WO2003103769A1

WO2003103769A1 - Magnetic pulse stimulation apparatus and magnetic field source thereof

Info

Publication number: WO2003103769A1
Application number: PCT/BY2002/000002
Authority: WO
Inventors: Vladimir V. Mikhnevich; Genadiy A. Govor
Original assignee: Mikhnevich Vladimir V; Govor Genadiy A
Priority date: 2002-06-05
Filing date: 2002-06-05
Publication date: 2003-12-18
Also published as: EA200401545A1; EA007347B1; AU2002344851A1; UA78051C2

Abstract

The present invention relates to medical equipment, and more particularly to an apparatus for pulse magnetic stimulation of various organs and systems of various organs and systems of the systems of the human body. To provide wide application of a method of magnetic stimulation and to design a fairly inexpensive and reliable apparatus for magnetic stimulation, it is necessary first of all to increase the rate of magnetic induction rise up to the value of ΔB/Δt=10⁵ T/sec. The latter will make it possibleto considerably reduce energy consumption of an apparatus for magnetic stimulation, to increase pulse repetition up to 1-2 kHz with the duration of an efficient stimulation pulse being increased up to t_s= 1 sec and more, i.e. to provide the modulation frequency of about 30 pulse trains per minute. The object of the invention set forth in the apparatus for magnetic stimulation comprised of an oscillatory circuit containing a capacitor and a source of unipolar magnetic field pulses, said source further comprising a winding, a power supply unit and a storage capacitor, said capacitor connected in parallel to a capacitor in the oscillatory circuit, and also first and second controlled switches, and first and second diodes is achieved by providing said magnetic field source with a core made of soft magnetic composite material.

Description

MAGNETIC PULSE STIMULATION APPARATUS AND MAGNETIC FIELD SOURCE THEREOF

Apparatus for Magnetic Pulse Stimulation and Magnetic Field Source Thereof

Field of the Invention

The present invention relates to medical equipment, and more particularly to an apparatus for pulse magnetic stimulation of various organs and systems of the human body.

Prior Art

Stimulation of the central and peripheral nervous systems as well as of various internal organs of the human body by pulsed magnetic fields principally consists in the optimization of the process of creating in the desired area of an induced electric field with the magnitude of said field being sufficient to promote the activity of one or other process.

Hence the process of stimulation optimization incorporates the definition of the shapes and duration of magnetic field pulses, their repetition rate and magnetic field distribution near a magnetic field source. The problem of magnetic stimulation has been investigated in a number of references: Alexander et all. Treatment of Urinary Incontinence by Electric Pessary. British J. of Urology (1980), 42,184-190 - [1]; Mouchawar, Close-Chest Cardiac Stimulation with Pulsed magnetic field, Medical and Biological Engineering and Computer Mar. (1992), 162-169 - [2]; Yunokuchi et al. Developing More Focal Magnetic Stimulator. J. Of Clinical Neurophysiology (1991), 8, 102-111 - [3]; Cohen et al. Effect of Coil Design on Delivery of Focal Magnetic Stimulation. Electroencephalography and Clinical Neurophysiology. (1990), 75, 350-357 - [4]; Mills et al. Magnetic Brain Stimulation with a double coils. Electroencephalography and Clinical Neurophysiology. (1992), 85, 17-21 - [5]. Appropriate solutions have been proposed in some of them: Cadwell et. al. Method and Apparatus for Magnetically Stimulation Neurons. Patent US:4940453 - [6]; Konotchick et al. Magnetic Stimulation Device. Patent US:5267938 - [7]; Gluck et al. Method of and Apparatus for Magnetically Stimulation of Neural Cells. Patent US:5738625 - [8]; Abrams et al. Medical Magnetoictal Therapy. Patent US:5813970 - [9]; Edrich et al. Method and Apparatus for Focused Neuromagnetic Stimulation and Detection. Patent US: 6066084 - [10]; Davey.et al. Magnetic Nerve Stimulation for Exiting Peripheral Nerves. Patent Patent US:5725471 - [11]; Eaton et al. Magnetic Nerve Stimulator. Patent US:5066272 - [12]; Lin et al. Treatment of Excretory Problems. Patent US:6306078 - [13]; Abrams et al. Prevention of Seizure Arising from Medical Magnetoictal non-Convulsive Stimulation Therapy6132. Patent US:6117066 - [14]; Poison et al. Magnetic Stimulator for Neuro- muscular Tissue. Patent US:5766124 - [15]; Epstein et al . Transcranial Brain Stimulation. Patent US: - [16]. The analysis of the references cited demonstrates the ambiguity and contradictory nature of the solutions proposed for the above-described problems of magnetic stimulation.

