The RF shield in the LHCb upgrade 2

1. The RF shield in the LHCb upgrade 2 Back to basics: why do we have the RF shield ? Requirements (from LHCb and LHC) So what ? And now ? Lessons learned, what more do we know compared to X years ago ? How bold can we be ? The way forward (IMHO) What needs to or could be investigated See also NicoloBiancacci, Elba workshop, May 2017

2. The reason for a RF box • detectors • cooled to <0 C • sensitive to RF • mirror image • sensitive to pipe geometry and resistivity Add RF shield • beam • in vacuum • high frequency fields • high intensity • transverse size changes 1/E => aperture requirements change with E minimize material => vacuum on both sides

3. RF shield, RF box, ... • We need a transparent box that protects the VELO without affecting the beams and with no material in the acceptance • Challenges: • must be “zero mass” • must be electrically conducting • must not grossly increase the LHC impedance • must be mechanically stable • must not interfere with silicon sensors (mechanically, electrically) • must not cause beam-induced emission/desorption effects • must be safe (interlocks!) • must be movable • must accommodate silicon detector geometry (complex! Overlaps!) • must withstand heavy irradiation • must withstand temperature cycles (about -30 to 160 oC) • etc...

4. RF box: current VELO Upstream / Downstreamwakefieldsuppressor

5. All done ! • Pressed/deformed Al foils (300 um + 500 um) of very complex shape (corrugations, anticorrugations), welded together • Coated detector side of foil with insulator (Torlon) • Coated beam side with NEG, activated • Connected to beam pipe with Au-coated 70 um thick CuBewakefield suppressors, deformable • Checked impedance with MAFIA/ABCI... • Checked dynamic vacuum / multipacting effects with VASCO & ... • Performed extensive RF field measurements with wire-method • Checked deformation versus differential pressure • Measured overall shape • etc... But it wasn't easy. It took several years from concept to final boxes.

6. RF box: LHCb Upgrade 1 • New production method (NIKHEF/VU) • Mill the shape out of a block (5-axis precision milling machine) • Deckel-Maho DMF 260 FD • More flexibility to change the shape • Prototype shown here (one of two) • Wall thickness 0.5 mm / 0.25 mm • Leak tightness: good • Thickness homogeneity: improving! A lot of info on https://wiki.nikhef.nl/lhcb/Upgrade/RFFoil http://www.nikhef.nl/pub/departments/mt/projects/lhcb-vertex/ http://www.nikhef.nl/pub/departments/mt/projects/lhcb-vertex/production/UpgradeRFbox/ 0.25mm 0.5mm

7. RF box: What’s in the balance ? Detector performance: • Impact parameterresolution • multscatt and extrap • Secondary interactions, bkg RF: • EMI……. • Wake field / impedanceeffects • Affect beamoperation • Heat dissipation Stillvalidparameters?

8. RF box: What’s in the balance ? Environmental: • Vacuum: => dynamic vacuum (withbeampresent!) • Affects beamoperation • Potentially, also affects the sensors (ion/e-bombardment) • Irradiation • Temperature (contraction) • Magneticfield ?? (couldbeproposed for Upgrade 2 ?) Operational: • The aperture must becomplywith Machine protection outside and during STABLE BEAMS • Movable detector seemsunavoidable • Machine and detector protection: failure scenarios must beconsidered

9. RF box: enters Impact Parameter resolution • Reducing the mass before the first hit will improve the IP resolution • Reducing the VELO distance of approach to the luminous region will make the Impact Parameter resolution better ! … Measurement errors (detector resolution) 12 = z2 - z1 Extrapolation error Multiple scattering error Z1 Z2 Multiple scattering Impact parameter (IP) Impact parameter (IP) Deviated track True origin vertex True origin vertex

10. RF box: enters Impact Parameter resolution Aperture reduction (5.5 to 3.5mm) Shouldn’t we plot IP2 vs pT-2 ? Thinning the central backbone We know thatthinningawayfrom the backbonebringsless see note LHCb-ANA-2014-35 by T.Head, T.Ketel, D.Vieira

11. Aperture: how close can we come ? VELO closed, axial symmetric Orthogonal plane: separation IP Look at beam in transverse plane at z=d

12. Aperture: how close can we come ? Pointlike beams Zero separation Zero Xing angle Pointlike beams Zero separation With Xing angle Beam position at z=d Extended beams With separation With Xing angle Pointlike beams With separation With Xing angle R

13. Aperture: putting it together “VELO aperture considerations for the LHCb Upgrade”, R,. Appleby et al. CERN-LHCb-PUB-2012-018 ; CERN-ATS-Note-2012-101 • The constraint due to and is directly depending on: • LHCb operational luminosity and the reserve factor (Lvirtual/LLHCb) • larger luminosity => smaller beam sizes and/or smaller beam separation • Bunch charge • larger bunch charge => larger beam size and/or more beam separation • But (interestingly) it is not directly depending on: • Beam energy • Beam emittance and/or beta star Although these affect the choice of crossing angle!

