Adjustable waveform spark source

A capacitor is charged to a high voltage, and then is discharged through a circuit comprising an analytical spark gap and first and second inductive elements, whereby the discharge current is oscillatory. The waveform of the discharge current is modified by a diode rectifier shunted across the series combination of the analytical spark gap and the second inductive element. To provide for adjustment of the waveform over a wide range, the second inductive element is adjustable in inductance. The first inductive element is also preferably adjustable in inductance. Provision may also be made for changing the capacitance of the capacitor. A reversing switch makes it possible to reverse the polarity of the diode rectifier. One or more control spark gaps may be connected in series with the analytical spark gap. An electronic switching device may be connected across one of the control spark gaps to initiate the discharge of the capacitor. Either or both of the inductive elements may be replaced with other reactances or impedances, such as transmission lines. Coupling may be provided between the inductive elements. Inductance or impedance may be provided in the diode circuit. The capacitor may be replaced with some other source of alternating current or pulses. The spark may be ignited by high voltage, developed at a radio frequency by a quarter wave line. The diode rectifier may be replaced with a silicon controlled rectifier, silicon controlled switch, or some other active element.

This invention relates to spark sources, which will find many applications, 
but are especially advantageous as light sources for spectroscopic 
analysis, particularly optical emission spectrometry and 
spectro-chemistry. 
Spark type light sources have an established place in optical emission 
spectrometry and other spectroscopic analytical techniques, whereby the 
composition of test samples can be determined. In such techniques, spark 
discharges are produced across an analytical spark gap. Material from the 
sample to be analyzed is introduced into the spark gap in that some of the 
material is vaporized by the spark discharges. The light emitted by the 
spark discharges is analyzed spectroscopically to determine the 
composition of the material. Generally speaking, the emitted light 
produces complex spectra containing many spectral lines. The spectroscopic 
analysis involves noting the presence of various spectral lines and 
measuring their relative intensities. By such spectroscopic techniques, it 
is possible to make accurate analyses of a wide variety of materials. Such 
techniques are especially valuable in analyzing metals and metal alloys. 
The basic composition of a metal alloy and the quantities of any 
impurities present in the alloy can be measured with a high degree of 
accuracy and sensitivity. Moreover, the test sample of the material to be 
analyzed can be very small in size. 
The spark discharge is generally produced by charging a capacitor and then 
discharging it across the analytical spark gap. The capacitor is charged 
from a direct current power supply which delivers a high voltage. 
The discharge current through the spark gap tends to be oscillatory because 
of inductance in the discharge circuit. There is always a certain amount 
of inherent inductance. Moreover, an inductive element, such as an 
inductance coil, may be provided in the discharge circuit to provide for 
an oscillatory discharge at a definite frequency. Thus, the discharge 
current generally comprises a damped train of alternating current pulses. 
The damping is provided by the effective resistance of the spark gap and 
the resistance of the other elements in the discharge circuit. 
One object of the present invention is to provide a spark type light source 
which is constructed and arranged so that the waveform of the oscillatory 
discharge current can be adjusted. Such adjustment makes it possible to 
carry out the spectroscopic analyses with considerably improved accuracy 
and facility. By providing for adjustment of the waveform, it is possible 
to produce a discharge current waveform having a high peak value in one 
half cycle, followed by a low peak value in the next half cycle. Thus, the 
current pulses are alternately at high and low peak values. The high value 
pulses have the advantage of producing high vaporization of the material 
to be analyzed. The alternate low value pulses make it possible to take 
spectroscopic photographs having improved contrast and resolution, 
particularly as to various weak spectral lines which tend to be obscured 
by background radiation. The accurate determination of intensities of such 
weak spectral lines is an important factor in the analysis of various 
materials, particularly as to the presence of impurities in metal alloys. 
It is a known technique to take spectroscopic photographs which are 
resolved or spread out as to time, so that the spectral lines 
corresponding to different portions of the oscillatory discharge current 
can be separately measured. 
In accordance with the present invention, a diode rectifier is shunted 
across the analytical spark gap and a reactive element is connected in 
series therewith. The reactive element may comprise an inductance coil, a 
transmission line or some other impedance. Usually, the reactive element 
predominantly comprises inductive reactance but capacitive reactance and 
resistance will always be present to some degree. The diode rectifier 
modifies the waveform of the oscillatory discharge current, so that the 
pulses are of alternately high and low peak values. The reactive element 
may be adjustable in inductive reactance. By adjusting this reactance, it 
is possible to adjust the waveform of the discharge current over a wide 
range. In particular, the magnitude of the smaller pulses can be changed 
without greatly affecting the magnitude of the alternate larger pulses. It 
is even possible to reverse the polarity of the intermediate smaller 
pulses so that the spark discharge current is undirectional. Preferably, 
the discharge circuit also includes another reactive element which is not 
shunted by the diode rectifier, but is the major factor in determining the 
basic frequency of the oscillatory discharge. The additional reactive 
element may comprise an inductance coil, a transmission line or some other 
impedance. Here again, the reactive element usually predominantly 
comprises inductive reactance, but capactive reactance and resistance will 
also be present. The additional reactive element may also be adjustable or 
variable in reactance. The variation of either reactive element affects 
the waveform of the discharge current. The value of the storage capacitor 
may also be adjustable. In some cases, the discharge circuit includes one 
or more control gaps, in series with the analytical spark gap. The 
discharge of the capacitor may be initiated by triggering an electronic 
switching device, connected across one of the control spark gaps. 
Various other objects, advantages and features of the present invention 
will appear from the following description, taken with the accompanying 
drawings, in which: 
FIG. 1 is a schematic circuit diagram of an adjustable waveform spark type 
light source to be described as an illustrative embodiment of the present 
invention. 
