Patent Publication Number: US-2005142035-A1

Title: Micro-discharge sensor system

Description:
The present application claims priority as a continuation-in-part to co-pending U.S. Nonprovisional application No. 10/749,863, filed Dec. 31, 2003, by Ulrich et al., and entitled “MICRO-PLASMA SENSOR SYSTEM”, which is incorporated herein by reference. 
    
    
     BACKGROUND  
      The present invention pertains to detection of fluids. Particularly, the invention pertains to plasma structures, and more particularly to the application of the structures as sensors for the identification and quantification of fluid components. The term “fluid” may be used as a generic term that includes gases and liquids as species. For instance, air, gas, water and oil are fluids.  
      Aspects of structures and processes related to fluid analyzers may be disclosed in U.S. Pat. No. 6,393,894 B1, issued May 28, 2002, to Ulrich Bonne et al., and entitled “Gas Sensor with Phased Heaters for Increased Sensitivity,” which is incorporated herein by reference.  
      Related art fluid composition analyzers may be selective and sensitive but lack the capability to identify the one or more components of a sample mixture with unknown components, besides being generally bulky and costly. The state-of-the-art combination analyzers GC-GC and GC-MS (gas chromatograph—mass spectrometer) approach the desirable combination of selectivity, sensitivity and smartness, yet are bulky, costly, slow and unsuitable for battery-powered applications. In GC-AED (gas chromatograph—atomic emission detector), the AED alone uses more than 100 watts, uses water cooling, has greater than 10 MHz microwave discharges and are costly.  
      Micro gas chromatography (μGC) detectors should be fast responding (&lt;1 ms), sensitive but not selective to specific compounds, of simple construction and low-cost, compact, and low-power (˜mW). Presently available or conceived μGC detectors are either not very sensitive, such as thermal conductivity sensors (&gt;10 to 100 ppm of analyte); too selective to specific compounds such as fluorescence and electron-capture detectors; relatively high-cost such as the typical price tags in year 2003 of about $600, $3000 and upwards for many GC detectors; prone to drift due to soiled optics as micro-discharge devices (MDDs) monitored via spectral analysis; or relatively high-power such as the AEDs (atomic emission detectors) which consume over 100 W.  
      Related art NO x  (and to an extent NH 3 , SO x , CO x , O 2 , VOC, and the like) sensors to monitor and/or control such emissions from (internal and external) combustion processes are not suited for use in unsupervised, stationary or automotive combustion systems. They are either too costly (chemiluminescence (CL) and even multi-layered ZrO 2  sensors), too bulky (chemiluminescence and IR absorption if the detection limit is to be near 5 ppm), too fragile (CL and IR long-path cell) or not stable enough (SnO 2 /WO 3  and wet-electrochemical sensors) or too costly especially for automotive applications. Other known problems of optical sensors is their high maintenance cost, as needed to keep the optics clean, and the short life of the electrical contacts to any electrical-powered sensor exposed to harsh combustion exhaust conditions  
      Related art optical gas sensors (NO, CO, NH 3 , SO 2 , CH 4 , . . . , CWA) based on spectral analysis of glow discharge emission are not suited for compact, low-cost, wide-wavelength-range packaged systems because they lack a rugged, low-cost and compact multi-channel analyzer. They are either too costly and bulky (e.g., FTIR or conventional dispersive spectrometers, or even new, compact palm-top-size spectrometers), too fragile (spectrometers), not transmissive enough (narrow band-pass filters need fairly good collimation of light to avoid band broadening, i.e., need low aperture operation resulting in low-light transmission) or not versatile enough (small number of channels with individual, narrow band-pass filters). Also, a problem of these optical sensors is their high maintenance cost, such as keeping their optics clean.  