One of the major problems of magnetic stimulation is the optimization of magnetic field distribution near the source, including the orientation of a magnetic induction vector in relation to the surface being stimulated. While considering this problem two basic types of stimulation can be distinguished i.e. bipolar stimulation and unipolar one.

In case of bipolar stimulation the direction of a magnetic induction vector Bn will be parallel in relation to the surface being stimulated. In this instance the circulation of an induced electric field is normal in relation to the surface being stimulated:

rot E = - dB/dt (1)

The induced circular electric field results in the creation of an electric current with the value of said current depending on the total specific resistance (p) of the internal layers pi and the skin p_s.

P = pi + p_s (2)

Since the specific resistance of the skin is much higher than the resistance of the internal layers,

Ps » pi , (3)

the magnitude of an induced current in case of bipolar stimulation will be minimal.

This drawback has been eliminated in a method of unipolar stimulation, wherein the direction of a magnetic induction vector B is normal in relation to the surface being stimulated. The latter determines the circulation direction of an electric field intensity vector directly in soft tissues featuring low specific resistance. As a result, the magnitude of current induced with unipolar stimulation is substantially higher than that induced with bipolar stimulation.

As follows from the above, the efficiency of a unipolar method of stimulation by pulsed magnetic fields will be substantially higher than that of bipolar stimulation. In this connection the magnetic stimulation by a bipolar stimulation circuit disclosed in [11] and in a number of other references seems to be less efficient as compared to a unipolar one.

Much higher efficiency of a unipolar method of stimulation by pulsed magnetic fields determines the design parameters of a pulsed magnetic field source.

Most essential for magnetic stimulation is the optimization of magnetic field pulse characteristics i.e. its shape, amplitude and duration.

In [12] the most frequently occurring shapes of magnetic field pulses and the circuit for generating thereof are described. A magnetic field pulse similar in shape to a triangular one is generated during the discharge of the capacitor bank C while the switch S is closed into the induction coil L. The circuit parameters i.e. inductance L, capacitance C and resistance R define a rise time of a magnetic field pulse as in the case described to t_r -100 microseconds. After the capacitor C is fully discharged causing the current through the inductance coil and the value of the magnetic field to decrease, the energy of the magnetic field is converted into the heat energy on resistor R. The circuit of the above-described type is very inefficient, since the energy supplied to generate a magnetic field is further converted into the heat energy.

A magnetic field pulse similar in its shape to a semi-sinusoidal one is generated when the capacitor Cι is discharged into the inductance coil L after the closure of the switch Si, and the magnetic field energy is further discharged into the capacitor C₂ after the closure of the switch S₂. In this case the total duration of a magnetic field pulse for the above circuit parameters is t_p =350 microseconds for pulse rise and decay times being in symmetry.

An important distinguishing feature of the above-described circuits is the divergence in the shapes of an electric field induced voltage pulse. In the first case the shape of an induced voltage pulse is practically unipolar due to significant difference between pulse rise and pulse decay times. In contrast to it in the second case the shape of an induced voltage pulse is similar to a sinusoidal one.

As it follows from the analysis of the data concerning the research on magnetic stimulation both shapes of magnetic field pulses have nearly similar applicability.

One of the most vital problems concerning the efficiency of magnetic stimulation is the duration of a magnetic field pulse i.e. pulse rise and pulse decay times. In [12] the duration of a magnetic field pulse makes up the value of t_p =350miroseconds. As opposed to the above, in one of the recent papers [13] the value of the proposed duration of a magnetic field pulse similar in shape to a triangular one is given as t_p = 0,4 sec. Approximately the same duration of a magnetic field pulse similar in shape to a semi-sinusoidal one is reported in [14].

As shown by the research of the influence of the magnetic field pulse duration on the stimulation efficiency of the central and peripheral nervous systems, the stimulation efficiency is increased with the increase in pulse duration. In this connection to improve the stimulation efficiency in some apparatus there is incorporated a parallel connection of several discharge chains [15].