.[.FIG. 2 comprises.]. .Iadd.FIGS. 2a-2d comprise .Iaddend.a series of 
diagrams illustrating the manner in which the diode rectifier and the 
associated reactive element modify the waveform of the discharge current. 
.[.FIG. 3 comprises.]. .Iadd.FIGS. 3a-3d comprise .Iaddend.a series of 
waveform diagrams, showing the manner in which the waveform of the 
discharge current is varied by changing the inductance of the shunted 
reactive element. 
.[.FIG. 4 comprises.]. .Iadd.FIGS. 4a-4d comprise .Iaddend.a series of 
waveform diagrams, illustrating the manner in which the waveform of the 
discharge current is varied by changing the inductance of the unshunted 
reactive element. 
.[.FIG. 5 comprises.]. .Iadd.FIGS. 5a-5h comprise .Iaddend.a series of 
oscillograms showing the changing waveform of the spark gap current with 
changes in the values of the reactive elements. 
FIG. 6 comprises a series of graphs showing the changes in the 
spectrograms, produced by changes in the waveform of the spark gap 
current. 
FIG. 7 comprises a series of graphs constituting a continuation of FIG. 6. 
FIG. 8 is a schematic circuit diagram showing a simplified version of the 
spark source shown in FIG. 1. 
FIG. 9 is a generalized version of the circuit diagrm of FIG. 8. 
FIG. 10 is a circuit diagram of a modified spark source. 
FIG. 11 is a circuit diagram representing a generalized and modified 
version of the spark source of FIG. 8. 
FIG. 12 is a circuit diagram representing a modified and generalized 
version of the spark source of FIG. 10. 
FIG. 13 is a circuit diagram similar to FIG. 11, but showing a modified 
spark source. 
FIG. 14 is a circuit diagram representing a generalized version of FIG. 13. 
FIG. 15 is a circuit diagram, similar to FIG. 14, but showing a further 
modification. 
FIG. 16 is a circuit diagram similar to FIG. 8, but showing another 
modified spark source. 
FIG. 17 is a circuit diagram representing a generalized version of FIG. 16. 
FIG. 18 is a circuit diagram similar to FIG. 17, but showing further 
modifications. 
FIG. 19 is a circuit diagram showing another modified spark source in which 
the spark is ignited by high voltage energy at a radio frequency. 
FIG. 20 is a circuit diagram showing a modified and generalized version of 
the spark source of FIG. 19.

As already indicated, FIG. 1 is a schematic wiring diagram of a spark 
source 10, which will find various applications but is particularly 
advantageous as a light source for use in making spectroscopic analyses. 
The light source 10 comprises an analytical spark gap 12 into which the 
material to be analyzed is to be introduced. It will be seen that the 
spark gap 12 comprises spaced electrodes 14 and 16. The material to be 
analyzed may be placed on one or both of the electrodes 14 and 16; or it 
may constitute one of the electrodes or exist as a gas between the 
electrodes. 
The analytical sparks are produced by discharging a capacitor 18 across the 
spark gap 12. The capacitor 18 is adapted to store an electrical charge at 
a high voltage. The abrupt discharge of the capacitor 18 across the spark 
gap 12 produces a discharge current of great magnitude, so that some of 
the material to be analyzed is vaporized and ionized. Accordingly, the 
spark produces light containing the optical emission spectra of the 
various constituents contained in the material being analyzed. 
The capacitor 18 is preferably arranged so that its capacitance is variable 
or adjustable. As will be understood by those skilled in the art, this may 
be achieved by subdividing the capacitor 18 into various capacitive 
elements and providing a switching arrangement or other means whereby the 
various capacitive elements can be selectively connected into the circuit. 
Variable capacitor elements may also be employed. 
Means are provided to charge the capacitor 18 to a high voltage. For this 
purpose, the illustrated apparatus 10 comprises a charging circuit 20 
which utilizes a transformer 22 and a rectifier 24, together with a 
variable regulating transformer 26 and a current limiting impedance 28. 
The transformer 22 is of the step-up type having primary and secondary 
windings 30 and 32. The regulating transformer 26 supplies the primary 
winding 30 with an alternating voltage of variable magnitude. In this 
case, the regulating transformer 26 is of the autotransformer type 
comprising a single winding 34 having an adjustable tap 36. The winding 34 
is connected across a pair of input line conductors 38 supplied with 
alternating current at 220 volts and 60-cycles, or any other suitable 
voltage and frequency. 
The primary winding 30 of the step-up transformer 32 is connected between 
the adjustable tap 36 and one of the power lines 38. Thus, the voltage 
developed by the secondary winding 32 may be changed by adjusting the tap 
36 on the regulating transformer 26. 
Because of the high voltage involved, the rectifier 24 preferably comprises 
a stack or series of diode rectifiers 42. Voltage equalizing resistors 44 
of high value are preferably shunted across the diodes 42. 
The rectifier assembly 24 and the current limiting impedance 28 are 
connected between the transformer 32 and the capacitor 18 so that the 
capacitor 18 is charged through the rectifier assembly 24 and the 
impedance element 28. The impedance element 28 may simply take the form of 
a resistor which limits the peak charging current through the rectifier 
diodes 42. It will be understood that the alternating current supplied by 
the transformer secondary 32 is rectified by the rectifier assembly 24 so 
that the capacitor 18 is charged to a high direct voltage. 
The storage capacitor 18 and the analytical spark gap 12 are connected into 
a discharge circuit 46, whereby the capacitor is discharged across the 
spark gap 12. To provide a switching action, the illustrated discharge 
circuit 46 comprises one or more control spark gaps which are employed to 
control the spark discharge across the main analytical gap 12. As shown, 
the discharge circuit 46 comprises two control spark gaps 48 and 50. 
