Seungyong Hahn, Youngjae Kim, Jungbin Song, John Voccio, Yong Chu, Juan Bascuñàn, Masaru Tomita, and Yukikazu Iwasa

October, 2014

Francis Bitter Magnet Laboratory, Plasma Science and Fusion Center

Massachusetts Institute of Technology Cambridge MA 02139 USA

This work was supported by the National Institute of Biomedical Imaging and Bio-engineering of the National Institutes of Health under Award R01EB006422. Submitted to IEEE Trans. Appl. Supercond.

PSFC/JA-15-39

Operation of a 130-MHz/9-mm Compact HTS Annulus Magnet With a Micro-NMR Probe

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Operation of a 130-MHz/9-mm Compact HTSAnnulus Magnet with a Micro NMR ProbeSeungyong Hahn, Youngjae Kim, Jungbin Song, John P. Voccio, Yong Chu, Juan Bascunan,

Masaru Tomita and Yukikazu Iwasa

Abstract—In this paper, we report final operation results ofour compact NMR magnet, named YP2800, with a homemademicro-NMR probe in a bath of liquid helium at 4.2 K. YP2800comprises of a stack of 2800 YBCO “plate annuli,” 0.08-mmthick, either 46-mm or 40-mm square, each having a 26-mm holemachined at the center. By the field cooling technique, YP2800was energized at 130 MHz (3.05 T); an overall peak-to-peakhomogeneity of 487 ppm within |z| < 5 mm was measured ata moment when a field drift of 11 ppm/hr was reached in 3days after the field cooling. Due to the small (9.2-mm) bore size,no commercial probes fit into the bore; a 8.5-mm micro-NMRprobe was designed and constructed. This paper presents thedesign and construction details of the micro NMR probe and itsoperation results with YP2800. An NMR signal was captured,with a dimethyl sulfoxide sample of φ4.4 mm and 5-mm long, ata base frequency of 130 MHz, of which the half-peak bandwidthis 60 kHz; this 461 ppm frequency error is chiefly due to thespatial field error, 487 ppm within |z| < 5 mm.

Index Terms—Compact NMR magnet, Micro NMR, NMRProbe, trapped field, YBCO annulus

I. INTRODUCTION

THIS paper is a final report of our 5-year effort todevelop a compact HTS (high temperature superconduct-

ing) NMR (nuclear magnetic resonance) magnet comprisingYBCO “plate annuli” [1]–[5]. The magnet, named “YP2800,”consists of a stack of 2800 YBCO plate annuli, 0.08-mmthick, either 40-mm or 46-mm square, each having a 26-mm hole machined at its center. To energize YP2800, theso-called “field-cooling” technique [6] is used, i.e., YP2800is cooled down to its operating temperature under a targetbackground field and then the background field is removedfor YP2800 to “trap” the target magnetic field. Key benefits ofthe trapped-field type NMR magnet, such as YP2800, over itsconventional current-driven counterpart include: 1) no need forcurrent leads due to the “passive (field trapping)” energizingprocedure, which leads to a simple and compact overallstructure; 2) intrinsic persistent-mode operation that has neverbeen achieved so far in HTS NMR magnets. Similar researches

Manuscript received July 15, 2013. This work was supported by theNational Institute of Biomedical Imaging and Bioengineering of the NationalInstitutes of Health under Award Number R01EB006422.

S. Hahn, Y. Kim, J. Song, J. P. Voccio, J. Song, Y. Chu, J. Bascunan andY. Iwasa are with Francis Bitter Magnet Laboratory, Massachusetts Instituteof Technology, Cambridge, MA 02139, USA (corresponding author phone:+1-617-2534-4161; fax: +1-617-253-5405; email: syhahn@mit.edu)

Y. Chu was with Francis Bitter Magnet Laboratory, Massachusetts Instituteof Technology, Cambridge, MA 02139, USA; he is now with the NationalFusion Research Institute, 169-148 Gwahak-ro, Yuseong-gu, Daejeon 305-806, Korea (e-mail: ychu@nfri.re.kr).

M. Tomita is with the Applied Superconductivity, Materials TechnologyDivision, Railway Technical Research Institute (RTRI), 2-8-38, Hikari-cho,Kokubunji-shi, Tokyo 185-8540, Japan (e-mail: tomita@rtri.or.jp).

