EXPERIMENTAL STUDY OF THE SUPERCONDUCTING MICROSTRIP ANTENNA AS A PROTECTIVE DEVICE OF THE RECEIVER FROM ELECTROMAGNETIC DAMAGE

The paper presents the results of experimental studies of a superconducting protective antenna, which consists of a high-temperature film deposited by a magnetron or a laser beam on a substrate. The work is carried out: − analysis of the selection criteria for the substrate type (Al2O3, Y2O3, SrTiO3, MgO) and the method for depositing a high-temperature superconducting film (HTSF) on its surface – YBaCuO − analysis of the methods of making contacts, which allow to reduce losses when passing a signal from the superconducting microstrip antenna to the waveguide path. The aim of this article is determination of the parameters of a prototype sample of a microstrip antenna device made from HTSF: transient characteristics of high-temperature superconductors, HTSF impulsive characteristics, recovery time of the superconducting state after the action of a powerful pulse on the protective device, the amplitude-frequency characteristics of the protective device in the superconducting state. This will allow to evaluate the possibility of using a microstrip antenna device made from high-temperature superconductors to protect the receiving systems from electromagnetic damage. The absence of a unified theory of high-temperature superconductivity leads to the need to select an analytical form of the functions of the amplitude-frequency characteristics of superconducting protection, and for this mathematical models are used in the programs “APPROX”, “MathCAD14.0”. The reliability of the obtained results of mathematical modeling of the processes of protection and recovery of the superconducting state after electromagnetic shock are confirmed in the course of experimental studies (an error of 0.15 %).


Introduction
Numerous studies have shown that an electromagnetic pulse (energy-10 3 −10 8 J, duration 10 -6 −10 -12 s), which disables the radioelectronic equipment of radio engineering systems, is formed using the existing methods of generating powerful electromagnetic influences used in the basis of electromagnetic weapons.
The response time of existing types of protective devices, with the exception of the superconducting microstrip memory, is longer than the duration of the electromagnetic pulse, and therefore they do not allow reliable protection of the radioelectronic equipment (REE) from penetrating the EMR into the receiver.  To obtain epitaxial HTSFs one can use single crystals which unit cell parameters are close to the corresponding parameters of HTSFs, or they differ approximately by a factor of two. In the first case, the crystallographic directions of the HTSF <110> lie along the directions of the <110> type of substrate and the error can be considered relative to the value of "а"/Y 2 of the substrate [4]. For superconductor electronics and microwave technology, both monocrystalline and polycrystalline films are required. In the first case, it is necessary to achieve a high-quality epitaxial fusion of a high-temperature superconductor film with a substrate by adjusting the lattice parameters of the substrate material and its thermal expansion coefficient to the corresponding value of these parameters for HTSF.
In the second case, the cracks and pores formed as a result of the difference in the lattice parameter and the CTE of the substrate material and HTSF are, as it were, closed, "healed" due to the chaotic distribution of the crystallites forming the polycrystalline film [5].
For the Y-Ba-Cu-O compound, which has lattice parameters ( Fig. 1), a=0.38; b=0.39 and c=1.17 nm, one of the most suitable materials is strontium titanate (a 0 =0.39 nm), with which the discrepancy with the parameter "a" is 2 %, and between "c" in YBaCuO and "c" in SrTiO 3 -2 %. It is this circumstance that explains the wide use of substrates from SrTiO 3 ( Table 1)  Fundamental and applied physics structure, resulting in a high ordering of the crystals in the films, in which the "c" axis is usually perpendicular to the substrate plane and, as a consequence, high values of jc (up to 5×10 6 A/cm) and small DТ (1 K) [5,7,[10][11][12][13][14]. High dielectric permittivity ( Table 1) significantly limits the use of SrTiO 3 , especially in microwave devices.
In addition, it should be borne in mind that when high-temperature treatment (annealing) is separated from SrTiO 3 strontium and replaces barium in YВa 2 Cu 3 O 7 , decreasing the value of Dt pd .
It can be concluded that for the creation of a high-temperature superconductor in a test sample it is possible to use a substrate of LaAlO 3 on which the YBa 2 Cu 3 O 7 film is deposited.

