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Field quality of Fermilab’s Nb 3 Sn magnets

Field quality of Fermilab’s Nb 3 Sn magnets. Director’s Review of the Fermilab High Field Superconducting Magnet Program. G.Velev for the HFM group January 31, 2006. Introduction. Five nearly identical shell-type dipole models (HFDA02-06) were built and tested.

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Field quality of Fermilab’s Nb 3 Sn magnets

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  1. Field quality of Fermilab’s Nb3Sn magnets Director’s Review of the Fermilab High Field Superconducting Magnet Program G.Velev for the HFM group January 31, 2006

  2. Introduction • Five nearly identical shell-type dipole models (HFDA02-06) were built and tested. • One magnet, HFDC01, with common coil geometry was built and tested • The test offered a unique opportunity of systematic study of the field quality in Nb3Sn accelerator magnets. • The field quality was measured under various conditions and compared with the theoretical predictions. Field quality of Fermilab’s Nb3Sn magnets

  3. HFDA Magnet design Common features: • 2-layer shell-type coil; • 43.5 mm bore, cold iron yoke; • Same cable dimensions. Major differences: • HFDA02-04 1-mm MJR strand; • HFDA05-06 1-mm PIT strand; • HFDA02-03 25 mm stainless steel (SS) core between the strands; • HFDA04-06 no SS core. Field quality of Fermilab’s Nb3Sn magnets

  4. Measurement/naming convention • A 250 mm probe was utilized in HFDA02-04/06 and a 43 mm probe was used in the HFDA05. All the probes are 25 mm in diameter. • The field harmonics were represented by the following expansion: • A probe centering correction was performed by zeroing the unallowed by the dipole symmetry a8 and b8. • The main field was assumed to be pure normal (no skew dipole component) and a corresponding field angle was assigned. • All harmonics are reported at r0=10 mm. Field quality of Fermilab’s Nb3Sn magnets

  5. Geometrical harmonics The geometrical harmonics were determined as the average values between up and down ramps at 3 kA in the body. Geometrical field harmonics, 10-4 HFDA02-04/06 had large skew or normal quadrupole components. Possible explanation top-bottom or left-right asymmetry in the coils due to heat treatment of the assembled two half-coils with no initial prestress. Field quality of Fermilab’s Nb3Sn magnets

  6. Slice studies HFDA02 coil was cut in the straight section (with the yoke in place) and the block coordinates were measured by an optical inspection system. Block deviations from the nominal position y 6 3 5 2 1 4 X • Major findings: • Radial position  systematic shift towards the center in all the blocks; • Azimuthal position  significant random deviations from the nominal in all the blocks; • Midplane gap  larger than the nominal by 200150 mm. Field quality of Fermilab’s Nb3Sn magnets

  7. Corrective actions • Action: • a thick steel plate was introduced between two half-coils of HFDA04 during the heat treatment. • Result: • the normal quadrupole component was improved; • the skew quadrupole got larger, possibly due to the opposite half-coil orientations with respect to the gravity vector during the coil heat treatment. • Action: • the half-coils of HFDA05 and HFDA06 were reacted and impregnated individually with the same orientation relatively to the gravity vector. • Result: • the harmonic measurements in HFDA05/06 (except for a2 in 06) magnet showed the best geometrical field quality among HFDA models. Field quality of Fermilab’s Nb3Sn magnets

  8. Persistent current effect • The persistent current effect was similar and well predictable in HFDA02-04 made of 1-mm MJR strand. • In order to reduce the persistent current effect, simple passive correctors based on thin iron strips were developed and successfully tested. • The passive correction has effectively reduced the sextupole variation in the field range of 1.5-4 T during the field up-ramp from 19.410-4 to 3.710-4. Iron strips Field quality of Fermilab’s Nb3Sn magnets

  9. Flux jump effect • Conductor instabilities are affecting also the field quality. • While it may not be relevant for the short magnet development, a successful accelerator magnet needs to demonstrate the “accelerator” field quality along with the reliable quench performance. Field quality of Fermilab’s Nb3Sn magnets

