Anal. Chem. 1906. 58.923-932
CONCLUSION This chemiluminescence detector, based on the gas-phase reaction of molecular fluorine with organosulfur compounds, has proven to be highly selective and sensitive when operating in either the gas chromatography or liquid chromatography mode. Much work remains to be done to elucidate the reaction mechanisms; however, it appears that compounds containing a reduced sulfur atom give the best response in the detector. Compounds containing sulfur atoms in or adjacent to ring structures respond moderately, and highly oxidized sulfur species respond poorly. Applications work has been limited, and we anticipate that the detector will be useful for the analysis of sulfur-containing biological compounds, petroleum products, food and drug samples, pesticide residues, and atmospheric samples in both the GC and LC mode. Efforts are currently under way to adapt the detector to a capillary GC system for the analysis of dimethyl sulfide and dimethyl disulfide in atmospheric gas samples (24). The detector could probably be interfaced to a supercritical fluid chromatographic (SFC) system, since the solvents typically used in SFC do not respond in the wavelength region of interest when reacted with F2. The use of molecular fluorine as the reagent gas in the detector is the major drawback to the general use of the system. We are currently investigating the potential use of nontoxic sources such as SF6and CF4, which are known to produce F2when passed through a microwave discharge (2) and may also produce F2 when passed through an electrical discharge such as an ozonizer. ACKNOWLEDGMENT The authors wish to thank Richard A. Henry of Keystone Scientific, Inc., for much assistance with the use of microbore systems and John Larmann from Du Pont for supplying the 3-pm Zorbax. We. especially thank Udo Brinkman for suggesting the adaptation of the organosulfur GC detector to HPLC. Registry No. Fz, 7782-41-4; 2-mercaptoethanol, 60-24-2; ethanethiol, 75-08-1; o-aminobenzenethiol, 137-07-5;2-methyl2-propanethiol,75-66-1; 1-butanethiol, 109-79-5;1-hexanethiol, 111-31-9; 1-octanethiol, 111-88-6;dimethyl sulfide, 75-18-3; di-
923
methyl disulfide, 624-92-0; ethyl sulfide, 352-93-2; allyl sulfide, 592-88-1; phenyl sulfide, 139-66-2;tert-butyl sulfide, 107-47-1; n-butyl sulfide, 544-40-1; n-butyl disulfide, 629-45-8; dimethyl selenide, 593-79-3;thiophene, 110-02-1;2-mercaptoethanol, 6024-2; Malathion, 121-75-5. LITERATURE CITED
(21) (22) (23) (24)
Nelson, J. K.; Getty, R. H.; Birks, J. W. Anal. Chem. 1983, 55, 1767-1 770. Nelson, J. K. Ph.D. Dissertation, University of Colorado, Boulder, CO, 1984. Cooke, N. H. C.; Olsen, K.; Archer, B. G. LC Mag. 1984, 2 , 514-524. Popovlch, D. J.; Dlxon, J. 6.; Ehrllch, 8. J. J . Chromafogr. Sci. 1979, 17, 643-650. Plrkle, W. H.; House, D. W. J . Org. Chem. 1970, 4 4 , 1957-1960. Hill, D. W.; Walters, F. H.; Wilson, T. D.; Stuart, J. D. Anal. Chem. 1979. 57, 1338-1341. Reeve, J.; Kuhlenkamp, J.; Kaplowitz, N. J . Chromatogr. 1980, 794, 424-428. Reed, D. J.; Babson, J. R.; Beatty, P. W.; Brodie, A. E.; Ellis, W. W.; Potter, D. W. Anal. Eiochem. 1980, 706, 55-62. Plrkle, W. H.; House, D. W.; Flnn, J. M. J . Chromafogr. 1980. 792, 143- 158. Bishop, C. A.; Kltson, T. M.; Harding, D. R. K.; Hancock, W. S. J . ChrOtMtOgr. 1981, 208, 141-147. Ingebretsen, 0. C.; Farstad, M. J . Chromafogr. 1981, 270, 522-526. Skaaden, T.; Grelbrokk, T. J . Chromafogr. 1982, 247, 111-122. Shlmada, K.; Tanaka, M.; Nambara, T. Anal. Chim. Acta 1983, 147, 375-380. Van Langenhove, H.; Van Acker. M.; Scharnp, N. J . Chromafogr. 1983, 257, 170-173. Bossle, P. C.; Martln, J. J.; Sarver, E. W.; Sommer, H. 2. J . ChromafOgr. 1984, 283, 412-416. Duewer, W. H.; Setser, D. W. J . Chem. Phys. 1973, 58, 2310-2320. Bogan, D. J.; Setser, D. W. J . Chem. Phys. 1976, 64, 586-602. Jones, W. E.;Skolnik, E. G. Chem. Rev. 1978, 76, 563-592. Getty, R. H.; Blrks, J. W. Anal. Lett. 1979, 72, 469-476. Glinskl, R. J.; Nelson, J. K.; Birks, J. W. Chem. Phys. Lett. 1985, 177, 359. Glinskl, R. J.; Mishalanie, E. A.; Blrks, J. W., unpublished work. Glinskl, R. J.; Mlshalanle, E. A.; Birks, J. W. J . Am. Chem. Soc., In press. Bogan, D. J; Setser, D. W.; Sung, J. P. J . Phys. Chem. 1977, 87, 888-898. Tawnier, J., National Center for Atmospheric Research, personal communlcation, 1985.
RECEIVED for review August 5,1985. Accepted October, 25, 1985. This work was funded by the Environmental Protection Agency, Grant No. R-810717-01-0, and was performed in partial fulfillment of the Ph.D. degree (E.A.M.) from the Department of Chemistry, University of Colorado, Boulder,
co.
Theta Pinch Discharge Designed for Emission Spectrochemical Analysis: Design and Electrical Characterization Gregory J. Kamla a n d Alexander Scheeline* School of Chemical Sciences, University of Illinois, 1209 West California Avenue, 79 RAL, Box 48, Urbana, Illinois 61801 A theta plnch dlscharge was deslgned and constructed as a hlgh-temperature plasma source for appllcatlon to the emlb slon spectrochemlcal analysis of solld samples. Thls source has been operated up to 32 kV (1.33 kJ), with a peak dlscharge current of 60 kA. The peak magnetic fleld generated by thls discharge through a double helical colt Is 28 kG, resultlng In a calculated peak plasma current of 32 kA. Thorough electrical characterlzatbnand deelgn spectflcatlons are reported.