The duration of a magnetic field pulse is used to define the energy consumption required for magnetic stimulation. Pulse duration being of t_p=350microseconds [12], the magnitude of pulse current of l_p =3000 A and capacitor voltage of U=1500 V, when the capacitor is discharged into the coil without a magnetic core, the energy required for generating a magnetic field pulse will be

E = U-l_p t_p = 1 ,5 κJ (4)

Similar high-energy storage apparatus is heavy and costly and not very reliable in operation.

The most closely related example of an apparatus and a method in accordance with the present invention is that described in [9]. The apparatus for magnetic stimulation is comprised of an oscillatory circuit containing a capacitor and a source of unipolar magnetic field pulses, said source further comprising a winding, a power supply unit and a storage capacitor, said capacitor connected in parallel to a capacitor in the oscillatory circuit, and also first and second controlled switches, and first and second diodes. A method of magnetic stimulation by the apparatus described consists in locating a butt-end of a source winding in the desired area on the patient head and further generating unipolar magnetic field pulses. In implementation of a method described a magnetic field source is placed in a Dewar vessel for decreasing its resistance due to superconductivity effect. The complexity of the design is beyond question.

Some of these characteristics could be improved by making a magnetic field source of high-induction ferromagnetic material such as vanadium permendur [16]. However, in this case the rate of magnetic field rise up due to eddy current losses could not be high and amounts to 20-30 T/sec. With the capacitor bank being discharged into the coil without a magnetic core, this value becomes substantially higher and makes up about 2000-3000 T/sec.

The rate of magnetic induction rise up actually defines the efficiency of magnetic stimulation i.e. the value of an electric field induced potential. Naturally, an increase in the rate of magnetic induction rise up will make it possible to reduce the utmost realizable induction of a magnetic field and hence the energy consumption to achieve the same values of an induced voltage.

The indicated rate values of magnetic induction rise up for coils both with and without a magnetic core are practically utmost realizable, since an increase in a magnetic field magnitude requires a substantial increase in a capacitor bank i.e. L, C, R parameters of a discharge circuit and, as a result, a decrease in the rate of magnetic induction rise up.

All' this results in the fact that despite great promise offered by a method of magnetic stimulation, the apparatus thereof have very limited application due to their high cost, large dimensions and low operating reliability.

The most closely related prototype of the source of unipolar magnetic field pulses is that described in [10] which is comprised of at least two helix-shaped windings with a butt-end of one of said windings being located on the patient body.

Summary of the Invention To provide wide application of a method of magnetic stimulation and to design a fairly inexpensive and reliable apparatus for magnetic stimulation, it is necessary first of all to increase the rate of magnetic induction rise up to the value of ΔB/Δt = 10⁵ T/sec. The latter will make it possible to considerably reduce energy consumption of an apparatus for magnetic stimulation, to increase pulse repetition up to 1-2 kHz with the duration of an efficient stimulation pulse being increased up to t_s = 1 sec and more, i.e. to provide the modulation frequency of about 30 pulse trains per minute.

The object of the invention set forth in the apparatus for magnetic stimulation comprised of an oscillatory circuit containing a capacitor and a source of unipolar magnetic field pulses, said source further comprising a winding, a power supply unit and a storage capacitor, said capacitor connected in parallel to a capacitor in the oscillatory circuit, and also first and second controlled switches, and first and second diodes is achieved by providing said magnetic field source with a core made of soft magnetic composite material.

A positive lead of an oscillatory circuit capacitor is connected to the first lead of a magnetic field source winding through the first switch and to the second lead of said source through the first diode, while a negative lead of an oscillatory circuit capacitor is connected to the second lead of said source through the second switch, and the first lead of said source is connected through the second diode, while said first and second diodes are connected in reverse direction.

The above-mentioned controlled switches are made as high frequency transistors featuring low output resistance with the control electrodes of said transistors serving as inputs for a device for supplying the control signals.

The transistors described above are IJBT-transistors.

Positive leads of a storage capacitor and an oscillatory circuit capacitor are coupled through a high frequency choke and their negative leads are directly connected.

The apparatus is provided with first and second variable delay means connected in series to corresponding diodes In a method of magnetic stimulation therapy consisting in stimulation by a magnetic stimulation therapy apparatus comprised of an oscillatory circuit containing a capacitor and a source of unipolar magnetic field pulses, said source further comprising a winding, a power supply unit and a storage capacitor, said capacitor connected in parallel to a capacitor in the oscillatory circuit, and also first and second controlled switches, and first and second diodes, said stimulation being effected by means of locating a butt-end of said source winding on a desired part of the patient body and further generating unipolar magnetic field pulses, the aim set forth is achieved by providing said source with a core made of soft magnetic composite material.