The discharge current across the analytical spark gap 12 is normally 
oscillatory, because of the presence of capacitance and inductance in the 
discharge circuit. To regulate and control the oscillatory character of 
the discharge current, it is preferred to provide first and second 
inductive or reactive elements 51 and 52 in the discharge circuit 46. It 
will be seen that the reactive elements 51 and 52 are connected in series 
with the analytical spark gap 12. In the illustrated arrangement, the 
reactive elements 51 and 52 are predominantly inductive, and hence will be 
sometimes referred to as inductive elements. However, it will be 
understood that the reactive elements 51 and 52 include components of 
capacitive reactance and resistance, as well as the predominant inductive 
reactance. The reactive or inductive elements 51 and 52 may comprise 
separate inductance coils, or may be combined into a single coil with a 
tap between the two inductive elements. Preferably, at least one of the 
inductive elements 51 and 52 is variable or adjustable in inductance. As 
shown, both inductive elements 51 and 52 are adjustable. This 
adjustability may be achieved in any known or suitable manner. Thus, each 
of the inductive elements 51 and 52 may be in the form of a variable 
inductor of any known or suitable construction. The variable inductors may 
utilize movable coils, movable taps or movable cores. The adjustability of 
the inductive elements may also be achieved by providing a plurality of 
taps and a switching arrangement. Plug-in or interchangeable inductive 
elements of various values may also be employed to achieve the desired 
adjustability. 
In the illustrated discharge circuit 46 the control spark gaps 48 and 50 
are connected in series with the main analytical spark gap 12. The control 
spark gaps 48 and 50 may be adjustable to control the distribution of the 
voltage. 
In the illustrated arrangement, the discharge of the capacitor 18 is 
initiated by connecting a switching device 54 across one of the control 
spark gaps, in this case the gap 50. The shunting of the control gap 50 
increases the voltage across the gaps 12 and 48 so that the spark 
discharge is initiated. 
The switching device 54 is preferably of the electronic type, illustrated 
as a Thyratron or other arc discharge tube 56. It will be seen that the 
anode and the cathode of the tube 56 are connected across the control 
spark gap 50, a current limiting resistor 58 being connected in series 
with the anode. The control electrode of the tube 56 is connected to an 
input terminal 60 through a coupling capacitor 62. The tube 56 is 
triggered into a conductive state by applying a positive pulse to the 
input terminal 60, relative to the cathode, which is grounded. The tube 56 
is initially maintained in a nonconductive state by a negative biasing 
voltage, applied to the control electrode by a bias supply 64. A return 
resistor 66 of high value is preferably connected between the control 
electrode and the cathode of the tube 56. 
The initial voltage across the control gap 50 is preferably established by 
a voltage divider 68 comprising resistors 70 and 72 connected across the 
capacitor 18. The control gap 50 is connected across the resistor 72. When 
the arc discharge tube 56 is triggered into a conductive state, the 
control spark gap 50 is virtually short-circuited, inasmuch as the shunt 
path through the tube 56 and the resistor 58 has a very low resistance. 
Thus, virtually the entire capacitor voltage 18 is applied across the 
spark gaps 12 and 48, which are adjusted so that these gaps will break 
down to initiate the spark discharge. 
To modify the waveform of the discharge current across the analytical spark 
gap 12, it is preferred to provide a diode rectifier 74 adapted to be 
shunted across the series combination of the analytical spark gap 12 and 
the second inductive element 52. It is preferred to provide means whereby 
the polarity of the diode rectifier 74 can be reversed. As shown, such 
means take the form of a reversing switch 76. Normally, the diode 
rectifier 74 is polarized so that it is initially nonconductive. Thus, the 
initial high voltage across the analytical spark gap 12 is applied 
inversely to the diode 74. To aid in discharging capacitor 18, a resistor 
78 of high value is preferably shunted across the analytical spark gap 12. 
It will be evident that the diode 74 must be capable of withstanding the 
entire voltage developed across the capacitor 18. To provide the necessary 
voltage rating, the diode rectifier 74 may actually comprise a stack or 
series of individual diodes. 
When the capacitor 18 is discharged across the spark gaps 12 and 48, the 
discharge current tends to be oscillatory, because of the presence of both 
capacitance and inductance in the discharge circuit. The oscillatory 
character of the discharge is clearly illustrated in the oscillograms of 
FIG. 5, which represent the waveform of the analytical spark gap current 
for various values of the inductive elements 51 and 52. It will be seen 
tha the oscillatory discharge is damped in that the alternating pulses 
decay or decline in magnitude with the passage of time. 
The pulses of the discharge current alternate in polarity, so that the odd 
numbered pulses are of one polarity, while the even numbered pulses are of 
the opposite polarity. The diode 74 is normally polarized in a direction 
opposite to the initial current pulse or surge. Thus, the diode does not 
have any substantial shunting effect upon the initial pulse. The same is 
true of the odd numbered pulses which have the same polarity as the 
initial pulse. However, the shunting effect of the diode 74 tends to 
attenuate the second pulse and the other even numbered pulses. 
By adjusting the first and second inductive elements 51 and 52, 
particularly the second inductive element 52, the shunting effect of the 
diode can be greatly changed, so that the waveform of the spark gap 
current can be varied over a wide range. This is graphically illustrated 
by the successive oscillograms of FIG. 5. FIG. 5(a) represents the spark 
gap current when the value of the second inductive element 52 is 
essentially zero. For this condition, the diode has no appreciable effect 
upon the waveform. FIGS. 5(b), (c), (d), (e), (f) and (g) represent the 
spark gap current for successively increased values of the second 
inductive element 52. It will be seen that the shunting effect of the 
diode 74 becomes progressively more pronounced. While the first and other 
odd numbered pulses 81 are relatively unaffected, the second and other 
even numbered pulses 82 are progressively attenuated. In the oscillograms 
of FIGS. 5(f) and (g), the shunting effect of the diode 74 is so 
pronounced that the polarity of the even numbered pulses 82 is reversed. 
