WO2012173584A1

WO2012173584A1 - Device for mechanical balance of superconductive gradientometer at unshielded location

Info

Publication number: WO2012173584A1
Application number: PCT/UA2011/000124
Authority: WO
Inventors: Yurii Dmytrovych MINOV; Mykola Mykolaiovych BUDNYK
Original assignee: Minov Yurii Dmytrovych; Budnyk Mykola Mykolaiovych
Priority date: 2011-06-16
Filing date: 2011-12-12
Publication date: 2012-12-20
Also published as: UA103062C2

Abstract

The invention relates to supersensitive magnetometry and proposes device for mechanical balancing SQUID input antenna, i.e. axial wire gradientometer of the 2^nd order, on 3 orthogonal components of the magnetic field with help of three shorted superconducting (trim) rings. According to the invention trim-rings are made of thin niobium wire and are fixed to rods so that their axes are orthogonally. Gear is shifted each ring separately, ring to balance on the axial (transversal) component is placed outside (inside) antenna and is shifted up-down between receiving and middle coils (near upper coil). Also some modifications are proposed such as other material, form and drive, application to other gradientometer design. The device achieves imbalance of 20 ppm at unshielded location in the presence of strong industrial noises in urban areas.

Description

IPC (2011.01) G01 R 33/035

G01 R 33/02

DEVICE FOR MECHANICAL BALANCE OF SUPERCONDUCTIVE GRADIENTOMETER AT UNSHIELDED LOCATION

FIELD OF INVENTION

The invention relates to the field of supersensitive magnetic measurements and refers to balancing input antenna of SQUID (Superconducting Quantum Interference Device)-magnetometers.

PRIOR STATE-OF-ART

When registering weak magnetic signals without reducing industrial noise, they thousands of times greater than the useful signal. To improve the signal-to-noise ratio (SNR) at the magnetometer it is used input special antennas, so-called superconducting gradientometer. For near sources R^«B (base) weakening of the signal at the gradientometer input is minor, and for far sources signal is proportional to 1/R^(3+M), where M - gradientometer order. However, a strong suppression takes place only at high degree of balance, which depends on the gradientometer design and method for its balancing.

Gradientometer coils differ in size, shape and parallelity because initial balance of wire gradientometer equal nearly to 2000÷4000 ppm (part per million = 10^~6), which is not enough to reduce noises generated by power sources. From the other hand, supersensitive magnetic measurements in unshielded environment require a degree of balance no worse than a few dozens of ppm.

For wire gradientometer it is known two ways:

a) improving the design and technology of antennas; b) improvement of means for the mechanical balancing of antennas.

In the direction of a) during the last decade it is known, for example, the design according to WO 2002/027332 and UA 6882 (2006).

In WO 2002/0227332 it is proposed axial gradientometer of 1÷3-th order with a frame constructed from glass PYREX, coefficient of heart expansion of which is close to niobium (wire material), spiral and vertical grooves are fashioned on the frame, in which the wire-wound coils are tensioned and glued by cyanoacrylate glue, and two twisted wires (bifilyar) are wrapped into the vertical grooves.

As a result, it is received balance equal to 400÷800 (2000) ppm for the vertical Z (horizontal X&Y) field components. Further, the balance is improved by electronic noise suppression system (ENSS) based on the reference vector magnetometer (RVM). However, only ENSS without mechanical balance is not enough for reliable operation of SQU ID- magnetometer without magnetic shielding room (MSR), for example, in the medical clinic.

Therefore, it is known use of balancing (trimming) elements (disks, plates, etc.) from superconductors (way b). Their action is based on pushing the magnetic field from superconductors, which is ideal diamagnetics. So, near trim-element magnetic field is distorted, and its direction and amplitude it also changed at the plane of the antenna turns. As a result of shift of the orthogonally-oriented trim-elements one can equalize the effective area of coils, that allows reach a high degree of balance.

It is known such devices for mechanical balancing:

1. US 3,976,938, G01R 33/02, Superconducting magnetic sensor with improved balancing system, W. Hesterman, Supercond. Technology, 1976.

