Warm-Cold Changes in the Sextupole Harmonic in the Quadrupole Magnets for the BEPC-II Luminosity Upgrade Animesh Jain Brookhaven National Laboratory Upton, New York , USA 14th International Magnetic Measurement Workshop Geneva, Switzerland, September 26-29, 2005

IMMW14, Geneva, Switzerland, Sept , 2005 Animesh Jain: BNL 1 BEPC Quad Construction BEPC Quadrupoles (SCQ) consist of 4 sets of double- layer coils, directly wound on a support tube. Field quality was measured (warm, at ±1A) after each double-layer serpentine coil set was wound. The coil pattern for the next layer was modulated to cancel any large harmonics measured at the previous stage. The final “as-wound” field quality was very good – well below the specification of 3 units at 50 mm. There were small changes after the final skew layers were wound, but the sextupole was still quite good.

IMMW14, Geneva, Switzerland, Sept , 2005 Animesh Jain: BNL 2 Low Sextupole Content

IMMW14, Geneva, Switzerland, Sept , 2005 Animesh Jain: BNL 3 Cold Field Quality Measurements Cold field quality measurements were carried out in a vertical dewar. The quadrupoles (SCQ) were measured at currents ranging from 20 A to 550 A. The variation of sextupole terms (in Tesla at 50 mm) was linear with current, as expected. The “geometric” sextupole terms were derived from the slope of a straight line fit. Sextupole in “units” is calculated by comparing this slope with a similar slope for the quadrupole term.

IMMW14, Geneva, Switzerland, Sept , 2005 Animesh Jain: BNL 4 Quadrupole Slope = T.m/kA Magnet #1

IMMW14, Geneva, Switzerland, Sept , 2005 Animesh Jain: BNL 5 Quadrupole Slope = T.m/kA Magnet #1

IMMW14, Geneva, Switzerland, Sept , 2005 Animesh Jain: BNL 6 Quadrupole Slope = T.m/kA Magnet #2

IMMW14, Geneva, Switzerland, Sept , 2005 Animesh Jain: BNL 7 Quadrupole Slope = T.m/kA Magnet #2

IMMW14, Geneva, Switzerland, Sept , 2005 Animesh Jain: BNL 8 Warm to Cold Discrepancy The geometric values of sextupole derived from the cold data were much larger than the warm values measured before cold test. QHG202:b 3 = 5.3 units cold (was 0.35 warm) a 3 = 4.5 units cold (was –1.28 warm) QHG203:b 3 = 1.5 units cold (was 1.80 warm) a 3 = –5.5 units cold (was –1.63 warm) Possible sources: Distortion under cool down; persistent current effects from other layers; measurement errors due to a tilt of the measuring coil with respect to magnet axis, iron in and around the dewar,.....

IMMW14, Geneva, Switzerland, Sept , 2005 Animesh Jain: BNL 9 Warm to Cold Discrepancy Out of the possible sources, distortion under cool down, persistent current effects, and iron around the dewar seemed to be very unlikely causes. Distortion: Unlikely that only one harmonic will be affected. Also, no such effects were seen in earlier magnet productions. (Distortions also ruled out by measurements at 35 K in magnet #2: to be discussed later) Persistent Currents: Should produce a hysteresis (Up Ramp to Down Ramp difference), which is not seen. Iron around the Dewar: Should affect both magnets in a similar way, since they were tested in the same dewar, and were mounted similarly.

IMMW14, Geneva, Switzerland, Sept , 2005 Animesh Jain: BNL 10 Warm-Cold Difference: Tilt of Coil For long magnets, and magnets with negligible end harmonics, a tilt of the measuring coil with respect to the magnet axis does not affect the measurement of harmonics in a dipole or a quadrupole magnet. The BEPC magnets are short, with serpentine coil design, and have large end harmonics. The skew octupole harmonic is large, and of opposite sign, in the lead end and non-lead end of the magnet. A tilt of the measuring coil will cause a sextupole term by feed down, and the contributions from the two ends will add up.

IMMW14, Geneva, Switzerland, Sept , 2005 Animesh Jain: BNL 11 A tilt of the measuring coil implies offsets of opposite sign at the two ends. This, coupled with the opposite signs of the skew octupole, will cause a spurious sextupole due to feed down.

