LIU Wire Scanner Production Readiness Review

29 March 2017

INDICO: https://indico.cern.ch/event/618262/

Present:

Federico Roncarolo, Lars Jensen, Jose Sirvent, Malika Meddahi, Giovanni Rumolo, Jonathan Emery, Patrik Andersson, Jose Somoza, Benoit Salvant, Klaus Hanke, William Andreazza, Thibaut Lefevre, Dimitry Gudkov, Nicolas Chritin, Jocelyn Tan, Bettina Mikulec, Alessandro Dallocchio, Alexandre Mariet, Emilien Rigutto, Stephen Jackson, Christine Vollinger, Thomas Kaltenbacher

Scope (Ray):

Questions to be addressed in this review

Is the mechanical design satisfactory and ready to proceed for series production?
Do we have confidence that this instrument will fulfil the specification for LIU?
Do we have confidence that the instrument:
- Will be qualified for vacuum acceptance?
- Will not have a major impact on machine operation due to impedance?
- Should not suffer wire breakage due to impedance heating?
Is the series budget and schedule understood and are the costs justified?

Mechanical Design (Dimitry)

New design to address issues/limitations with existing scanners:

Motion is transferred through crank mechanism
Bellows for motion transfer
Inaccuracies due to the relatively complex mechanics (crank-slider mechanism)
High deformation of forks during movement
Polymer materials used in mechanism design
Custom Helicoflex flange and seal

Initial SPS prototype needed to be improved in terms of:

Weight and size (wasn’t designed for the PSB)
Non-standard ConFlat flange (DN400)
Optical encoder components are inside the vacuum volume
Drum-based design, complicated and expensive for production

New design in close collaboration with IWG, TE-VSC, EN-MME

2 main components - vacuum tank & kinematic unit
Kinematic unit the same for all machines
- Only fork size & position on shaft changes
4 variants of vacuum tank
- H&V for SPB, PS & SPS
Geometry optimised for UHV - pumping holes included for all possible trapped volumes
- Design approved by VSC
Reliability
- SPS prototype tested to 75000 cycles with no degradation observed
- Long term tests to over 100000 cycles foreseen for 2nd PSB prototype

Components

Stepped chamber - most complicated mechanical part to produce
- Requires electron beam welding
Optical encoder
- Glass disk currently used, but favoured solution is LESS Al disk
Forks
- 3D additive Ti manufacturing - tested for vacuum compatibility - OK from VSC

Status

SPS & PSB scanners installed
Integration
- PSB H&V - underway with 3D drawings to be prepared
  - Looks to be interference between SEM grid on ring 3
  - ACTION: BI-ML to check integration of SEM grid & WS for PSB
- PS & SPS - 3D integration complete

Malika: Can SPS prototypes be re-used?

No - not compatible with final design.
Used to verify design which led to optimisation of
- Weight & size - non-standard flange replaced
- Motor & control
- Drum based design - changed to cantilevered shaft
- Optical components - moved out of vacuum

Mechanical Production (William)

Tanks - MS & IT required

Documents prepared & meeting with spec committee done
Aim - contract signed 3rd quarter 2017
Delivery 1st quarter 2018

Stepped chamber

Price enquiry for machining with EB welding at CERN
Delivery 1st quarter 2018

Mechanical components

Procurement handled by MME for ~2500 parts
Delivery 1st quarter 2018

Supports

Design ongoing, price enquiry MME
Delivery 3rd quarter 2018

Motor

Single tender (ALXIOM) with price request submitted
Delivery 4th quarter 2017

Questions:

Instrument bakeable? - not required & not tested but design in principle was for bakeable instrument
Main worry for schedule - no critical item identified - aim to have all ready for start of LS2
- VSC qualification comes last & can always give surprises & lead to delays. Try to test components and ensure cleanliness during assembly process – hence the need for the clean assembly area.
MME on-board - e.g. e-beam welding? Yes

Vacuum Acceptance (Jose)

Mechanical components - Individual tests - all OK

RGA of full system

Issues after cleaning/assembly required to repeat the process
Origin of contamination not fully understood
Cleaning and assembly of different components is critical

Co-axial Kapton cable

Identified as main outgassing item
Diffusion of water in very thin Kapton® layer
Bakeout with subsequent exposure to air does not help long term outgassing
- Only helps when bakeout in situ
For PSB
- FWS out of spec at 24h but in spec at >300h

Summary

Once exposed to air Kapton re-absorbs water so no improvement in pumpdown rate
With ion pump added even current Kapton cable OK for PSB & SPS
Not OK for PS
- Need ion pump & change of wire

