NBS MONOGRAPH 109

Investigations of tlie Exploding

Wire Process as a Source for

High Temperature Studies

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
NATIONAL BUREAU OF STANDARDS

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Foreword

Pulsed electrical discharges in metal wires and in gaseous systems provide a means of producing high-energy molecules and ions corresponding to

high temperatures.

The investigation of transient phenomena, such as exj^loding wires and

pulsed gaseous discharges, requires a number of different experimental techniques with high time resolution. This monograph summarizes a series of

studies of experimental methods for the exploding wire process and presents

some of the results. Since the experimental methods may have other uses, they

have been described in some detail.

The research described in this monograph was performed at the National

Bureau of Standards for the Advanced Research Projects Agency and the

Propulsion Division of the Air Force Office of Scientific Research. The sponsorship of these agencies is gratefully acknowledged.

A. V. Astin, Director.

Ill
Contents

Page

Foreword iii 1. Introduction 1

2. Calculations of the composition, enthalpy, enti-opy, and density of

the explosion mixture 2

2.1. Introductory abstract 2

2.2. Assumptions 2

2.3. Method of calculation 2

2.4. Results and discussion 2

8. Electrical measurements 5

3.1. Introductory abstract 5

3.2. Experimental apparatus 5

3.3. Calibration 7

3.4. Exploding wire results 8

3.5. IMscussion 10

4. Optical studies and techniques 11

a. Introductory abstract 11

4.1. High-speed shutter system 11

a. Introduction 11

b. Fast-opening shutter 12

c. Results and diseussion 12

d. Fast-closing shutter 16

e. Conclusions 16

4.2. Photographic, photoelectric, and preliminary spectroscopic

studies 17

a. Introduction 17

b. Experimental apparatus and procedure 18

c. Wire, vessel, and environment 19

d. Spectroscopic instrumentation 19

e. Photomultiplier system 20

f. Exploratory photographic and photoelectric results 22

g. Conclusions from photographic and photoelectric results 29

h. Exploratory spectroscopic observations 30

i. Conclusions from exploratory spectroscopic studies 32

.1. Estimate of temperature from spectroscopic measurements— 33

4.8. Time-resolved photographic studies of the explosion spectrum— 84

a. Introduction 34

b. Experimental apparatus and procedure 35

c. Results and discussion 36

d. Spectral results with aluminum wires 36

e. Spectral results with titanium wires 40

4.4. Time-resolved observations of the spectrum of AlO 42

a. Introduction 42

b. Apparatus and Procedure 42

c. The A — X 22 Blue-Green System 44

d. Ultraviolet systems 44

e. Conclusions 49

5. References

iv
Investigation of the Exploding Wire Process as a

Source of High Temperature Studies

Esther G. Gassidy, Stanley Abramowitz, and Gharles W. Beckett

Numerous experiments with electrically exploded wires are described. The results

include time-resolved measurements of electrical energy, power, voltage, and current during the discharge; periodic still and high-speed photographs of the entire explosion

process ; integrated and time-resolved measurements of the intensity and spectral distri- bution of the radiation emitted ; and time-resolved absorption spectra from the products of the discharge, with emphasis on observations of the spectrum of the AlO molecule.

The apparatus, instrumentation, and fast-measurement techniques developed in order to permit these direct experimental observations and measurements, under the extreme and

transient conditions of the explosive discharge, are also described. Results from calculations of the composition, entropy, enthalpy, and density of the explosion mixture are given.

Key Words : Electrical discharges, exploding wire, high current, high speed, high

speed photography, high temperature, high voltage, light sources, timeresolved electrical measurements, time-resolved spectroscopy.

1. Introduction

The explosion generated by passing a large

I electrical current through a thin metal conductor

' has been the subject of numerous and diverse

studies, ranging from the early works of Anderson and Smith [1,2,3] ^ to those presented more

recently at the three Conferences on Exploding

Wire Phenomena [4,5,6]. In addition, as the

j ever-increasing number of publications [7,8] sugj gests, the process has found significant applicaI tions in modern technology. It is used to ignite

explosive devices on rockets and satellites, to

simulate nuclear explosive and micrometeorite

effects in the laboratory, to pump lasers, to produce light for high-speed photography, and so

on. Unfortunately, because of the diversity of

these applications and of the goals of the various

research groups, the results reported in the literature sometimes seem to be inconsistent or even

in conflict. One must, however, be alert to the

fact that all exploding wires do not have the

same behavior. There is no "ideal" wire explosion. The gross features of an explosion depend

upon the design and operating conditions employed, and one should be aware that a change

in even a single parameter (e.g., in wire material or dimensions, in energy input, in environment), may so affect the discharge and explosion

behavior that one process, rather than another,

' becomes important. For consistent, comparable

results with a particular system, therefore, the

behavior of the system over a range of operating

conditions should be determined by systematic

observations.

It is the purpose of this monograph to bring

together and report the results of numerous ex1 Figures in brackets indicate the literature references at the end of this paper.

ploding wire experiments performed at the National Bureau of Standards over the past five

years. In all cases, our principal goal was not to

accumulate knowledge about exploding wires,

but to develop a system and measurement techniques suitable for high temperature (above

2000 °K) thermodynamic and spectroscopic studies. The substances selected for study in the ex- periments were those of current interest in high

temperature and chemical propulsion work.

In view of our purpose, the experimental setup

and procedure were somewhat different from

those employed in most exploding wire experiments. Perhaps the most significant differences

were the following: (1) The stored energy available was high (ranging from 1,500 to 12,000 J)

relative to the mass of sample wires (about 2.5

mg). (2) The wires were exploded in a sealed

vessel. To date, the majoritj^ of investigators

have worked in the open air with energies of not

more than a few joules. (3) The explosion behavior was studied over the entire duration of

light emission. Most studies have been limited

to the time required for the electrical discharge,

times of up to 200 fisec. Kantenberg and Johnson

[9], who studied the aluminum-oxygen reaction

in commercial photoflash lamps, and Baker and

Warchal [10], who studied metal-water reactions

by this technique, are among the few who have

investigated the phenomena into the millisecond

range. (4) A relatively slow (total discharge

time 90 fisec, ringing frequency 50 kH)

discharge circuit was employed for generating

the explosions.

For the sake of clarity, different phases of the

overall program are introduced and described in

1
separate sections. Each section gives the experimental apparatus, the results, and the fast meas- urement techniqvies developed during that particular phase of the work. In addition, some

practical rules are provided for design of a system suitable for direct experimental observations

and measurements under the extreme and tran- [ sient conditions of the explosion. It is hoped that f the effort described will serve as a contribution

both to pure exploding wire research and to the

work of developing methods for measurement of

high-temperature data.

2. Calculations of the Composition, Enthalpy, Entropy, and Density

of the Explosion Mixture

2.1. Introductory Abstract

Because of the complex effects which the vari- ous operating conditions have upon exploding

wire phenomena, it is often difficult to select conditions which are favorable for a given type of

experiment. To assist in this task, a theoretical

calculation of the composition, entropy, enthalpy,

and density (as functions of pressure and temperature) of the explosion mixture was made.

A brief description of the method and some typical results of the computations are given in this

section.

2.2. Assumptions

The mixture considered was that generated by

explosion of an aluminum wire in a closed vessel

containing air under normal laboratory conditions. The mixture was assumed to pass through

a series of high-temperature, high-pressure states

where all of the particles in the mixture are in

thermodynamic equilibrium. This assumption was

considered reasonable because the time (10"^ to

10~^^ sec) [11] required for establishment of equilibrium conditions is very short compared to the

time required for the explosion. (Experiments

by the authors have shown that the current in

the discharge lasts about 10~* sec, and that the

radiation emitted lasts about 4 x 10~^ sec.)

The composition of the system before explosion

was assumed to be 100 parts air and 10 parts AI2

(79 parts N2, 21 parts O2, and 10 parts AU).

These ratios were selected primarily because they

approximated the conditions expected to prevail

in our experiments. The components of the gas

phase of the explosion mixture were assumed to

be AI2, N2, O2, Al, AlO, AI2O, AI2O2, N, NO,

NO2, N2O, and O. Solid and liquid phases were not considered. Other possible components,

such as N2O4 and the ions of O2''' and N2"'", were

neglected because they were thought to make up

only a very small pei'centage of the mixture. Free

electrons and the ion Al"*" were also not included.

This latter assumption is in error at the higher

temperatures (near 6000 °K) and lower pres- sures (near 0.01 atm). However, the error is probably small in the other pressure-temperature

regions where it is expected that the aluminum

oxides are the predominant aluminum component. Finally, it was not possible to include still other components, such as the aluminum nitrides

and the molecule Al203(g), because of the lack

of data in the literature. To the authors' knowledge, no data on the former have been reported.

The only reference uncovered on AI2O3, which is probably an important constituent of the mixture, was that of Medvedev [12] who, although

he has not observed Al203(g), states that he believes it may be present in the experimental mixtures of Brewer and Searcy [13].

2.3. Method of Calculation

The molar enthalpies Hj and the molar entropies Sj of each component j (1 to 12) at nor- mal pressure (1 atm) for each 500° temperature

interval T between 1500 and 6000 °K, as reported

by McBride et al. [14], were taken as input data.

The composition of the explosion mixture (i.e.,

the volume fraction r, of each component) at six

constant pressures, between 0.01 and 1000 atm

was calculated from the mass balance and equilibrium equations by an iterative method as described by Neumann and Knoche [15]. The calculations were performed with the aid of a high

speed digital computer, using the general purpose program OMNITAB [16].

In a second set of calculations, the enthalpy

per unit mass i7, the entropy per unit mass /S, and the density D of the mixture at each pres- sure and temperature were calculated from the

following equations

12

i = 1 H

12

12 = 1

S

D

} = 1 p 12

The molecular weights Mj in the above equations

were also taken from reference 14. R is the universal gas constant = 1.98726 cal mole-i°K-i.

2.4. Results and Discussion

Some typical results from the calculations of

the concentrations of the various components in

the explosion mixture are presented graphically

2
t

' in figures 1 through 3. Some points of interest

which may be noted from the figures are discussed in the following paragraphs

(1) The concentration of A12 molecules is very

small (always less than 0.01% of the mixture)

for all the levels of pressure and temperature

, considered. One may, therefore, conclude that

I for observations of the AI2 molecule, the wire

should not be exploded in an air or oxygen en- vironment, where the calculations indicate that

the formation of oxides will predominate the

mixture. A noble gas environment should rather

be preferred for AI2 observations.

(2) At the lower temperatures (less than

3500 °K) nearly all the aluminum in the mixture

is in the form AI2O2 for all pressure levels. This

I system should, therefore, be favorable for observation of the AI2O2 molecule.

(3) At the lower pressures the aluminum oxides are the predominant aluminum components

2000 3000 4000 5000

TEMPERATURE,°K

Figure 1. Eqnilibrium concentration of the various components in the explosion mixture for p = 0.01 atm.

> 0.01

0001

: N2

02 /

: A

UjO/

NX*' 0

- NO/

'/l }

2000 3000 4000 5000 6000

TEMPERATURE, °K

Figure 2. Equilibrium concentrations of the various components in the explosion mixture for p = 1 atm.

01

Ng : -

02

-

-no/

/ //

/o 1

\a«202

2OOO 3000 4000 5000 6000

TEMPERATURE, °K

Figure 3. Equilibrium concentrations of the various components in the explosion mixture for p = 100 atm.

up to 3500 °K. Beyond 3500 °K, the oxide concentrations drop to zero, and the Al concentration increases. Therefore, as a rough estimate, if

spectroscopic observations of the explosion show

oxide bands and if it has been determined that

the mixture passes through a series of instanta- neous equilibrium states, one may conclude that

the temperature of the explosion mixture does

not exceed 3500 °K. In making such an estimate,

however, one must be sure that the observed

oxide bands are due to radiation from an equilibrium state rather than from chemical luminescence.

(4) At the lower pressures, the figures show

that the concentration of a given oxide is high

only in a very small temperature range (e.g., the

AI2O concentration is high only between 2000

4

p=0OI/^

p--oi^^-^ —

p=10

p=IO0^^

p=IOOO_^^

2000 3000 4000 5000 6000 TEMPERATURE ,°K

Figure 4. Entropy diagram for the explosion mixture.

3
and 3000 °K) . At the higher pressures, the temperature range for high concentration of a given

oxide is larger (e.g., ALO concentration is high

between 3000 and 6000 °K).

The entropies, enthalpies, and densities of the

explosion mixture are shown graphically in fig- ures 4 through 6. Figure 7 is a Mollier Diagram

10,000

1000

2000 3000 4000

TEMPERATURE °K

5000 6000

Figure 5. Enthalpy diagram for the explosion mixture.

10-0

10-2

10-3

: 10-

I0-'

p=iooo :

pHOO -

p = IO -

p=l -

p = QI

p=ooi ;

2000 3000 4000 5000 6000

TEMPERATURE T. °K

FiGUEE G. Density diagram for the explosion mixture.

{

showing the entropy of the mixture at the vari- ous pressures and temperatures plotted as 3\ function of the enthalpy. This latter diagram is of interest as a guide in the practical planning

of experiments. To use the diagram effectively, one must assume the changes from one equilibrium state to another, in the various stageSf

of the explosion, are adiabatic. Further, althoughf

parts of the explosion are surely not reversiblei- one may, in order to approach suitable condition^

for experiment, assume that these changes are, isentropic. Under these assumptions, one mayl^

follow an isentrope on the diagram (e.g., the,

isentrope 2.0 cal g"^°K~^) from pressure to pres-[ sure, and determine from the intersecting iso-,_ therms what the corresponding temperature|

change would be (e.g., from approximately 6000j,

°K at 1000 atm to approximately 2500 °K at^

1 atm). Also, the approximate energy dissipated^,

in the process may be determined from the en-j

thalpy scale (e.g., for the above case about I700i

cal g~^).