Methods of making contacts for microstrip HTSFs
The problem of obtaining stable low-resistance contacts to HTSFs has two characteristic features. The first is a high current density (up to 5×10 6 A/cm 2 ), which can only be obtained in the HTSFs if the contact has a low transition resistance. Although, from a different point of view, the high chemical activity of HTSF causes the oxidation of metal contacts and the formation of a dielectric layer at the HTSF-metal interface. In addition, often the HTSF near-surface layer is dielectric even in the process of obtaining films as a result of a violation of stoichiometry in it or a crystal structure. Therefore, just before deposition on the surface of the HTSF-metal (usually Ag and Au), it is necessary to remove the near-surface dielectric layer or to restore in it somehow superconductivity.
In practice, ionic purification in inert gas plasma (more often argon) is used, but the thickness of the nonsuperconducting surface layer decreases, however, purification does not completely destroy gases, since structural changes occur with a depth of up to 4 nm (Cu-O bonds breakdown and oxygen diffusion). Cleaning in oxygen plasma causes a smaller change, but the films are very critical to the cleaning regime (the duration of the process, the angle of incidence of ions on the surface). Purification by removal of the surface layer of the HTSF material is also accompanied by destruction of the superconducting properties of the surface. Consequently, the surface of HTSF is a very unstable formation, and therefore it is necessary to protect it with films of other materials immediately after application. However, a very limited number of materials can be used for this purpose.
Most metals enter into chemical reactions with the HTSF material, causing the superconducting properties of near-surface layers of the HTSF to break down and to oxidize in the materials in contact with it. There are only a limited number of materials: Ag, Au, Pt, Ir, Ru, Rh, Pd, Os, Hg, which oxides have a lower binding energy than Cu-O. Only with the deposition of Ag and Au on the surface of YBaCuO does not form an oxide film, in all other cases an oxide film with a thickness of 4 nm for Cu and up to 14 nm for Al appears. The process of oxidation of materials is accompanied by the destruction of superconductivity and the formation of a semiconductor surface layer 2-5 nm in thickness.
Of all the currently known methods of obtaining ohmic contacts, two are the most suitable. In the first of these, before the deposition of Ag or Au, the surfaces of HTSFs are purified from the nonsuperconducting phase by ion sputtering. In the second method, YBaCuO films are grown in the presence of oxygen and annealed at T=850 °C and a high rate of temperature increase, while Au precipitates at T=20 °C immediately after cooling the YBaCuO film without violating the vacuum. The minimum achieved resistance of the interface of HTSF -normal metal is 10 -10 Ω cm 2 [6].

