High-voltage spark source

A spark source circuit has a discharge circuit including reactive means connected in circuit between a capacitor and an analytical spark gap and switching means for effectively removing the capacitor from the discharge circuit after an initial flow of current through the spark gap so that the current flow through the spark gap is essentially unidirectional. At the start of the capacitor discharge, the discharge circuit is essentially of the classical RLC oscillatory discharge type; and the switching action converts the discharge circuit to essentially a classical RL discharge type circuit for the remainder of the spark discharge. The circuit is simple and efficient, permits use of ceramic capacitors, and may be housed in a sealed enclosure.

This invention relates to spectroscopic analysis and more particularly to 
high-voltage spark sources for use in spectroscopic analysis. 
In spectroscopic analytical techniques using high voltage spark sources, 
material from the sample to be analyzed is introduced into an analytical 
spark gap and spark discharges across the spark gap vaporize some of the 
sample material. The vaporized sample material may be excited to 
spectroemissive energy levels sufficient to emit reliably detectable 
radiation characteristic of all the elements in the sample by the spark 
discharges themselves or by supplemental excitation. The resulting emitted 
radiations are analyzed spectroscopically to determine the composition of 
the sample material. Such techniques are especially valuable in analyzing 
metals and metal alloys. Spark sources of these general types are 
described in Walters, "Historical Advances in Spark Emission 
Spectroscopy", Applied Spectroscopy, 23, 317 (1969); Walters, "An 
Adjustable-Waveform High-Voltage Spark Source for Optical Emission 
Spectrometry", Analytical Chemistry, 40, 1672 (1968); Coleman et al, "An 
Electronic, Adjustable-Waveform Spark Source for Basic and Applied 
Emission Spectrometry", Spectrochimica Acta, 31B, 547 (1976); and in U.S. 
Pat. Nos. 3,749,975 and 3,973,167. 
In accordance with one feature of the invention, a spark source circuit has 
a discharge circuit including reactive means connected in circuit between 
a capacitor and an analytical spark gap and switching means for 
essentially removing the capacitor from the discharge circuit after a 
brief initial flow of current through the spark gap so that the current 
flow through the spark gap is essentially unidirectional. At the start of 
the capacitor discharge, the discharge circuit is essentially of the 
classical RLC oscillatory discharge type; and the switching action 
converts the discharge circuit to essentially a classical RL discharge 
type circuit for the remainder of the spark discharge. The waveform of the 
current through the spark gap is a substantially nonoscillatory 
triangularly-shaped pulse which rises rapidly towards peak and then 
gradually falls, and has a typical duration of about 100 microseconds. 
In preferred embodiments, the switching means is rectifier circuitry 
connected directly to the capacitor for charging the capacitor to a high 
voltage and the rectifier circuitry, in response to the voltage on the 
capacitor reaching zero volts (which in preferred circuits occurs in less 
than 5 microseconds), conducts and shunts flow of the current around the 
capacitor for the remainder of the spark discharge. The circuit is simple 
and efficient, permits use of ceramic capacitors, and may be housed in a 
sealed enclosure. Among the advantages are reduced heating and reduced 
radio frequency emission. 
While a variety of spark initiating arrangements may be used, including 
devices such as blower gaps and thyratrons, in accordance with another 
feature of the invention, in preferred embodiments, a long-life, low 
trigger power switch which includes a stack of semiconductor switch 
devices connected in series between the capacitor and the spark gap is 
employed. In response to a triggering signal, the semiconductor switch 
devices are initially switched serially into conduction followed by an 
essentially concurrent switching of the remaining semiconductor switch 
devices into conduction, effectively closing the switch and transferring 
the voltage on the charged capacitor to the spark gap to initiate the 
spark discharge. In a particular embodiment, each semiconductor switch 
stage includes a controlled rectifier and voltage distributing means, and 
the concurrent switching action is responsive to the voltage across each 
nonconducting switch stage exceeding a predetermined value. Also, in that 
particular embodiment, the spark initiating switch is triggered as a 
function of the voltage on the capacitor in an arrangement which provides 
uniform waveform characteristics of successive spark discharges through 
the analytical spark gap.

DESCRIPTION OF TICULAR EMBODIMENTS 
The analytical spark source circuit shown in FIG. 1 has supply terminals 
10, 12 connected through circuit breaker 14 and fuses 16 to contacts 18 of 
start relay 20 that is controlled by switch 22. Blower 24 is energized 
when circuit breaker 14 is closed. Contacts 18 are connected through 
resistor 26 to the primary winding 28 of transformer 30. The secondary 
winding 32 of transformer 30 is connected through resistors 34, 36 to 
rectifier circuit 38 that includes four high voltage diodes 40. 
