Patent Publication Number: US-2011056272-A1

Title: system for analysing plasma

Description:
FIELD OF THE INVENTION 
     The present invention relates to a system for analysing plasma. The present invention more particularly relates to a system including a sensor for providing an analog signal representative of a characteristic of plasma and an analog-to-digital converter for converting the analog signal into a digital signal for facilitating digital processing thereof. 
     BACKGROUND 
     Plasma generators for generating plasma are well known in the art. Typically, plasma generators comprise a plasma discharge chamber filled with a working gas such as helium, argon or a mixture of gases. Electrodes are located within the discharge chamber to which a power supply applies a voltage to affect a plasma discharge. Plasma is used for various applications, for example, for waste gas abatement, liquid treatment, surface coating and modification, and biomedical applications such as functional/structural thin films; plasma sterilisation/inactivation. Depending on the application it is desirable that the plasma has certain characteristics. For example, it is known in the art that parallel-plate atmospheric pressure plasma driven at a frequency below ˜20 kHz operates in a self-organised filamentary or primary glow mode. In contrast, atmospheric pressure plasma driven a frequencies above ˜20 kHz operates in a secondary glow mode. The transition between these modes may be controlled as is commonly known in the art. 
     Various types of instrumentation are used to analyse plasma to establish its characteristics. Depending on the instrumentation used, the analytical technique may be based on optical spectroscopy, ion mass spectroscopy or electrical. Analysing plasmas using optical or ion mass spectroscopy is expensive and is typically limited to providing line of sight information. Furthermore, locating and configuring optical or ion mass spectroscopy instrumentation in small geometry plasma generators is difficult and requires an extremely skilled operator. 
     Electrical monitoring of plasma in the low frequency range (50 Hz to a few 100 KHz) to very high frequency range to ultra high Frequency range (13.56 MHz to 2.56 GHz) using instrumentation known heretofore is also expensive. Oscilloscopes and spectrum analysers which would be typically considered instruments for such analysis have limited band width. For example, at least two oscilloscopes (or spectrum analyzers) are typically required to span the frequency range from DC-pulsed to 2.45 GHz unless complex aliasing or down conversion frequency circuitry is used. 
     There is therefore a need for a system for analyzing plasma which is relatively inexpensive and non-invasive. 
     These and other features will be better understood with reference to the followings Figures which are provided to assist in an understanding of the teaching of the invention. 
     SUMMARY 
     These and other problems are addressed by providing a system for analysing plasma which includes a sensor for providing an analog signal representative of a characteristic of plasma and an analog to digital converter which converts the analog signal into a digital signal suitable for digital processing. 
     Accordingly, a first embodiment of the invention provides a system as detailed in claim  1 . The invention also provides a system as detailed in claim  33 . Advantageous embodiments are provided in the dependent claims. 
     These and other features will be better understood with reference to the followings Figures which are provided to assist in an understanding of the teaching of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described with reference to the accompanying drawings in which: 
         FIG. 1  is a perspective view of a system for analysing plasma in accordance with the present invention. 
         FIG. 2  is a block diagram a detail of the system of  FIG. 1 . 
         FIG. 3  is a perspective view of a detail of the system of  FIG. 1 . 
         FIG. 4  is a schematic circuit diagram of a detail of the system of  FIG. 1 . 
         FIG. 5  is a perspective view of another system in accordance with the teaching of the present invention. 
         FIG. 6  is a perspective view of another system in accordance with the teaching of the present invention. 
         FIG. 7   a  is a perspective view of another system in accordance with the teaching of the present invention. 
         FIG. 7   b  is a circuit block diagram of a detail of the system of  FIG. 7   a.    
         FIG. 7   c  is a block diagram of a component of the circuit of  FIG. 7   b.    
         FIG. 8  is a perspective view of another system in accordance with the teaching of the present invention. 
         FIG. 9  is a perspective view of another system in accordance with the teaching of the present invention. 
         FIG. 10  is a detail of the system of  FIG. 9 . 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     The invention will now be described with reference to some exemplary systems which are provided to assist in an understanding of the teaching of the invention. 