3. The main technological methods of applying HTSF onto the substrate
For the production of high-temperature superconducting films, in fact, the entire arsenal of available methods is used, but those that are organically fit into the general production cycle of production are most promising for use in industrial production. Such methods at the moment are laser and magnetron sputtering. Laser sputtering is perhaps the most flexible method of preparing HTSFs. Spray can be almost any of the composition of the target. Quite the high cost of equipment is paid off by its efficiency and ease of control of the process. Another advantage is the possibility of using the same technique for profiling the surface. In Fig. 2 of [4,8] is presented in a generalized form the scheme of the experimental equipment used for laser deposition of HTSF materials. The radiation from the laser, as a rule, at an angle to the surface, is focused on the target surface inside the vacuum volume. The sprayed material is deposited onto the heated substrate. Precipitation can be conducted both in a vacuum and in an atmosphere of a certain composition [8−10].
One of the drawbacks of the laser sputtering method is the presence of microdroplets in the laser torch. If the cluster particles containing dozens and hundreds of molecules contribute to the preservation of the stoichiometric composition and do not disturb the morphology of the surface of the films, the microdroplets, depositing on the surface, make it rough, cause various irregularities, and, in general, reduce the quality of the films. To avoid (or at least reduce) the impact of microdroplets on the surface of films, different screens are used [11−14]. The nature of laser ablation has been studied in sufficient detail in many works [11−13]. The heat treatment regimes are also studied in detail. Still, it is perhaps impossible to draw up complete recommendations on the production of films by laser sputtering. Sputtering conditions are very different both for the laser part (energy and power density, wavelength, focal spot size), and for other parameters (chamber pressure, substrate temperature, distance from the target). Nevertheless, the technology of obtaining films by the laser method is very popular, since all these parameters easily vary and allow to quickly find the optimum at which good films are obtained. Single-crystal films with high critical characteristics are obtained by a laser method. The use of lasers, as already noted, is also attractive because it is simultaneously possible to use laser radiation for other purposes. Fast high-temperature laser annealing is used [10], irradiation of the substrate during the sputtering with the same laser radiation [14], which is used for sputtering the target, lasers use to apply films of a certain pattern.
Another method of obtaining HTSFs is magnetron sputtering, for which magnetron sputtering systems (MSS) are used (Fig. 3).
Magnetron sputtering systems are essentially diode systems (Fig. 3), in which the interaction of electric and magnetic fields in combination with the shape of the sputtered target surface creates a configuration of magnetic traps for electrons, in which the electron drift currents are self-locked.
As a result, three maxima coincide in the MSS [4,5]: − the electron energy distribution function; − the number of ionization acts that create one electron per unit of the path of its directed movement; − the number of electrons knocked out by one ion from the target. c d Fig. 3. MSS constructive options. a, b, c, d -options for depositing films: 1 -spraying target; 2 -substrate holder; 3 -magnetic system; 4 -power supply; 5 -gas supply system; 6 -matching device; 7 -additional magnetic system; 8 -mesh electrode; 9 -displacement source The main MSS advantages, providing the possibility of their wide industrial use, are: higher energy efficiency of the process compared to SS of diode and triode types, and hence a higher rate of precipitation of the films.
The fact is that in MSS the high efficiency of the plasma formation process due to the presence of a magnetic field is combined with a high sputtering coefficient at an ion energy of 400−500 eV, which is characteristic for MSS; − obtaining the required stoichiometry while simultaneously sputtering several targets by controlling the power supplied to each of the targets; − the possibility of controlling the sputtering zone as a result of the use of several independent magnetic systems; − the possibility of changing the structure and properties of films due to simultaneous sputtering of several targets with different velocities, changing the pressure and composition of the gaseous medium (percentage of Ar and O 2 ), the bias potential on the substrate, and other methods; − low porosity and high adhesion of films even at small thicknesses; − a lower (in comparison with conventional diode SS) radiation and thermal effects on the structures being processed and the possibility of its further reduction during the thermalization of the deposition process; − inversion of the process, which makes it possible to use MSS for both preliminary cleaning (etching) of substrates, and for deposition of a wide class of materials; − the versatility of the process, allowing in a single vacuum cycle to deposit on the substrate films of virtually any materials and compatibility of the process with other operations of the technological cycle of production of integrated circuits.
During the last decade, there have been intensive development and improvement of MSSs, typical design variants of which are presented in Fig. 3.
The laser method is much more reliable, provides a high deposition rate (<50 nm/min) and a stoichimetric composition of the resulting film without the use of complex instrumentation. How-a b Reports on research projects (2017), «EUREKA: Physics and Engineering» Number 5

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Fundamental and applied physics ever, it does not provide the proper level of quality of films with a thickness of less than 1 μm. To create such films, the magnetron sputtering method is most suitable.
To create a superconducting protective MA, the magnetron sputtering method is used [16].

Experimental study of the superconducting HTSF element
The article presents experimental studies of the obtained protective device based on high-temperature superconductors [16].
2. HTSF pulse characteristics of the limiter.
3. Time to restore the superconducting state after the action of a powerful pulse on the protective device.
4. Amplitude-frequency characteristics of the protective device in the superconducting state. 5. Characteristics of the electrical strength of a protective device based on HTSF.
The measurements are carried out for a superconducting sample − a high-temperature superconducting film (YBa 2 Cu 3 O 7 ) 0.1 μm thick deposited on the substrate (LaAlO 3 ) by the coaxial line РК-5. Contact of HTSF with the residential coaxial cable is carried out using vacuum or magnetron sputtered gold on a part of the film in contact with the holder.