Several capacitive discharges have been used as plasma sources for analytical emission spectroscopy. These discharges 0003-2700/86/0358-0923$01.50/0
have found application in the analysis of solid materials, due to their ability to directly sample solids with minimal sample preparation. The high-voltage spark discharge has been used for many years as an analytical source for multielement emission spectrochemical analysis (1-3). Spark discharges have the ability to directly analyze solid, conductive materials. Spark generated working curves are in many instances nonlinear and dependent on sample matrix, thus requiring standards matched to the sample type. Reductions in sample matrix effects have been realized by increasing the plasma source energy. A less common analytical plasma source, which employs a higher energy discharge than is used for the high voltage spark, is the electrically vaporized thin film (4). This source does not require the sample to be one of the discharge 0 1988 Arnerlcan Chemlcal Soclety
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
electrodes and can therefore directly analyze both conducting and nonconducting materials. When applied to refractory powder samples, electrically vaporized thin films have yielded linear working curves over several orders of magnitude, with few or no matrix effects for sample particle sizes less than 30 wm diameter (5). Theta pinch discharges use magnetic compression to produce plasmas of high density and temperature. In this configwation, a single-turn coil is located outside of and concentric with an insulated discharge vessel. To produce a theta pinch plasma, typically two discharges are employed, a preionizer discharge and a main discharge. The preionizer discharge produces a diffuse, ionized plasma within the discharge vessel. The main discharge sends a fast, high current pulse through the single turn coil, causing compression and heating of the plasma created by the preionizer discharge. The current through the coil induces a current within the plasma, and a current sheet forms around the plasma. This plasma current sheet acts to shield the inner plasma region from the magnetic field produced by the coil. Since the magnetic field is then greater outside the plasma current sheet, there is a net inward magnetic force which compresses and heats the plasma (6, 7). Originally designed for thermonuclear fusion research, theta pinch machines have been used to produce plasmas with Emission observed from these temperatures up to 1keV (8,9). plasmas ranges from the infrared to the X-ray region, with neutron emission also possible under the appropriate conditions. In fusion studies, it has been observed that one of the major mechanisms for energy loss is radiation by impurities ablated from the discharge vessel walls and from other materials placed within the discharge vessel ( 1 0 , I I ) . While this effect is undesirable in fusion research, it may be of analytical utility for emission analysis of solid samples placed within the discharge vessel. The observation of impurity emission in pinch discharges, combined with the proven analytical capability of other high-energy discharges, has suggested the possible use of a theta pinch discharge as an analytical emission source. The high temperature and density of a pinched plasma, higher than is typically used analytically in other discharges, should show advantages in the analysis of solid materials. Specifically, this high energy discharge should improve the analysis of highmelting point and nonconducting solids, materials which have been difficult to directly sample previously. The purpose of the work described here is to investigate the application of a theta pinch discharge as a source for the emission analysis of solid samples. To this end, a theta pinch plasma source has been designed, built, and characterized. This is the first of two papers on this discharge source. In this paper, the design and electrical characterization of the theta pinch discharge will be described. In the second paper (12),discharge spectral characteristics will be described. The two mechanisms by which a theta pinch discharge deposits energy into a plasma are irreversible shock heating and reversible adiabatic compression. Formation of the plasma current sheet causes resistive heating of the gas at the plasma surface. The heating of this cylinder of gas, along with the constriction of the current sheet, causes formation of a high velocity shock wave, which implodes into the center of the plasma. With intense shock waves, relaxation times are very short and shocked material is rapidly ionized (13, 14). The extent of compression by the shock/current sheet is determined by the magnitudes of the magnetic field and the kinetic pressure of the plasma. Magnetically driven shocks of this type can produce translational energies greater than 100 eV per ion (15, 16). The adiabatic component of compression results from the increasing magnetic field from the main discharge. For a
slowly changing magnetic field, the magnetic moment and the action change slowly and can be considered constants of motion, or adiabatic invariants (6). If the magnetic moment is constant, as a particle travels into a region of increasing magnetic field, its velocity perpendicular to the field must increase also. Due to conservation of energy, the velocity parallel to the field must therefore decrease. If the magnetic field becomes great enough, the velocity parallel to the field must become zero, or even change sign (cause the particle to reverse directions). The plasma produced by a theta pinch is diamagnetic; the current sheet shields the inner plasma from the magnetic field of the coil. The current sheet acts as a magnetic mirror where ions and electrons are reflected from the region of high field outside the sheet and are thus magnetically confined inside the plasma. The action is defined as the momentum of a particle integrated over a closed path. For a theta pinch plasma, the current sheet sets the boundary for the closed path. As the magnetic field slowly increases, the current sheet compresses and the distance traveled by the particle around the closed path decreases. If the action is constant, this decrease in distance must be offset by an increase in particle velocity, thereby increasing the particle energy. Theta pinch discharges were first designed in the 1950s for work on controlled fusion research. Typically, the magnetic field produced by a high current pulse through a single-turn coil was used to compress and heat a plasma. Approximate values attainable with these discharges were 1keV temperatures and lo1' cm-3 electron densities. In contrast, highvoltage spark discharges at atmospheric pressure have typical temperatures and electron densities of 3 eV (17) and 6.7 X 10l8cm4 (It?), respectively. The cylindrical coils used for fusion research typically had dimensions on the order of 1 m in length and 10 cm in diameter (8, 10, 19, 20). Insulating discharge vessels, fitting within the cylindrical coil, were generally operated between 0.1 mtorr and 1torr. Preionizer discharges were used to ionize the gas within the discharge vessel prior to firing the main discharge, Spark discharge (21, 22), radio frequency (rf) discharge (23, 24), and direct current (dc) hollow cathode discharge (25)have all been used as preionizers. Such diagnostics as plasma emission, Thomson scattering, laser interferometry, and magnetic flux measurement are common. The main discharge high-current pulse, which is used to create the magnetic compression field within the theta coil, is produced by a capacitive discharge. Megajoule capacitor banks have been assembled that, when discharged, are capable of delivering over 10 MA peak current through the pinch coil (22,24,26).The duration of these discharges, and concurrent plasma lifetime, is on the order of 50 ps. The values of temperature and electron density observed in a plasma produced by a theta pinch are dependent upon the type and density of fill gas within the discharge vessel. For fusion research, the discharge vessel is generally fiied with deuterium at pressures of 0.1 to 100 mtorr. Decreasing the fill gas pressure causes an associated drop in plasma density along with an increase in particle (ion and electron) temperature (6,10). At a constant discharge energy, a plot of total radiated power vs. gas filling density yields a sigmoidal curve (27). Although greater plasma temperatures can be attained by use of low atomic number fill gases, a greater plasma compression and associated density can be attained by using a higher atomic number fill gas (28). Under identical experimental conditions, greater plasma compression has been observed for argon plasmas than for hydrogen plasmas. Due to radiative losses and increased ionization levels, the argon plasma will have a lower kinetic temperature and therefore a lower kinetic pressure. This decrease in kinetic pressure,
