Electronically-controlled, current-injection spark source for basic and

Electronically-Controlled, Current-Injection Spark Source for Basic and Applied Emission Spectrometry J. P. Waltersl and T. V. Bruhns2 Department of Chemistry, University of Wisconsin, Madison, Wis. 53706

A spark source of high versatility is described. Using a crystal-controlled oscillator and frequency multiplying amplifiers to feed a 500-watt power tube, a 162megahertz sine wave is delivered to a quarter-waveresonant, rigid, coaxial line containing the spark gap as an integral termination. The rapid buildup of a standing wave of progressively increasing voltage on the open end of the line forms the spark with high temporal precision. A current injection point is located at the sending end of the quarter-wave line where a radio-frequency voltage node exists, allowing maximum flexibility in generating a current waveform to sustain the discharge. The source offers freedom from radio-frequency interference, low-power electronic control, and continuously adjustable repetition rates up to three kilohertz. THISSPARK SOURCE was designed to fulfill the need for producing a discharge at atmospheric pressure to carry current of waveform easily adjustable on both short and long time scales. Use of modern, small, low-voltage components for current generation was sought. Additionally, high temporal precision in spark formation, completely electronic control of spark formation using modern, low-power trigger sources, continuously adjustable discharge repetition rate, and maximum freedom from conducted and radiated radio-frequency interference were sought. As opposed to conventional electronically-ignited, high-voltage spark sources, e.g. (I), and other combinations of mechanically-ignited, coupled high and low voltage combinations, e.g. (2))the formation of the spark and delivery of current to it was sought here without the use of massive transformers, high-voltage cabling, and inductive high-voltage, low-voltage circuit isolation to prevent the resulting high residual source impedances and physically bulky arrangements associated with these approaches. The above needs are well established in experiments designed to study the discharge by optical means (3))as well as in those spectrochemical applications where closed loop computer control of the source Din its radiation output is sought. I n the former case, temporal precision in forming a train of identical discharges synchronized to externally-generated trigger signals is important. Similarly, fine control of the discharge current on a short-time basis allows investigation of such rapid events as electrode space charge formation and stepwise excitation of sampled electrode material by timeresolved spectroscopy (4). In the latter case, experimental control of the output radiation from the spark for analytical purposes ultimately will be traceable to refined instantaneous control of the discharge current ( 5 ) as well as to such bulk To whom requests for reprints should be sent. Present address, United States Navy, SN B865603, Batt. A, Co. F, NATTC, Naval Air Station Memphis (63), Millington, Tenn. 38054 1

2

(i) (2) (3) (4) (5)

A. Bardocz, Spectrochim. Acta, 7,307 (1955). M. F. Hasler and H. W. Dietert, J. Opt. Soc. Am., 33,218 (1943). J. P. Walters, ANAL.CHEM., 39, 770 (1967). Zbid., 40, 1540 (1968). Ibid., p 1672.

1990

ELECTRODE SUPPORT SYSTEM

/ SYSTEM

COAXIAL RESONANT

MODULAR CURRENT SOURCE

aJ

R E S I S T I V E LOAD

Y

/I-D IG I TA L TRIGGER SYSTEM

Figure 1. Components of current injection spark source

manipulations as feedback adjustment by digital computer of the repetion rate of the source according to photoelectrically derived signals from a desired spectral line. In either case, the mechanical-electrical properties of the spark source should suggest and encourage such experiments, rather than stand as a limitation to their convenient execution, and this goal prompted the specific apparatus described here. TRANSMISSION LINES

AND THE QUARTER-WAVE CONDITION

While conventional capacitor discharge sources seldom involve functional integration of energy storage, current delivery, and electrode support into a mechanical unit, this source unit succeeds in part in meeting its design criteria precisely because of this integration. The source is defined conceptually in Figure 1. The heart of the system is a quarter-wave transmission line (coaxial resonant line). Gas control for the analytical spark gap, the spark gap proper, and the equivalent of conventional high-voltage cabling are integral with the transmission line. A radio-frequency driver stage, operated by a digital trigger system, feeds the quarter-wave line through a purely resistivq load, the latter being mechanically integral with the transmis-

ANALYTICAL CHEMISTRY, VOL. 41, NO. 14, DECEMBER 1969

Figure 2. Establishing the quarter-wave condition on an open-ended line Four conditions are shown. For each condition there are three photographs showing ( a ) a single positive test pulse sent down the line, received, reflected, and sent back to the generator to define the electrical length of the line; (b) a positivenegative pair of drive pulses to be sent down the line; and (c) the results of sending and receiving the positive-negative drive train. The ordinate is 5 volts/cm and the abscissa i s 0.2 psec/crn In the upper left, twice the electrical line length (time A B) is greater than the time between the positive and negative drive pulses (time D). The positive pulse of the train is sent and received (time B = time A), and, since the impedance of the generator is less than that of the line, is partially reflected as a negative pulse back to the receiving end of the line (time C). The negative pulse of the train is sent (time D) and received (time E). In the upper right and lower left, the same events occur for lines of progressive shorter electrical length as the quarter-wave condition is approached (times A, B, and C decrease). In the lower right, the quarter-wave condition is achieved to give voltage cancellation at the sending end of the line and amplification at the receiving end

+

sion line housing, but displaced electrically from the quarterwave point at the sending end. A fully modular current source is connected to the sending end of the transmission line at the quarter-wave point using any mode of cabling desired. To establish a spark discharge capable of conducting current, it is necessary first to ionize the interelectrode supporting gas to a degree sufficient for space charge formation at the electrodes (6). In an atmospheric pressure system, this is accomplished most readily through the rapid and precise application of a high voltage to the electrodes, holding the potential at a high value only as long as necessary to ionize the discharge supporting gas. This is then followed by application of current to the ionized gap at a voltage level only in excess of that required to sustain an ionizing potential fall in the electrode space charges.

(6) H. Kaiser and A. Wallraff, Ann. Phys., 34, 297 (1939).