(a) (b)

Fig. 1: (a) YBCO “plate annulus”; (b) YP2800

for trapped-field NMR magnets have been conducted by othergroups [7]–[11]. In 2011, Ogawa, et al. reported MR imagestaken from a 200-MHz/20-mm NMR magnet comprising 6EuBCO bulk annuli of 60 mm OD, 28 mm ID, and 20 mmhigh [7].

In our latest paper [2], we reported trapped-field test resultsof YP2800 in a bath of liquid helium at 4.2 K. As a final step,this paper focuses on obtaining an NMR signal using YP2800.Due to the small room-temperature (RT) bore size (9.2 mmin diameter) of YP2800, no commercial NMR probes fit intothe RT bore. Therefore, we designed and constructed a microNMR probe of which the overall diameter is 8.5 mm. Thispaper presents design and construction details of our micro-NMR probe, and its operation results of YP2800 to capturean NMR signal.

II. COMPACT NMR MAGNET: YP2800

A. Magnet Construction

Fig. 1a shows a picture of YP2800 of which key parametersare summarized in Table I. 2800 square plates were cutfrom either 40-mm or 46-mm wide YBCO tapes, originally

TABLE I: Key Parameters of YP2800

Parameters ValuesWidth of REBCO plate [mm] 40 or 46 squareThickness of REBCO plate [mm] 0.08Diameter of center hole [mm] 26Manufacturer AMSCTotal number of plates 2800Overall height [mm] 224Peak trap field at 77 K [T] 0.6Peak trap field at 4.2 K [T] 6.0 (estimated)

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manufactured by AMSC, and then 26-mm center hole wascreated by machining to accommodate an NMR probe afterthe 2800 plate annuli were stacked in a way that the bestannulus, in terms of trapped field capacity tested in a bath ofliquid nitrogen (LN2) at 77 K [12], was placed at the center ofYP2800 and the annuli having the lower trapped field capacitytowards the top and bottom of YP2800. The overall height ofYP2800 is 224 mm and the maximum center field at 77 Kwas measured to be 0.6 T.

B. Final Operation at 130 MHz: Spatial Homogeneity andTemporal Stability

For the final test to capture an NMR signal, YP2800 wasoperated in a bath of liquid helium (LHe) at 4.2 K witha trapped field of 3.05 T (130-MHz 1H NMR frequency),∼50% of its maximum trapped field capacity at 4.2 K inorder to improve the spatial field homogeneity [3]; 130 MHzis large enough to avoid the FM radio frequency in the Bostonarea, 88.1 - 107.9 MHz. As previously reported [12], thetrapped field “enhancement” at the center of YP2800 was alsoobserved in the final operation at 130 MHz, chiefly due to themagnetic coupling among 2800 YBCO plates having differenttrapped field capacities. A temporal stability of 11 ppm/hrwas reached 3 days after the field cooling procedure wascompleted. Table II presents harmonic field errors measuredjust before NMR signals were captured. The overall fieldhomogeneity within |z| <5 mm was 487 ppm.

TABLE II: Spatial harmonic field errors of YP2800 at 130MHz (3.05 T)

Harmonic Errors ValuesZ1 [ppm/mm] 0.294Z2 [ppm/mm2] -23.1Z3 [ppm/mm3] -0.463Z4 [ppm/mm4] 0.0684Z5 [ppm/mm5] 1.86×10−3

Z6 [ppm/mm6] -0.258×10−3

Overall (|z| <5 mm) [ppm] 487

III. DESIGN AND CONSTRUCTION OF MICRO NMR PROBE

Due to the small (9.2-mm) room temperature (RT) boresize, no commercial probes fit into the bore. Therefore, wedesigned and constructed a micro NMR probe of which theoverall diameter is 8.5 mm.