14. Aperture: RF foil tolerances “Determination of the aperture of the LHCb VELO RF foil”, MFL, T. Latham, C. Wallace, LHCb-PUB-2014-012. Measurement on the current VELO RF boxes from secondary interactions

15. Aperture: positioning and geo of VELO halves “Performance of the LHCb Vertex”, LHCb VELO group, Locator2014 JINST 9 P09007, CERN-LHCb-DP-2014-001 => Message to our LHC colleagues: We do not have any relevant aperture uncertainty on the position and geometry of the VELO halves! +/-10um

16. Putting it together “VELO aperture considerations for the LHCb Upgrade”, R,. Appleby et al. CERN-LHCb-PUB-2012-018 ; CERN-ATS-Note-2012-101 • The constraint due to and is directly depending on: • LHCb operational luminosity and the reserve factor (Lvirtual/LLHCb) • larger luminosity => smaller beam sizes and/or smaller beam separation • Bunch charge • larger bunch charge => larger beam size and/or more beam separation • But (interestingly) it is not directly depending on: • Beam energy • Beam emittance and/or beta star Although these affect the choice of crossing angle! => Message to our LHC colleagues: We do not have any relevant aperture uncertainty on the position and geometry of the VELO halves! => Message to our LHC colleagues: We do not have any relevant aperture uncertainty on the position and geometry of the VELO halves! if the Tturns out to be systematically larger than foreseen, we would ask for a smaller  ...

17. Wake fields and impedance lhcb-99-041, 99-043, 99-044, 2001-082, N. A. van Bakel, J.F.J. van den Brand, MFL VELO as now: wedideverythingourselves • Calculatefrequency modes of VELO structure • Calculateimpedance of corrugated RF foil • Estimateeffect of shieldingstrips, of depths of corrugations near the beam, etc • Made a mock-up withwire to performmeasurements (Nikhef, F. Kroes)

18. Wake fields and impedance VELO upgrade 1: • done by professional, Olga Zagorodnova (DESY) & BenoîtSalvant (BE-ABP) • Computing power (and codes) have vastlyimproved over 10 years and still…

19. EMI lhcb-2001-081, N. A. van Bakel, MFL • In ~2000: itwas tough to estimate how the spurious RF fieldswould affect the sensors • Start from ~GV/m near the beam, end up with <1V/m at the sensorsor asics • Complicated RF spectrum (depends on filling pattern structure, bunchlength, …) • Hard to mimick the LHC spectrum and power in the lab • Hard to simulate the complex RF spectrum and itspenetrationthroughthin foils and itseffect on complicatedsilicon IC • Weconcludedthat an Al foil of 150um wouldbesafe • But isit an overkill ? What about now ?

20. VELO protection Principle of the “last line of defense”: the Beam Conditions Monitor BIC (beam dump) If beam moves to VELO (or vice versa), halo particle interactions in VELO material will be detected by BCM (both sides) which will trigger a beam dump (beam dumped within 1 turn, 88us). Question: if the intense beam approaches the VELO RF foil, will the halo interaction trigger a beam dump (via BCM) before the impedance heat dissipation damages the foil ? LHCbUpgrade: more intense and smaller beams!! BCM signal scales with rate of particles traversing the diamond sensors BCM (diamond) VELO RF foil Beam

21. VELO protection Halo particle interaction rate vs power loss Assuming the tails are at least as large as Gaussian, BCM will dump if distance between wall and beam passes threshold d < 5  Hence, as long as  > 50um, the BCM protection will dump before the power reaches ~40W (150W) at 1.15e11 p (2.2e11 p). For smaller beams it becomes somewhat worrying. =50um Possible mitigation: add a “shadowing” mini-scraper upstream of VELO boxes BCM BCM Would move with the VELO and define the VELO aperture VELO RF foil Beams

