# Presentation: 820083

## Tevatron Dipole Reshimming

- Dave Harding,
- Jamie Blowers, Ray Hanft, John Carson
- 2 December 2003

## Outline

- The creep problem
- Shutdown reshimming activity
- Closeout plans
- A new issue
- Further studies

## Tevatron Dipole Cross-section

**Notes:**
The collared coil in a Tevatron dipole is held in place in the warm iron yoke at nine places down its length.  
Eight of those nine locations look like this, with rectangular blocks of G-11, known as suspensions, providing the mechanical link between the bolts that push on the cryostat from outside and the collared coil on the inside.
During the shutdown we have added shims to the bottom support points to compensate for shrinkage in the suspensions, lifting the collared coil back to about where it was when it was built.
Our basic measure of what is going on inside is the lift, measured from the outside of the smart bolt to the top of the plunger.
This number increases as the magnet is cooled from room temperature, to liquid nitrogen to liquid helium.
It decreases as you add a shim to the bottom and as you warm up.

At the center location the same basic geometry holds, but with several significant differences in the details.
Instead of a die spring in the smart bolt, we have a stack of Belleville washers.
Instead of the suspensions, two stacked blocks of G-11, we have anchors, cylinders of G-10.
And, instead of the suspensions sliding longitudinally during a thermal cycle, the anchors are anchors to both the cryostat and the collared coil to prevent either longitudinal sliding or rotation within the yoke.

## The Creep Problem

- Collared coils are positioned by glass-epoxy “suspensions” secured by “smart bolts”
- Suspensions deform with time
- Coil moves relative to iron yoke
- Magnetic field changes (skew quad)
- Bad fields disrupt beam in the Tevatron

## Recognition of the Problem

- Convergence of beam measurements and mechanical measurements
- Beam measurements showed conclusively that the betatron “coupling” was distributed.
- “Lift” measurements, then vs. now, show coil movement.

**Notes:**
Most of you know the story.  In February, or so, had a gratifying convergence of ...

## Measured Change in Cold Lift

- No change in (Q1-Q2)/2 left-right OK
- 0.15 mm change in (Q1+Q2)/2  0.1 mm lowering

## Shutdown Activity

- Reshim 106 chosen dipoles
- Retrieve original warm and cold lift data
- Measure lift cold in tunnel
- Issue traveler, kit
- Measure lift cool or warm
- Reshim
- Measure lift cool or warm
- Measure lift cold

**Notes:**
Due to various delays, the digging the production data out of the travelers did not get started until fairly late and we did not have time to study the data as thoroughly as we would have liked before the shutdown started.
We were only able to get modern cold data on the chosen magnets in the last days before the shutdown and, again, did not have time to adequately study the data in advance.  As I will show you, the “cool” data taken at about liquid nitrogen temperature is not an adequate substitute for cold data.
The reshimming itself went rather smoothly.  As we expected, there were a handful of magnets with problems.  Some we explicitly anticipated, such as inaccessible stations, stripped threads on bolts, and dropping a stack of shims.  Others, we did not anticipate, but dealt with.
Our tunnel time once the Tevatron was back at liquid helium temperature was again limited, and we did not get reliable cold measurements on quite all the magnets that been reshimmed.  We have a pending request to be called in if there is a suitable opportunity for an access.

## Slide 8

**Notes:**
This photo is just to remind you that reshimming in the tunnel is not an easy job.
So we prepared...

## Off-line Work

- Choose tools
- Assemble materials
- Develop procedures
- Review measurements
- Analysis data
- Deal with discrepancies

- John Carson
- Bill Robotham
- Glenn Smith
- Matt Cullen
- MTF
- MC
- Safety
- A dozen technicians

**Notes:**
Before the technicians first entered the tunnel there was a lot of preparation.
A substantial effort went into finding and choosing the best tools for the job, the air driven power torque wrenches and so forth.
Every shim was checked in advance to be sure that it was the correct thickness and kits were assembled with the correct number of shims for each magnet.
The procedure was though through carefully, written up, tested, and revised.  Reshimming trials in IB2 and MTF.
During the shutdown we reviewed measurements daily.
We spent endless hours analyzing the data, interpreting the observations, digging deeper to solve puzzles.
Most discrepancies were easily recognized, but others required thought to disposition.

## “Normal” magnet

**Notes:**
This is the basic QC plot that we settled on after some development.  It shows the change in lift, before minus after.  
The left half is one side of the magnet and the right half is the other side of the magnet.  Each point is one station.
Ideally this should be a little under 4 mils straight across.  The least count on the electronic depth gauge is half a mil.
The difference calculation shows the ‘normal’ amount of noise.