     Referring to the drawings and initially to  FIG. 1  there is provided a system  100  for analysing plasma. The plasma is generated by a plasma generator  105 , which may be implemented in accordance with known methodologies. Such known plasma generators  105  typically comprise a housing  107  defining a plasma discharge chamber  110  which may be filled with a working gas. A pair of electrodes  115  are located in the chamber  110  and are coupled to a power supply  116 . The power supply  116  operably applies a voltage to effect a plasma discharge in the chamber  110 . The power supply  116  is electrically coupled to the electrodes  115  via a power line  118 . The electrodes  115  can be arranged in various configurations, for example, parallel-plate, coaxial, and reel to reel. The working gas may be any suitable type of gas such as helium, argon or a mixture of gases such as oxygen, nitrogen, and a liquid precursor such as polydimethylsiloxane. The drive frequency of the power source may be direct current (DC) power supply or it can be an alternating current AC power supply. Typically frequencies used in plasma generators vary from 50 Hz to 2.456 GHz. 
     The present inventors have realised that the mechanical, electrical and chemical components of the plasma generator  105  results in an electrical noise and an electro-acoustic emission noise signal on the power line  118  between the power supply  116  and the electrodes  115  which is related to the characteristic of the plasma. Specifically, this electrical noise signal on the power line  118  is a summation of the drive frequency of the power supply  116  and the non-linear plasma discharge current-to-voltage (IV) characteristic of the generated plasma in the chamber  110 . As the discharge mode changes within the plasma, the generated electrical noise signal on the power line  118  also changes. In accordance with the teaching of the invention, the electrical noise signal on the power line  118  can therefore be used to analyse the condition of the plasma discharge process in the chamber  110 . 
     To provide for such analysis, the system  100  comprises a sensor which may for example be based on capacitive sensing and include a capacitive means, such as a capacitive clamp  125  for sensing electrical noise on the power line  118 . The capacitive clamp provides as an output an analog signal indicative of the sensed characteristic. This analog signal is coupled via an analog to digital convertor to a processing means such as that provided in a computer where a comparable digital signal may be analysed. To provide for such coupling, a preferred implementation provides for the capacitive clamp  125  to be electrically coupled to a sound card  128  installed on a computing means, typically, a laptop computer  130 . It will be understood that the term sound card is intended to include any audio card or interface to a computer such as those provided as a computer expansion card to facilitate the input and output of audio signals to/from a computer under control of one or more computer programs. The sound card  128  may be operably coupled to the laptop computer  130  through a SPDIF (Sony/Phillips Digital Interface) or other suitable interface depending on the specific arrangement of sound card used. Alternatively the sound card, may be incorporated into the mother board of the laptop  130 . 
     As shown in  FIG. 2 , an exemplary sound card  128  that may be usefully employed within the context of the present invention typically comprises four ports, namely, a microphone socket  129 , a headphone port  127 , a MIDI port  131 . A volume actuator  132  is provided on the soundcard  128  for controlling the volume of an output signal to headphone socket  127 . The capacitive clamp  125  is coupled to the MIDI port  131  for delivering the electrical noise signal to the sound card  128 . An audio mixer  134  is provided on the sound card  128  for mixing the analog signal received from the capacitive clamp  125  with other signals allocated to the sound card  128 . The audio mixer  134  mixes the analog signal with an audio signal for facilitating audio analysis thereof. An analog-to-digital converter  136  is also provided on the sound card  128  for converting the analog noise signal received from the capacitive clamp  125  via the audio mixer  134  into a digital signal consisting of binary code with a time stamp. A bus  140  interconnects the analog-to-digital converter  136  with a processor  138  of the computer  130  such that the processor  138  receives the digital signal outputted from the analog-to-digital converter  136 . As the digital signal is in binary format and includes a time stamp it is suitable for digital processing which is carried out by the processor  138 . The software module (firmware) installed on the computer  130  instructs the processor  138  of the laptop  130  to manipulate the digital signal in a predetermined manner. 
     The capacitive clamp  125  operates like a parallel plate capacitor which is charged by the electrical noise on the power line  118 . The analog-to-digital converter  136  on the sound card  128  reads the analog voltage signal generated by the capacitive clamp  125  and converts the electrical noise signal into a comparable digital signal. In an exemplary arrangement shown in  FIG. 3 , the capacitive clamp  125  is integrated with a junction box  133 . The junction box  133  comprises a power line  135  extending between a first BNC connector  137  and a second BNC connector  139 . The first and second BNC connectors  137 ,  139  are used to releasably couple the respective opposite ends of the power line  135  to the power line  118  of the plasma generator  105  between the chamber  110  and the power supply  116 . The capacitive clamp  125  may be provided in the form of a copper clad cable  143  which surrounds or encapsulates the power line  135  intermediate the first and second BNC connectors  137 ,  139 . An output line  145  is electrically coupled at one end to the copper clad cable  143  and terminates at the opposite end in a third BNC connector  147 . The third BNC connector  147  is releasably coupled to a cable  149  interconnecting the sound card  128  and the capacitive clamp  125  together. In this way, the sound card  128  is able to receive the electrical noise signal on the power line  118  as captured as a voltage signal by the copper clad cable  143 . While a preferred arrangement for a capacitive clamp has been described it will be understood that the capacitive clamp  125  can be provided in numerous alternative arrangements, for example, as a copper foil wrapped around the power line  118 . Indeed any sensor that can be used to sense the electrical noise provided on the power line could be usefully employed within the teaching of the present invention and it is not intended to limit the teaching of the present invention to a capacitive clamp. 
