# Presentation: 793059

## Rotatable Collimators as a Possible US Contribution to the LHC Luminosity Upgrade Project

**Rotatable Collimators ****as a Possible US Contribution ****to the LHC Luminosity Upgrade Project**

- 24 April 2008
- LARP CM#10
- Tom Markiewicz/SLAC

*BNL - FNAL- LBNL - SLAC*

_**US LHC Accelerator Research Program**_

## Vision

- DOE/LARP management perception is that rotatable copper collimators are relatively simple mechanical devices that could be offered as a low risk component of a US LHC Luminosity Upgrade Project, offsetting risk of not delivering Nb3Sn magnets, their main interest.

## The Problem

- Suggesting that the US manufactures 36 of these devices for the 30 reserved Phase II secondary collimator lattice slots (plus 6 spares)
- Soft peddles fact that
-  1st single jaw has only now been brazed and still needs some weeks of work before thermal-mechanical tests can begin
- SLAC has not begun, the “almost” fully defined, process of building the 1st full beam testable double jaw collimator
- Ignores the CERN collimation “White Paper” plan calling for construction and mechanical, vacuum & beam testing of three “complementary” designs that can bring LHC luminosity from 1034 to 1035 before a production decision is made
- The Cu secondary RCs do not give x10 improvement in luminosity
- Destructive testing of the by-design damageable Copper may be prove extensive enough to remove RC from consideration
- Does not address adequately the quality control issues seen in CERN’s Phase I collimator production nor has there yet been any talk or agreement with CERN on interface, installation, or engineering support issues.

## My Opinion

- LARP, through SLAC, is participating fully in the Phase II Collimation project as outlined in the CERN “White Paper” plan and any action on LARP’s part that looks, as this does, like it is circumventing the agreed to schedule damages our relationship with our colleagues and is doomed to failure.
- If, after the R&D process concludes in October 2010, a decision is made to produce some number of rotatable copper collimators, SLAC is ready to participate.
- This exercise should only be taken at the “CD-0” level: mission need and ballpark cost estimate, and is hopefully, laying the bureaucratic groundwork for eventual US participation in construction of collimators

## References

**Executive Summary**
***Second Phase LHC Collimators***
- R. Assmann
- 2007-04
***Description Phase 2 Collimator Project***
- R. Assmann
- 2007-07-02
**Collimation Issues for the Two LHC+ Scenarios and Future Plans**
- R. Assmann
- Beam’07
- 2007-10-01

## White Paper Plan

- The R&D and prototyping of at least three different concepts for advanced collimation designs, each addressing different possible limitations to LHC luminosity.
- Operational experience of the LHC with the Phase I collimation system to understand the nature of any luminosity limit and the best ways to ameliorate deleterious effects with already installed systems.
- Testing of prototypes at CERN to their technological limits before any installation in the LHC
- Installation and testing of qualified Phase II prototypes in the LHC during the second full year of LHC operation with a decision on phase II design and production, which may require more than one technology, at the end of the year.
- Production of the collimators during years 3-4 of LHC operation, installation during regularly scheduled shutdowns and use beginning with year 5 of LHC operation.

## White Paper Schedule

## CERN Phase II R&D Budget

## CERN Pre-Production Manpower vs. Time

## Production Estimated at 8M CHF

## NLC Consumable Collimatorrotatable jaws – 500 to 1000 hits

- 6.0

**Note short high-Z material.**

**< 10 W per jaw =>radiative cooling!**

**Aperture control mechanism – 5********m accuracy & stability**

**Alignment BPMs upbeam & down**

**Movers align chamber to beam based on BPMs**

## LHC Phase II Base Concept

- physical constraints
- current jaw design

** ****beam spacing: geometrical constraint**
** ****Length available 1.47 m flange - flange**
** ****Jaw translation mechanism and collimator support base: LHC Phase I**
** ****>10 kW per jaw Steady State heat dissipation (material dependent)**

**Cu coolant supply tubes twist  to allow jaw rotation**

**Hub area**

**Glidcop     Cu     Mo**

**Cantilever Mo shaft @ both ends**

**Helical cooling channels**
**25mm below surface**

**20 facets**

## Cu Jaw-Cu Hub-Mo Shaft Design

**Cu Jaw-Cu Hub-Mo Shaft Design**

**2mm shaft-jaw gap gives x5 improvement in thermal deformation over solid shaft-jaw design**
**1260 um ******** 236 um (60kW/jaw, ********12min)**
**  ****426 um ********   84 um (12kW/jaw, t=60min)**
**Rather than Cu, Moly shaft improves**
**Gravity sag x3: **
**     ****200 um ******** 67 um**
**Thermal bulge 30%: **
**     ****339 um ******** 236 um**

## Brazing Each Moly Shaft End to a Central Copper Hub

- After much R&D, developed method to braze Molybdenum to Copper for inner shaft

## Three Braze Cycles

- Three main brazing steps.
- Brazing materials set to melt at gradually lower temperature.
- 1.) Braze each shaft end to a central half-hub
- 2.) In one go:
- Braze shaft hubs to Mandrel
-  25% Gold, 75% Copper
- Braze copper cooling coil to Mandrel
-  35% Gold, 65% Copper
- 3.) Braze jaw quadrants to mandrel surface
-  50% Gold, 50% Copper

## Inserting Molybdenum Shaft Ends into Mandrel then Wind Coil Around Mandrel with Ends of Coil Protruding Out Each End

## Braze Step#1 Shaft Assembly & Coil to Mandrel

- On support stand and ready for insertion in baking oven
- Carbon block used to hold thermally expanding copper against central hub and shaft (moly and copper)
- Next time may use carbon block full length of mandrel

## Filling Coil-Mandrel Keystone Gaps

**Filling Coil-Mandrel Keystone Gaps**

- Three brazing cycles needed before coil-mandrel ‘keystone’ gaps filled adequately
- On 3rd cycle excess braze material attaches support stand to mandrel, which warps

-  Pix of 2nd braze cycle

## Recovery after Excess Braze Material Attaches Mandrel & Shaft to Inox & Inconel Braze Supports

## Measure & Machine Quadrants to Mandrel. Assemble & Braze

- Using 50-50 Au-Cu brazing material ($$)

## Results of Jaw Brazing 22 April 2008

- Looks good!
- QA measurements will be done 23-Apr
- Next step is machine flat facets and grooves for heater tests and thermocouple holes.