## “Suspect” anchor magnet

**Notes:**
Here is a non-normal magnet.  Quadrant 2 anchor does not show the correct movement as a result of reshimming.  I’m going to come back to this issue later on.  The point is that anomalies in the shimming do show up in this kind of plot.  That the measurements have to be taken, the plots have to be generated, the results have to be interpreted to identify interesting situations. When something did arise, a detailed look at all the was required, sometimes digging back into the travelers again to understand all of the circumstances surrounding the measurements.  
And all of this allow had to be done fairly quickly to allow correction of the appropriate problems in the tunnel.
So, although the anchor caught our eyes here, we also looked at the suspensions...

## “Normal” magnet

**Notes:**
When something interesting showed up, we looked at all the data in detail for patterns.  This is the same normal magnet showing the absolute lift measurements for the suspension stations (not the anchors).  That’s 10 mil per division, running from 1.610 to 1.670 inches.
Note the “fingerprint”, and the expected shifts due to temperature and creep.
We start with the warm measurement during production, the purplish open circles across the bottom.
When the magnet was cooled to helium temperatures, the collared coil shrank and the lift increased to the dark blue diamond values.
Over time the suspensions crept, shrinking and leading to the additional 6 to 8 mil increase shown in the square magenta data.
During the shutdown the magnets were allowed to warm to about nitrogen, and the collared coil expanded, reducing the lift.
We added 4 mil shims, reducing the lift some more, to the red x’s.
Finally, on recooling to helium temperature the collared coil shrank and the lift increased to the brown open square data.  Note that the goal was to bring the cold lift about half way back to where it had started and that here we have done that fairly well.

## Another issue emerges

**Notes:**
This graph doesn’t show the anchor that caught our attention to start with.  Rather it shows that when you start looking, you see other interesting things.  Here the fingerprint shows station Q2-7 had something funny happen between warm production testing and MTF. Maybe someone removed 12 mils of shim instead of adding.  We can’t tell.  The allowed window was rather large.  This was corrected by adding 24 mils to the smart bolt to bring the lift back into line.  We didn’t add to the dumb bolt because we didn’t want to change the field from what it had been and we didn’t want to push the collared coil up toward the instability point.

## Reshimming differences (cool)

**Notes:**
Over all the correction averaged the 4 mil that we expected.  There was some spread.  This histogram is of the data that was available as we proceeded.  It is the average over each magnet of all the differences.  The temperature is somewhere between LN2 and room temperature, and not always the same before and after.  The standard deviation is 0.88 mil.

Spread of pre-post reshim lift differences when the magnets were a LHe temperatures. Note that 4 magnets have not yet had cold post reshim measurements, 2 magnets were replaced, and 2 magnets have differences which can’t be correct.  The general shape of the distribution is tighter, reflecting the more stable thermal conditions.  However there are more outliers.  We attribute those to the rushed conditions under which both the initial and the final measurements were taken.  There is nothing that we can do now about the before data, but we are interested in repeating some of the post reshimming measurements.  

## Closeout plans

- Finish collecting final cold lifts
- Summarize measurements
- Assess equipment and procedures
- Record resource usage, reshimming rate
- Define needs for future reshimming projects
- Three months?

## A new issue

- Some anchor lifts didn’t change with reshimming
- Also don’t change with temperature

## “Normal” magnet

**Notes:**
Anchors behaving as they should (temperature and creep effects).
Production warm
Production cold and the lift increases
Creep and the lift increases more
Partially warm and the lift decreases
Shim and the lift decreases more
Cool and the lift increases to where we want it

## “Suspect” anchor magnet

**Notes:**
Notice Q2-5 does not move with temperature or reshimming. This is evidence that something is wrong.

## Broken anchors?

- We suspect that these magnets have broken “anchors”.
- There appear to be many of them in the ring
- Some magnets may have two broken anchors
- Anchors can be broken on the bench
- We don’t know the effect on the magnets
- But we have to study the situation

## Broken Anchors

**Notes:**
Here are some anchor assemblies out of our inventory of spare parts.  
Note that the anchor fits with an interference fit into grooves in the two stainless steel parts.  
The curved steel part fits against the cryostat and is welded to the cryostat.
The flat part mates to a flat surface machined in the coil’s collar.
The copper washer gets soldered to the liquid nitrogen temperature heat interceptor.
Note the small hole drilled in the anchor to allow air to escape readily during pump down.
Notice also that the cylinders are fabricated by wrapping the fiberglass around a cylindrical mold, meaning that the forces that we apply are not in a direction that compresses the layers.

In assembly the shimming was adjusted so that the smart bolts applied up to 3100 pounds to the anchors.
Two of the samples going around are ones that we recently tested to failure.
You can see the change in color where the fracture occurred, and see that the fractures started at the relief hole where the stresses were uneven.

## Further studies

- Study magnet from tunnel at MTF
- Look at its anchors
- Study spare magnets with same symptoms
- Open “dead” magnets to look at anchors
- Study tunnel data, acquire more data
- Gather more historical data
- This will take time, people
- Eight months?

**Notes:**
We have tunnel data on one more house, but no one to extract traveler data.
We need more tunnel data.
We need more traveler data.
We need autopsy and biopsy time, technician time.
We need MTF time.