Connected between plus terminal 42 and negative terminal 44 of the 
rectifier circuit 38 is storage capacitor 50 which is composed of 75 
ceramic capacitors in a series parallel arrangement of five groups of 
fifteen capacitors each to provide a capacitance value of 0.03 microfareds 
and a voltage rating of fifteen kilovolts. Adjustable air core inductor 52 
is connected between the negative terminal 54 of capacitor 50 and 
electrode 56 of the analytical spark gap 60 which receives sample material 
58 and through which argon gas is flowed. Tungsten counter electrode 62 of 
spark gap 60 is connected through RF ammeter 64 to ground. High voltage 
resistor 66 is connected across the analytical spark gap 60. 
Connected to the positive terminal 68 of capacitor 50 is a voltage divider 
network or resistors 70, 72, 74, 76, and 78 and a high voltage switch 80 
that consists of ten semiconductor switch stages 82-1 - 82-10 that are 
connected in series between the positive terminal 68 of capacitor 50 and 
ground. 
The circuit also includes a trigger circuit 84 and a power supply 86 that 
provides a regulated 25 volt supply to trigger circuit 84. Power supply 86 
includes isolation transformer 90, diode 92, capacitor 94, and two zener 
diodes 96, 98. The trigger circuit includes transistor 100, resistors 102, 
104, capacitor 106, programmable unijunction transistor 108 and 
potentiometer 110. The base 112 of transistor 110 is connected to the tap 
114 between resistors 76 and 78. The output voltage from transistor 100 
charges capacitor 106 through resistor 104. The positive terminal of 
capacitor 106 is connected to the anode of PUT 108, the cathode of PUT 108 
is connected to the control input 116 of electronic switch 80, and the 
gate terminal 118 of PUT 108 is connected to the slider 120 of 
potentiometer 110. Test point 122 provides convenient monitoring of the 
trigger circuit. 
Electronic switch 80, as indicated above, includes ten semiconductor switch 
stages 82 connected in series. Each switch stage 82 is mounted on a 
separate heat sink and includes a controlled rectifier (SCR) 130 with the 
anode 132 of each stage connected to the cathode 134 of the SCR of the 
next stage. Connected across anode and cathode of each SCR is a resistor 
136 and a capacitor 138, resistor 136 providing equivoltage distribution 
and capacitor 138 providing capacitance balancing. Connected between the 
anode of each SCR and its gate 140 is a series circuit of resistor 142 and 
break-over diode 144. A reverse voltage protection diode 146 is connected 
between each gate 140 and cathode 134. Also connected to gate 140 of each 
SCR via current limiting resistor 148 is the secondary winding of a pulse 
transformer 150. A capacitor 152 is connected in parallel with each 
capacitor 138, stages 82-1 - 82-9 having the capacitor 152 connected 
between the pulse transformer primary winding of that stage and its SCR 
cathode 134. Each semiconductor switch stage 82 has a trigger pulse input 
line 154 and an output line 156 that couples a trigger signal to the next 
stage. The first stage (stage 82-1) of switch 80 has its trigger input 
154-1 connected to trigger terminal 116 and a trigger output on line 
156-1. Switch stages 82-3 - 82-8 are identical to stages 82-2 and 82-9. 
Set out in the following table are values of components employed in the 
circuit of FIG. 1. Those skilled in the art will understand that the 
values of various components can be varied widely to suit various 
conditions: 
______________________________________ 
Component 
Reference No. Value 
______________________________________ 
Resistor 26 10 ohms - 200W 
Transformer 
30 120V/6000V 
Resistor 34 3K - 25W 
Resistor 36 3K - 25W 
Diode 40 FMC EFLH 20 
Capacitor 
50 0.03 microfared 
(75 Sprague 30GA 
0.01 .mu.f capacitors) 
Inductor 52 0-560 microhenries 
Resistor 66 54 ohms - 100W 
Resistor 70, 72, 74, 76 2.2 M - 2W 
Resistor 78 27K - 1/2W 
Transformer 
90 120V/120V 
Diode 92 1N4007 
Capacitor 
94 4 microfared 
Resistor 95 15K - 2W 
Zener Diode 
96, 98 1N5243B 
Transistor 
100 2N6014 
Resistor 102, 104 10K - 1/2W 
Capacitor 
106 0.01 microfared 
Transistor 
108 2N6116 
Potentiometer 
110 100K 
SCR 130 FMC 40C100 
Resistor 136 100K - 2W 
Capacitor 
138 0.01 microfared 
Resistor 142 1K - 1/2W 
Diode 144 Brown Boverie BOD1-10 
Diode 146 1N4007 
Resistor 148 2.2K - 1/2W 
Transformer 
150 1:1 
Capacitor 
152 0.01 microfared 
______________________________________ 
In operation, closure of circuit breaker 14 applies power to start relay 
contacts 18, blower 24, and power supply 86. Closure of control switch 22 
energizes relay 20 to close contacts 18, applying power through step-up 
transformer 30 to rectifier network 38 to charge capacitor 50. The voltage 
at the positive terminal 68 of capacitor 50 rises as the rectified line 
voltage increases from the zero voltage crossing. The voltage is monitored 
by the voltage divider network of resistors 70-78 and the voltage at tap 
114 is applied to the base of transistor 100 to charge capacitor 106. When 
the voltage across capacitor 106 exceeds the voltage applied to gate 118 
of PUT 108 by potentiometer 110, transistor 108 is fired and capacitor 106 
discharges through that transistor to apply a trigger pulse to terminal 
116 of electronic switch 80. 