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4,APRIL 1986 925 relative to a hydrogen plasma with the same total energy, will result in an increase in compression by the theta pinch magnetic pressure. For a 5-torr discharge in argon, the plasma was measured t o have an electron density and electron temperature of 6 X 10le cm-3 and 3.6 eV (29). For a theta pinch plasma to be successfully applied to sample analysis, it should be able to directly sample solid materials, especially those which a t present are difficult to sample with typical analytical discharges. Several studies suggested that this was possible. Ablation of discharge vessel walls was observed in early fusion research (10). In a study of the interaction of a theta pinch plasma with the quartz walls of the discharge vessel, emission was observed from oxygen and silicon ablated from the vessel walls, whose blackbody temperature never exceeded 200 "C (30). Polystyrene spheres (50-150 pm diameter) have been placed in the center of a theta pinch discharge vessel to study the interaction of solid pellets with hot plasmas (31). After exposure to the plasma, the pellets appeared slightly elliptical and finely polished. For fill gas pressures ranging from 45 to 90 mtorr, the ablation rate was proportional to fill gas pressure, with 1-6 pm ablated from the pellet surface per discharge. Ablation has also been observed from solid end plugs used to contain the axially expanding plasma (11,26, 32-35). A theta pinch discharge constructed by Goode and Pipes (36) has been shown to increase the emission intensity of a microwave-induced plasma created within a sealed quartz tube. The theta pinch was produced by capacitive discharge through a single-turn coil of 12-gauge copper wire wrapped around the plasma tube. With 5 torr neon fill pressure, discharge of the theta pinch caused a 100% increase in the intensity of neon emission at 6506 A. In another experiment, 1% of CO was added to a 1torr neon tube. Carbon atom along with carbon monoxide molecule emission were monitored at 2479 A and 2665.3 A, respectively. Again an increase in atomic emission intensity due to the pinch was observed, but the pinch had little effect on the molecular emission. This work by Goode and Pipes is the first reported use of a theta pinch for analytical spectroscopy. EXPERIMENTAL S E C T I O N In the design of this theta pinch for analytical studies, two factors influenced the entire system design. These were the energy storage, and associated size, of the main discharge, and the compatibility of this theta pinch source with the optical rail system (37) existing in our laboratory. From a practical point of view, it was necessary to downsize the theta pinch machine for analytical, compared to fusion, applications, both with regard to energy dissipation and physical size. For example, meter long coils are not necessary for an analytical emission source. The discharge vessel and surrounding coil need only be large enough to contain the analytical sample. The plasma gas kinetic temprature sought is on the order of 10-40 eV, which is an order of magnitude above that which is commonly used analytically in such sources as the high-voltage spark. With these criteria of a smaller coil and lower temperature, the energy requirements of this system could be met using either a single large capacitor or several small capacitors, rather than a large capacitor bank. The main discharge circuit is illustrated in Figure 1. It consists of a high-voltage capacitor, a three-electrode triggered spark gap switch, a theta coil, and transmission plates connecting these components. Incorporated into the electrical design of the main discharge are two other components, a Rogowski coil to monitor discharge current and a safety crowbar. The crowbar is a component used to drain the main discharge capacitor if necessary. In order to maximize the efficiency of energy transfer between the main discharge circuit and the theta coil, the inductance of the circuit components, excluding the theta coil, must be minimized. By decrease of the total circuit inductance, the discharge oscillation frequency increases. The increase in discharge dI/dt results in an associated increase in the theta coil dB/dt, improving
D
Figure 1. Main discharge circuit. D is the main discharge coil. S is one of two crowbar switches. T, Y, and X are the triggered spark gap
electrodes. C1 is the main discharge capacitor. The triggered spark gap bias circuit consists of C2 (250pF, 60 kV), R1 (100 Ma, 45 kV, 10 W), and R2 (50 MQ, 45 kV, 20 W). Vc is the capacitor charging voltage and Vt is the spark gap trigger pulse. the coupling efficiency between the coil and the plasma. Also, by decreasing the inductance of circuit components other than the theta coil, the theta coil will have a greater percentage of the total circuit inductance, and a greater percentage of the main discharge energy can be dissipated by the plasma coupled to the theta coil field. To minimize the total circuit inductance, the inductance of the capacitor and triggered spark gap switch must be low, and rigid transmission plates, having a low inductance' and a high current capacity, are used to connect the circuit components. The capacitance value and voltage rating of the main discharge capacitor determine the energy of the main discharge. Since the efficiency of the theta pinch is dependent on the discharge oscillation frequency, a low-capacitance, high-voltage capacitor is the preferred energy storage method and was chosen for the main discharge circuit. A 2.87-kF, 60-kV capacitor was chosen. The physical characteristics of this capacitor are given in Table I. The most common shape used by others for the theta pinch coil is a cylinder. This coil can easily be constructed by wrapping a copper sheet around a cylindrical form. The main discharge was designed modularly so that different coils, having different shapes, could be used. Coils were designed using the following standards: (1) 38.1 mm coil i.d.; (2) 6.4 mm centered slot for side-on observation through the coil; (3) 25.4 mm thick by 133.4 mm wide transmission plate connection; and (4) 120.7 mm distance from coil center to end of transmission plate connection. Three different coil designs were considered, cylindrical, spherical, and helical. The spherical-shaped coil reduces axial plasma loss because of the greater magnetic field near the discharge tube ends (21). The multiple turn design has a greater inductance and generates a greater magnetic field than either of the two single-turn designs. The cylindrical coil was fabricated first. The poor coupling observed with this coil showed that greater magnetic fields were needed, and the helical coil was then designed and constructed. Because of the improved coupling characteristics of the helical coil, the following discussion will focus on the main discharge/helical coil system. No spherical coil has been tested to date. The helical coil utilizes two, four-turn solenoids connected in parallel. Current flowing through a solenoid can be described as having a circular, or theta, component and a linear, or z , component. The use of two equal coils wound in opposite z directions causes the z current components of each coil to be in opposite directions, and thus to cancel. With this arrangement, the coupling between the main discharge and a linear preionizer is minimized, thus protecting the preionizer circuit from inductively coupled current injection. The helical coil is made by wrapping 6.4 mm 0.d. copper tubing around a nylon winding form, which provides mechanical support. The inner diameter of the winding form is 38.1 mm. The form is designed so that each solenoid has an inner diameter of 50.8 mm, four turns per solenoid, and total length of the two solenoids is 114.3 mm.
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Table 1. Theta Pinch System Components Main Discharge Circuit capacitor: Model no. 32132, Maxwell Laboratories, Inc., 2.87 pf. 60 kV, 80% voltage reversal, 20 nH, 279 mm X 356 mm X 635 mm, 100 kp spark gap switch Model no. 40044, Maxwell Laboratories, Inc.. 2C-50 kV, 50 kA,0.3 C/shot. 20 nH spazk gap driver: Model no. TG-40, Impulse Electronics, Inc., 40 kV peak open circuit, 1.3-A peak short circuit, nominal 1 pa fwhm Rogowski coil: Model no. 301X, Pearson Electronics, Inc., 0.01 VIA, 50 kA peak, 400 ARMS, 0.002%/pa droop, 200 ns rise time, 3.2 Hz low frequency 3 dB point resistive crowbar: 6-ACI-RPC BBV resistors, American Components, Inc., 200 MR, 30 kV, 5 W, lo%, connected as three parallel sets of two series resistors High Voltage Power Supply voltage doubler: 1OO:l high-voltage stepup transformer; each capacitor, 5 OF600-202 Capacitors in parallel, Plastic Capacitors, Inc., each 0.002 pF, 60 KVDCW each diode. 8 VC80 diodes, Varo, Inc., 1 A forward, 8 kV, and 8 ACI-RPC BBR resistors, American Components, Inc., 100 MR, 15 kV, 3 W, 10%. parallel pairs of a diode and resistor connected in series; isolation resistor, 6-25 kR, 200 W resiators connected in series with negative output current limiting resistors: 5-250 0, 225 W, wire-wound resistors in parallel, connected in series with high-voltage transformer primary triggerable interlock: Type MR5D relay, Potter and Brumfield, 12 Vdc, 8 A, SPDT, solenoid driven by 2N2222 NPN transistor, relay connected in series with power supply interlock line Discharge Vessel Vacuum System vacuum pump: Model no. 14058, Sargent-Welch Scientific Co., Duoseal vacuum pump, two stage. 60 Lpm. thermocouple pressure gauge: Thermovac Model TR201/NPT, Leybold-Haraeus Vacuum Products, Inc. thermocouple gauge controller: Tbermovac Model TM210, Leybold-Haraeus Vacuum Products, he., 0.001-760 torr. gas inlet needle valve: Model no. 22RS4, Whitey Co.
