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Engineering Note DocumentTest Results of LCLS-II Cryomodule Magnet SPQA129Document Number: LCLSII-4.5-EN-NNNN-R0 of 33

Test Results of LCLS-II Cryomodule Magnet SPQA129J. DiMarco, V. Kashikhin, O. Kiemschies, T. Strauss, M. Tartaglia

I. IntroductionHE production magnet SPQA129 for LCLS-II Linear Accelerator [1] was built by Milhous and tested at Fermilab. The magnet package is conductively cooled by LHe inside the Cryomodule, and has a splittable in

the vertical plane configuration (see Fig.1). The magnet was built in an agreement with the magnet physical requirements LCLSII-2.4-PR-0081-R0, and the specification LCLSII-4.5-ES-0355-R0. The magnet design, and previously tested magnets are described in [2] – [10], [13-14].

T Two prototype magnets SPQA01 and SPQA02 were built for testing the verification of the magnet design and the conduction cooling (See LCLSII-EN-0612). This note describes the bath cooling test results of SPQA128 magnet including high precision magnetic measurements by a rotational coil system. The magnet view inside SCRF Cryomodule is shown in Fig. 1.

Fig. 1. Magnet package inside the LCLS-II SCRF Cryomodule.

The main goal of the magnet test at FNAL Technical Division Stand 3 is to test magnets in the bath cooling mode, and prove their magnetic performance in the wide range of operating currents and operating scenarios. One of the most critical magnet specifications is to provide quadrupole, and dipole corrector field reproducibility in the range of ±1 %. Most of uncertainty in the magnet strength is caused by the iron core hysteresis effects which substantially increase at low field levels. To reduce these effects, degaussing and standardization procedures were used. All three magnet power supplies are bipolar and provide the full current operational range of ± 20 A for each magnet circuit, which allows up to 2.5 T integrated quadrupole field gradient. Bipolar cycling will be used for magnet degaussing. During operation, the quadrupole magnet will not change in polarity, except possibly the first magnet, QCM01, and unipolar full or partial cycling will be used for the quadrupole standardization procedure. The test results were used to formulate the magnet operation procedures in the accelerator to obtain reproducible magnet strength vs. current variations for quadrupole and dipole magnets.

The main magnet parameters shown in Table 1, and the magnet cold mass in Fig. 2. The magnet test plan described in LCLSII-4.5-PP-0731-R0.

The only official copy of this file is located in the LCLS-II Controlled Document Site. Before using a printed/electronic copy, verify that it is the most current version.


TABLE ILCLS-II MAGNET PACKAGE PARAMETERS

Parameter Units ValueIntegrated peak gradient at 10 GeV T 2.0Integrated peak gradient at 0.4 GeV T 0.05Clear bore aperture mm ≥78Ferromagnetic pole tip bore diameter mm 90Effective length mm 230Peak quadrupole gradient T/m 8.67Quadrupole field harmonics at 10 mm radius % ≤1.0Quadrupole magnet inductance (DC) H 0.66Number of superconducting coil packages 4Number of superconducting sections in the coil package 3Number of turns in the quadrupole section 426Number of turns in vertical/horizontal dipole sections 39Peak superconductor current A ≤20NbTi superconductor diameter mm 0.5Superconductor filament size µm 3.7Dipole corrector integrated strength T-m 0.005Max magnetic center offset in Cryomodule mm ≤0.5Magnet physical length mm 340Magnet width/height mm 322/220Quantity required 35

Fig. 2. The quadrupole magnet package cold mass.



II. Magnet Package Tests The magnet SPQA129 was cold tested in March/April 2018. Fig. 3 shows an overview of the similar SPQA03 magnet and top plate assembly ready to install in the helium Dewar, with a 30 mm warm bore tube mounted through and centered in the magnet aperture for magnetic measurements, the right of Fig. 3 is a close-up of SPQA102, note the missing aluminum shield. Warm electrical checks of the assembly and instrumentation were performed prior to cool down, and repeated when cold. The quench performance was tested individually for the quadrupole, vertical dipole and then the horizontal dipole. Each magnet was ramped at 0.5 A/s to 20 A, with no quenches. All three circuits were then powered simultaneously at 20 A for several minutes with no quench, before ramping down to 0 A. High current magnetic measurements were then completed, again with no quenches.