2. US 4,320,341 , G01 R 33/02, Method and apparatus for balancing the magnetic field detecting loops of a cryogenic gradiometer using trimming coils and superconducting disks, C. Lutes, Sperry Corporation, 1982. 3. US 3,956,690, G05F 7/00, Trimmed superconductive magnetic pickup coil circuits, L. Rorden, Develco Inc., 1976.

4. US 3,965,411 , G01R 33/02, Gradient wound trim coil for trimming a primary pickup coil, V. Hesterman, Develco Inc., 1976.

5. US 3,962,628, G01 R 33/02, Adjustable magnetic gradiometer, C. Rein, US Secretary of the Navy, 1976.

6. US 4,549,135, G01 R 33/035, Magnetic field gradiometer with trimming element, A. Vaidya, EMI Limited, 1985.

7. UA 16882. G01 R33/035, Superconductive gradientometer of magnetic field, Yu. Minov, M. Budnyk, 2006.

8. Fine balancing second derivative gradiometer, US 4,523,147, G01 R 33/02, S. D'Angelo et al., Consiglio Nazionale Delle Recerche, 1985.

Known prior devices for balancing on the 3 field components are based on orthogonal superconducting trimming disks [US 3,976,938 and US 4,320,341]. But an essential drawback is the strong dependence of the degree of balance from the direction and homogeneity of magnetic noise due to mutual influence of trim-elements. As a result, after antenna at the factory setting into uniform magnetic field and move it to measuring place with non-homogeneous noises the degree of balance may be become worse on 100 or more times.

In addition, according to US 3,976,938 firstly it is conducted a rough balancing, after gradientometer is removed from the cryostat, discs are mechanically fastened and again immersed into liquid helium for fine balancing. But due to thermal cycling the size of the antenna parts are changed, so this method can not provide a sufficient degree of balance.

Also devices based on other approaches are known. Thus, according to US 3,962,628 device includes a cam mechanism where by rotation cams the gradientometer coils are deformed, causing to change their effective areas. In the patent US 4,549, 135 balancing element includes an electric heater, which is switched of superconductors to the resistive state. However, the implementation of both of above devices is too complex and is focused on certain types of antennas and special conditions of measurements, so that the current state of technology does not demonstrate their further development.

A more close approach [US 3,965,411 and US 3,956,690] is based on the orthogonal superconducting rings, which are included in galvanic manner into the magnetic flux transformer. Rings are made in gradient configuration and links with the magnetic field is controlled by shifting a superconducting magnetic shield.

The main disadvantage of this approach is the introduction of additional inductances to flux transformer, which deteriorates the sensitivity of the SQUID-magneto-meter to the input signal. In addition, trim loops must be accurately prepared and oriented, and in case of gradientometer configuration also have own high degree of balance.

In analogue UA 16882 balancing element on Z-axis is made in form of superconducting ring placed inside the antenna frame and driving gear for moving along the frame axis. The ring has inductive (but not galvanic) connection with the compensation (but not pick-up) coil of the antenna. The disadvantage is that device intended to balance only the vertical field component.

The prototype device according to US 4,523,147 includes wired axial gradientometer of the 2nd order with the upper, central and lower (receiving) coil. The frame consists of two co-axial parts, the first of which fixed the central and lower coil and the second one - the upper coil, and the upper and lower coil have N turns, and central has 2N turns. The antenna frame has 3 vertical holes in which the balansing mechanisms onto 3 orthogonal field components is placed, trim elements are made as lead plates. Balancing consists in moving said plates relative to coils for equalization of their effective areas. A distinctive feature of the prototype is micron changing of distance between the upper and central coils by shifting the top part of frame relatively bottom one. For this a shift rod and two screw are served which convert rotational motion of the screw to the rod shifting. Balancing is performed by the screw turning, thus the distance between the antenna coils is changed by a shifting up and down the upper frame part. The degree of balance is increasing due to compensation of vertical gradient of noise. The result is confirmed by decreasing cut-off frequency of 1/f noise at the magnetometer output from 10 Hz to 0.4 Hz.