To prevent expansion of the coil from magnetic force, a twopi- nylon cylindrical clamp is placed around the windings and form, and nylon screws are used to hold the clamp in place. The inner diameter of this nylon cylinder is made equal to the 63.5 mm outer diameter ofthe copper winding and nylon winding form and provides not only strength but also electrical insulation. Strength calculations (38)were used to determine the cylinder thickness required to withstand the worst ease 1400 PSI pressure exerted by the solenoids. A photograph of the helical coil can be found in Figure 2. In this figure, the top half of the nylon cylindrical clamp has been removed to show the copper tubing wrapped around the nylon winding form. Due to the flexibility of the copper tubing, a free standing coil would eventually droop from the weight of the nylon support pieces. Optical alignment of the coil is maintained by mounting the ends of the nylon winding form on Plexiglas vee blocks. A spark gap switcb is used to provide electrical triggering of the high voltages and high currents associated with the main discharge circuit. This switch is described in Table I (39). External connections are provided for the three electrodes, along with gas fittings used to control the internal pressure of the switch. The spark gap was purged with dry air between firings and used at a pressure appropriate for the voltage in use. To operate the switch, an external resistive divider and blocking capacitor are q w e d (40,41). The resistive divider biases the trigger electrode at onethird the voltage of the energy storage capacitor. The total potential a c r w the switch is thus divided in the ratio of the gap lengths to minimize the static stress on the two gaps before fuing. To fire the gap, a positive high-voltage trigger pulse is fed through a blocking capacitor to the trigger electrode by a spark gap driver circuit, Model TG-40.The TG-40 specifications are listed in Table I. This circuit was originally designed to accept
Fl#ure 2. WlIcal theta coil assembb (1) nelkal wlndlng; (2)n y b winding form; (3)nylon clamp (lower naif): 14) coillupper bansIrddon
plate transtion. a 7 T L trigger input but wm later modified to accept a fiber optic input trigger (42.43). To allow operation of the switch in air above 30 kV, a 229 mm diameter guard ring was fabricated from 3.2-mm Plexiglas. This guard ring was attached to the switch near the high-voltage electrode using a silicone adhesive. Addition of the guard ring allows operation of the switch up to 50 k V (40). The switch has been successfully operated at voltages between 10 and 32 kV, using operating pressures between 1.95 and 2.8 atm. After extended use of the switch. the operating pressure needs to be increased tu prevent static breakdown. The decrease in the static breakdown potential with use is caused by electrode erosion. The main discharge circuit haa been used to deliver peak current up to 60 kA, 20% in excess of the switch rating. Such operation is ac. ceptahle but decreases switch life expectancy (40). The Rogowski coil specifications are listed in Table 1. To monitor the main discharge current. the coil is placed around the center post of the capacitor, enclosed in a Plexiglaq box to insulate the coil from the capacitor cenkr post and p u n d buses. As listed in Table I, the output of the Rogowaki mil is 0.01 V/A of primary current meanured. To match the coil output with oscilloscope amplifier input modules. a lo0.I frequency compensated voltage divider circuit was fabricated. This voltage divider was modeled after the Model P6015,1OOO:l high voltage probe manufactured by Tektronix. Inc. 144). Though the coil is only rated for peak currents up to 50 kA.linearity of response was demonstrated for the 60 kA peak currents monitored. Copper transmission plates are used to provide high-current capacity, low-inductance connections between the capacitor, the spark gap switch, and the theta coil. The configuration of the components and transmission plates used in this design is illustrated in Figure 3. The length of the plates was set so that the coil extends over the optical tahle, placing the coil center at the optical system focal point. In order lo improve the stability of the coil. a support clamp was designed and placed between the coil and the vertical transmission plate. With the 25.4 mm thick transmission plates, deflections frmn mapetirally induced mechanical stresses are less than 0.02 mm. To prevent arc-over between the two closely spaced horizontal transmission plates, a 6.4 mm thick hy 184 mm wide sheet of polyethylene is placed between the sheets, providing dielectric holdoff of 112 kV. Crowbars are used to allow manual dissipation of the main diecharge capacitor energy by shorting the spark pap switth. This safety feature is used to drain the charged capacitor and is also engaged when the source is not in use to prevent accumulation of static charge on the main discharge capacitor. Two crowbars are employed in this design. The first is a low-resistance m w k constructed of copper, which provides a dead short connectiun acww the spark gap switch. and is engaged when the capacitor is at a low voltage. The second crowbar has a resistance of 133 MR and can he u d when the capacitor is fully c h a q d to slowly dissipate the capacitor energy. For component vnlues see Table 1.
The main discharge circuit is mounted with the energy storwe capacitor suspended between the optical rails using a hybrid of components described by Walters (37). Figure 3 shows the whole system assembled. Detailed blueprinrs are available from the authors, The main discharge rirmii mechanical d e s w relies on
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986 RI
I I
R2
R3
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R4
I i
Figure 3. Main discharge dlagram: (1) main dlscharge capacitor; (2) Rogowski coli; (3) spark gap switch; (4) crowbars; (5) transmission plates; (6) cylindrical theta coil; (7) discharge vessel; (8) optical rails; (9) optlcal table; (10) pneumatic isolation mount.
the optical rails not only for alignment but also for stability. The general shape of the main discharge assembly is that of an inverted triangle and utilizes the rails under the oversize riders to stabilize the assembly. A pneumatic isolation mount is used to support the weight of the discharge assembly (45). A single-phase, high-voltage power supply, which has previously been used for analytical high voltage spark discharges, was modified to charge the main discharge capacitor. This power supply used a bridge-rectified, 100:l step-up transformer to generate charging voltages up to 25 kV from a single-phase, 208-V input. The modifications made to this power supply include: (1) voltage doubler circuit in the high voltage transformer secondary to increase the maximum voltage rating from 25 kV to 50 kV (46); (2) voltage divider circuit at the high voltage output to monitor the capacitor/power supply voltage; (3) current limiting resistors in the transformer primary to limit the current during initial charging; and (4)triggerable interlock in the transformer primary for remote turn-off of the power supply before the main discharge is fired. For component values, see Table I. A circuit diagram for the frequency compensated voltage divider appears in Figure 4. The triggerable interlock is used to remotely turnoff the power supply. Just prior to firing the main discharge, a trigger pulse is sent to the triggerable interlock relay, which turns off the power supply and thus prevents immediate recharging of the main discharge capacitor after firing. Due to the speed of the relays, the input trigger pulse must remain high for at least 20 ms to be assured of power supply turnoff. The preionized plasma is produced by electrical discharge between two electrodes in the discharge vessel. Both high voltage spark discharges and glow discharges have been used as preionizers. The spark discharge, used first as the preionizer, is produced by a thyratron-triggered, adjustable waveform spark source (47). The particular source employed was built by Thang and Scheeline (48).A minor modification has been made to the spark source to decrease the time jitter in spark breakdown. The gap shunt and thyratron shunt diodes are not single diodes, but rather diode stacks composed of multiple diode elements in series, each consisting of two diodes, a resistor, and a capacitor in parallel. The original gap shunt diode stack had an overall capacitance of 1500
Figure 4. High-voltage power supply 10000: 1 voltage divider circuit: W resistor; R6, R1-4, 100 M a , 15 kV, 3 W resistor; R5, 33 kQ, 20 kQ, trim pot; C1-4,25 pF, 15 kV capacitor; C 5 0 . 0 5 2 pF capacitor; HV, high-voltage connection.