Here, the quarter-wave transmission line is responsible for generating the initial high voltage in a rapid manner upon electronic command, itself being shut off upon ionization of the gap, and the sustaining voltage is supplied by the current source immediately upon gap ionization. Switching between functions is accomplished by the dynamic impedance of the spark gap reflected through the transmission line and the radio-frequency driver coupling impedance, in a manner such that the quarter-wave point at the sending end of the line remains at ground potential at all times. I n a qualitative sense, if the transmission line is opencircuited at the receiving end (the spark gap) and an electric impulse is sent into it, the impulse will be reflected upon arrival at the receiving end and return to the sending end with no change in velocity. Then, if a second impulse of opposite direction is started down the line at the moment the first reflected impulse arrives at the sending end, the two impulses will add to increase the voltage at the receiving end. If im-


0

1991

pulses of alternating polarity are started, separated by intervals equal to the time required for each impulse to travel down the line and back, they will add to produce a standing wave of continually increasing voltage at the receiving end of the line until the impedance there changes from an infinite value, i.e., until a spark forms. If external manipulations are performed o n the spark gap that would increase the voltage necessary to came its cumulative-ionization, such as increasing the interelectrode spacing, decreasing the pressure to low values, or changing the supporting gas, the line ideally will continue to build up voltage on the gap until ionization occurs. Since the above sequence of events is critical to the operation of this source, a quarter-wave line was constructed with conventional coaxial lines and pulse generators. The approach to the quarter-wave condition is shown with this model in Figure 2. Using only the first two pulses from a hypothetical train of many, both voltage cancellation at the sending end of the line to provide a ground potential point and voltage addition at the receiving end to provide a standing wave of continually increasing amplitude are illustrated. The principle is quite old, being discussed by Steinmetz in 1909 (7). With the several hundred feet of transmission line required to prepare Figure 2, voltage amplification at the receiving end of the line is partially negated by line losses. In the quarter-wave line used in this spark source, losses were kept to a level sufficiently low to assure a rapidly increasing voltage amplitude at the receiving end of the line. This gives precise temporal ignition of the spark and is best understood through a quantitative description of the quarter-wave condition. The voltage and current on a transmission line may be described in terms of two completely general equations:

(3) (4) The subscripts zero and one imply the receiving and sending ends, respectively. Equations 3 and 4 may be combined to give : Eo

=

Zl - E1 lo

(5)

Equation 5 indicates that €or a lossless line with the receiving end open (lo = 0), the voltage on the receiving end will approach infinity. Clearly, the losses on any transmission line are not zero, and, Equation 5 is at best an idealized approximation. Thus, in the second case, line losses are considered, but r and g are assumed small with respect to x and b such that a = d 1 / 2 ( z y rg - xb) and @ = The quarter-wave condition is set such that a1 = ./@ ( ~ / 2 )and , ./@ is arranged to give :

dx

+

The integration constants in Equations 1 and 2 may now be solved in terms of a: for I = 0 and 1 = 1.

(7) EO=

dF[

jZ1

"1

-p2

Zo

For the open-end condition (ZO = 0), the absolute value of the maximum voltage developed on the receiving end of the line is given by : The integration constants A1 and A2 can be evaluated using boundary conditions for an open-ended line. Line parameters of concern are: r = resistance x = inductive reactance (2" fL) g = conductance b = capacitive susceptance (2" fC) I = distance along the line z = impedance = r - j x where z is the scalar magnitude Y = admittance = g jb where y is the scalar magnitude

+

f f =

attenuation constant

8 =

phase shift constant

=

4: + -(zy

rg

- xb)

2 Eo = - E1 P"

(9)

The driving or input current necessary to sustain this voltage on a continuous basis is given by:

For the transmission line used here, C = 4.5 x 10-12 0.12 ohm, p = 1.07 farads, L 5 X 10-7 henries, g 21 0, r x x N 5.1 ohms, and the maximum estimated output voltage for a 3000-volt line drive at 162 MHz is 178 kV. If this voltage could be sustained on a continuous basis without spark formation, an estimated drive current of 530 A would be required.

=

CURRENT INJECTION The quarter-wave boundary condition is set by putting p i = ~ 1 2 . Two cases can be considered. In the first case, the line is assumed lossless (r = g = 0). Then, a = 0 and p = with I = 7r/2&. The integration constants are evaluated to give the absolute values of the voltage and current on the sending and receiving ends of the line :

a,

(7) C . P. Steinmetz, "Transient Electric Phenomena and Oscillations," McGraw-Hill, New York, 1909, see p 313 f. 1992

One significant feature of the quarter-wave line as it is used in this spark source is that the sending end of the line remains close to ground potential regardless of the condition of the spark gap. Because of this, the current source requires only minor voltage protection prior to, during, and after spark formation. Any device that can operate into a few ohms resistance can be used to generate the current waveform. The current may be said to be injected directly into the formed spark with minimal reactive circuit isolation. To ensure that the line operates with its sending end close to ground potential during and after spark formation, the radio-

* ANALYTICAL CHEMISTRY, VQL. 41, NO. 14, DECEMBER 1969

GENERATOR SECTION I RADIOFREQUENCY

PARALLEL RESONANT COUPLINQ SECTION ( H I G H , PURE RESISTANCE)

-VS$LJ/TEEI

-

QUARTER-WAVE RESONANT LINE (TWO-VALUE PURELY RESISTIVE LOAD)

I

_h_

MLAkTiPLY

M U L T ~PLY BY 2

M U L ~PLY I BY 2

0 . 0 4 9 yH ( + I 49n1 MASTER CRYSTAL

6000 n

P L A T E LOAD RESISTANCE

2OpF

(-14911)

m

m

:... ................................................ j BEFORE BREAKDOWN

........................................................

I

m I

AFTER BREAKDOWN

Figure 3. Radio-frequency generator coupling section and line characteristics necessary for current injection Figure 4. Block diagram of electrical design of source

frequency line driver must be properly coupled to the quarterwave section. The specifications for the driver and resistive coupling section used here are established in Figure 3. With an operating frequency of 162 MHz and the power tube operating at 2500 V plate drive, approximately 625 watts of drive power are coupled through 3 reactive ohms to a parallel resonant circuit that serves to drive the quarter-wave line. The inductive portion of the coupling section is part of the transmission line proper, and the capacitive section is added as a plate tuning device for the drive tube. The parallel circuit shows at resonance a pure resistance to the generator with a value dependent on the amount of actual ohmic resistance in the circuit, which here is that of the analytical spark gap reflected through the 210-ohm, quarter-wave line. Prior to spark formation, the receiving end of the quarterwave line is open-circuited and its impedance is reflected back to the sending end as a dead short to ground. This completes the parallel resonant coupling circuit to ground with no added resistance, and the net impedance shown to the generator is high and purely resistive. The generator then supplies its full voltage to the coupling section to drive the line. Upon formation of the spark, the receiving end of the quarter-wave line is terminated in a dynamic resistance of value dependent on the conditions in the spark gap, but typically on the order of two ohms (8). This is reflected back to the sending end of the line as approximately 20 kilohms, terminating the coupling section with that resistance to ground. The coupling section then shows a pure resistance to the generator to ground of approximately one ohm, effectively shorting the plate of the generator to ground. This holds the sending end of the quarter-wave line close to ground potential by shutting down the generator drive to the line. If the current source is switched on at this time to sustain ionization in the formed spark gap, the generator is held in the off condition and the current source works into mainly the dynamic impedance of the spark. If for some reason the spark gap goes out of conduction, the coupling section impedance immediately increases. The generator once again drives the line to hold the sending end at ground potential and force the gap back into an ionized state. ELECTRICAL DESIGN