However, the implementation of frame from two parts complicates the design and increases the cost of the device. In addition, drawbacks of superconducting plates are similar to disks (see mentioned US 3,976,938 and US 4,320,341 ).

During unshielded measurements the noise suppression can be made by the additional balancing at measurement place. The main criterion in this case is the maximum suppression of noise at magnetometer output. However, such balance makes sense only for stationary measuring systems, the location of which remains unchanged relative to noise sources during long time.

Also in mobile systems using superconducting plates require additional balancing in each new measuring location. In addition, the balancing procedure is complex and many-iterative due to strong mutual influence of the plates.

SUMMARY OF THE INVENTION

The task stated is achieved by:

- Implementation of hand-driven moving gear for balance the axial wire-wound gradientometer of the 2nd order for longitudinal and two transverse components of magnetic field, which includes three separate rods to separate move the three short-closed, superconducting rings along the axis of a cylindrical frame of gradientometer,

- Manufacture of balancing rings with the thin niobium wire and their attachment to the rods so that their axes are mutually perpendicular,

- Making the movement drives enabled to move each ring separately along the gradientometer axis to the maximum distance, no longer than baseline distance B (base) of gradientometer,

- Placing the ring to balance on the longitudinal field component outside gradientometer and moving it up-down between receiving and middle coiuls,

- Placing two rings for balancing on the transverse component inside the gradientometer frame and moving them up-down relative to the upper turn to a distance of no more than half of the base;

- Making at least one said ring of superconducting material other than niobium;

- Implementation at least one rings of other form than circular, for example such as gradientometer shape;

- Placing at least one ring for balancing on the transverse component outside the gradientometer;

- Bring at least one gear into action does not move manually, but using the electromechanical drive or any other mechanical device;

- Bring at least one said gear in action under control of a computer program using a computer or any other automatized way;

- Use of a device for balancing wire-wound axial gradientometer of form other than circular, such as a polygon;

- Use of a device for balancing wire-wound axial gradientometer of different order than the 2^nd order;

- Use of a device for balancing wire-wound gradientometer of non-axial type; - Use of the device for balancing non-wire-wound gradientometer as, for example, thin-film planar or volume (bulk).

The essence of invention is device for balancing of superconductive gradientometer, which differs from the prototype that as superconducting trim-elements are used superconductive rings with small diameter, which does not exceed the diameter of the wire from which the antenna coils are made. Said rings does not nonlinear distort the magnetic field near the coils and substantially does not affect its symmetry of gradientometer with respect to another field components. The degree of balance is adjusted by changing inductive connection between trim-ring and antenna coils, and the sign and strength of the effect depends on its position relative to coils.

The novelty of the proposed invention consists in:

1 ) combination of the device design according to prototype US

4,523,147 and analogue UA 16882;

2) expanding opportunities of device for balancing onto Horizontal field components;

3) refusal of the application of 3-component RVM and ENSS for zneshum-tion signal;

4) possibility of placing a balancing ring for vertical field component outside gradientometer that allows to apply the device to the antenna with a diameter of less than 1 cm;

5) possibility for creation of multichannel measuring systems with minimal mutual influence between measuring channels;

6) shifting a balancing element for the vertical field component between the middle and receiving turns;

7) increasing distance between the vertical and horizontal rings, which is not less than the base distance of gradientometer that minimizes their mutual influence;

The basis of the invention is the task improving the design of the device for mechanical balansing of superconductive gradientometer onto 3 orthogonal magnetic field component, which provides to achieve a degree of balance up to 20 ppm for all components sufficient for measurements in unshielded environment, simplifying and accelerating procedures for balancing by minimizing the deterioration of balance degree for two components during establishing the third one, the possibility of using the device to the multichannel systems and for its mobile performance.

