CN112782281B - Ultrashort pulse microwave thermoacoustic imaging method and device based on waveguide output - Google Patents
Ultrashort pulse microwave thermoacoustic imaging method and device based on waveguide output Download PDFInfo
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Abstract
The invention discloses an ultrashort pulse microwave thermoacoustic imaging device based on waveguide output, which comprises an ultrashort pulse microwave excitation source, a long tapered monopole coupling antenna, a waveguide for adjusting microwave output, an optical fiber frequency controller and a thermoacoustic excitation component, wherein the thermoacoustic excitation component is connected with a thermoacoustic signal acquisition component and a data processing component, the thermoacoustic excitation component, the thermoacoustic signal acquisition component and the data processing component are sequentially connected, and the thermoacoustic signal acquisition component comprises an annular ultrasonic transducer, 64-channel signal amplifiers and a high-speed digital acquisition card. The invention has great improvement on the thermoacoustic excitation source, breaks through the traditional excitation mode of generating microwaves based on a magnetron in the past, adopts the technical means of generating pulse microwaves based on high-voltage electric pulses to generate ultrashort pulse microwaves, and uniformly radiates the ultrashort pulse microwaves into a space after shaping by a self-designed long conical monopole coupling antenna and a waveguide, so that a microwave radiation uniform area in a certain area is formed in the space, and the pulse width of the output pulse microwaves reaches 10ns.
Description
Technical Field
The invention relates to the technical field of microwave thermoacoustic imaging, in particular to an ultrashort pulse microwave thermoacoustic imaging method and device based on waveguide output.
Background
The microwave thermoacoustic imaging technology has high resolution of ultrasonic imaging and high contrast of microwave imaging, and effectively overcomes the interference problem of ultra-broadband microwave imaging direct waves.
The traditional microwave thermoacoustic imaging excitation source adopts a microwave generator based on a magnetron technology, the magnetron is expensive, the service life is short, the long-time stable work is difficult, the pulse width of the generated pulse microwave is in the us level, and the resolution of the microwave thermoacoustic image is limited by the long pulse width, which is also a technical bottleneck faced by the microwave thermoacoustic imaging field at present.
Disclosure of Invention
The invention aims to solve the defects in the prior art, such as: the existing microwave generator based on magnetron technique has short service life, and is difficult to work stably for a long time, and the proposed ultrashort pulse microwave thermoacoustic imaging method based on waveguide output and the device thereof are provided.
In order to achieve the purpose, the invention adopts the following technical scheme:
an ultrashort pulse microwave thermoacoustic imaging method based on waveguide output comprises the following steps:
firstly, ultra-short pulse microwaves transmit pulse microwaves with the central frequency of 434M through a long conical monopole coupling antenna, the pulse microwaves are output at the mouth surface of a waveguide after being adjusted by the waveguide, the mouth surface of the waveguide is sealed by a polytetrafluoroethylene plate with the thickness of 20mm, and SF6 gas with 2.5 atmospheric pressures is filled in the mouth surface of the waveguide;
secondly, after the ultrashort pulse microwave source is connected with 220v voltage, the frequency switch controlled by the optical fiber outputs repetition frequencies to the microwave pulses of 1HZ, 5HZ, 10HZ and 20HZ;
thirdly, the pulse microwave adjusted by the waveguide is radiated to a sample through the opening surface of the waveguide, and the sample absorbs electromagnetic energy to rise and expand instantly to generate pressure waves, namely ultrasonic waves;
fourthly, dividing the 256-channel annular transducer into 4 plugs, wherein 64 channels are arranged in each plug, and switching each plug to receive the ultrasonic wave in 360 degrees respectively;
fifthly, detecting the generated ultrasonic signals by a 256-channel annular ultrasonic transducer through No. 25 insulating transformer oil, amplifying the ultrasonic signals by a 64-channel amplifier, collecting the ultrasonic signals by a high-speed digital acquisition card, and storing the collected data in a computer in a matrix form;
and sixthly, calculating the acquired data in a computer, extracting the acquired signals into 256 data groups, respectively extracting the amplitude of the thermoacoustic signals detected after the pulse in each group is triggered, and reconstructing a microwave absorption image by using a filter projection algorithm.