Finally, the authors would like to caution_the[

reader that the results of the present calculations^ are intended to serve only as examples. One must

expect errors due to the assumptions given. How-

•ever, the method applied and the computer program devised for the calculations are valid, and

it is felt that such calculations would be usefulj

guides to conditions for selective exploding wire^

studies.

6000

I 4000

1=6000/

/Is

/ /?

/ / ^

/ yT=5500

// V /

? / / /

( / / ly^i -A

/ yT=5000

71=4500

r=40oo 3500

-f / //

/

yi =3oc

/ /T=2000

iL/T= l500

0

ENTROPY S, [cal/g °k]

Figure 7. Entropy-enthalpy diagram for the explosion

mixture.

I
3. Electrical Measurements ^

3.1. Introductory Abstract

The problem of measuring voltage, power, and

energy dissipation in a transient, high-current

discharge has long been a difficulty in electrical

discharge and exploding wire research. In recent

years, a number of workers have approached the

problem with varying degrees of success. Fiinfer

et al. [17] used a divider arrangement to measure the voltage drop across a sample and a current measuring resistor. They estimated an error

of less than 8 percent in electrical energy integrated between times when the electromagnetic

field was zero. Bennett et al. [18] employed a divider method, with special care to isolate the

sensing probe from the surge current field. A cor- rection [Z d//dt] was introduced to compensate

for the induced-voltage errors in the voltage

data. They estimated that the error in their re- sults did not exceed 5 percent. Ostroumov and

Shteinberg [19] corrected for the induced emf

by inserting (clirectly in the voltage-measuring

circuit) a toroidal coil which introduced an equal

and opposite compensating emf. Moses and Korneff [20] used a similar but more sophisticated

system, equipped with two voltage dividers and

several amplifiers. Neither of the latter groups

assigned an accuracy to their measurements.

Others, such as Brady and Dedrick [21], have

made pulse voltage measurements with carefully

constructed, precision capacitive dividers. In

1963, Ettinger and Venezia [22] applied the

electro-optic Kerr effect for such measurements.

Wunsch and Erteza [23] also employed a Kerr

cell technique. This method, which is characterized by complete electrical isolation between the

main discharge circuit and the voltage-measuring

circuit, high frequency (to 100 MHz) res^Donse,

and high accuracy (Wunsch and Erteza report

that peak voltage measurements should be accurate to within ±1%), appears to hold much

promise for future work.

This section discusses a method of voltage

measurement, devised for the purpose of determining the energy dissipated in any selected

section (e.g., across an exploding wire sample)

of a high-current, high-voltage circuit as a function of time. Since the reactive component of the

voltage makes no contribution in this process, the

calculations and corrections required for timeresolved energy determinations may be reduced

considerably by measuring only the energy-dissipating, resistive component of the instantaneous

voltage. We therefore undertook to make direct

measurements of the latter by a method which

is an extension of an earlier technique by Zimmerman [24].

2 Portions of this section are taken from a paper entitled,

Time-resolved electrical measurements in high current Discharges, by E. C. Cassidy, S. W. Zimmerman, and K. K. Neu- mann, Rev. Sci. Instr. 37, 210 (1966).

A compensated, two-divider system was used

for the voltage measurements. The double-divider

arrangement was preferable because it precluded

the possibility of arcing within the recording

oscilloscope. (The measured voltages were re- corded differentially, for reasons to be given

later, by use of a differential preamplifier in the

oscilloscope. A second divider was required to

drop the voltage at the low side of the sample

to a level which could be handled by the oscilloscope without danger of internal arcing.) With

this setup, it was possible to measure the voltage

across any portion of the high voltage, high-current circuit; i.e., the measurements were not re- stricted to portions of the circuit which either

have one end connected directly to ground or which have one end very close to ground potential. Corrections for voltages across adjacent

portions of the discharge circuit (for example,

that across the current-measuring resistor) were

not required. Compensation for the emf induced

in the sample and voltage measuring circuit by

the discharge was provided by a small coil inserted in one of the divider circuits. It was therefore possible to record directly the resistive component of the voltage on an oscilloscope.

Simultaneously, the voltage across a coaxial

Park-type current shunt [25] was recorded differentially on the same oscilloscope (dual beam).

The discharge current, voltage across the sample,

sample resistance, power, and energy dissipation

in the sample were calculated as functions of

time from these oscilloscope displays.

3.2. Experimental Apparatus

The present voltage divider system was designed specifically for use in exploding wire ex- periments with a 60 ixF, 20 kV capacitor bank.

The main discharge circuit with the voltage and

current measuring circuits is shown in the schematic diagram in figure 8. The apparatus with

the divider assembly and current shunt installed

in place is shown in figure 9. The ringing freHtlN DISCHtRGE CIRCUIT

SPARR_

C4P £^ fXPlODINEME

SSMPLE rxxa

TT.TT

CAPACITOR B4N« (60/;f,20kVmax )

V0LT4CE DIVIDER STSTEH

0

, f 50ft COAXIAUABLE h

. 1

TERMINATION WITH DIf f EfiENTIAlOUTPJT /

50fl COAXIAL CABLE

GAPS FOR CD X 'CABLE OFF" PROTECTION IT

50fl COAXIAL CABLE

REFERENCE GROUND

TERMINATION WITH DIFFERENTIAL OUTPUT^

BEAM

DISPLATING (C-D)

DUAL- BEAM

OSCILLOSCOPE

BEAM

DISPLAYING (E-FI

FiGuuE S. Schematic of main discharge circuit and elec- trical measurement circuits.

5
FiGUEE 9. The current shunt and voltage divider installed on the

discharge circuit.

POSITION PROBE

NO.l

Figure 10. The voltage divider assembly.

quency and calculated inductance of the discharge circuit were about 50 kHz and 0.16 /^H,

respectively.

The construction of the voltage divider and

compensating coil is shown in figure 10. The

high-voltage portion (high side) of each divider

was made from several resistors connected in

series. Although noninductive resistors of the

type described by Park and Cones [26] are preferable, seven 2 W carbon-composition resistors

sufficed for the present work, where in most cases (during the calorimeter experiments and during

the gaseous, "restrike" portions of the exploding

wire discharges) the pulse voltage applied acrossi

each resistor was only about 100 V maximum.^j

Three resistors in parallel were used for the low'

voltage portions. The total resistance of eachi

divider was about 1100 Q. The divider ratios]

3 The voltage coefficient of resistance stated by the manufactuier was about 0.003 percent per volt for each resistor, giving

a ma.ximum change in divided resistance of about 0.3 percent]

during the "restrike," and of about 5 percent during the very

initial portions ("first pulse" and "dark pause") of the ex-!' ploding wire discharges. Carbon composition resistors are notj

recommended for future work, especially for measurements at very high voltages such as those encountered during the "first! pulse" and "dark pause" portions of exploding wire discharges.i

!
(about 200:1 with cable impedance included)

were matched to within 1 percent. The reactance

of the compensating coil was not a significant

factor since its inductance was only about 10 /aH

and the frequency of the oscillatory discharge

was only about 50 kHz.

The compensating coil was installed in the

high side of Divider No. 1 (see fig. 8). The coil

(20 turns of No. 24 copper wire) was wound on

an acrylic resin rod (3.81 cm diam x 3.81 cm

long), and mounted on a rotatable fiber shaft.

Stops limited rotation to 180°. An external dial

with divisions from 0 to 100 indicated the orientation of the coil, with the axis of the coil parallel to the current path (the position of no compensation at 50). In practice, the coil was adjusted to the unique orientation with respect to

the magnetic field in which the coil-induced voltage was equal and opposite to the reactive component in the measurement. At this point, simultaneous display (on a dual-beam oscilloscope) of

the current (/) through and the voltage {E)

across the sample showed similar, in-phase waveforms. With the present setup, about four discharge experiments were required to adjust the

coil for results with <5 < 6 deg, where * is the

angular displacement between E and /.

The divider probes (not shown), which were

3.174 mm diam brass rods, were inserted at the

positions indicated (in fig. 10) in sturdy insulating bushing assemblies. Firm electrical con- nection between the probes and the circuit directly adjacent to the wire sample was insured

by spring loading. The entire assembly was fixed

rigidly on a thick plastic plate and mounted in

a fixed position with respect to the discharge

circuit. The rigid assembly prevented small

changes in the areas of the divider loops. Repeated adjustment of the compensating coil was

therefore not required when identical samples

were used.

The divider signals were carried to the measuring oscilloscope by shielded, concentric cables

(see fig. 8). The cable sheaths and fittings were

insulated from the main circuit by polytetrafluoroethylene bushings. Grounding gaps (N2

neon tubes) were installed to prevent arcing

within the divider and to insure safety at the

ends of the cables. (A hazard exists if a discharge is fired with the voltage probes connected

while the measuring cables are not connected to

the oscilloscope.)

The voltage attenuation provided by the system may be seen from figure 11, which indicates

the relative voltage levels of the various points.

For matched dividers,

4

Eao

Ebo 62

where Eaoi ^boi ^i, and 62 are the voltage drops

indicated in the figure. Therefore,

DIVIDER NO.l

COMPE^SATINC COIL

Figure 11. Schematic indicating the potential levels of

various points in the voltage measuring system.

Eab = (61-62)

Ebo

62 = (61-62)^2,

where the divider ratio R2 = -^i- With the use

of differential amplifiers, the monitoring oscilloscope displays (61-62). The voltage across the

wire sample is therefore

Eab = CRO Voltage X Ri,2.

Should the divider ratios Ri and B2 differ

where

then

Eao

ei

k =

k

Ebo

e2

Ri

R2

EAB (kei -62)

Ebo

62

In this case, evaluation of Eab requires a knowledge of both of the divider ratios, measurement

of (ei — 62), and measurement of either ei or 62. The percentage error A introduced to measure- ments by divider mismatch may be determined

from A = {1 — k)

61 (kei - 62)

X 100.

3.3. Calibration

Calorimetric experiments of the type described

by Tsai and Park [27] were performed in order

to determine the accuracy of the voltage- and

current-measuring systems. A carefully machined

Inconel tube was used in place of the sample as the "resistance calorimeter." The temperature

rise (AT) in the tube as a result of the capacitor

discharge was determined by extrapolating thermocouple readings to the time of the discharge. It

was estimated that, for the usual 20 °C increase,

the temperature change was determined with an

error not in excess of 0.5 percent. The heat was

calculated as g = Mcp AT, where M is the mass

and Cp the specific heat of the tube. With errors

in the determination of M and Cp about 0.1 percent and 0.3 percent (cp measurement by Douglas

and Victor [28]), respectively, the energy calcur
lated iq) was thought to be in error less than

1 percent.

The measured voltage, current, electrical en- ergy input, and instantaneous power from one

of the calibration experiments are shown in fig- ure 12. Effective cancellation of the reactive components in the voltage measurements is evident;

the power curve and the slope of the energy

curve are always positive.

For the calibration, the electrical measure- ments were compared with the heat measure- ments which were taken as the "standard." The

total electrical energy input to the "calorimeter"

iSJ EI dt) was r'^termined by integrating the

product of simultaneous measurements of the voltage {E) across and the current (/) through the

"calorimeter" sainple over the duration (t) of

the discharge. The ohmic heating (JJ PR dt)

was determined from the current measurements

and known resistance {R) of the "calorimeter."

The results of several experiments are given in

table 1. With the uncertainty in R less than 0.2

percent, the error in the current measurements

is estimated from the PRdt determinations to

be less than 1 percent. The error in the voltage

measurements is estimated from the EI dt re- sults to be less than 2 percent.

Table 1. Results of calibration experiments

MCp^T J"^ I2Rdt Deviation Eldt Deviation

Joules ° Joules from q "Joules from q

Percent Percent 268 270 + 0.7 276 +2.9 412 416 +1.0 423 + 2.7 417 419 +0.5 429 + 2.9

3.4. Exploding Wire Results

Preliminary discharge experiments with the

"calorimeter" and with aluminum rods showed

that the proper setting of the compensating coil

was distinctly different with different samples.

Identical wire samples were therefore used in

adjusting the coil for exploding wire experiments. The voltage and current records from

two of the coil-adjusting experiments (with * =

2 deg and * = 24 deg) are given in figure 13.

The first portions ("first pulse" and "dark

pause") of the voltage curves were not included

in order to show the details of the gaseous, sec- ond portion ("restrike") of the discharge where

about 99 percent of the energy was delivered.

The extent of the errors introduced into the

energy and power calculations by the rather

large 24° phase angle may be seen from figure 14.

The power curves differ particularly during the

first half of the discharge. Also the dashed

power curve is at times negative (indicated by

arrows) . However, the energy curves are similar,

Figure 12. Time-resolved voltage, current, energy, and

power during a calibration experiment.

Sample : Inconel "calorimeter" tube, 2.5 cm o.d., 0.8 mm wall,

8.8 cm long ; stored energy : 60 iiF at 8.2 kV.

e -500

50,000

50,000

100,000

150,000 40 60 80 100 TIHE-fisec

Figure 13. Measured voltage and current from exploding

loire experiments toith different settings of the compensating coil. Sample : Aluminum wire, 0.14 mm diam., 6.2 cm long ; stored energy : 60 ytiF at 9.0 kV (solid curve) and 9.1 kV (dashed curve).