     Typically audio sound cards have an input impedance of 1 kΩ to 2 kΩ. As such it will be understood that when sound cards are used as a spectrum analyzer, the input impedance influences the measured circuit and short-circuits low frequency input signals. As a consequence commercial oscilloscope probes, which are designed to operate at impedances of about 1 MΩ cannot be directly used. To enable usage of such probes, the present invention provides for use of a buffer  150 , such as that shown in  FIG. 4 , that may be operably coupled between the third BNC connector  147  and the sound card  128  so that the output signal from the capacitive clamp  125  is buffered prior to reaching the sound card  128 . The buffer  150  isolates the input impedance of the soundcard  128  from the output impedance of the capacitive clamp  125 . 
     The buffer  150  may be provided by a unity gain amplifier  151  which includes inverting and non-inverting inputs and an output. In such an arrangement, an input capacitor  152  is coupled between the capacitive clamp  125  and the non-inverting input of the amplifier  151 . An output capacitor  153  is coupled between the output of the amplifier  151  and the sound card  128 . A resistor divider  154  coupled between a Vdc power supply and ground is connected to a intermediary node  155  common to the input capacitor  152  and the non-inverting input of the amplifier  151 . A feedback path comprising a feedback resistor  156  in parallel with a feedback capacitor  157  is provided between the inverting input and the output of the amplifier  151 . The input and output capacitors  152 ,  153  provide DC isolation up the rated values of the capacitors. 
     To enable an analysis of the digitised representation of the sensed capacitive signal it is desirable that the computing device incorporates some analysis software. In an exemplary arrangement, the software module installed on the computer  130  may include atmospheric pressure plasma spectrogram software which has been programmed in the National Instruments LabVIEW 8.2 graphical programming language. The programme is designed around the LabVIEW Express sound acquire.vi and the Order Analysis Toolkit (OAT) Spectral Map (water fall).vi. The program may be used to provide the captured information in the frequency domain. The program transforms the signal from the capacitive clamp  125  from the time domain to the frequency domain. Viewing the electrical noise and acoustic noise in the frequency domain allows specific frequency bands to be allocated a registration tag that corresponds to a specific part of the circuit. The specific part being mechanical, electrical or plasma related. The amplitude verses frequency data is visually displayed as a function of process time using the OAT waterfall plot.vi and displayed on the colourmap.vi. A High pass filter (1-2 KHz) is used to remove very low frequency environmental noise that swamps the plasma signal. A peak search programme is used to numerically display all peaks within a given frequency span and above a given threshold value. The OAT spectral map.vi collects the spectral data by defining a block of data into a specified number of frames. Each frame equals a set number of FFT points to create a spectrum waveform. These frames (waveforms) are manipulated into a colormap display. The data is saved to a LabVIEW measurement (.lvm) file as a block or as a Microsoft Excel file. The block contains the header information and the number of specified frames in columns. 
     In operation, the capacitive clamp  125  is connected to the power line  118  by the first and second BNC connectors  137 ,  139  so that the power line  135  of the junction box  133  is in series with the power line  118  of the plasma generator  105 . The sound card  128  is coupled to the capacitive clamp  125  via the third BNC connector  147 . The power supply  116  applies a voltage across the electrodes  115  such that a discharge current flows between the electrodes  115  to effect plasma discharge in the chamber  110 . The mechanical, electrical and chemical components of the plasma generator  105  results in an electrical noise signal on the power line  118  between the power supply  116  and the electrodes  115 . The capacitive clamp  125  is charged by the electrical noise on the power line  118 , and the resulting voltage signal is fed to the analog-to-digital converter  136  on the sound card  128 . The analog-to-digital converter  136  reads the analog voltage signal from the capacitive clamp  125  and converts it into a digital signal which is fed to the processor  138  of the computer  130  via the bus  140 . The software module installed on the laptop computer  130  analyses the digital signal. The digital signal which is representative of electrical noise signal on the power line  118  is suitable for digital processing and is used to analyse the condition of the plasma discharge process in the chamber  110 . 