In the condition represented by the oscillogram of FIG. 5(h), the value of 
the first inductive element 51 has been decreased. This change increases 
the basic frequency of the oscillatory discharge current, while also 
increasing the shunting effect of the diode. 
The following table gives illustrative values of the inductive elements 51 
and 52 for the oscillograms of FIGS. 5(a)-(h). 
______________________________________ 
Inductances in microhenrys 
FIG. L 51 L 52 
______________________________________ 
5(a) 150 0 (residual) 
5(b) 150 0.8 
5(c) 150 2.4 
5(d) 150 5.6 
5(e) 150 8.8 
5(f) 150 12.0 
5(g) 150 16.0 
5(h) 16.0 
______________________________________ 
Illustrative values of the other components shown in FIG. 1 are as follows: 
______________________________________ 
Transformer 
22 23 KV. rms. 
Rectifier 24 RCA CR-110 
Rectifier 74 Westinghouse #1-18M-lH/441B-D 
Tube 56 RCA 5563-A 
Capacitor 18 0.0625 microfarad, 20 KV. 
Capacitor 62 1 microfarad, 600 v. AC 
Resistors: 28 1500 ohms, 200 W. 
44 20 megohms, 5 W. 
58 50 K, 100 W. 
66 47 K, 1 W. 
70 20 megohms, 5 W. 
72 20 megohms, 5 W. 
78 1 megohm, 5 W. 
Inductor 51 Self-inductance, 0-300 microhenrys 
52 Self-inductance, 0-16 microhenrys 
______________________________________ 
Those skilled in the art will understand that the values of the various 
components can be varied widely, to suit various conditions. 
FIG. 2 illustrates the shunting action of the diode in various stages of 
the oscillatory discharge. In the diagrams of FIG. 2, the effective or 
dynamic impedance of the analytical spark gap 12 is represented by a 
resistor 92. The other components shown in FIG. 2 are the same as in FIG. 
1. These components comprise the capacitor 18, the first and second 
inductive elements 51 and 52, and the diode 74. 
FIG. 2 is a qualitative representation of the wave shaping action of the 
diode 74 and the inductive elements 51 and 52. In FIG. 2(a) the discharge 
has been triggered and the first quarter cycle of the discharge current is 
in progress. The discharge current flows through the spark gap impedance 
92 and the inductance 52 because the diode 74 is reverse biased and is 
nonconductive. This current flow is indicated by the arrows in FIG. 2(a). 
During the time interval represented by FIG. 2(a), the first peak current 
of the oscillatory discharge appears in the spark gap impedance 92. The 
diode 74 sustains a reverse bias until the current peaks, because the 
potential drop across the diode is determined by the inductance 52 in 
series with the spark gap impedence 92. 
In FIG. 2(b), the current has peaked and the capacitor voltage is just 
beginning to reverse. At approximately this time, the diode 74 goes into a 
forward biased condition, because the decreasing current produces an 
induced voltage of reverse polarity in the inductance 52. The forward bias 
establishes the current shunt path through the diode 74. The point in time 
with respect to the main oscillatory current at which the diode 74 
sustains a forward bias depends upon the phase angle of the voltage drop 
across the diode, relative to the oscillatory current. Due to the 
inductive reactance of the inductive element 52, the voltage drop across 
the diode 74 leads the discharge current by a phase angle which increases 
as the inductance of the element 52 is increased. This angle approaches 
90.degree. when the inductive reactance is increased so that it greatly 
exceeds the dynamic impedence 92 of the spark gap. Inasmuch as a typical 
spark gap impedence may be only about 2 ohms, the inductive reactance of 
the element 52 does not have to be very great to produce a phase angle 
approaching 90.degree.. Thus, the voltage drop across the diode 74 
reverses in polarity to produce a forward bias, shortly after the peak of 
the discharge current. 
After the diode 74 is in a forward biased condition so as to be conductive, 
the bias is current controlled and is determined largely by the voltage 
drop across the forward impedence of the diode itself. The forward 
impedence of the diode is generally substantially lower than the impedence 
of the inductance 52 and the dynamic spark gap impedence 92. The diode 
impedence is substantially resistive, so that the forward bias is in phase 
with the current through the diode. As long as the forward current is 
sustained through the diode 74, it continues to be conductive. Thus, there 
are two distinct biasing regions for the diode 74 with respect to the main 
discharge current. The first biasing region controls the turn-on of the 
diode. In this region, the biasing voltage assumes a leading phase angle 
with respect to the discharge current, as determined by the inductive 
impedence of the inductive element 52 in series with the spark gap 
impedence 92. The second biasing region controls the diode turn-off. The 
turn-off point is not reached as long as the forward biasing current is 
maintained. After the diode has been turned on, the forward biasing 
current is initially maintained by the relaxation of the inductance 52, 
which produces a loop current through the diode 74, the spark gap 
impedence 92 and the inductance 52, as indicated by the arrows in FIG. 
2(b). This loop current is due to the voltage induced in the inductance 52 
by the decreasing discharge current through the inductance. The loop 
current tends to sustain and prolong the initial discharge current through 
the spark gap impedence 92. During this interval, prior to the reversal of 
the main oscillatory discharge current, the only current in the diode 74 
is due to the field relaxation in the inductance 52, because the main 
discharge current is still in a direction that would cause reverse bias in 
the diode 74. 
FIG. 2(c) illustrates the condition after the reversal of the main 
oscillatory discharge current, due to the reverse discharge of the 
capacitor 18. The main discharge current is now in a direction to cause 
forward bias in the diode 74. Thus, the main discharge current combines 
with the loop current to sustain the forward bias of the diode 74. This 
situation continues during the entire second half cycle of the main 
dishcarge current, because the main discharge current continues to flow in 
a direction such as to sustain the forward bias. Thus, the diode 74 is 
turned on slightly after the peak of the first half cycle of the main 
discharge current, and is maintained in a conductive state until 
completion of the entire first current cycle. When the diode is 
conductive, the spark gap current is modified by the loop current, due to 
the relaxation of the inductance 52. During the second half cycle of the 
main discharge current, the spark gap current is substantially attenuated 
by the bypassing of the current through the conductive diode. 