A. Design

The main design goal for an NMR probe is to design anRF (radio frequency) resonance circuit of which the termi-nal impedance is matched to a standard NMR spectrometerimpedance of Z0, typically 50 + j0 Ω (pure resistive), ata target NMR frequency of f0 [13]. Fig. 2 shows a circuitdiagram of the micro NMR probe design. It consists of threecomponents: 1) an RF coil surrounding an NMR sample,which is modeled as an inductor (L) and a resistor (r) con-nected in series; 2) a set of tunning capacitors (CT ) connectedin parallel to the RF coil; and 3) a set of matching capacitors(CM ) connected in series to the RF coil and the tunning

Fig. 2: Circuit diagram of the micro NMR probe

Fig. 3: Quality factor (Q) vs. Capacitance (CT , CM , and CT +CM )

capacitors. For an NMR probe, a quality factor, Q, may bedefined by (1), where L, f0 and r are, respectively, RF coilinductance, target NMR frequency, and RF coil resistance. AnNMR probe with a higher Q value has a better sensitivity inprinciple. Once Z0 and f0 are given, CT and CM may beobtained as a function of Q by (2) and (3), respectively, froman analysis of the probe circuit in Fig. 2.

Q = 2πStored Energy

Energy Loss per Cycle= 2πf

LI2

rI2=Lω

r(1)

CT =Q±

√r(1 +Q2)/Z0 − 1

2πf0r(1 +Q2)(2)

CM =1

2πf0√r(1 +Q2)Z0 − Z2

0

(3)

The RF coil is a 3.5-turn single layer solenoid wound withuninsulated Cu wires of AWG 29 (0.29 mm in diameter).Winding diameter and height of the coil are 6.1 mm and 2.9mm, respectively. Self-inductance of the RF coil inductanceis then calculated to be 88 nH. With f0 of 130 MHz and Z0

of 50 Ω, Fig. 3 shows CT (blue circles), CM (red triangles),CT +CM (black squares) graphs with respect to Q. Based onthese analyses, a total of three capacitors, two variable (CT1

and CT3) and one fixed (CT1), are used to have a total tuningcapacitance (CT = CT1 + CT2 + CT3) ranging from 7 to 18pF, while two variable capacitors (CM1, CM2) to have a totalmatching capacitance (CM = CM1 ‖ CM2) ranging from 1.2to 3.8 pF. Key parameters of the RF coil, matching and tunningcapacitors are summarized in Table III

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TABLE III: Parameters of 9-mm NMR probe

Parameters ValuesRF CoilUninsulated Cu wire diameter [mm] 0.29 (AWG 29)Winding diameter [mm] 6.1Turn; layer 3.5; 1Self-inductance [nH] 88Tuning CapacitorsCT1 (variable) [pF] 3 – 10CT2 (fixed) [pF] 2CT3 (variable) [pF] 2 – 6Total CT [pF] 7 – 18Matching CapacitorsCM1 (variable) [pF] 3 – 10CM2 (variable) [pF] 2 – 6Total CM [pF] 1.2 – 3.8

Fig. 4: Picture of 3.5-turn single-layer solenoid RF Coil

B. Construction

Fig. 4 shows a picture of the RF coil. An NMR samplecontainer, made of Kel-F, was used as a bobbin for the RF coil;a groove was made directly on the container, around whichCu wires were wound (Fig. 4) to avoid an electrical short andprovide accurate spacing between turns. A dimethyl sulfoxide(DMSO) sample was placed in a cylindrical cavity of φ4.4mm and 5.0 mm long within the container; a snug-fit coverwas used to seal the container.

Fig. 5 shows: (a) a picture of the 8.5-mm micro NMRprobe; and (b) a wiring diagram for capacitors, both tuning andmatching. The five capacitors, three tuning (CT1, CT2, CT3)and two matching (CM1, CM2), are placed on a 7.6 mm ×35.6 mm rectangular board as seen in Fig. 5(b). A coaxial

(a)

(b)

Fig. 5: (a) Picture of the micro NMR probe; (b) capacitorwiring diagram

(a)

(b)

Fig. 6: Test results of: (a) 9-mm micro NMR probe; (b) 25-mmcommercial NMR probe

cable having a line impedance of 50 Ω, is used for wiringthe NMR probe terminal to an NMR spectrometer. A Cu tubehaving an outer diameter of 8.5 mm housed the completedprobe.

IV. OPERATION OF YP2800 WITH HOMEMADE MICRONMR PROBE

A. Probe Test

To check the performance of the micro NMR probe, itsfrequency response was tested with a network analyzer andcompared with that of a commercial probe having an overalldiameter of 25 mm. Fig 6 shows the frequency responses ofthe two probes, (a) micro and (b) 25-mm. The qualify factor,Q, of an NMR probe can be obtained from the frequencyresponse by (4), where f0 is a base frequency and ∆f is afrequency difference at a response of -3 dB in Fig. 6.