The trigger pulse applied at terminal 116 is coupled by transformer 150-1 
to fire SCR 130-1, i.e., effectively short circuiting anode 132-1 to 
cathode 134-1. Before firing, commutating capacitor 152-1 has been charged 
to the voltage across SCR 130-1. When SCR 130-1 fires, capacitor 152-1 is 
discharged through the primary of transformer 150-2, inducing a current 
pulse on the secondary of transformer 150-2 that fires SCR 130-2. As this 
serial triggering process is repeated the voltage across each of the 
remaining unfired SCR's increases. When the voltage exceeds the voltage 
limit of break-over diodes 144, the break-over diodes conduct and pass 
current pulses which concurrently fire the remaining nonconducting SCR's. 
This triggering action effectively closes switch 80, connecting the plus 
terminal 68 of charged capacitor 50 to ground and transferring the voltage 
of charged capacitor 50 to spark gap 60. Gap 60 breaks down, completing a 
circuit for capacitor 50 to discharge through inductor 52 and gap 60. 
A waveform diagram of currents and voltage during the spark discharge is 
shown in FIG. 2 (with these parameters indicated in FIG. 3--a simplified 
diagram of the circuit shown in FIG. 1). Current flow I.sub.C from charged 
capacitor 50 commences in response to breakdown of gap 60 and flows 
through gap 60, rising rapidly and reaching a peak of about eighty amperes 
approximately 3.5 microseconds after switch 80 is closed. The voltage 
E.sub.C on capacitor 50 concurrently falls from its initial value of about 
6700 volts and reaches zero volts approximately 3.5 microseconds after 
switch 80 has been closed. At that time, and in response to that capacitor 
voltage, rectifier diodes 40 conduct (the diode current being indicated at 
I.sub.D) and essentially prevent the voltage on capacitor 50 from swinging 
negative. The conduction of diodes 40 provides a current flow path around 
capacitor 50, effectively shunting capacitor 50 so that the capacitor 
current I.sub.C falls and the spark gap discharge circuit is converted 
from a classical RLC oscillatory circuit to a classical RL circuit in 
which the gap current I.sub.G (supplied by inductor 52) falls to zero in 
about 100 microseconds. The circuit thus provides a spark discharge in the 
form of a substantially nonoscillatory triangularly shaped current pulse 
that has a rapid rise and a gradual fall. 
The specific current waveform across spark gap 50 depends on circuit 
parameters, such as the trigger voltage, the values of capacitor 50 and of 
inductor 52, residual impedances in series with the diodes 40 and 
distributed inductances of the connecting leads for example, which may be 
changed to provide different spark gap current pulses as desired. In a 
typical analysis cycle, the spark source circuitry of FIG. 1 generates 240 
sparks per second for a period of thirty seconds. The spark source 
circuitry operates normally (without harm) with the spark stand short 
circuited. Also, should sample 58 be left out of the spark stand, there 
will be no spark but capacitor 50 will discharge through resistor 66 in 
response to each trigger signal. These features provide increased circuit 
reliability. 
Shown in FIG. 4 is a spark gap circuit similar to the FIG. 1 circuit that 
uses a conventional control blower gap 80' as the discharge triggering 
switch. The waveforms shown in FIGS. 5A, 5B, and 5C are oscillograms of 
the gap current waveform in the circuit of FIG. 4 with a capacitor 50' of 
0.03 microfared value and an inductor 52' with 40 turns (FIG. 5A); 70 
turns (FIG. 5B); and 80 turns (FIG. 5C) respectively. The oscillogram of 
FIG. 5A shows a spark gap current pulse with a peak amplitude of about 90 
amperes and a duration of about 80 microseconds; the oscillogram of FIG. 
5B shows a spark gap current pulse with a peak amplitude of about 65 
amperes and a duration of about 120 microseconds; and the oscillogram of 
FIG. 5C shows a spark gap current with a peak amplitude of about 55 
amperes and a duration of about 130 microseconds. Other waveform 
variations can be obtained by changing other circuit parameters. 
While particular embodiments of the invention have been shown and 
described, other embodiments will be apparent to those skilled in the art, 
and therefore it is not intended that the invention be limited to the 
disclosed embodiment or to details thereof and departures may be made 
therefrom within the spirit and scope of the invention.