JP5022884B2 - Probe system, ultrasonic system, and ultrasonic generation method - Google Patents

Description

  The present invention relates generally to inspection techniques, and more particularly to non-destructive testing techniques using plasma discharge.

  Ultrasonic inspection of materials is a commonly used technique for detecting and quantifying material defects and subsurface damage of industrial parts. However, one of the limitations of conventional ultrasonic inspection techniques is the need for a liquid interface between the probe and the material to be inspected, from the voids that may otherwise exist at the interface. This is to prevent reflection of excessive acoustic energy. Non-contact ultrasonic inspection is particularly attractive for materials that can be damaged by water, for high temperature materials, and when the equipment for supplying water to the interface is expensive or difficult. Non-destructive inspection technology. One possible non-contact inspection technique is by air-coupled ultrasound. However, air-coupled ultrasound has a relatively poor signal-to-noise ratio (approximately 40-80 dB lower than liquid-coupled ultrasound).

  Commonly used techniques for generating ultrasonic waves in materials through voids include localized laser heating or cauterization (laser ultrasonics) on the part surface, acoustically matched to air High power piezoelectric or capacitive membrane devices and electromagnetic acoustic transducers (EMAT) that generate mechanical vibrations in materials via electromagnetic forces are included. Where each of these techniques is subject to certain limitations. For example, laser ultrasound is very effective at generating ultrasound in metals and some composites, but it causes minor damage to the surface of the material and is very expensive to perform. high. Also, with respect to conventional high power air matched ultrasonic transducers and EMAT, it has been observed that these techniques generally limit maximum power.

  Therefore, there is a need for an improved ultrasonic inspection system that addresses the above-mentioned problems.

  According to one embodiment, a probe system is provided. The probe system includes an inner conductor coupled to an alternating current (AC) voltage source. The probe system also includes an outer conductor disposed about the inner conductor and having one end coupled to an AC voltage source. The other end of the outer conductor forms the first electrode. The probe system further includes a second electrode separated from the first electrode by an air gap for initiating a plasma discharge within the air gap.

  According to another embodiment of the invention, a shielded probe system is provided. The shielded probe system includes a central conductor having one end coupled to an AC voltage source and another end forming a first electrode. The shielded probe system also includes an outer conductor disposed about the central conductor and having one end coupled to an AC voltage source. The other end of the outer conductor forms a second electrode, and the first electrode and the second electrode are separated by a gap to initiate a plasma discharge in the gap. The shielded probe system further includes a shield disposed concentrically with the central conductor for shielding electromagnetic interference.

  According to another embodiment of the present invention, an ultrasound probe system is provided. The ultrasound probe system includes a carrier signal source for providing a radio frequency (RF) carrier signal. The ultrasound probe system also includes an acoustic modulator for providing an envelope signal. The ultrasound probe system also includes a mixer for mixing the RF carrier signal and the envelope signal to provide a modulated RF signal. The ultrasonic probe system further includes a ground electrode. The ultrasound probe system also includes a radio frequency plasma probe that is separated from the ground electrode by an air gap and is configured to receive a modulated RF signal and generate a plasma in the air gap. The plasma generates ultrasonic waves.

  According to another embodiment of the present invention, a method for generating ultrasound is provided. The method includes providing a modulated radio frequency signal to a radio frequency plasma probe. The method also includes initiating a plasma discharge via the radio frequency plasma probe. The method also includes stabilizing the plasma discharge via a radio frequency plasma probe. The method further includes modulating the intensity of the plasma discharge. The method also includes generating a plurality of ultrasonic waves using a plasma discharge.

  These and other features, aspects and advantages of the present invention will be better understood when the following detailed description is read with reference to the accompanying drawings. In the drawings, like parts are designated by like reference numerals throughout the drawings.

  The present invention provides an improved transmitter design that increases the maximum acoustic power that can be delivered to the part to be inspected to overcome the disadvantages of the conventional air-coupled ultrasonic inspection techniques described above. In this aspect, the signal level of the measurement is increased. As described in detail below, embodiments of the present invention include a system for initiating and stabilizing a plasma discharge. Embodiments of the present invention also include a method of generating ultrasound using an ultrasound system and a plasma. “Plasma discharge” as used herein refers to plasma generated at radio pressure at atmospheric pressure and with a frequency greater than about 1 MHz. The generated ultrasound is a high pressure acoustic wave used in nondestructive testing to look for defects in the material and any subsurface damage. Non-destructive testing using ultrasound is referred to herein as “air-coupled” ultrasound, in which the probe that generates the plasma and the material to be inspected are separated by a gap.