     SUMMARY  
      The invention may be a sensor system having a discharge gap formed by electrodes. A fluid to be sensed may enter the vicinity of a discharge at the gap. An optical coupling may include a waveguide proximate to the discharge gap. Cleanliness of the optical coupling and one or more electrodes may be maintained by the discharge. A processor may be coupled to the waveguide. The electrodes and waveguide may have various configurations and arrangements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       FIG. 1  shows a micro discharge device optically coupled to an optical multi-channel analyzer based on light inputs through interference filters;  
       FIG. 2  is a close view of a discharge gap to optical fiber interface;  
       FIG. 3  shows a discharge gap housing attached to an exhaust pipe;  
       FIG. 4  shows a micro discharge device optically coupled to an optical, single-channel, wavelength-modulated analyzer based on a scanning Fabry-Perot filter;  
       FIG. 5  is a close view of the Fabry-Perot type analyzer;  
       FIG. 6  shows a micro discharge device optically coupled to a spectrometer;  
       FIG. 7  is a graph of the relative intensity versus wavelength for a spectral emission of a glow discharge with a mixture of NO in N 2 ;  
       FIG. 8  is a table of angular sensitivity data for materials of various refractive indexes;  
       FIG. 9  is a table of Fabry-Perot filter design parameters for wavelength modulation in gas sensing;  
       FIG. 10  is a graph of a wavelength scan of a Fabry-Perot filter;  
       FIG. 11  is a sensor system having a silica chip to support the micro discharge electrodes;  
       FIG. 12  is a sensor system having no silica chip to support the micro discharge electrodes;  
       FIG. 13  shows a sensor situated in a spark-plug-like package;  
       FIGS. 14 and 14   a  are cross-section diagrams of sensors revealing electrode enclosures;  
       FIG. 15  is a cutaway of the a sensor showing overlapping edges of the electrodes;  
       FIG. 16  shows a sensor having a concentric electrode relative to a light waveguide;  
       FIG. 17  is a cross-section view of concentric electrodes forming an annular discharge gap;  
       FIGS. 18   a  and  18   b  are top views of an optical fiber and a pair of electrodes separated by a number of optical fibers;  
       FIG. 19   a  is a side cross-section and top view of two electrodes separated by an optical fiber; and  
       FIG. 19   b  is a side cross-section and top view of two concentric electrodes separated by a light waveguide. 
    
    
     DESCRIPTION  
      The present optical spectral/molecular emission-based NO (and other chemicals) sensor system may be a low power, low-mass and compact (the emissive glow discharge plasma of each element may be 10 to 100 microns in diameter). The system may have its plasma operate at about 1100 degrees C. Also, the system may be low-cost, rugged (no precision optical alignments needed) and maintain operational stability various kinds of environments. With adequate air filtering, sensor system operation may occur without noble gas purging, such as for exhaust gas composition measurements, along with high temperature plasma self-cleaning, signal processing and advantageous low-cost, compact and rugged packaging.  
      MDD may be used for optical transmission surface cleaning and for maintaining electrode isolation in an MDD detector application; that is, the same plasma discharge may be used to keep the observation window clean, by plasma-etching away any combustion-product deposits such as condensable tars and carbon-soot. The same or a similar glow discharge may maintain cleanliness and (more importantly) the required electrical isolation of the soot-sensor electrodes of soot sensors. One may co-locate a spectral-emissive and a soot sensor in one package. In other words, it is compatible and easy to integrate with soot sensor systems.  
      The silica chip may be eliminated and the discharge may be operated between two free-standing electrodes. The same plasma discharge may maintain the required electrical insulation of the non-grounded micro-discharge electrode (magnified view of one example electrode tip in  FIG. 2 ), or achieve such insulation by periodic power-cleaning cycles, which may or may not cause a pause in the measurement and the self-check cycle.  
      With little power, an electrostatic field across the impactor “baffles” or between the “fins” of the cyclone element can improve the capture and retention of the smaller particles. Housing with louvers (and cyclonic and impactor particle separators) may be less costly than the sensor system frit. Integrated particle removal via cyclone and impactor plate may occur with low Δp to sample gas flow ( FIG. 14 ).  
      Smart positioning between the end of the optical fiber and the photodiode may be used to detect optical fiber light components of small angles, as required by the chosen bandpass filter width.  
      The present system may be more compact, rugged and lower cost than chemiluminescence-based sensor systems. It may be more stable than metal-oxide or catalyst-based and conventional optical sensor systems and less energy consuming than ZrO2-based sensor systems. The present system may be more tolerant to temperature change than other sensor systems, and more manufacturable than multi-layer ZrO2, metal oxide or catalyst based sensor systems.  
      The present system may be lower cost than previous MDD-based NO x  sensor systems. It may permit observation of NO spectral emissions in the IR. Also, it may allow co-planar design with one MDD as source and another as detector.  
      Concerns about water condensation may be obviated with removal or preferably made harmless via sensor heating. Another sensor may be packaged into the same housing of the system to reduce cost, total bulkiness and incorporate plasma-cleaning synergies.  
      Spectral analysis of the MDD emission may rely on a scanning, narrow band-pass, MEMS Fabry-Perot (FP) filter, i.e., it is compact, versatile (having many channels), highly effective light intensity (despite the high mirror-etalon reflectivity if many (100 to 1000) MDDs are operated in parallel) and low-maintenance because the FP-filter operates in a sealed environment, and the only other optical surface exposed to sample gas is self-cleaned by the MDD.  