pf, and the thyratron shunt diode stack had an overall capacitance of 1200 pf. Reduced jitter was achieved with a thyratron shunt diode capacitance of 18 pF. A glow discharge has also been used to create the preionized plasma. This discharge is produced using the same three-phase power supply as used with the spark source. A current limiting resistor is connected to the high-voltage output of the power supply, and the power supply high-voltage output is then connected directly to the discharge vessel electrodes. This discharge has a lower peak current than the high-voltage spark discharge and produces a more positionally stable, diffuse discharge at higher pressures than the spark. The discharge vessel is placed inside the theta coil, and the pinched plasma is created inside this vessel. The vessel design features included (1)optical ports for viewing the discharge either end-on or side-on, (2) preionizer electrodes, one at each end of the discharge vessel, for generating the initial, ionized plasma, (3) gas fittings used for control of the type and pressure of fill gas, and (4) sample introduction hardware for reproducible sample placement within the vessel. The discharge vessel consists of a 25.4 mm diameter central glass or quartz tube with a brass cap at each end. The brass end cap has a Cajon Ultra-Torr fitting (Cajon Co., Macedonia, OH) to seal the brass cap to the glass tube. The other end of the brass cap has a flange with an O-ring vacuum seal. Various configurations of the discharge vessel can be made from solid brass flanges, flanges with quartz windows, or a flange with a sample probe. The flange with the quartz window allows end-on viewing of the discharge plasma in conjunction with a hollow preionizer electrode. The sample probe, which is shown in Figure 5, allows reproducible placement of a sample, radially centered in the discharge vessel. Three adjustment screws are incorporated into the end knob of the brass rod for fine adjustment of sample position. Sample rods have been constructed from brass and Macor (Corning Glass Works, Corning, NY), a machinable glass. With the brass sample rod, the sample is electrically connected to the brass end cap, and the sample potential thus follows that of the preionizer. With the Macor sample rod, the sample is electrically insulated. Hollow cylindrical electrodes were used for preionization. These allowed unobstructed end-on viewing, and the large orifice of the electrodes improved the pumping efficiency of the vacuum system. The preionizer electrodes are directly connected to the brass end caps. The discharge vessel fill gas is controlled by a vacuum system, which includes a pump, cold trap, needle valve, and thermocouple pressure gauge. Components are listed in Table I. The cold trap is cooled by a dry ice/ethanol mixture, which prevents back diffusion of the pump oil into the discharge vessel. In an attempt to minimize the radiated radio frequency interference (RFI) generated by the main discharge, the portion of the high-voltage section of the main discharge circuit above the optical riders is enclosed in a Faraday cage. The Faraday cage acts as a grounded antenna, collecting the radiated RFI from the main discharge, and shorting the collected signal to ground. This cage consists of brass screen attached to a Plexiglas shell with aluminum strips. One point on the brass screen is connected to the main discharge capacitor ground bus using a copper braid
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flgun 5. Discharge vessel sample probe: (1) end flange; (2) Caion Ultra-Twr filling; (3)adjushent screws; (4) Maccf sample rod; ( 5 ) alumlnum sample.
cahle. All Faraday cage feedthroughs employ attenuating stub antenna waveguides to prevent the eacape of radiated RFI through holes in the cage. Thex waveguides are made from copper tubing and are connected to the brass screen. For a circular waveguide, the waveguide critical frequency is given hy (49) = 0.289c/a
(1)
where uI is the critical frequency in hertz, c is the speed of light in centimeters per second, and a is the waveguide radius in centimeters. For a circular stub antenna waveguide the attenuation of frequenciea legs than the critical frequency is greater than loo0 for waveguides whrme length is twiee the waveguide diameter (50). The minimum cutoff frequency for the waveguides used was 2.73 GHz. Along with the Faraday cage, which minimizes the radiated RFI, a single point grounding scheme is used to minimize conducted RFI by eliminating ground loops. All electrical components associated with the preionizer and main discharges are isolated from the power line ground and grounded to the main discharge capacitor ground bus using copper braid cahle. A single piece of copper braid cahle then conneds the main discharge capacitor ground bus to dead earth ground. A control circuit has been assembled that generates trigger pulses for the two discharges and the main discharge power supply. The timing diagram for this circuit is shown in Figure 6. A zero-crossing detector circuit generates clock pulses a t 120 Hz, synchronized to the zero-crossing of the 60 Hz power line (47). The spark preionizer is triggered first and runs for 600 sparks, which at the clock rate of 120 Hz lasts for 5 s. This repetition rate is used,rather than running the spark single-shot, to decrease the time jitter in spark breakdown. Synchronous with the 6ooth, or last, spark in the train, a trigger pulse is sent to the main discharge, firing the main discharge and causing compreasion and heating of the plasma. Prior to the main discharge trigger pulse, on the 596th clock pulse, a trigger pulse is sent to the main discharge power supply, turning off the power supply. The RC time constant for the main discharge capacitor/power supply combination is such that the voltage on the capacitor decays less than 0.01% of ita peak voltage between power supply turnoff and discharge initiation. The circuit diagram for the timing circuit is shown in Figure I. The input trigger wurce is switchable between a manual push
hutton and an external fiber optic trigger input. This input trigger is connected to the trigger input of a flipflop. The paitive output of the flipflop and the clock input are connected to a NAND gate. When the flipflop is first triggered, its positive output goes high, and the clock frequency will appear at the output of the NAND gate. With this arrangement, the timing sequence can he aborted before the main discharge is fired by sending a second input trigger, toggling the flipflop low, and turning off the clock. The output of the NAND gate is sent to the counter section and to the preionizer trigger line. Two 74393 dual four-hit binary counters are used with quad NAND gates to count the 596 and 600 pulses needed for the main discharge power supply and main dischge triggers, respectively. On the 596th pulse, a trigger pulse is sent to the main discharge power supply line. On the 600th pulse, a trigger pulse is sent to the main discharge trigger line and to the stop monostable. The negative output of the stop monostable is connected to the flipflop clear line. The monrmtahle is used to hold the clear line low for the duration of the main discharge. This holds the output of the flipflop low and preventa the RFI transient, generated hy the TG-40 spark gap switch driver high voltage pulse, from restarting the timing sequence. Both the main discharge and spark preionizer trigger lines have two monrmtahles. One has a trim pot and is used to adjust the delay between the main discharge and preionizer pulses. This allows for optimization of coupling between the two discharges by appropriately t i m i i the triggering of the two discharges. T h e second monostable uses a fixed resistor to set the trigger pulse width. The main discharge power supply trigger line has only one monostable with a fixed resistor, also used to set the trigger pulse width. Each trigger line includes a 50-Rline driver, used to drive the coaxial cables initially used with this circuit to transmit the trigger pulses. This timing circuit was later modified for fiber optic transmission by installing fiber optic transmitters at the output of each line (42). This control circuit is also used with the glow discharge preionizer. In this case, the preionizer trigger pulses are not used. When the glow discharge preionizer is used, the preionizer is turned on prior to initiation of the control circuit and remains on until after completion of the main discharge.
RESULTS A N D DISCUSSION T h e preionizer spark discharge is bath spatially and temporally stable in argon at pressures up to 3.8 torr. As is the emission indescribed in the accompanying paper (E), tensity of an aluminum sample exposed to a theta pinch discharge increases as the pressure is increased to 3.8 torr. This suggested p'msible improvements in analyte sampling by the theta pinch a t higher discharge vessel pressures. Preionizer spark discharges are not stable above 3.8 torr, and therefore a glow discharge preionizer was first used to measure aluminum emission at higher fill pressures. The glow discharge produces a diffuse, ionized dc plasma within the discharge vessel. This discharge is positionally stable up to pressures of 12 torr. This upper pressure limit is set not by the stability of the discharge hut rather by the combination
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Flgun 7. Theta pinch timing circuit. All resistor values listed in
ohms. and a11 capacitor values listed in microfarads. FR and FT refer lo HewleaPackard R-2501 and T-1501 nber oDUc receivers and transmnters. respectively.