  Conventional electronic components used to initiate a plasma discharge are generally found to be inefficient to achieve the desired voltage for the plasma discharge. Moreover, the plasma discharge is inherently unstable at atmospheric pressure due to the decrease in the electrical impedance of the plasma discharge as the current increases. Furthermore, plasma at atmospheric pressure requires a high voltage to start. Beneficially, the present invention provides a stable, low intensity, narrow band plasma source at atmospheric pressure.

  Referring now to the drawings, FIG. 1 is a schematic diagram of a system 10 for initiating and stabilizing a plasma. System 10 can also be referred to as plasma probe 10. The plasma probe 10 includes an inner conductor 12 that is coupled to an alternating current (AC) voltage source 14. In the illustrated embodiment, an outer conductor 16 that includes a plurality of windings 18 is disposed around the inner conductor 12. One end of the outer conductor 16 is coupled to the AC voltage source 14 and the other end on the opposite side forms the first electrode 20. In the illustrated embodiment, the second electrode 22 is separated from the first electrode 20 by an air gap 24 to initiate a plasma discharge 26. In certain embodiments, the inner conductor 12 is a cylindrical shaped conductor. In another embodiment, the inner conductor 12 is a wire. In one example, the outer conductor 16 is a solenoid. In the exemplary embodiment, inner conductor 12 is connected to ground 28. In another embodiment, outer conductor 16 is connected to ground 28 via AC voltage source 14. In one example, the plasma discharge 26 can initiate an atmospheric pressure radio frequency plasma having a frequency of about 6 MHz to about 7.5 MHz. In certain embodiments, inner conductor 12 and outer conductor 16 are configured to boost the voltage supplied by AC voltage source 14 to initiate plasma 26 by about 20 dB or more. Although FIG. 1 shows a solenoid 16 extending around a solid inner conductor 12, in another embodiment (not shown), the inner conductor 12 is a solenoid disposed inside the outer conductor 16. The outer conductor 16 is a solid (eg, hollow tube) metal electrode.

FIG. 2 is an equivalent circuit 30 of the plasma probe 10 of FIG. The inner conductor 12 as described in FIG. 1 and the outer conductor 16 as described in FIG. 1 are configured as a series LC circuit, where L is the inductance of the outer conductor 16 represented by an inductor 32, and C _S is a capacitance between the inner conductor 12 and the outer conductor 16. Induct 32 may have a small real-valued resistance represented by resistor 36. In certain embodiments, resistance 36 can vary between about 10 ohms and about 100 ohms. The plasma probe 10 as described in FIG. 1 acts as a quarter-wave transformer with a distributed transmission line 38 with an inner conductor 12 and an outer conductor 16. The outer conductor 16 can include a number of windings 18 as described in FIG. 1 to increase the inductance per unit length. Winding 18 may be between each of these windings 18 including the parasitic capacitance 40, the parasitic capacitance 40 is significantly smaller than the capacitance 34 denoted by C _S. As a result, the capacitance 40 can be ignored. The electrical circuit 30 can also include a leakage resistor 42 of capacitance 34, which is represented by G. The leakage resistance 42 is very small and may therefore be negligible.

  Before the plasma is started, the AC voltage source 14 looks at the open circuit transmission line when unloaded. The equivalent circuit 30 resonates at a quarter wavelength. The quarter-wave resonance occurs because the frequency of the AC voltage source generates a standing wave in the transmission line 38 so that the physical length of the transmission line 38 is a quarter of the wavelength of the standing wave. It is time to let them. Thus, the AC voltage source 14 is considered shorted at the end 44 of the transmission line 38 that produces the load 46. As a result, a large amount of current is driven into the transmission line 38. The large current generated in this way passes through the inductor 32 and the capacitor 34. Furthermore, when f is the frequency of the AC voltage source 14, the impedance of the inductor 32 represented by 2πfL is large when both f and L are large. Accordingly, the voltage drop across the inductor 32 is large, and as a result, a high voltage is generated at the end 44 of the transmission line 38. This high voltage initiates the generation of plasma at the load 46.