Table IV summarizes test results of the two probes at abase frequency of 100 MHz. The 25-mm probe has Q of 63,1.7 times larger than that of the 8.5-mm micro probe, 38.The peak response of the 25-mm commercial probe is -48dB, while that of the 8.5-mm micro probe is -34 dB. Therelatively poor performance of the micro NMR probe may beoriginated mainly from the small inductance of the RF coil and

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TABLE IV: Test Results of 9-mm Micro and 25-mm Com-mercial Probes

Parameters 9-mm Micro 25-mm CommercialTuned frequency, f0 [MHz] 100 100Peak reflection [dB] -30 -48∆f at -3 dB [MHz] 2.6 1.6Q 38 63

Fig. 7: Configuration of the final test setup

the consequent low Q. Currently, a research is in progress toimprove the performance of the micro NMR probe.

Q =f0∆f

(4)

B. Final Test: Capture of NMR Signal

Fig. 7 shows an overall configuration of the final test setup.YP2800 was at 130 MHz (3.05 T) in a bath of liquid heliumat 4.2 K. Fig. 8 shows an NMR signal obtained from theexperiment. With a base frequency of 130 MHz, the half-peakfrequency of 60 kHz was measured, which corresponds to 461ppm. Provided that the overall peak-to-peak field error within|z| <5 mm is 487 ppm (Table II), most of the signal errorsmay be originated from the spatial field inhomogeneity. Noshimming techniques, either active or passive, were adoptedfor the experiment.

C. Discussion

Although capturing NMR signals with YP2800 demon-strates a strong potential of our compact annulus magnet forNMR, still its field homogeneity needs to be significantly im-proved for actual NMR applications. We have experimentallydemonstrated that the conventional active shimming technique,where a set of shim coils are placed radially outside themagnet, were not effective for YP2800 chiefly due to thelarge screening currents induced within YP2800 [3]. HTS“inner” shim coils, which we recently published in [14], canbe placed in the cold bore of an annulus magnet owing to theircompactness, and thus more effective for active shimming. Theconventional ferromagnetic shimming technique may be stilluseful for an annulus magnet, though the primary challenge

Fig. 8: NMR signal captured with YP2800.

is to design the ferro shims under the field-cooling chargingenvironment. Use of an NMR magnet to generate a back-ground field for field cooling is also proven to be effectiveto improve the pre-shim field homogeneity [7]. Also, thetemporal instability of an annulus magnet needs to be furtherstudied, especially the trapped field enhancement that we wereable to explain with an equivalent circuit model [2], to achievethe actual NMR-quality field homogeneity, typically <0.1 ppmwithin a target DSV.

V. CONCLUSION

Since 2009, we have developed a compact HTS NMRmagnet, named Y2800, comprising 2800 YBCO plate annuli.As a final step, YP2800 was energized, by the field coolingtechnique, at 130 MHz (3.05 T) in a bath of liquid helium at4.2 K. A field drift of 11 ppm/hr was measured 3 days afterthe field cooling procedure had been completed. Due to thesmall (9.2-mm) room-temperature bore size, no commercialprobes fit into the bore and therefore a 8.5-mm micro NMRprobe was designed and constructed. The qualify factor of themicro NMR probe was measured to be ∼60 % of that of acommercial 25-mm probe but an NMR signal was successfullycaptured in operation with YP2800. The half-peak bandwidthof the captured NMR signal with a base frequency of 130 MHzwas 60 kHz, i.e., 461 ppm. This corresponds to the measuredpeak-to-peak spatial field error of YP2800, 487 ppm within|z| <5 mm. No shimming techniques, either active or passive,were adopted for the experiments. Currently, a continuationprogram is under preparation, where we will investigate thefield shimming technology to improve the spatial homogeneityof trapped fields, and the performance improvement of themicro NMR probe.

ACKNOWLEDGMENT

The authors would like to thank Dr. Tony Bielecki, Dr. Tao-Chung Ong, and Dr. Vladimir K. Michaelis for their valuablecontribution in design and operation of the micro NMR probe.

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