Technical result is achieved using 3 orthogonal superconducting rings, which separately move along the gradientometer axis, while for balancing on each field component is used single ring of given orientation, which does not worsen the balance of other two components of magnetic field. Technical result consists in:

1) achieving the degree of balance on the orthogonal components of the field better than 20 ppm, sufficient for measurements in unshielded environment;

2) stability of the achieved degree of balance in multiple temperature cycles of device "cooling-warming";

3) simplifying the design of the adjusting device for the mechanical gradientometer balancing that provides movement of the ring;

4) an opportunity to perform balancing at measurement location as often as it is required when direction and degree of heterogeneity of external disturbances is changing;

5) an opportunity to perform balance at the significant changes in amplitude and degree of noise heterogeneity when mobile magnetometer is moved to different location (for example, using a specialized vehicle);

6) the possibility of use in multichannel systems without over- dimensional increase the diameter of the cryostat by reducing the diameter of the antenna;

7) achieving a minimum mutual influence between trim-rings designed for balancing the various components of the field;

8) magnetometer is cheaper due to refusal of RVM. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows location of balancing rings and axial gradientometer of the 2nd order, associated with SQUID: 1 - the bottom or receiving antenna coil, 2 - middle compensation coils, 3 - top compensation coil, 4 - gradientometer frame, 5 - input coil of the magnetic flux transformer, 6 - SQUID sensor, 7, 8, and 9 - superconductive rings for balancing on Z, X and Y field component, respectively.

Figure 2 presents general view of the device in preferred embodiment with axial gradientometer of the 2nd order: 1-9 - items similar to Figure 1 , 10 - cylindrical frame for mounting the Z trim ring, 11 , 12, and 13 - rod for fixing and shifting trim ring X, Y and Z, respectively.

Figure 3 illustrates view of moving drive to balance gradientometer on 3 orthogonal coordinates: 11-13 - the same as at Figure 2, 14 - bearing, 15 - slider, 16 - rotation clutch, 17 - power screw.

Figures 4 and 5 display spectra of output noise of SQU ID- magnetometer before and after the mechanical balancing antenna, i.e. gradientometer of the 2nd order.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The mutual arrangement of short-circuited superconducting rings and axial gradientometer of the 2nd order, linked with the SQUID, is shown at

Figure 1 illustrating the principle of the invention.

At the main embodiment device according to invention is intended to balance the axial gradientometer of the 2nd order, which registers the 2nd spatial derivative of the vertical (axial) field component. Gradientometer consists of 4 circular coils 1-3 of 11 mm radius and distance between them

1-2 and 2-3 (base) equal to 60 mm.

The receiving coil 1 is intended for registration of useful signal, two middle 2 and top coils 3 - to compensate magnetic noises from far sources.

Middle coils are wounded in the opposite direction relative to the top and bottom coils. The antenna is made from whole piece of superconducting wire, coils are connected by the twisted wire (bifilar winding) placed into vertical grooves in the frame.

Used in the preferred embodiment design provides achieving the initial balance of axial (transverse) field component in the range 400÷800 (200÷400) ppm. Useful signal is introduced into the SQUID sensor 6 via input coil 5, which together with the antenna creates a magnetic flux transformer. Gradientometer and transformer designs are presented only for illustration and are known from prior art [WO/2002/027332, UA16882].

Further, balance of said antenna on Z is improved to 20 ppm according to UA 19997 [Method for mechanical balancing superconducting gradientometer of magnetic field, G01 R33/035, M. Budnyk, Yu. Minov, 2006]. Here, firstly, trim-ring is placed midway between the middle and top compensation coils, and balancing is made by its shift along the antenna axis between these coils.

However, it was found that level of horizontal industrial noise penetrating into the SQUID-sensor was too strong due to magnetic contamination in the large city (Kyiv). So, we need to improve the degree of antenna balance on horizontal components, similar to the vertical one.

According to the proposed invention balancing is made by 3 superconducting shorted trim-rings 7-9 (see Figure 1 ). In the basic embodiment trim-rings are made from the same niobium wire 0 50 μιη as gradientometer. Ring 7 for balancing on axial Z component is placed outside and in the middle between receiving and middle coils in a plane parallel to the turn plane. Ring 8 (9) to balance the horizontal component X (Y) is placed inside the top coil and ring axis is oriented along the X (Y).