  Due to the generation of plasma, the transmission line 38 is short-circuited by the load 46. In such a state, the current driven into the transmission line 38 can have two paths. One path is a path through the inductor 32 as described above, and the other path is a path that bypasses the inductor 32 and passes through the load 46. Thus, the current is divided into two paths and the current through the inductor 32 is relatively small. This results in a lower voltage drop across the inductor 32 and a lower voltage across the generated plasma at the load 46. Accordingly, the current flowing into the plasma is reduced, resulting in an increase in plasma resistance. Thus, a very fast negative feedback loop is created to stabilize the plasma generated at load 46. Thus, the plasma probe 10 plays an important role in starting the plasma and stabilizing its voltage to a desired value so that the plasma is not extinguished while at the same time preventing any possible burnout.

  In another exemplary embodiment of the present invention, FIG. 3 is a shielded probe system 60 for initiating and stabilizing a plasma discharge. The system 60 includes a central conductor 62 that is coupled at one end to an AC voltage source 64 and at the other end opposite the first electrode 66. System 60 also includes an outer conductor 68 having a plurality of windings 70 disposed about a central conductor 62. In the illustrated embodiment, the outer conductor 68 has one end coupled to the AC voltage source 64 and the other opposite end forming a second electrode 72. As shown, the first electrode 66 and the second electrode 72 are separated by a gap 74 to initiate a plasma discharge 76. The system 60 further includes a shield 78 disposed concentrically with the central conductor 62 for electromagnetic interference shielding. The magnetic field generated by the outer conductor 68 is compressed and exists in the space between the center conductor 62 and the shield 78. The center conductor 62 and the outer conductor 68 can be configured to boost the voltage supplied by the AC voltage source 64 by about 40 dB. An insulator 80 can be disposed between the center conductor 62 and the shield 78. In certain embodiments, the insulator 80 is a ceramic insulator. In one example, the shield 78 includes an inner conductive cylinder and an outer conductive cylinder that are concentrically connected to each other and an outer conductor 68 between the inner and outer conductive cylinders. Is placed. The shielded probe system 60 initiates a warm plasma discharge 76. As used herein, “warm plasma” refers to plasma that is hot enough to burn paper but not hot enough to melt copper. Although FIG. 3 shows a solenoid 68 extending around the center conductor 62, in another embodiment (not shown), the inner conductor 62 is a solenoid disposed inside the outer conductor 68, and the outer conductor Reference numeral 68 denotes a solid (for example, hollow tube) metal electrode.

FIG. 4 is an equivalent circuit 90 of the shielded probe system 60 of FIG. The central conductor 62 in FIG. 3 and the outer conductor 68 in FIG. 3 are configured as a series LC circuit. The presence of shield 78 as described in FIG. 3 reduces the effective inductance in circuit 90 and increases the effective capacitance. The effective inductance is equal to (LM) represented by the inductor 92, where L is the inductance of the center conductor, and M is between the center conductor 62 and the shield 78 due to eddy current loss. Mutual inductance. The effective capacitance is equal to (C _S1 + C _S2 ) represented by the capacitor 94, where C _S1 is the capacitance between the center conductor 62 and the outer conductor 68, and C _S2 is the shield 78 and the outer conductor 68. Capacitance between the two. Inductor 92 may have a small real-valued resistance represented by resistor 96 that includes resistance due to eddy current loss. Capacitor 94 may include a negligible leakage resistance represented by leakage resistor 98. The circuit 90 can also include negligible parasitic capacitance represented by the capacitor 100 between each of the windings 70 as described in FIG. Since the effective inductance is smaller and there is more loss in the form of eddy currents, the voltage required to initiate the plasma discharge increases. As a result of the start of the plasma discharge, a short circuit occurs at the end 102 having the load 104. Accordingly, it is possible to obtain a plasma probe having a relatively low efficiency as compared with the plasma probe described in FIG.

Beneficially, the probes 10, 60 provide a low intensity narrow band plasma source that is stable at atmospheric pressure in air. The plasma is relatively damaging to most industrial materials such as metals, plastics and composites and does not cause excessive electromagnetic interference. This plasma source is also very efficient and does not require a very large power supply voltage to operate.
Probes 10, 60 can be used to generate a plasma discharge for use in a high temperature environment to measure the length of a void in an object such as a turbine engine. Furthermore, the probes 10 and 60 can be used to generate a plasma discharge for generating ultrasound that can be used to detect defects, as described below.