It will be evident from this discussion that the second half cycle of the 
current through the spark gap can be varied over a wide range by adjusting 
the value of the inductance 52. This changes the inductive reactance of 
the element 52 and varies the extent to which the main discharge current 
is bypassed by the diode 74. Moreover, the variation of the inductance of 
the element 52 changes the turn-on point of the diode 74 to some extent. 
FIG. 3 comprises a series of graphs, showing the manner in which the 
waveform of the spark current may be changed by adjusting the value of the 
shunted inductance 52. These graphs show the waveforms of the main 
discharge current, the diode current, and the spark current. The four 
graphs of FIG. 3 are drawn for four different values of the shunted 
inductance 52, but the unshunted inductance is held constant at 100 
microhenrys. The values of the shunted inductance 52 for the conditions 
represented by these graphs are as follows: 
______________________________________ 
FIG. 3(a) 0.8 microhenrys 
3(b) 5.8 microhenrys 
3(c) 10.5 microhenrys 
3(d) 15.1 microhenrys 
______________________________________ 
As already indicated, the main discharge current has a typical damped 
oscillatory waveform in all four cases. As already discussed, the diode 
becomes conductive shortly after the first peak of the main discharge 
current, at the end of the first quarter cycle, and the diode current 
continues throughout the remainder of the complete cycle. The diode 
current rises to a peak and then declines to zero, shortly after the 
beginning of the second complete cycle of the main discharge current. The 
spark current is the algebraic difference between the main discharge 
current and the diode current. 
It will be clearly evident from FIG. 3 that the peak diode current 
increases with the increasing values of the shunted inductance 52. This is 
due to two factors: increased loop current and the increased bypassing 
effect of the diode with the increasing shunted inductance. 
It will be seen that the peak of the diode current coincides roughly with, 
but is somewhat ahead of the second peak of the main discharge current. 
For relatively low values of the shunted inductance, the peak diode 
current is less than the second peak of the main discharge current, as 
shown in FIGS. 3(a) and (b). The second peak of the spark current is 
diminished or attenuated, relative to the main discharge current, due to 
the subtraction of the diode current therefrom, but the second peak of the 
spark current is still of unchanged polarity, despite the shunting effect 
of the diode. For larger values of the shunted inductance, as represented 
by FIGS. 3(c) and (d), the peak diode current actually exceeds the second 
peak of the main discharge current, so that the second peak of the spark 
current is reversed in polarity, relative to the main discharge current. 
Thus, the higher values of the shunted inductance result in a spark 
current waveform which is unidirectional, in that there is no change of 
polarity between the first and second peaks. This effect is clearly 
evident in FIGS. 3(c) and (d). 
It is worthy of note that the changes in the shunted inductance 52 do not 
appreciably change the magnitude of the first peak of the spark current. 
This is due to the fact that the diode 74 does not start to conduct until 
the first peak is reached, or shortly thereafter. The magnitude of the 
first peak is determined primarily by the values of the storage capacitor 
18 and the total inductance in the discharge circuit. The unshunted 
inductance 51 is usually much larger than the shunted inductance 52, and 
thus is the dominant factor in determining the first peak current. 
Increasing the value of the capacitor increases the first peak current. On 
the other hand, the first peak current is decreased by increasing the 
total inductance in the discharge circuit. 
The graphs of FIG. 4 illustrate the effect of varying the unshunted 
inductance 51, while keeping the shunted inductance 52 at a constant 
value. Here again, the waveforms of the main discharge current, the diode 
current and the spark current are shown. The shunted inductance 52 is held 
constant at 8.8 microhenrys. The values of the unshunted inductance 51 are 
as follows: 
______________________________________ 
FIG. 4(a) 230 microhenrys 
4(b) 127 microhenrys 
4(c) 74 microhenrys 
4(d) 31 microhenrys 
______________________________________ 
It will be seen that the first peak of the main discharge current is 
increased by decreasing the unshunted inductance 51. At the same time, the 
frequency of the oscillatory discharge current is increased. 
The peak diode current also increases when the unshunted inductance is 
decreased. This is due to two factors: The increased first peak of the 
main discharge current, which increases the loop current due to relaxation 
of the shunted inductance; and the increased inductive reactance of the 
shunted inductance, due to the increased frequency of the main discharge 
current. The second factor increases the bypassing action of the diode. 
In the situation represented by FIG. 4(a), the second peak of the spark 
current is decreased in amplitude from that of the main discharge current, 
but the polarity is the same. In the situations represented by FIGS. 4(b), 
(c) and (d), the diode current is so great that the second peak of the 
spark current is reversed in polarity relative to the main discharge 
current. Thus, the spark currents of FIGS. 4(b), (c) and (d) are 
unidirectional. 
In making certain types of spectrograms it is a great advantage to vary the 
waveform of the spark current, in the manner made possible by the present 
invention. The graphs of FIGS. 6 and 7 were made from certain spectrograms 
of the spectra produced by an aluminum alloy with a copper impurity. In 
making these spectrograms, the analytical spark gap 12 comprised an anode 
made of silver and a cathode made of the aluminum alloy with the copper 
impurity. FIGS. 6A, B and C and FIGS. 7D, E and F are graphs representing 
the relative line intensities of certain closely spaced spectra lines. As 
noted in FIGS. 6 and 7, one of the lines was produced by the silver in the 
anode, and the other two lines were produced by the copper impurity in the 
cathode. 
