# Presentation: 869725

## Suspected Broken Anchors in Tevatron Dipoles

- David Harding, Ray Hanft, Jamie Blowers, John Carson
- 10 September 2003

**Notes:**
As the title of the talk says, and as you probably know from your invitation, we think that we have identified a potential problem with broken anchors in some Tevatron dipoles.
We have been looking at lift data as part of the quality control process in reshimming Tevatron dipoles, and stumbled on this effect.
This seems to affect perhaps as many as 20% of the magnets that we have looked at, but probably fewer.
We can make the argument that since the condition is so widespread and the Tevatron works, this cannot be a problem.
On the other hand, the Tevatron does not work as well as we would like, so we feel that it is prudent to investigate any abnormalities that we encounter, especially ones that have the potential cause unpredictable changes in the magnet behavior.
We are not here to advocate a specific action, certainly not any remedial action.
We just want to report this observation because we think that you should know about any interesting conditions in your machine.  
We also want to get a sense of how vigorously we should pursue a further understanding of this situation.  
Since one possible avenue of investigation would involve swapping out a suspect magnet or two, with the obvious impact on the shutdown schedule, you need to make a management decision on how worried you are.

## Outline

- Anchors – form, function, failure test
- Lift measurements – suggest a problem
- Implications
- Possible further studies

**Notes:**
What we think that we see is broken anchors in Tevatron dipoles.
I will start out by reviewing the function and form of the anchors and some recent bench top measurement on a limited sample of anchors.
I will review the lift measurements, which started out as a QC check but which turned out to hold even more interesting features.
We can discuss the implications of this finding for the magnet, and hence machine, performance.
And, finally, I will outline some possible directions of further attempts to understand what is going on.

## Tevatron Dipole Cross-section

**Notes:**
As you know, the collared coil in a Tevatron dipole is held in place in the 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.

## 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.

## Stress – strain curve on one recently tested anchor

**Notes:**
One of the samples failed at about 8000 pounds.  
You can see the initial seating of the anchor in the grooves, its elastic compression, and then its failure.
There is a suggestion that after giving a little, the remaining material is reseating and again providing some resistance.

## Stress – strain curve on another recently tested anchor

**Notes:**
The failure here came at 4000 pounds, still well more than the nominal but disturbingly different from the 8000 pounds at which the first one failed.
Given this range in the first two random samples, it would not surprise us to find anchors that failed at lower forces.
Notice the partial recovery that this anchor shows as well.

Now that we have a picture of what we are talking about, let me show you the data that caught our attention and the further pondering that we have done.

## Lift data

- Legacy (production) data (poor QC?):
- Warm production data
- Cold pre shimming production data
- Cold post shimming production data
- Modern data (better QC: use of PDA, etc.):
- Cold tunnel data
- “Cool” pre-reshimming data *
- “Cool” post-reshimming data *
- Cold post-reshimming data (after Tev is cold again)
- * 87 dipoles at LN2 in houses B1, C4, D1, and 19 dipoles at room temperature in A4

**Notes:**
First, used data to identify creep problem
Second, used data to look for anomalous magnets to avoid reshimming
Third, planned to use data as quality control check to assure that reshimming had been done correctly

As I said earlier, it was in the QC phase that we observed some anomalies that caused us to look deeper.
Our intended use of the measurements has biased the data collected, and the need to monitor progress in the tunnel has meant that we have not put as much effort into studying other effects as we might otherwise have done.
Based on our model of the magnet behavior, we expect fairly well-defined changes from one measurement to the next, subject to measurement errors and modest variability in the materials.
The only measurement that we can repeat is the most recent one, but the redundancy often allows us to make a good guess about which inconsistency should be attributed to measurement problems.

## Reshimming Quality Control

**Notes:**
Each magnet has an individual character and it is rare to find one that is perfect in every way to display as the “typical” magnet.
That said, we have chosen magnets to illustrate particular points.  They may have other defects that I could use to make other points, but I would rather not dwell on those.
This magnet shows the use of the before and after measurements as we intended to use them.
The numbers are lift before minus lift after, chosen in that order to make the numbers come out positive.
The horizontal axis runs down one side of the magnet (Quadrant 1), then down the other side (Quadrant 2).
Initially we had some trouble with these measurements because we didn’t realizing that the temperature was not stable, but rather warming up then cooling down.  Especially when we made the before and after measurements a day or even two days apart, this became quite significant compared to the measurement precision, though not large compared to the size of the mistakes that we were identifying.

This is illustrative of the mistakes we found.  

We found a few cases where a bolt was not completely tightened.  
We found one case where a shim pack had been dropped on the floor and some shims that rolled under the magnet were not recovered.  
These have all been corrected, but this required vigilance above ground monitoring the daily data dumps.
For every actual problem, there are many anomalous results that require careful study before they can be dismissed as measurement artifacts or other innocuous effects.  Ray and Jamie wind up examining every magnet in detail.  Although the last magnet was reshimmed on October 2, the analysis continues, with some adjustments here and there.