Flpura 8. Currem waveform 01 main d scharge circun: insat A. cylindrlcal coil; abscissa. 5 usldw sion: ora nate. 20 kAldwision: insat E. helical coil: abscissa. 10 #sldiv#sion:oramale. 20 kA1division.
of vacuum system gas inlet pressure. gas inlet needle valve orifice size, and pumping speed. The current waveforms for the main discharge circuit with the cylindrical coil and the helical coil are given in Figure 8, insets A and B. The peak current of the cylindrical coil discharge is 60 kA, and the peak-tc-peak current is greater than 100 kA. This discharge has an oscillation period of 4.25 us and a duration of 35 us. The peak current of the helical coil discharge is approximately .56 kA, and the peak-&peak current is greater than 100 kA. This discharge has an oscillation period of 7.6 ps and a duration of 95 us. The peak current of the main discharge circuit is limited by the 50 kA rating of the spark gap switch and Rogowxki coil. These components can operate above their rating. and the 60-kA peak current obtained with the cylindrical coil was arbitrarily set as the upper limit for the main discharge rircuit. With the helical coil, the increase in circuit inductance decrease8 not only the discharge oscillation frequency hut also the discharge peak current, at constant capacitor charging voltage. For this reason, the capacitor voltage could be increased, increasing the energy stored, while maintaining a p
F@m 8. E W of main discharge circuit on highvoltage spark curent wavefwm: inset A. spark current waveform without firing main discharge; abscissa. 5 psldivision; ordinate, 5 Aldivision: inset E. spark current waveform with synchronous firing 01 cylindrical coil main d i 5 charge; abscissa. 5 psldivision: ordinate. 5 Aldivision; inset C. Spark current wavefum WMsynchroms firing of helical coil mah bscharge; abscissa. 10 psldivision; ordinate, 5 Aldivision.
proximately the same peak current. The energy storage eapability increased from 574 J (22 kV)with the cylindrical coil to 1.33 kJ (32 kV) with the helical coil. Coupling between the main discharge circuit and the discharge vessel plasma can be observed hy monitoring the preionizer discharge current waveform. Figure 9 shows the spark preionizer current with and without firing the main discharge circuit Inset A of Figure 9 is the preionizer current without firing the main discharge circuit. Inset B of Figure 9 is the preionizer current with the cylindrical coil main
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discharge circuit fired. The timing delays of the preionizer and main discharges were set so that the main discharge circuit is triggered after the preionizer, with the main discharge circuit trigger coinciding with the time of peak preionizer current. Coupling of the main discharge circuit with the plasma is evident on the trailing edge of the preionizer current waveform; oscillations in the preionizer waveform correspond to the oscillations in the main discharge circuit current. The oscillations in the preionizer waveform are due to a decrease in plasma impedance caused by the increased ionization of the plasma by the theta coil current. Inset C of Figure 9 shows the preionizer current with the helical coil main discharge fired. Coupling is more substantial in this case. The duration of the preionizer discharge increases from 12 ps to greater than 80 ps. The oscillations in the preionizer current decay occur at the same frequency as the main discharge circuit oscillations, verifing that the increase in plasma ionization is due to the main discharge circuit. Comparison of insets B and C illustrates the improvement in ionization obtained with the multiturn, helical coil design. The assignment of the increased preionizer duration to a decrease in the plasma impedance was also verified by computer simulation of the preionizer waveform (51). The magnetic field generated by the theta coil can also give an estimate of the expected effect of the main discharge circuit on the plasma. Approximating the the\ta coil as an infinitely long solenoid, we can calculate the magnetic field using the following equation (52)
B, = 2?rIn/5b
(2)
where B, is the magnetic field in gauss, Z is the current in amperes, n is the number of coil turns, and b is the length of coil in centimeters. For the helical coil, the peak discharge current was 56 kA. Since the two parallel coils are dimensionally equal, the peak current for each coil, Z, is half the peak discharge current, 28 kA. For each coil, the coil length, b, is 5.08 cm, and the number of coil turns, n, is 4. With these values, the peak magnetic field is 27.7 kG. This contrasts with a 5.65 kG peak magnetic field for the cylindrical coil arrangement, demonstrating the greater efficiency of the helical coil. The magnetic field can be related to the plasma kinetic pressure to estimate the plasma temperature. For adiabatic compression with the trapped magnetic field within the plasma equal to zero, the gas kinetic temperature is given by
kT = /3B2/8an
(3)
where P is the ratio of the plasma kinetic pressure to the external magnetic pressure, n is the particle number density per cubic centimeter, kT is the energy in ergs, and B is the magnetic field in gauss. Under typical operating conditions, the discharge vessel was filled with argon at a pressure of 1.5 torr. At standard temperature, 1 torr corresponds to 3.24 X 10l6particles ~ m - ~If .the plasma is approximated as having a constant volume, and the argon is singly ionized, the particle density is 9.72 X 10l6~ m - ~By. use of this value for n, and assuming f? = 1, the plasma temperature is 196 eV. This calculation ignores the effect of plasma compression. If instead the assumption is made that the plasma radius is compressed to 0.2 of its original value (28),the particle density is increased by a factor of 25, and the plasma energy decreases, by a factor of 25, to 8 eV. It should be emphasized that these calculations are for a particle kinetic pressure and do not take into account the effects of excited electronic states, or multiple ionization levels. This estimate of plasma temperature is thus an overestimate. The shape of the current waveform from the main discharge circuit is that of a damped harmonic oscillator, which can be described by the following equation
(4) where t is the time, A is the amplitude constant, 7 is the damping constant, and w is the oscillation frequency. At the peak values of the wave form, sin (ut) is approximately equal to 1, and the above equation simplifies to
t, = -7 ln(lI(tp)l) + T In A
(5)
AS shown in eq 5, a plot of In (II(t,)l) VS. t , yields a straight line with the slope equal to the negative of the damping constant, -7,and the y intercept equal to r In A. If In (IZ(t,)l) vs. t, is plotted for the main discharge current waveform, the damping constant for this circuit can be computed. Waveform peak current and oscillation period data were collected by measurement of the current waveform oscilloscope traces, which were recorded photographically. A slight difference in oscillation period was observed between the two cases. With the plasma present, the oscillation period was 7.55 ps. Without the plasma, the oscillation period was 7.57 /AS. These two measured periods are almost identical, considering the accuracy of measurement from an oscilloscope photo. It is questionable if the last significant figure is valid. When a more precise means of measurement of these waveform data becomes available, the waveforms should be reacquired, and the data verified. For the following discussion, the above oscillation period values will be assumed correct. In (IZ(tp)l) was plotted vs. t, for the main discharge circuit with and without the discharge plasma. The plot showed a linear relationship exists between the two parameters at early times in the discharge, with a roll-offapparent at later times. The roll-off at later times could be due to a change in the resistance of the spark gap switch as the main discharge current decreases. To calculate the slope of the data, linear regression was used on the first 15 points of the curve, where the relationship appears to be linear. The slope calculated for the data with the plasma was -27.29 ps, and the correlation coefficient was -0.9990, For the data without the plasma, the calculated slope was -31.34 ps, and the correlation coefficient was -0.9992. The main discharge circuit has been modeled as a series RLC circuit. In this model, the capacitance has been taken as arising solely from the main discharge capacitor, and thus is assigned a value of 2.87 pF. The inductance and resistance values of the circuit were modeled as the series sum of the inductances and resistances of all the main discharge components, capacitor, spark gap switch, transmission plates, and theta coil. For an RLC series circuit, the damping constant, r , and oscillation frequency, w, are related to the circuit values of R , L , and C by the following equations (53):
R = 2L/r 1/C
L=
+ [(l/C)' 202
(6)
- (wR)~]'.~ (7)
With the known values for 7,w , and C , the above equations can be solved iteratively. Without the plasma present, the calculated circuit resistance is 32.2 ma, and the inductance is 505 nH. With the plasma present, the calculated resistance is 36.8 ma, and the inductance is 502 nH. Thus, the presence of a plasma in the discharge vessel increases the apparent main discharge circuit resistance by 4.6 ma, or 1470,and decreases the apparent circuit inductance by 3 nH, or 0.6%. The energy dissipated by the plasma can be calculated from the apparent resistance change in the main discharge circuit. The 4.6-mil resistance attributed to the plasma is one-eighth of the total resistance of the circuit. For a total stored energy of 1.33 kJ, 166 J are dissipated by the plasma. The discharge duration is 95 ps, and thus the average power dissipation by
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
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MICROSECONDS Figure 11. Computer simulated maln discharge and plasma current waveforms: A, calculated main discharge current; B, calculated plasma current.