IMMW14, Geneva, Switzerland, Sept , 2005 Animesh Jain: BNL 12 Sextupole term with no tilt. Integral ~ 0 unit

IMMW14, Geneva, Switzerland, Sept , 2005 Animesh Jain: BNL 13 Sextupole with tilt: Integ. b 3 = 3 unit Integ. a 3 = 1.7 unit Axis used for computations:

IMMW14, Geneva, Switzerland, Sept , 2005 Animesh Jain: BNL 14 Is Tilt Really the Cause? If the measured warm to cold difference is indeed a result of the tilt of the measuring coil, then even a warm measurement in the vertical dewar should show equally large sextupole. Measurements were carried out in the vertical dewar in magnet #1 after the cold tests were completed. Warm sextupole in dewar was much smaller than the cold value, although not as low as the initial warm measurements. Magnet #1 was also warm measured horizontally after the cold test, and was found to have low sextupole, matching the initial warm values before cold test. An estimate of maximum effect from tilt was also obtained by measuring with the coil deliberately tilted. A comparison of final warm measurements horizontally and vertically gives an estimate of the actual effect of tilt.

IMMW14, Geneva, Switzerland, Sept , 2005 Animesh Jain: BNL 15

IMMW14, Geneva, Switzerland, Sept , 2005 Animesh Jain: BNL 16 Estimates of Tilt-Corrected Sextupole (Cold) A tilt of the measuring coil perhaps contributed to about unit of b 3 and about +0.2 unit of a 3 in magnet #1 (see the table in the previous slide). Subtracting this contribution from the cold values, the best estimates of cold sextupole harmonics in the magnet #1 are: b 3 = +3.6 unit, and a 3 = +4.3 unit. A similar exercise for magnet #2 gives estimates of cold sextupole harmonics as: b 3 = 1.2 unit, and a 3 = –5.2 unit. Although the tilt correction improves sextupole a little, it is still mostly larger than the 3 unit limit.

IMMW14, Geneva, Switzerland, Sept , 2005 Animesh Jain: BNL 17 Distortions due to Cool Down? In view of the surprising results in magnet #1, we carried out extensive studies during cool down of magnet #2 to investigate any effect of cool down itself. Measurements were carried out at ±1 A in the vertical dewar before cool down, and then at various stages of cool down at 35 K and 80 K. The temperatures were chosen to be high enough such that no superconductor magnetization effects are present, but low enough that nearly all the mechanical contraction had already taken place. No significant differences between the warm and the cold harmonics (at ±1A) were seen, thus ruling out any distortions as a possible cause of the sextupole change.

IMMW14, Geneva, Switzerland, Sept , 2005 Animesh Jain: BNL 18

IMMW14, Geneva, Switzerland, Sept , 2005 Animesh Jain: BNL 19 Effect of Support Tube Magnetic Properties? The support tube material was chosen to be stainless steel 316L, and is certified to be seamless by the vendor. We measured the ferrite content around the circumference of the support tube in magnet #2 before it was cooled down. The ferrite number varied azimuthally from 0.02 to 0.9 near the non-lead end, and from 0.05 to 0.6 at the lead end. (A ferrite no. of 1 is  ~ 0.3) These ferrite numbers are quite large, and represent significant azimuthal asymmetry in the magnetic properties, affecting mostly the low field measurements.

IMMW14, Geneva, Switzerland, Sept , 2005 Animesh Jain: BNL 20 A Closer Look at the Cold Data It is expected that the ferrite content in the support tube will affect mostly the low field measurements. At higher fields, the small ferrite particles saturate, and the permeability becomes essentially ~1. If this is true, significant non-linearity should be seen at the low field region of the cold data. A departure from the high field slope was indeed found for currents below ~25 A in both the magnets. The very low field slopes match very well with the warm measurements.

IMMW14, Geneva, Switzerland, Sept , 2005 Animesh Jain: BNL 21 Low Field Sextupole in Magnet #2 Low Field slope: a 3 = –2.3 unit High Field slope: a 3 = –5.4 unit Additional cold data taken in magnet #2 in 5 A to 40A range

IMMW14, Geneva, Switzerland, Sept , 2005 Animesh Jain: BNL 22 Conclusions Large discrepancies have been seen between the warm and the cold sextupole harmonics in both the BEPC quadrupole magnets. A number of possible causes of this discrepancy have been eliminated by a series of very carefully planned warm and cold measurements. The source is proved to be a non-linearity at very low fields (below ~25 A), rather than any real warm to cold effect, or superconductor effects. This non-linearity is most likely caused by the magnetic properties of the support tube.