8
2,000

5 1,000

500

40

1

1

>S 1

1

///

//

ll

NEAR PR )PER COHPENS4TI0 N,<#>= 2°

1

1

- 1

) 1

tl 1

1 1 1

0 20 40 60 80 100

TIME-/j,sec

Figure 14. Time-resolved energy and power dissipation as determined from the rriltage and current measurements

in figure 13.

even with the 24 deg displacement. For example,

the total energy for the interval from ^ = 0

through t ^ 20 ixsec as measured with * = 24

deg differed from that measured with * = 2 deg

by only about 15 J out of 1000 J. The total

energies determined at the end of the discharge

differed by about 3 percent (1640 J for 4> = 24

deg and 1595 J for $ = 2 deg.). This difference

may be attributed in part to the difference in the

energies stored by the capacitor bank._

On the basis of these calculations, it was con- cluded that precise adjustment of the compensating coil does not appear to be essential for determination of total energy input to the sample if a large portion of the final total energy has been

delivered. However, more precise adjustment of

the coil is required for measurement of power,

for determination of \ ^ Eldt where and t2 are

times (where I 0) early in the discharge, or

for measurement of j' Eldt where i is a time

(/

^

0) before much of the stored energy has

been delivered (i.e., when a given error would be

a greater percentage of the total energy delivered to that point).

Energy and power determinations from ex- periments with energy storage at three different

voltages are given in figure 15. The resistance of

0 20 40 60 80 100 TIME -/xsec

Figure 15. Time-resolved energy and poiver determinations in a series of exploding toire experiments.

Sample : Aluminum wire, 0.14 mm diam., 6.2 cm long ; stored energy : 60 /iF at 8, 9, and 10 kV.

the exploding sample as a function of time from

the 8 kV experiment is given in figure 16. Since

corrections for the reactive component of the

voltage were provided by the action of the compensating coil, the resistance was determined

directly from Ohm's law. As a check on the

method, the resistance of the Inconel * tube

("calorimeter"), as a function of time, was calculated from time-resolved voltage and current

data. These results are also plotted in figure 16.

The resistance of this tube, as measured by standard steady-state methods, is 0.00153 fi at 26 °C

and 0.00154 at 50 °C. During the discharge

experiment performed for figure 16, the temperature of the tube rose from 20.2 to 41.5 °C. In the

absence of skin effects (considered negligible un- der the conditions of the experiment), one would

expect the resistance of the tube to be between

these values. The calculated values of the "calorimeter" resistance showed a variation from

about 0.00153 to 0.00160 O, with larger deviations

from the expected results during periods when

4 Certain commercial materials and equipment are identified

in tliis paper in order to specif.v ndequatel.v the experiinental

procedure. In no case does such identification imply recom- mendation or endorsement by the National Bureau of Standards, nor does it imply that the material or equipment identified is necessarily the best available for the purpose.

9
in 0.6 E

g 0.2

-WIRE -VAPOR

CALORIMETER'

0.006 It

0.004 iU

20 40

TIME -/isec

Figure 16. Time-resolved determinations of resistance

during high-current discharge experiments: (a) with

"calorimeter" in figure 12. stored energy: 60 /xF at 10.0 kV; and (b) ivith exploding aluminum ivire, O.I4 mm

diam. x 6.2 cm long, stored energy : 60 fiF at 8 kV.

E and / are small. The accuracy was reduced at

these times because the measuring instrumentation was adjusted for viewing very large values

of E and /. Further, since $ was not exactly zero ($ 2 deg), the calculated instaneous resistance

was periodically infinite (at /=0), zero (at

E=0), and negative. With both the wire and

the tube sample, the curves were not plotted in

the vicinities of these points.

3.5. Discussion

One may approach the problem of measuring

voltage and energy in high-current discharge

experiments in a variety of ways. One may, for

example, use the method of Fiinfer et al. [17]

and make energy determinations only at times

when the magnetic field is zero. In other cases, determination of the total energy input to a sample over the entire discharge process may be

sufficient, and one may employ a method such as that used by Tsai and Park [27] in their calibration experiments. However, if time-resolved

energy, power, and/or resistance determinations

are to be derived from the voltage (and current)

measurements, one must in some way eliminate

the reactive components from the instantaneous

voltage records. This may be accomplished by

applying calculated corrections (using the timederivative of the current) to remove induced

effects. Bennett et al. [18] have, for example,

followed this procedure. However, such correc- tions require: (1) a knowledge of the self-induct- ance of the sample and other parts of the circuit

included betAveen the measuring probes, (2) a knowledge of the mutual coupling between the

measuring probes and the main circuit, and (3)

in the case of an exploding wire, a time-resolved

knoAvledge of the change effected in each of the

latter by the changing sample.

The present method provides direct compensation for most of the induced effects. However,

the authors feel that errors due to the changing

sample are still a source of some concern. For

more accurate measurements with a changing

sample, one may, for example, be limited to

short intervals during which a given setting of

the compensating coil is nearly perfect (* 0).

Fortunately, the present experiments, with a

single coil setting, have shown that the change

in sample geometry did not introduce appreciable errors during the "restrike" portion of the

exploding wire experiments. However, during

the "restrike," changes in the geometry of the

hot gas column (arc) were relatively insignificant, and adjustment of the compensating coil

to a single setting which provides adequate compensation was a rather simple matter. During the

early ("first pulse" and "dark pause") portions

of the explosion, the wire expanded from 1 to

about 60 diam with the present setup. With

faster, higher voltage systems, expansion may be

exjDected to be even more rapid. It seems clear

at this point that the ever-increasing demand for

accurate, time-resolved measurements at higher

voltages (> 100 kV) will require still further,

more refined work in this area. Perhaps the more

novel Kerr cell technique [22,23] will provide

the speed and accuracy needed for reliable re- sults even under the more extreme conditions.

In the present work the errors in the voltage

and current measurements are estimated from

the calibration experiments to be about 2 and

1 percent respectively. The errors in the energy

and power calculations are thought to be less

than 3 percent. The exploding wire resistance

determinations are in general agreement with

earlier results by Reithel and Blackburn [29],

Webb et al. [30], and others who have shown

that the resistance of the wire first increases

rapidly as the temperature rises and as the interatomic distance is increased by the rapid expansion of the wire material. According to O'Day

et al. [31], the pressure of the expanding vapor

soon (within a microsecond or so) drops to the

point where an arc forms and ionization by impact produces ample carriers for the current. In

figure 16, the "restrike" follows and the resist- ance drops to a very small value where it re- mains nearly constant for a relatively long period of about 30 /Asec (from t = 10 /^sec to t 40 ju,sec). Eecent calculated results (by a different method) by Good [32] show a similar behavior. The resistance determinations during the

"restrike" are thought to be accurate to the same

degree as those for the "resistance calorimeter,"

i.e., to a few percent during the first half of the

discharge and to about 4 percent during the lat- ter half of the discharge when E and / are relatively small.

10
4. Optical Studies and Techniques

a. Introductory Abstract

This section is divided into four parts : 4.1

through 4.4-D. Section 4.1 describes a special

high-speed shuttering system developed to facilitate the photographic studies. This system, which

is intended for use with the high-speed cameras,

is made up of a fast-opening and a fast-closing

I shutter. The fast-closing action is obtained from

blackening of a window by exploding a series of

parallel lead Avires against the window surface.

This technique was first developed by Edgerton

and Strabola [33]. The fast-opening part of the

shutter was develoj^ed specifically for this proj gram. Its use for spectroscopic studies was reI cently reported by Carter et al. [34].

In section 4.2, the various high-speed framing

camera, combined framing camera and drum

camera, drum camera, periodic still, and ultra-

] violet photographic techniques employed for

time resolution of the explosion are described

' and illustrated. The design and operation of a

special six-channel photomultiplier system for

time-resolved measi;rements of the radiation intensity at selected wavelengths and a preliminary

attempt to estimate the temperature of the exj plosion mixture from some photomultiplier

! measurements are also described. Although this

latter effort Avas not successful, the results suggest that the method may be a promising technique for determining the temperature of the

exploding vapor column, provided that the meas- urements are made at selected wavelengths during selected intervals of the explosion.

Section 4.3 gives the results from time-resolved

studies of the spectra produced by explosions of

aluminum and titanium wires in various controlled atmospheres. The techniques developed

for photographic time resolution of the spectra

are described.

Finally, the techniques employed for higher

resolution, absorption studies of the spectrum of

the AlO molecule are given in section 4.4. The

results, which include observation of a considerable number of new bands in the ultraviolet re- gion, are arranged in Deslandres tables.

4.1. High-Speed Shutter System ^

a. Introduction

In recent years a number of high-speed photographic shutters have been developed (see for

example, references 33 through 39).

This section reports the development of a fast5 Portions of this section are given in a paper entitled New

fast-opening, large-aperture shutter for liigh-speed photography

by E. C. Cassidv and D. H. Tsai, J Kes. N.B.S. 67C (Engr. and

Instr.) No. 1, 65 (1963) ; reprinted in J. of SMPTE 72, 531

(1963) ; reprinted in Instrumentation and High-Speed Photography (Soc. Mot. Picture and Television Engineers, 9 East 41st

Street, N.Y.), p. 108, 1963. U.S. Patent No. 438,438 was al- lowed on July 22, 1966. Patents have been issued in Canada,

France, Austria, Italy, Great Britain and Switzerland. Patents

are pending in Germany and Sweden.

opening, large aperture, high transmission shutter. The details of its design and operation, as determined by original experimental studies, are

included. An Edgerton-type, fast-closing shutter

[33], developed for use in conjunction with the

opening shutter, is described at the end of the

section.

The fast-opening shutter consists of a piece of

metal foil clamped between electrodes in a ca- pacitor discharge circuit. The foil is placed in

front of the camera so that the camera lens is

completely covered. When a heavy current is passed through the foil by discharging the ca- pacitor, each current filament in the foil reacts

with the magnetic field set up by the other cur- rent filaments in the foil. The direction of the

force is such that the filaments are drawn to- gether [40]. As a result, the edges of the foil are compressed toward the center filament, and the

fast-opening action is achieved.

An improved arrangement consists of two foils

mounted side by side in the same plane (but insulated from each other along their common

edge) and clamped to a common conductor at

the top and to two electrodes at the bottom. The

foils thus form the two arms of a loop circuit.

When current is passed through the foils, each

foil is compressed by the electromagnetic forces

described above, and, in addition, each foil is repelled by the other because they carry current

in opposite directions. The opening action is therefore faster. After some experimentation

with modified versions of this design, opening

FLASHTUBE

TRIGGER HIGH VOLTAGE

SIGNAL POWER SUPPLY

Figure 17. Schematic of the looped-foil shutter and dis- charge circuit.

11
Foil before discharge

FiGUKE 18. Single foil shutter.

Foil after discharge

times as short as 20 /^sec were obtained for a

1 in X 3 in aperture.

b. Fast-opening shutter

Experiments were made to test the performance of several shutters of both the single- and

the looped-foil type with foils of different materials and dimensions. Figure 17 is a schematic

drawing of the discharge circuit with a loopedfoil shutter. In this model, the foils were

mounted on opposite sides of a thin sheet of

transparent plastic. The foils overlapped slightly

in order to prevent the passage of light at the

center. The plastic sheet served to support the

clamps at the top and the bottom, and to insulate the two bottom clamps and the foils from

one another. The clamps were made rather massive in order to reduce distortion when tightened,

and to provide good electrical contact with the

foils.

The shutter assembly was installed in series

with a high-voltage capacitor and a spark gap

which served as a switch [11]. The capacitor was

charged by means of a high voltage power supply. The spark gap was fired by means of a thyratron trigger circuit which supplied a highvoltage pulse to break down the spark gap. With

this setup, the initiation of the discharge could

be controlled to within 1 ixsec. The duration of

the discharge, for the foils tested, ranged from

about 10 fisec to about 50 fxsec. The motion of the

foils due to the discharge was observed by means

of a high-speed framing camera, focussed on the

foils. The clischarge was monitored, for purposes

of comparison with the framing camera record,

by observing (on an oscilloscope) the voltage

induced in a coil of copper wire mounted near

the foil holder.

Figure 18 shows a single-foil shutter before

and after the passage of current. With an aluminum foil, 1 in X 3 in X 0.0005 in thick, it was

found that the width could be compressed from

1 in to in in about 100 ju,sec. Figure 19 shows the experimental apparatus

with two 2 in X 3 in X 0.0005 in aluminum

FiGUKE 19. A looped-foil shutter (before discharge)

installed in the experimental setup.

foils installed in the loop arrangement. In this

setup, the foil holder and the spark gap assembly were mounted on a 15 /i,F, 20 kV energy

storage capacitor. This setup was employ^ed to

study experimentally the effects of a variety of

operating conditions on the performance of the

shutter, as described in the next section.

c. Results and Discussion

The opening action of the shutter (the motion

of the foils) is controlled by the magnitude and

duration of the electromagnetic forces, and by

the mass and stiffness of the foil. The rather

complex problems of solving the equation of

motion for the foil and of optimizing the speed

12
I of the opening action were not undertaken in the

I present investigation. However, the qualitative

I effects of some of the design and operating con-

; ditions on the opening of the shutter were ex-

! amined from simplified equations relating the

j various parameters, in an effort to determine

I conditions of more favorable operation. In the

following paragraphs, the results of this investigation are briefly discussed and compared with

some typical experimental results. For convenI ience, these results are summarized in table 2.

