16-18 January 2012 CERN Christian Boccard

Outline

1 - COLD BPM

2 - STRIPLINE COUPLER

3 - WARM BPM

4 - COLLIMATOR BPM

Christian Boccard, (CERN BE/BI)

Cold BPMs

Arc Beam Position Monitor (Arc type+ DS) 864

Special BPM for end of arcs 12

BPM with Rotated Beam Screen 20

Enlarged Aperture BPM 36

Directional Stripline Coupler 8

Combined pick-ups for RF 16

Warm BPMs

Warm LHC BPM adapted for Elliptic shape 16

Enlarged Warm BPM 28

Enlarged Warm BPM with Quick flanges 16

Diam 80 Warm BPM for Roman Pots 12

Warm Directional Stripline Coupler 16

Beam Dumping System

BPM Aperture 80mm for Interlock System 8

BPM Aperture 130mm for Interlock System 8

Other Special BPMs

Trigger for Experiments 8

Long Striplines for Transverse Damper,

Tune & Chromaticity Measurements 14

RF Wideband Transverse pick-up 2

Total 30 Equipment Codes 1092

Dump RF &

BI

Cleaning Cleaning

Buttons …. or Striplines ?

Simple & robust More expensive to build

Removable from outside of chamber More parts: less reliable ?

Less cumbersome Require precise machining

Well adapted to LHC short pulses More sensitive but more heat load

Little effect on longitudinal impedance

Final choice (1996) was Buttons for the arcs (and Directional Couplers around

experimental areas…)

• Our feedtroughs are the only vacuum chamber non welded component ! (use Helicoflex ®

gaskets)

• Choice of feedtrough technology: Glass-metal was preferred to Brazed Ceramic

• Systematic LN2 cycling tests done by contractor

• Tests and pairing at CERN:

• vacuum tests & mechanical data on samples

• AB/BI measures final capacitance on all buttons

• Pairing is done on amplitude response

Brazing

Ceramic

Metallic

Pin

Stainless Steel Body

Glass-Ceramic

Classical Ceramic Seal Glass-Ceramic Seal

Prone to cracks

& fissures

Excellent adhesion

to all surface blemishes

BPM are housed in the

Technical Service Module

We must share this

crowded area.

BPM is also used for

Interconnect and VAC:

responsibility change with

function.

BPM became a Vacuum

component when welded to

the beam screen.(BPMs are

used to feed beam Screen

capillaries with super fluid

helium at 1.9 K )

Then became an

Interconnect component

when welded to the cold

mass.

Exceptions require same

amount of work !

Cooling Tube Feedthroughs

BPM Body

Beam Screen

BPM Body

Cooling Tube Feedthroughs

Button Electrodes Drift tube

Cooling Tube Feedthroughs

BPM Body

Beam Screen

BPM Body

Cooling Tube Feedthroughs

Button Electrodes Drift tube

Tooling development of the ‘monolithic’ first BPM for Arcs was lengthly and costly.

Later the design of Enlarged cold BPM and Coupler evolved to a modular design basis.

To standardize components and tooling.

Re-use of VAC Plug-In module from dipoles

Cooling Tube Feedthrough Module

BPMY Support

BPMY Body

Cold Bore

Beam Screen Enlarged cold BPM

Stripline Coupler

• Tight mechanical tolerances: nice but…

• Should be maintained during production

• Machining cost is proportional

• 3 axis Dimensional checks are also expensive

• Help to design: ANSYS 3D analysis of temperature or mechanical constraints.

• Cold Material: Specify forged austenitic stainless steel of type AISI 316LN

• ~ 60 leaks found in LHC (not on BPMs) due to macro-inclusion in material. Can affect thickness up to some milimeters.

• (CERN specification 1001-Ed. 3-02.08.1999)

• Electroplated inner copper layer of the BPM body is 100 um 20 um (for longitudinal impedance)

• witness samples are required to assure clean material and Copper plating quality:, e.g. adherence, blisters.

• nickel not allowed, use gold strike.

• Welds should be re-qualified regularly during production.

from A. Gerardin

Deformation contours due to Helicoflex washers surface pressure

• Insulator Spec: UHV, cryo temp, radiations, losses -> Ultem

• Coupler Feedthroughs: same technology as buttons

• Gold - Rhodium Sliding contact do not stick at cryo temp

• Electron Beam welding of a 2mm pin on washer

• Electrode gap is not easy to trim

• Achieved directivity : 24 dB at 70MHz -> not enough

• 50um error from cross-talk for 25ns spaced beam

• False triggers on electronic.

• Cryo tests to measure contact resistance

Strong specification: – The cables must be cryogenically compatible, radiation

resistant, mechanically preformed , have low heat leak and VSWR.

– In addition the cable must be electrically stable when subjected to the above mentioned radiation, flexing, and temperature gradients.

– Leading to the choice of silicon dioxide foam (SIO2) dielectric

– Contract awarded for 4500 units in 2001

– electrical length difference less than 10ps for the 4 cables of each BPM.

– Gold reference used to ease production and logistic

Integration is a challenge

– First prototype was not mountable in SSS !

> new design produced

– Forming not as accurate as specified

Additional Configurations

– Design of 5 different configurations required for other SSS types

– cabling of BPMS in Q2 is done in the tunnel during interconnection

– Connector nuts are locked by twisted wires

Taylor Hobson sphere

Support of fiducial

Magnetic centering

• Forms part of a Work Package Sub-contracted for the 480 SSS magnets

• Procedure – Weld beam screen to BPM body – Select pairs of buttons for mounting – Mount button feedthroughs – Perform leak test of assembly – Insert BPM/beam screen assembly in magnet – Spot weld BPM to support – Measure and adjust position and tilt of BPM – Weld BPM to support – Measure position and tilt of BPM – Install and connect cryo cables – Mount warm feedthrough on cryostat – Perform electrical test of BPM system

• Worries – Sub-contracting started slowly – Technology transfer repeated many times – Magnet does not wait: Non Conformities to be treated rapidly.