Fig. 3. Overview (left) of SPQA03 attached to the header and close-up (right) of the SPQA101 magnet assembly for installation in the stand 3 Dewar for cold testing in 4.5 K liquid helium bath.

III. Magnet Package Magnetic Measurement Results The magnet package magnetic measurements were performed by rotational coils at FNAL Stand 3. The rotational coil system utilizes a PC Board design [11] and provides a measurement accuracy of ~1 unit (10 -4). The probe rotates in an anti-cryostat (warm bore tube) placed within the magnet aperture as the assembly is suspended in the LHe vessel. The probe radius is limited by the ~30mm inner diameter of the warm bore. The PCB is 1m long and extends out both ends of the magnet. However, owing to the magnet and warm bore position in the cryostat, the probe is not centered in the magnet, and only extends out the far end by about 100mm; ~200 mm short of capturing the full end field. The board is a spare from a previous project [12]. All harmonics are reported here at a reference radius of 10mm. Compared to previous tests of the sPQA004 prototype, the rotational coil was fully centered and the previously existing current overshot during the ramping was addressed by tuning a resistor within the power supply controller. The magnet SPQA129 was fabricated by Milhous; this test must be compared with the measurements obtained for the SPQA03 and SPQA04, as well as the other production units. For reducing the remnant and hysteresis field effects, degaussing and standardization procedures were developed. For degaussing the following current drive formula was used:

I ¿k ⋅e−tτ ⋅sin (t 2/m ) ,

Where k, τ, m are coefficients that define the peak current, the current amplitude decay, and the cycle period.



For the Imax = 20 A peak degaussing current were used following parameters: k=44 A, τ = 30 s, m = 380 s2. This degaussing current fluctuation were approximated for more robust power supply regulation by the control system scripts having intervals of cosine and linear functions with predetermined dI/dt (see Fig. 4).

Fig. 4. Current variation during degaussing.

The TF hysteresis loops were measured once (see Fig. 5, Fig. 6 for TF and Fig. 7 for current profiles). The reproducibility is better than 0.5 %, and meets specification. In comparison with SPQA04 [10] one can clearly see a very similar behavior of the transfer function.



Fig. 5. A zoom of the current vs time plot shows a clean powering profile for all current plateaus.

Fig. 6. Quadrupole TF variations for different current ramps shown in Fig. 7.



Fig. 7. Current variations for the full hysteresis cycles, the variations for the unipolar ramps and for the partial

standardization cycles in Fig. 5 and Fig. 6.

We could identify noise in the quadrupole transfer function at a level ten times higher compared to previous tested magnets. Extensive test and debugging of the test stand 3 was performed and a second measurement was taken, the noise level stayed the same. A magnet tested afterwards (SPQA128) performed flawless, so the origin of the problem seems to be related to the sPQA129 magnet.

The integrated magnetic field quality was also investigated. Fig. 8 shows the quadrupole field harmonics at different currents in the range of 0.2 A – 20 A.



Fig. 8. Quadrupole field harmonics at different currents.



One could see that for most of the entire range of currents all field harmonics are less than 5 units (1 unit=10 -4) at 10 mm reference radius, except for the a3, b3, a4 and b4. This can be attributed to a rotation and tilt between the two magnet halves due to mechanical tolerances in the alignment between the two halves. Large allowed harmonics are also seen in the allowed a6 and b6. However, all harmonics are less than 1 % at 10mm, which is the design specification according to the Table 1.



Fig. 9 and Fig. 13 show the variation of the quadrupole magnetic center displacement for different combinations of the horizontal and vertical dipole corrector currents. The plots have the slope removed after a linear fit, the fit parameter is given in the legend. It should be noted that dx and dy displacements are fully decoupled, they are shown with removed slope in Fig. 10 and Fig 12, respectively; the slope is nearly identical to zero.