     Referring now to  FIG. 5  there is illustrated another system  200  in accordance with the teaching of the present invention. The system  200  is substantially similar to the system  100 , and like components are indicated by the same reference numerals. In this arrangement, instead of providing the sensor as a capacitive clamp  125  as described heretofore, the sensor is provided as a microphone  205  for capturing an acoustic characteristic (sound energy) associated with the plasma generated by the plasma generator  105 . The present inventors have realised that plasma in the pressure range of between 200 Torr and 760 Torr supports acoustic and electro-acoustic pressure waves and that these energy pressure waves may be detected by use of a microphone. The microphone  205  is coupled to the microphone socket  129  of the sound card  128  by a cable  208 . The analog-to-digital converter  136  receives the analog audio signal from the microphone  205  and converts it into a digital signal including binary code and a time stamp which is suitable for digital processing. The digital signal outputted from the analog-to-digital converter  136  is representative of sound energy associated with the plasma. It will be appreciated by those skilled in the art that the microphone  205  provides a non-invasive and cost effective method for monitoring plasmas that are driven at high voltage levels (1 to ±15 kV). Monitoring a plasma driven by such high voltages would be dangerous for an operator using the capacitive clamp  125  as there is a risk of the operator being electrocuted, however as the microphone is a passive device that does not require direct coupling to the plasma generator it can be safely used for such scenarios. The microphone  205  may be omni-directional or directional. Typically, plasma generators are located in noisy environments and a directional microphone may be used to de-select unwanted environmental noise resulting from electrical fans, motors, etc. 
     In operation, the microphone  205  is placed in the proximity of the plasma generator  105  outside the chamber  110 . The microphone  205  picks up electrical and electro-acoustic energy generated by the plasma and generates an electrical signal representative of the detected energy which is then fed to the analog-to-digital converter  136  on the sound card  128 . The software module installed on the computer  130  is programmed to analyse the digital signal outputted by the analog-to-digital converter  136 . It will be appreciated by those skilled in the art that two or more microphones  205  may be used to capture the acoustics associated with the plasma. Where provided as two microphones, one of the microphones  205  may be a mono-microphone and the other a stereo-microphone, or both microphones may be stereo. 
     Referring now to  FIG. 6  there is illustrated another system  300  in accordance with the teaching of the present invention. The system  300  is substantially similar to the system  100 , and like components are indicated by the same reference numerals. Instead of providing the sensor as a capacitive clamp  125 , two sensors are provided. The first sensor is provided as a microphone  205  such as described in the system  200  with reference to  FIG. 5 , and the second sensor is provided as an image capture device for capturing an image associated with the generated plasma. A suitable image capture device is for example a digital camera  305 . The digital camera  305  is coupled to the computer  130  via a USB port, serial, port or LAN port. The processor  138 —described previously with reference to FIG.  2 —operates as a cross referencing means which cross references the digital images from the digital camera  305  with the electro-acoustic data obtained from the microphone  205 . By combining the output of the camera and the sound card it is possible to generate an audio-visual display of both images and audio data. It will be appreciated by those skilled in the art that a video camera with a live feed may be used instead of the digital camera  305 . 
     In operation, the digital camera  305  is placed proximal to the plasma generator  105  but outside the chamber  110 . The digital camera  305  captures an image of the plasma in the chamber  110  through the transparent walls of the chamber  110  which is fed to the sound card  128 . The camera  305  may also be placed in the proximity of an expanding plasma plume that is eminating out of the plasma chamber  110 . A software module installed on the digital camera  306  may be accessed by the processor  138  to provide machine vision data such as object recognition (micro discharge, streams and arcs). The computer  130  may be suitably programmed to analyse the digital image received from the camera  305  and display the digital image on the visual display unit of the computer  305  beside the processed audio data. 
     Referring now to  FIGS. 7   a ,  7   b  and  7   c , there is illustrated another system  400  which comprises two sensors. The first sensor is provided by the capacitive clamp  125  as described in the system  100 . The second sensor is provided by the microphone  205  as described in the system  200 . The microphone  205  picks up sound energy originating from the plasma. 
