Session 7: Open discussion: what are the guideline to steer the production and actions to be taken, as emerged from the workshop?

Session Chair: O. Brüning, L. Rossi, Scientific Secretaries: Massimo Giovannozzi, Christine Vollinger

1. Topic: values and steering of the magnetic length

Concerning the value of the magnetic length and its variation, there is an agreement that the magnetic length has to be in tight control with little variation. In order to steer the dipole integrated strength, the following suggestions were made:

Change the end laminations (as presented in the talk by Paolo Fessia).

Reduction of the length of the mandrel.

During the discussion Lyn Evans pointed out that special efforts were made during construction of the SPS machine to bring BdL within limits. Furthermore, he stated clearly that the mandrel should not be changed. Lucio Rossi wanted to have absolute certainty that the field of dipoles from Firm 3 is higher than that in the other manufacturers and suggested to wait for one more measurement before corrective actions are taken. He also explained that as long as the coil sizes are within limits, the company cannot be held responsible for the B₁ value. He then favoured the change in laminations as the easiest solution compared to a reduction of the mandrel size. Jos Vlogaert favoured the option of reducing the mandrel since a change of the laminations at the present production phase limits the corrective range of future actions. Paolo Fessia remarked that a change of mandrel, if considered, should happen now, since currently a number of mandrels are in production. Concerning the existing mandrel size, he already compared the mandrels of Firm 3 and Firm 2 and could not find any difference in the radii. Ezio Todesco pointed out that for the case that all companies show a trend in direction of a low B₁ later on, this might not be recovered, if the laminations are already added today. Davide Tommasini remarked that the mandrel should not be touched as long as the source of the high B₁ is not found, however, in case it is proven that the length is wrong, the mandrel has to be corrected.

Conclusion

Agreements were found on the following points:

• The magnetic length of the main dipoles should be corrected by changing the end laminations of the individual magnets. The mandrel will not be touched at the moment.
• From beam dynamics point of view, this adjustment has to be done symmetrically around the magnetic centre of the dipoles.
• In order to keep the costs for the corrective action as low as possible, or even cost-neutral, nested laminations between the different companies should be exchanged in a first step. Furthermore, Jos Vlogaert will start the discussions with the companies right now in order to profit from the already ongoing negotiations concerning the coil tolerances.
• A more detailed analysis of the behaviour of the dipole integrated strength at cold should be carried out to obtain a complete picture of this problem. In this respect, additional cold measurements of Firm 3 magnets have the highest priority.

2. Topic: odd order multipoles (b₃, b₅, b₇)

The value of odd order multipoles should be modified to bring them within the beam dynamics limits. Thus, a second correction of the coil cross-section is suggested (see also talks by Ezio Todesco and Stephane Fartoukh).

Stephane Fartoukh recommended a reduction of multipole b₃ to about 2-3 units on high field level. He insisted that from beam dynamics point of view, b₃ is still considered the most dangerous to control. Andrzej Siemko suggested changing only the insulation thickness on the copper wedges, instead of changing the size of the copper wedge, to reduce the costs. Ezio Todesco concluded as it was already presented in his talk, i.e. the only way to reduce the three multipoles b₃, b₅ and b₇ with one single correction is a change in the mid-plane thickness. Davide Tommasini commented that a change in the mid-plane thickness means little cost involvement as long as a commercially available insulation thickness is chosen. Jean-Pierre Koutchouk asked about the requirements by which the accuracy on the b₅ has to be set (answered by Stephane Fartoukh to approximately 20 %). Lucio Rossi suggested taking solution no. 4 which was proposed in the talk by Ezio Todesco. Solution no. 4 suggests an increase of the mid-plane insulation of 0.1 mm. This would still leave the possibility to take an additional correction (e.g. solution no. 1, same talk), if later on necessary. Solution no. 1 foresees a change of the outer polar shim of 0.1 mm. Lucio Rossi asked whether the expected values of the multipoles after this correction are satisfactory from beam dynamics point of view, explicitly b₇. Stephane Fartoukh could confirm this, also for multipole b₇ that seems to be acceptable, even if it is not within the desired beam dynamics bounds.

Rob Wolf pointed out that a step change on b₃ and b₅ (1 unit and 0.2 unit, respectively) has to be expected due to change in cable. However, this change will mainly affect the persistent currents, which have to be considered at injection only, when the correctors have their maximum power.

Conclusion

Agreements were found on the following points:

• The mid-plane thickness will be changed according to the solution no. 4 being presented in the talk by Ezio Todesco.
• In the case that an additional adjustment is necessary, the solution no. 1 from the presentation by Ezio Todesco, can be applied to further modify b₃.
• A cost estimate should be given by Davide Tommasini as soon as possible and before the meeting of the closed Panel following this workshop.

Minutes by Massimo Giovannozzi and Christine Vollinger