1 A framing camera result, typical of those obtained in the experiments, is shown in figure 20.

This shutter employed two aluminum foils (2 in

X 3 in X 0.0005 in) installed in the loop arrangement. The foils were folded in the configuration

of the last entry of table 2 with two thicknesses

of foil on the outer edges of the 1 in X 3 in

aperture. The third frame in the third column

(from the left) is a 3 sec time exposure of the

foils before the discharge. (It should be noted

that this time exposure is shown in place of the

high-speed frame at 90 fis&c. The next frame

shows the foils after 100 ^usec, and so on.) The

lines on the transparent plastic sheet were made

in order to indicate distance; the distance between the vertical lines is V2 ii^? ^i^d that between

the horizontal lines is 1 in. The time between

frames is 10 /usee, the sequence of the frames

being from top to bottom beginning with the

extreme left column. In the first frame, the two

foils are overlapped and no light from the flashtube, which was used to backlight and silhouette

the motion of the foils, passed through to the camera. The crumpling and opening of the foils

then follows upon initiation of the capacitor

discharge. This particular shutter opened to a

1 in X 3 in area in 30 to 40 /xsec. It is interesting

to note from the later frames that some of the

foil was shattered into tiny particles. It has been

suggested that these particles may act as a filter,

of unknown characteristics, in the light path.

The record shows, however, that these particles

were accelerated with the foil away from the

center of the aperture by the repulsive electromagnetic forces, and that they continued their

outward motion long after the discharge was

completed (the discharge, which was oscillatory,

lasted about 45 /xsec). These particles, therefore,

do not limit the light-transmitting characteristics of the shutter.

Optimum Energy Input—The speed of the

opening action of the shutter may be increased

by increasing the current through the foil [40].

However, with a given foil and discharge circuit,

the maximum speed of the action is limited by

two practical considerations. (1) The total electrical energy input to the foil must not be so high as to cause combustion of the foil,^ because

the flash from the combustion would expose the

film in the camera. (2) The electromagnetic

6 All of the energy input goes into heating of the foil, because

the heat loss during the discharge is negligible.

Table 2. Opening of a looped-foil shutter under various

conditions

(Aperture = 1 In. X 3 in.)

Material Width Thickness

Con- Cafigura- paci- tion tance

Stored

Energy

OpGH"

ing

Time

Inch Inch ( iiffirtifl^ ( .TniiJpft ^ Copper 1.0 0.001 Plane 15 920 61 Copper 1.0 .001 Plane 30 920 60

Aluminum 1.0 .001 Plane 15 580 50

Aluminum 1.0 .0005 Plane 15 298 57

Aluminum 1.5 .0005 Plane 15 450 51

Aluminum 2.0 .0005 Plane 15 580 42

Aluminum 2.0 .0005 Rolled 15

Edges

580 40

Aluminum 2.0 .0005 Pleated 15 580 39

Aluminum 2.0 .0005 Folded 15

Double

580 39

Aluminum 2.0 .0005 Partly 15

Folded

580 35

Aluminum 2.0 .0005 Partly 15 Folded

580 32

forces at any instant should not be so high as to cause excessive stresses in the foil, because this

could shatter the foil and interrupt the current

path, and consequently reduce the driving electromagnetic forces. These considerations, therefore, determine the "optimum" energy input to a foil.

The "optimum" energy input depends primarily on the thermochemical and the mechanical

(stress-strain relationship) properties of the foil

material. For a given foil the "optimum" energy

level may be obtained by a trial-and-eri'or

method, through adjustment of the initial en- ergy stored in the capacitor. Experience has

shown that for a given material the "optimum"

energy (initial stored enei'gy) per unit mass of

foil remained approximately constant. With our

discharge circuit, the "optimum" energies per

unit mass for copper and aluminum foil were

approximately 4.7 X 10^ J/lb and 9.9 X 10^ J/lb,

respectively. It was also found, of three materials tested (copper, aluminum, and monel), that

copi^er and aluminum were limited by burning,

whereas monel was limited by shattering.

Effect of Circuit Parameters—The capacitance,

resistance, and inductance of the circuit affect

the current as well as the duration of the discharge. These parameters therefore affect the

impulse to the foil and the shape of the impulse

during the discharge. The problem of obtaining

the "optimum" impulse (highest force without

shattering or burning, shortest discharge time)

from a given level of stored energy is difficult,

and a satisfactory solution was not obtained in

the present investigation. However, experimental

tests showed the following results, which may be

of interest to one who is seeking more favorable

operating conditions.

With a given foil, for which the energy input

was held fixed at the "optimum" level, a change

13
in the capacitance in the circuit had essentially

no effect on the opening of the shutter over the

range tested (from 15 to 30 /xF). As for the re- sistance in the circuit, it would seem that, for

faster opening action, a lower resistance would

be desirable, inasmuch as a lower resistance

would allow a higher current, and thus a higher

electromagnetic force. However, the resistance

in the shutter circuit is mainly that of the foils.

If the size of the foils is kept the same, then the

resistance can be changed only by changing the

material (or the resistivity) of the foil. This, of

course, involves a change in the "optimuna" en- ergy and in the mass (density) of the foil. To

determine the effect of changing the resistance

(foil materials), therefore, one must balance the

effects of these concurrent changes. In the present investigation, experiments were performed

with two shutters, one with copper foils and the

other with aluminum foils. The dimensions of

the foils in each case were 1 in X 3 in X 0.001

in. For the copper foils, the' "optimum" stored

energy was 923 J, the density was 8.9 g/cm^, and

the resistivity was 1.8 X 10"® O-cm; for the aluminum foils, these values were 528, 2.7, and

2.8 X 10"®, respectively. It was concluded, from

a consideration of these changes, that the shutter

with aluminum foils should open faster. The ex- perimental results (in table 2) show that this

was indeed the case. The shutter with aluminum

foils opened to an area 1 in X 3 in in about

48 /xsec, while the shutter with copper foils re- quired about 60 /xsec to open the same area. Aluminum foil is also a good choice because of its

availability and low cost. The incluctance of the circuit is determined

primarily by its geometry, and is not easily

adjustable in an actual experiment. The effect of

inductance was therefore not investigated experimentally. However, generally speaking, a low

inductance results in a high peak current and

a short duration of discharge. For fast-opening

BEFORE DISCHARGE

FOILS OVERLAPPED

FOIL I FOIL 2

DISCHARGE INITIATED

TIME ZERO

CRUMPLING ACTION

(OPENING) BEGINS

III

30 [ISEC-OPENED TO

I IN. X 3 IN, APERTURE

FiGUKE 20. High-speed framing camera photographs (10 /xsec betiveen frames) shotoing opening action of a looped-foil

shutter, and a time exposure (frame 11) of the foils before actuation of the shutter.

14
action, therefore, the inductance in the circuit

should be minimized. The inductance should not,

however, be so low that the peak current and

thus the compressive force and the mechanical

stresses in the foil become excessive and cause

shattering of the foil.

Ejfect of Size of the Foil—In the present

work, the length of the foil was considered as fixed by the size of the camera aperture, and

was not investigated. To determine the effects of

changing the width or changing the thickness of

the foil, experiments were performed with several pairs of plane aluminum foils installed in

the loop arrangement. Foils of two thicknesses,

0.0005 in and 0.001 in were tested. (The width

in both cases was 1 in.) It was found that the

shutter with the thicker foils opened faster. This

result was expected inasmuch as the electromagnetic forces on the foil filaments are proportional

to the current squared (and thus to the thickness

squared), while the mass per filament is directly

proportional to the thickness. The stiffness is not

a factor if the foil is kept thin enough to permit

buckling, because after the initial buckling of

the foil the stiffness is greatly reduced. It was

concluded, therefore, that if the electromagnetic

forces are sufficient to cause the foil to buckle,

then the shutter with foils of greater thickness

should open faster.

Experiments were also made with plane foils

of the same thickness (0.0005 in) but different

widths, the widths tested being 1.0, 1.5, and 2.0

in. With the 2 in foils, in the loop arrangement,

the inner edges of each foil attained an initial

speed of 1.7 X 10* in/sec, and the shutter opened

to an area of 1 in X 3 in in about 42 /^sec. The

1.5 in and 1.0 in foils required 50 /xsec and 57

ynsec, respectively, to open to the same area. These

results were also expected inasmuch as increasing the width of the foil allows a greater current,

without affecting the mass per filament or the

stiffness of the foil. Increasing the width should,

therefore, increase the opening speed of the

shutter. Another advantage of a wider foil is

that the interruption of the current path, caused

when the edges of the foil are torn from the

clamping electrodes in the process of opening, is less significant for a wider foil. The foil should

therefore be as wdde as is practical with the

stored energy available.

Another, more effective, way of utilizing a wider foil should be to use a corrugated or pleated foil. This arrangement would permit a more compact placement of the current-carrying

foil filaments. The interacting electromagnetic

forces should therefore be stronger, the forces

being inversely proportional to the distances between the filaments. To test this thesis several

plane aluminum foils were folded to the various

configurations shown in (the bottom half)

table 2 and tested under identical experimental

conditions. Each of the foils was 2 in wide (before folding) and 0.0005 in thick. The time re- quired in each case, for opening to a 1 by 3 in

aperture, is given in the table. The results show

that the opening speed was indeed increased by

folding the foils into more compact arrangements. However, there was some difficulty with

clamping the pleated foils and the foils with

rolled edges. These configurations are, therefore,

not considered feasible unless one designs special

clamps. Better success was obtained with the foils

which were folded flat to one or more thicknesses. In this case the foils were entirely suitable

COPPER

STRIPS I in. X 3in. APERTURE

TRANSPARENT

PLASTIC

(O) (b)

Figure 21. Looped-foil shutter loith conducting strips along edges of aperture, before (a) and after (b) actuation.

15
for clamping in the simple arrangement shown

in figure 17.

The highest opening speeds (with foils alone) were obtained with foils folded to give a single

thickness over the camera aperture but two or more thicknesses on the outer edges of the aperture, as shown by the last two entries in the

table. This arrangement was most effective because, in addition to allowing a more efficient

placement of the foil filaments, it also minimized

the mass of foil to be removed from the aperture.

In some final experiments with this fast-opening portion of the shutter system, the time re- quired for opening the shutter was reduced to

about 20 /i,sec by mounting heavier copper strips

in (electrical) parallel with plane foil elements.

Figures 21 and 22 show experimental models of

this design before (a) and after (b) actuation,

with copper strips mounted along the 3 in edges

of a 1 in by 3 in aperture. The heavier strips of

conducting material provide two advantages

(1) they reduce the resistance in the discharge

circuit, and thereby they allow a heavier discharge current; (2) they provide a more compact current path. Therefore, since the driving

electromagnetic forces increase as the square of

the current and since these forces are inversely

related to the distances between the currentcarrying foils and strips, the driving forces are increased and faster opening action may be

achieved.

d. Fast-closing shutter An experimental model of the Edgerton-type

[33] closing shutter, adapted for use in conjunc-

* m #

FiGUEE 22. Close-up vieio of foils in shutter of figure 21

after actuating discharge.

tion with the foil shutter, is shown in figure 23.

In this case, a series of lead wires (0.010 in

diam) were wrapped around a rectangular, transparent plastic sheet (3 in X 2 in X Vie iii)? and

installed between two clamping electrodes in a capacitor discharge circuit similar to the one

used with the foils. Best results were obtained

when the wires were spaced about y^g in apart.

The entire assembly was enclosed in a sealed

chamber which prevented the toxic lead vapor

produced by explosion of the wires from escaping into the laboratory. (The front of the chamber is removed in the figure.)

At the desired instant, the capacitor discharged through the wires, which were installed

in series with the capacitor discharge circuit, and

exploded them. The rectangular Plexiglass sheet

was thus blackened by lead particles, and the

closing action was achieved. The electrical energy

input to the wires was adjusted, as with the foils,

to the highest level permissible without emission

of a flash from combustion of the wires. A closing time of about 30 /^sec for the 1 in X 3 in

aperture was achieved with this technique. Explosion of the wires and thus closing of the shutter was synchronized with the opening foil

shutter and the portion of the high-speed event

to be viewed by use of a pulse and delay generator system.

e. Conclusions

The two shutters which have been described in , this section 4-A are intended for combined use. i There may, howeA'er, be many experiments where

j use of only one is required. For example, the

flash photolysis experiments reported by Carter

et al. [34] required only a fast-opening shutter, j while the more frequently encountered problem \ of photographic "rewrite" (double exposures) \ may be overcome by use of only a fast-closing i shutter. On the other hand, in cases where one j

Figure 23. H. E. Edgerton's fast-closing shutter

(before discharge).

16
HIGH- I- VOLTAGE 1 TRIGGER

CIRCUIT SPARK GAP

SYNCHRONIZING DELAY SYSTEM

TRIGGER

MONITORING OSCILLOSCOPE

BANK

XTXl

/T T. T T

WIRE SAMPLE

CAPACITOR

TRIGGER

r

CURRENT SHUNT

OIFFERENTIAL\

INPUT, A-B /

PHOTOGRAPHIC EQUIPMENT

TWO - OIVIDEF VOLTAGE MEASURING

CIRCUIT

DIFFERENTIAL NPUTTO

OSCILLOSCOPE

C-D)

i —COMPENSATING

,0 '"'^

— REFERENCE

GROUND

Figure 24. Schematie of discharge circuit and associated

instrumentation.

wishes to employ high-speed photographic, photoelectric, or spectroscopic methods for study of

selected portions of a high-speed event, an opening and closing technique must be employed. In

some cases, when the experiment is conducted

under low light conditions and when no light is emitted by the event, a high-intensity, strobetype pulsed light source may be used to effect

the shuttering action. In cases where light attenuation by the shutter is not of concern or when

extremely short (10"^ sec) exposure times are

desirable, a Kerr cell or Faraday-type shutter

may be used. When light amplification is a re- quirement because of the low light level of an

experiment, an image converter may be preferable. When the duration of light emission by the

studied event is short (no longer than a few

milliseconds) and ultra high speeds are not re- quired, a simple rotating disk shutter (such as the one described in sec. 4.3) may be an inexpensive but yet effective solution.