Preferably, the high-speed digital acquisition card is a real-time acquisition system based on LABVIEW software control, and can correspond the excited region to the data one by one.
Preferably, the ultrasonic wave generated in the second step is expressed by the following formula:
wherein C is p Is specific heat capacity, beta is medium volume expansion coefficient, p is thermoacoustic pressure, H = u a I is the microwave energy absorbed per unit area and time, I represents the intensity of the microwave pulse, u a Showing the microwave energy absorption profile.
The ultrashort pulse microwave thermoacoustic imaging device based on waveguide output comprises an ultrashort pulse microwave excitation source, a long tapered monopole coupling antenna, an adjusting microwave output waveguide, an optical fiber frequency controller and a thermoacoustic excitation component, wherein the thermoacoustic excitation component is connected with a thermoacoustic signal acquisition component and a data processing component, the thermoacoustic excitation component, the thermoacoustic signal acquisition component and the data processing component are sequentially connected, and the thermoacoustic signal acquisition component comprises an annular ultrasonic transducer, 64-channel signal amplifiers and a high-speed digital acquisition card.
Preferably, the microwave waveguide device further comprises a microwave waveguide output part, wherein the microwave waveguide output part comprises a square waveguide main body, a barometer, a trachea valve, a long tapered monopole coupling antenna, a polytetrafluoroethylene plate and an insulating sleeve, and the long tapered monopole coupling antenna is arranged at the microwave waveguide output part.
Preferably, the ultrasonic transducer is an annular phased array transducer, the number of channels is 256, the distance between every two channels is 2mm, the inner diameter size is 150mm, the dominant frequency is 2.5MHz, and the bandwidth is 85%; the sampling rate of the high-speed digital acquisition card is 50MHz, and the high-speed digital acquisition card has 32 data channels and 12-bit sampling precision; the acquisition control program and the signal processing process installed on the computer are compiled by Labview and Matlab programs; the microwave center frequency emitted by the pulse microwave excitation source is 434M, the pulse width is 10ns, and the repetition frequency is 1HZ, 5HZ, 10HZ and 20HZ; the microwave pulse excitation source is controlled by a frequency controller through an optical fiber to generate heating/detection pulses with adjustable repetition frequency.
Compared with the prior art, the invention has the beneficial effects that:
1. the microwave radiation source has great improvement on a thermoacoustic excitation source, breaks through the traditional excitation mode of microwave generation based on a magnetron in the past, adopts the technical means of generating pulse microwave based on high-voltage electric pulse to generate ultrashort pulse microwave, and is shaped by a self-designed long conical monopole coupling antenna and a waveguide and then uniformly radiated into a space to form a microwave radiation uniform area in a certain area in the space, the pulse width of output pulse microwave reaches 10ns, the limitation of the pulse width us level of the previous thermoacoustic imaging microwave is broken through, the microwave radiation main frequency is 434M, and the microwave radiation source is a microwave frequency band applied in medicine.
2. The microwave radiation field is relatively uniform in a certain area, so that the pixel value difference of thermoacoustic images caused by the uneven microwave radiation can be greatly reduced, the thermoacoustic imaging technology based on the microwave absorption difference imaging in the tissue has positive significance, the thermoacoustic imaging technology is well promoted to be clinically applied, and the thermoacoustic imaging technology has huge clinical application prospects.
Drawings
FIG. 1 is a schematic structural diagram of an ultrashort pulse microwave thermoacoustic imaging device based on waveguide output according to the present invention;
FIG. 2 is a schematic diagram of an antenna of the present invention;
FIG. 3 is a schematic cross-sectional structural view of an output portion of an ultrashort pulse microwave waveguide in an ultrashort pulse microwave thermoacoustic imaging device based on waveguide output according to the present invention;
FIG. 4 shows the electric field amplitude distribution along the x-axis at the waveguide facet;
FIG. 5 shows the electric field amplitude distribution along the y-axis direction at the waveguide aperture plane;
FIG. 6 shows the electric field energy density distribution along the x-axis at the waveguide aperture;
FIG. 7 shows the electric field energy density distribution along the y-axis at the waveguide aperture;
in the figure: 1 waveguide body, 2 barometers, 3 trachea valves, 4 long tapered monopole coupling antennas, 5 polytetrafluoroethylene plates and 6 insulating sleeves.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments.