22. Dynamic vacuum effects • Static vacuum is not such an issue • LHC (beam lifetime) and LHCb (bkg) could easily survive with a stable pressure of up to 10-7 mbar (or more ?), while VELO easily can reach below 10-8 mbar. • But beams can affect the pressure ! • electron multipacting: free electrons are accelerated by the beam RF fields, hit the surfaces and release more electrons (or ions/atoms) • Ion bombardment: gas atoms are ionized and kicked to the surfaces, thus releasing more atoms • Synchrotron radiation also releases particles from the surfaces • These effects, under certain circumstances, can run away • See e.g. in “LHC vacuum system”, O. Gröbner, CERN-OPEN-2000-288 CAS - CERN Accelerator School : Vacuum Technology, Snekersten, Denmark, 28 May - 3 Jun 1999, pp.291-306, https://cdsweb.cern.ch/record/455985. • Vacuum group can simulate these effects

23. Dynamic vacuum effects A. Rossi, N. Hilleret, LHC Project Report 674 • Calculate critical current Icrit, defined as current at which gas pressure diverges. In LHC experiments (2 beams in the same chamber), it must be verified that: Icrit/ 2> 2 × 0.85A (2 × 1.6A for HL-LHC)

24. Dynamic vacuum effects Beamenergy Beamintensities VELO pressure

25. Dynamic vacuum effects B. Henrist, N. Hilleret, C. Scheuerlein, M. Taborelli, CERN EST/2000-007 (SM) • SecondaryElectron Yield • In LHCb, NEG isexposed to tinyamount of air (spuriousleaks), if VELO detector is open to air • M. Taborelli's team will check effect of air exposure in smallquantities • Will defineourspecs for box leak-tightness 1L = 10-6 Torr  s

26. Dynamic vacuum effects We are Somewherebetweenthesetwo OK

27. Differential pressure Detector vacuum ~10-4…-7 mbar • Current VELO (NB: Upgrade 2 has additionnaltertiary vacuum) • Two vacuum volumes «separated» by thin RF box • DeltaP < 10mbar (plastic deformations at ~O(100) mbar) • Protection by DeltaPsensors and microswitches and valve • Onlycriticalduringevacuating or venting to 1bar withultrapureneon (required for NEG) • Future: avoidtwoseparated volumes ? Ambient air 1 bar ∆P<10mbar Beam vacuum ~10-7…-10 mbar Low SEY coating (NEG now, C in future ?)

28. Differential pressure Detector vacuum ~10-7 mbar • A) Same vacuum ? • Depends how «clean» the modules + cables can be made • B) Differential pumping between detector and beam vacuum ? • Never truly separated, both have their powerful pumps • Or separatable by a valve, only for degassing ? (warm up) • Sacrificial window to protect box • Accept saturation of NEGs (no pumping capacity, but good SEY) • Possibility to regenerate (bake out) more easily than now ? Ambient air 1 bar Beam vacuum ~10-7…-10 mbar Low SEY coating (NEG now, C in future ?)

29. So what ? • Can we get rid of the RF box altogether ? • We can at least imagine to drop the «vacuum barrier» functionality. The sensors can be designed such that they sit directly in the beam vacuum. • Proper choice of materials, put enough pumping speed for static vacuum • But what about the dynamic vacuum and RF ? • The surfaces exposed to the beam bombardment must have low secondary electron yield, low ion desorption yield, … • The detector must be insensitive to (or protected against) high frequency electromagnetic fields • The mirror charge must be allowed to flow smoothly from entry to exit of VELO • New technologies ? • Al thinning/polishing • Carbon fibre with coating ? • 3D print a metallic mesh ? • RF screen as part of modules ? (attached to modules) • What IP resolution performance do we really need ?

30. One bigadvantage! • We can test a prototype, just upstream of the VELO • A valve will allow us installing/removing equipment without breaking LHCb/VELO beam vacuum • The best way to check that EMI will not be an issue is to test it in the LHC (End of Run3 ? EarlyRun 4?)

31. Summary and conclusions • The RF “box” is crucial in the VELO design • Dominates multiple scattering effects on resolution • It is our interface to the powerful LHC beams • My feeling: vacuum barrierfunctionalitycanbedropped • If proper design of detectors • Remain the RF and dynamic vacuum aspects => innovative solutions ? • Main worry: EMI, how to check / validate the design ? • Modern simulation codes ? • Test in situ ! ?? (upstream of VELO) • Just likeweaskourselveswhetherweshouldmove the sensorsawaybecause of radiation, weshouldaskourselveshow badlyweneed(or not) to make a thinner RF shield.