In any case, one should evaluate the various

types of shutters available in the light of the

requirements of their particular experiment. The

fast-opening shutter described in this paper is

of particular value in experiments where a large

aperture and high light transmission, coupled

with high speed (yusec), are important considerations.

4.2. Photographic, Photoelectric, and

Preliminary Spectroscopic Studies

a. Introduction

High-speed photographic studies by Zernow

et al. [42,43], Muller [44,45], Korneff et al. [46],

etc. have provided detailed information about

the exploding wire process. However, since the

7 Portions of this section are reported in a paper entitled : Photofrraphic and spectroscopic studies of exploding wires in a

sealed vessel by E. C. Cassidy and K. K. Neumann, Proc. 7tli International Congress on High-Speed Photography, Zurich,

Switzerland, 1965 ; O. Helwich, Ed. (Verlag Dr. O. Helwich,

Darmstadt, (1967), pp. 178-185.

process may be expected to vary significantly

with design and operating conditions, such studies were considered an essential part of the overall j)rogram reported here, and numerous highspeed photographic experiments were conducted.

The main goals of these studies were (1) to observe and thus learn something about the general

nature of the explosion process with our particular system; (2) to observe the mixing process

between the wire material and the air environment; (3) to observe the gas volume outside the

arc column and thus to estimate the extent of its

interference with spectroscopic observations;

(4) to measure the volume of the selected system; (5) to determine the velocity of the expansion of the hot-gas column; and (6) to determine

time interval (s) and place (s) inside the hot-gas

cloud in which a fairly uniform temperature

and mixture might be expected. The latter purpose was consiclered most important for meaningful temperature measurements and thermodynamic studies.

Concurrent photoelectric measurements of the

intensity of the radiation emitted in narrow

wavelength regions were also conducted for cor- relation with the photographic results. Later,

upon completion of a special photomultiplier

system, intensities were recorded at several se- lected wavelengths simultaneously, in an effort

to estimate the temperature of the mixture.

A number of typical results from these experiments are presented and evaluated in this section.

In addition, since most of the equipment, apparatus and instrumentation was developed and

initially used during this phase of the program,

the main discharge circuit, the explosion chamber, and other principal components of the

system are described here. The fast-measurements

teclmiques employed for time-resolved optical

studies of the mixing process between the wire

and the confined air environment during the

various stages of the explosion are also discussed.

17
Figure 25. Experimental apparatus with 20 kV,

60 fiF capacitor bank.

h. Experimental Apparatus and Procedure

The discharge circuit and some of the instrumentation are shown schematically in figure 24.

A 15 fiF, 20 kV capacitor and later a 60 fiF, 20

kV capacitor bank were used for energy storage.

The setup with the 60 /xF bank is shown in fig- ure 25. The capacitors were connected, as shown,

in a parallel, flat-plate arrangement. The wire

was mounted along the central axis of a cylindrical, aluminum current return path. Whenever

possible, areas were minimized and symmetrical

geometry was preserved in order to minimize

circuit inductance. The ringing frequency and

calculated inductance of the main discharge circuit Avere about 50 kHz and 0.2 /xH, respectively.

The discharge was initiated by ionizing a thyratron-triggered spark gap. The capacitor

bank then discharged through the sample wire

and exploded it. All electrical instrumentation

and recording equipment were synchronized with

the discharge by use of an adjustable waveform

and delay generator system.^

The general instrumentation included a framing camera (which was sometimes used together

with a drum camera for observations in the millisecond range),^ a photomultiplier tube with nar- row band pass filters for recording the light intensity versus time, a spectrograph (f/6.3, focal

length = 0.75 m) Avith interchangeable plane

gratings (one blazed at 5000 A with dispersion

•20 A/mm in the first order and the other blazed

8 Tektronix 160 Series.

9 Beckman and Whitley Model 189 Framing Camera and

Model 224 Drum Camera.

at 3000 A with dispersion 5 A/mm in the first

order), and a 70 mm air driven drum camera for

j continuous recording of the explosion spectrum,

For safety reasons, all equipment was operated

[ from a remote location by means of control in-

| strumentation. The control equipment (including ' a high-voltage switch bank, voltmeters and power i supply, the framing and drum camera control

units, the delay and thyratron-trigger units, and

some of the recording oscilloscopes) used for

| most experiments is shown in figure 26.

, Electrical measurements of the discharge cur- 1 rent and of the impulse voltage across the samj pie wire were made by use of a Park-type coaxial i shunt [25] and a differential, compensated volt- I age divider (sec. 3). The electrical and photo-

| electric measurements were recorded on and i photographed from the screens of common-time

base, dual beam oscilloscopes.^"

| During the experiments, interference from

| electrical pickup and ground currents generated

by the discharge proved to be a rather serious

difficulty. Special precautions were required in ; order to minimize its effect on the measurements. | Extensive shielding Avas used and triaxial cables i Avith suitably grounded outer sheaths were re- , quired for all measuring circuits. A differential [ measuring method Avas required in order to re- duce pickup in the long cables. All cables with

direct connection to the main discharge circuit

Avere isolated from the measuring circuits. To

reduce errors from ground currents, the main

j

10 Tektronix Type 551.

18
Figure 26. Remotely located control room

instrumentation.

discharge circuit ground was isolated from the

instrument ground. Finally, damping resistors

were installed in the capacitor charging line to

minimize parasitic reflections during the discharge. Some of the details of the efforts made

to reduce interference errors are given in the

descriptions of the photomultiplier and electrical

measurements systems.

c. Wire, Vessel, and Environment

The sample wire, which in most cases was

99.999 percent pure annealed aluminum (0.14

mm diam, 6.2 cm length) , was clamped between

two metal electrodes, enclosed in a sealed vessel,

and installed in the capacitor discharge circuit.

The vessel was sealed in order to contain the

chemical reactions and products generated by

the explosion. OFHC (oxygen-free, high-conductivity) copper was found to be the most desirable

electrode material, principally because it introduced the least number of impurities to the ex- plosion mixture. (High-conductivity Type 61-ST

aluminum electrodes, which were very effective

for the early photographic experiments, were

discarded in later spectroscopic studies because

they caused numerous, intense Fe i and Ti i lines

in the spectrum.) High-purity (99.99% pure)

Type 1199 aluminum electrodes also proved

unsatisfactory because they were extremely diffi- cult to machine and because they suffered excessive melting damage with each discharge.

11 Supplied by Sigmund Cohn Company, Mount Vernon, N.Y.

12 This special alloy was provided for these experiments by

the ALCOA Research Laboratories, New Kensington, Pa.

As for the vessel and explosion environment,

Plexiglass cylinders (dimensions: 7.1 cm i. d.,

0.6 to 1.2 cm wall thickness, 6.2 cm length) con- taining air under normal laboratory conditions

were used in these early experiments. In this connection, several practical considerations were

found to be important, particularly for longtime (msec) observations. (1) The vessel should

be larger than the "footprints" (burned, scarred

area) of the arc on the electrodes which clamp

the wire. If the intense heat of the arc column

is allowed to touch the vessel wall, such conse- quences as burning and blackening of the walls,

the introduction of impurities, and/ or explosion

of the vessel ensue. In addition, confinement of

the arc in a small-diameter vessel forces increased current density in the arc column and

therefore increased burning and evaporation of

the clamping electrodes. This not only promotes

blackening of the walls but also introduces an

increased unknown quantity of metal to the ex- plosion mixture. (2) The materials used for

fabrication of the vessel (and electrodes) should

be selected carefully, and preferably tested under

the conditions of experiment. For example, we

found that Bakelite should not be used with oxygen or hydrogen, because such vessels were ignited by explosions in these gases. Similarly,

Teflon was not suitable for spectroscopic studies

of explosions in nitrogen, vacuum, or argon, be- cause it introduced impurity bands from C2 and

CN in the spectrum. (3) The diameter of the

vessel should, if possible, be large enough to permit complete combustion of all the metal before

the vapor reaches the walls. This precaution re- duces blackening of the walls and windows by

metal vapor or droplet deposition.

In our work blackening and burning of the

vessel were not encountered in the earlier experiments, where the wire was immersed in air. Similarly, in later experiments with an oxygen en- vironment, the interior of the vessel was quite

clean after an explosion. Explosions in vacuum

or argon produced melted metal droplets which

were deposited on the walls and windows. Explosions in nitrogen or hydrogen prodviced a fine

black powder which stuck to and coated the vessel interior. Frequent cleaning (soaking in hydrochloric acid) was requ.ired in these cases. For future work, a large-diameter (5 cm) glass

vessel witli demountable windows seems most

desirable, particularly if spectroscopic studies are to be conducted. Such a vessel was used for

spectroscopic work in the later stages of this

program. Details of its design and construction

are given in section 4.3.

d. Spectroscopic Instrumentation

A schematic drawing of the optical setup used

for the earlier stages of the spectral studies is shown in figure 27. The spectrograph was

equipped with an adjustable 10 to 200 ju, slit.

19
DISCHARGE

CIRCUIT y

IMAGE OF EXPLODING WIRE ENTRANCE SLIT

SYNCHRONIZING TRIGGER PHOTOGRAPHIC

DELAY SYSTEM EQUIPMENT

TERMINATION WITH DIFFERENTIALINPUT FiGUBE 27. Schematic of optical system and associated recording

instrumentation.

Quartz lenses were used to focus the interior re- gion of the explosion chamber on the entrance

slit. Spectral time-resolution was achieved by

photographing the spectra with the 70 mm drum

camera, or by photographing oscilloscope displays of the response signals from a six-channel

photomultiplier as indicated in the figure. The

latter setup permitted observation of radiation

intensity at six different wavelengths simultaneously. Photomultipliers with quartz envelopes were used in the wavelength region from 2,000

to about 4,000 X. Glass envelopes were used for

the longer wavelengths. The photomultipliers

were driven with voltages between 550 and 800 V.

Each tube was equipped with an emitter-follower

amplifier and suitably terminated coaxial cables

(50 ft long for remote operation). Interference

from electrical pickup and ground currents (generated by the discharge) was reduced to an

acceptable level by differential measurement of

the response signals. The spectrum from an aluminum arc was used for adjusting the multipliers' position to selected wavelengths. A calibrated tungsten ribbon lamp with a quartz

window was used to determine the multipliers'

sensitivities (at 2400 °K) at the selected Avavelengths. A more detailed description of the photomultiplier system is given in the following

paragraphs.

e. Photomultiplier System

The six-channel photomultiplier system, which

was installed at the focal plane of the spectrograph for recording of the spectrum, was made

up of six photomultiplier tubes. Both end-on and

side viewing tubes were used. The end-on tubes

were RCA types Y0042A and 70042D, and the

side viewing tubes were type Both types

of tubes were selected because they employ S-20

13 This system was designed specifically for this -work by Mr. Malcolm S. Morse of the NBS Electronic Optical Development

Section, Measurement Engineering Division. The spectral re- sponse data in figures 28, 20, and 30 were determined in tests conducted by Mr. Milton L. Kuder, also of the NBS Electronic

Optical Development Section.

14 Obtained from the Applied Physics Corporation of Califor- nia and manufactured by Hitachi, Ltd. of Japan.

00 1 ^—

1 1 1 1

RELATIVE OUTPUT

Vs WAVELENGTH FOR 70042A 8i 70042D TUBES 80 \ 2.3.11 ^.24

60 2.3.32 \ X2

6.3.6\

40

X2^^ ^

20

0 1 400 500 600 700 800

WAVELENGTH MILLIMICRONS

j Figure 28. Spectral response of RCA 70042A and 70042D ! photomultiplier tubes.

j

1 type surfaces, which are reported to be most

efficient over a wide spectral range. The R-136 I and 70042D tubes, which have quartz envelopes, were selected for use in the ultraviolet region. | The spectral responses of each of the selected i tubes, under steady-state light conditions, are ' given in figures 28, 29, and 30.^^ The system was , also tested under transient, pulsed light condi-

[ tions, by use of a Strobotac ^® for simulation of I the explosion flash. This instrument provided

light flashes with rise times of less than 2 /tsec,

and witli sufficiently broad spectral distributions

to enable the system to be tested with the tubes I in place on the spectrograph. It was found from

these tests that close coupling of the tubes to i emitter-follower amplifiers, designed to drive the > cables which connected the system to the recording oscilloscopes, was necessary for viewing short

j

15 The test data shown in figures 28, 29, and 30 pertain only

to the particular tubes tested. There is no implication that

other tubes of the same lot or type will show comparable re-

| suits. I 16 General Radio Type 1531-A.
RELATIVE OUTPUT OF

HITACHI LTD. R-I^B PM. TUBES

300 400 500 600

WAVELENGTH MILLIMICRONS

700

Figure 29. Spectral response of Hitachi Ltd. R-136

photomultiplier tubes.

rise time pulses. A circuit diagram of the amplifiers adapted for this purpose is given in figure

31. The voltage gain under operating conditions

was measured to be 0.995. The computed rise

time for the system was less than 1 X 10"'^ sec. Figure 32 shows the six photomultipliers in

position at the exit plane of the spectrograph.