In the description of the present invention, it is to be understood that the terms "upper", "lower", "front", "rear", "left", "right", "top", "bottom", "inner", "outer", and the like, indicate orientations or positional relationships based on the orientations or positional relationships shown in the drawings, are merely for convenience in describing the present invention and simplifying the description, and do not indicate or imply that the device or element being referred to must have a particular orientation, be constructed and operated in a particular orientation, and thus, should not be construed as limiting the present invention.
Referring to fig. 1, the ultrashort pulse microwave thermoacoustic imaging method based on waveguide output includes the following steps:
the method comprises the following steps that firstly, ultrashort pulse microwaves with the central frequency of 434M are transmitted by a long conical monopole coupling antenna, the pulse microwaves are output from a waveguide port surface after being adjusted through a waveguide, the waveguide port surface is sealed by a polytetrafluoroethylene plate with the thickness of 20mm, SF6 gas with 2.5 atmospheres is filled in the waveguide port surface and used for preventing the single pole of the antenna from being broken down with a shell, and the microwave radiation has relatively good energy uniformity in a certain area after being adjusted through the waveguide;
secondly, after the ultra-short pulse microwave source is connected with 220v voltage, the microwave pulse output repetition frequency of 1HZ, 5HZ, 10HZ and 20HZ can be realized through the frequency switch controlled by the optical fiber;
the ultrasonic waves generated here were represented by the following formula
Wherein C is p Is specific heat capacity, beta is medium volume expansion coefficient, p is thermoacoustic pressure, H = u a I is the microwave energy absorbed per unit area and time, I represents the intensity of the microwave pulse, u a Representing a microwave energy absorption profile;
thirdly, the pulse microwave adjusted by the waveguide is radiated to a sample through the opening surface of the waveguide, and the sample absorbs electromagnetic energy to cause instant temperature rise expansion and generate pressure wave, namely ultrasonic wave;
fourthly, dividing the 256-channel annular transducer into 4 plugs, wherein each plug is internally provided with 64 channels, and respectively switching each plug to realize 360-degree reception of ultrasonic waves;
fifthly, detecting the generated ultrasonic signals by a number 25 insulating transformer oil through a 256-channel annular ultrasonic transducer, amplifying the ultrasonic signals by a 64-channel amplifier, collecting the ultrasonic signals by a high-speed digital collecting card, storing the collected data in a computer in a matrix form, wherein the data collection is a real-time collecting system based on LABVIEW software control, and the excited areas can be in one-to-one correspondence with the data;
and sixthly, calculating the acquired data in a computer, extracting the acquired signals into 256 data groups, respectively extracting the amplitude of the thermoacoustic signals detected after the pulse in each group is triggered, and reconstructing a microwave absorption image by using a filter projection algorithm.
Referring to fig. 1, the ultrashort pulse microwave thermoacoustic imaging device based on waveguide output includes an ultrashort pulse microwave excitation source, a long tapered monopole coupling antenna, a waveguide, the long tapered monopole coupling antenna is disposed in the waveguide through a side wall of the waveguide, an optical fiber frequency controller, a thermoacoustic excitation assembly, the thermoacoustic excitation assembly is connected with a thermoacoustic signal acquisition assembly and a data processing assembly, the thermoacoustic excitation assembly, the thermoacoustic signal acquisition assembly and the data processing assembly are sequentially connected, the thermoacoustic signal acquisition assembly includes an annular ultrasonic transducer, 64-channel signal amplifiers, and a high-speed digital acquisition card;
as shown in fig. 2, the long tapered monopole coupling antenna is made of stainless steel, the whole shape is long taper, burrs of the whole cone are polished smoothly, the whole length is 181.26mm, the long needle is 110.58mm, the needle head part is a nut thread, the diameter of the nut is M8, the thread length is 15mm, the diameter of the bottom of the cone is 42.19mm, the edge is chamfered, the radius of the chamfer is 0.3mm, the length of the cone is 70.68mm, the length of the front cylindrical part of the cone is 18.12mm, and the included angle between the edge of the cone and the center of the cone is 17.03 degrees.