The basic electrical design of the source is shown in block form in Figure 4. Synthesis of the radio-frequency drive signal begins with a 6.75 MHz crystal-controlled oscillator, chosen

here for frequency stability. A 162-MHz signal is derived from the oscillator by harmonic multiplication through five stages of tuned amplification (Blocks 2-6). This method is chosen to conserve both signal fidelity and power. The 162-MHz signal is applied to a final stage, 500-watt, power amplifier (Block 14) via a gated, push-pull, power stage (Block 10). A low-power, external trigger signal (Block 7), which may be derived from the triggering optics of a timeresolved spectrometer (3) or independently synthesized from a pulse generator or digital logic circuitry, is fed through a buffer trigger generator for rise time shaping if required (Block 8) and then to a SCR pulser (Block 9). The latter provides sufficient power to rapidly gate on the power amplifier. Upon receipt of radio-frequency power, the quarter-wave line (Block 13) begins to form the spark (Block 15), which upon formation accepts current injected at the sending end of the line from the current source (Blocks 11 and 12). If desired, the current source may be independently synchronized to the external trigger signal through either the buffer trigger generator or the SCR pulser. Alternately, it may be designed to operate as a relaxation oscillator, fired by spark formation to deliver current to the spark immediately upon its formation. The latter case will be illustrated here. The 162 MHz oscillator-amplifier proper is shown in Figure 5. The design follows accepted radio-transmitter practice and is unique only in the frequency at which it operates. Detailed design and operation considerations are available for similar units at related frequencies (9) and will not be presented. For driving the quarter-wave line, any stable, highfrequency oscillator is acceptable, including commercial V.H.F. radio transmitters. The push-pull gated amplifier and SCR pulser circuits responsible for electronic initiation of the spark (Blocks 9 and 10 of Figure 4) are shown in Figure 6. Tube V5 functions as a radio-frequency gate, with both its plates and screen grids driven by the output of the SCR to effect the gating. This operation mode was chosen over other possibilities, since here only one high frequency element need be switched on and off, allowing all previous tank circuits in the 162 MHz oscillator and the V5 control grid circuits to be fully energized prior to application of the gate pulse. Thus, the 162 MHz drive signal can be applied directly to the final power amplifier and quarterwave line without degradation of the rise time of the gated rf

~

(8) J. P. Walters and H. V. Malmstadt, Appl. Spectry., 20, 80

(1966).

(9) E. P. Tilton, ed., “Radio Amateurs’ V. H. F. Manual,” Amer. Radio Relay League, Newington, Conn., 1968, p 105fl

ANALYTICAL CHEMISTRY, VOL. 41, NO. 14,DECEMBER 1969

e

1993

E;

162 Mhz OUTPUT TO PULSED

DRIVER

15

(NOT S H O W N )

J3, J4.J5 ARE TEST POINTS

Figure 5. Electrical schematic of the 162-MHz crystal controlled oscillator kl-L2 multiply X 2 L3-L4 multiply X 2 L5-L6 multiply X 2 L7-L8 multiply X 3 Specific components listed in Table I

Table I. Components for Figure 5 33 pF, 1 kV, IO%, ceramic Miller, 41AOOOCBI/16 T., ct., 5 X 44 litz, 6/18 in. long, L6 100 pF, 1 kV, lo%, ceramic ~ 2 . pH 5 0.002 uF, 1 kV, GMV dual-disk, ceramic 5 T., ct., 16 GA tinned copper, lS/an-in.i d . X l6/a8 in. L7 part of C3 long, =0.6pH 15 DF.500 V. lox, ceramic L8 4 T., ct., 16 GA tinned copper, 13/az-in.i.d. X l7/dn. 10 bF; 500 V; 10%; tubular ceramic long, ~ 0 . p3 H 0.01 p F , 1 kV, GMV disk ceramic 82 kn, W, lo%, carbon R1 0.002 uF, 1 kV. GMV dual-disk ceramic 47 kn, 1 W, 10% carbon R2 part of C8 . 100 a, W, lo%, carbon R3 0.002 uF. 1 kV. GMV dual-disk ceramic 68 ka, 11%W, l o % , carbon R4 part of CIO R5 1 kn, 'I2W, lo%, carbon 0.002 pF, 1 kV, GMV dual-disk ceramic 33 ko, 1 W, lo%, carbon R6 part of C12 100 n, 'I2 W, lo%, carbon R7 330 pF, 1 kV, IO%, ceramic R8 68 ka, W, lo%, carbon 330 pF, 1 kV, lo%, ceramic R9 1 kn, 'I2W, lo%, carbon 330 pF, 1 kV, lo%, ceramic 33 kn, 1 W, 10%. carbon R10 1.8-5.1 pF variable butterfly (Johnson 160-205) R1f 100 3, W, lo%, carbon 330 pF, 1 kV, lo%, ceramic 27 k0, 1 W, lo%, carbon R12 330 pF, 1 kV, lo%, ceramic 270 0, '1%W, lo%, carbon R13 330 pF, 1 kV, lo%, ceramic 39 kn, 1 W, lo%, carbon R14 0.01 p F , 1 kV, GMV disk ceramic 56 0, 11%W, lo%, carbon R15 0.01 pF, 1 kV, GMV disk ceramic RFCl 2.5 mH, powdered iron core (Miller 4666) 6773.97 kHz fundamental crystal, HC 6/V (6.75 MHz in. i.d. X g/le-in.long RFC5 12 T., 25 GA, plastic insul. nominal) . X Q/~~-in. long RFC6 12 T., 24 GA, plastic insul. ' / ~ i n i.d. Miller, 41AOOOCBI/18T. 34 cotton/enamel, PI wound, 5763 v1 =7 pH 5763 v2 Same as L1 5763 v3 Miller, 41AOOOCBI/19T. 5 X 44 litz close wound, -8 63GO v4 Jones, P-306-AB PH P1 Miller, 41AOOOCBI/17T. 5 X 44 litz close wound, -3.5 Johnson, 105-202-200 53 Johnson, 105-202-200 J4 /-a Miller, 41AOOOCBI/13T. 5 X 44 litz, 1/4-in.long, ~ 1 . 5 J5 Johnson, 105-202-200

c1 c2 c3 c4 c5 C6 C7 C8 c9 c10 c11 c12 C13 C14 C15 C16 c17 C30 C31 C32 c34 c35 CR1

'