## Lift Summary Graph (TB0877 D13-5)

**Notes:**
The second tier of graph shows five measurements for each station and quadrant.  
Warm production
Cold post shimming production
Cold tunnel
Cool (or warm) pre-reshimming
Cool (or warm) post-reshimming
These are now absolute measurements, not differences.  
These are only suspension points, not anchors, because the geometry is too different to show on one graph.  I’ll come back to the anchors in a few minutes.
Notice the finger print of the magnet.  
Within the measurement precision the all the points go up and down together.
From warm to cold the lift increases with the material shrinkage.
From production cold to now cold, there is some creep in the suspensions and the lift increases a little more.
As the magnet warms from helium to nitrogen temperature, the lift decreases.
We add shims, and the lift decreases some more.

## Reshimming Quality Control

**Notes:**
On this magnet I call your attention to quadrant 1, station 5. This one has exactly zero change as a result of being reshimmed.
So, we go to the absolute lift data, as we did before.

## Anchor Lift Summary Graph (TC1177 C45-5)

**Notes:**
Here, instead of the two sets of eight suspensions, we have the two anchor lift measurements under the same five standard conditions.
In quadrant 2, we see the expected behavior.  
From warm to cold in production we see an appropriate shrinkage of the collared coil and anchors, leading to an increase in the lift.
From production to now we see a shrinkage in the anchors, and small increase in lift.
As the magnet warms to nitrogen temperature, we see a decrease in the lift of about the expected amount.
We reshim and the lift decreases again.
We expect that after returning to helium temperature the lift will be where we want it.

In quadrant 2, we see that when the magnet was initially cooled down it behaved as expected.  
Over the years, somehow, it has changed dramatically.
Then, when we warm up to nitrogen temperature, there is very little change.
And again, when we apply a shim, there is again no change.

Our interpretation of this is that an anchor has broken inside the cryostat.  We can’t tell which one, upper or lower.

## How widespread is the situation?

- Measure #1 –Pre-reshim lift cool to post reshim lift cool
- Measure # 2 –Modern cold to modern cool (pre-reshim)
- Measure #3 –Production (MTF) cold to modern cold
- Measure #4 –Production warm to production (MTF) cold

**Notes:**
Both measures 1 and 2 are reflect the response to modern changes.  Both are small and they are in the same direction.  So, we add the two together.  4 mil, 3 mil in anchors.
In measure 3 we expect to see roughly the same ” creep” in all the magnets.  An unusually large creep might be an indication of some failure.
Measure 4 should be a fairly large but uniform change.  24 mil in suspensions, 14 mil in anchors.  On a few we see no change, suggesting a problem.
On any any of these you have to decide what is within the normal spread of measurement uncertainty and what is a real anomaly. So, we make histograms to guide us in applying cuts to the data..

## M1+M2: Modern cold – post reshim cool“Small” response unexpected!

**Notes:**
As I said, we added the response to warming from helium to nitrogen temperature to the response from adding a 4 mil shim.  This was smeared out more than we expected, at least in part due to the unexpected variation in the so-called liquid nitrogen temperature.  
We plot the data from Q1 and Q2 separately.  Initially we did this because it was quicker than combining the data.  Here we see that the distribution doesn’t look quite the same for the two, as you would expect it to.
We have, somewhat arbitrarily, said that any anchor bolt that doesn’t respond by at least 2 mil to these changes is suspicious and deserving further investigation.

## M3: Production (MTF) cold – modern cold“Large” creep unexpected!

**Notes:**
We set the cut at 

## M4: Production (MTF) cold – production warmSubstantial deviation from 0.014 unexpected!

**Notes:**
A couple of clunkers.  We set the cut at

## M3 vs. M1+M2

**Notes:**
You might say that if an anchor met one criterion or another for being bad, then it was suspicious, but that if it met two of the criteria it was almost surely bad.  So, we make a scatter plot.  
You can clearly see some that are especially suspicious, failing two tests by being high and to the left.  Others seem to fail the reshimming test, though they aren’t off in the extra creep measure or vice versa.
This gives a sense of how many magnets are involved and identifies a collection of magnets for further study.  This study includes looking closely at the behavior of the whole magnet, using the graphs that I showed earlier.  It includes going back to the travelers to make sure that nothing was missed, for example a later rework of the magnet that would have changed the initial data.  And it includes doing some additional experiments in the tunnel.
For example, we learn that Txnnn ...
And that 
And that 
And that