As shown in the figure, the tubes were installed

in pairs and enclosed in metal housings. Each

housing contained two tubes, which monitored

the light from two different but proximate (only

40 A apart) wavelengths. Resolution to about

20 A was achieved by placing a 1 mm slit before

each tube. Because of this limited resolution, the

600 1/m grating was satisfactory for all studies

conducted with this system. In the text which

follows, each tube and its associated circuitry are referred to as channel 1, 2, 3, 4, 5, or 6.

RELATIVE OUTPUT OF HAMMAMATSU TV. R-136 PM. TUBES

400 500 600 700

WAVELENGTH MILLIMICRONS

FiGxiEE 30. Spectral response of Hammamatsu TV R-186

photomultiplier tubes.

The tubes were adjusted independently to the

desired wavelengths by sliding the housings

along horizontal supports at the focal plane of

the spectrograph. An arc with one aluminum

electrode was used to obtain a spectrum suitable

for adjustment of the multipliers to selected

wavelengths. A calibrated tungsten ribbon lamp

with a quartz window vras used to calibrate (at

2400 °K) the photomultipliers at each of the

desired wavelengths. A carbon arc was used for

calibration at 3800 °K.

In making the exploding wire intensity measurements, it was desirable to view the photomultiplier signal prior to, during, and after the exPHOTOMULTIPLIER TUBE AND VOLTAGE DIVIDER

200K

200K

200K

26V.

O-lOV,

Output to Oscilloscope

51 n cable

0,802= 2N3I36

03= 2N3I34

D, = '^W ,5.IV Eener diode

-H.V. SUPPLY INPUT

Figure 31. Photomultiplier and emitter folloiver circuit.

21
FiGUEE 32. TUe six-channel photomultiplier system in- stalled at focal plane of spectrograph.

plosive discharge. Because of this requirement,

major electrical interference problems were en- countered. These were aggravated by the wide

signal bandwidth necessary, the sharp discharge

rise time, and the physical separation (necessary

for remote operation) between the photomultipliers and the recording oscilloscopes. In addition, operating convenience required that the

capacitor charging power supply and disconnect

switch be in the same room with the oscilloscopes.

With this arrangement, the power supply cable,

which was laid in a bundle with the measuring

and control cables, was in effect charged and

discharged with the capacitor bank, thus causing

appreciable pickup in the measuring cables. Extensive shielding, isolation, and installation of a damping resistor at the capacitor-bank end of

the supply cable were required to eliminate this

pickup.

60 Hz interference and pickup from the ca- pacitor discharge were also troublesome, especially in cases where the signals to be measured

were small and use of the more sensitive ranges

(0.1 and 0.050 V/cm) of the oscilloscopes vertical amplifiers Avas necessary. Unfortunately, conventional filtering methods could not be used to

eliminate these difficulties since it was required

that the measuring instrumentation have a frequency response to dc. It was found, however, that isolation of the

measuring circuits from ground reduced the interference considerably. Further reduction, to an

acceptable level, was achieved by differential re- cording (at the oscilloscopes) of the photomultiplier signals. The central conductor and inner

shield of triaxial cable, with the outer shield

grounded at both ends, were found most satisfactory for connection of each photomultiplier to

the differential input terminals of a plug-in preamplifier at the oscilloscope. A special differential-type termination which matched the cable

impedances and facilitated connection to the preamplifier terminals, was used at the oscilloscope

end of the cables. Its design and assembly are given in figures 33 and 34. This arrangement for

differential measurement of the photomultiplier

signals not only reduced interference from 60 Hz

pickup and other extraneous signals common to

both measuring cables, but it also served to isolate the measuring circuits from ground loop

errors.

f. Exploratory Photographic and Photoelectric Results

High-speed photographic experiments were

performed at different energy levels: with the

single 15 fiF capacitor at voltages between 10

and 14 kV and with the 60 ju,F bank between

OUTER SHIELD

CENTER CONDUCTOR

TRIAXIAL CA8LE

TO

PHOTOMULTIPLIER TUBE

INNER SHIELD i

I.OuF

AND

560n,''2W

21

0.003yu.F

wwv

I00n,i/2W

7-45pF

0.005;u.F

INPUT A

DIFFERENTIAL AMPLIFIER

OSCILLOSCOPE GROUND i

INPUT

FiGUKE 33. Schematic of differential cable termination.

22
Figure 34. Construction of differential cable termination.

10 and 14 kV. All photographs were taken using

the light from the explosion only; i.e., there was

no backlighting. Each record shows the explosion process for one wire only; i.e., the figures are not composites made up of single photographs from the explosion of many wires. Kodak

Linagraph Shellburst film was used with the

drum-framing camera combination. Kodak

Tri-X, Plus-X or Ektachrome ER. (color) film

was used with the framing camera. Suitable

filters were inserted in order to reduce intensity

and to permit photographs with radiation in

selected wavelength regions. The exact operating

conditions for each experiment reported are given in the figure captions and text. In the initial experiments, the explosion was

photographed over the entire duration of light

emission (several milliseconds), in order to obtain an overall understanding of the explosion

process and to search for intervals of quasi-uniformity in the explosion mixture. This task was

achieved by combined use of the drum and framing cameras. The drum camera was mounted on

the framing camera, so that the drum camera

(speed «=! 50 rps) recorded one frame per revolution of the turbine in the framing camera

(speed !=« 6000 rps). It was thus possible to ob-
1

1 1 1 1 1 1 1 1 1 1 1 1

1 11 1 1 1 1 1 1 1 1 1 1 1 1

- DIAMETER [

-

VOLUME -

-

1 1 1 1 1

1 1 1 1 1 1 1 r 1

1

11

1

11

I

11

0 1000 2000 3000 TIME.^sec

Figure 36. Measurements of the average diameter and

volume of exploding tvire vapor as functions of time.

tain continuous, sequential framing-camera observations over the entire duration of light emission (several msec). Figure 35 is a portion of one of these records. The first frame, which was

taken at t ^

50 fisec, shows several brighter

streaks in the exploding vapor column. These

are the current paths of the arc. At about 90

fisec (between frames 1 and 2), the current

stopped completely. However, the record shows

that irregular paths of hot gas, left by the current, endured for some time. There is, however,

no evidence of an overall temperature gradient,

and there is a relatively sharp boundary between

the radiant volume and the surrounding cold gas.

After about 600 jusec, the vapor column became

quite irregular and there was more extensive

mixing with the surrounding air. At ^ 1100

/Asec, a spherical cloud of hot gas, which grew

in size and intensity long after the discharge had

ceased, began to form. It should be noted, however, that the cloud did not reach the vessel wall

during the time when it was hot enough to emit

radiation. This is a desirable condition, because

of the practical point made earlier in the discussion of the size of the vessel. The approximate,

average diameter and volume of the zone of intense radiation, as measured from this record, are plotted in figure 36. (The arc was assumed

to be a uniform column for the volume calculations. Verification of this assumption by end-on

photographic observations was not attempted,

because, with the present system, a viewing port

in the clamping electrodes would have distorted

the arc.)

Figure 37 shows the earlier stages of the ex- plosion in more detail. The framing camera

(speed 97 rps) alone was used for this experiment; the frequency of the frames was therefore much higher than in figure 35. However,

because of the slow turbine speed, the exposure

time per frame was much longer (about 15 fisec).

} The first frame is consequently quite interesting;

j it shows the initial, very rapid expansion of the i wire (during the "first pulse", "dark pause", and i early "restrike" portions of the discharge) in , composite form. The line of intense radiation

along the central axis is the wire at a very early ! stage. The superimposed larger column of radiaj tion shows the extent of the vapor's expansion

after about 15 jusec. Between 50 and 200 jusec | (frames 2 through 6) the heated gas looks

fairly uniform. The boundary between the ra- I diant volume and the surrounding is sharp. There ' are no signs of ejected radiant particles or of an i enveloping cloud of cooler but radiant gas. This , interval, therefore, appears to be the most suit- I able time during the explosion for spectroscopic

temperature-measurement studies.

j Graphical representation of the average ap- | proximate (to within 0.1 cm) diameter of the j radiating gas is shoAvn in figure 38. Here again | it is evident that rapid expansion of the hot-gas i column ceased at about 150 /xsec. The column ! then contracted and expanded in a series of | periodic (frequency 8.8 kHz) pulsations. The

existence of these pulsations, which are believed i due to reflecting pressure or shock waves, was

reaffirmed by photoelectric measurements (and

later by spectroscopic measurements) . Figure 39, I for example, shows pulsations (frequency ' 8 kHz) in the intensity of radiation emitted in i the narrow wavelength region between 5600 and j 5900 A, as monitored by the filtered photomulti- | plier described earlier. Figure 39 also shows that

the intensity (in this wavelength region) was . maximum at a time long after (at ^ ^ 750 yasec) I the discharge current had ceased (the current ; flow lasted only about 90 ^sec), and that radia- ! tion endured for several milliseconds. It is in- ! teresting to note that the duration of the radia- | tion and the timing of the maximum intensity

j indicated by these results are quite different from

those shown in "typical" radiation versus time | curves published by Muller [44,45] and Fiinfer ! et al. [17]. (They observed light emission, with '

a peak intensity at the time of peak current, for

less than 30 ;u.sec.) We have attributed this difj ference to the fact that Muller and Fiinfer per- | formed their experiments in the open atmosphere

(where the reflecting shock or pressure wave

phenomenon would be less pronounced), rather

than in a sealed vessel. The photomultiplier re- i sponse in figure 40 (from an experiment performed under identical conditions in a vessel

with an open, 0.5 cm X 2 cm window) supports

this viewpoint. The pulsations are aperiodic, the

radiation is less intense, and the light output en- , dured only about 1.5 msec.

Results from experiments at higher energies

j (60 (uF at 10 to 14 kV) are given in figures 41 | and 42. Very dense filters were used for these i
29o| SlOftsec

I

Figure 37. Framing camera record of earlier stages of an explosion.

Exposure time ~ 15 usee ; neutral density filter of 20 percent transmittance ; energy stored : 15 /iF at 14 kV.

200 400

TiME,/isec

600

3 I

I I I I I I I

I I I

I M I I I I

1 M I I

I I I I iI II M I I I I I M I

1000 2000

TlME,;j.sec

3000 4000

Figure 39. Photomultiplier record of the radiation in- FiGURE 38. Average diameter of radiating vapor during tensity (wavelength region: 5600 to 5900AJ during an

the first 600 /isec of an explosion. explosion.

25
T i

I—I—I—

I

I—I—I—I—I—I—I—I—I—

I—I—I—I—

TIME.yu-Sec

Figure 40. Photomultiplier response from an experiment

with wire exploded in a vessel with an open loindow.

photographs. The overall behavior of the explosions was much the same as in the lower energy

experiments. However, the intensity of the radiation was very much greater than in figures 35

and 37, where no filters were used. Another significant difference is that the entire event was

much faster. At the lower energies the heated

gas maintained the shape of a column for about

700 /xsec, and then formed a spherical cloud in

the center of the vessel. At higher energies, ra- diating gas filled the entire vessel in about

20 /xsec. Figure 41 shows the very early stages of a higher energy explosion. A No. 18A Wratten

filter was used for this experiment because it permitted passage of radiation only in the range

where Al-lines and AlO bands were observed

spectrographically. This technique, which was

employed earlier by Anderson [47] permitted

near-ultraviolet photographs of the high-temperature column, thus preventing visible light

(from surrounding cooler gases) from masking

the column. The "striations," which are so well

known in exploding wire woi"k, show up clearly

in the first frame {t ^ S /xsec). Between ^ 10

and t 15 /xsec the arc is sharply defined and

apparently quite uniform. At ^ <=« 16 /xsec, ejected

wire material and reaction products begin to

envelop the column, thus posing a disturbance

to spectroscopic measurements. However, the

period between ^ 10 to 15 /^sec, which appears

suitable for such studies, was found to be quite

reproducible. At the lower energies, where the

explosion was less rapid, this phase of the explosion endured from t ^ 50 to t ^ 200 fisec. Figure 42 shows an experiment under the same

conditions with 2 /xsec between frames, using

only a neutral density filter (no spectral filter).

In the first frames, the boundary of the arc col-

} umn is not so smooth as in figure 41 which was | taken with ultraviolet light only. This indicates , that a small layer of cooler but still radiant gas i is surrounding the current path. After about 20

'

fxsec, the smaller diameter of the radiant volume | near the electrodes (at either end) indicates that

the arc column is considerably smaller in diameter than the radiating hot-gas cloud. This figure

(and those from a number of other experiments) | shows a disturbance at the center of the column

which is at first (at about 17 ftsec) very intense

| and later (at about 31 /xsec) less intense than the j rest of the column. This behavior suggests the , presence of pressure waves traveling in the direc- I tion of the wire. The fact that the last frame is i more intense than some of the earlier frames | (e.g., ^ 31 through ^ 37 /xsec) also suggests

'

that the effects of shock reflections and/or chemical reactions are quite pronounced at this stage

of the explosion. The radiant but less intense

envelope of gas, which begins to surround the ; column in the center of the vessel (see fig. 41) i at ^ ^ 15 /xsec and then expands rapidly towards ' the electrode ends of the vessel, also suggests the I existence of pressure or shock waves along the \ length of the wire vapor. i Figure 43 shows another interesting phenome- '. non. This sequence, which was taken with the

framing camera using high-speed color film, re- ' veals the motion of an apparent wave front or : head of ionized gas [48] through the metal vaj por. (The latter is evident in the frames showing i s« 2 to 10 /xsec.) The front seems to reach the I left electrode (the anode) at the instant when

the "restrike" begins. It is also interesting to note I from the first few frames that the gas inside the } vessel (between the electrodes which clamped i the exploded wire) was glowing in a manner ' suggestive of corona discharge [48] between the

[ electrodes. (At this point, the electrodes form,

[ in effect, a spark gap with a high-pressure metal

[ vapor between its electrodes.) In any case, the

| authors feel that these novel observations should | be noted as suggestive of an area for future study

| of the mechanisms of the initial "first pulse,"

"dark pause," and early "restrike" portions of

tlie explosive discharge.