L1 L2 L3 L4 L5

PH

burst due to activating the entire oscillator and gate circuits from an unloaded condition. The principle is important for achieving rapid buildup of a high voltage standing wave on the quarter-wave transmission line, limited in rate mainly by the rise time of the output pulse from the SCR. The SCR pulser in Figure 6, while arranged in a conventional synchronized relaxation oscillator configuration, is critical to the applied use of this source with respect to its triggering. The gate is operated with respect to the cathode through a low impedance (R23) and is activated through a pulse transformer (T1) wired for current amplification. With this combination, a ten- to fifteen-volt pulse of 0.1 psec duration may be applied to the primary of T1 to start regenerative 1994

e

firing of the SCR and removed from the gate be€ore the SCR achieves full conduction. This then isolates the SCR gate from the trigger pulse generator, such that the risetime of its output pulse is dependent mainly on the time constant of the anode capacitor-cathode load network. Here, the cathode load is the parallel combination of R19 with R18 and L11 in series, and was empirically matched to the anode capacitor, C26, to achieve the maximum output voltage rise time commensurate with the charging time of the R22-C26 anode network. It is this compromise that establishes the minimum source repetition rate at approximately three kilohertz. An additional feature of the gate circuit in Figure 6 is the


1

V 5 6360

7 2

162 M H Z

L IO

INPUT

162 MHZ OUTPUT

FROM MULTI- L7 PLlER OSCILLATOR ~ 1 7 TEST POINT

1 I

C27

c21

R19

I

Kvfl

t 400

Figure 7. Operation of circuit in Figure 6 to produce a 162-MHz burst for line drive

AAJI

TRIGGER INPUT

Figure 6. Electrical schematic of SCR pulser and gated push-pull power amplifier Darkened lines show gate drive voltage path from SCR 1 to V 5 Specific components listed in Table I1

option of continuous mode operation of V5 through selection of the position of Switch SW1. In the “pulsed” position, the cathode network of SCRl is returned to ground, and when SCRl is out of conduction, the plates and screen grids of V5 similarly are returned via R19. In the “continuous” position, the anode supply voltage for the SCR bypasses the cathode load network by forward biasing diode D1, to put V5 in conduction, the proper screen current being determined by the parallel combination of R20-R21. The quarter-wave line then receives rf power continuously. If the electrode spacing at the line receiving end is made sufficiently large to prevent full spark formation, a continuously sustained 162 MHz, plasma “torch” can be formed. I n this mode, the current source typically is disconnected. Electrical operation of the pulser-gate combination to drive the final power amplifier load is shown in Figure 7. The figure is self-explanatory. The output rf envelope from V5 closely follows the shape of the SCR output pulse voltage. This provides a rise time for application of rf power to the quarter-wave line of approximately 50 nsec. While this is the limiting time for electrically establishing a spark, the actual time for the gap to change from an insulator to conductor is determined by the physical properties of the interelectrode gas and the electrode separation and can be significantly less than 50 nsec. The final stage for driving the quarter-wave line is shown in Figure 8. A 4CX250B beam power tube (Vl) is used. The majority of components in Figure 8 are simply radio-frequency filters to protect the filament, plate, and grid supplies of V1. Tube drive is aiu L1, L2, and the tube capacitance is neutralized via C, to prevent continuous oscillation. The plate supply voltage is brought into the line at a region physically close to the quarter-wave point and follows the line center conductor to V1 to eliminate the need for high voltage protection of the plate supply leads. Capacitance C4 feeds radio-frequency changes in plate voltage to the parallel resonant circuit previously discussed (Figure 3), the latter consisting of approximately four inches of the line proper and tuning ca-

All high frequency measurements shown here and in subsequent figures taken with a Tektronix 556 oscilloscope and 1Al or 1S1 sampling plug-ins

pacitor C3. Current is injected at jacks 52 and 53. A few turns of wire are used in conjunction with capacitor C17 as a 162 MHz filter to protect the quarter-wave injection point for the first few cycles of line drive (ca. 12 nsec) while the standing wave is first forming. The line drive current is monitored at

C18 c19 c20 c21

c22 C23 C24 C25 C26 C27 C28 C29 c33 L9 L10 L11

R17 R18 R19 R20 R21 R22 R23 RFC2 RFC3

sw1

v5

TI D1 SCRl J1 J2

Table 11. Components for Figure 6 330 pF, 1 kV, IO%, ceramic 1.8-5.1 pF variable butterfly (Johnson 16C-205) 2.3-14.2 pF variable (Johnson 160-107) 330 pF, 1 kV, lo%, ceramic 0.01 p F , 1 kV, GMV disk ceramic 0.01 pF, 1 kV, GMV disk ceramic 330 pF, 1 kV, lo%, ceramic 330 pF, 1 kV, 10% ceramic 0.01 pF, 1 kV, GMV disc ceramic 50 pF, 1 kV, IO%, ceramic 0.01 pF, 1 kV, GMV disc ceramic 0.01 pF, 1 kV, GMV disc ceramic (power socket, J1, bypass, not shown) 0.01 pF, 1 kV, GMV disc ceramic 5 T., ct., 16 GA tinned copper, la/s2 in. i.d. X in. long, FZ 0.5pH IT, 16 GA tinned copper, wrapped around L9 110 pH,no tolerance, hand wound and peaked 15 kn, 1 W, IO%, carbon 100 a,1 W, lo%, carbon 1 kO, 1 W, 5 z , carbon 56 kO, 1 W, IO%, carbon 56 kO, 1 W, lo%, carbon 120 ka, W, IO%, carbon W, lo%, carbon 270 a, 12 T., 20 GA tinned copper, l/d-in. i.d., X Q/16 in. long 2.5 mH, powdered iron core, (Miller 4666) SPDT, 5A. 125 V. (J-B-T type JMT 123) 6360 Technitrol, 11 RGD PS04 1:l :I pulse transformer connected for 2 :1 stepdown Sarkes-Tarzian, 1N2484/F6 RCA, 2N3228 UG-290 A/V input connector UG-290 A/V output connector