      A micro discharge device (MDD)  11  is shown in systems  10 ,  20  and  30  of  FIGS. 1, 4  and  6 , respectively. Device  11  may have one electrode  31  and another electrode  32  with ends facing each other to form a gap for providing a micro glow discharge  18 . The gap may be enclosed in a glass tube or hollow pipe  33 . Device  11  may have a soot electrode that may be kept clean of soot build-up. The glow of device  11  may have a UV/visible spectrum as shown in a graph of  FIG. 7 . That graph shows relative intensity versus wavelength in nm for a spectral emission of a glow discharge using 22.9 ppm of NO in N 2  in an environment of 700 Torr. To date, noble gases (N 2 , Ar, He) have been used to study the characteristics of such micro discharges.  
      The glow discharge device  11  may be a part of system  10  as illustrated in  FIG. 1 . The system may consist of the building blocks as outlined in  FIGS. 1, 11 ,  12  and  13  (like  FIG. 3 ). System  10  may have a sample gas filter  13  connected to an exhaust pipe  14  at an opening  15 . Filter  13  may remove PM (particulate matter) and condensables from an exhaust sample  16  from exhaust  17 . Then sample  16  may flow into the vicinity of glow discharge  18  situated in a glass pipe  33  and affect the emission of the discharge according to the composition of sample  16 . Light  27  from discharge  18  may propagate through fibers  21 , filters  22  and be converted to electrical signals by detectors  23 . The electrical signals may go to amplifiers and microprocessor  24  to be processed into output signals indicating the composition of sample  16 .  
      Glow discharge  18  may be about 10 to 500 microns in diameter. The discharge may be started and sustained with about a 100 to 400 volt AC/DC power supply in series with about a 1 to 15 Meg-ohm resistor  19 , which generates the spectral band emissions shown in  FIG. 7 . Power supply  28  may be connected to metal electrode  31  via resistor  19  and to metal electrode  32 . The glow discharge  18  may be started and maintained between electrodes  31  and  32  due to the presence of the voltage from the power supply  28 . Electrodes  31  and  32  may be coated with an insulative material  46  such as, for example, MgO. Other materials may be used.  
      Optical fibers  21  may be optically connected to the glow discharge device  11  at optical interface or window  25  and be used with filters  22  for NO at 247.2 or 258.8±1.4 nm, a reference N 2  at 336.9 or 357.5±2 nm, other band pass filters for O 2 , CH, C 2 , CO, SO 2 , as needed, and off-NO and N 2  at 251.2±2.5 and 362.3±4 nm, respectively. The optical filters  22  may be deposited at the flattened ends of the optical fibers  21 , which would have narrow band pass half-width of about three nm (to match the ˜2.8 nm NO emission half bandwidth (HBW)) to 20 nm. Also shown in  FIGS. 1, 11  and  12 , are photo detectors  23  (Si-diode, Si-phototransistor, sensitized for UV) proximate to filters  22 . Outputs of the photo detectors  23  may go to amplifiers and signal processor  24  which may output a referenced signal about NO, VOC, CO, SO x , or the like in the sample  16 , with a ppm indication signal at output  35  of amplifiers and processor  24 .  
      For operation, device  11  may be designed to force the micro discharge  18  to glow close to and impinge on the side of the observation fibers  21 , as shown in  FIG. 1 . The mild discharge  18  sputter action may be intended to maintain a high level of optical transmission of the window  25  in  FIG. 1 , despite the known tendency of combustion exhaust gases to darken optical surfaces they come in contact with, in a short time. However, there may be cleaning action on the window  25  by the plasma of discharge  18 . Also, the electrodes may be kept clean.  
      Significant elements of the system  10  in  FIG. 1 , and other systems described herein, may include optical fiber-cables  21  with deposited filters  22  at their ends with the other ends facing the glow discharge  18 , and the PM filter or filters  13 . Materials of these fiber and filters may include those that are low-cost, temperature resistant (not a high need due to the intermediate PM filter  13 , which may cool sample gas temperatures) and of a high index, in order to minimize the angular sensitivity of the band-pass filters  22 , which may be given by a few exemplary filters described in-terms of peak transmission wavelength, λ o , vs. deviation angle, φ, of the incident beam from one parallel to the fiber axis: 
 
λ φ =λ o ( n   e   2 −sin 2  φ) 0.5   /n   e . 