FIGS. 6G and 7H are graphs showing the waveforms of the spark currents, 
employed in making the six sets of spectrograms. It will be understood 
that the spectral graphs of FIGS. 6A, B and C were made with the spark 
waveforms designated A, B and C in FIG. 6G. Waveform A was produced by 
using a low value of shunted inductance, while waveforms B and C were 
produced with higher values. Thus, the second peak of the spark current in 
waveform A is attenuated very little, while the second peaks in waveforms 
B and C are more severely attenuated. This also applies to the other even 
numbered peaks. In these waveforms, the shunting action of the diode is 
not so great as to cause reversal of the polarity of the even numbered 
peaks. 
FIG. 6A comprises five graphs of time resolved spectrograms made at the 
first five even numbered peaks of the spark current. The first four of 
these instants of time are marked by the numerals 1-4 in FIG. 6G. 
It will be seen that the spectrographs of FIG. 6A show high background 
levels, and relatively poor resolution of the closely spaced lines, 
particularly the silver line at 3280.7 angstroms and the copper line at 
3274.0 angstroms. For the spectral graphs of FIG. 6B, the background is 
decreased and the resolution is improved. FIG. 6C also represents a highly 
favorable situation in this regard. Thus, it is highly advantageous to use 
the modified waveforms represented by graphs B and C in FIG. 6G. For 
spectro-chemical uses, the low background produces the advantage of a high 
signal to noise ratio. 
FIG. 7 represents the situation with still higher values of shunted 
inductance. The spark waveforms are shown in FIG. 7H and are designated D, 
E and F. The spectrographs of FIGS. 7D, E and F were derived from 
spectrograms made with these spark waveforms. In these three cases, the 
shunted inductance was so high that polarity reversal was produced for the 
even numbered peaks of the spark current. This is clearly evident in FIG. 
7H. 
FIGS. 7D, E and F represent the same spectral lines as in FIGS. 6A, B and 
C. It will be seen that FIG. 7D represents a fairly favorable situation, 
with a fairly low background and good resolution of the spectral lines. 
However, the background is greatly increased in FIGS. 7E and F, while the 
resolution is not as good as in FIG. 7D. 
It will be evident from FIGS. 6 and 7 that the best spectrograms are 
obtained when the pulses of spark current are small in magnitude and brief 
in duration. Such relatively small spark currents minimize the spectral 
background and improve the resolution of the closely spaced spectral 
lines. Such relatively small spark currents are in the ordinary arc 
discharge range. Higher spark currents overexcite the ions and produce 
more complex spectra, resulting in high background. 
Nevertheless, it is a distinct advantage to retain the high spark currents 
in the first pulse and the other odd numbered pulses of the spark current. 
These high currents result in very rapid vaporization of the material to 
be analyzed, so that the spark gap is kept full of the vaporized material. 
The high concentration of the material to be analyzed results in better 
spectrograms, particularly when it is necessary to make accurate 
measurements of an impurity or other constituent which is present in only 
a very small percentagte. The spectralgraphs of FIGS. 6 and 7 represent a 
situation of this kind, in which the copper is present in the aluminum 
alloy as an impurity in a proportion of only about 1 percent. 
As already indicated, the first peak of the spark current can be increased 
by decreasing the value of the unshunted inductance 51. This would 
normally increase the frequency of the main discharge current, but the 
value of the capacitor 18 can be increased to compensate for the decrease 
in the unshunted inductance, so that the frequency will remain constant. 
The present invention is highly advantageous in that the waveform of the 
spark current can be changed over a wide range, simply by adjusting the 
inductance of one coil. The apparatus of the present invention is highly 
versatile, yet highly economical. 
FIG. 8 represents a simplified and somewhat modified spark source which is 
generally very similar to the spark source of FIG. 1. Thus, the capacitor 
18 is adapted to be charged by the charging circuit 20, and is adapted to 
be discharged through the series circuit comprising the inductance coils 
51 and 52 and the spark gap 48. A switch 90 is also connected into this 
series circuit, for the purpose of initiating the discharge of the 
capacitor 18 across the spark gap 48. The switch 90 may comprise various 
switching means, such as the switching arrangement of FIG. 1, utilizing 
the control spark gaps 48 and 50 and the electronic switch 54. Various 
other electronic switching arrangements may be employed. The switch 90 may 
also simply comprise a mechanically operable switch. Normally, the switch 
90 is adapted to be operated repetitively so that a series of sparks is 
produced. Thus, the switch 90 may be rotary or vibratory in operation. 
However, it generally is preferable to employ a suitable electronic 
switch. 
The spark source of FIG. 8 also includes the rectifier 74, which is 
connected in the same manner as in FIG. 1, so that the rectifier is 
shunted across the portion of the circuit comprising the spark gap 48 and 
the second inductance coil 52. As shown in FIG. 8, this portion of the 
circuit also includes the switching means 90. In FIG. 8, the reversing 
switch 76 has been omitted, but it will be understood that the polarity of 
the rectifier 74 can be reversed, if desired. 
While the inductance coils 51 and 52 are predominantly inductive in 
reactance, these components should be regarded generally as reactive 
elements or impedances, having inductive, capacitive and resistive 
components. This concept is illustrated to better advantage in FIG. 9, in 
which the first reactive element 51 is illustrated as comprising 
inductive, capacitive and resistive components 91, 92, and 93. Similarly, 
the second reactive element 52 is illustrated as comprising inductive, 
capacitive and resistive components 94, 95 and 96. These components will 
inherently be present to some degree in the reactive elements 51 and 52. 
It will be understood that additional capacitive reactance, inductive 
reactance or resistance can be provided in the form of lumped components, 
if desired. Inductive reactance generally predominates in the reactive 
elements 51 and 52, but in some cases the capacitive reactance or the 
resistance may predominate. 
