1442 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 17, NO. 2, JUNE 2007

Drift Compensation of 600 MHz NMR MagnetA. Otsuka, T. Kiyoshi, S. Matsumoto, K. Kominato, and M. Takeda

Abstract—Although high-temperature superconductors (HTS)are very promising for high-field generation over 25 T, it is diffi-cult to apply them to an NMR magnet because of their low indexvalues and the difficulty caused by superconducting joints. Wehave developed a drift compensation technique to apply HTS toa high-resolution NMR using a 14 T (600 MHz) vertical NMRmagnet. The magnet had a poor magnetic field stability of about

0.7 ppm/hour, so a drift compensation unit based upon a fluxpump method was added. The unit consisted of nested inner (sec-ondary) and outer (primary) coils. The inner coil was connectedin series to the main coil circuits, and the outer coil was connectedto the auxiliary power supply to sweep the output current veryslowly. While the current of the outer coil was changed at an ade-quate sweep rate by the power supply, the current was induced byinductive coupling in the inner coil. The induced current canceledout the decay of the main coil current that caused the poor drift ofthe magnet. With the drift compensation unit, the magnetic fielddrift was improved to less than 0.0001 ppm/hour for 3 days at 14T. This period was long enough for one NMR measurement.

Index Terms—Drift compensation, flux pump, induced current,magnetic field stability, NMR magnet.

I. INTRODUCTION

THE Tsukuba Magnet Laboratory (TML) of the National In-stitute for Materials Science developed 920 MHz (21.6 T)

and 930 MHz (21.8 T) NMR magnets using and NbTiwires [1]. High temperature superconductors (HTS) will be nec-essary to achieve an NMR magnet over 1 GHz (23.5 T). How-ever, it is difficult to apply HTS to the NMR magnet because ofthe low index values and the difficulty of the superconductingjoints. It appears that the properties of HTS cause poor magneticfield stability in the persistent mode operation.

Flux injection has been developed elsewhere to resolve thisstability problem [2]. This method is effective in raising themagnetic field in small increments. However, the proceduresof this method involve the rapid movements of the magneticfield accompanying the operation of the persistent switches. Therapid movements are not preferred for precise NMR measure-ment.

Another current injection has used windings already presentin a main magnet [3], [4]. In this method, a change in field ho-mogeneity occurs with increasing injected current.

Manuscript received August 27, 2006.A. Otsuka, T. Kiyoshi, and S. Matsumoto are with the National In-

stitute for Materials Science, Tsukuba Magnet Laboratory, 3-13 Sakura,Tsukuba, Ibaraki 305-0003, Japan (e-mail: OTSUKA.Akihiro@nims.go.jp;KIYOSHI.Tsukasa@nims.go.jp; MATSUMOTO.Shinji@nims.go.jp).

K. Kominato and M. Takeda are with Japan Superconductor TechnologyInc., 1-5-5 Takatsukadai, Nishi-ku, Kobe 651-2271, Japan (e-mail: Komi-nato-jastec@kobelco.jp; Takeda-jastec@kobelco.jp).

Digital Object Identifier 10.1109/TASC.2007.898525

TABLE ICOIL PARAMETERS OF DRIFT COMPENSATION UNIT

In order to improve the magnetic field stability continuously,we developed the drift compensation technique using the fluxpump technology and applied it to a 14 T (600 MHz) verticalNMR magnet. The magnet had poor magnetic field stabilitythough the field homogeneity of the magnet was equivalent toactual NMR magnets.

II. METHOD OF DRIFT COMPENSATION

A. Drift Compensation Unit

The drift compensation unit is a key component for the fluxpump technology. Table I shows the coil parameters of the driftcompensation unit. The inner (secondary) coil was wound onthe former made by GFRP, and the outer (primary) coil waswound on the inner coil. The brass wire was wound for structuralprotection on the outer coil, and finally, this unit was treated withthe vacuum impregnation.

B. Main Coil Circuit With Drift Compensation Unit

Fig. 1 shows the main coil circuit combined with the driftcompensation unit.

When the magnet was being operated in the persistent mode,the equation of the coil voltage was described as

(1)

The main coil involves a slight resistance . The secondarycoil (self inductance ) is connected in series to the main coil(self inductance ) and coupled inductively with the primarycoil (mutual inductance ). The primary coil current is en-ergized by the auxiliary power supply. The mutual inductancebetween the main coil and the drift compensation unit is negli-gible.

1051-8223/$25.00 © 2007 IEEE

OTSUKA et al.: DRIFT COMPENSATION OF 600 MHz NMR MAGNET 1443

Fig. 1. Main coil circuits with the drift compensation unit.

The resistance is defined by the original field decaywithout the drift compensation.

(2)

(3)

When the induced current in the secondary coil caused by theflux pump cancels out the original field decay, the sweep rate ofthe primary coil is defined as

(4)

The measured original field decay was 0.665 ppm/hour.The operating current of the main coil was 166.7 A at thecentral magnetic field 14.1 T. The main coil inductance was76 H. Using these values, we obtained the resistance valueof and the sweep rate of the primary coil

. This means thatthe primary coil transferred the energy to thesecondary coil by the flux pump.

C. Auxiliary Power Supply for the Primary Coil

The required sweep rate for the primary coil is so slow thatanalogue circuits can not achieve it. Therefore, we applied thedigital sweep method using a D/A converter. When the max-imum output current 10 A of the power supply was con-trolled by a 16-bit D/A converter, the minimum resolution wasabout 0.3 mA. Changing the output current every seven secondsincrementally with the above resolution realized the sweep rateof 2.6 mA/min. The specification of the power supply is shownin Table II.