 
      This influence of the index, n, on λ φ  is illustrated by the data in the table of  FIG. 8 , for λ o =250 nm and φ=10° and 20°. The highest index of the listed materials, i.e., sapphire with n=1.845 at 250 nm still may cause a shift by about 5 nm for φ=20°, but only about 1.5 nm for φ=10° (see  FIGS. 2 and 14  for positioning the fiber  21  at some distance from window  25 , so that φ≦10°), which may be one approach. Another one would be based on using a wider band-pass filter that covers all bands of NO with a half-width of 23 nm: λ o =247.2±12 nm; the down-side of this approach is an approximately 5× loss in NO sensitivity and a greater probability for cross-sensitivity to other gases that might have spectral emissions in that same band. If this approach is chosen, the manufacturability, cost of the filter and its shifts due to angular and temperature variations may become less critical.  
      The concern about the influence of temperature is based on the fact that λ o  tends to shift to longer wavelength with increasing temperature (and vice versa) due to the thermal expansion of the coating materials, as suggested here. 
 
λ T =λ o +α ΔT, 
 
 with α˜0.01−0.2 nm/deg. C. 
 
      This may shift λ o  by 10 nm for only a 100 degree C. rise in temperature and α=0.1 nm/deg. C., if the above information is correct. One would expect a value for α′˜10 −6 −10 −5 /deg. C. or α˜2·10 −4 −2·10 −3  nm/deg. C.  
      It may be useful to calculate the maximum diameter, d, possible for a single-mode optical fiber, which also may have a more limited acceptance angle, which could keep the band-pass half-width of an associated interference filter small. For single mode optical fiber operation, the quantity V&lt;2.405, where V=(πd/λ) {n(core) 2 −n(clad) 2 )} 0.5 , so that the d&lt;2.405·λ·{n(core) 2 −n(clad) 2 )} 0.5 , which for an example based on sapphire (n=1.6) optical fibers, operation near 300 nm, and a Δn˜0.3 would require that d&lt;673 nm. For single-mode fibers, the numerical aperture (=sine of largest acceptance angle, which is half-angle of the cone within which the light is totally internally reflected by the fiber core), NA=0.15 for single mode fiber and 0.3 for multi-mode fibers. 
 
 NA =sin( q   max )=( n   1   2   −n   2   2 ) 0.5 . 
 
      Manufacturing costs may be low due to inexpensive parts and assembly as preliminarily noted here. The parts may include one grounded and one insulated wire in a tube  33  (glass, quartz, sapphire) to support the plasma in a spark-plug-like environmental package  44  as shown in  FIG. 3 , optical fibers  21  with deposited interference filters  22 , two to four Si photo-diodes  23 , a power supply  28  with a DC-to-DC converter (100-400V), an amplifier  24  for the photo-diodes  23 , and a microprocessor  24  for signal processing and logic functions, a PM filter  13  and sample gas flow channels. Also, automated assembly and calibration may be implemented to reduce costs. A very little scrap would be expected from the making of the present micro-plasma sensor system  10 .  
      NO x  sensing via MDD may have been done by others, with noble gas purge in one micro channel leading to the MDD, but has not been done without such purge, directing only the sample gas to the MDD. Features of the sensing system in  FIG. 1  include: operating the MDD without noble purge gas; using MDD for window cleaning and for maintaining electrode isolation in an MDD detector application; observing no spectral emissions in the IR; designing a co-planar MDD as source another MDD as detector; and co-locating a spectral-emissive and, for example, a soot sensor in one package.  
      There may be self-cleaning of the optical surface  33  on the MDD side and facing the optical fiber, i.e., window  25  of  FIGS. 1, 2 ,  4  and  6 . No noble gas purge cleaning is needed. The sensor system  10  may include use of plasma discharge device  11  for exhaust gas composition measurements, but without noble-gas purge; use of the plasma discharge  18  to keep the observation window clean, by plasma-etching away any combustion-product deposits such as condensable tars and carbon-soot; use of the same plasma discharge to maintain the required electrical insulation of the non-grounded micro-discharge electrode (see magnified view of one example electrode tip in  FIG. 2 ); use of a plasma discharge to maintain the required electrical insulation of the non-grounded electrode by additional periodic power-cleaning cycles, which may or may not cause a pause in the measurement and the self-check cycle; use of an associated PM filter  13  to cool and clean the sample gases after soot sensing but before spectral MDD sensing, in order to minimize temperature-induced wavelength shifts in the bandpass filter; use of smart positioning between the end of the optical fiber and the photodiode to detect only optical fiber light components of small angles, as required by the chosen bandpass filter width; and use of the same or a similar glow discharge  18  to maintain cleanliness and (more importantly) the required electrical isolation of the soot-sensor electrodes (not shown  FIGS. 1-3 ).  
