LHCb Week, CERN, may ‘00M. Ferro-Luzzi LHCb Vertex Detector System: An Update Review of TP design mechanics, wake field suppression, vacuum system, cooling

LHCb Week, CERN, may ‘00M. Ferro-Luzzi

LHCb Vertex Detector System:An Update

• Review of TP design mechanics, wake field suppression,

vacuum system, cooling system.

• Difficulties with this design center frame stiffness, accessibility, ...

• New requirements more cooling power,

intermediate Y positions of two halves

• An (unfinished) improved design vacuum tank and flange, center frame

• Foil thickness what should we start with ?

• Milestones


Mechanics: “TP” design

Side flange

Bending hinges

Detector support and cooling

Bellows (22000signal wires)

Support frame

Si detector

moves by 30 mmonly two positions:open or closed !!

See LHCb 99-042/VELOtop half = bottom half


Marco Kraan, Martin DoetsFEA for TP center frame

See http://www.nikhef.nl/pub/departments/mt/projects/lhcb-vertex/calculations/centerframe/

The two detector support frames (for the Si detectors halves) are mounted on a “center frame” standing on three legs.

FEA: deflection on center frame when loaded should be less than 0.1 mm.

Materials used: Aluminum/Stainless Steel


FEA Model 1

Starting point.

The maximum deflection is 0.7 mm


FEA Model 2To decrease the deflection an open rib is added to the frame.

The maximum deflection is 0.4 mm


FEA Model 3

Investigate deflection with a solid rib.

The maximum deflection is 0.24 mm.


FEA Model 4

Stiffen frame withribs under frame.

The maximum deflection is 0.38 mm.


FEA Model 5

Decrease torsion with hollow box added at the rear of the frame and “more solid” ribs.

The maximum deflection is: 0.054 mm for Al, 0.018 mm for SS.

Conclusion:possible, but not very handy...


Wake field suppressionPerformed set of MAFIA calculations

Resonant effects frequency domain (“E3”)Short range effects time domain (“T3”)

1) complete VD with/without strip shielding(600000 mesh points, problems with disk space and CPU time)

2) reduced VD model (no strip shielding)a) position open/closedb) no strips, vary (reduce) cavity depthc) various designs

See LHCb 99-041/VELO LHCb 99-043/VELO LHCb 99-044/VELO


Reduced VD model

d

Without strip shielding:at what “depth” d of cavities does RF couplingbecome acceptable ?d varied: 160, 20, 5 mm


Varying cavity “depth”: results

E3 calculations show that cavity depth d of less than about 20 mm results in acceptable resonant losses(without strip shielding)

fine tune design

Selected modes (those whichexhibit significant losses):

Almost ok

ok


Why not use wake field suppressor strips ?• design very difficult (we could not find, so far, a satisfactory solution) Remember: four 130 cm long, very thin strips which must

(a) be connected to exit foil (b) put under tension (thermal expansion and inward motion of exit foil, few mm)(c) be retractable(d) not touch the detector boxes

• even assuming unrealistic† thickness of 10 m (stainless steel), then multiple scattering corresponds to about (3.4 - 83) 10 m 8.9/1.76 170 - 4200 m of “perpendicular” Al foil

†Due to resistive overheating. Compare to HERA-B case: current Iherab = Ilhcb /10 (and only one beam), resistance Rherab = Rlhcb 10m/5m 7mm/12.7mm Rlhcb 1.1 heat load Plhcb = Pherab 102 2 / 1.1 180 (goes mostly to Si modules!)and HERA-B observed already a strip temperature increase.

• have to come closer to the beams than the detector boxes r 4 mm more background, reduced efficiencies

• introduce a risk: what if the aperture requirements at startup are worse than currently assumed ? see e.g. HERA-B .

• WF calculations showed that, in “open” position, strips do not shield sufficiently (few kW of WF losses)• what if one strip breaks at runtime? (and, in fact, strips did break at HERA-B)


Foil design options

Variant AVariant B


Foil design options (continued)

Away from beam region(both variants A and B)

In beam region: variant A:12 mm clearance variant B: tapering as shown


Various foil designs: E3 results

E3 results: ok for both variants, with detectors closed and open. But... see T3 results.


Various foil designs: T3 results

T3 results: ok for position closed, not ok for open, but probably due to the fact that beam sees vacuum tank.Further calculations will be carried out with shielding around the detector boxes (hide tank from beam).

Longitudinal loss factor


Wake field suppressionCriterium: losses < 100 W (time domain, or any resonant mode)

Conclusions:1) complete VD with/without strip shielding

“deep” cavities require strip shielding2) reduced VD model (no strip shielding)

a) position open/closedrequires shielding on sides of enclosures

b) reduce cavity depthok for depth of < 20 mm

c) various designs ok, analog to b)

tough with TP design!no space, no access!