  FIG. 5 is a schematic diagram of an ultrasound probe system 110. The ultrasound probe system 110 includes a carrier signal source 112 for providing a radio frequency (RF) carrier signal 114. In certain embodiments, the frequency of the RF carrier signal 114 can vary within a range of about 10 MHz to about 50 MHz. System 110 also includes an acoustic modulator 116 for providing an envelope signal 118. In one example, the acoustic modulator 116 includes at least one function generator configured to provide an envelope signal 118 having a frequency between about 0.001 MHz and about 2 MHz. The ultrasound probe system 110 also includes a mixer 120 for mixing the carrier signal 114 and the envelope signal 118 to provide a modulated RF signal 122. In the illustrated embodiment, the modulated RF signal 122 can be transmitted to an RF plasma probe 124 that is separated from the ground electrode 126 by an air gap 128. A plasma 130 is generated in the air gap 128 and the plasma then generates a plurality of ultrasonic waves 132. In one example, the RF plasma probe can be an unshielded plasma probe. In another embodiment, the RF plasma probe can be a shielded plasma probe as described in FIG. In certain embodiments, the air gap 128 between the RF plasma probe 124 and the ground electrode 126 is less than about 1 mm. Although FIG. 5 shows a grounded electrode 126, in another embodiment (not shown), the electrode 126 is not grounded.

  In another exemplary embodiment of the present invention as shown in FIG. 6, an ultrasound probe system 140 that uses a ground plane 142 is presented. System 140 includes a carrier signal source 112 as described in FIG. 5 for providing an RF carrier signal 114. In certain embodiments, the frequency of the RF carrier signal 114 can vary within a range of about 10 MHz to about 50 MHz. System 140 also includes an acoustic modulator 116 as described in FIG. 5 for providing an envelope signal 118 as described in FIG. In one example, acoustic modulator 116 includes at least one function generator configured to provide an envelope signal 118 having a frequency between about 0.001 MHz and about 2 MHz. The ultrasonic probe system 140 also mixes the RF carrier signal 114 and the envelope signal 118 to provide a modulated RF signal 122 as described in FIG. including. In the illustrated embodiment, the RF carrier signal 122 is transmitted to an RF plasma needle 144 that is separated from the ground plane 142 by an air gap 146. In certain embodiments, the RF plasma needle can be configured to provide a current of about 1 mA to about 100 mA and a frequency of about 10 MHz to about 50 MHz. Plasma is generated in the air gap 146. In one example, a dielectric barrier 148 can be disposed in the air gap 146 between the RF plasma needle 144 and the ground plane 142. In addition, a wire mesh 150 can be placed on the end of the dielectric barrier 148 facing the RF plasma needle 144. The plasma generated in the gap 146 generates a plurality of ultrasonic waves 152. The system 140 can also include an acoustic mirror 154 that reflects the generated ultrasound 152 to increase efficiency.

  FIG. 7 is a block diagram illustrating the signal modulation system 170 in the ultrasound probe system described in FIGS. The RF signal 114 as described in FIG. 5 from the carrier signal source 112 and the envelope signal 118 as described in FIG. 5 from the acoustic modulator 116 are supplied to the mixer 120. In certain embodiments, the carrier signal source 112 may be a function generator operating at 7.2 MHz. In another embodiment, the acoustic modulator 116 may be a function generator that provides a 900 kHz single cycle sine wave for amplitude modulation. The modulated RF signal 122 from the mixer 120 as described in FIG. 5 can be supplied to the divider 172. Divider 172 provides an RF spectral signal 174 having a central peak at 7.2 MHz and two sidebands at 900 kHz. The occurrence of such a central peak is useful for maintaining the plasma discharge when no ultrasonic wave is generated. The RF spectrum signal 174 is provided to a power amplifier 176 that modulates the envelope of the RF carrier signal 122. The modulated envelope signal 178 from the power amplifier 176 is supplied to the RF plasma probe 124 as described in FIG. When the amplitude of the RF spectrum signal 174 increases, a resulting warm plasma is generated, and when the amplitude decreases, a resulting cold plasma is generated. Rapid cooling and heating of the plasma causes high and low pressure changes in the plasma, which in turn cause acoustic wave propagation.