      Additional design features related to quasi state-of-the-art PM filters may include mechanisms for overcoming concerns about water condensation (removal or made harmless via sensor heating), and packaging the soot sensor electrode into this same housing to reduce cost, total bulkiness and plasma-cleaning synergies.  
      Another implementation of glow discharge device  11  is system  20  shown in  FIG. 4 . A scanning Fabry-Perot filter  26 , shown with more detail in  FIG. 5 , may be adapted to the band pass and wavelength range desired for the desired application. A PM filtered gas  16  may enter the glow discharge device  11  and enter the vicinity of the glow discharge  18 . Discharge  18  may be enclosed in a glass capillary or pipe  33 . The discharge  18  may be started and sustained by a voltage of about 100 to 400 volts from power supply  28  connected to electrodes  31  and  32  from which the discharge emanates. A light pipe  34  or other optical conveyance mechanism may be optically connected to the glass pipe  33  at a window  25  to carry the light  27  of the discharge to a non-dispersive, Fabry-Perot, narrow band-pass, scanning filter  26 . Filter  26  may provide a spectral analysis of the light  27 .  
      Filter  26  may be a Fabry-Perot (FP) based MEMS spectrometer for MDD emission analysis. Light pipe  34  may be optically coupled to a Pyrex or quartz window  36  of filter  26 . Window  36  may be a UV blocking filter. As shown in  FIG. 5 , light  27  may propagate through window  36  into a FP cavity having about a 5 mil (25 micron) high cavity  37  with an etalon  38  that may move up or down to adjust cavity  37  to a particular frequency of interest to be passed through or filtered out. The movement of etalon  38  may be effected with a control signal line  45 . This adjustment may determine the wavelength of light  27  to be passed or blocked. Cavity  37  may be formed with a sapphire base  38  and window  36  with an environmental hermetic seal  39  formed around the perimeter of cavity  37  to provide space in the cavity and a seal between window  36  and sapphire base  38  to seal the cavity from its environment. The portion of light  27  that passes through cavity  37  may be sensed by an array of detectors  41 . The detectors  41  may be in a form of a linear or another kind of array, and be composed of AlGaN/GaN or other appropriate or workable material. Detectors  41  may convert the light signals  27  into electrical signals that are input into a readout integrated circuit  42 . Circuit  42  may have a processor to analyze the signals to provide information about the sample gas  16 . A package  43  may be utilized overall to enclose at least a portion of filter  26 . The output of circuit  42  may provide a spectral analysis of light  27 . This analysis may imply the composition of the sampled gas  16  passing through the glow discharge  18 .  
      During operation of filter  26 , one may envision that only one (and not many in parallel) tine (=transmission peak of the Fabry-Perot comb-filter) of about 1 nm to 3 nm half width does the scanning, while the others may be designed to be outside of the scanning area. The table in  FIG. 9  shows parameters of FP-based wavelength modulation for gas sensing. It gives some examples of the FP-filter design parameters needed to accomplish this application of the MMD as well as for other applications (CO and O 2  sensing). The parameters shown in  FIG. 9  may include the gas sensed, band center, tine spacing, line width, ν/Δν, FP spacing, dither, band limits and finesse, among other parameters.  
      As the FP-spacing layer  38  of cavity  37  is dithered by a given amount, the Δλ line-width band-pass may scan around the band center by ± the tine spacing in cm −1  or nm, or between the shown band limits in nm. The computed Fabry-Perot band width and spectral position (and including the response of the AlGaN detector array) for the last row in the table in  FIG. 9  may be shown in  FIG. 10  for the minimum, center and maximum wavelength position, respectively, with the corresponding etalon mirror spacing.  FIG. 10  shows percentage of transmission versus wavelength for a wavelength scan of a MEMS FP filter. The wavelength position may be limited in the computed example in  FIG. 10  by the available wavelength sensitivity range of the AlGaN detectors, which is about 290 to 360 nm.  
      Features of system  20  in  FIG. 4  may be taken as exemplary emission bands for which the scanning FP-filter and detector  26  would need to achieve the following measurement performance (λ and ±Δλ/2): with NO at 247.2 or 258.8±1.4 nm, reference N 2  at 336.9 or 357.5±2 nm, and off-NO and N 2  at and 251.2±2.5 and 362.3±4 nm, respectively.  
      One may consider the known influence of f-number on achievable FP-filter  26  finesse, which may be even more constraining here. However, one may design the FP-filter  26  to be less sensitive to temperature-induced drift of the wavelength band-pass, but also limited by the temperature range rating of the discharge device  11 .  
