PSFC/JA-17-54 A Field-Shaking System to Eliminate the Screening Current-Induced Field in the 800-MHz HTS Insert of the MIT 1.3-GHz LTS/HTS NMR Magnet: A Small-Model Study Jiho Lee, Dongkeun Park, Philip C. Michael, So Noguchi, Juan Bascuñán, and Yukikazu Iwasa August 2017 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 Bioengineering and the National Institute of General Medical Sciences. Reproduction, translation, publication, use and disposal, in whole or in part, by or for the United States government is permitted.
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PSFC/JA-17-54
A Field-Shaking System to Eliminate the Screening Current-Induced Field in the 800-MHz HTS Insert of the MIT 1.3-GHz LTS/HTS NMR Magnet:
A Small-Model Study
Jiho Lee, Dongkeun Park, Philip C. Michael, So Noguchi, Juan Bascuñán, and Yukikazu Iwasa
August 2017
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 Bioengineering and the National Institute of General Medical Sciences. Reproduction, translation, publication, use and disposal, in whole or in part, by or for the United States government is permitted.
Mon-Af-Po1.03-03
Template version 8.0, 27 July 2017. IEEE will put copyright information in this area
See http://www.ieee.org/publications_standards/publications/rights/indes.html for more information.
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A Field-Shaking System to Eliminate the Screening Current-Induced Field in the 800-MHz HTS Insert of the MIT 1.3-GHz LTS/HTS NMR Magnet: A Small-
Model Study
Jiho Lee, Dongkeun Park, Philip C. Michael, So Noguchi, Juan Bascuñán, and Yukikazu Iwasa
Abstract— In this paper, we present results, experimental and
analytical, of a small-model study, from which we plan to develop
and apply a full-scale field-shaking system to minimize or even eliminate the screening current-induced field (SCF) in the 800-MHz HTS Insert (H800) of the MIT 1.3-GHz LTS/HTS NMR
magnet (1.3G) currently under construction—the H800 is com-posed of 3 nested coils, each a stack of no-insulation (NI) REBCO double-pancakes. In 1.3G, H800 is the chief source of a large error
field generated by its own SCF. To study the effectiveness of the field-shaking technique, we use two NI REBCO double-pancakes, one from 2 coils (HCoil2 and HCoil3) of the 3 H800 coils, and place
them in the bore of a 5-T/300-mm room-temperature bore external magnet. The external magnet is used not only to induce SCF in the double-pancakes but also eliminate it by the field-shaking. For
each run, we induce SCF in the double-pancake at an axial location where the external radial field Br > 0, then for the field-shaking, move to another location where the external axial field Bz >> BR.
To examine if other SCF eliminating techniques, e.g., the current-sweep-reversal (CSR) method, is applicable to H800 even when L500 and H800 are series-connected, we perform similar se-
quences of test for other combinations of the double-pancake axial locations. In this paper, we report 77-K experimental results, de-velop an analysis that satisfactorily explains the results, and apply
the analysis to design a field-shaking system for 1.3G at full oper-ation.
Index Terms—High field magnet, HTS insert, REBCO, SCF, Screening current
I. INTRODUCTION
OR the development of >1 GHz NMR, with LTS outsert
coil, HTS insert coil must be used owing to its large in-field
current-carrying capacities [1]. For a high-resolution NMR, its
magnetic field must be uniform with an error field of ~0.01 ppm
over a sample volume, making field shimming a must in the
NMR magnet. The screening current-induced field (SCF), a di-
amagnetic field generated by each turn of HTS coil, is major
field error to incorporate an HTS insert for a high field
LTS/HTS magnet. The magnitude of diamagnetic field, SCF is
This work was supported by the National Institute of Biomedical Imaging
and Bioengineering and the National Institute of General Medical Sciences.
J. Lee, D. Park, P. C. Michael, J. Bascunan, and Y. Iwasa are with the Magnet Technology Division, Francis Bitter Magnet Laboratory, Plasma
Science and Fusion Center, Massachusetts Institute of Technology, Cambridge,
proportional to the superconductor size and critical current den-
sity [2]. Various studies have reported the screening current-
induced field (SCF) generated by HTS coils [3-5].
Although much less serious with LTS magnets than with
HTS magnets, the so-called field-shaking technique to mini-
mize SCF error fields was proposed in 1986 for LTS magnets
[6-8]. This technique has since been demonstrated, theoretically
and experimentally to be applicable to HTS magnet [9-15].
Since SCF by an HTS insert can be >100 times greater than
those typical by an LTS outsert, it is critical to minimize or even
eliminate the SCF-generated error field.