1995

r.f. MONITOR

I I

I I

J4

4

1 3 2 5 6 TO POWER SUPPLY J 2

Figure 8. Electrical schematic of the beam power tube 500watt amplifier, quarter-wave line, and current injection jacks Specific components listed in Table III Table 111. Components for Figure 8 3-40 pF (Hammarlund CT2E100 with 8 rotor and 8 stator plates removed) c2 3-20 pF (Hammarlund CT2E100 with all but 4 stator and 5 rotor plates removed) c3 0.1-20 pF insulated copper disk capacitor for plate tuning (see Figures 12 and 13) c4 3 X 100 pF, 15 kV (Centralab 857-1OON) c5 1500 pF, 1 kV (Johnson 124-113-1) 330 pF, 1 kV, GMV disk ceramic (not shown; parallels C5a C5) 180 pF, button mica (MIL CB2lPD 181 K) C6 same as C6 c7 same as C6 C8 C9-C16 330 pF, 1 kV, lo%, ceramic 100 pF, 15 kV (Centralab 857-100N) C17 0.005 pF, 3 kV, lo%, ceramic C18 same as C18 c19 1-in. capacitive probe, 18 GA tinned copper Cn L1 1 T., 16 GA tinned copper L2 2 T., 16 GA tinned silver plated copper Plate tank line, 210 (see Figure 11) L3 Rectangular loop l/4-in. X i/&, 18 GA tinned copper, L4 connected to J7 for rf monitoring RFCl 25 T., 18 GA copper, 7/az-in.i d . close wound RFC2 2 1 T., 22 GA copper, 3/16-in.i.d. close wound RFC3 27 T., 18 GA copper, =/&-in. i.d. close wound RFC4 2 T., " 4 - h i.d. braid plus 1 ferrite bead, connected to RFC 9 and line center conductor RFC5 25 T.,18 GA copper, ai8-in.i.d. close wound RFC6 12 T., 18 GA copper, 7Iaz-in.i.d. close wound RFC7 Same as RFC6 RFC8 Same as RFC6 RFC9 2 T., 14 GA tinned copper, l/z-in. i.d. open wound RFClO Same as RFC9 i.d. close RFCll 43 T., 16 GA Teflon insul. tinned copper, wound RCA 7203/4CX250B beam power tube v1 Blower, Fasco Industries 50747-30 G B1 UG 262/U, BNC bulkhead socket J1 50-239, UHF bulkhead socket 52 Johnson 108-903 53 Jones P-306-AB 54 Millen 37001, high voltage connector J5 Johnson 108-903 (below J5 for grounding) 56 UG 290 A/U BNC chassis socket 57 Spark gap (see Figure 15) Gl

c1

1996

Table IV. Components for Figure 9 C1 200 pF, 350 V, electrolytic C2 Same as C1 c3 0.1 pF, 200 V, paper 50 p F , 50 V, electrolytic c4 c5 0.1 pF, 200 V, paper C6 50 ,uF, 50 V, electrolytic 200 pF, 350 V, electrolytic c7 C8 200 pF, 350 V, electrolytic 1N2484/F6, Sarkes-Tarzian D1 D2-D40 Same as D1, compose bridge rectifier with 10 diodes/leg D41 lN2070/10D4, International Rectifier D42 Same as D41 D43 1N24841F6, Sarkes-Tarzian D44-D46 Same as D43. compose F. W. Rectifier with 2 diodes/ leg D47 1N24841F6, Sarkes-Tarzian D48 Same as D47 D49-D56 Military surplus bridge rectifier assembly, unmarked, 2 diodesileg with only D53-D56 conducting current F1 10 A: 250 V, Bussmann ABC F2 5 A, 125 V, Littelfuse 3AG or 3AG slo-blo F3 10 A, 250 V, Bussmann ABC L1 Military surplus, filter choke, unmarked Military surplus, filter choke, unmarked L2 M1 0-500 pA, DC, panel meter, Simpson 1227 P1 Hubbell 4716 PL1 Dialco 0111-201, lamp No. 47 Same as PLl PL2 R1 10 0, W, lo%, carbon R2 9 kn, 10 W, 5%, wirewound R3 10 kn, 1 W, lo%, carbon R4 10 kQ, 1 W, lo%, carbon R5 5 kn, 4 W, lo%, wirewound potentiometer R6 1 kn, 1 W, 10% carbon R7 Wirewound, to drop 0.4 V at operating conditions R8 9 kfi, 10 W, 5%, wirewound R9 9 kn, 10 W, 5%, wirewound R10 1 kQ, W, lo%, carbon R11 3.3 kQ, ' 1 2 W, lo%, carbon R12 10 kQ, W, 10%. carbon potentiometer R13 5 kQ, 50 W, 5%, wirewound R14 390 kQ, 2 W, IO%, carbon R15-17 Same as R14 R18 5 k0, 50 W, 5%, wirewound adjustable R19 1 kn, 1 W, 5 % , carbon R20 12.5 0, W, carbon, selected for 0-20-mA-F.S. meter shunt R21 400 kn, selected 390 kn carbon for 0-200-V-F.S. meter multiplier R22 2.5 Q, wirewound, selected for 0-100 mA-F.S. meter shunt R23 1 MQ, 2 W, 1%, carbon film R24 1.25 Q, wirewound, selected for 0-200 mA-F.S. meter shunt 1 MQ, 2 W, 1% carbon film R25 430 52, metallic film, 3 parallel resistors selected for 0.5 V R26 F.S. meter multiplier DPDT, 115 VAC, 1OA. Potter and Brumfield RV1 KRPI IAG 4PDT, 24 VDC, 3A, Magnecraft W224.5 X 146 RV2 SPDT, 125V, 6A, Toggle s1 __ S2-S4 Same as S1 s5 SPDT pushbutton SPDT, 250 V, 3A, Toggle S6 SPDT, 125 V, 1OA: microswitch s7 S8 SPDT, 125 V, IOA, microswitch DPGT, ceramic rotary s9 UTC S-48, 1000/1250/1500 VAC, 500 mA secondary T1 Military surplus, secondary 115 VAC to 6.4 VAC at T2 20 A, 5.0 VAC at 8 A, 150 VAC ct. at 300 mA Military surplus, secondary 530 VAC ct. at 150 mA, T3 6.3 VAC at 5 A, 5.0 VAC at 3 A Military surplus, secondary 690 VAC ct. at 150 mA, T4 6.3 VAC ct. at 5 A, 5.0 VAC at 3 A J1, 52 Jones S306-AB Millen 37001 high voltage connector 53 Superior DF31BC 54 Mallory S C l A phone jack J5


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1997

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Figure 10. Electrical schematic of filter section for smoothing the beam power tube plate voltage the quarter-wave point by terminating jack J7 in a 50-ohm load at the input of a high-speed or sampling oscilloscope. The power supplies for all circuits presented are conventional. All supplies are unregulated, since no circuit component or sub-assembly requires more than a few per cent voltage regulation. The plate supply for the 4CX250B beam power tube is filtered to allow continuous source operation at .all repetition rates from 0 to 3 kHz. The main power supply is shown in Figure 9 and the plate supply filter in Figure 10. MECHANICAL DESIGN

The mechanical features of this source arise from the electrical need for low-loss operation, the need for reproducible electrode positioning without excessive restrictions on electrode physical form, and the general stipulation of reasonable overall size for placement in a spectrometer optical train. Since the entire source, with the exception of the current supply, is constituted within the coaxial line proper, these considerations reflect its successful operation as much as the necessary electrical circuitry. Essential mechanical features are shown diagrammatically in Figure 11. By choosing a line drive frequency of 162 MHz, the overall line length, including the inductive portion of the parallel resonant coupling section, does not exceed 3 feet. The line itself is machined from 6-in. 0.d. brass pipe with I/S-in. wall