      The sensor system  20  may be based on the following: plasma micro discharge device (MDD) for gas sensing via spectral emission analysis of unknown gas mixture samples, using non-dispersive (Fabry-Perot-based) spectral analysis (rather than a dispersive spectrometric analysis) or interference filters; the Fabry-Perot (FP) wavelength scan performed via a MEMS-based FP-filter design; new use of the above assembly (of MDD and FP-based spectral filter) as high speed gas chromatography peak (GC) analyzer, and independently, as stand alone gas sensor for NO, O 2 , SO 2 , . . . in one unit; new use of above assembly (MDD+FP+GC), whereby the GC is a μGC or a μGC-μGC or a μGC-μGC-MDD gas mixture analyzer, of low probability for false positives, P fp ; and a design of the MDD in which the discharge self-cleans the window  25  and operates without a noble gas purge.  
      Successful implementation of systems  10  and  20  may enable the achievement of low false positive probabilities when using this discharge device  11  and detector as part of a GC-CG-MDD micro-analyzer, as represented by PHASED.  
      The sensing systems  10  and  20  may offer the following advantages over previously proposed or offered exhaust gas composition sensing systems. They are more compact, rugged and lower cost than chemiluminescence-based sensor systems. They are more stable than metal-oxide or catalyst-based and conventional optical sensor systems. They are less energy consuming than ZrO 2 -based NO and O 2  sensor systems and more temperature change tolerant than other ZrO 2 —NO/O 2  sensor systems. They are more manufacturable than multi-layer ZrO 2 , metal oxide or catalyst based sensor systems. They are compatible and easy to integrate with a soot sensor system.  
      System  30  of  FIG. 6  may have a discharge gap device  11 , like that of systems  10  and  20 , except that light  27  may be conveyed via a light pipe  34  to a dispersive spectrometer  47  for analysis of the emission of the discharge  18  to reveal information about the sample gas  16 . Light  27  may be conveyed to an optical grating  48  for reflection of various wavelengths of light  27  to various pixels, respectively, of a CCD light detecting array  49 . Electrical signals from array  49  may go to a processor  51  for analysis and interpretation.  
      One sensor system is depicted in  FIGS. 11, 12 ,  13 ,  14  and  14   a,  while other systems are shown in  FIGS. 1, 2  and  6 . The MDD (micro discharge device)  60  of  FIG. 11  may generate an optical emission  56  that is characteristic of the gaseous environment around the electrodes  53  supported by a ˜2×2 mm silica chip, as protected from particulates of exhaust gases  65  by a screen or stainless frit  55  which may be regarded as an enclosure or housing for at least partially containing the electrodes and the light waveguide or waveguides. Exhaust gases  65  may be turbulent and reach temperatures up to 1100 degrees C. (2012 degrees F.). MDD  60  may be housed in a spark plug type of package or housing  57  like that shown in  FIG. 13 . The housing  57  may be threaded to be in a fitting  58  having, for example, M14 threads  61 . Or the fitting  58  may be formed or welded to an exhaust pipe  59  or other fixture that may have a fluid to be tested. Housing or package  57  may have a hex fitting like a nut or bolt so that the housing and package may be screwed in or out with a tool such as a wrench.  
      Light may be conveyed from discharge  56  through fiber  63  to optical filters  22 . Fiber  63  may be a silica fiber having an outside diameter of about 20-200 microns. The filters  22  may have a delta wavelength of about 2-5 nm. The filtered light may proceed on to be detected by photo diodes, phototransistors or generically “light detectors”  23 , on a one filter to detector basis, respectively. Electrical conductors may connect the detectors  23  to terminals  115  of connector  64 . Terminals  115  may be connected to a processor  24  (as shown in  FIG. 1 , though not shown in  FIGS. 11 and 12 ).  
      A mode of failure could be via contact problems between the electrical leads  62  fed through or around the silica chip  54  and those embedded in the spark plug package  57 . Alternatively, there may be a simple spark plug housing package  57  which includes a pair of self-supporting discharge electrodes  53 , or the ends of leads  62  which may be electrodes, a discharge  56  that maintains the optical fiber  63  inlet surface  64  clean, and without a MDD silica chip  54 . The electrodes may be kept clean also. This cleaning of surface  64  and electrodes may occur without the presence of a noble gas. The leads  62  may be extended from connector  64  to electrodes  53 . The length of the leads  62  and waveguide  63  may be about 10 to 40 cm.  