     Atmospheric pressure plasma (APP) devices are powered by power supplies using a number of drive frequency bands. These drive frequency bands are typically, in the ranges of 16 kHz to 24 kHz, 10 kHz to 100 kHz and the Industrial Scientific and Medical (ISM) frequency of 13.56 MHz. The maximum pass-band of most commercial sound cards is 48 kHz. The first drive frequency band of 16 kHz to 24 kHz may be sampled within the soundcard frequency pass-band. However, the second frequency band of 10 kHz to 100 kHz band breaches compact disk (CD) specification. The ISM frequency of 13.56 MHz is also outside the maximum pass-band of soundcards. To sample these out of band drive frequencies a radio frequency mixing circuit  215  may be used to generate an intermediate frequency signal (IF) within the soundcard pass-band by mixing the frequency signal received from the capacitive clamp  125  with a sampling signal (fs). Sound cards designed for DVDs have a pass-band which may accommodate the second frequency band. 
     The mixing circuit  215  comprises a radio frequency mixer  165  which mixes the drive signal (fd) received from the capacitive clamp  125  with a sampling frequency (fs). A voltage control oscillator (VCO)  168  operates as a local oscillator (LO) for providing the sampling frequency (fs). The sampling frequency (fs) is set by a dc voltage supply  230 . The radio frequency mixer  165  mixes the sampling signal (fs) with the drive signal (fd) and generates an intermediate signal (IF) which is within the pass band frequency of the sound card  128 . The sampling frequency (fs) is provided by a voltage controlled oscillator (VCO)  168 . For example, when the drive frequency (fd) from the capacitive clamp  125  is 13.562 MHz, the mixer  165  mixes the drive signal with a sampling frequency (fs) of 13.56±100 kHz such that the intermediate signal (IF) fed to the sound card  128  is approximately 20 kHz. The intermediate signal (IF) of approximately 20 kHz is within the pass band frequency of the sound card  128  and is fed into one of the stereo channels of the soundcard  128 . The intermediate signal (IF) is passed to a splice circuit  224  which merges the Mono intermediate signal (IF) signal received from the radio frequency mixer  165  with a Mono audio signal received from the microphone  205 . A stereo jack  228  which is plugged into the microphone socket on the sound card  128  receives the two signals from the splice circuit  224 . A monitor  233  may be coupled by a coupler  235  intermediate the VCO  168  and the radio frequency mixer  165  for visually monitoring the signal from the VCO  168 . To provide the correct voltage level for each stage of the down conversion, an amplifier is placed at the radio frequency mixer  165  input and attenuation pads  240  are placed on the mixer RF and IF ports. The software module installed on the computer  130  is programmed to cross reference the electrical noise captured by the capacitive clamp  125  with the audio sound energy captured by the microphone  205 . Thus, the software module operates as a cross referencing means. Cross referencing audio sound energy with plasma electrical noise floor, drive oscillator phase noise (at the fundamental or harmonic) with plasma-surface interactions provides a powerful process diagnostics. 
     The use of the radio frequency mixer  165  is a cost effective method of acquiring out of band frequency information. When the drive frequency is between 50 kHz and 2.45 GHz, the radio frequency mixer  165  is used to capture harmonic related and non-harmonic electrical noise. A software utility such as that provided by the program LabVIEW (version 8.2 or greater) may be used to process and display the down conversion data together with the audio data but on separate frequency traces. 
     Referring now to  FIG. 8  there is illustrated another system  500  in accordance with the present invention. The system  500  is a combination of the systems  100 ,  200 ,  300 . Like components are indicated by similar reference numerals. Instead of providing a single sensor as described in each of the systems  100 ,  200 , and  300 , the system  500  comprises three sensors. The first sensor is provided by the capacitive clamp  125  as described in the system  100 . The second sensor is provided by the microphone  205  as described in the system  200 . The third sensor is provided by a low cost USB camera  510  with VGA resolution that has no onboard processing software. The digital camera is connected to the computer  305  using a USB port. 
     In operation, the digital camera  510  is placed in the proximity of the plasma generator  105  outside the chamber  110 . The digital camera  305  captures an image of the plasma in the chamber  110  through the transparent walls of the chamber  110  which is fed to the sound card  128 . The camera may also be placed in the proximity of an expanding plasma plume that is emanating out of the plasma chamber  110 . The digital image from the camera  510  is displayed on the visual display unit of the computer  130 . This arrangement allows visual cross referencing with the electrical and electro-acoustic processed information. The optical image of the plasma provides visually perceptible information on the plasma state. 
     Referring now to  FIG. 9  there is illustrated another system  600  in accordance with the teaching of the present invention. The system  600  is substantially similar to previously described systems and like components are indicated with similar reference numerals. The main difference is that the system  600  includes a photo-acoustic cell  605  in fluid communication with a plasma source  610  and defines a measuring area  618  for receiving ionised atmospheric gas (plasma) from the plasma source  610 . The atmospheric plasma under test is surrounded by the cylindrical (or similar) open-ended cell  605  or chamber whose dimensions are chosen to produce acoustic resonances in the kHz range. In this exemplary arrangement the cell  605  is cylindrical and its resonance frequencies (f jmq ) can be calculated using the following equation: 
     
