SURVEY OF ELECTRICAL RESISTIVITY MEASUREMENTS ON 16 PURE METALS IN THE TEMPERATURE RANGE TO 273 K
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Each layer 44 is preferably formed by a homogeneous metal foil, but may generally comprise more than one monolithic structure or layer. Conductor 40 may be stacked onto or surrounded by a non-conductive support material such as quartz, sapphire, glass, or a ceramic material.
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Conductor 40 includes an internal conductive layer 46 , and two identical external conductive layers 48a-b. Each of layers 46, 48a-b is formed by a normal non-superconducting metal. Layers 48a-b laterally enclose layer 46 , and are attached to layer 46 on opposite sides of layer The thickness of each layer 48a-b can range from tens to hundreds of microns, and is preferably on the order of one to a few hundred m tenths of a mm. The thickness of each layer 48a-b is preferably chosen to be higher than the skin depth of each layer 48a-b.
Current flow through conductor 40 occurs primarily through external layers 48a-b , and the net resistivity of conductor 40 is determined in large part by the resistivity of layers 48a-b. Layer 46 can have a higher resistivity and magnetoresistance than external layers 48a-b. The volume magnetic susceptibility of internal layer 46 is opposite in sign to the magnetic susceptibility of external layers 48a-b.
For example, if internal layer 46 is diamagnetic, external layers 48a-b are paramagnetic.
Using layers of opposite magnetic susceptibilities allows compensating for the magnetic susceptibilities of each individual layer, thus reducing any distortions introduced by the RF coils into applied magnetic fields. The net or effective magnetic susceptibility of conductor 40 is preferably substantially equal to that of its surroundings. If conductor 40 is embedded in a support material, the net magnetic susceptibility of conductor 40 is preferably equal to the susceptibility of the support material.
If conductor 40 is surrounded by vacuum, the net susceptibility of conductor 40 is preferably close to zero, e. The thicknesses of layers 44 can be chosen so as to provide a desired net magnetic susceptibility for conductor In the preferred embodiment, external layers 48a-b are formed by pure aluminum, while internal layer 46 is formed by a diamagnetic material such as copper. Other diamagnetic normal metals suitable for layer 46 include silver, gold, beryllium, and lead. The purity of the aluminum forming layers 48a-b is preferably higher than Impurities and other defects in layers 48a-b increase the resistivity of conductor 40 and reduce the quality Q factor of the RF coil.
Maintaining low levels of impurities and dislocations within layers 48a-b is particularly desirable since layers 48a-b are held at a cryogenic operating temperature. At low temperatures, the mean free path of electrons is typically much longer than at room temperature. While at room temperature the mean free path of electrons is typically limited by phonons, at low temperatures the mean free path can be limited by impurities and dislocations. Aluminum is preferred for the outside layers because of its relatively low resistivity and magnetoresistance, which allow achieving higher Q-factors with aluminum coils than with conventional copper coils, particularly in the presence of applied magnetic fields.
Coils made from Table 1 shows several empirically-determined Q-factors for pure aluminum and copper coils, in the presence and absence of an applied static magnetic field B 0. The measurements were taken at 20 K. The aluminum and copper foils were commercially obtained from Goodfellow Corp. Preliminary measurements on a susceptibility-compensated, 0. For a free-standing conductor formed by aluminum and copper, a total copper thickness of about twice the total aluminum thickness is preferably used to achieve susceptibility compensation.
If the conductor is formed by an Al-Cu-Al stack with identical Al layers on both sides, the thickness of the internal copper layer is preferably about 4 times the thickness of each external aluminum layer. In practice, exact layer thicknesses and coil annealing conditions can be tailored empirically to achieve a desired level of susceptibility compensation at the operating temperature of the coil. Table 2 shows exemplary measured susceptibilities for an annealed and an unannealed Al-Cu-Al foil at room temperature K and at a cryogenic temperature 25 K.
As illustrated, the measured effective susceptibilities for the Al-Cu-Al foils are substantially lower than the susceptibilities of pure copper and aluminum. Further susceptibility compensation can be achieved by empirically tailoring layer thicknesses. Other suitable materials for the layers of conductor 40 include silver, gold, platinum, palladium, lead, and beryllium.
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The data shown in Table 3 are the lowest values in the survey by Hall. Suitable materials such as the ones listed above do not contain nuclei that interfere with typical NMR measurements, and can be manufactured to controlled thicknesses. An Al-Cu-Al clad metal foil as illustrated in Fig. Individual Cu and Al foils are hot-pressed together to form a layered structure as illustrated in Fig.
Annealing reduces dislocations such as slip-planes in the crystalline lattice of a material. Reducing dislocations allows reducing the material resistivity. Annealing can also slightly affect the magnetic susceptibility of the foil, as illustrated above in Table 2. Conductor 50 includes a composite internal layer 54 comprising two separate monolithic layers 52a-b.
Layer 54 is clad on both sides by external monolithic layers 56a-b. The magnetic susceptibilities of the various layers of conductor 50 are chosen such that the difference between the net susceptibilities of the paramagnetic layers and the diamagnetic layers is substantially equal to the net susceptibility of the surroundings of conductor It will be clear to one skilled in the art that the above embodiments may be altered, for example, the coil conductor may include a composite structure with more than three layers.
Various normal metals may be used for the diamagnetic and paramagnetic layers of the coil. The above experimental results are given for exemplary purposes only, and arc not intended to limit the invention. A susceptibility-compensated cryogenic radio-frequency coil 30 for a nuclear magnetic resonance probe 20 , comprising: a an internal copper layer 46, 52a-b ; and.
The coil of claim 1, wherein an impurity fraction in the external aluminum layers is less than 10 The coil of claim 1 or 2, wherein said internal layer 46 is a copper foil clad on each of two opposite sides by an external aluminum foil, the foil forming the external layers 48a-b. The coil of claim 3, wherein the aluminum foil on each side of the copper foil has a thickness substantially equal to one fourth of a thickness of the copper foil. A nuclear magnetic resonance probe 20 comprising: a a susceptibility-compensated cryogenic radio-frequency coil 30 according to any of the preceding claims for applying a radio-frequency magnetic field to a sample; and.
A nuclear magnetic resonance spectrometer 12 comprising: a a magnet 16 for applying a static magnetic field B 0 to a nuclear magnetic resonance sample; and. USB1 en.
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EPB1 en. JPB2 en. DET2 en. NMR spectrometer and method of operation with the stabilization of the transverse magnetization in NMR superconducting resonators. NMR probe having an inner quadrature detection coil combined with a spiral wound outer coil for irradiation.
NMR resonator, formed as a conductive coating on both sides, insulating film, and related low-resolution NMR spectrometer. Cryogenically cooled superconductor gradient coil module for magnetic resonance imaging.
JPA en. NLA en. USREE en. NMR spectrometer with superconducting coil having rectangular cross-section wire.
DEC2 en. USA en. CAC en. Apparatus and method for compensation of magnetic susceptibility variation in NMR microspectroscopy detection microcoils. Magnetic susceptibility control of superconducting materials in nuclear magnetic resonance NMR probes. AC magnetic susceptibility control of superconducting materials in nuclear magnetic resonance NMR probes. Laminated body not exerting magnetostriction and sample coil for nuclear magnetic resonance apparatus using the same. DED1 en. EPA3 en. EPA2 en. Wu et al. AUB2 en.
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