      To avoid contact problems with the electrodes  53  and leads  62 ,  FIG. 12  shows another version of the sensor package, where the silica chip  54  may be eliminated, with the electrical leads  62  coming through the spark plug housing  57  and extending into the exhaust gas side of the sensor. The leads  62  may be strong enough to hold their position and shape without the need of the silica chip  54  to support them. There may be an alumina insulator  69  about the leads  62  and waveguide  63  within a package, housing or structure  57 . The separate discharge electrodes  53  which were fashioned out of deposited thin-film metal material may be dispensed with in favor of the ends of leads  62 , the latter of which may be regarded as substituting electrodes  53 . The shape and space between the latter electrodes may be formed via mechanical cutting, shaping and bending, to result in the typical electrode shape and spacing of 10 to 30 microns, not unlike self-supporting spark-plug electrodes. There may be savings in eliminating the cost of the fabricating the silica chip  54 , its feed-throughs for leads  62 , the deposition of the electrodes (ends of leads  62 ) with contact metals, chip dicing, and the positioning and bonding to the spark-plug-like housing  57  feed-throughs. The ends of the leads may be regarded as electrodes  53  or  62 .  
      There may be an elimination of potential performance problems caused by less than the ideal and clean optical interface between the silica chip  54  and the optical fiber  63 , and a reduction of the losses in transmission through the interface between the silica chip  54  and fiber  63 .  
      During operation, successful system designs may cause the micro discharge  56  to glow close to and impinge on the side of the observation optical surfaces, such as those of the silica fiber  63 , chip and/or window. The mild discharge sputter action may be intended to maintain a high level of optical transmission of the optical transmission surface, despite the known tendency of combustion exhaust gases to darken optical surfaces they come in contact with, in a short time, due to a deposition of tarry and soot-containing materials. Such is the cleaning action of discharge plasmas. Excessive discharge action may etch the material of surfaces close to the glow discharge, while insufficient action (i.e., power) may create deposits.  
      A capillary electrode discharge (CED) approach may be used to generate atmospheric plasmas, and surface cleaning such as the removal of organic material on glass substrates. Helium or hydrogen (He or H 2 ) may be used as an ignition and discharge gas and oxygen (O 2 ) be added as a reactive gas. Low frequency AC power supply with a sine wave voltage (20 kHz to 20 GHz) may be used to generate plasmas under atmospheric pressure. The electrodes may be composed of a 10 micron thick capillary dielectric and have a diameter of about 300 microns. He/O 2  plasmas generated in the space of a few mm between the capillary electrode and the substrate (ground) may be uniform and very stable. The optical emission spectroscopy and the I-V characteristic of the discharges may be used to characterize the capillary electrode discharges. A removal of organic materials such as photoresist, in addition to He/O 2  plasmas, the effects of other gases like those of He additive gases such as CF 4 , Ar, and N 2 , may be implemented. Cleaning rate of organic material on glass higher than 100 Å/min may be observed after exposure to He/O 2  plasmas. There may be some effects of various gas mixtures in addition to those of He/O 2  on the cleaning rate of organic material and surface chemical composition of the remaining residue as might be measured by x-ray photoelectron spectroscopy.  
       FIG. 14  shows a sensor system  80  having a housing or enclosure  66  with a particle suppression structure  67  having louvers, a sample gas input/output, and a swirl and cyclone separator. Housing or enclosure  66  may be regarded as containing at least partially the electrodes and the light waveguide or waveguides. Structure  67  may have impactor plates  68  for aiding separation of particles of fluid  65 . The flow of exhaust  65  through structure  67  may be induced by head pressure and venturi action, assisted by components of enclosure  66 , including a 0.3 mm opening at the top of the enclosure or structure inserted in the flow of the fluid or exhaust gas  65 . Also, fluid or exhaust  65  may be rather turbulent in the pipe or conveyance  59 , and structure  67  may provide a way of sampling exhaust  65  for the micro discharge  56  without disrupting the latter for an accurate reading with sensor system  80 . Electrodes  53  may provide the basis for the micro discharge  56 . The reaction of the discharge  56  to the fluid  65  being sensed may be optically transmitted by optical fiber  63  to a light detector for conversion to electrical signals. The signals might be digitized for computer processing and analyses of the fluid. The light conveyed from the discharge  56  may be IR, visible and/or UV. Leads or electrodes  62  and fiber  63  may be contained and structurally supported within housing  57  having a ceramic insulator  69 . Electrodes  62  may be attached to connector pins  71  which may have a 0.3 mm dimension. The discharge  56  may keep the electrodes and the input surface of waveguide clean without the need of noble gas purging. The filters, detectors, power supply and processor associated with system  80  may similar to that of systems  60  and  70 .  