Magnetic field pulses are generated with the shape similar to a triangular one.

Pulse decay time of a magnetic field pulse is controlled in the range from 10^"5 to 10^~3 seconds.

The trains of magnetic field pulses are generated.

The trains of magnetic field pulses are generated with the frequency of pulses within a train varying from 10 to 1000 Hz.

The repetition rate of the trains of magnetic field pulses is made ranging from 20 to 60 trains per minute.

In a source of magnetic field pulses having at least two helix-shaped windings with a butt-end of one of said windings intended for locating on the patient body the aim set forth is achieved by providing said source with a core made of soft magnetic composite material.

The core of a magnetic field source is made in the shape of a hollow cylinder with an outer surface of said cylinder enclosed by one of said helix-shaped windings, while the other winding connected in opposition is enclosed by the inner surface of said core.

The height h and the outer diameter d_e of the core are chosen from proportion h/4<d_e<h with the outer diameter of the core d_e exceeding its inner diameter dj by 10 to 30 mm.

Brief Description of the Drawings Fig.1 is a generalized block-diagram of an apparatus in accordance with present invention

Fig.2 is a block-diagram of an apparatus with two magnetic field sources

Fig.3 are time charts of a magnetic field created by the apparatus in accordance with the present invention

Fig.4 is a time chart of a voltage-induced pulse

Fig.5 is a schematic representation of a source of magnetic field pulses shown in a lengthwise section.

Fig.6 are time charts of magnetic field pulses generated by the source in accordance with the present invention (A), by a magnetic field source without a core [12] (B) and by the source with a core made of vanadium permendur [13] (C)

Detailed Description of the Invention and Preferable Examples

A generalized block-diagram of a magnetic stimulation apparatus in accordance with the present invention is shown in Fig.1. An apparatus in accordance with the present invention is comprised of an oscillatory circuit containing a capacitor 1 and a source 2 of unipolar magnetic field pulses, a power unit 3, a storage capacitor 4. Positive lead of an oscillatory circuit capacitor 1 is connected to the first lead of the source 2 through the first switch 5 and to the second lead of the source 2 through the first diode 6. Negative lead of the oscillatory capacitor 1 is connected to the second lead of the source 2 through the second switch 7 and to the first lead of the source 2 through the second diode 8. First and second diodes are connected in reverse direction. Positive leads of capacitors 1 and 4 are coupled through a high-frequency choke 9. Control electrodes of said transistors serve as inputs for a device for supplying control signals. In the example described these are supplied with similar signals. First and second variable delay means 10 and 11 are connected in series to diodes 6 and 8.

The method of the present invention is implemented as follows.

The magnetic field source 2 is located on the part of the patient body suffering from a disorder. It may be head, extremities or others parts of body. The capacitor 1 is supplied with the voltage from the power supply unit 3 until the capacitor is fully charged.

When the control electrodes of transistors i.e. switches 5 and 7 are simultaneously supplied with a pulsed signal, the magnetic field source 2 will generate a magnetic field pulse as shown in Fig.3.

Should the magnitude of a magnetic field be decreased after said transistors are turned off, a decay time of said magnetic field pulse is formed. The decay time of said magnetic field pulse can be varied from 5.10^"5 to 10^"3 seconds by means of first 10 and second 11 variable delay means.

An induced voltage of reverse polarity in the source 2 is inverted by means of diodes 6 and 8, and the energy is returned into the capacitor 1 in an oscillatory circuit.

The recovery of active losses in an oscillatory circuit is effected simultaneously by supplying the energy through the high-frequency choke 9 from the storage capacitor 4. Thus, during a decay time of a magnetic field pulse the voltage on the capacitor 1 is- restored to its rated value.

The second magnetic field pulse could be generated actually after the first discharge pulse is terminated.

The control electrodes of the transistors, i.e. switches 5 and 7, are supplied with the series of pulsed signals of pre-defined frequency and in pre-defined amount to generate the trains of magnetic field pulses in the way described with the frequency of said pulses within a train varying from 10 to 1000 Hz and repetition rate of the trains of magnetic field pulses varying from 20 to 60 trains per minute.