As shown in figure 3, the long tapered monopole coupling antenna 4 is fixed on the side wall of the waveguide through an insulating sleeve 6, the needle head part of the coupling antenna is screwed into the center of the insulating sleeve through the screw thread of M8, the width of the opening surface of the waveguide is 276.22mm, the length of the opening surface of the waveguide is 400mm, the height is 384.33mm, the width of the top part of the waveguide is 139.17mm, the outer diameter of the insulating sleeve 6 is 62.03mm, the inner diameter is 40mm, the length of the insulating sleeve is 120.02mm, the connecting part of the waveguide and the insulating sleeve is 135mm, the diameter of the outer ring is 14.50mm, the diameter of the inner ring is 70mm, the width of the inner ring is 48.39mm, the insulating sleeve is positioned at the left side of the waveguide, 2 is a pressure indicator, 3 is an inflation tube valve, the diameter of the inflation tube is 10mm, and the material is stainless steel, the long-tapered monopole coupling antenna is used for filling SF6 gas into a waveguide port, the height of a pressure indicator 2 from the top of a waveguide is 79.57mm, the height of the distance between the top of the waveguide and the top of the waveguide is 100.66mm, the distance between the top of the waveguide and the needle tip of the long-tapered monopole coupling antenna is 203.83mm, the height of the needle tip of the long-tapered monopole coupling antenna from the bottom of the waveguide is 180.5mm, 5 of the long-tapered monopole coupling antenna is a polytetrafluoroethylene plate and used for sealing the opening surface of the waveguide, the thickness of the long-tapered monopole coupling antenna is 20mm, the distance between the outward expansion part of the waveguide and the opening surface of the waveguide is 98.58mm, the long-tapered monopole coupling antenna is positioned in the waveguide, the waveguide is made of stainless steel, the distance between the top of the taper of the long-tapered monopole coupling antenna and the right side of the waveguide is 45.32mm, and the distance between the left side of the waveguide is 93.85mm.
The oil groove is filled with insulating transformer oil, the annular ultrasonic transducer is immersed in the insulating transformer oil, the sample is placed in an inner ring of the annular transducer, when the thermoacoustic signal of the sample is measured, the sample is immersed and fixed in the ultrasonic coupling liquid of the insulating transformer oil in the oil groove, and the microwave pulse waveguide is fixed right above the sample; heating/detecting pulse microwaves emitted by the long conical monopole coupling antenna are radiated on a sample through the waveguide; the annular ultrasonic transducer is immersed in the ultrasonic coupling liquid of the insulating transformer oil and is used for receiving a thermoacoustic signal excited by pulse microwave of a sample; an LABVIEW data acquisition control platform and an MATLAB program for data processing are arranged in the computer; and (3) after the thermoacoustic signals generated by the detection pulses are acquired by the high-speed digital acquisition card, obtaining a microwave absorption reconstruction image of the sample to be detected by using an MATLAB through a filtering back-projection algorithm.
The integral microwave waveguide output part comprises a square waveguide main body, a gas pressure meter for measuring SF6 gas in the sealed waveguide, a gas pipe valve for filling the SF6 gas into the sealed waveguide, a polytetrafluoroethylene plate with a small microwave absorption coefficient and used for sealing a waveguide port, and an insulating sleeve, wherein the insulating sleeve is used for preventing high-voltage pulse from puncturing air, and a long conical monopole coupling antenna is fixed on the side wall of the waveguide through an insulator and is used for inputting the microwave into the waveguide.