Francis Bitter Magnet Laboratory (FBML) of Massachusetts
Institute of Technology (MIT) has developed a high-resolution
S. Noguchi is with the Graduate School of Information Science and Technol-ogy, Hokkaido University, Sapporo 060-0814, Japan (email: [email protected]).
F
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the field-shaking in absence or presence of the external radial
field.
II. TEST COILS AND EXPERIMENTS PROCEDURE
In this study, for SCF induction and field-shaking tests, we
used 2 NI REBCO DP coils, one from each of 2 H800 coils
(Coils 2–3). Specifications of 2 REBCO DPs are shown in Ta-
ble I.
The procedures to induce the screening current and eliminate
the SCF caused by the screening current are as below:
1. Place the NI REBCO DP, at room temperature, at an ax-
ial location, 𝑧 = −175 𝑚𝑚, below the mid-plane of the
external magnet.
2. Apply the magnetic field of 5 T to the DP set. Each coil
experiences the external radial field, the perpendicular
field to the tape surface, at this location.
3. Turn the DP set into the superconducting state, by cool-
ing in a bath of liquid nitrogen.
4. Reduce the magnetic field to zero, the process of which
induces SCF in the double-pancake.
5. Relocate the double-pancake to the mid-plane of the ex-
ternal magnet.
6. Apply a shaking field with the external magnet using the
trapezoidal field injection, confirmed as effective for
elimination of SCF.
7. Map the radial field along the z-axis at each double-pan-
cake’s innermost turn
The magnetic field near the ends of the H800 contains signif-
icant radial field component. To better simulate conditions
throughout the H800, we examine the effect of field-shaking on
SCF in the absence or presence of radial field. This can be
achieved by testing each NI REBCO DP at selected position
above or below the mid-plane of 5-T external magnet. From the
geometry of the coil winding of 5-T external magnet, that se-
lected position which has the maximum radial field, 2 NI
REBCO DPs experience, is 𝑧 = ±175 𝑚𝑚.
III. INDUCTION OF SCF AND FIELD-SHAKING TEST
A. Induction of SCF and field-shaking at mid-plane
Following the above noted procedure, we induced the screen-
ing current of HCoil2 DP at 𝑧 = −175 𝑚𝑚 and moved the
double-pancake to 𝑧 = 0 𝑚𝑚 to eliminate its SCF by field-
shaking without the radial field. Fig. 2 shows the magnetic flux
density at magnet center of the external magnet to induce SCF
and perform the field-shaking. The measurement of magnetic
flux density distribution along the z-axis were repeated after
every field-shaking experiment. Fig. 3 shows the measured ra-
dial field, BR at the innermost turn of HCoil2 DP before and
after field-shaking. From the first attempt of field-shaking of
0.6 T based on the magnet flux density at magnet center, the
considerable SCF was decreased to 20% of its originally in-
duced one. However, after then, even with the multiple at-
tempts, same field-shakings of 0.6 T didn’t show the notable
effect on the elimination of SCF.
Fig. 4 shows the measured BR at the innermost turn of HCoil3
DP before and after field-shakings. From five field-shaking of
0.6 T lowered SCF to 55-50% of its originally induced one. And
then four more field-shaking of 0.8 T and 1.2 T lowered the
SCF to 44-48% of the original magnitude. To eliminate SCF to
~20% of the originally induced one same as the HCoil2 DP test
result, three field-shaking of 1.6 T was needed.
Fig. 1. Position of 5-T/300-mm external magnet (black), HCoil2 DP (blue),
and HCoil3 (red)
TABLE I
SPECIFICATIONS OF TESTED HTS DOUBLE PANCAKE COILS
Parameters HCoil2 DP HCoil3 DP
Conductor (REBCO) Width; thickness [mm; µm] 6.02; 76 6.04; 75 Cu stabilizer thickness [mm] 0.01 0.01 Ic @ 77 K [A] 188 190 NI Double pancake Coil ID [mm] 151.04 196.90 OD [mm] 169.18 211.50 Height [mm] 12.198 12.200 Turn per pancake 120 96 Self-inductance [mH] 18.06 16.60 Characteristic resistance, Rc [µΩ]
322.1 491.9
Time constant, τ [s] 56.07 33.75 Ic @ 77 K, self-field [A] 42.81 67.41
TABLE II
SPECIFICATIONS OF 5-T/300-MM ROOM-TEMPERATURE BORE EXTERNAL
B. Induction of SCF at mid-plane and field-shaking at end-
plane
To simulate the conditions of the end-plane of H800 which
has the maximum external radial field, another field-shaking
test about HCoil2 DP was performed without changing its axial
location. When the magnetic flux density of the external magnet
is 0.98 T, the axial and radial magnetic flux densities are 0.6
and 0.14 T, respectively. To compare the effect of the pres-
ence/absence of the radial field on field-shaking, shaking field
of 0.6 T with radially shaking field of 0.14 T was injected to
HCoil2 DP. Fig. 5 shows the test result of the field-shaking with
the radial field. With the radial field of 0.14 T, 0.6 T field-shak-
ing didn’t show the effective reduction/elimination of SCF
comparable than the test result shown in Fig. 3 which is the re-
sults of field-shaking without the radial field.