For disclosure of the device principle one can consider trim-ring in an external magnetic field. Because the effect of magnetic flux quantization in a ring, it is induced screening current, which compensates flux applied into the ring. When trim-ring is introduced between the antenna coils due to the magnetic linkage the mutual shielding of currents is take place, which leads to a reduction of inductances both antenna and ring.

The principal possibility of balancing is that at a certain ring position magnetic flux introduced

nnay become equal in magnitude and opposite in sign to parasitic flux Φ_Α=α^χΦ, entered into gradientometer due to non-ideal balance (imbalance), where a - initial (technological) antenna imbalance, Φ - flux penetrating into receiving coil, a_BAL=M_A /L - imbalance, transmitted by trim-ring to antenna, M_A - mutual inductance between the antenna and trim-ring, L - inductance of trim-ring.

The condition of balance is equal to zero total flux in the antenna

where we have link a=-My_ between imbalance a, inductance L and mutual inductance M_A, which is subject to regulation. As a rule, inductances of coils are equal L_1I2,3=L_P, than M_A=Kx (Lp/x L)^1/2 and condition (1 ) gives

a = - Kx (L_P/L)^{1 2} , (2) where K=K1-2xK2+K3 and K1,2,3 are coefficients of mutual inductance between ring with whole antenna and turns (here considered that middle coil of the 2nd order gradientometer is wounded in the opposite direction).

When trim-ring is positioned between receiving and middle coils, mutual inductance between ring and top coil is negligible K3 = 0. According to invention, trim-ring on Z component has a diameter close to the coils' diameter, which provides the approximate equality of inductances L*L_Pl then (2) is simplified and has the form

2xK2- K1 = a . (3)

When trim-ring is positioned between said coils K1=K2=K0, and a = K0. If ring is shifted to up magnetic link with the middle (receiving) coil is increased (reduced), i.e. K2>K1. At the highest position (plane of middle coils) K1 «1 , K2 1 , then K 2. When turn is down shifted, vice versa, magnetic link is decreased with middle coils and is increased with receiving one (K2 <K1). In some ZO ("zero point") position it is satisfied condition K2=K1/2, in which the influence of trim-ring to antenna is absent, i.e. K=0. At the lowest position (plane of the receiving coil) K2«1, K1∞ 1, then K∞- 1.

Thus this device allows you to change imbalance made by trim-ring into antenna for Z field component, in the range -1<a_BAL<2. Balance equation (3) requires a shift ring so that the difference of mutual-inductance coefficients was equal imbalance. Taking into account that the initial imbalance is in thousand-ppm range, the optimal position according to (3) is near "zero point" Zopt«Z0. So, shift of Zopt to up or down relative to ZO is determined by the sign of initial imbalance.

According to invention, design of the balancing device intended for measurements at unshielded environment is shown at Figure 2. Antenna includes coils 1-3 and frame 4 and described above (see Fig.1 ). Regulating device consists of 3 shorted trim-rings 7-9, designed for balancing on the Z, X & Y field component. Rings 8 (X) and 9 (Y) are mounted on nonmagnetic rods, 11 & 12, respectively. Ring 7(Z) attached to rod 13 by a cylindrical holder 10, which provides fixation of trim-ring in the transverse plane.

At Figure 3 design of moving gear to balance gradientometer on 3- orthogonal coordinates is shown, comprising bearing 14, slider 15, clutch 16 and power screw 17. Rotation of clutch 16 causes rotation of screw 17, which, in turn, leads to a shift up-down along the slider 15 axis and rod attached 11. In basic embodiment the shifting speed is determined by pitch and is 5 turns per mm. Power screw 17, inserted into the bearing 14 hole, is part of moving gear allows smooth movement of trim-ring (see Fig. 3).

In result, presented design provides to achieve the balance degree over all field components better than 20 ppm, which is sufficient for measurements in unshielded environment without using RVM. Also, the design has sufficient rigidity to mechanical deformations and shiftings caused by thermal expansion of materials of. device parts. This leads to a permanent position of trim-rings and maintains the same degree of balance due to multiple termo-cycles from cryogenic to room temperature.