FIG. 10 illustrates a spark source which is the same as shown in FIG. 8, 
except that the inductance coils 51 and 52 are replaced by transmission 
lines 101 and 102. For example, such transmission lines may be in the form 
of lengths of coaxial cable. The line 101 is represented as including 
inductive, capacitive and resistive elements 103, 104, and 105. Similarly, 
the line 102 is represented as comprising inductive, capacitive and 
resistive components 106, 107 and 108. The inductive reactance generally 
predominates in each of the transmission lines 101 and 102. It has been 
found that the transmission lines produce interesting and useful 
modifications in the waveform of the spark discharge current. Generally, 
the transmission lines introduce stepped patterns into the waveform, 
apparently due to reflections travelling along the lines. 
FIG. 11 shows a generalized and modified version of the spark source shown 
in FIGS. 8 and 9. The reactive elements 51 and 52 are replaced with 
generalized impedances 111 and 112, which may include inductive, 
capacitive and resistive components in various proportions. A third 
impedance 113 is shown directly in series with the rectifier 74. This 
impedance also modifies the waveform of the spark discharge current. As in 
the case of the impedances 111 and 112, the impedance 113 is usually 
arranged so that inductive reactance predominates. However, in some cases 
the capacitive component or the resistive component could predominate. 
In FIG. 11, the switch 90 is the same as discussed in connection with FIG. 
8, and is preferably of the electronic type. The switch 90 may be 
triggered by repetitive pulses from a control pulse source 116. 
FIG. 11 also shows an additional switch 118 which illustrates the fact that 
the switch may be located anywhere in the series circuit between the 
capacitor 18 and the spark gap 48. The switch 118 may be regarded as being 
the same in construction as the switch 90. As shown, the switch 118 is 
located in the series circuit between the capacitor 18 and the first 
impedance 111. 
FIG. 12 represents a spark source which constitutes a still further 
generalization of the spark sources shown in FIGS. 10 and 11. The 
transmission lines 101 and 102 of FIG. 10 are replaced with complex 
impedance networks 121 and 122, illustrated as comprising series 
impedances 123 and 124 and shunt impedances 125 and 126. All of these 
impedances can be varied to modify the waveform of the spark discharge 
current. 
In the spark sources as thus far described, the rectifier 74 is in the form 
of an ordinary diode, or a series of such diodes. However, the rectifier 
74 may be replaced with various other active elements, adapted to cause 
shunting of the spark gap during a portion of each cycle of the main 
discharge current. Thus, for example, FIG. 13 illustrates a spark source 
which is generally the same as illustrated in FIG. 11, except that the 
rectifier 74 has been replaced with a silicon controlled rectifier (SCR) 
134 having an anode 135, a cathode 136 and a gate or control electrode 
137. The SCR 134 has a rectifying action, and also is capable of carrying 
out switching functions, in response to pulses or other signals applied to 
the gate 137. Thus, the SCR 134 can be controlled so as to be conductive 
during a variable portion of the cycle. For example, by changing the 
signals on the gate 137, the conductive portion of the cycle can be 
progressively reduced. 
It is convenient to utilize the control pulse source 116 to provide the 
control signals for the SCR 134, in addition to providing the control 
signals for the electronic switch 90. The control pulse source 116 can be 
arranged so that the interval between conduction of the switch 90 and 
conduction of the SCR 134 can be varied to change the waveform of the 
spark current. 
FIG. 14 illustrates a spark source which is the same as that shown in FIG. 
13, except that an active element 144 is shown in place of the SCR 134. 
Various active elements may be employed, in addition to the SCR, already 
discussed. Thus, the active element 144 may take the form of a silicon 
controlled switch, a Triac, an integrated switching circuit, various other 
solid state switches, or an electron discharge tube, for example. All such 
active elements are capable of controlling and varying the portion of the 
cycle during which the spark gap 48 is shunted. It will be understood that 
the active element 144 may be employed in any of the illustrated spark 
sources. 
As already indicated, the capacitor 18 constitutes means for producing an 
oscillating or alternating current through the spark gap circuit. The use 
of such a capacitor is advantageous, because the capacitor is capable of 
delivering a high peak current. However, other sources of alternating 
current signals or pulses can be employed. 
Thus, FIG. 15 illustrates a modified spark source, similar to the 
generalized spark source of FIG. 14, but utilizing an alternating current 
signal source 148 in place of the capacitor 18. Various types of sources 
can be employed. For example, the source 148 can be constructed and 
arranged to develop alternating current signals electronically, usually by 
electronic switching circuits. The control signals for the electronic 
switch 90 and the active element 144 should preferably be timed to occur 
in a suitable relationship to the alternating current signals from the 
source 148. Thus, a control link 149 is shown between the control pulse 
source 116 and the alternating current signal source 148. The basic timing 
can be established by either source 116 or 148. The control link 149 is 
then utilized to supply timing pulses or signals to the other source. It 
will be understood that the generalized alternating current source 148 can 
be employed with any of the disclosed spark sources. 
FIG. 16 shows a spark source which is a modified form of the source shown 
in FIG. 8. As already indicated in connection with the discussion of FIG. 
1, the reactive elements 51 and 52 can take the form of a single inductnce 
coil with a tap thereon. This arrangement is illustrated in FIG. 16, which 
shows a single inductance coil 150 having portions 151 and 152 which 
provide the reactive elements. The rectifier 74 is connected to a tap 153 
on the coil 150. The tap 153 constitutes the dividing point between the 
inductive portions 151 and 152. The coil 150 is preferably arranged so 
that the inductance of the portions 151 and 152 can be changed, as before. 
In the spark source of FIG. 16, there is inherent mutual inductance or 
coupling between the two portions 151 and 152 of the coil 150. This 
coupling modifies the waveform of the current across the spark gap 148. 
During the portion of the cycle when the rectifier 74 is conductive, the 
current along the inductive portion 152 is affected by the current in the 
inductive portion 151, due to the coupling therebetween. 
It will be understood that the coupled inductance elements 151 and 152 of 
FIG. 16 may be replaced with other coupled reactive elements or 
impedances. This modification is illustrated in the spark source of FIG. 