In a direct comparison to the sPQA04 measurement one will note that the slopes of the dx and dy displacement in Fig. 9 and Fig. 13 are within 3% or better.

Fig. 9. Quadrupole magnetic center displacement in the vertical plane at different Horizontal Dipole Corrector

currents.



.

Fig. 10. Quadrupole magnetic center displacement in the horizontal plane at different Horizontal Dipole Corrector

currents.



6

Fig. 11. Quadrupole TF function at different Horizontal Dipole Corrector currents.



Fig. 12. Quadrupole magnetic center displacement in the vertical plane at different Vertical Dipole Corrector currents.



Fig. 13. Quadrupole magnetic center displacement in the horizontal plane at different Vertical Dipole Corrector currents.



Fig. 14. Quadrupole TF function at different Vertical Dipole Corrector currents.

Fig. 15. Quadrupole and dipole currents variations for Fig. 9 to Fig. 14 with a single repetition. Plot is taken from

SPQA04 current measurements.

The test results indicate an unstable quadrupole transfer function versus time.



At zero dipole current the field harmonics are less than 15 units (1 unit=10 -4) at 10 mm reference radius, once the dipole correctors are ramped, once can see an increase in the harmonics up to a factor one hundred in the a3 in case of the horizontal dipole corrector and b3 in case of the vertical dipole corrector, as shown in Fig. 16 and Fig. 17. During field measurement results discussion Chris Adolphsen raised the question on why the dipole currents affect the quad TF more than one would expect; it changes TF by 0.5% (see Fig. 11 or 14) when the dipoles are set for 2 mm offsets, under the assumption that the dipole symmetry would not produce a quad component. Additionally, it was asked if the changes in the n=3 harmonic agree with expectation given that the dipole windings produce a skew Sextupole component.

We simulated 2D, and 3D magnetic fields for the quadrupole, dipole with both currents. The results of field harmonic analysis at 1 A current and reference radius R=10 mm is shown in Table 2.

Table 2

So, the main field absolute harmonics at 1 A current (in the table shown coil ampere-turns):B1=-1.171e-3 T (dipole), B2=-5.305e-3 T/m (quadrupole), B3=-5.644e-5 T/m^2 (Sextupole). Solving equation:

F ( x )=B1∙ IdIq

+B2∙ xR

+B3∙ IdIq∙ x

2

R2=0,

where Id, Iq – dipole and quadrupole currents, gives the magnetic center shifts Xo for different currents. This F(x) function derivative at Xo defines the field gradient in the shifted quadrupole magnetic center.

dF (x)dx

=B2∙ 1R

+2 ∙B3 ∙ IdIq∙ xR2.

One could see that the quadrupole field gradient coupled with the magnetic center shift caused by the Sextupole.The quadrupole relative gradient variations in % shown in Fig. 16. This parabolic dependence has about ideal fitting by the function:

Y ( x )=−0.471 ∙( IdIq

)2

, %.

Fig. 17 shows the quadrupole transfer function deviation for measured data, compare with Fig. 11 and 14 for data.

Magnetic measurements showed that vertical and horizontal dipole fields fully decoupled. So, we could use the analytic gradient correction formula for both dipoles to correct the quadrupole strength for different dipole currents:

dG ( x )=−0.471 ∙10−3 ∙( IvdIq

)2

,



dG ( y )=−0.471 ∙10−3 ∙( IhdIq

)2

,

where Ivd, Ihd – vertical and horizontal dipole currents.

Fig. 16. Quadrupole gradient transfer function deviations at different C=Id/Iq.

Fig. 17. Approximated (left) and measured (right) quadrupole transfer function deviations.

It should be noted, because the fitting coefficient was received from magnetic field simulations, and provides independently the good fitting of measurement data, this formula could be distributed for all production magnets.