• Carefully design and integrate your instrument

• Then do your best to make reality close to that …

• Fiducialize the BPM on their supports

• Helicoflex aluminium gaskets proved to be leaking above temperature of 280 C. Conflat more adapted to this case.

• Bakeout temperature of BPM was then limited to 200 C to avoid any problem (lower temperature is compensated by a longer time)

• But hard to control this, errors are not admitted. the colour of stainless steel after bakeout reveal the reached temperature :

– No change <200

– Yellow 220

– Brown 260

– Purple 280

– Blue 300

• Budget – Cryo cables budget was under estimated

• Design

– Reliability was one of the important parameter taken in account during the design phase leading to technology and material choices.

– Experience from previous machine at CERN and similar machines proved to be extremely valuable

• Specification

– Many of the issues related to material could have been avoided if treated at the project level

• Integration – We learn all along the project. “What I would change if I would do it once again”

• Construction & Installation – Everything was slower then planned due to the large scale of the project

• Operation

– No leaks except overheated BPMs – Not enough directivity from couplers to avoid false triggers.

First development of a variable aperture

BPM for the LHC collimation system.

0.2 mm Left jaw (Carbon

example)

Graphite or

Tungsten

Material jaw

The LHC Beam Collimator

6s

LHC Collimators must clean the

beam halo at given positions so that

the rest of the machine is protected.

To this purpose collimators insert

absorbing materials into the vacuum

pipe.

Absorbing jaws are movable and

can be placed as close as 0.25 mm to

the circulating beam !

Nominal distance at 7 TeV: ≥ 1 mm.

80 kg TNT

High stored energy : 360 MJ at 7TeV !

The LHC collimation was conceived as a staged

system:

Phase I collimators:

• Designed to ensure maximum robustness against

abnormal beam losses in operating conditions.

Phase II collimators:

• Complement phase I and able to reach nominal intensity

and energy.

Constraints:

• Improve collimation efficiency.

• Keep low longitudinal impedance

• gain factor ≥10 in set-up time

Phase I jaw

Phase II

jaw

Standard setup method relies on centering

collimator jaws by detecting beam loss.

• Procedure is lengthy and can only be

performed with pilot fill.

• Big worries about risks, reproducibility,

systematic effects and time lost for physics.

The integration of pick-ups into jaws will

allow:

• deterministic centering of jaws around

circulating beam.

• Improvement in set-up time.

• Continuously follow orbit drifts,

• Allow tighter collimator settings, …

High precision (<10 um) and stability

(averaged, not bunch by bunch)

And:

• Protect components from accident cases.

• Withstand a bakeout temperature of 250 º C

during 48H.

• Operate under strong radiation (200 Mgy/y).

• Maintain Ultra High Vacuum.

• Very accurate geometric stability .

• Low-Z material.

Center pair of

buttons (in case ..)

button in the tapering

Prototype build with the implementation of 4 buttons in Jaws

Graphite

Cu jaw

support

Ineffectiveness of center pair of buttons on

closed jaw operation

Based on experience gained with demonstrator -> Implementation of 2 BPMs

Cross-section of jaw tapering.

fine positioning

system

BPM housing

cooling circuits

molybdenum back-stiffener

Embedded BPM

GlidCop®

absorbing material

• Flat surface for housing the button in recess

of the beam

• Less smooth transition.

• longitudinal trapped modes are mainly

generated by the transition region.

Phase I jaw tapering Tapering with button Phase II

Key component: the RF cables

The coaxial cables needed should:

• Be small, robust and flexible enough to

follow the jaw motion during the thousand of

cycles expected.

• Be vacuum compatible (choice of materials,

reliability, cleanliness and outgassing rate)

Only SiO2 Cables meet the

specification ! • They also provide exceptionally low

hysteresis vs temperature and motion, with

phase and loss values returning to the same

values.

But:

• SiO2 dielectric is Hydrophilic and cables are

backfilled with Neon gas (chosen for low

molecular mass).

cable routing up to the button fixture.

-> Issue in case of leak in the beam vacuum !

BPM Cables

But if vacuum degrades we stop LHC !

• Stress simulations show that

they should survive at least 30

000 cycles.

• Cable failure means no BPM

anymore because cover tank is

welded.

Agreement of measured non

linearities with simulations results.

No noise seen due to

upstream scraping.

correlation between the centres

measured with the in-jaw BPMs and

the BLM dependent method.

Adjustable

Stand

Collimator

assembly

Embedded BPMs Collimators:

• Mission to replace all moveable collimators in

the Ring with new collimators equipped with

BPM

• Start production in 2012 of 20 tertiary collimators

(TCTP) with installation of 8 of them during LS1.

• Production of 2 Secondary collimators (TCSP)

for point 6.

• Baseline for other collimators, and ideas for

Beam Beam Long Range Compensator.

• Development of a calibration system.

• Tertiary Collimators with Embedded

BPMs will be based on Phase II concept.

• BPM design is based on experience and

good results gained with demonstrator

installed in the SPS.

• Should achieve requested precision and

stability performance.

From demonstrator installed in the SPS to… Tertiary Collimators with Embedded BPMs