VSC recommendation to add ion pump & change of wire for PS

Retrofit cable on all scanners if suitable replacement found

Expected Performance (Federico)

Wire position determination

Old system
- Rotational WS: Relies on calibration tables
- Linear : direct use of potentiometer
New system
- Beam size: inferred from angular position from optical disk - no calibration required
- Beam positon (offset) : still requires calibration

Sources of uncertainty for beam size measurement

Wire position uncertainty
- Optical disk
  - 10urad precision and reproducibility
  - Translates to error < 1.5 um
- Mechanical plays
- Fork deformation
- Wire vibration
Acquisition electronics
- Detector
- Acquisition
- Fit routine

SPS Prototype Coasting Beam Results

270GeV with PMT down to 1.5 spread in measurements
- 12um sigma on ~800um beam size
- Compared to 22um for linear scanner
400GeV pulsed with single bunch
- 12um on beam centroid
- 107nm! on beam size

ACTION:

BE-BI to provide a table summarising wire position uncertainty & calibration test bench results

Alessandro: FE model for comparison would be good to have

Malika: Would like full table on expected performance in all machines to be able to eventually discuss staged approach e.g. in SPS.

Comments regarding results from calibrations:

1) Concerning the measurements made with the prototype WS in the SPS, you quote as spread observed with the AWAKE single bunches at 400 GeV: '107nm! on beam size’. We actually had quite a long discussion with Federico on the point that the measurements of beam size exhibited also a linearly growing trend (and I guess the spread is calculated removing this slope). We agreed that this seems bizarre, because the machine was being cycled and receiving always the same bunch from the upstream injectors AND the current operational WS did not exhibit the same trend (rather the opposite). So I guess that there is still something to explain about these measurements.

Comments regarding scope for use in the SPS:

One of the limitations of the present WS is that we cannot measure the emittance of full LHC beams at 450 GeV in the SPS. This is because 1) there is a limitation on the total intensity of the beam and 2) the resolution would be poor due to the small size. I am still not very clear whether we can assess that these issues are overcome with the new device and for typical LIU beam parameters.

Impedance (Christine)

PSB - simulations & measurements:

Simulations compared to probe-probe measurements and probe-coupler measurements
2 strong modes simulated & measured at 812 MHz & 867 MHz
- Well above beam spectrum
Effective impedance negligible

Simulations for SPS & PS:

Effective impedance also found to be negligible

Conclusion: no visible showstopper expected from transverse or longitudinal impedance

Wire heating

Highest induced power observed at 523 MHz and 882 MHz at 135 deg position
- PSB expect no influence (above beam spectrum)
- PS 0.01 W/turn from 874MHz mode at 0 degree (parking)
- SPS
  - 0.11 W/turn from 523 MHz mode at 0 degree (parking)
  - 1.98 W/turn from both modes at 135 degree
  - Need to check units!!
Heating will depend on interaction with beam modes
- With ferrites will always interact, i.e. heat
- Without ferrites will depend on mode frequency
- No ferrites therefore preferred as long as modes are not hit

Discussion: Heating of the wire is not currently simulated. Experimentally, BI saw changes in the resistance of operational SPS scanner wires, suggesting impedance heating. However, no resistance change has been observed in the new prototype scanner.

ACTIONS:

Bench measurement with real wire (ACTION: Impedance WG with help from BI)

MD with high intensity in SPS
- Possibility to instrument SPS prototype wire with higher frequency readout (ACTION: BE-BI in collaboration with Impedance WG)

Cost & Schedule (Ray)

Unit costs for production:

Material + vacuum tank & chambers + instrument + support
- 57 kCHF/ instrument
Control Electronics + Acquisition system
- 23.5 kCHF/ instrument
Cabling average
- 33.4 kCHF/ instrument

Budget:

1258 kCHF for series production
780kCHF for cabling, vac interface, remaining design
133kCHF for assembly & calibration benches

2.2MCHF TOTAL BUDGET for installed scanners

1.3MCHF remaining in EVM

0.9MCHF additional required

Spares - 280kCHF

Options - 308kCHF

Summary

Decision on go-ahead needed or mechanics early May for tendering to start mid-May

VSC OK
Impedance OK
Mechanical performance to be finalised
- LIU needs input on final expected mechanical performance by end April

MAIN ACTIONS:

BE-BI to produce table of precision uncertainty for mechanical system and address the comments regarding calibration and scope for use in the SPS.
LIU management to see how additional costs can be covered in discussion with CONS
BE-BI to organise follow-up meeting end of April to present & discuss this table