A direct comparison with the SPQA04 and SPQA101 measurement shows nearly identical harmonic values for the a3 or b3 components between the two magnets during the horizontal and vertical dipole variations, a very comparable result. The results are plotted in Fig. 18 and 19.It should be noted that the maximum needed dipole corrector strength is reached at 20 % of the quadruple current, to compensate possible 0.5 mm quadrupole magnetic center shift caused by magnet installation and cooling. In this case, the Sextupole field component will be much lower than at full current.



6

Fig. 18. Quadrupole field harmonics at 0.4A quadrupole current for different horizontal dipole currents.



Fig. 19. Quadrupole field harmonics at 0.4A quadrupole current for different vertical dipole currents.



A separate set of harmonic measurements was obtained with 0A Quadrupole Current. Each Dipole was ramped in 0.5A steps from 0 to 2A, to negative 2A and to 0. Three repetitions allowed to obtain the harmonics and transfer function of each Dipole, presented in Fig. 20 and Fig. 21. Clearly the Dipole has a large Quadrupole component. This is to be expected for this type of corrector coils where a remnant field of the quadrupole is still present after degaussing due to the factor 1000 difference in strength between dipole and quadrupole.



Fig. 20. Horizontal Dipole field harmonics at 0A quadrupole current for different horizontal dipole currents.



Fig. 21. Vertical Dipole field harmonics at 0A quadrupole current for different vertical dipole currents.



The transfer function of each Dipole is presented in Fig. 22 and Fig. 23. The results agree with previous measurements of SPQA002 and SPQA102.

Fig. 22. Horizontal Dipole transfer function at 0A quadrupole current for different horizontal dipole currents.



Fig. 23. Vertical Dipole transfer function at 0A quadrupole current for different vertical dipole currents.



Additional measurements which were requested by SLAC. For simplicity, the measurements are labeled CA1- CA4.

CA1: Compare unipolar TF results to the results from a random sequence of currents. As expected the TF varies between the minimal and maximum transfer function values of the full hysteresis curve, depending on the step profile.

Fig. 24. Current of Fig. 25.



Fig. 25. TF for a random current walk between 0 and 10 A. As expected the random current falls in between the up and down hysteresis curves as shown in Fig. 25. There is a consistency of the results between the SPQA series measurements.



A second measurement, CA 2, asked for a degauss cycle followed by setting a current and varying the current is within a 15% range, the measurement shows that the resulting transfer function is within 2% range of the original value. Fig. 26 and Fig. 27 show the results, each with a sub-selection of sampled currents shown in Fig. 28.

Fig 26. Variation of the transfer function for 15% changes in current around a given set point, data set 1.




Fig. 28. Currents for the current variation for Fig. 26 and Fig. 27, between each measurement a degauss cycle was performed, followed by the variation (see zoom). Sample currents from SPQA04 measurement.

Additional measurement CA3 was requested with a similar profile as the measurement CA2, however between each current variation the Quadrupole current was set to zero. Shown in Fig. 29 and Fig. 30 are the results, the shift in the transfer function between stays within 2%. The current of a sub-sample is shown in Fig. 31.




The only official copy of this file is located in the LCLS-II Controlled Document Site.

Before using a printed/electronic copy, verify that it is the most current version.



Fig 31. Current measurement of the 6A main current for the Fig. 29 and Fig. 30. Sample current from SPQA103.

In Fig 32 – Fig. 34 we compare directly the measurements for CA2 and CA3. The TF is not influenced by the way the plateau was reached, contradictory to previous results on all other magnets.




A fourth CA4 measurement involved powering both the Quadrupole and the Dipole correctors was requested. The goal was to vary the magnetic center in the range of ±300 µm for three quadrupole currents: 0.4 A, 1 A, and 4 A. Ideally, they should result in a straight line with the uniform slope.

Given the problems of a non-uniform TF for the quadrupole, uniformity being needed to check the results, we did NOT analyze the data of this measurement.