1. Transient characteristics of protective device
The transient characteristics of an HTSF-protective device (PD), that is, the dependence of the output voltage on the load from the input, are removed when a pulsed and sinusoidal voltage is applied.
Schemes for measuring transient characteristics are shown in Fig. 5. A voltage pulse with amplitude up to 130 V and a duration of t i =45 ns is supplied from the generator И1-7 to a matched load of 50 Ω and a voltage divider formed by the HTSF sample under study and a load resistance R l =50 Ω simulating the input resistance of the receiving electronics.  The parameters of the pulses of the input and output voltages are measured with an oscilloscope C8-14, which has a rise time of the transient response t r <7 ns, which is clearly seen from a typical oscillogram of the generator voltage pulse on the coordinated load shown in Fig. 6.
Similarly, the characteristics are removed when a sinusoidal voltage of 100 kHz and amplitude of 0.1 to 11 V is applied from the ГЗ-34 generator.
The experimental sample is obtained by the magnetron method on a common substrate of LaAlO 3 .
The investigated protective device has the following initial characteristics [1][2][3]: -resistance at room temperature R 2 = 100kΩ; -resistance near the superconducting transition (at T> Tc) Ro=(50) kΩ; -contact resistance r con =10 -2 Ω [12,16]. It should be noted that there is a parasitic capacitance (C) (Fig. 7) between the screen and the superconductor. This capacitance has little effect on the propagation of the signal at low frequencies (Xc=1/jwC in Fig. 7), but turns out to be significant at high frequencies of the transmitted signal ( Fig. 6 -distortion of the front and the decay of the pulse).
The transient characteristics of the sample and the corresponding coefficients are shown in Fig. 8, 9. These characteristics are measured for the time instant t 0 =50 ns on the plane vertex of the pulse (quasi-stationary mode). It can be seen from the above dependences that the superconducting state is observed for the amplitudes of the input pulses u in <10 V, after which the amplitude gradually falls to 7−8 V. This value u in corresponds to the current in the circuit corresponding to the current density through the sample, which approximately coincides with the critical current density of the superconductor.
The absence of a sharp break in the transient characteristic when a critical current is reached is associated with a transient region of current superconductivity, which means that the restoration of the nonsuperconducting resistance of the sample occurs at low currents, and this transient process has no connection with the inertia of the superconducting properties of the film.
As the cross section of the superconductor decreases, the current reaches its critical value much faster. In this case, the amplitude of signals transmitted without significant distortion is less. If the input effect (voltage) has a sinusoidal form, then the considered PD also limits the level of the output voltage at the level of 9 V.
The superconducting state is observed for u in =1−7 V, after which the permanent screening begins to the value u in =8 V.
Consequently, the limiter based on HTSF transmits without distortion voltage signals up to 8 V, and if this level is exceeded, the output voltage does not exceed 12 V (Fig. 6).  Fig. 10 shows the oscillograms of the transient processes obtained for average amplitudes of the input pulses (H307 is used). When comparing (Fig. 10) the figures show that the rise time of the front of the output pulse will be 10 ns (5 %). This distortion of the front is related to the parasitic integrating capacitance of the sample on the common bus (Fig. 7), which is actually equivalent to the capacity of the sample with respect to an infinitely remote point having a zero potential.  (Fig. 10) when the voltage is applied (Fig. 7) 1 2 a t a t t t 1 2

2. Transient characteristics of pulse processes
R 1 -the resistance of the HTSF sample in the n-state, R 2 -the load resistance, (Fig. 11) The shape of the diagram (Fig. 11) of this voltage approximately (error up to 8 %) [6] coincides with the oscillogram in Fig. 6. Analysis (4) and data in Fig. 6 shows that in this case: Consequently, the integrating capacitance phenomenon is decisive in determining the shape of the output voltage [6,16].
Experimental studies have confirmed that the transition of a high-temperature superconductor (the main element of the prototype PD sample) from a superconducting state to a nonconducting state (i. e., the PD switching process) lasts less than 1 ns and the output voltage does not exceed 12 V.

Time to restore the PD superconducting state
A comparison of the results obtained with each other (Fig. 10) shows that in the case of sufficiently large amplitudes of the input voltage pulses (more than 100 V) there is no decay of the plane vertex of the output pulse during operation of the memory with a superconducting element (HTSF). This effect [9,16] due to the heating of the superconductor by a flowing current, as a result of which its resistance increases in comparison with the resistance in the region of the critical point of the transition. An increase in the amplitude of the input voltage by a factor of 20 will lead to a similar drop in the amplitude of the output voltage. Consequently, the question of restoring the superconducting state after passing through powerful pulses is arises.
The scheme for measuring the recovery time is shown in Fig. 12. From the generator И1-7 to the input of the circuit, along with a powerful pulse, the pulse of the generator Г5-60 with amplitude of 0.15 V is delayed. The outputs of the generators are separated by pulsed diodes. In the superconductivity regime, the amplitude of the output pulse from the second generator should be 0.12 V. The smaller amplitudes of the output pulse mean that the sample did not completely transfer to the superconducting state. Changing the delay of the pulse of the second generator, it is possible to determine the interval during which the superconductivity is restored. The smallest interval of pulse separation time is 0.45 ns. Complete restoration of superconductivity occurs in 3 μs, which corresponds to calculations (an error of 0.15 %) [16].
As the amplitude of the input pulse increases, the recovery time will increase. If assume that this growth is directly proportional to the electric power output (and this is the worst case, when the pulse is fed to a high-temperature superconductor element with a duration of 100 ns and amplitude of 40 kV, the recovery time does not exceed 5 μs).