The ultrasonic transducer is an annular phased array transducer, the number of channels is 256, the distance between every two channels is 2mm, the inner diameter size is 150mm, the dominant frequency is 2.5MHz, and the bandwidth is 85 percent; the sampling rate of the high-speed digital acquisition card is 50MHz, and the high-speed digital acquisition card has 32 data channels and 12-bit sampling precision; the acquisition control program and the signal processing process installed on the computer are compiled by Labview and Matlab programs; the central frequency of the microwave emitted by the pulse microwave excitation source is 434M, the pulse width is 10ns, and the repetition frequency is 1HZ, 5HZ, 10HZ and 20HZ, and is adjustable; the microwave pulse excitation source is controlled by a frequency controller through an optical fiber to generate heating/detecting pulses with adjustable repetition frequency; the ultrasonic coupling liquid filled in the oil groove is No. 25 insulating transformer oil, and the temperature is room temperature.
The method comprises the steps of generating ultrashort pulse microwaves with the pulse width of 10ns by using a high-power electric pulse technology, outputting the microwaves, radiating the microwaves through a long conical monopole coupling antenna, outputting a relatively uniform microwave radiation field after wave guide shaping, radiating the microwaves to the surface of a sample, receiving excited ultrasonic signals through an annular ultrasonic transducer around the sample, storing acquired data in a computer, and analyzing a gray graph, namely a thermoacoustic image, absorbed by the sample microwaves through a filtering back-projection algorithm.
The ultrashort pulse microwave generator emits pulse microwaves through the long conical monopole coupling antenna, the pulse microwaves are adjusted through the waveguide and then radiated to the surface of a sample, the position between the sample and the waveguide port is adjusted, and the sample is placed in a region with relatively uniform microwave radiation;
connecting a microwave trigger source to a 256-to-64-channel adapter plate on an acquisition card through a coaxial line to realize the synchronization of microwave trigger and signal acquisition; absorbing microwave heating expansion in the microwave radiation uniform area to generate a thermoacoustic signal; detecting thermoacoustic signals by a 256-array-element annular ultrasonic transducer, collecting the signals by an NI (5752) acquisition card through a self-made 64-channel signal amplifier, and storing the signals into a computer, wherein in the process, the plug of the annular ultrasonic transducer is converted for 4 times to realize the 360-degree signal acquisition of a sample;
processing and calculating the collected thermoacoustic signals on a computer, analyzing 256 channel signals on four ultrasonic probes one by one according to the sequence of collection angles, and performing microwave absorption reconstruction on the 256 channel signals by utilizing a filter back-projection algorithm compiled by the computer to generate a microwave absorption gray image; processing and calculating the collected thermoacoustic signals on a computer, analyzing 256 channel signals on the four ultrasonic probes one by one according to the sequence of collection angles, and performing microwave absorption reconstruction on the 256 channel signals by utilizing a filter back-projection algorithm compiled by the computer to generate a microwave absorption gray image; and in the fourth step, the acquisition of the thermoacoustic signals uses a signal acquisition system based on labview software control, and the thermoacoustic signals of 64 channels can be monitored in real time through a real-time display screen on an acquisition panel.
The horizontal direction is defined as an x-axis direction, the width direction of the waveguide is a y-axis direction, the height direction of the waveguide is a z-axis direction, fig. 4 is electric field distribution in the x-axis direction at an opening surface of the waveguide, wherein an origin of coordinates is located at the center of the waveguide surface, a measurement range is that the center of the measurement range is 0 point, the left and right sides of the measurement range are 6cm respectively in the x-axis direction, the total measurement length is a length range of 12cm, a vertical coordinate of a graph represents the magnitude of the measured electric field, a test result shows that the maximum value of the electric field amplitude change in the range of 12cm in total from the left and right 6cm of the center of the waveguide is 12, the variation of the electric field in the x-axis direction is small, and the electric field distribution in the range of 6cm from the center in the x-axis can be considered to be relatively uniform.