Figure 11. Mechanical schematic of quarter-wave line

thickness, both for mechanical rigidity and to minimize resistive radio-frequency current losses. The outer wall of the line is physically earthed. The center conductor is composed of two sections of telescoping brass tubing and is accurately centered within the line by two half-inch thick, low-loss polystyrene insulating disks. The physical connection of the transmission line to the 4CX250B beam power tube is important, in that the coaxial geometry of the line should be preserved to ensure efficient power transmission through the coupling capacitors and parallel resonant coupling section. The method chosen is shown in Figures 12 and 13. The methods used to introduce the plate voltage for the beam power tube and the current injection jacks and filter are shown in Figure 14. The latter is important in that this simple connection plus the length

Figure 12. Mechanical arrangement of beam power tube and rotor of plate tuning capacitor View is looking down at sending end of line with top section removed 1998


Figure 13. Mechanical arrangement of beam power tube coupling capacitor and sending end of quarter-wave line This section mates with that shown in Figure I2 to form the complete line

of the transmission line center conductor constitute the entire impedance presented by the line to the current source. Electrode geometry and positioning are major factors in any spark source, and particularly so for those intended for research use. Here, care must be taken in the design of the electrode connected to the line center conductor. Should this assembly become too large, or massive amounts of irregularly shaped metal be associated with its construction, the ratio of inner to outer conductor diameters will change and disturb the line impedance. The resulting reflections could prove detrimental to the beam power tube. For this reason,

the center conductor and spark electrode are made the anode for current injection, One possibility currently in use for inserting and adjusting the grounded cathode is shown in Figure 15. The electrode desired is made as a shaped pin approximately inch long and inserted over the end of the drill rod cathode support shaft. The entire micrometer assembly and support tube then is inserted through the brass end cap of the coaxial line. The cathode is lowered to contact the anode pin and backed off to establish the desired gap spacing. The electrode assembly shown in Figure 15 represents only

Figure 14. Sending end of quarter-wave line with beam power tube and chassis removed View is external to show physical locations for plate supply input to power tube, 55, and current injection jacks, 52, 53 ANALYTICAL CHEMISTRY, VOL. 41, NO. 14, DECEMBER 1969

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Positioning the line and power tube plus chassis along a spectrometer optical train is not as straightforward as encountered with cable-fed enclosed spark stands because of the line’s overall length. However, this partially is outweighed by the mechanical ease of vertically supporting a symmetrical cylinder. The source typically is supported by the brass pipe proper with the beam power tube and chassis being carried free. The line diameter is sufficiently small to impose few restrictions on the placement of image transfer optics, and, if desired, these can be made an integral part of the external line wall. In current practice the line has been mounted at the end of an optical bar or table and the optic axis positioned relative to it when a small monochromator was used. For positioning with larger commercial spectrometers it would be necessary to place the line off the optical rail and fold the optic axis onto it with a transfer mirror. The relative merits of this compromise, however, must be weighed against the great simplicity and versatility resulting with this source compared to conventional massive, fixed-frequency, electronicallyignited, high-voltage spark sources and their attendant insulated stand and cabling complications. OPERATIONAL CHARACTERISTICS

Figure 15. Electrode support system used for pin cathodes Anode insert falls directly below drill rod cathode support in practice

one possibility, and that specifically chosen for current research use in this laboratory. For example, if it were desired to spark to metal flats or disks, the entire line could be modularly shortened to bring the line end cap within sparking distance of the anode insert. The metal flat could then be placed on the outside of the cap and spacing and sparking accomplished as with a conventional Petrey table. In fact, improved physical convenience and operator safety over conventional Petrey stands would result because of the open-air accessability of the grounded sample and lack of cabling and electrode jaws found in conventional spark stands. In this source, radiation presently is observed through one of four 1-in. diameter light ports symmetrically placed at 90° intervals in the line outer wall. Atmosphere control is established by filling the top chamber of the line with the desired sparking gas, or by flowing the gas around the gap via conventional, but nonmetallic, jets or stabilization devices. A moderate vacuum can be drawn in the electrode chamber. With improved window and end cap sealing, the electrode P chamber could be fully evacuated. Although the spark gap must physically be located one quarter of a wavelength from the generator and coupling sections of the line, this dimension is sufficiently flexible to allow up to 12-mm electrode separations without significantly altering temporally precise spark formation. In present use, continuous changes in gap spacing between I/z and 5 mm have been made during sparking without detectably altering spark formation due to instrumental limitations. If a higher drive frequency and shorter line were used, this consideration would become more important. It thus appears that the line dimensions used here approach an optimum for both reproducible basic and convenient spectrochemical use. 2880

e

At the time of this writing, many of this source’s research possibilities and spectrochemical uses remain in the prototype stage. The fact that essentially any device capable of supplying voltage sufficient to sustain the electrode falls in a formed spark can be used in a truly modular fashion for current injection would make a specific current circuit inclusion here premature and far too restrictive. Similarly, genuinely continually adjustable, high repetition rate, electronic spark sources yet have not been in sufficient use to define even the instrumental limitations to be overcome for efficient spectrochemical use, much less to predict their physical and radiative properties. Thus, primarily to define only the key experimental factors that accompany current injection with this source, a capacitor discharge type current source was constructed for initial use, with additional experiments being done with prototype charged transmission lines. The above current sources are sufficiently common to eliminate the need for detailed circuit descriptions, and their inclusion is not meant to imply exclusivenessor restrictiveness. Here, the capacitor or line used was charged either from a conventional full-wave power supply with a L-section output filter of 200 ohms resistance and 170 pF capacitance or seriesparallel combinations of Mallory type BA1315/U, 136-V dry batteries. Argon, argon-5 Z hydrogen, nitrogen, and air were used as supporting atmospheres, with operation in argon and nitrogen defining typical electrical extremes. Operation in Argon. Because of the low voltage drops required to sustain a discharge in argon at atmospheric pressure and the relative ease with which the gas is ionized, it is to be expected that operation of this source in argon would represent an optimum. This has proved true with respect to injecting current into long spark gaps at low voltages and the rapidity with which a spark can be formed. Examples of the latter point are shown in Figure 16. For an 0.5-mm gap, the generator power is shut off in three complete half-cycles of the 162-MHz line drive (ca. 15 nsec) indicating that total gap ionization and €ormation of stable electrode falls has been completed in this time. As the electrode separation is increased, rf voltage on the gap prior to breakdown increases as does the time lag from the application of rf drive until its collapse. The number of cycles of the 162-MHz drive required to complete breakdown also increases with