4. Frequency responce of a superconducting PD
Measurement of the frequency response is carried out with the help of the device P4-11. The measurement scheme is shown in Fig. 13. To determine the transmission coefficient, the value of the output signal of the limiter is given to the value of the input signal.

Fundamental and applied physics
The measurement is carried out in the 0¸2500 MHz band and for two characteristic measurement areas of the input signals: u 1in =40 mV, when the sample is in the superconducting state and u 2in =400 mV, when the sample begins to limit the amplitude.
Let's first consider the small amplitudes of the input signal, which make it possible to judge the undistorted frequence responce of the superconducting limiter. Fig. 14 shows the frequency response of the HTSF-microstrip line [16]. It follows from the results of the research that the considered limiter based on the superconducting coaxial-waveguide junction is normal (the coefficient is 1 with a thousandth of an error) operates over a wide range of operating frequencies (studied in the range from 100 MHz to 3 GHz). In carrying out all the above experimental studies, there is no organic breakdown of the HTSF sample. Physical destruction means the electrical breakdown of the superconducting strip on the screen. The absence of breakdown is explained by the fact that, with a high resistance of an HTSF microstrip in its normal state, even with kilowatt amplitudes of input voltage pulses, the current through the sample does not reach the limiting values (10−30 mA), and also because the nitrogen in which the sample is located is itself a dielectric ( cnitrogen cair.

Approximation of the results of experimental studies
As a result of the experimental studies, an array of values of the input and output voltages of the memory are determined, which collectively represent the transfer characteristic of HTSF-based PD in the case of a powerful pulse signal (Fig. 8, 9, 14). The type of approximating functions of the obtained experimental data is determined in the work ( Table 2). The "APPROX" mathematical package is used to select functions (Fig. 15, 16): Table 2 A set of functions offered by "APPROX" (Fig. 15) for approximating the results of the experiment shown in Fig. 8 No.
Approximating By the same principle "APPROX" program found expressions that mathematically can describe the result of the experiment in Fig. 9 are presented in Table 3 and Fig. 16. From Fig. 15 and Table 2 it follows that function (5) gives the minimum standard deviation (SD), function (3) has also a small SD.
However, the accuracy of the formulas is verified by substituting new experimental points, and it turned out that function (3) most accurately describes the transientr characteristic of HTSFbased PD under pulse action.
Thus, in Table 4 the results of the function selection by "APPROX" program that most accurately summarizes the results of the experimental studies presented in Fig. 8, 9. Table 3 A set of functions proposed by "APPROX" for approximating the results of the experiment presented in Fig. 9 No.
Approximating Fundamental and applied physics Table 4 Functions approximating the experimental results

Conclusions
− the manufactured sample of the microstrip antenna protective device (HTSF 1 micron deposited by a magnetron or laser method on a substrate (Al 2 O 3 or SrTiO 3 )) does not disturb the wave structure upon arrival and transmission; − the superconducting protective device allows to reduce the signal level of high power 1000 times (up to 1 volts); − the recovery time of a superconducting protective device after an electromagnetic failure does not exceed 3 μs; − APPROX package for testing formulas (Fig. 15, Table 2) is possible to determine that the most accurate approximation of the experimental results (Fig. 8, 9) with the help of functions  Table 4). In the MathCAD14.0 mathematical package, the accuracy of the "APPROX" package is confirmed (Fig. 16), (error 0.15); − the use of high-temperature superconducting antenna devices will exclude the possibility of electromagnetic damage to its semiconductor elements, Fig. 17.