D. Installation of the Drift Compensation Unit

1) Considerations: We considered the following things be-fore installation of the drift compensation unit:

• The secondary coil was in the stray field of the main coil,and the current was the same as the main coil current.

TABLE IISPECIFICATION OF THE POWER SUPPLY FOR THE PRIMARY COIL

Fig. 2. Installed position and direction of the drift compensation unit in theliquid helium vessel.

Therefore, the electromagnetic force worked upon the driftcompensation unit.

• The influence of the main coil quench should be mini-mized.

• The stray field produced by the drift compensation unit didnot disturb the main coil field homogeneity.

For these reasons, the drift compensation unit was set at aposition apart from the main coil, and the axis of the unit wassituated perpendicular to the main coil axis, as shown in Fig. 2.

2) Electromagnetic Force: The magnetic fields at the centerof the drift compensation unit (point shown in Fig. 2) were

, , , and. The electromagnetic force working

upon the drift compensation unit had a rotating torquecentering on the axis that was perpendicular to the magnetic

1444 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 17, NO. 2, JUNE 2007

Fig. 3. View of the drift compensation unit installed on the 600 MHz NMRmagnet.

field direction. This electromagnetic force was considerable, sowe applied the reinforced bracket to fix the drift compensationunit as shown in Fig. 3. The bracket is made from 5-mm-thickstainless steel and welded to the stand used for the plate of themagnet parts.

III. TEST RESULT

A. Procedure

At first, the main coil was energized to the full field andmoved to the persistent mode. The magnetic field was measuredby the METROLAB PT2025 Teslameter. Then, we started to en-ergize the primary coil to compensate the field decay.

B. Result of Drift Compensation

The test result is shown in Fig. 4. Before energizing the pri-mary coil, the magnetic field decay of the main coil was 0.665ppm/hour. However, while energizing the primary coil for driftcompensation, the magnetic field was kept at approximately thesame value for over 73.5 hours. After the primary coil currentreturned to zero, the magnetic field also returned to the originalfield decay line.

Fig. 5 shows the expanded view of the magnetic field driftduring the drift compensation. The averaged field stability was

, which was small enough for an NMRmeasurement.

C. Field Control Technique

As shown in Fig. 5, the behavior of the magnetic field seemsto include a measuring noise. However, this fluctuation wascaused by the following field control technique. Fig. 6 shows therelationship between the primary coil current and the mag-netic field for over one hour. The primary current ramp rate wasvaried to maintain control of the magnetic field within the spec-ified range of 0.03 ppm with a defined upper and lower limit.The sweep rate of the was maintained at either a positive ornegative 3.2 mA/min.

Fig. 4. The magnetic field and the primary coil current vs. time plots.

Fig. 5. Expanded view of the magnetic field drift during drift compensation.

Fig. 6. The magnetic field and the primary coil current vs. time plots.

While the was running up, the magnetic field was in-creased by the flux pump of the drift compensation unit. Whenthe magnetic field was larger than the upper limit, the waschanged to run down. Then the magnetic field also began todecrease. When the magnetic field was smaller than the lowerlimit, the was changed to run up again to increase the

OTSUKA et al.: DRIFT COMPENSATION OF 600 MHz NMR MAGNET 1445

magnetic field. The above running up and down control of thewas repeated about every nine minutes.

Consequently, the actual magnetic field was controlled towithin about 0.06 ppm. It was larger than the set width of0.03 ppm. The above difference was caused by the followingdelay to control the . The magnetic field values measuredby the Teslameter were averaged for 30 seconds for higheraccuracy. In that time, there was a slight difference between theaveraged value used to control and the actual magnetic field atthe real time. Furthermore, there was the time lag between themoment of alternating the output current of the power supplyand the actual changing of the . This time lag was due to theprotection resistance (about 0.1 ohm) connected to the primarycoil in parallel.

IV. CONCLUSION

We developed the drift compensation technique using the fluxpump technology and applied it to a 14 T (600 MHz) NMRmagnet. As a result of the drift compensation unit controlledby running up and down the primary coil current , the poormagnetic field stability was remarkablyimproved .

The test results make clear the following advantages of themethod:

• The only modification of the magnet system is the instal-lation of the drift compensation unit. It is very simple andeasy, so this method could be applied to any other magnets.

• The magnetic field can be changed continuously by theflux pump. The rapid field movements in the other methodusing additional switches do not occur.

On the other hand, there is the following disadvantage:• The period during drift compensation is limited by the

maximum output current of the power supply for the pri-mary coil.

Although the operating period is limited, it is a sufficientlylong time to make NMR measurements. The drift compensationunit combined with the auxiliary power supply is a very effectivemethod to improve the poor magnetic field stability.

REFERENCES

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[2] Y. Iwasa and H. Lee, “NMR/MRI superconducting magnet technolo-gies: Recent activities at MIT francis bitter magnet laboratory,” Journalof Korea Institute of Applied Superconductivity and Cryogenics, vol. 5,no. 1, pp. 1–12, 2003.

[3] W. D. Markiewicz, “Current injection for field decay compensation inNMR spectrometer magnets,” IEEE Trans. Applied Superconductivity,vol. 12, pp. 1886–1890, 2002.

[4] R. Fu, W. W. Brey, K. Shetty, P. Gor’kov, S. Saha, J. R. Long, S.C. Grant, E. Y. Chekmenev, J. Hu, Z. Gan, M. Sharma, F. Zhang, T.M. Logan, R. Bruschweller, A. Edison, A. Blue, I. R. Dixon, W. D.Markiewicz, and T. A. Cross, “Ultra-wide bore 900 MHz high-resolu-tion NMR at the national high magnetic field laboratory,” Journal ofMagnetic Resonance, vol. 177, pp. 1–8, 2005.