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Can CUDI be used for decay and snap-back reduction and/or prediction? Arjan Verweij, AT/MAS-SC

Can CUDI be used for decay and snap-back reduction and/or prediction? Arjan Verweij, AT/MAS-SC. Talk Nicholas:  Experimental data from SM18 give the best input of the expected decay in the different magnet types for a number of given current histories.

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Can CUDI be used for decay and snap-back reduction and/or prediction? Arjan Verweij, AT/MAS-SC

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  1. AT-MAS/SC A. Verweij 17 April 2007 Can CUDI be used for decay and snap-back reduction and/or prediction?Arjan Verweij, AT/MAS-SC

  2. AT-MAS/SC A. Verweij 17 April 2007 Talk Nicholas: Experimental data from SM18 give the best input of the expected decay in the different magnet types for a number of given current histories.  Scaling, which is required because tested magnets are not equal to the statistical mean in the machine, seems to work well. Forecast to minimise decay not available. Limited data available for different current histories Possible solution: simulate the cause/origin of the decay, and then use these results to calculate the decay.

  3. AT-MAS/SC A. Verweij 17 April 2007 Origin of decay/snap-back Decay is due to current re-distribution among the strands, causing field variations inside the cable, which in turn cause M variations because |dM/dB| is completely different for positive en negative B. The origins of current redistribution are: - Redistribution of the transport current, due to non-uniform joints/splices. - Boundary-Induced Coupling Currents (BICCs), induced during ramping, mainly due to variations in dB/dt along the cable. B M

  4. AT-MAS/SC A. Verweij 17 April 2007 Origin of decay/snap-back In a dipole magnet there are (per aperture): - 5 joints - about 2x4x15=120 strong d(dB/dt)/dz variations (inner layer) - about 2x4x25=200 strong d(dB/dt)/dz variations (outer layer) Conclusion: The decay will be dominated by the BICCs. Each boundary (or non-uniformity) in d(dB/dt)/dz will cause BICCs, diffusing through the cable. The local diffusion speed and amplitude increase depend on the local contact resistances. Interference of the BICCs will occur because diffusion lengths are larger than half the magnet length.

  5. AT-MAS/SC A. Verweij 17 April 2007 Exact calculation Exact quantitative calculation of the BICCs and hence the decay is impossible in a magnet because: - the local Rc values are unknown - interference of the BICCs depends strongly on the cable transposition length However, using CUDI –even on a relatively simple cable configuration- seems to give good qualitative correlation with the measured data.

  6. AT-MAS/SC A. Verweij 17 April 2007 CUDI (electrical module) ► Typical discretization: 1-4 mm3 Picture courtesy of R. de Maria, CERN-AB Nodal equations: Loop equations: Conservation of transport current:

  7. AT-MAS/SC A. Verweij 17 April 2007 Output parameters Input parameters Cable geometry:incl. return lead, mixed cables (s.c. strands + copper strands) 2 independently applied fields:arbitrary direction, variations along length and across width Ra and Rc:variations along length and across width, local variations, random distributions, soldered cables, zebra type cables, cables with non-uniform strand coating/oxidation, cores Critical current:incl. variations per strand, local Ic variations (e.g. due to edge degradation, broken filaments etc) SC-normal transition:‘n-power’ or ‘matrix resistivity’ models Strand resistivity:incl. local strand resistances (cold welds, broken strands, soldering to another cable) Voltage taps:On single strands, or entire cable Transport current:uniform or non-uniform distribution at the cable ends (simulating non-uniform cable joints) Energy pulse:local or global (e.g. for stability and Minimum Quench Energy calculation) Heat flow parameters:along the strand, to the adjacent and crossing strands, and to the helium Currents: in strands, and in contacts Ra and Rc Powers:in strands, and in contacts Ra and Rc Inter-filament coupling power Resistivities of the strands Voltages: resistive and inductive Temperatures: of strands and surrounding helium Self-field Field along arbitrary line in space Magnetisation Heat flows: in the strands, between the strands, to the helium

  8. AT-MAS/SC A. Verweij 17 April 2007 The Program Definition of the parameters (LabView) csv Data from other sources (field maps etc) Input Data CUDI.exe (executable FORTRAN code) Output Data (csv format) Excel or LabView based visualisation of the results

  9. AT-MAS/SC A. Verweij 17 April 2007 CUDI (electrical module) • Define a cable geometry • Define a I(t) and B(t) cycle • CUDI will then calculate, at user defined times: • the exact currents in all the elements of the circuit (i.e. Itransp + BICCs + ISCCs), • the field pattern next to the cable, or elsewhere (as a result of Itransp, BICCs, and ISCCs), • the magnetization M in each element of the circuit. Variation of M at constant Itransp is a measure for the decay.

  10. AT-MAS/SC A. Verweij 17 April 2007 Model multiturn coil by a simple straight cable with a few RA, RC and dB/dt variations Constant RA & RC, 2 (dB/dt)/dz boundaries constant Rc dB/dt z Non-constant RA & RC, 4 (dB/dt)/dz boundaries low Rc medium Rc high Rc medium Rc low Rc dB/dt z

  11. AT-MAS/SC A. Verweij 17 April 2007 Results for standard SM18 cycle start injection end injection start precycle

  12. AT-MAS/SC A. Verweij 17 April 2007 Results for standard SM18 cycle

  13. AT-MAS/SC A. Verweij 17 April 2007 t_FT

  14. AT-MAS/SC A. Verweij 17 April 2007 I_FT

  15. AT-MAS/SC A. Verweij 17 April 2007 dI/dt

  16. AT-MAS/SC A. Verweij 17 April 2007 Decay reduction by adding 400 s in 5 different ways

  17. AT-MAS/SC A. Verweij 17 April 2007 Decay/snap-back reduction: Results

  18. AT-MAS/SC A. Verweij 17 April 2007 You need time to get decay to 0 !!!!

  19. AT-MAS/SC A. Verweij 17 April 2007 Discussion/Conclusion • CUDI calculates the cause of M decay/snap-back, i.e. the BICCs, and can therefore be used as a predictive model. • A very simple cable geometry gives already good qualitative agreement with data from SM18. Better quantitative agreement could be obtained by using a more sophisticated cable geometry. However, one should not expect to get perfect quantitative agreement. • Using CUDI seems a good way to select the best options for minimising the decay/snap-back in the machine, which then in turn can be experimentally validated in SM18. • It would be possible to run CUDI on-line with the machine, which would then give at any moment and for any current history approximative values of the BICCs, the field pattern along the magnet, the magnetization, and maximum possible decay/snap-back.

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