       FIG. 14   a  shows a sensor  81  having the same kind of structural and electrical parts as sensor system  80 , except that the former has a housing that is somewhat different. Structure  82  may have an input of fluid  65  from the sides and a procession of the fluid through a number of impactor plates and into the micro discharge  56  area, and an exit out through the end or top of housing or structure  82 . On the other hand, structure  82  may be designed to have both an input and output of fluid at the top of it. Systems  60 ,  70 ,  80  and  81  may be used to sense other fluids besides exhaust gases  65 .  
      There may be ruggedized MDD sensor systems.  FIGS. 16, 17 ,  18   a  and  18   b  show sensor configurations that expand on those in  FIGS. 14 and 14   a.  The preferred range of the electrode  62  gap  83  for atmospheric MDDs may be 30 to 70 microns.  FIG. 15  details the electrode gap  83  area shown in  FIG. 14 . The shape of the two electrodes  62  at the discharge location may enable the spacing between the two electrodes to be within a desirable range, after being kept apart by the thick optical fiber  63  cladding. However, a “hook” at the electrode  62  tips may add unacceptable cost during manufacture, and the cavity may plug up with deposits during use in a car exhaust environment.  
       FIG. 16  shows an arrangement  113  that may avoid the plugging and spacing problem by using a tubular bottom electrode  84  and a (grounded) top electrode  85 , with a tip  86 , which lines up with the centered optical fiber  87 , having a thin (≦λ/2) cladding  88 . The fiber with the cladding may have a diameter  116  of about 70 microns. The risk of 30 to 70 micron discharge gap misalignment may be high and possibly costly to overcome. The gas discharge area may be represented by the two elliptical, shaded areas  89 .  
       FIG. 17  shows an arrangement  114  having a qualitative (not to-scale) side view of a tube-shaped optical fiber  91 , where the discharge (elliptical shaded areas  92  shown to represent the cross section of a ring-shaped or annular micro discharge) occurs between the center  93  and outer  94  electrodes and over the top rim of the tube-“fiber”. The tube-fiber  91  may be replaced with a number of optical fibers adjacent to each other parallel to one another to the electrodes. The overall diameter of the device  114  may be about 3 mm. The center electrode  93  may have a diameter of about o.5 mm. The tube-fiber or optical fibers  91  may have an outside diameter of about 0.6 mm.  
       FIGS. 18   a  and  18   b  show arrangements  111  and  112 , respectively, with a single 40 to 70 micron diameter  117  optical fiber  95  with cladding of a small thickness, and four of such fibers  95  having positions and being fastened between two larger-diameter metal electrodes  96  and  97  of about 300 to 500 microns (0.3 to 0.5 mm) in diameter. Both electrodes  96  and  97  may be anchored within the same ceramic block  69  as shown in  FIG. 14 . The discharge light  56  may be aligned with the optical fibers  95 , and the assembly may be manufactured with relative ease. The use of separate fibers  95 , besides lending mechanical flexibility, also enables design flexibility on how to separate different wavelength channels (via band-pass filters  22  of  FIG. 1  or a dispersive element).  
       FIGS. 19   a  and  19   b  represent two versions  101  and  102  of the sensor system related to  FIGS. 17, 18   a  and  18   b,  with respect to capturing the discharge light  103 , ease of manufacture (gap adjustment and stability, despite temperature fluctuations, via a common ceramic anchoring block) and design flexibility. Version  101  shows electrodes  104  and  105  with a fiber  106  between the electrodes  104  and  105  to convey light from discharge  103  of the assembly  101 . One may note that the tube-shaped optical fiber  107  in  FIG. 19   b  may be broken into or replaced with a number (10 to 20) of individual fibers  108  that are positioned along a circular order or path about the central electrode  109 . A circular discharge  103  may be initiated between the center electrode  109  and circumferential electrode  110 . The circular fiber arrangement  107 ,  108  may convey the light from the discharge  103  of the assembly or arrangement  102 . Arrangement  102  may bear much resemblance to arrangement  114  of  FIG. 17 , except that the tubular fiber  91  of arrangement  114  may be in lieu of a plurality of fibers  108  in arrangement  102 .  
      The arrangements  112  and  102  shown in  FIGS. 18   b  and  19   b,  respectively, appear to provide a significant area for discharge  92  and  103 , and optical fiber  91  and  107 ,  108 , for conveying light of the discharge from the arrangements to appropriate optical and electrical mechanisms for signal processing and analyses.  
      Although the invention has been described with respect to at least one illustrative embodiment, many variations and modifications will become apparent to those skilled in the art upon reading the present specification. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.