       
         
           
             
               
                 
                   
                     f 
                     jmq 
                   
                   = 
                   
                     
                       c 
                       2 
                     
                     · 
                     
                       
                         [ 
                         
                           
                             
                               ( 
                               
                                 
                                   α 
                                   jm 
                                 
                                 R 
                               
                               ) 
                             
                             2 
                           
                           + 
                           
                             
                               ( 
                               
                                 q 
                                 L 
                               
                               ) 
                             
                             2 
                           
                         
                         ] 
                       
                       
                         1 
                         / 
                         2 
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   1 
                 
               
             
           
         
       
     
     Where: 
     f jmq  is the resonance frequency of the cell, 
     c is the velocity of light 
     R is the radius of the cell, 
     L is the length of the cylinder, 
     α jm  is the j th  zero derivative of the m th  Bessel function divided by π, 
     q is the longitudinal mode index, 
     m is the azimuthal mode index, and 
     j is the radial mode index. 
     In operation of the system, a photo-acoustic signal is generated inside the cell. The cell  605  acts as a cavity resonator and provides for the amplification of the photo-acoustic signal. The dimensions, specifically in this exemplary arrangement the radius and length of the cell  605  facilitates the generation of standing wave patterns and resulting resonance frequencies, which are representative of the characteristics of the plasma within the cell. 
     The gas is ionised with an electrical drive circuit  615  with appropriate electrode configuration such that plasma is formed prior to entering the cell  605 . The cell  605  maybe fabricated from an insulating material (plastic) or a conducting material (metallic). If the cell  605  is fabricated from a conducting material the inner wall  640  of the cell  605  is anodized/insulated in order to prevent plasma arcing to the cell walls. An exciting means, in this case, a light source  625  excites the plasma in the measuring area  618 . The excitation light causes a heat release from the atmospheric plasma/gas in the measuring area  618  due to the relaxation of absorbed light energy through molecular collisions. The release of heat in the measuring area  618  results in the generation of acoustic energy and thermal waves. Thus, the cell  605  may be considered to be an acoustic generator. 
     The generated acoustic energy is recorded with one or more microphone(s)  205  arranged relative to the plasma to detect acoustic energy in the measuring area  618 . The microphone  205  is in communication with a sound card  128  installed on a computer  130  which converts the acoustic energy to digital data for facilitating digital processing of the acoustic energy. The recorded acoustic energy is a measure of the energy absorbed which depends on the intensity of the excitation light and also on the characteristics of the plasma in the measuring area  618 . The intensity of the excitation light may be determined and as such the system may be used for providing a characterisation of the plasma in the measuring area  618 . For example, the characteristics of the plasma in the measuring area  618  may be determined from the acoustic energy generated in the measuring area  618  using the following equation. 
     