Each of the above-described magnetic field pulses induces an electric field potential pulse in the patient tissues as shown in Fig.4. It is evident that the shape of this pulse and hence its specific stimulation efficiency are dependent on the magnetic field pulse being generated. For a very short decay time of a magnetic field pulse (curve (a) in Fig.4) said potential pulse is a bipolar one (curve a in Fig.4), and, with the magnetic field decay time being increased, a potential pulse becomes a unipolar one (curve (b) in Fig.4 respectively). The circuit diagram of an apparatus with two magnetic field sources (15 and 16 respectively) made with the cores of soft magnetic composite material with the diameter of said cores d_e =100 and 40 mm is shown in Fig.2. The peculiar feature of the apparatus circuit in accordance with the present invention is that the energies of the capacitors 17 and 18 in the oscillatory circuits are much higher than the energy of a magnetic field pulse:

Eιoo« Ci₇.U²/2 ιmιι E₄₀« Cι₈.U²/2,

In this connection, should the transistor switches, e.g. 19 and 20, be opened and the capacitor 17 be discharged into the magnetic field source 15, the voltage on the capacitor 17 is decreased by maximum 20%. After the closure of transistor switches 19 and 20 the voltage on a magnetic field source is inverted with the energy of a magnetic field pulse being returned into the capacitor 17 after opening of diodes 21 and 22. Simultaneously the recovery of active losses in an oscillatory circuit occurs by means of the energy being supplied from storage capacitors 24 and 25 through the high- frequency choke 23. Actually during the decay time of a magnetic field pulse the voltage on the capacitor 17 is restored to its rated value. In content of all the above-said the second magnetic field pulse can be generated just after the first discharge pulse is terminated.

The operation of the second oscillatory circuit (magnetic field source 15, capacitor 18, transistor switches 19' and 20', diodes 21' and 22' and high-frequency choke 23') is similar to the first one but it occurs with a time-shift equal to the duration of a discharge pulse in the first oscillatory circuit.

A magnetic field source of the present invention is illustrated in Fig.5.

To provide the circulation of an induced electric current inside the patient body in the plane parallel to its surface a magnetic field source is provided with two helix-shaped windings 12, 13 and the core 14 made of soft magnetic composite material. The core 14 is made as a hollow cylinder with an outer side surface of said cylinder enclosed by the winding 12, while the other winding 13 connected in opposition is enclosed by the inner surface of the core 14. To enhance a magnetic field induction in the pulse up to B_m=2,2 T on the surface of a magnetic field source and to increase a magnetic field penetration depth, the height h of the core 14 is chosen from the proportion h/4 < d_e < h,

where d_e is an outer core diameter. The outer diameter d_e of the core exceeds its inner diameter dj by 10 to 30 mm.

The soft magnetic composite material is an iron-based composite with each iron particle being covered by a thin layer of magnetic dielectric, e.g. SMC-500 of Hoganas AB Co. (Sweden) and, as opposed to metal ferromagnetic material, it does not feature any eddy current losses. As a result a magnetic permeability frequency characteristic is shifted towards higher frequencies as compared to that of metal ferromagnetic material.

The field dependencies of the magnetic induction for metal magnetic material and for soft-magnetic composite material greatly differ in the initial part of the curves. When the intensity of the magnetic field reaches the value of more than H= 10 kA/m, the behavior of magnetization curves will be actually the same. Soft-magnetic composite materials are characterized by almost linear variation of magnetic induction with magnetic field variation. The major characteristics of a soft-magnetic composite material are as follow:

Magnetic permeability mean value - μ =200 Magnetic induction saturation value - B_m = 2,2 T

The induction L of a magnetic field source with a core of soft-magnetic composite material actually retains its value just as for a magnetic field source without a core due to the reduction in a number of turns:

L = μ₀-μ-n²-V = 20-10^"6 H

The capacitance value of the capacitor 1 in an oscillatory circuit is decreased as compared to the previous cases to C =10 μF. As a result the magnetic induction rise time is determined by

T/4 = 0,5τ L-C = 20-10^"6 sec

Accordingly the rate of the magnetic induction rise time will be come to

ΔB/Δt = 10⁵ T/sec The total duration of a magnetic field pulse with rise up and decay times being in symmetry will be t_p = 50 microseconds.