In other embodiment, one, two or all three trim-rings are made with the other superconducting material than niobium. Thus, any of the rings can be made of another form.

In another embodiment at least one trim-ring for balancing the horizontal component is placed outside (but not inside) the gradientometer frame. This simplifies the device production because the moving gear does not need to be located inside the said frame, which is very inconvenient if the gradientometer diameter is small.

Said embodiment is promising for multi-channel systems, because allow to increase the number of channels without increasing the diameter of the cryostat, but by reducing the gradientometer diameter. As a result, cryostat having acceptable diameter provides lower specific expenses of liquid helium per one channel.

In additional embodiment one or more gears to shift trim-rings put in action by not hand-driven and using electro-mechanical drive or any other mechanical drive. This movement of a drive is controlled by using the computer with help of the software or any other automatized way.

Such embodiment is perspective for the mobile variant of magnetometer, because it frees staff from routine work including often balancing at the each new measurement location. Also it is increased the reliability and accuracy due to the independence of personnel skills. This balancing procedure is done automatically and can be part of the self-diagnostic program of magnetometer, which is performed at its power-on.

In another embodiment the axial gradientometer has other, differ than round, shape, such as a proper polygon. Also wired axial gradientometer may be a different order than the 2nd, such as 1st or 3rd. In another alternate embodiment it can be used wired gradientometer of non-axial type or non-wired, but another type, such as thin-film planar or volume (bulk).

The given embodiments of the device in the invention are described in detail only for the purpose of illustration. It is clear that in practice people experienced in the SQUID-magnetometry and cryogenic technology can make some changes and modifications in the design of the proposed device. However, we consider that if said modifications and changes are made without significant deviations from the essence and claims of proposed invention, they fall under this patent.

Claims

1. Device for mechanical balance of superconductive gradientometer at unshielded location at a high level of industrial interference, comprising:

a regulatory mechanism with the hand-driven moving gear for balancing gradientometer in three orthogonal (longitudinal and two transverse) components of magnetic field,

a three rods, which perform separate movement of three balancing elements, made as short-circuited superconducting trim-rings along the axis of a cylindrical frame of wire-wounded gradientometer of the 2nd order, formed by a receiving coil and two compensating coils, located at the lower, middle and upper end of frame, respectively,

characterized in that the balancing rings are made of thin niobium wire and are fixed to rods so that their axes are mutually perpendicular, said moving gears are made capable to shift each ring along the gradientometer axis at the maximum distance, no more than the gradientometer base distance,

said ring for balancing on the longitudinal field component is placed outside gradientometer and is shifted up-down between receiving and middle coils,

said two rings for balancing on the transverse field component are placed inside the gradientometer frame and are shifted from the top coil to a distance not more than half-base.

2. Device for the mechanical balancing according to the claim 1 , characterized in that at least one shorted balancing ring is made with another superconducting material than niobium.

3. Device for the mechanical balancing according to the claim 1 or claim 2, characterized in that at least one trim element is made other form than the ring.

4. Device for the mechanical balancing according to anyone of the claims 1-3, characterized in that at least one balancing ring on the transverse component is placed outside gradientometer.

5. Device for the mechanical balancing according to anyone of the claims 1-4, characterized in that at least one the said hand-driven gear is activated by electromechanical drive or any other mechanical drive.

6. Device for the mechanical balancing according to anyone of the claims 1-5, characterized in that at least one said gear is activated by computer under control of software or any other automatized method.

7. Device for the mechanical balancing according to anyone of the claims 1-6, characterized in that the regulatory mechanism is used to balance the wire axial gradientometer other form than round, e.g., polygon.

8. The said device according to anyone of the claims 1-7, characterized in that the regulatory mechanism is used for balancing the axial wire gradientometer of different order than the 2^nd order.

9. The said device according to anyone of the claims 1-8, characterized in that the regulatory mechanism used for balancing the wire gradientometer of non-axial type.

10. The said device according to anyone of the claims 1-9, characterized in that the regulatory mechanism is used for balancing non-wire gradientometer, such as thin-film planar or volume (bulk) one.