In a final series of exploratory photographic

experiments, the interior of the vessel was ob- ! served (without magnification) for some time ; after radiation had ceased. At first the entire

j volume was clear and transparent. There was no

evidence of burning of the vessel walls. After

^ about a minute, gradually growing solid parti- < cles (assumed to be AI2O3) became quite appar-

'

ent. The particles were observed to move con- tinuously about the vessel in a churning action.

After About an hour, the particles settled into 1 parabaloidal webs. Figure 44 shows the web

| formation hanging from the walls of the vessel ! about 1/2 1^1' f^fter the explosion.

1
19/xsec

Figure 41. Framing camera record (loith near-ultraviolet light) of interval between 8 and 19 usee after current starts.

Exposure time ~ 0.4 Aisec ; 10 percent N.D. filter and wratten filter 18 A ; energy stored : 60 fiF at 10 kV.

27
Figure 42. Framing camera record under conditions of figure Ifl rising a 0.1 percent transmittance N.D. filter.

Exposure time 0.75 /isec.

28
MtKBSMn mttKKMm Ki

nnn m/KKKMrn mKtKME mtttKmM

nnn

Figure 43. Framing camera record shoioivg motion of a wave front along length of wire;

energy stored: 60 /iF at 10 kV.

g. Conclusions from Photographic and

Photoelectric Results

In ^reneral, these results showed that the overall behavior of wires exploded under similar

operating conditions was fairly reproducible.

The wire particles and vapor first expanded

quite symmetrically about the axis of the wire.

The radiant vapor column then contracted and

expanded in a series of pulsations in diameter,

which were apparently caused by shock or pres- sure waves reflect ino; from the walls of the cylindrical explosion chamber. Similar pulsations were

obseiwed in the intensity of the radiation emitted.

There was also some evidence of reflecting; shock

or pressure waves along the length of the wire

vapor column (between the electrodes which

clamped the exploded wire).

For purposes of thermodynamic studies, the

results showed no time interval (during the lightemitting stages of the explosion) when there was

an ideal mixing behavior between the metal

vapor and the air in the vessel; i.e., no interval

was observed when the metal vapor was mixed

uniformly Avith all of gas in the vessel. However, in spite of such disturbances as the apparent shock or pressure wave phenomena, "striations." ejected radiant particles, etc., nearly all

of the experimental records showed time intervals when the radiant vapor column appeared

quite uniform in composition and temperature.

29
Figure 44. Interior of the exploding tvire vessel about

i/i hr after an explosion.

During these periods, a sharp boundary existed

between the expanding vapor column and the sur- rounding cold gas, and there was no evidence of an overall temperature gradient along the radius

of the column. These intervals were found to be

quite reproducible, enduring from ^ 50 [isec

until t ^

200 fxS^Q. with the lower energy system

(stored energy about 1500 J) and from ^ 10

to ^ 15 /isec with the higher energy system

(stored energy about 3000 J). Some results from

spectroscopic studies during these intervals are given in the following pages.

h. Exploratory Spectroscopic Observations

Our main purpose in these studies was to determine a suitable method for making temperature measurements in the explosion mixture. As

a first step, the spectrum of the integrated light

output from the entire duration of the explosion

in the wavelength range from 2,000 to 10,000 k

was observed and photographed. A vessel with

a quartz window was used for the range from

2,000 to 3,000 A, because the Plexiglass vessel

absorbed the radiation in this region. The 15 /xF

capacitor charged to 14 kV was used for these

experiments.

The spectroscopic records showed lines (3082,

3093, 3944, and 3962A) from neutral Al atoms

and bands from AlO molecules (the AlO green

system with the transition A^S^X^S and part

of the AlO ultraviolet system at and below

3113 A). The region between 3900 to 6500 A is shown in figure 45.

For thermodynamic measurement purposes, it

was important to show that there was no record

from heated air at this level of energy input

(about 750 J), indicating that, at the times

during the explosion when the temperature of

the mixture was high enough to emit radiation

from air components, there was very little mixing between the metal and the surrounding air.

One should bear in mind, however, that this may

not be the case when higher energies and/or

faster discharge circuits are employed for ex- plosion of the wire, because (as the photographic

results have indicated) mixing occurs more rapidly and temperatures will surely be higher in

such cases. At this point, it was interesting to see if the

integrated spectral results could be correlated

favorably with the calculated concentrations of

the principal components of the mixture, as presented in section 2. From the calculations for

1 atm, which is shown graphically in figure 2, one would expect the following: maximum con- centration (alDout 7% of the mixture) of AlO

3944 4470

1 4842 53 37

3962 4648 5079

Al AlO

Figure 45. Spectrum (3900 to 5500 A) of the integrated light emitted

during an exploding wire experiment.

30
at about 3500 °K, about 10 percent Al above

4000 °K, several percent A120 between 2500 and

3500 °K, several percent AI2O2 at temperatures

lower than 3000 °K, and no noticeable concentration of AI2 at any temperature. The spectral

results show Al lines, AlO bands, and an absence

of AI2 bands as the calculations predict. A120

and A1202 bands were not observed; however,

no electronic spectrum for these components has

been observed. It would seem, therefore, that

the calculations give at least a fair estimate of

the composition of the mixture to be produced

in an experiment. However, as our later experiments (sec. 4.3) show clearly, the spectrum

emitted by an explosion is highly sensitive to

impurities in the system, e.g., to the iron, titanium, magnesium, etc., in the wire. One should

therefore expect to observe spectral features from

such impurities in the explosion mixture. However, no Fe, Ti, Mg, etc., lines were evident in

the integrated spectral results (e.g., in fig. 45).

As a result, their interference with later photoelectric intensity measurements was unfortunately not anticipated. This problem will be discussed later with the results of the temperature

calculations in figure 49.

Upon completion of the studies of the integrated spectrum, a series of experiments was con- ducted in which tlie intensity of the radiation

emitted by the explosion was measured at four

selected wavelengths simultaneously. Figure 46

shows these measurements (Traces B, C, D and

E) from an experiment performed using the 60

fiF bank. Time resolution was achieved by use

of the multichannel photomultiplier system at

the focal plane of the spectrograph, as shown in

figure 32. Trace A is a time-resolved record of

the main discharge current, measured by use of

a Park-type coaxial shunt [25]. The curves shown, which were taken directly from the oscilloscope traces with no calculations involved, are intended only for qualitative comparison. However, the photomultiplier traces B and C may be

compared quantitatively by multiplying the intensity units by the factors 1.0 and 2.44 respectively; D and E may be compared by applying

the factors 1.0 and 0.625 respectively. These factors were calculated from calibration experiments with a calibrated tungsten ribbon lamp.

It is interesting to see that the intensity traces

show a first pulse which corresponds to the wellknown "first pulse" in current trace (A). Similarly, the "dwell" or "dark pause" and the

"restrike" are evident in all the traces. The difference in the waveforms of traces B and C

(Al-line and nearby continuum, respectively)

and of traces D and E (AlO band and nearby

continuum) indicates that line and band radiation was emitted, probably by ejected particles

which reacted with the surrounding air, from

the very beginning of the discharge. Further

comparison shows that after the restrike the Al

line B and the AlO band head D rise to maxi60

TIME ./xsec

Figure 4G. Simultaneous measurements of the discharge

current and radiation intensity; stored energy: 60fiF

at 12 KV; A = discharge current; B, C, D, and E =

radiation intensities at 3962 A (A\-line), 3992 A (continuum), 5019 A (AlO hand head), and 5119 A (continuum), respectively.

mum intensity much faster (in about 8 /tsec)

than the continuum intensities C and E (in

about 15 /j,sec), thus indicating that the line and

band head appear in emission during this rise.

Their intensities then go down to a much lower

level than that of the nearby continuum, indicating that these features now appear in absorption. No doubt the absorption takes place in the

cooler layers of Al-vapor which by this time

have enveloped the heated arc column. On the

other hand, comparison of traces C and E shows

that the behavior of the continuum intensity at

different wavelengths is about the same (the

waveforms are essentially the same). This indicated that the continuum radiation in different

wavelength regions seems to be from the same

source.

31
:

B -

-

C -

-

0 20 40 60 80 100 120 140 TIME./xSec

FiGTJKE 47. Relative intensity of Al line at 3962 A as function of time in three identical experiments ; stored

energy: 60/iF at 10 kV.

The continuum should therefore be a reliable source for quantitative temperature-dependent

intensity measurements. Further, traces C and E

and similar results from other experiments show

that the continvium is intense enough to allow

such measurements. It also seems that the meas- urements should be made as far as possible into

the ultraviolet region in order to avoid interfer- ence from the ejected particles mentioned earlier.

This is desirable because the particles outside

the hot column are cooler and will therefore

emit radiation in the longer wavelength regions.

However, the effect of scattering, which increases

as V*, should be investigated as one moves into

the ultraviolet.

Figure 47 shows the relative intensity of the

Al line at 3962 A as observed in three experiments

where all conditions of operation were thought

to be identical. The intensity units are the same

for A, B, and C. The traces show that the radiation output is not exactly reproducible. However,

the times of the maximum and minimum intensities are quite reproducible, and it seems that a correlation of these peaks with detailed highspeed photographic, photoelectric, and electrical

measurements might be helpful in determining

the hydrodynamic behavior of the system.

The radiation intensity at 5119 A (continuum

from experiments performed at three different

energy levels, as shown in the caption, are given

in figure 48. All other conditions were held fixed.

The intensity units are again equal for each

curve. The curves show several points of interest

30

20

10

10

10

1 L

-

\I

-

-

A -

-

-

- \

-

-

B -

- - U

C -

-

0 20 40 60 80 100 120 140 TIME.^SeC

Figure 48. Relative intensity at 5119 A (continuum) as function of time at three different energy levels: A = 16 kV; B = IJ, kV; and C = 12 kV. [ Capacitance = 60 /iF '

i

I (1) the time integral over the light output in- creases with increasing charging voltage, (2) the \ intensity of the radiation during the first cur- I rent pulse (before the "dark time") increases ; with higher voltages, and (3) the maximum in- I tensity after "restrike" is reached faster and ' maintained longer at higher voltages. Experi- | mental results showed that these points were also

j valid at other wavelengths. '

i. Conclusions from Exploratory Spectroscopic Studies ! Analysis of the spectroscopic data led to the ' following conclusions: (1) The behavior of an

exploding wire and thus of the radiation emitted

j varies widely with the condition of operation. I Therefore, for temperature calculations from , spectroscopic data, one must first detei-mine time

intervals and volumes in the explosion mixtures

(generated by a given system under given condi- ' tions), which are suitable for meaningful measurements. (2) Spectra from numerous experiments with the given experimental setups showed

emission lines from neutral Al-atoms and bands

from AlO molecules against a background con- tinuum. At certain times during the explosion

the lines and bands appeared in absorption. ^ These wavelength regions ai'e therefore unsuit- | able for temperature-dependent intensity meas32
urements. (3) With the experimental setups

described here, it seems that temperatures may

be determined most readily from comparison of

the continuum radiation intensity at several different wavelengths, under a grey-body assumption. The \\'avelengths used for these comparisons

should not be near the observed lines and band

systems (i.e., they should be in the continuum).

The wavelengths selected should be close enough

together to ensure the fact that the radiation is emitted by the same reactions. On the other hand,

they must also be as far apart as is feasible in

order to permit maximum accuracy in the results.

An attempt to apply this technique for estimating the average temperature of the exploding

vapor column is described in the following paragraphs.

j. Estimate of Temperature from Spectroscopic

Measurements

As indicated in the previous paragraphs, it

was not possible to use the intensity of the lines or band heads in the explosion spectrum for

temperature-dependent intensity measurements,

because the radiation at these wavelengths was

at times absorbed by the cooler gas outside the

main exploding vapor column. However, the

strong continuum radiation emitted by the ex- plosion at certain intervals did appear to be suitable for such measurements, under the assumption that the radiation emitted was that of a grey- or near-blackbody. A series of experiments

(with energy storage of 8, 9, and 10 kV at 60 fxF)

was therefore conducted in an attempt to estimate the temperature of the explosion mixture

by this method.

Four channels of the photomultiplier system

were placed at the focal plane of the spectrograph for measurement of the intensity of the

continuum at four different wavelengths. (A photographic plate of the integrated spectrum, such

as that shown in figui'e 45, was used for selection

of the wavelengths.) For purposes of calibration,

the response of each of the four channels to a tungsten ribbon lamp (calibrated and running

at 2673 °K) was recorded from the screen of an

oscilloscope. The intensity / (in cal/cm^-sec) of

a blackbody at this temperature was then calculated from Planck's law:

/ =

where Ci = 0.893 x 10-12 cal-cm2/sec

1.438 cm-°K

Ai = 4832 A

A2 = 5754 A

As = 3381 A

A4 = 3421 A

T = 2673 °K (2400 °C).