Lp-M
RP
Main discharge circuit model with inclusion of plasma inductance and resistance: inset A, circuit model of the main discharge with inclusion of inductively coupled plasma resistance and inductance; inset B, equlvalent clrcuit of inset A used for calculations; C, main discharge capacitor; Rs, main discharge source resistance; Rp, plasma resistance; Ls, total main discharge source inductance; Lc, helical coil inductance; Lp, plasma inductance; M, helical coil/plasma mutual inductance. Figure 10.
the plasma is 1.75 MW. The uncertainty in the discharge period measurement does not affect the calculation of plasma resistance or dissipated power by more than 170. The effect of the plasma on the main discharge current waveform suggests that the simple, RLC series circuit model is no longer adequate for describing the main discharge circuit. A more appropriate circuit model, which includes the plasma inductance and resistance, is illustrated in inset A of Figure 10. This model provides inductive coupling between the main discharge circuit and the plasma. The theta coil acts as the transformer primary winding, and the plasma as the transformer secondary. An equivalent circuit, which describes this transformer model, is given in inset B of Figure 10. In this circuit, the two transformer windings have been replaced by three inductances: two self-inductances and their mutual inductance. The values of C,R,,and L, correspond to the main discharge source circuit parameters calculated from the discharge current without the plasma, 2.87 pF, 32.2 m0, and 505 nH, respectively. By use of the model in inset B of Figure 10, the changes in the circuit inductance, AL,and resistance, AR, are given by the following equations: L&PW2
AL,=
L,2w2 - R,2
(8)
R&PW2
A R = RP2- L,2u2
(9)
L,, R,, and M correspond to the circuit model plasma resistance, plasma inductance, and mutual inductance. AL and AR, previously calculated from the discharge current wave forms, are -3 nH and 4.6 m0. To solve the above equations for the plasma resistance and inductance, the mutual inductance was experimentally determined. The plasma was physically modeled with a copper cylinder having approximately the same dimensions as the plasma. This copper
cylinder was 1.6 mm thick and 152.4 mm long and had a diameter equal to the inside diameter of the discharge vessel glass tube, 22 mm. With a vector impedance meter, the inductance of the helical coil was measured both with and without the copper cylinder positioned inside the coil. From these two measurements, along with the calculated self-inductance of the copper tube of 3 nH, the mutual inductance was calculated to be 11 nH. The plasma inductance and resistance were then calculated from the above equations to be 10.2 nH and 15.7 m0. Taking the uncertainty in change of system inductance when the plasma is present of f 3 nH, the range of plasma resistances calculable is from less than zero (physically unacceptable) to 18.2 mQ. The latter figure corresponds to a net system inductance change of zero. The circuit model of Figure 10, inset B, is similar to models used to describe high voltage spark sources (54). The differential equations, which describe the discharge currents of this circuit model, have been solved in closed form. Figure 11 shows a plot of the main discharge current, and induced plasma current, obtained from this model using the circuit parameter values from above. The peak current for the main discharge circuit is calculated to be 70 kA, higher than the measured value of 56 kA. The reason for this discrepancy is unclear but may be due to a lowering of the capacitance value by changes in the capacitor plate spacings from the electrostatic stress at high voltage. The peak current for the plasma occurs on the second oscillation, rather than the first as for the main discharge. The peak value of the plasma current is -32 kA. With the plasma resistance value of 15.7 mQ, the peak power dissipated by the plasma is 16 MW. The circuit model just described assumes a constant value for plasma resistance and inductance. The plasma is not static; it undergoes changes in both physical dimensions and temperature during compression. A more accurate model would include dynamic values for the plasma resistance, the plasma inductance, and the mutual inductance between the helical coil and the plasma. Also, dynamical processes in the plasma, such as collisional energy transfer, ambipolar diffusion, and radiation would be included. Note Added in Proof. Related work has recently been reported (55). ACKNOWLEDGMENT The authors thank R. Harrison, M. Funkhouser, E. Lash, and N. Vassos for assistance in construction. LITERATURE CITED (1) Walters, J. P. Appl. Spectrosc. 1969, 23, 317-331. (2) Walters, J. P. Science 1977, 198, 787-797. (3) Scheeline, A. Prog. Anal. A t . Spectrosc. 1984, 7 , 21-65.