Fig. 2. ”Time-versus-external magnetic flux density” profile of 5-T/300-mm
external magnet to induce the screening current and to eliminate its SCF
through field-shaking with trapezoidal waveform, for HCoil2DP test
Fig. 3. Measured radial magnetic flux density, BR, at innermost turn region of HCoil2 DP along the z-axis when coil position during field-shaking was 0
(𝑧 = 0 𝑚𝑚)
Fig. 4. Measured radial magnetic flux density, BR, at innermost turn region
of HCoil3 DP along the z-axis when coil position during field-shaking was 0
(𝑧 = 0 𝑚𝑚)
Fig. 5. Measured radial magnetic flux density, BR, at innermost turn region of HCoil2 DP along the z-axis when coil position during field-shaking was -
175 (𝑧 = 175 𝑚𝑚)
Fig. 6. Normalized value of the line integral ∫|𝐵𝑠𝑐𝑓| 𝑑𝑧 according to the
field-shaking iteration
4
C. Test summary
To quantify and compare the effect of field-shaking for the
elimination of SCF, we calculated the line integral of the meas-
ured magnetic flux density along z-axis, ∫|𝐵𝑠𝑐𝑓| 𝑑𝑧 which has
the unit of Wb/m and compare their normalization values. Fig.
6 shows the normalization value of that line integral according
to the field-shaking iterations. Three tests successfully elimi-
nated the SCF to approximately 20% of their initially induced
values. Once the degradation of SCF shows the tendency of sat-
uration, unless the shaking-field was increased, the repetition of
the field-shaking didn’t make the elimination of SCF more. The
field-shaking of 0.6 T with the radial field of 0.14 T which
shows 9% reduction (100→89%) makes the field-shaking of 0.6
T which shows 80% reduction (100→20%) significantly less
effective.
During the induction of SCF to each NI double-pancake and
the field-shaking, the electromotive force developed by the ex-
ternal magnet caused the azimuthally induced current due to its
NI winding technique which makes the closed loop inside the
coil. Electromotive force is proportional to the change rate of
the external magnetic field, which is proportional to the ramp
rate of the power supply current for the external magnet. During
the induction of SCF for HCoil3 DP, the trace of the coil voltage
shows superconducting-to-normal transition. HCoil3 DP which
has the bigger inner/outer radius than HCoil2 DP should expe-
rience the higher radial field. Therefore, its critical current at 3-
4 T could be high enough to make the superconducting-to-nor-
mal transition. Most of NI HTS double-pancakes are used for
high-field magnets themselves or their insert to elevate the
magnetic flux density at magnet center. Therefore, the field-
shaking could make the operation of HTS coils closer to the
critical condition.
IV. CONCLUSION
The measurement of SCF and the effect on the field-shaking
for its elimination was studied experimentally. Two NI REBCO
double-pancakes, one from each of two H800 coils (Coils 2–3)
were tested. Screening current and SCF were induced at axial
location which experiences the largest radial field. The field-
shaking tests were performed at various amplitudes of shaking-
field and the axial location. Based on the experimental results
and analyses to date, we may conclude that:
Field-shaking method was successfully applied to elimi-
nate the considerable amount of SCF in a couple of at-
tempts. The higher shaking field is more effective on the
elimination of SCF.
Presence of the radial field during field-shaking made
field-shaking significantly less effective than absence of
one. Though with the same z-axial shaking field of 0.6 T,
the presence of the radial field, 0.14 T makes the field-
shaking almost ineffective.
Double-pancakes applying NI winding have the azimuth-
ally induced current due to the electromotive force which
is proportional to the ramping-rate of field-shaking magnet.
In case of operating condition, this azimuthally induced
current could be helpful to eliminate SCF similar to the cur-
rent sweep reversal (CSR) method or make the HTS coils
closer to their operating current margin.
REFERENCES
[1] Yukikazu Iwasa, Juan Bascuñán. Seungyong Hahn, John Voccio, Youngjae Kim, Thibault Lécrevisse, Jungbin Song, and Kazuhiro