10,000

2,000

01 \ I 1 \ I

\ 1 I I I

0 20 40 60 80 100

TIHE,fisec

Figure 49. Average temperature of the explosion vapor

as determined from simultaneous intensity measurements.

From these photomultiplier responses (in volts)

and calculated intensities, a sensitivity factor

(in cal/cm^-sec per volt of photomultiplier re- sponse) was derived for each photomultiplier

channel.

Then, one may write for any given pair of

channels

In /i - In /a = 5 In A2 + In e - 5 In Ai -

or

J, ^

\Ai A2 /

5 (In A2 - In Ai) + In h - In A

"

The average temperature T of the exploding

vapor column was calculated from this latter

equation, where /i and I2 are intensity measure- ments recorded at wavelengths Ai and A2. Since

four photomultiplier channels were used for the

experiments, six (one for each possible combination of the four channels) calculations of the

temperature were made for each experiment.

The advantage of a multichannel system for

such measurements is therefore quite clear. All

of the temperature curves calculated will be

identical if the assumptions and measurements

are correct. This provides an excellent check on

the reliability of the results.

The calculated temperatures from an experiment at 8 kV are shown graphically in figure 49.

The Avavelengths monitored were those given

above. The curves were computed and plotted by

33
AlO A^I-X^I BLUE -GREEN SYSTEM

K

(a)

-I -2 -3 Nal

Figure 50. Spectra emitted by cm exploding aluminum wire : (a) integrated plate from entire explosion (60 /iF at 14 kV); (b) time-resolved plate at t ^ 900 fisec (60 fiF at 14 kV) ; (c) at t ^ 1.1 /isec ivith lower energy input

(15 fiF at 14 kV).

a high speed automatic computer. A special program was written, using OMNITAB [16],

for this purpose. The intensities at ^ < 5 ;usec (during the "dark pause") were too small to be

recorded; the curves are therefore not shown

prior to this time. The results indicate that some

of the measurements and/or assumptions made

were not reliable. During the interval from t 10 to 15 fjLsec, when the explosion mixture appeared nearly uniform in the framing camera

results, only three of the curves (those from

combined measurements by channels 1-3, 2-4,

and 3-4) are in fair agreement, yielding an

estimated temperature of about 4800 °K.

In conclusion, the authors would like to point

out that the failure of these initial attempts to

estimate the temperature of the explosion mixture does not necessarily indicate that the method

described is at fault. Subsequent time-resolved

spectroscopic studies (see sec. 4.3) showed nu- merous lines and bands from impurities in the

system. The existence of these features, which

were overexposed by intense continuum radiation

in the integrated spectrum (fig. 45), was not

anticipated. It is now certain that interference

from these features contributed to the failure of

the measurements described here. Further, in

view of the results reported in section 4.4, it is also clear that higher spectral resolution is re- quired in order to avoid interference from impurity features in the spectrum. If these diffi- culties are eliminated and if the photomultiplier

system is calibrated more carefully at several

very high temperatures (e.g., by use of a calibrated carbon arc source), the authors feel that

the present techniques provide a promising

method for future high-temperature studies.

4.3. Time-Resolved Photographic Studies

of the Explosion Spectrum ^'^

a. Introduction

Because of its convenience as a laboratory

technique for generating high temperatures, the

exploding wire has long been of interest as a source for sjiectroscopic studies of high-temperature atomic and molecular species [49,50,51]. The

investigations described in this section were con- ducted: (1) to explore further the feasibility of

17 Portions of this section were taken from papers entitled

Time-resolved spectroscopic studies of exploding wires in controlled atmospheres bv Esther C. Cassidy and Stanley Abramowitz, Appl. Spect. 21, 360 (1967) and Studies of some exploding

wire light sources by Esther C. Cassidy and Stanley Abramowitz,

J. Soc. Motion Picture and Television Engineers 75, 735 (1966).

34
adapting the exploding wire process for such

studies, (2) to develop techniques of fast meas- urement Avhich would permit time-resolved spectroscopic study of the explosion, and (3) to

search the explosion spectrum for features from

previously unobserved species (with emphasis on

the search for species of interest in modern technology) . To date, except for a few such as Bartels and

Bortfeldt [52], Nagaoka et al. [63], and Teeple

[51], most workers have presented integrated

(from the entire explosion) spectral results.

However, it is clear that integrated methods are not suitable for observation of weaker or highly

transient features in the explosion spectrum, be- cause the extreme intensity of the initial con- tinuum radiation overexposes the photographic

plate and thus desensitizes it to weaker radiation.

Figure 50, which shows an integrated plate (a)

and two time-resolved plates (b and c), illustrates this effect, and shows the importance of

time resolution. Plates (b) and (c) were taken,

using a rotating shutter disk (described later in

this section) about 1 msec after initiation of discharges at two different levels of energy storage.

This time was known from drum camera experiments (as will be seen later) to be favorable for

observation of AlO features in the explosion

spectrum. A neutral density filter with approximately 5 percent transmission was used for (a)

to permit approximately the same light exposure

as in (b) and (c), where the shutter disk limited

the exposure time to 180 jusec .The Ay = 0 through

+ 3 sequences of the AlO spectrum are not distinguishable on the integrated j^late. The intense

continuum emitted at the very beginning of the

explosion has overexposed all but the most intense features. On plate (b), the A-y = —3 through

+3 sequences are evident. On plate (c), the i^v =

—3 and -1-3 sequences are barely visible. However,

here it is clear that these features were not, for

some reason (probably because the energy input

was not sufficient) produced by the explosion.

Without a complete time history of the spectrum

one could not distinguish whether these features,

observed in emission in (b), were overexposed by

the continuum, or whether they were actually not

produced by the explosion.

The following pages describe a procedure for

thorough, time-resolved photograj^hic observation of exploding wire spectra. Results from ex- periments with several different explosion systems are presented.

b. Experimental Apparatus and Procedure

For most of the experiments described here,

the wire was enclosed in a sealed cylindrical vessel (i.d. = 7.6 cm, length = 9.5 cm), equii^ped

with demountable quartz Avindows. (ilass (minimum wall thickness about 4 mm with the pres- sures used) Avas found to be the most suitable

vessel material, because, as mentioned earlier, it had no effect upon the explosion spectrum. A typFiGUBE 51. Assembled explosion chamber.

ical chamber Avith a Avire clamped in place is shoAvn in figure 51. Prior to each experiment,

the chamber Avas pumped to a vacuum (approximately 4 X 10 * torr), and then charged Avith

argon, oxygen, hydrogen, or nitrogen to a controlled pressure betAveen 0.01 and 1 atm. Hoavever, since the atomic and molecular features of

the spectra Avere found to be more intense and

more enduring at reduced pressures, most of the

exjieriments Avere conducted Avith the gas pres- sure fixed at about 76 torr. During the explosion, a continuous timeresolved record of the spectrum was photographed

from the focal plane of the spectrograph by use

of the 70 mm drum camera. The speed of the

camera (600 rps max) was adjusted to give

maximum time resolution over the 31.9 cm length

of the film (70 mm Kodak Eoyal-X Pan). Diafine tAvo-bath developer Avas used for processing

the film.

After this time survey of the spectrum, timeresolved photographic plates AA'ere taken of the

spectrum at selected intervals (during the explosion) Avhen intermediate species of interest were

knoAvn (from the drum records) to exist. This

pi'ocedure permitted broader Avavelength cover- age and more detailed examination of the sj^ectrum. (The optical components of the drum

camera permitted observations only in the visible region, and they reduced the effective dispersion of the system). Time resolution on the

plates Avas achieA-ed by placing a rotating shutter

disk, similar to that described by Bartky and

Bass [55], before the entrance slit of the spectrograph. Two slots, one for shuttering the

35
spectrograph and one for generating time pulses, were machined in the disk. It Avas thus possible

to prevent radiation from the explosion from

passing into the spectrograph except during a

desired, preselected interval of the explosion.

The disk and associated equipment used for

the present experiments are shown in figure 52.

The disk was driven by a reversible synchronous

motor (1800 rpm, 1/50 hp). The 2° shutter slot

permitted a 180 /xsec exposure of the photographic plate for each revolution of the disk.

The radiation from the explosion ceased in

less than 33 msec (the period required for one

revolution) ; double exposures were therefore not

a problem. Synchronization of the slot's arrival

at the entrance slit of the spectrograph with the

interval of interest was achieved by use of a photomultiplier tube. The photomultiplier received

light from a miniature six volt lamp through

the timing slot (175° removed from shutter slot

and 0.005 in Avide—see figure 52) at a fixed time

before the shutter slot passed over the entrance

slit. The photomultiplier signal was then passed

through the delay and pulse-generating circuits

shown for triggering of the explosion at a controlled time. The circuit arrangement in the fig- ure was quite convenient, because it utilized

on-hand general-purpose equipment. A special

inverter-amplifier was, for example, not I'equired

this function was performed by the triggering

and gate circuits of an oscilloscope.^^

During the experiments, a record of the timing

(during tlie explosion) of the exposure was obtained by photographing a dual-beam oscilloscope display of the trigger signal from the photomultiplier tube and the voltage induced by the

discharge in a small coil placed near the capacitor discharge circuit. With the present system,

this timing could be controlled to within ±50

SIDE VIEW

fxsec.

c. Results and Discussion

In general, the results of the photographic

studies of the spectrum showed that all of the

explosions first emitted intense continuum radiation, with broadened, very intense line features

from constituents of the wire material. After the

initial continuum sharp line and/or band spectra

from the various species produced by the explosion were observed. In most cases, this radiation

endured for a relatively long time (several milliseconds), compared to the total discharge time

(~90 fisec). The pulsations in intensity mentioned earlier were also evident in the explosion

spectrum for about 400 /^sec. It is interesting to

see that the frequency of these pulsations (~ 10

kHz) is nearly the same as the frequeue ies

(~ 9 kHz) of the volume pulsations observed in

the high-speed photographic studies (see fig. IS)

and of the intensity pulsations recorded in the

exploratory photoelectric experiments (see fig.

39).

18 Tektronix 160 series waveform and delay generator system

and type 551 oscilloscope.

lOkfl

OSCILLOSCOPE GATE OUT

(TRIGGER IN ( + 20V) SINGLE SWEEP)

WAVEFORM AND

DELAY GENERATOR

+ 50V

TO SPARK GAP-I5kvTHYRATRON AND

PULSE TRANSFORMER

Figure 52. Block diagram of shutter disk and triggering

circuit.

d. Spectral Results With Aluminum Wires

A drum camera record from an aluminum wire

exploded in vacuum (pressure about 3 X 10^

torr) is shown in figure 53. The mercury line at 5461 A was added for reference after the explosion. The record shows that radiation strong

enough to expose the drum camera film was

emitted for only about 100 /^sec, and that no

molecular bands appeared. Principal features of

the spectrum are emission lines from the various

impurities in the aluminum wires and in the

electrodes used to clamp the wire. Since the stock

from which the wire was drawn was certified by

the manufacturer to be 99.99912 percent pure, the

electrode material (Type 6061 aluminum) was

assumed to be the source of the impurities. However, results obtained with electrodes made from

99.99 percent pure aluminum (Type 1199) still showed many lines from impurities. Two conclusions therefore seem clear: (1) Not surprisingly, the method is an extremely sensitive technique for detecting the constituents of a substance; and of greater import (2) it is extremely

difficult to construct an exploding wire apparatus

for experiments with a selected system, free from

the effects of impurities. These conclusions were

found to be particularly true ^ohen the wires

loere exploded in gases (such as nitrogen or argon) lohich are not conducive to chemical re- action xoith the toire material.

The upper portion of figure 54 shows the first

300 fxsec of a drum camera record from an ex- plosion in pure, dry nitrogen (pressure = 76

torr). Higher wavelength resolution and a broader range of wavelengths are given in the

time-resolved spectrum shown under the drum

camera result. The plate Avas taken, using the

shutter disk, about 1 msec after initiation of the

36
5890/96 Nal

I X

o

LJ

-J

UJ

>

<

5

5461

4680

200

TIME - //sec

Figure 53. Spectrum from aluminum loire exploded in vacuum as recorded by drum camera (15 /iF at H kV).

discharge. The spectrum from an iron arc in air

is shown at the bottom for ready identification

of the many Fel lines in the explosion spectrum. Though the discharge current lasted only

about 90 />isec, atomic lines from the wire and

electrode vapor endured well into the millisecond

range.

Figure 55 shows a joortion of a drum camera

record of the spectrum from an aluminum wire

exploded in argon. The time-resolved (rotating

shutter) plate at the bottom was taken about

1 msec after initiation of a similar explosion in

nitrogen. The spectral distribution was essentially the same with both gases. As the intensity

of the continuum waned, numerous atomic lines

from the wire material (and its impurities) became gradually more distinct, and endured for

several milliseconds. Most of the lines were observed in emission, with the exception of the

All and Nal doublets at 3944 and 5890 A, respectively, which were at times self-reversed. The

interval between t = 600 and 1100 /xsec, where t is the time after initiation of the discharge current, showed the lines most distinctly. Here again,

it is clear that time resolution of the spectrum

is quite important for observation of phenomena

occurring later in the explosion. Integrated photographic plates, overexposed by the continuum,

could not possibly show the weaker spectral features w'hich appear later in the process. