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High sensitivity NMR measurements at low temperature and frequency
Physica
B 194-196
(1994)
PHYSICA[
159-160
North-Holland
High sensitivity NMR measurements at low temperature and frequency V. Ruutu, J. Koivuniemi, f0. Parts, A. Hirait, and M. Krusius Low Temperature Laboratory, Helsinki University of Technology, 02150 Espoo, Finland The sensitivity of continuous-wave NMR can be increased with a high-Q resonance tank circuit by using superconducting wire and a GaAs FET preamplifier operating at liquid helium temperatures. We discuss impedance matching to obtain an optimal signal-to-noise ratio for input circuits operating at -- 500 kHz, with Q -- 103-104 at an impedance level of 1 M~2 and a noise voltage of 1.3 nV/'~Hz. High rf compensation of 1: 105 is achieved with orthogonally oriented transmitter coils and an astatically wound pick-up coil. The performance is evaluated on the basis of resolution that we achieved in the measurement of the NMR line in normal liquid 3He; this is better than 1 part in 103 of the peak amplitude, while in the integrated intensity the standard deviation from the mean is better than 0.3%. I. SIGNAL COIL OPTIMIZATION In cw NMR measurements the signal voltage across the tank circuit in a parallel resonance configuration is proportional to the number of turns N in the pick-up coil and the quality factor Q of the tank circuit. High Q-values are achieved with superconducting wire. In Table 1 we compare the Q's obtained with different wires for saddle-shaped coils, which were wound on a hollow cylinder with an outer diameter of 4 mm and a length 8 mm. The opening angle of the winding ¢p = 120 °, as recommended in Ref. [1] for maximum homogenous coverage of the sample volume. The highest Q-value is obtained with the smallest wire without a normal metal coating or matrix. This reduces eddy currents in the wire material and restricts rf losses mainly to dielectrics, such as the wire insulation (heavy Formvar) and the coil former (Stycast 1266 epoxy).
found. Next, one or two layers of windings were removed from the coil, the capacitance was increased to maintain f0--- 400 kHz, and the experiments were repeated. In Fig. 1 we plot the measured Q and L, the apparent parasitic parallel capacitance Cp = 1/(O9o2L)- C and the impedance Z0 = QO)oL of the tank circuit at resonance as a function of N, or more precisely, the number of layers. The results can be fit to yield Q N-L with 7= 1.06 _+0.04, L o~ N ~ with • = 1.94 + 0.01, Z0 o~ N, and Cp ~ -N. Thus the signal voltage ,,~ NQ becomes independent of the number of layers in the pick-up coil. As seen from Fig. 1, the Q value . . . .
o --
I
. . . .
I . . . .
I
,,
q"
~' ¢:~
5
Table 1: Q-values of saddle-shaped rf coils at ~ 400 kHz with 60 turns in each half and L = 122 laI-I. Wire material Nb, single strand ¢ 25 gm CuNi matrix ¢ 50 gm, Nb/Ti filaments, 35 x ¢ 3 gm Cu-coated ~ 25 gm, Nb/Ti single strand, ¢ 16pro
Q 8430 2670
-=
/
\/
190
To find the dependence of Q on N we made a series of measurements on a solenoidal coil with a diameter of 4 mm and a length 8 mm. 12 layers of ¢ 50 gm Nb wire were wound on an epoxy coil former using varnish for fixing the turns. The inductance L, the Q-value, and the resonance frequency f0 were measured for a tank circuit consisting of the coil and a parallel capacitance C. Both silver-mica and high Q porcelain capacitors [2] were used, but no clear difference in the measured Q of the tank circuit was
~:~_*
Io
.V.,~p,"k,~ )C:(,
°
/
~ >
•
Q
&
L
=
/ Zx Cp :
,"-X. ,,,.. o "0"
....
I,,
IoN@ =
mm~n
,,
I,,
500 1000 Number of turns N
, , I 1500
Fig. 1. Measurements of Q, L, Cp and Z0 vs N, starting from a single layer solenoidal coil with 130 turns up to 12 layers and with a total of 1550 turns.
t Department of Physics, University of Kyoto; deceased. 0921-4526/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved S S D I 0921-4526(93)E0658-4
.
160
of the solenoidal coil with a single layer of 130 turns is 5 times higher than that of the equivalent saddle-shaped coil in Table 1. The high Q-values can only be exploited if the amount of normal conductor is kept to a minimum within the tank circuit and a liquid helium temperature high input impedance preamplifier is used. The pick-up coil, its tuning capacitance, the twisted pairs of connecting leads, and the preamplifier were all placed inside separate superconducting shields to minimize rf losses and interference. 2. C O O L E D P R E A M P L I F I E R In Fig. 2 a cascode amplifier is shown with two Sony 3SK166 GaAs MESFETs [3]. The cascode design features good stability by decoupling input and output, but is compromised by a power consumption of 80 m W at 4 K with drain-source current Ids = 9 mA. The 47 ~2 resistance at the input gate and the 47 pF capacitance at the midpoint of the cascode are for suppressing parasitic oscillations in the GHz range. The gain G = gmRL = 40 mS • 500 f~ = 20 is stable from dc up to = 900 kHz and linear within 10% for input levels up to 60 mV (the transconductance gm = A / d s / A g g s = 40 mS at 4 K). The input resistance was typically measured to be Rin = 80 G~2 at dc, corresponding to a gate leakage current of 10 pA with Vg s = -0.85 V, while at 500 kHz one may estimate Rin = gm/((_o2 Cgs 2) = 0.5 Gf2 [4] where the measured value of the input capacitance Cgs = 3pF. At 500 kHz we found a noise voltage of 1.3 n V H H z and a noise current of 1 - 2 fA/~/Hz. For efficient coupling of the tank circuit to the preamplifier, impedance matching is important. The signal power Ps ~ (NQ) 2 while the noise power Pn = en2+(inZo) 2. Substituting for Q = aN-7 L = fiN~. and optimizing the signal-to-noise ratio Ps/Pn with respect to N, one finds that the optimal tank circuit im+0.9 V
5 nF 150 pF RL=
I
T
r o +8 V
pedance is on the order en/i n = 1 Mr2 [5]. With superconducting wire, Q = 1/N and not independent of N, as was found in Ref. [5] for normal wire; thus the best matching in our case is achieved with a single layer solenoid. 3. NMR M E A S U R E M E N T SETUP For high resolution NMR, large rf excitation levels are needed. Compensation of the induced voltages from the excitation field is then required, such that the read-out instrument, a phase-locked detector, can be maintained within its dynamic range. Our measuring setup, with all coils fabricated from superconducting wire, is shown in Fig. 3. Two pairs of orthogonal excitation coils J Compensation coil Astatic pair of ] ; pick-up coils End corrected polarization solenoid 3He samlSle Fig. 3. NMR setup with compensation up to 1:105 of the induced feed-through from the rf excitation. 4. C O N C L U S I O N The measured NMR linewidth of normal 3He at 500 kHz corresponds to an overall static inhomogeneity of IAH/HI = 3" 10.4 and displays no serious deterioration from the superconductors in the different coils. This set up has been used for studying quantized vortex lines in rotating superfluid 3He-B; the signature from a single vortex has been resolved [6]. This work was supported by the Academy of Finland. REFERENCES
T
100 pF ~
5 nFm -
1
output
l q.qK166 ~47pF
r I-_' ~
' _
--
' [_~3SK166 --J
~
E /////
- l- - ~ resonance ~- - J circuit 68~F~
v t
5nF
1
10Mr2
-0.85V
U--] _.3__ °
Fig. 2. 4.2 K preamplifier.
1. D.M. Ginsberg and M.J. Melcher, Rev. Sci. Instrum. 41 (1970) 122. 2. Series 100 and 175 capacitors, A m e r i c a n Technical Ceramics Corp., One Norden Lane, Huntington station, N.Y. 11746-2102, USA. 3. D.V. Camin, G. Pessina, E. Previtali, and G. Ranucci, Cryogenics 29 (1989) 857. 4. F. Ayela, J.L. Bret, and J. Chaussy, Rev. Sci. Instrum. 62 (1991) 2816. 5. J. Lepaisant, D. Bloyet, and E. Varoquaux, Rev. Sci. Instrum. 55 (1984) 521. 6. LI. Parts et al, this conference.
B 194-196
(1994)
PHYSICA[
159-160
North-Holland
High sensitivity NMR measurements at low temperature and frequency V. Ruutu, J. Koivuniemi, f0. Parts, A. Hirait, and M. Krusius Low Temperature Laboratory, Helsinki University of Technology, 02150 Espoo, Finland The sensitivity of continuous-wave NMR can be increased with a high-Q resonance tank circuit by using superconducting wire and a GaAs FET preamplifier operating at liquid helium temperatures. We discuss impedance matching to obtain an optimal signal-to-noise ratio for input circuits operating at -- 500 kHz, with Q -- 103-104 at an impedance level of 1 M~2 and a noise voltage of 1.3 nV/'~Hz. High rf compensation of 1: 105 is achieved with orthogonally oriented transmitter coils and an astatically wound pick-up coil. The performance is evaluated on the basis of resolution that we achieved in the measurement of the NMR line in normal liquid 3He; this is better than 1 part in 103 of the peak amplitude, while in the integrated intensity the standard deviation from the mean is better than 0.3%. I. SIGNAL COIL OPTIMIZATION In cw NMR measurements the signal voltage across the tank circuit in a parallel resonance configuration is proportional to the number of turns N in the pick-up coil and the quality factor Q of the tank circuit. High Q-values are achieved with superconducting wire. In Table 1 we compare the Q's obtained with different wires for saddle-shaped coils, which were wound on a hollow cylinder with an outer diameter of 4 mm and a length 8 mm. The opening angle of the winding ¢p = 120 °, as recommended in Ref. [1] for maximum homogenous coverage of the sample volume. The highest Q-value is obtained with the smallest wire without a normal metal coating or matrix. This reduces eddy currents in the wire material and restricts rf losses mainly to dielectrics, such as the wire insulation (heavy Formvar) and the coil former (Stycast 1266 epoxy).
found. Next, one or two layers of windings were removed from the coil, the capacitance was increased to maintain f0--- 400 kHz, and the experiments were repeated. In Fig. 1 we plot the measured Q and L, the apparent parasitic parallel capacitance Cp = 1/(O9o2L)- C and the impedance Z0 = QO)oL of the tank circuit at resonance as a function of N, or more precisely, the number of layers. The results can be fit to yield Q N-L with 7= 1.06 _+0.04, L o~ N ~ with • = 1.94 + 0.01, Z0 o~ N, and Cp ~ -N. Thus the signal voltage ,,~ NQ becomes independent of the number of layers in the pick-up coil. As seen from Fig. 1, the Q value . . . .
o --
I
. . . .
I . . . .
I
,,
q"
~' ¢:~
5
Table 1: Q-values of saddle-shaped rf coils at ~ 400 kHz with 60 turns in each half and L = 122 laI-I. Wire material Nb, single strand ¢ 25 gm CuNi matrix ¢ 50 gm, Nb/Ti filaments, 35 x ¢ 3 gm Cu-coated ~ 25 gm, Nb/Ti single strand, ¢ 16pro
Q 8430 2670
-=
/
\/
190
To find the dependence of Q on N we made a series of measurements on a solenoidal coil with a diameter of 4 mm and a length 8 mm. 12 layers of ¢ 50 gm Nb wire were wound on an epoxy coil former using varnish for fixing the turns. The inductance L, the Q-value, and the resonance frequency f0 were measured for a tank circuit consisting of the coil and a parallel capacitance C. Both silver-mica and high Q porcelain capacitors [2] were used, but no clear difference in the measured Q of the tank circuit was
~:~_*
Io
.V.,~p,"k,~ )C:(,
°
/
~ >
•
Q
&
L
=
/ Zx Cp :
,"-X. ,,,.. o "0"
....
I,,
IoN@ =
mm~n
,,
I,,
500 1000 Number of turns N
, , I 1500
Fig. 1. Measurements of Q, L, Cp and Z0 vs N, starting from a single layer solenoidal coil with 130 turns up to 12 layers and with a total of 1550 turns.
t Department of Physics, University of Kyoto; deceased. 0921-4526/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved S S D I 0921-4526(93)E0658-4
.
160
of the solenoidal coil with a single layer of 130 turns is 5 times higher than that of the equivalent saddle-shaped coil in Table 1. The high Q-values can only be exploited if the amount of normal conductor is kept to a minimum within the tank circuit and a liquid helium temperature high input impedance preamplifier is used. The pick-up coil, its tuning capacitance, the twisted pairs of connecting leads, and the preamplifier were all placed inside separate superconducting shields to minimize rf losses and interference. 2. C O O L E D P R E A M P L I F I E R In Fig. 2 a cascode amplifier is shown with two Sony 3SK166 GaAs MESFETs [3]. The cascode design features good stability by decoupling input and output, but is compromised by a power consumption of 80 m W at 4 K with drain-source current Ids = 9 mA. The 47 ~2 resistance at the input gate and the 47 pF capacitance at the midpoint of the cascode are for suppressing parasitic oscillations in the GHz range. The gain G = gmRL = 40 mS • 500 f~ = 20 is stable from dc up to = 900 kHz and linear within 10% for input levels up to 60 mV (the transconductance gm = A / d s / A g g s = 40 mS at 4 K). The input resistance was typically measured to be Rin = 80 G~2 at dc, corresponding to a gate leakage current of 10 pA with Vg s = -0.85 V, while at 500 kHz one may estimate Rin = gm/((_o2 Cgs 2) = 0.5 Gf2 [4] where the measured value of the input capacitance Cgs = 3pF. At 500 kHz we found a noise voltage of 1.3 n V H H z and a noise current of 1 - 2 fA/~/Hz. For efficient coupling of the tank circuit to the preamplifier, impedance matching is important. The signal power Ps ~ (NQ) 2 while the noise power Pn = en2+(inZo) 2. Substituting for Q = aN-7 L = fiN~. and optimizing the signal-to-noise ratio Ps/Pn with respect to N, one finds that the optimal tank circuit im+0.9 V
5 nF 150 pF RL=
I
T
r o +8 V
pedance is on the order en/i n = 1 Mr2 [5]. With superconducting wire, Q = 1/N and not independent of N, as was found in Ref. [5] for normal wire; thus the best matching in our case is achieved with a single layer solenoid. 3. NMR M E A S U R E M E N T SETUP For high resolution NMR, large rf excitation levels are needed. Compensation of the induced voltages from the excitation field is then required, such that the read-out instrument, a phase-locked detector, can be maintained within its dynamic range. Our measuring setup, with all coils fabricated from superconducting wire, is shown in Fig. 3. Two pairs of orthogonal excitation coils J Compensation coil Astatic pair of ] ; pick-up coils End corrected polarization solenoid 3He samlSle Fig. 3. NMR setup with compensation up to 1:105 of the induced feed-through from the rf excitation. 4. C O N C L U S I O N The measured NMR linewidth of normal 3He at 500 kHz corresponds to an overall static inhomogeneity of IAH/HI = 3" 10.4 and displays no serious deterioration from the superconductors in the different coils. This set up has been used for studying quantized vortex lines in rotating superfluid 3He-B; the signature from a single vortex has been resolved [6]. This work was supported by the Academy of Finland. REFERENCES
T
100 pF ~
5 nFm -
1
output
l q.qK166 ~47pF
r I-_' ~
' _
--
' [_~3SK166 --J
~
E /////
- l- - ~ resonance ~- - J circuit 68~F~
v t
5nF
1
10Mr2
-0.85V
U--] _.3__ °
Fig. 2. 4.2 K preamplifier.
1. D.M. Ginsberg and M.J. Melcher, Rev. Sci. Instrum. 41 (1970) 122. 2. Series 100 and 175 capacitors, A m e r i c a n Technical Ceramics Corp., One Norden Lane, Huntington station, N.Y. 11746-2102, USA. 3. D.V. Camin, G. Pessina, E. Previtali, and G. Ranucci, Cryogenics 29 (1989) 857. 4. F. Ayela, J.L. Bret, and J. Chaussy, Rev. Sci. Instrum. 62 (1991) 2816. 5. J. Lepaisant, D. Bloyet, and E. Varoquaux, Rev. Sci. Instrum. 55 (1984) 521. 6. LI. Parts et al, this conference.