Si detector encapsulations

Current solution: Al encapsulation.Try: Cu foil, Cu on Al, low-emission coatings (Ti, NEG), ...

• Test setup needed !• System cannot (?) be baked out in-situ !• No valve to NEG-coated chamber !

300 m

100 m

Design must take into account: • multiple scattering• wake field effects • vacuum compatibility


2z 15 cmQb 2 × 10-8 C

r 6 mm

Detector box

E < 1V/m (?)

Peak E-field on inside surface for moving bunch:

Epeak = 3.4 × 109 V/m

E-field for fundamental harmonic (n=1, 40 MHz):

En=1 Epeak / 21 1.6 × 108 V/m

For condition E < 1 V/m we need skin attenuation of about 108. In fact, about 19 skin depths !!(Al, 40 MHz) 13.3 m foil thickness > 250 m !!

RF field attenuation

· Qb

23/2 0 · z ·r

For non-cylindrical complex geometry, EM fields will be even higher.

7500


Vacuum constraints

LHC:• beam life time: static density of 10-7 mbar 2 m (H2 300K) 0.01 % of LHC limit for integrated density ( 2.7 106 cm 1.6 109 molecules/cm3 )

• beam stability: dynamic effects must be taken into account

LHCb:• 10-7 mbar 1.2 m (H2 300K) 1.5 % of LHCb nominal luminosity

Difficult to achieve with silicon detectors, electronics and signal wires directly in LHC vacuum ! differential pumping.

(rough!)

See LHCb 99-045/VELO


Static pressure in VDConsider outgassing by: assuming outgassing rates of:

(mbar • l • s-1 • cm-2)

11 m2 Kapton (signal wires, pumped 40 hours) 10-7 H2O 2.3 m2 Al housing (per half) 10-10 H2

1.5 m2 bellows (per half) 10-9 H2

8 m2 SS vessel 10-10 H2

Pumps in detector volume: 140 l/s (per half) H2OPumps in tank: 4000 l/s H2

Bypass tube: 200 mm 4 mm pumped in the middle.

Calculate using a static flow model.Result: 1•10-4 mbar in detector volume

1•10-8 mbar in VD tank2•10-8 mbar • l • s-1 from det. vol. to VD tank


Aluminium exit foil

20 m beam pipe

NEGDetector volume 1

Detector volume 2

Pump station 1

Pump station 2

RestrictionClean gas

Reducing valve

Overpressure safety

Mechanical pump

Ion-getter pump

Penning gauge

Ionization gauge

Pirani gauge

Electromechanical valve

Self-opening valve

pressure sensor

QMA


Summary table:(Data are approximate. QLHCb_total = estimate for the full vertex detector, i.e. both halves.)

Item Outgassing rate of item QLHCb_total

[mbar l s -1] Kapton foil, after 40 hrs pumping 1 E-7 mbar l s -1 cm-2 n/a sample Kapton flat cable QPI 3 E-5 mbar l s -1 130 E-4 male/female pair of PEEK D-type 25-pin connectors 6 E-6 mbar l s -1 / pair 50 E-4 male/female pair of stand. D-type 25-pin connectors 1 E-5 mbar l s -1 / pair 100 E-4 Liverpool carbon-fiber Si support 1 E-8 mbar l s -1 cm-2 ~ 1 E-4

Outgassing measurements

Continue: measure all unknown outgassing rates of components in a detector station


Dynamic Vacuum

Beam-induced particle bombardment desorption, emission

Ions, photons, electronsenergies up to keV

• Local pressure runaway (ion/electron-induced desorption)• Local static charge increase (electron multipacting)

LHC beam instability

See Adriana Rossi’s presentation


Dynamic Vacuum (continued)

Perhaps a solution:

use coating of surfaces by Tiadvantages: low SEY , low , local pumping

Design issues: • better surfaces ? (NEG ?)• in-situ coating required or not ? • thickness of layer needed ?• what re-coating rate ?• affordable cathode temperature in-situ ? • wake field / RF properties ?• side effects ? (peeling, ...)

We need , for:• different materials • surface conditions (un)baked, saturated, activated, etc. • different impact energy spectra

Data available only in a few months ! (Mahner et al.)


New requirements on VD design

Recent VELO workshop in Amsterdam (April ‘00):

• Top/bottom halves should have intermediate steps (not only open/closed), with “relaxed” overall position accuracy (0.3 mm). Only useful if we can acquire meaningful data in intermediate positions ! trigger algorithm

• Higher cooling power required (20 W/module 40 W/module) to accommodate for backup solution SCTA/velo. Should be no problem for CO2 cooling system, but is it realistic at all ?

thermal modeling needed


Summary of difficulties with TP design

• Recent FEA results: to obtain sufficient stiffness of center frame, a rather complex structure needs to be used

reduced space and accessibility

• MAFIA: WF suppressors on sides of the two detector boxes are needed not easy to find a solution with current design because center frame is “in our way”and because of the fact that the two boxes are mounted separately into the tank

• Dynamic vacuum: we must have the option to insert a Ti evaporator not easy with current designmust enter from the front of tank and, again,center frame is “in our way”


Towards a modified design

in which difficulties become challenges.