  FIG. 8 is a block diagram illustrating an air coupled ultrasonic receiver system 190 for inspecting an object. System 190 includes a carrier signal source 192 for providing an RF carrier signal 194. In certain embodiments, the frequency of the RF carrier signal 194 can vary within a range of about 10 MHz to about 50 MHz. System 190 also includes an acoustic modulator 196 for providing an envelope signal 198. In one example, the acoustic modulator 196 can include at least one function generator configured to provide an envelope signal 198 having a frequency between about 0.001 MHz and about 2 MHz. The air coupled ultrasound system 190 also includes a mixer 200 for mixing the RF carrier signal 194 and the envelope signal 198 to provide a modulated RF signal 202. In the illustrated embodiment, the modulated RF signal 202 is provided to a transmitter 204 that is separated from the ground electrode 206 by an air gap 208. In one example, transmitter 204 is an RF plasma probe. A plasma is generated in the air gap 208, which in turn generates a plurality of ultrasonic waves 212. In one example, the RF plasma probe may be an unshielded plasma probe as described in FIG. In another embodiment, the RF plasma probe can be a shielded plasma probe as described in FIG. In certain embodiments, the air gap 208 between the RF plasma probe 204 and the ground electrode 206 can be less than about 1 mm. The generated ultrasonic wave 212 is received by the receiver 216. In the illustrated embodiment, receiver 216 is spaced from the transmitter by air gap 218. In one example, the gap 218 can be about 10 mm.

  FIG. 9 is a flowchart illustrating various exemplary stages for a method 230 for generating ultrasound. Method 230 includes providing 232 a modulated radio frequency signal to a radio frequency plasma probe. The process of providing a modulated radio frequency signal includes providing a radio frequency carrier signal, providing a modulated signal, and then mixing the modulated signal and the radio frequency carrier signal to form a modulated radio frequency carrier signal. . A radio frequency plasma probe initiates a plasma discharge at step 234. Once started, the plasma is stabilized at step 236 by a radio frequency plasma probe. This stabilizing step can include providing feedback to the radio frequency plasma probe. The intensity of the plasma discharge is modulated at step 238. In one example, modulating the intensity can include modulating the current through the radio frequency plasma probe. As a result of the modulation of the intensity of the plasma discharge, a plurality of ultrasonic waves are generated in step 240.

  An advantage of the above-described ultrasound system and method is that it can generate relatively large amplitude acoustic vibrations in the air. Furthermore, the above-described plasma probe has a device efficiency comparable to existing transducers. There are no practical restrictions on the sound output, and the device efficiency is comparable to existing transducers. Still further, the plasma generation technique described above is less expensive than laser ultrasound, and the plasma generation occurs slightly away from the part so that it does not damage the part during testing.

  While only certain features of the invention have been illustrated and described, various modifications and changes will occur to those skilled in the art. Accordingly, it is to be understood that the claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.

1 is a schematic diagram of a system for initiating and stabilizing a plasma at a radio frequency in accordance with an embodiment of the present invention. FIG. 2 is an equivalent circuit diagram of the system of FIG. 1. 1 is a schematic diagram of a shielded probe system for initiating and stabilizing a plasma discharge in accordance with an embodiment of the present invention. FIG. 4 is an equivalent circuit diagram of the shielded probe system of FIG. 3. 3 is a block diagram illustrating a system for generating ultrasound using the system of FIGS. 1 and 2 and a ground electrode in accordance with an embodiment of the present invention. FIG. FIG. 3 is a block diagram illustrating a system for generating ultrasound using the system of FIGS. 1 and 2 and using a ground plane as an electrode in accordance with an embodiment of the present invention. It is a block diagram showing the signal modulation system used for the ultrasonic wave generation system of Drawing 5 and Drawing 6. 1 is a block diagram illustrating an air coupled ultrasonic receiver system 190 for inspecting an object. FIG. 2 is a flow diagram illustrating various exemplary stages for a method of generating ultrasound.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Plasma probe 12 Inner conductor 14 Alternating current (AC) voltage source 16 Outer conductor 18 Winding 20 1st electrode 22 2nd electrode 24 Air gap 26 Plasma discharge 28 Ground 30 The equivalent circuit of the plasma probe 10 32 Inductor 34 Capacitance 36 Resistor 38 Distributed Transmission Line 40 Parasitic Capacitance 42 Leakage Resistance 44 End 46 Load 60 Shielded Probe System 62 Center Conductor 64 AC Voltage Source 66 First Electrode 68 Outer Conductor 70 Winding 72 Second Electrode 74 Gap 76 Plasma Discharge 78 Shield 80 Insulator 90 Equivalent circuit of shielded probe system 60 92 Inductor 94 Capacitor 96 Resistor 98 Leakage resistor 100 Capacitor 102 End 104 Load 110 Ultrasonic probe system 112 Carrier signal source 114 Line frequency (RF) carrier signal 118 Envelope signal 120 Mixer 122 Modulated RF signal 128 Air gap 130 Plasma 132 Ultrasound 140 Ultrasonic probe system 146 Air gap 148 Dielectric barrier 150 Wire mesh 152 Ultrasonic wave 154 Acoustic mirror 170 Signal modulation system 174 RF spectral signal 178 Modulated envelope signal 190 Air coupled ultrasound system 194 RF carrier signal 198 Envelope signal 200 Mixer 202 Modulated RF signal 208 Gap 212 Ultrasound 218 Gap 230 Method of generating ultrasound