       
         
           
             
               
                 
                   F 
                   = 
                   
                     1 
                     
                       2 
                        
                       π 
                        
                       
                         √ 
                         L 
                       
                        
                       
                           
                       
                        
                       C 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   2 
                 
               
             
           
         
       
     
     Where: 
     f is the change in frequency of the power source of the drive circuit, 
     L is the inductance of the measuring area, and 
     C is the capacitance of the measuring area. 
     As the frequency of the generated acoustic energy in the can be measured, the only unknown is the equation 2 is LC. However, L is made up of a cell component Lc and a plasma component Lp. Similarly, C is made up of a cell component Cc and a plasma component Cp. Thus, equation 2 may rewritten as shown in equation 3: 
     
       
         
           
             
               
                 
                   f 
                   = 
                   
                     1 
                     
                       
                         ( 
                         
                           Lc 
                           + 
                           Lp 
                         
                         ) 
                       
                        
                       
                         ( 
                         
                           Cc 
                           + 
                           Cp 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   3 
                 
               
             
           
         
       
     
     As Lc and Cc are product of the cell  605  and drive circuit  615  f is the change in frequency of the power source caused by the plasma inductance (Lp) and the plasma capacitance (Cp) in the measuring area  618 . Lc and Cc may be changed by adjusting the experiment set up. For example, the length, colour, diameter and the material of the cell  605  may be varied. It will therefore be appreciated by recording f for different experimental setups and subtracting these readings the contribution of plasma may be obtained. 
     The light source  625  may provide a modulated or pulsed light beam to the measuring area  618 . The light source  625  may be a light emitting diode (LED) or laser or any other suitable means. The modulated light beam may be modulated using for example a sine or a square wave of varying duty cycles, or of some other modulation type. The light source could also be frequency modulated and of any wavelength, or indeed a continuum spectrum. The excitation light may also be polarized. In general the aim is to match the wavelength of excitation light source to the absorption frequency of the plasma and the duration to the life time of the plasma. The light beam emitted by the light source  625  is usually narrow so as to avoid reflections from the sidewalls of the cell  605 . In many situations, the excitation light beam may be coaxial with the longitudinal axis of the cell or parallel to the longitudinal axis of the cell. Alternatively, the excitation beam may be at angle relative to the longitudinal axis of the cell. An image capture device, in this case, a webcam  305  may be provided to record graphical images of the plasma in the measuring area  618 . These images may be provided to the computer  130  via a USB port, serial, port or LAN port. The processor  138 —described previously with reference to FIG.  2 —operates as a cross referencing means which cross references the digital images from the webcam  305  with the audio data obtained from the microphone  205 . By combining the output of the camera  305  and the sound card  128  it is possible to generate an audio-visual representation of both images and audio data extracted from the measuring area  618 . The capacitive clamp—also described previously with reference to  FIG. 2  is coupled to plasma drive circuit  615  for delivering an electrical noise signal representative of plasma in the measuring area  618  to the sound card  128 . The analog-to-digital converter  136  on the sound card  128  reads the analog voltage signal generated by the capacitive clamp  125  and converts the electrical noise signal into a comparable digital signal. The processor  138  is also operable to cross reference the digital signal derived from the electrical noise with both the digital images from the webcam  305  and the acoustic data obtained from the microphone(s)  205 . By using feeds from three sensors (microphone, camera, and capacitive clamp) enables the simultaneous measurement of three characteristics of the plasma in the measuring area  618 . 
     It will be appreciated that any desired number of microphones may be used to record the generated acoustic energy and may be located in the measuring area or externally thereof. As illustrated in  FIG. 10  two microphones  205  are provided in the measuring area  618  of the cell  605 . While in  FIG. 9  a single microphone  205  is provided and located on the wall of the cell. It will be appreciated by those skilled in the art that the positions and quantity of microphones  605  can be varied to optimise the photo-acoustic output to the sound card  128 . 
     It will be understood that what has been described herein are some exemplary embodiments of systems for analysing plasma. The plasma may be atmospheric plasma or liquid plasma. While the present invention has been described with reference to some exemplary arrangements it will be understood that it is not intended to limit the teaching of the present invention to such arrangements as modifications can be made without departing from the spirit and scope of the present invention. In this way it will be understood that the invention is to be limited only insofar as is deemed necessary in the light of the appended claims. 
     Similarly the words comprises/comprising when used in the specification are used to specify the presence of stated features, integers, steps or components but do not preclude the presence or addition of one or more additional features, integers, steps, components or groups thereof.