## Internally actuated drive and jaw mount for rotating after beam abort damages surface Completed 27 May 2007

- Rotation drive with “Geneva Mechanism”

- Universal Joint Drive Axle Assembly
- Thermal expansion
- Gravity sag
- Differential transverse displacement

## Up Beam end detail beam side view of Current Idea for RF Transition to Beam PipeLCR meter, Contact Resistance, Trapped Mode Calculations

- Spiral style backing springs reside inside
- “Sheath” (sheath not shown)

- Round to Square Transition

- Thin sheet metal RF “Curtain”

## Summary of Mechanical Design

- Complete braze procedure developed, but want to look at...
- Full length graphite choke when brazing coil to mandrel
- Keystone reduction
- Blowing up coil with compressed air
- Wedging copper between mandrel between coil slots to drive it towards coil
- Using 360 quarter length jaws with braze wire rings cut on ID rather than 90 quarter length jaws
- Anything to reduce number surface prep requirements for brazes
- Support & rotation done
- Well developed ideas on RF features that (in principle) are far from critical path

## Performance Risks

- Will thermal-mechanical testing of the first jaw validate the shaft-hub-jaw design that should limit maximum jaw deflection into the beam area to 230 microns for 10 second bursts of 60kW beam absorption
- Will the mechanical accuracy of the device be adequate (40 um)
- Will the basic jaw-adjustment and jaw-rotation work as planned
- Will the effective impedance and RF characteristics of the jaw and the transition to the vacuum tank aperture be adequate
- Will the device vacuum be sufficiently low
- When damaged by an aborted beam, will the extent of damage be sufficiently localized to allow rotation to a clean surface, as planned

## Exact Nature & Extent of Damaged Region

- Thin Cu sample in FFTB electron beam at SLACHole = Beam Size

- 2000um 500 kW 20 GeV e- beam hitting a 30cm Cu block a few mm from edge for 1.3 sec  (0.65 MJ)

- FNAL Collimator with .5 MJ

## Accident Case: Permanent deformation AND Molten copper

**Case:** beam abort system fires asynchronously, **8 full intensity bunches into jaw**
**Model:**   - increased resolution 3-D ANSYS & FLUKA models
-               - Thermal heating/cooling analysis followed by quasi-static stress analysis
-               - Jaw ends constrained in z during 200 ns, released for 60 sec cool-down
-               - 0.27 MJ deposited in 200 ns
-               -  Molten material removed from model after 200 ns
**Result: **- 57e3 peak temperature (ultra fine model)
-               - **54 ********m permanent deformation** (concave)

## Braze Test #3: Vacuum tests: No improvement

- 3rd Jaw Braze Test Assembly has been vacuum baked at 300 degrees C for 32 hours. Results in slightly lower pressure.
- Inclusion of longitudinal grooves in the inner length of jaws for better outgasing
- Test Chamber setup similar to previous test.

|  | Old | New |
| --- | --- | --- |
| Baseline | 3.2E-9 Torr | 2.4E-9 Torr?? |
| w/ jaw assy. | 3.7E-9 Torr | 3.4E-9 Torr |
| Presumed  jaw assy. pressure | 4.5E-10 Torr | 10E-10 Torr?? |
| LHC requirement | 7.5E-10 Torr | 7.5E-10 Torr |

- Under Investigation...

## Efficiency StudiesChiara Bracco (CERN)

- Performance (Efficiency) of collimation system depends on # particles getting into the cold aperture (>10) per unit length as a function of location around the LHC given the optics model AND an aperture model
- 500E6 Halo protons tracked over 200 turns in 10cm steps by Bracco et al to get loss maps for each magnet
- Beam intensity limitations are due to losses in the dispersion suppressor above the quench limit.

- Phase I to II Global Improvement

- Phase I Beam Intensity Limit

## Intensity Limit at “Dispersion Suppressor” with Phase II at Nominal Collimation Settings

- Losses due to particles that hit primaries but that do not see the secondaries

- While philosophy is every bit helps (take the x3.6) “ultimate” luminosity may require different collimation settings or completely different collimators (crystals as primaries?)

## Graphite -> Copper

- Switching to copper will decrease the imaginary part of the impedance but increase the real part
- This is either good or bad depending on how you want to damp beam instabilities (Landau Damping versus Feedback)
- So, impedance still dominated by copper collimators

## Unscrubbed Unit Cost Based on work done to date: $250k

## Project Cost “x2 Estimate”

- Unit Cost for Tank, 2 jaws, rotation & support, RF	$250k
- Unit Cost for Rack & pinion, motors, LVDTs		$50k
- These are probably underestimates:
- Assembly, test & qualification at SLAC			4 FTEs
- 	36 units over 2 years					$200k
- EDI&A							6 FTE
- Lab overhead						42%

## Project Schedule

- Sure to be dominated by braze operations and braze oven cycles
- 72 jaws with 3 braze cycles per jaw
- imply that while not excluded 2 years of production is aggressive
- 1 year construction seems impossible