IV. ConclusionThe splittable conduction cooled magnet package SPQA129 was thoroughly tested and showed a good

performance for the dipoles and a noisy performance for the Quad. The magnet package combines a quadrupole with orthogonal dipole correctors. During cold tests the following features were observed and verified:- The field quality and reproducibility of the dipoles are acceptable. - The field quality and reproducibility of the quad is not acceptable. - Field geometric harmonics are low and meet the specification.- The magnet was successfully excited to 20 A without quench (20 A is the peak power supply, and operating

current).- This is a production magnet by the vendor Milhous, the results were compared with previous obtained

measurements of SPQA magnets and found in consistency for dipole and disagreement for the quad.

The completed tests pointed out a mechanical problem with SPQA129. From the test we conclude that Eddy currents on the inner aluminum cooling strip, or other mechanical vibration due to the bolting down of the coil package or magnet halves create a noisy in the quadrupole. The effect is large enough that the repeatability of the measurements between current cycles is a factor 10 worse compared to all other magnets.



Acknowledgment The authors would like to thank Prof. Akira Yamamoto (KEK), Chris Adolphsen, Paul Emma (SLAC) for very useful discussions. We are very grateful to the SLAC team for providing and commissioning regulated power supply, and to all FNAL Technical Division personnel involved in the design, fabrication and tests of these magnets.

REFERENCES

[1] “Linac Coherent Light Source,” LCLS-II, 2015:https://portal.slac.stanford.edu/sites/lcls_public/Pages/Default.aspx

[2] V.S. Kashikhin, et al., “Test results of a superconducting quadrupole model designed for linear accelerator applications,” IEEE Transactions on Applied Superconductivity, vol. 19, Issue 3, Part 2, June 2009, pp. 1176-1182.

[3] V. S. Kashikhin, N. Andreev, J. Kerby, Y. Orlov, N. Solyak, M. Tartaglia,and G. Velev, “Superconducting splittable quadrupole magnet for linear accelerators,” IEEE Trans. Appl. Supercond., vol. 22, no. 3, p. 4002904, June 2012.

[4] N. Andreev, V. S. Kashikhin, J. Kerby, N. Kimura, M. Takahashi, M. A. Tartaglia, T. Tosaka, and A. Yamamoto, “Conduction cooling testof a splittable quadrupole for ILC cryomodules,” IEEE Trans. Appl. Supercond., vol. 23, no. 3, p. 3500305, June, 2013.

[5] N. Kimura, N. Andreev, V. S. Kashikhin, J. Kerby, M. A. Tartaglia, and A. Yamamoto, “Cryogenic performance of a conduction cooling splittable quadrupole magnet for ILC cryomodules,” Adv. Cryogenics Engineering, 59A, 2014, pp.407-415.

[6] R. Carcagno, et al, “Magnetic and Thermal Performance of a Conduction-Cooled Splittable Quadrupole,” IEEE Trans. Appl. Supercond., vol. 24, no. 3, p. 4001604, June, 2014.

[7] V.S. Kashikhin, et al., “Compact Superconducting Magnet for Linear Accelerators,” IEEE Trans. Appl. Supercond., vol. 25, Issue: 3, p. 4001705, 2016.

[8] V.S. Kashikhin, et al., “Performance of Conduction Cooled Splittable Superconducting Magnet Package for Linear Accelerators,” IEEE Trans. Appl. Supercond., vol. 26, Issue: 4, p. 4103405, 2016.

[9] J. DiMarco, V. Kashikhin and M. Tartaglia, “TestResults of LCLS-II Cryomodule Magnet SPQA03”, LCLS-II-4.5-EN-0826-R0

[10] J. DiMarco, V. Kashikhin, T. Strauss and M. Tartaglia, “TestResults of LCLS-II Cryomodule Magnet SPQA04”, LCLS-II-4.5-EN-0930-R0

[11] J. DiMarco, et al.,”Application of PCB and FDM Technologies to Magnetic Measurement Probe System Development”, IEEE Trans. on Appl. Supercond., vol. 23, no. 3, 9000505, June 2013.

[12] J. DiMarco et al., “A Fast-Sampling, Fixed Coil Array for Measuring the AC Field of Fermilab Booster Corrector Magnets”, IEEE Trans. on Applied Superconductivity, Vol. 18, No. 2, June 2008.




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