotron light distribution, including absorber: Power END-RADIATION ABSORBER W/cm2 • For an ideal beam, this end absorber can reduce the maximum power density more than 4 times on the slope area, reducing the possibility to have a thermomechanical failure 61 W (12.6% Tot.P.) 800W/cm2max` Beam direction • Redirecting as much SR as possible to the absorber, not only the cooling efficiency improves, the total outgassing is also decreased. It can be done changing the reflector’s shape or reducing its roughness Carnot efficiency of refrigerators Credit: I. Bellafont, ALBA, EuroCirCol Meeting, 7/11/2016, Barcelona

Synchrotron light distribution, including absorber: Flux Photons/s/cm2 % of Total BM Flux, 2.1E18 ph/s Ribs 29.6% Reflectors 21.4% Cold bore 0.27% Inner copper 4.9% End absorber 12.8% Drift space 5.9% • 78% of the incident flux is reflected by the reflector Credit: I. Bellafont, ALBA, EuroCirCol Meeting, 7/11/2016, Barcelona

Synchrotron light distribution, including absorber: Flux MISALIGNMENT STUDY (w/ newergeometry) W/cm2 • To perform an approach to the real geometry conditions, a misalignment study has been carried out, moving the beam 1mm upwards • The leaked power is slightly lower, while the molecular density is slightly higher • The radiation is directed forward with a narrower angle, increasing the power on the end absorber. • Stretching the sharp point of the reflector can produce the same effect in ideal conditions Aligned Misaligned Molecular density @72h 9.88·1013 H2/m3 9.93·1013H2/m3 Cold bore power 0.034 W 0.032 W 63 W 86 W Absorber power Inner copper power 3.3 W 3.7 W Ribs power 165 W 73 W Credit: I. Bellafont, ALBA, EuroCirCol Meeting, 7/11/2016, Barcelona

Gas density distribution: Molflow+ Simulations Main slot aperture Pumping Holes length Holes area coverage Reflector roughness 67.6 % 5mm outwards 10mm σ/T = 0.0032 Molecular density profile; only PSD H2 Drift space between dipoles 1.43m ~1.5 m-long Drift 14.3 m-long Dipole Temperatures Ribs 60K Main reflector 60K Cold bore 1.9K Inner copper 47K Beam direction End absorber 100K Bending magnet 14.3m Outgassing per BM Other areas 50K 4.97·10-6 mbar·l/s 9.88·1013 H2/m3 Average arc density PSD data Average arc pressure 6.56·10-10 mbar Bk. 316 SS H2 293K 4 keV Conditioning Time 72h Credit: I. Bellafont, ALBA, EuroCirCol Meeting, 7/11/2016, Barcelona  QUESTION: IS THIS DESIGN ACCEPTABLE FOR IMPEDANCE REASONS??? 

Last but not least: e-cloud mitigation • Several possibilities exist, namely: • laser ablation (LA) (*): • at COLDEX, now; • At ANKA, early ‘17 (room T only); • amorphous carbon (a-C): • at COLDEX (already done); • at Photon Factory, KEK (room T and LN2); • clearing electrodes: • at ANKA, late 2016-early 2017 (room T only); • Titanium nitride (TiN); • Data at LN2 or lower T not available; • CERN has no experience on this!  • HTS: • See S. Calatroni’s presentation; (*) Note: it has recently been determined that “LESS” surface patterns are NOT suitable for low-impedance devices; “Laser Ablation” (LA) implements a different pattern, more “impedance friendly” (see O. Malyshev presentation at recent KIT workshop)

Conclusions, Part 3: Impedance Issues • The mechanical design of the FCC-hh BS has evolved a lot since its inception (FCC Week in Washington D.C.); • Concerning the impedance contributions (RW and geometric), calculations and possibly measurements on prototypes need to be made as soon as possible; • We (the vacuum people) need to have a confirmation/validation of the proposed design for: • the height of the double longitudinal slot (presently set at 5 mm); • effect on impedance of a tilted or vertically-offset beam; • the geometry of the end-dipole photon absorber (a lumped narrow triangular cone place at the end of each dipole); • the dimensions (internal diameter) and shape of the interconnect area between arc dipoles (presently modelled as a conical taper, followed by cylindrical pipe with RF sliding fingers and BPM block, followed again by another conical taper to the following dipole); • the suitability of LHC-style RF fingers (as long as impedance is concerned); • the choice of materials for the BS: please note that ONLY the internal side (i.e. facing the beam) of the two-half BS will have 300 mm-thick co-laminated copper, while the “reflectors” tips are made of P506 steel; • the new proposed “extra-large” size of the pumping holes: we hope there will be no HOM trapping in the coaxial space where the ribs are placed, which could lead to power deposition onto the cold-bore; • the size and spacing of the ribs: presently they are EVENLY spaced, at ~ 70 mm distance

Vacuum performance of the beam screen