Non-contact ultrasonic techniques have advantages over several advantages over conventional contact methods. Non contact methods can be:-
- Totally remote.
- Operate on hot or moving samples.
- Work in hostile environments.
- More easily automated giving higher reliability.
One of the most versatile techniques for generating ultrasound without contact is laser generated ultrasound [see laser ultrasound section]. However, in some situations it is almost impossible to ensure that laser generated ultrasound is totally non destructive, as is the case for generation on painted or coated surfaces where the surface coating will typically be removed or damaged by the laser beam impact. One technique that tackles this problem of generating on easily damaged surfaces is the use of plasma generated ultrasound  as has been used in generating ultrasound in real on-line applications . Strictly speaking we do not have a true plasma but rather a mixture of plasma and a hot expanding gas travelling at supersonic velocity, which for simplicity will be referred to as a plasma in this paper. In previous work  the plasma has been generated using a pulsed TEA CO2 laser as shown in the schematic diagram of figure 1. We are now also looking at techniques for generating the plasma using alternative methods such as the plasma igniter [3-5].
Schematic diagram of how plasma generated ultrasound is realised using a pulsed TEA CO2 laser. This is a method of generating high energy ultrasonic waves in samples without causing damage - it is particularly useful in generating sound on painted samples, plastic samples and paper.
What materials is plasma generation typically used on?
We have had some success in using laser induced plasma generation in generating ultrasound on painted beverage cans on production lines . The plasma is hot and has high internal pressure. Thus is a sample is placed too close (less than 1mm) to the generation point of the plasma then there may be some small amount of damage on the most delicate samples.
Some samples that it is difficult to perform non-contact ultrasonic generation on that we have demonstrated the totally non-destructive nature of plasma generation include:-
Thick metal blocks (painted).
Metal strip, including beverage cans (painted).
Plastics and polymers including carbon fibre.
It will of course generate ultrasound on anything but these help illustrate the potential.
The next generation of plasma based ultrasound generators
How does the plasma igniter work?
Consider the schematic diagram of the igniter shown below in figure 2.When the initial spark from the 15kV pulse is generated within the plasma igniter core, the gas in the cavity becomes ionised and the stability of the cavity is destroyed. Hence the high voltage capacitor is very rapidly discharged through the central core electrode throughout the volume of gas inside the cavity. This liberates (assuming no energy losses) approximately 1J of energy into the small volume of gas, resulting in the formation of a plasma. As this plasma is essentially a very hot expanding gas it is ejected from the exit port and when colliding with a target will impart an impulsive shock to the surface of the target. This impulsive force generated by momentum transfer from the plasma to the sample generates a shock (ultrasonic) wave in the target.
Schematic diagram of our plasma igniter. This prototype has been run at repetition rates of up to a few Hz. The main limiting factor is the power supply as it determines how fast the capacitor can re-charge. The operation is shown schematically in the animation below.
What are the advantages of using a plasma igniter over a laser induced plasma source?
The plasma igniter is :-
What are the potential drawbacks with the plasma igniter?
Before you rush out to PLASMA IGNITERS-R-US there are of course some things that we should consider :-
Close proximity is required (as is the case for the laser induced plasma source)
Electrical noise (which can be almost eliminated by suitable filtering and 'earthing')
Acoustic noise (which is also a problem in the direct CO2 and laser induced 'plasma')
Shot-shot variation (which can be overcome by suitable experimental geometries)
Fundamental generation characteristics of the igniter
This will of course depend on the sample that ultrasound will be generated on. The source appears to have comparable high frequency content to a pulsed TEA CO2 laser. Some preliminary work we have done suggests that it may have relatively less low frequency components than the same laser source, but we will confirm this with measurements using a Michelson interferometer.
The directivity of the longitudinal wave is comparable to the directivity obtained with the 'classical' normal piston force (ablative laser source). The directivity is shown below in figure 3., where the igniter - sample separation was 5mm.
R-theta plot of directivity pattern produced by the plasma igniter. The plot is consistent with a normal force piston-like generator.
References for further reading
 Dixon S., Edwards C. and Palmer S.B., J. Appl. Phys. D: Appl. Phys. 29, 3039, (1996)
 Dixon S, Edwards C and Palmer S B, Review of Progress in QNDE, Vol. 17b, (1998) p.1929
 Weinberg F.J., Hom K., Oppenhiem A.K. and Teichman K., Nature 272, 341 (1978)
 Smy P.R., Clements R.M., Simeoni D. and Topham D.R., J. Phys. D: Appl. Phys. 15, 2227 (1982)
 Haley R.F. and Smy P.R., J. Appl. Phys. 27, 934, (1994)