Fig.6 shows minimum duration times of magnetic field pulses for the following sources: a source in accordance with the present invention with a core of soft magnetic composite material (curve 1), a source without a magnetic core (curve 2) and a source with a core of vanadium permendur (curve 3).

The energy required for generating a magnetic field pulse with induction of said field B = 1T for a source without a core of magnetic material is defined by

E₁ = B²t_p / 2-μ₀

For a source with a magnetic core made of vanadium permendur or soft-magnetic composite material the required energy is defined as E₂ and E₃ accordingly:

E₂, E₃ = B².t_p / 2.μ.μ₀

By substituting the values of the parameters in the above expression it is possible to define the proportion of energies required for generating a magnetic field pulse with the induction of said field B= 1 T by a source without a magnetic core E-i, a source with a core made of vanadium permendur E₂ and a source with core of magnetic composite material E₃:

E₁: E₂: E₃ = 600: 200 : 1

As follows from the above equation the energy required by a magnetic field source of the present invention is almost three times less than for a source without a magnetic core or with a core made of vanadium permendur. Relatively small energy (2-3 J) consumed during generating a magnetic field pulse by a source with a core made of magnetic composite material makes it possible to do away with traditional high-voltage components such as thyristors, high-power capacitors etc. and to make use of high- power transistors type IJBT.

Claims

Claims:

1. An apparatus for magnetic pulse stimulation comprising an oscillatory circuit, said circuit including a capacitor and a magnetic field source of unipolar pulses, said source further comprising a winding, a power supply unit and a storage capacitor, said capacitor connected a parallel to an oscillatory circuit capacitor, and also first and second controlled switches, and first and second diodes, wherein said magnetic field source is provided with a core made of soft magnetic composite material.

2. An apparatus according to claim 1 , wherein a positive lead of an oscillatory circuit capacitor is connected to the first lead of a magnetic field source winding through the first switch and to the second lead of said source through the first diode, while a negative lead of said oscillatory circuit capacitor is connected to said second lead of a magnetic field source through the second switch and to said first lead of a magnetic field source through the second diode, while said first and second diodes are connected in reverse direction.

3. Apparatus according to claims 1-2, wherein said controlled switches are high- frequency transistors with low output resistance, while the control electrodes of said transistors serve as inputs for a device for supplying the control signals.

4. An apparatus according to claim 3, wherein the transistors are IJBT transistors.

5. An apparatus according claim 1 , wherein positive leads of a storage capacitor and an oscillatory circuit capacitor are connected through a high-frequency choke and their negative leads are directly connected.

6. An apparatus according to claims 1-5, wherein said apparatus is provided with first and second variable delays means connected in series to corresponding diodes

7. A method of magnetic stimulation therapy consisting in stimulation by a magnetic stimulation therapy apparatus comprised of an oscillatory circuit containing a capacitor and a source of unipolar magnetic field pulses, said source further comprising a winding, a power supply unit and a storage capacitor, said capacitor connected in parallel to a capacitor in the oscillatory circuit, and also first and second controlled switches, and first and second diodes, said stimulation effected by means of locating a butt-end of a source winding in the desired area on the patient head and further generating the unipolar magnetic field pulses, wherein said magnetic field source is provided with a core made of soft magnetic composite material.

8. A method as in claim 7, wherein said magnetic field pulses are generated with the shape similar to a triangular one.

9. A method as in claim 7, wherein the decay time 'of magnetic field pulses is controlled in the range from 5.10^"5 to 10^"3 seconds.

10. A method as in claims 7-9, wherein the trains of magnetic field pulses are generated.

11. A method as in claim 10, wherein said the magnetic field pulses within the train are generated with frequencies ranging from 10 to 1000 Hz

12. A method as in claim 10, wherein the repetition rate of said trains of magnetic field pulses is made ranging from 20 to 60 trains per minute.

13. A source of unipolar magnetic field pulses comprised of at least two helix-shaped windings with a butt-end of one of said windings intended for locating on the patient body, wherein said source is provided with a core made of soft magnetic composite material.

14. A source as in claim 12, wherein said core is made as a hollow cylinder with an outer surface of said cylinder being enclosed by one of said windings, while the other helix-shaped winding connected in opposition encloses the inner surface of said core

15. A source as in claim 13, wherein the height h and the external diameter d_e of the core are chosen from proportion h/4< d_e<h, while the external diameter de of said core exceeds its internal diameter dj by 10 - 30 mm.