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(4) Sacks, R. D.; Goldberg, J. M.; Collins, R. J.; Suh, S. Y. Frog. Anal. At. Specfrosc. 1982, 5 (2-3), 111-154. (5) Goldberg, J. M.; Sacks, R. D. Anal. Chem. 1982, 54, 2179-2186. (6) Shohet, J. L. “The Plasma State”; Academic Press: New York, 1971. (7) Post, R. F. Rev. Mod. fhys. 1958. 2 8 , 338-362. (8) McKenna, K. F.; Bartsch. R. R.; Commisso, R. J.; Ekdahl, C. A.; Qulnn, W. E.; Siemon, R. E. fhys. Flu& 1980, 2 3 , 1443-1462. (9) Grlem, H. R.; Kolb, A. C.; Lupton, W. H.; Phllllps, D. T. Nucl. Fuslon 1982, (part 2), 543-551. (10) Boyer, K.; Elmore, W. C.; Little, E. M.; Quinn, W. E.; Tuck, J. L. fhys, Rev. 1980, 119, 831-843. (11) Ekdahl, C. A.; Commlsso, R. J.; McKenna, K. F. J . Appl. fhys. 1981, 5 2 , 3245-3248. (12) Kamla, G. J.; Scheeline, A. Anal. Chem., following paper in this Issue. (13) Wright, J. K. “Shock Tubes”: Wiley: New York, 1961. (14) Bond, J. W. fhys. Rev. 1957, 705, 1683-1694. (15) Kolb, A. C. fhys. Rev. 1957, 107, 345-350. (16) Kolb, A. C. Rev. Mod. fhys. 1980, 3 2 , 748-757. (17) Oberauskas, J.; Serapinas, P.; Salkauskas, J.; Svedas, V. Spectrochim. Acta, Part B 1981, 388, 799-807. (18) Scheellne, A.; Kamla, G. J.; Zoellner, M. J. Spectrochim. Acta, Part B 1984, 398, 677-691. (19) McKenna, K. F.; York. T. M. fhys. Flujds 1977, 2 0 , 1556-1565. (20) Commisso, R. J.; Grlem, H. R. fhys. Flulds 1977, 20, 44-50. (21) Panarelia, E. Can. J . fhys. 1980, 5 8 , 983-999. (22) Bodin, H. A.; Green, T. S.; Newton, A. A.; Niblett, G. B. F.; Reynolds, J. A. Plasma fhys . Controlled Nucl. Fusion Res. 1968, No. 1 , 192. (23) Jordan, H. L. Nucl. Fusion 1982, (part 2), 589-593. (24) Kolb, A. C.; Griem, H. R.; Lupton, W. H.; Phllllps, D. T.; Ramsden, S. A.; McLean, E. A.; Faust, W. R.; Swartz, M. Nucl. Fuslon 1982, (part 2) 553-559. (25) Ebihara, K. J . fhys. SOC.Jpn. 1980, 48, 958-964. (26) Commlsso, R. J.; Ekdahl, C. A.; Freese, K. B.; McKenna, K. F.; Qulnn, W. E. fhys. Rev. Lett. 1977, 3 9 , 137-139. (27) Costa, S.; DeAngelis, R.; Ortolani, S.; Puiattl, M. E. Nucl. Fusion 1982, 2 2 , 1301-1311. (28) Lee, S. Aust. J . fhys. 1983, 3 6 , 891-895. (29) Simard, P. A. Can. J . fhys. 1982, 60, 820-824. (30) Koppendorfer, W. J . Nucl. Mater. 1978, 76 & 77, 418-421. (31) Smith, D. L.; Krlstiansen, M.; Hagler, M. 0. J . Appl. Phys. 1977, 48, 4521-4527. (32) Malone, R. C.; Morse, R. L. fhys. Rev. Lett. 1977, 3 9 , 134-139. (33) DeSilva, A. W. Plasma fhys. 1979, 2 1 , 873-883. (34) Commlsso, R. J.; Bartsch, R. R.; Ekdahl, C. A.; McKenna, K. F.; Siemon, R. E. fhys. Rev. Lett. 1979, 43, 442-445. (35) Ebihara, K. Jpn. J . Appl. fhys. 1981, 20, 1135-1144. (36) W e , S. R.; Plpes, D. T. Spectrochim. Acta, Part B 1981, 368, 925-929.
(37) Waiters, J. P. I n “Contemporary Topics in Analytical and Clinlcal Chemistry”; Hercules, D. M., Hleftje, G. M., Synder, L. R., Evenson. M. A., Eds.; Plenum Press: New York, 1978; Val. 3. (38) Oberg, E.; Jones, F. D.; Horton, H. L. “Machinery’s Handbook”, 21st ed.; Industrial Press: New York, 1980. (39) Bulletin 807R, Maxwell Laboratories, fnc.: San Diego, CA. (40) Casper, D. Maxwell Laboratorles, Inc.: San Diego, CA, personal communication. (41) Engineering Bulletin No. 1007, Maxwell Laboratories, Inc.: San Dlego, CA. (42) White, J. S.; Scheellne, A., School of Chemical Sciences, University of Illinois, Urbana-Champaign, IL, unpublished research. (43) Mathews, S. E.; Ekimoff, D.; Walters, J. P. Appl. Spectrosc. 1982, 36, 617-626. (44) Tektronix Model P6015 Instruction Manual, Tektronix, Inc.: Beaverton, OR. (45) Coleman, D. M. Ph.D. Thesis, University of Wisconsin, Madison, 1976 (University Microfllms no. 77-3393). (46) Horowltz, P.; Hill, W. “The Art of Electronics”; Cambridge University Press: Cambridge, 1980. (47) Coleman, D. M.; Walters, J. P. Spectrochim. Acta, Part 8 1976, 3 18, 547-587. (48) Thang, T.; Scheeline, A. Appl. Spectrosc. 1981, 35, 536-540. (49) Morrison, R. “Grounding and Shielding Techniques in Instrumentation”, 2nd ed.; Wlley: New York, 1977. (50) Pressel, P. I . Bendix Corp., personal communication. (51) Scheeline, A. Appl. Specfrosc. 1984, 3 8 , 124-135. (52) Purcell, E. M. “Electricity and Magnetism”; McGraw-Hill: New York, 1965; Berkeley Physics Course Volume 2. (53) Spiegel, M. R. “Applled Dlfferential Equations”, 3rd ed.; Prentice-Hall: Englewood Cliffs, NJ, 1981. (54) Scheellne, A. Appl. Spechosc. 1981, 3 5 , 70-77. (55) Albers, D.; Johnson, E.; Tisack, M.; Sacks, R. Appl. Spectrosc. 1986, 40. 60-70.
RECEIVED for review September 13,1985. Accepted December 4, 1985. This work was supported in part by the National Science Foundation (Grant CHE-81-21809)and the Office of Basic Energy Sciences, U S . Department of Energy (Grant DEFG02-84-ER13218). Fellowship support (G.J.K.) by Dow Chemical Co., Phillip Petroleum Co., and the University of Illinois are appreciated. Portions of this work by G.J.K. were completed in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemistry.
Theta Pinch Discharge Designed for Emission Spectrochemical Analysis: Spectral Characterization Gregory J. Kamla and Alexander Scheeline* School of Chemical Sciences, University of Illinois, 1209 West California Avenue, 79 RAL, Box 48, Urbana, Illinois 61801
The hlgh-temperature plasma generated by a theta plnch discharge was studled as an emlsslon source for solid samples. Both sample geometry and the discharge fill gas affect plasma tormatlon and subsequent plasma/sample Interactlon. Ablation and emlsslon from solld samples have been demonstrated for alumlnum, tungsten, and stalnless steel samples exposed to the plasma. Tlme-resolved emlsslon data show neutral emlsslon occurring well afler cessation of Ion and contlnuum emission. Time resolutlon therefore removes the llmkatlons of hlgh backgroundand lon line Interference, whlch are associated with hlgh energy dlscharges.
A theta pinch discharge has been designed for use as a source for emission spectrochemical analysis. These discharges use magnetic compression to create high temperature plasmas.
Originally designed for fusion research, theta pinch discharges have been used to generate plasmas with temperatures and electron densities in excess of 1 keV and 1017 ~ m - ~In. fusion-related studies on these high-temperature plasmas, ablation has been observed from solid materials exposed to the plasmas ( 1 , 2 ) . These studies suggested that this discharge could be employed as an analybical emission source, where the discharge should be capable of sampling solid materials directly with little or no sample preparation. Ideally, a solid sample would be placed in the discharge vessel, the theta pinch would be fired, and the emission intensity from the sample measured. In the preceding paper (3),the design and electrical characterization of this discharge were discussed. In this paper, spectroscopic studies on the theta pinch plasma will be described. Spectral data have been acquired from the theta pinch discharge using both cylindrical and helical coils ( 3 ) .
0 1986 Amerlcan Chemical Society 0003-2700/86/0358-0932$01.50/0