Claims (10)

An inner conductor (12) coupled to an alternating current (AC) voltage source (14);
An outer conductor (16) disposed around the inner conductor (12) and having one end coupled to the AC voltage source (14), the other end forming a first electrode (20) An outer conductor (16),
A second electrode (22) separated from the first electrode (20) by a gap (24) for initiating a plasma discharge (26) in the gap (24);
A probe system (10) comprising: The inner conductor (12) has a cylindrical conductor and the outer conductor (16) has a plurality of windings (18) disposed around the inner conductor (12); The probe system (10) of claim 1. The probe system (10) of claim 1, wherein the outer conductor (16) comprises a solenoid. The inner conductor (12) and the outer conductor (16) are configured as an LC circuit, in which case the outer conductor constitutes an inductance L (32), and distributed capacitance between the inner conductor and the outer conductor. Constitutes a capacitance C (34) and a maximum voltage is generated at the end of the outer conductor (16) forming the first electrode (20) before the plasma discharge (26), The probe system (10) of claim 1. A central conductor (62) having one end coupled to an alternating current (AC) voltage source (64) and another end forming a first electrode (66);
An outer conductor (68) disposed around the central conductor (62), having one end coupled to the AC voltage source (64) and another end forming a second electrode (72) The first electrode (66) and the second electrode (72) are separated by a gap (74), and plasma discharge (76) is started in the gap (74). An outer conductor (68);
A shield (78) disposed concentrically with the central conductor (62) for shielding electromagnetic interference;
A shielded probe system (60) comprising: The shielded probe system (60) of claim 5, further comprising an insulator disposed between the central conductor (62) and the shield (78). A carrier signal source (112) for providing a radio frequency (RF) carrier signal (114);
An acoustic modulator (116) for providing an envelope signal (118);
A mixer (120) for mixing the RF carrier signal (114) and the envelope signal (118) to provide a modulated RF signal (122);
A ground electrode (126);
A radio frequency plasma probe (separated from the ground electrode (126) by a gap (128) and configured to receive the modulated RF signal (122) and generate a plasma in the gap (128). 124) a radio frequency plasma probe (124), wherein the plasma (130) generates a plurality of ultrasonic waves (132);
An ultrasonic probe system (110). A dielectric barrier (148) disposed in a gap (146) between the radio frequency plasma probe (124) and the ground electrode (126);
A wire mesh (150) disposed at the end of the dielectric barrier (148) facing the radio frequency plasma probe (124);
The ultrasonic probe system (10) of claim 7, comprising: Providing a modulated radio frequency signal to a radio frequency plasma probe (232);
Initiating a plasma discharge via the radio frequency plasma probe (234);
Stabilizing (236) the plasma discharge via the radio frequency plasma probe;
Modulating the intensity of the plasma discharge (238);
Generating a plurality of ultrasonic waves using the plasma discharge (240);
A method for generating ultrasound. An air coupled ultrasound system (190) for inspecting an object comprising:
A carrier signal source (192) for providing a radio frequency (RF) carrier signal (194);
An acoustic modulator (196) for providing an envelope signal (198);
A mixer (200) for mixing the RF carrier signal (194) and the envelope signal (198) to provide a modulated RF signal (202);
A ground electrode (206);
A radio frequency plasma probe separated from the ground electrode (206) by a gap (218) and configured to receive the modulated RF signal (202) and generate a plasma in the gap (218). A transmitter (204), wherein the plasma generates a plurality of ultrasonic waves (212);
A receiver (216) for receiving ultrasound from an object under test, wherein the receiver (216), transmitter (204) and the object (216) are air coupled;
An air coupled ultrasound system (190).