As shown in fig. 5, the electric field distribution in the y-axis direction of the waveguide aperture surface is shown, wherein the origin of coordinates is located at the center of the waveguide surface, the measurement range is a length range with the center of the waveguide surface being 0 point, the left and right sides of the waveguide surface are respectively 10cm in the y-axis direction, the total measurement length is 20cm, the ordinate of the graph represents the magnitude of the measured electric field, the test result shows that the electric field amplitude change is 1.3% in the range of 20cm in the y-axis direction of the waveguide aperture surface, which indicates that the electric field change is small in the y-axis direction of the waveguide, and the y-axis electric field distribution is relatively uniform in the range of 10cm in the center of the waveguide.
Fig. 6 and 7 correspond to the microwave energy density at the mouth face of the waveguide in the x-axis and y-axis, respectively, and show that the maximum attenuation of the microwave energy is 23% in the range of 6cm from the center in the x-axis direction and 2% in the range of 10cm from the center in the y-axis direction.
By integrating the electric field distribution and microwave energy distribution in the x-axis direction and the y-axis direction of the opening surface of the waveguide, it can be considered that the microwave energy distribution is relatively uniform within the range of 6cm at the center of the opening surface of the waveguide, and compared with the microwave energy distribution of the traditional antenna, the microwave energy distribution has good microwave uniform field distribution when pulse microwaves with main frequency of 434M are output in a waveguide mode.
The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art should be considered to be within the technical scope of the present invention, and the technical solutions and the inventive concepts thereof according to the present invention should be equivalent or changed within the scope of the present invention.
Claims (2)
1. An ultrashort pulse microwave thermoacoustic imaging method based on waveguide output is characterized by comprising the following steps:
firstly, ultrashort pulse microwaves transmit pulse microwaves with the central frequency of 434MHz through a long conical monopole coupling antenna positioned in a waveguide, the pulse microwaves are output after being adjusted through the waveguide, wherein the length of the opening surface of the waveguide is 400mm, the width of the opening surface of the waveguide is 276.22mm, the height of the opening surface of the waveguide is 384.33mm, the width of the top of the waveguide is 139.17mm, the length of the opening surface of the waveguide is 400mm, the opening surface of the waveguide is sealed by a polytetrafluoroethylene plate with the thickness of 20mm, and the waveguide is filled with SF6 gas with 2.5 atmospheric pressures; the long conical monopole coupling antenna is made of stainless steel materials, the whole shape is long conical, burrs of the whole cone are polished to be smooth, the whole length is 181.26mm, the length of a long needle structure is 110.58mm, the needle head part is a nut thread and is M8, the diameter of the nut is 15mm, the diameter of the bottom of the cone is 42.19mm, chamfering processing is carried out on the edge, the radius of the chamfering is 0.3mm, the length of the cone is 70.68mm, the length of a front cylindrical part of the cone is 18.12mm, the included angle between the cone edge and the center of the cone is 17.03 degrees, the long conical monopole coupling antenna (4) is fixed on the side wall of the waveguide through an insulating sleeve (6), the distance from the top of the waveguide to the needle tip part of the long conical monopole coupling antenna is 203.83mm, the height from the needle tip part of the long conical monopole coupling antenna to the bottom of the waveguide is 180.5mm, the distance from the top of the cone to the right side of the waveguide is 45.32mm, and the distance from the left side of the waveguide is 93.85mm;
second, after the ultra-short pulse microwave source is switched on with 220v voltage, the frequency is controlled by the optical fiberThe switch outputs the repetition frequency to the microwave pulses of 1Hz, 5Hz, 10Hz and 20Hz; the generated ultrasonic wave is expressed by the following formula:
wherein C is p Is specific heat capacity, beta is medium volume expansion coefficient, p is thermoacoustic pressure, H = u a I is the microwave energy absorbed per unit area and time, I represents the intensity of the microwave pulse, u a Representing a microwave energy absorption profile;
thirdly, the pulse microwave adjusted by the waveguide is radiated to a sample through the opening surface of the waveguide, and the sample absorbs electromagnetic energy to rise and expand instantly to generate pressure waves, namely ultrasonic waves;
fourthly, dividing the 256-channel annular transducer into 4 plugs, wherein 64 channels are arranged in each plug, and switching each plug to receive the ultrasonic wave in 360 degrees respectively;
fifthly, detecting the generated ultrasonic signals by a 256-channel annular ultrasonic transducer through No. 25 insulating transformer oil, amplifying the ultrasonic signals by a 64-channel amplifier, collecting the ultrasonic signals by a high-speed digital acquisition card, and storing the collected ultrasonic signals in a computer in a matrix form;
sixthly, calculating the acquired data in a computer, extracting the acquired signals into 256 data groups, respectively extracting the amplitude of the thermoacoustic signals detected after the pulse in each group is triggered, and reconstructing a microwave absorption image by using a filter projection algorithm; the high-speed digital acquisition card is a real-time acquisition system based on LABVIEW software control, and can correspond the excited area to the data one by one.