17, in which the inductance elements 151 and 152 are replaced with 
impedances 161 and 162 with coupling 163 therebetween. These impedances 
represent any desired or suitable reactive elements. Another impedance 164 
is shown in series with the rectifier 74. The impedance 164 may be present 
as a separate impedance, or may represent the mutual impedance resulting 
from the coupling 163. The impedances 161, 162 and 164 may contain 
inductive and capacitive reactances and also resistance. Coupled 
transmission lines may be utilized as the impedances 161 and 162, for 
example. 
It will be recognized that the spark source of FIG. 17 is similar to that 
of FIG. 11, except for the coupling 163 between the impedances 161 and 
162. FIG. 18 represents a spark source which is similar to that shown in 
FIG. 17, except for modifications of the character discussed in connection 
with FIGS. 13-15. Thus, in the spark source of FIG. 18, the rectifier 74 
is replaced with the active element 144, as described in connection with 
FIG. 14. The capacitor 18 has been replaced with the alternating current 
signal source 148, as discussed in connection with FIG. 15. 
In all of the spark sources disclosed thus far, the spark discharge is 
initiated or triggered by switching means, represented as a switch 90. The 
spark gap 48 is broken down by the high voltage to which the capacitor 18 
is charged. The switch 90 can be omitted if other means are provided to 
initiate the spark discharge. 
In the spark source of FIG. 19, the spark gap is broken down by a 
radio-frequency signal at a high voltage. The capacitor 18 is then 
discharged across the spark gap. With this arrangement, the voltage across 
the capacitor 18 can be greatly reduced, because the voltage does not have 
to be great enough to break down the spark gap. The reduced voltage across 
the capacitor provides much greater flexibility in the selection of 
reactive elements to be used in the discharge circuit, and also in the 
selection of the rectifier or someother active element. 
In the spark source of FIG. 19, the high voltage at a radio-frequency is 
developed by a resonant quarter wave line 170 in the manner disclosed and 
claimed in the co-pending application of John P. Walters and Thomas V. 
Bruhns, Ser. No. 8,462, filed Feb. 4, 1970. Radio-frequency energy is 
supplied to the line 170 by a radio-frequency source or generator 172. A 
suitable frequency is 162 megahertz, but the frequency can be varied over 
a wide range. The radio-frequency source 172 is adapted to be actuated or 
triggered by control pulses or signals from a source 174. Each pulse 
produces a train of radio-frequency signals at a voltage sufficient to 
break down the spark gap 48. 
The illustrated quarter wave line 170 is of the coaxial type, comprising an 
axial line conductor 176, disposed within a cylindrical outer line 
conductor 178. The output of the radio-frequency source 172 is connected 
between one end of the central conductor 176 and the corresponding end of 
the outer conductor 178. The length of the line 170 is such as to produce 
quarter wave resonance at the operating frequency. The spark gap 48 is 
connected between the opposite end of the axial line conductor 176 and the 
corresponding end of the outer conductor 178. Thus, the remote end of the 
line 170 is open before the spark gap breaks down. Due to the quarter wave 
resonance, the sending end of the line 170 affords a low impedance. Thus, 
the radio-frequency source delivers its output into a low impedance. 
In FIG. 19, the arrangement of the capacitor 18, the reactive elements 51 
and 52, and the rectifier 74 is the same as in FIG. 8. However, instead of 
being connected directly to the spark gap 48, the reactive element 52 is 
connected to the central conductor 176 of the line 170. The outer 
conductor 178 is connected to the grounded side of the capacitor 18. The 
connection between the reactive element 52 and the line conductor 176 is 
by means of the lead 180, connected to the line conductor 176 at the 
sending end thereof, adjacent the radio-frequency source 172. A small 
radio-frequency choke coil 182 may be connected in series with the lead 
180, to prevent the passage of any substantial radio-frequency current 
along the lead, but the provision of this choke coil is not always 
necessary, because of the low radio-frequency voltage which exists at the 
sending end of the line, due to the low impedance at this point. 
By virtue of the quarter wave resonance, the line 170 builds up the 
radio-frequency voltage so that it is sufficiently great to break down the 
spark gap 48. When the spark gap becomes conductive, the capacitor 18 is 
discharged through the reactive elements 51 and 52, the line 170 and the 
spark gap 48. The shunting action of the rectifier 74 modifies the 
waveform of the spark current, in much the same manner as discussed in 
connection with FIG. 1. However, the initial voltage across the capacitor 
18 may be much lower than before, with the result that components having a 
much lower voltage rating can be employed for the rectifier 74 and the 
reactive elements 51 and 52. 
When the spark gap 48 becomes conductive, the quarter wave line 170 
reflects an increased impedance to the output of the radio-frequency 
source 172, so that the source is effectively unloaded or decoupled from 
the line 170. 
FIG. 20 illustrates a spark source which is similar to that shown in FIG. 
19, but with some previously discussed modifications. Thus, the inductance 
coils 51 and 52 are replaced with the complex or generalized impedances 
121 and 122 of FIG. 12. The impedance 113 of FIG. 11 is employed in the 
shunting circuit. The rectifier 74 is replaced by the active element 144 
of FIG. 14. Instead of the capacitor 18, the spark source of FIG. 20 
employs the alternating current signal source 148 of FIG. 15. As discussed 
in connection with FIGS. 14 and 15, the active element 144 is triggered by 
pulses from the control pulse source 116. Pulses or signals from the 
source 116 are also employed to trigger the radio-frequency source 172. 
The spark source of FIG. 20 is well adapted for use of coaxial transmission 
lines as the complex impedances 121 and 122. The voltage involved can be 
kept down to a level within the rated voltage of commercially available 
coaxial cables. By using coaxial cables of various lengths, the waveform 
of the spark current can be greatly changed.