  10. 1-mm MJR/PIT strands Sextupole @10mm in HFDA magnets Noise in the sextupole HFDA06 HFDA05 HFDA02 b3 , 10-4 HFDA04 HFDA03 Time (s) • What looks like “noise” in HFDA02-04 measurements is actually reflection of the flux jumps in the field quality. • The noise level was ~50 times lower as shown in the plot above. Field quality of Fermilab’s Nb3Sn magnets

  11. 0.7-mm MJR strand Sextupole @10mm in HFDC01 magnets • The persistent current effect was ~7 times lower in the HFDC01 common-coil magnet than in HFDA02-04 magnets due to the specific coil layout. • However, the amplitude of field oscillations was lower by only a factor of ~1.5 that is consistent with 30% smaller deff. • Thus the effect of flux jumps can not be reduced by simply correcting the persistent current effect. ~ 8 units vs 55 units Field quality of Fermilab’s Nb3Sn magnets

  12. Unallowed harmonics Quadrupole in HFDA04 magnet • Large fluctuations in the normal and skew quadrupole components of HFDA04 dipole magnet are observed. • The flux jumps can happen in any region of the coil under favorable conditions (when the stability criterion is violated). • They are not necessarily complying with the magnetic field symmetry (e.g. dipole) and can produce fluctuations in allowed and unallowed harmonics. Field quality of Fermilab’s Nb3Sn magnets

  13. Reproducibility HFDA02/05 consecutive cycles • The field quality is presented for two consecutive current cycles in HFDA02/05 magnet. • Harmonic fluctuations are not repeatable from cycle to cycle. • They can not be predicted or measured in order to apply corresponding correction using passive or active correction system. • The only way to improve the field quality is to reduce the flux jump amplitude. HFDA02 b3, 10-4 HFDA05 B (T) Field quality of Fermilab’s Nb3Sn magnets

  14. Eddy current effect HFDA02-04 • Coil magnetization in HFDA05/06 was different from the first three magnets. • A similar behavior was observed in SSC dipole DCA312 with a low interstrand resistance. • Major difference between the first three and the last two magnets: RRRHFDA04~5, RRRHFDA05~110. • Db3 extrapolated to dI/dt=0 is consistent with the expected persistent current effect. HFDA06 B = 2 T Field quality of Fermilab’s Nb3Sn magnets

  15. Tevatron dipole b20 t0 Sextupole decay • The decay measurements were performed at constant currents around 1.5 T field after a pre-cycle with 20 A/s. • There was no significant b3 variation during 30 minutes in HFDA02-04, though a periodic oscillation in HFDA04 and HFDA06 was observed. • There was a distinct sextupole variation in HFDA05 that decayed by 8 units during the first 30 minutes at current plateau. • More data are needed to constrain a model for the dynamics effects in Nb3Sn magnets b3, 10-4 time (s) HFDA06 HFDA05 b3, 10-4 HFDA04 HFDA02 HFDA03 time (s) Field quality of Fermilab’s Nb3Sn magnets

  16. Conclusion • Five nearly identical short Nb3Sn dipole models were fabricated and tested at Fermilab. First systematic studies of field quality in Nb3Sn magnets were performed. • The geometrical harmonics were determined by the magnet fabrication tolerances. Noticeable improvements of the low-order geometrical harmonics were achieved after some optimization of the coil fabrication process. There is also a room for further improvements. • The persistent current effect was well predictable in all the magnets. A passive correction technique was developed and successfully tested. • The measured amplitude fluctuations of the low-order harmonics in the models due to flux jumps is in the order of 1-2 units. These random fluctuations may not effect the beam dynamics (need to be confirmed by AP) , they will reduce the accuracy of the field quality measurements. • The large eddy current effect observed in HFDA05 and 06 magnet is due to low interstrand contact resistance. It can be reduced by using a stainless steel core in the cables with high RRR. • Magnetic measurements are a powerful method for magnet diagnostics – they will provide important information for LARP magnet R&D Field quality of Fermilab’s Nb3Sn magnets

  17. Field quality of Fermilab’s Nb3Sn magnets

  18. Backups B I b3 b3 Time (s) Field quality of Fermilab’s Nb3Sn magnets

  19. HFDA06 Field quality of Fermilab’s Nb3Sn magnets

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