Vacuum tank• new FEA required (probably additional ribs are needed)• flange will be on inside of LHC ring • solves lack of space issue on outside of ring• all feedthroughs on the same big flange • all cables and repeater cards on same side (present choice of feedthrough limits total number of pins to about 25000 !!)


• optimally positioned short legs, no more problems of bending, stiff, stable structure• xy-table can move down enough to allow insertion of complete detector (tilting mechanism no longer needed)• frames not any longer “in our way” (more accessibility to critical items of VD)

Support frames


Detector and support frame

• both halves on same side• VD easier to mount and position in the tank• install complete VD at once• the two halves are no longer interchangeable


TP design

center frame

New design

Sideways accessibility (1)

Side wake field suppressors

Ti evaporatorNo room onthe sides !


Sideways accessibility (2)

• More access to critical WF suppressor• Much room left on one side of the tank 2ary vacuum pumps (pumping on the detector boxes is more efficient) if required, a Ti evaporator with much more favorable design conditions


Cooling system with mixed-phase CO2

Phase diagram CO2

1

10

100

-80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50

Temperature [°C]

Pre

ssur

e [b

ar]

vapor

liquidsolidgas

critical point

triple point

*


cool to 20 C

CO2 gas-liquid storage tank57.3 bar at 20 C

CO2 supply line

compresssor

P [W]

P [W]

P [W]

flow restrictions

supply lineexpansion valve cooling lines

gas only

pressure (temperature)regulating valve

heat to 20 C

Mixed-phase CO2 Cooling system

See LHCb 99-046/VELO


Report from LEMIC(LHC Experiment Machine Interface Committee)

• We presented the design of the Vertex Detector on 15-2-2000 (mechanics, vacuum, CO2 cooling, wake field studies)• Overall reaction was positive and constructive• Major points of discussion:

(a) foil resistance to differential pressure test, add self-opening valves

(b) dynamic vacuum issues partial bake-out ? Ti coating ?

(c) accuracy of WF calculations test setup, compare

(d) reliability of CO2 cooling system (vac.) demonstrate


Set Si design parameters march 2000 (?)positions, materials, tolerable foil thickness, ...

Calculate dynamic pressure effects march (?) desorption, multipactingBuild 1-1 prototype summer

test mechanics, RF, cooling, vacuumFormal approval of design from CERN/LHC summerTDR ready december

Outlook presented to LEMIC


Why not start with 250 m Al foil ?

Conservative approach, but, if physics permit, offers importantadvantages:

• RF penetration, pick-up Epeak 1.3 × 109 V/m

• Easier to manufacture (more freedom in design) corrugation depth• More resistance to differential pressure expect about ? mbar

Continue developments to reduce foil thickness, while acquiring experience with this foil.Upgrade after some time of operation.


List of available LHCb notes

LHCb 99-040, VELO, --- Heat dissipation studies for the LHCb microvertex silicon detector,N. van Bakel et al.

LHCb 99-041, VELO, --- A first study of wake fields in the LHCb vertex detector,N. Van Bakel et al.

LHCb 99-042, VELO, --- Mechanical design of the LHCb vertex detector : baseline solution,M. Doets et al.

LHCb 99-043, VELO, --- Wake fields in the LHCb vertex detector : strip shielding, N. Van Bakel et al.

LHCb 99-044, VELO, --- in preparation LHCb 99-045, VELO, --- Preliminary studies for the LHCb vertex detector vacuum system,

M. Doets et al.LHCb 99-046, VELO, --- Preliminary studies for the LHCb vertex detector cooling system,

M. Doets et al.LHCb 2000-019, VELO, --- Geant description of the aluminum shielding of the vertex detectors

T.J. Ketel


Topics for future LHCb notes (2000)

• Estimation of RF field attenuation through the Al shield• Description of modified VD design • FEA results for the VD vacuum tank and support frames• Test of the VD CO2 cooling system• Measurements of outgassing rates for the VD components• Measurements of the RF shield properties under differential pressure • Test of self-opening valves• Test of Ti evaporation on Al RF shield• Measurements of RF properties of the VD• Test of motion mechanics

Documents

LHCb Week, CERN, may ‘00M. Ferro-Luzzi LHCb Vertex Detector System: An Update Review of TP design mechanics, wake field suppression, vacuum system, cooling