## Shimming response

**Notes:**
What can we do to further understand the magnets?
We can check that the shim packs are correct by taking out the bolts and looking.
Perhaps more interesting, we can measure the response of the lift to additional shimming.  The smart anchor bolts have etched on the side the initial “free lift”, the lift before installation in the magnet. 
We can compare that to the current free lift to see if a Belleville washer in the bolt has deformed.
Then we can add shims, we chose 4 mil increments, and measure the response of the lift.  If the internal problem we are hypothesizing is real, then we expect the initial lift when installed to be equal to the free lift.  Then, as we add shims, we should eventually make solid contact with the anchor and see the lift change.
Staying away from the original values established for production, we limited the number of shims we would add to keep from applying a dangerous amount of force and risking further broken anchors.
As a control, we measured both anchors on the magnets – one suspicious and one apparently good.
This shows a sample of our measurements.  You can see a that some start out with exactly the response you expect for a good configuration – a linear decrease in lift with the number of shims added.  In other cases there is no, or little, response, initially, but the eventually the bolt engages the cryostat and the anchor, and the lift decreases linearly.

Tie back to bench top anchor failure measurements...

## Implications for Tevatron

- Maybe none
- Maybe movement on each ramp
- Maybe movement on quenches or thermal cycles
- So far, no more than one suspect anchor per magnet
- At least two dipoles have operated for 20 years in this state

**Notes:**
What can we imagine happening inside the magnet with a broken anchor.
The easiest thing for us to imagine is that nothing happens.  This could be wishful thinking, but the other three anchors, and even a broken anchor, would seem to hold the collared coil firmly in position.
If we strain, we can imagine a small movement of the coil with each ramp, as the unbalance Lorentz forces pull the collared coil one way or another, depending on the offset from center.  This would then add a current dependent a1 and b1 term.  We don’t think this is likely.  The remaining anchor pair will resist this motion.  
We can also imagine small movements in response to larger, stressful events like quenches and thermal cycles, but again it does seem likely.  The anchor system was designed to resist such movements, and it is easy to argue that it was over-designed, allowing the remaining three anchors to hold the collared coil in position.
As I said early on, perhaps the strongest evidence that this is not a fatal problem is that the Tevatron is still alive.  However, except for a few cases, we can’t know exactly when this condition developed – whether this is an infant mortality thing or where repeated stresses are finally taking their toll.
We invite anyone with an active imagination for help in developing disaster scenarios, especially testable ones.

## What can we do?

- No remediation planned during this shutdown
- If not broken but just short, could add more shims
- Broken anchors can be replaced, but only with significant effort in IB2
- Study one or more samples at MTF

## Potential studies

- Biopsy odd magnet(s) to see whether anchor(s) are really broken
- Magnetic measurements to see response to broken anchors
- Other?

**Notes:**
We could open up a magnet which shows a string indication of having a broken anchor to see what we find.  I call this a biopsy rather than an autopsy, because I don’t want to imply that the magnet is dead.  We would replace the broken anchor, weld it all up again, and have a good magnet.
For magnetic measurements, I would expect to put a probe in the center of the magnet and measure the field harmonics as a function of current, comparing with one or more good magnets.  Deviations from standard with a quadratic dependence on current would suggest movement in response to a magnetic force.

## Possible sample magnets

- One or more magnets from tunnel houses currently warm
- Suspect magnet(s) from spares pool
- Marginal performance magnet(s) from spares pool that we could break
- Need guidance

**Notes:**
There is nothing to compare with actually opening up a magnet that has shown this symptom in operation in the tunnel for knowing what is down there.  Other magnets just don't have quite the same history.  We do understand that this would be a blow to the schedule, so we are not advocating this.  Also, at this point A2 seems the most likely source of candidates, but we have not yet been able to gather the information to let us make a selection.
We haven’t looked at magnets in storage, but assume that we will find the same disease.  Some of them have had a fair amount or running time in the tunnel.  We don’t have as rich a collection of measurements to help assess the situation of individual magnets.  Once the shutdown is over, however, we could go through and do our lift versus shim response test to select candidates.  We cannot guarantee that we will find one, and we won’t be sure that it is broken in the same way.
Perhaps the richest test would be to choose a mechanically sound magnet or two with marginal quench performance, one that we would not go into the ring for 980 GeV operation, and break it.  This would let us do magnetic measurements on an intact system to establish a baseline.  We would break one anchor and remeasure.  Then we could break an anchor on the other diagonal and measure again.  We assume that we could squeeze one hard enough to break it, but we could not be sure that it was broken in the same way.
The big question is how worried are you?  What is the most useful information that we could learn?  Any of these would be a substantial effort in Technical Division and it wouldn’t take much to convince us to forget it all.  Pulling a magnet from the tunnel would be a big deal for Beams Division and the shutdown schedule.  Would the knowledge gained be worth the investment?  That is the one decision that needs to be made quickly.