Not all the points should be discussed. Any participant that has other
relevant topics is invited to contact the Discussion Session Charis and
bring it up.
SOME SUGGESTED TOPICS FOR THE EUCARD2 WORKSHOP DISCUSSION PERIODS: PROPOSED BY THE IAC
The process of setting the magnet requirements
(1) When choosing the maximum integrated strength to require, can the beam physicist trust the magnet engineer to provide that strength? Or does the beam physicist ask for 5% more strength than they really need, but the magnet engineer does not know that and adds another 5% to the current the computer model of the magnet predicts. Then the power supply engineer does not trust the magnet engineer and adds another 5% to the power supply current specs. So one ends up with a much stronger and more expensive magnet and power supply system than one needs.
(2) How precisely do beam physicists need to know the higher multipoles in a magnet? Before a magnet is fabricated we can only predict the allowed multipoles in a magnet. Prior experience in fabricating and measuring magnets gives one an idea of the typical range of sizes of all their multipoles. But do the beam physicists modeling the beam dynamics as it passes through the lattice use the same multipole values in all the magnets of the same style? That is not a realistic scenario, is it not better to use different multipole values in different magnets of the same style when simulating a beamline of magnets? Then a “tolerance” study can be carried out.
(3) Can lattice designers remember a physical magnet is longer than its effective length? If they are not sure how much longer to allow for the coils then they should consult with their magnet engineers.
(4) When the beam physicist chooses the magnet aperture can they specify if they’ve left room for a beampipe or not?
(1) Before starting to design a magnet the magnet engineer needs to know several other characteristics from the beam physicist: does it run in a DC mode or how is it ramped; does it need to run in a bipolar way; will the magnet style be needed in large quantities; will groups of the magnets be strung on one power supply; are there vibrational tolerances; does it matter how much heat the magnet gives off; how precisely will it have to be placed and aligned in the beamline; add your favourite characteristic?
(2) It is not always necessary to do a 3D computer model of a magnet, a 2D model can often produce a magnet that meets the field quality requirements.
(1) The un-allowed multipoles in any magnet arise from asymmetries created during the fabrication process, usually in the steel core but could also be in the coils. In order to estimate the mechanical tolerances to put on the magnet drawings one can use a set of coefficients developed by Klaus Halbach in 1969 which correlate different types of errors in the placement of poles with the creation of un-allowed multipoles, e.g. a sextupole component in a quadrupole. Magnets can be mechanically measured on a Coordinate Measuring Machine to find the fabrication asymmetries and then magnetically measured so one can evaluate the correlations predicted by Halbach’s perturbation analysis. Knowledge of such correlations can guide the setting of mechanical tolerances on future magnets.
Magnetically Measuring Magnets
(1) What parameters need to be measured on standard magnets?
(2) When is an integrated strength not sufficient, when is a 3D field map needed? Under what circumstances would one need to make a 3D field map of a solenoid?
(3) If a dipole is curved would a 3D field map around the ends of the magnet core be useful to the beam physicist?
(4) What order polynomial do you want to use to relate current to integrated strength; this will affect how many different currents to measure the strength at?
(5) How do you calibrate the magnetic measuring apparatus so its field accuracy is reliable? Should all types of apparatus be traceable to an NMR probe?
(6) How precisely do the higher-order multipoles need to be measured?
(7) Does a beam physicist need to know the sizes of multipoles beyond the 2nd allowed harmonic, e.g. in a typical quad do you need to know the 28-pole’s size, in a typical dipole do you need the 14-pole’s size?
(8) Does a beam physicist need to have separate measurements of the normal and skew higher-order multipoles? How precisely do they need the angles of the poles measured?
(9) If there are for e.g. 5000 quadrupoles of exactly the same style fabricated for a new accelerator, and there are not enough resources to magnetically measure every one, what is the lowest fraction of these quads that would need to be measured so a beam physicist would still be able to commission and operate the beamline? The rest would be characterized by average values.
Operating magnets in an accelerator beamline
(1) What criteria are used to determine if a magnet needs to sit on a mover that can adjust the magnet’s position in the beamline via motors under remote control?
(2) The beam physicist should have set a reproducibility tolerance on the integrated strength after the magnet has been turned off and back on. The magnet engineer will have developed a standardization procedure for the magnet to be taken through whenever it is turned on, that will guarantee it produces the same integrated strength at a set current to within that tolerance. If the beam physicist and accelerator operators get impatient and do not allow the magnets to be standardized when they have been off for any reason then the reproducibility tolerance may not be satisfied.
(3) When does online monitoring of the magnetic field value need to be done?