2. An ultrashort pulse microwave thermoacoustic imaging device based on waveguide output comprises an ultrashort pulse microwave excitation source, a long conical monopole coupling antenna, an optical fiber frequency controller, a thermoacoustic excitation component, a thermoacoustic signal acquisition component, a data processing component and a computer, wherein the long conical monopole coupling antenna is positioned in a waveguide, microwaves adjusted by the waveguide are output from a waveguide port face, the thermoacoustic excitation component, the thermoacoustic signal acquisition component and the data processing component are sequentially connected, the thermoacoustic signal acquisition component comprises an annular ultrasonic transducer, a 64-path signal amplifier and a high-speed digital acquisition card, the waveguide comprises a square waveguide main body, a gas pressure gauge, a gas pipe valve, a long conical monopole coupling antenna, a polytetrafluoroethylene plate and an insulating sleeve, the long conical monopole coupling antenna is made of stainless steel materials, the whole shape is a long cone, the whole cone burr is smooth after being polished, the whole length is 181.26mm, and the long needle structure is 110.58mm, the needle part is a nut thread, the diameter of the nut is M8, the thread length is 15mm, the diameter of the bottom of the cone is 42.19mm, the edge is rounded, the radius of the rounded angle is 0.3mm, the length of the cone is 70.68mm, the length of the front cylindrical part of the cone is 18.12mm, the included angle between the edge of the cone and the center of the cone is 17.03 degrees, the long conical monopole coupling antenna is fixed on the side wall of the waveguide through an insulating sleeve, the needle part of the long conical coupling antenna is screwed into the center of the insulating sleeve through the thread of the M8, wherein the length of the opening of the waveguide is 400mm, the width of the opening of the waveguide is 276.22mm, the height of the waveguide is 384.33mm, the width of the top of the waveguide is 139.17mm, the length of the waveguide is 400mm, the distance from the top of the tip of the long conical monopole coupling antenna is 203.83mm, the distance from the tip of the long conical monopole antenna is 180.5mm from the bottom of the waveguide, the outward expanding part of the waveguide is 98.58mm from the opening of the waveguide, the top of the long conical monopole coupling antenna is 45.32mm from the right side of the waveguide, the distance from the left side of the waveguide is 93.85mm, the polytetrafluoroethylene plate is arranged at the input port of the waveguide, the thickness of the polytetrafluoroethylene plate is 20mm, and SF6 gas with 2.5 atmospheric pressures is filled in the waveguide; the ultrasonic transducer is an annular phased array transducer, the number of channels is 256, the distance between every two channels is 2mm, the inner diameter size is 150mm, the main frequency is 2.5MHz, and the bandwidth is 85%; the sampling rate of the high-speed digital acquisition card is 50MHz, and the high-speed digital acquisition card has 32 data channels and 12-bit sampling precision; the acquisition control program and the signal processing process installed on the computer are compiled by Labview and Matlab programs; the microwave center frequency emitted by the ultrashort pulse microwave excitation source is 434MHz, the pulse width is 10ns, and the repetition frequency is 1Hz, 5Hz, 10Hz and 20Hz; the ultrashort pulse microwave excitation source is controlled by a frequency controller through an optical fiber to generate heating/detection pulses with adjustable repetition frequency.
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