ANALYTICAL CHEMISTRY, VOB. 41, NO. 14, DECEMBER 1969

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2.0 rnm GAP 4.0 rnm G A P

Figure 16. Formation of a spark capable of receiving current in argon The ordinate is uncalibrated, but is proportional to current at the sending end of the line, and thus voltage on electrodes at the receiving end. Signal monitored through 50 0 termination at 57 in Figure 8. No current was injected increasing electrode separation, the latter time reflecting the increased time required to change the gap impedance between its two limits. The reproducibility of the breakdown is quite high as evidenced by the fact that the gradual collapse of line drive can be photographed with minimal irregularity with a sampling oscilloscope. From Figure 16, it also is evident that operation in argon offers a good example of optimum impedance change for the formed discharge, even with no current injection. Prior to breakdown, the 162-MHz buildup on the gap is smooth and follows the output voltage from the SCR pulser (e.g. Figure 7), indicating that the gap is fully out of conduction and presenting essentially an infinite impedance to the generator and coupling sections of the line. At most, only a few cycles of line drive exist where the 162-MHz signal is eratic in amplitude from half cycle to half cycle, indicating a regular transition of the supporting gas out of its nonconducting state. For the gap spacings and times required to complete this transition, an ion velocity on the order of lo6 cmlsec is estimated, in agreement with previous studies of spark formation, e.g. (10). After breakdown, the line drive is efficiently shut off at the generator, indicating a transition into a conducting state of low dynamic impedance. (10) H. Raether, “Electron Avalanches and Breakdown in Gases,” Butterworths, Washington, 1964, p 129 ff.

Current injection into ionized argon is rapid, positive, and temporally precise. As shown in Figure 17, the potential at the quarter-wave injection point remains exceptionally close to ground before, during, and after spark formation, indicating that the current source, when it first is activated, will be working into only the voltage drop across the forming spark. In Figure 18, the onset of current injection is shown to occur in the time required for the spark to form. Only a small precurrent is detected when the rf drive is switched on the quarterwave line, verifying the virtually complete isolation of the two circuits. The smooth, transient-free, lead edge of the current waveform in Figure 18 is rewarding in that it reflects the low total reactive and resistive impedance into which the current source works. A further example of the impedance seen by the current source working into a spark in argon is shown in Figure 19. Here the discharge of 0.4 pF at 270 volts through 1 foot of RG l l j U cable produces a 1 psec current pulse in the 0.5-mm gap. An additional 2 feet of RG 55 A/U cable were connected in parallel with the capacitor, open-ended to deliberately produce the transients shown on the lead edge of the current pulse. As the gap spacing is increased to 4.5 mm, the pulse is decreased in amplitude and extended in duration in accord with conventional mathematical descriptions of a capacitor discharge through a series connected inductor and resistor (7). Of


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Figure 18. Precision onset of current injection into am argon spark in time coincidence with full spark formation

Figure 19. Veriffication that the quarter-wave current injection ofmt r ~ ~ a rat ~ nground s potential before, during, and after erk ~ ~ r minaargon ~ i ~ ~ ~

~

~ was taken a ~by monitoring ~ r voltage ~ to~ ground ~ directly n ~ Figure 8 with no current injected. Low amplitude signals are refleeted 162 MHz, and high are parasitic oscillations of coupling e a ~ a c i ~ ~ nG4 e ewith stray inductances at jack JZ in

importance here are the facts that there is insufficient residual inductance in the entire circuit to sustain more than one half cycle of current oscillation even when no attempt was made to utilize a non-inductive capacitor, and that the majority of circuit resistance is located in the spark gap proper. The significance of the above observations lies in the ease with which a current waveform may be adjusted experimentally. For example, when the primary source of resistance in a spark source is located in the discharge itself, the addition of small amounts of external resistance becomes a powerful

technique for current shaping. Similarly, when the residual inductance of the overall source circuit is this low, short lengths of conventional coaxial cable can be connected both in series and parallel to fine tune the lead edge of the current waveform. In current practice, smoothly rising and falling, unidirectional, current pulses as short as 0.5 psec. and of only 15-A peak magnitude have been generated from an 0.1 puF capacitor charged to only 135 V. This would be a complete impossibility with a conventional, cabled, high or low voltage spark source. Two examples from the wide variety of current pulses that can be shaped into a spark in argon due to the low overall circuit impedance are shown in Figure 20. A fully-oscillatory, high-current spark is formed from discharging 0.4 pF charged to 530 V into sufficient coaxial cable of the proper impedance to produce a definitively stepped lead edge to the current waveform. Using the same capacitor, the charging voltage and cable type and length may be changed to reduce the magnitude of the current pulse, force it into a unidirec-

Figure 19. Variation of dynamic impedance of a formed spark in argon with increasing electrode separation Maximum peak current corresponds to minimum electrode separation 2002

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Figure 20. Shaped current waveforms from discharge of a capacitor into argon


Figure 22. Current reversal in a current-summed discharge by voltage control Figure 21. Waveform synthesis by current summing

Low-current, nonreversing pulse vias generated as in Figure 21

tional mode, and greatly shape and exaggerate the transients on the pulse lead edge. Additional waveform synthesis in the same vein is possible through parallel-connection current summing of individually shaped pulses. In Figure 21, a conventional capacitor discharge was summed with the waveform resulting from the discharge of a 4-ohm transmission line to control the risetime of the resulting trapezoidal pulse, In Figure 22 the same summed pulse is caused to reverse direction by controlling the charging voltage of the line and capacitor to cause higher discharge currents, without altering significantly the temporal precision in forming the spark. In the above examples, parallel summing at the voltages and cable conditions shown is possible because of the low impedance of the discharge formed in argon. I n this sense, argon as a supporting gas is ideal. In those cases where the dynamic resistance of the formed spark is higher, larger voltages and

Figure 23. High-frequency electrical response of source while forming a spark in nitrogen

different impedances would be required to produce the same waveforms. However, the components required to produce the waveform are neither massive nor expensive, have trivial voltage ratings compared to conventional spark source components, and are not in the least responsible for introducing uncontrolled transients into the discharge waveform, As should be the case, those limitations to producing a desired current waveform are traceable directly to the physical and chemical properties of the atmospheric pressure discharge itself, and not restricted significantly by the apparatus required to form the discharge with temporal precision. Operation in Nitrogen. Theoretically optimum performance of this source occurs when a genuine spark is formed, Le., if the voltage drop across the electrodes is low and primarily confined to the electrode falls. There is then no question that the generator will shut off and current injection can and will begin immediately. However, if the properties of

Figure 24. Electrical evidence for rapid ionization and quenching while forming a spark in nitrogen without current Injection


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Figure 26. Variable repetition rate operation of source for current injection tlia a capacitor discharge Ordinate: Capacitor voltage, 100 V/cm Abscissa: Time, 1 msec/fm 0.5 rnm

(3

1.5 m m

0

2.0 rnm

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Figure 25. Electrical-optical correlations during formation of B spark in nitrogen without current injection

the sustaining gas are such that ionization started during one half-cycle of the 162-MHz drive can be completely or partially removed before peak voltage is achieved on the next halfcycle (ca. 3 nsec), then rapid spark formation cannot occur. I n this case the generator is alternately turned partially or completely on and off and a standing wave of progressively increasing voltage cannot form. Depending on the magnitude of the current injected, the discharge will differ in its early optical output when injection does occur. Such a case can occur for moderate to large gap spacings with pure nitrogen as the supporting gas. For example, Figure 23 shows electrical evidence of unique line response to spark formation in nitrogen. It is clear in the top two insets that the collapse of line drive requires more time than observed in argon. The power delivered to the line after breakdown also is greater than required with argon. When current is injected at a 500 kHz repetition rate, formation of the spark occurs more rapidly, but the discharge's higher dynamic impedance does not terminate the line drive as completely as observed with argon. If the magnitude of the injected current is low, or the gap spacing is adjusted to a small value without current injection, and the line drive is observed on a short time basis, the process of ionization and rapid quenching can be detected in the waveform of the 162MHz current at the sending end of the line. This is shown in Figure 24, displayed to bring out the temporal reproducibility of the effect on a spark to spark basis. It is important that the cyclic ionization-quenching effect observed with nitrogen as the supporting gas does not introduce temporal jitter into the electrical or optical output from the spark, but mainly extends the time required for current injection to begin after the source is triggered and rf power is applied to the line. This is because the effect is chemical in nature, as shown by correlating the electrical response of the 2004

e

line discussed above to the optical output of the discharge during its formation. For example, in Figure 25 are shown the earliest phases of discharge formation in nitrogen. The spectroscopic data were prepared with a photoelectric time resolving apparatus described previously (3). A 1P28 photomultiplier with 900 total applied dynode volts and 50 ohms anode load was used to detect the photon output through a Heath EU-700 1/3 meter monochromator with 50-micron slits with the source imaged vertically on the entrance slit. These data apply to the case where the quarter-wave injection point is physically grounded. In case A , for an electrode separation of 0.5 mm, radiation from the 3371.3, 0-0 band head of the second positive system of nitrogen is detected to rise, peak, and fall in a time only slightly greater than the combined electron transit and spread time for the lP28 type tube (11). Correspondingly, the 262MHz voltage on the gap begins collapsing, shows a short burst of eratic behavior, and then uniformly falls as the Nz radiation ceases and ionization builds up in the gap. That there is little or no optical temporal jitter in spark formation is evidenced by the crisp rise and fall of the Nz radiation, since the oscilloscope recording the photomultiplier output was triggered by the same pulse that triggered the SCR pulser to start line operation. In case B, increasing the electrode separation by 0.5 mm delays the peak Ne radiation by approximately 50 nsec from case A , and increases its half-intensity width by approximately 10 nsec. More significantly, the rise time of the radiation is now greater than the fall time (30 GS. 10 nsec, respectively). The increased risetime reflects the greater time needed for molecular dissociation to lead to ionization, the latter still occurring rapidly. The increased radiation rise time also is reflected directly into an increase in the time the 162-MHz gap voltage is responding eratically, verifying that both ionization and quenching are occurring simultaneously. Upon termi(11) RCA Technical Manual PT-60, "RCA Phototubes and Photocells," Radio Corporation of America, Lancaster, Pa., 1963, p 62.


nation of the Nz radiation, the 162-MHz signal uniformly collapses to settle at a new, stable level of magnitude greater than that observed in case A because of the increased impedance of the longer gap, The above process is exaggerated further as the electrode separation is increased in 0.5-mm steps. In case C, the Nz radiation fall time also has increased and the time duration of eratic line drive reflects this condition proportionally. In case D the radiation peaks off scale, but the significant increase in its rise time and proportionally longer line drive still is evident. In all cases, Nz radiation termination and the onset of collapse of the 162-MHz signal are sufficiently constant and removed from the radiation peak to ensure that it is the buildup of ionization that is largely responsible for the increased time to form the spark. The above phenomena in no way are unique to this spark source (e.g. Table 11, Ref. 12, and Figure 8 in Ref. 4, nor do they signify a limitation in its design. They are a basic property of the supporting gas itself and an: quite reproducible on a spark-to-spark basis. Where they constitute an instrumental limitation to versatile source operation, such as in the temporally precise formation of and current injection into long spark gaps, the best solution will be chemical. The electrical alternative of brute force application of a large and sustained step voltage for spark formation leads to substantial electrical transients, radio-frequency interference, and positional instability in the discharge formed. Repetition Rate Limitations. Figure 26 shows examples of source operation at four typical repetition rates for sparking in argon when an 0.4-pF capacitor was battery charged to approximately 270 V for the injection of 30-A, 2 psec unidirectional current pulses. A Tektronix type 114 pulse generator was used for triggering. While the SCR pulser used here can drive the line at repetition rates up to 3 kHz, the main limitations to sustained operation at these frequencies fall upon the power ratings of the current source, the deionization time of the gap, the perishability of the electlrode surfaces, and the

heat capacity of the electrodes, their supports, and any associated wiring. A typical non-instrumental limitation observed with argon supporting gas is sustained gap ionization leading to gross discharge hairpinning (Le., extrusion of the spark channel out of the gap) at approximately 2 kHz when 70-A, 2-psec, unidirectional current pulses are used. It is felt that this type of phenomena will prove the major limitation to high repetition rate operation, since in the worst case brute force wiring with massive capacitive smoothing of capacitor charging power supply output voltages can be used. Additionally, with current pulses in the 50-100 A range and of a few microseconds duration, surface damage to the cathode can be so massive at repetition rates above 1 kHz that the utility of such a combination is questionable. SUMMARY

The apparatus described here is unique among atmospheric pressure spark-sources in its versatility. The features of temporally precise, low-power variable frequency sparking with completely electronic control are valuable for basic research on the discharge. In addition, the high simplicity of current injection, with effectively little need for voltage protection of the current source, and the virtually complete absence of radio-frequency interference, make it possible to use high frequency, sensitive, solid state switches, oscilloscopes, digital computers, and photon counting apparatus (13) for both control and observation of an analytically useful spark in a manner previously impossible. Although only a few of the more obvious means for injecting variable waveform current pulses have been illustrated here, the major barrier to more sophisticated current injection should not be instrumental. RECEIVED for review July 17, 1969. Accepted September 23, 1969. Paper presented in part at 20th Mid-America Symposium on Spectroscopy, Chicago, May 12, 1969. Support of the National Science Foundation under grant GP-7796 is acknowledged.

~~

(12) J. P. Walters and H. V. Malmstadt, ANAL.CHEM., 37, 1484 (1965).

(13) M. L. Franklin, G. Horlick, and H. V. Malmstadt, ANAL. CHEM., 41, 2 (1969).


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Electronically-controlled, current-injection spark source for basic and

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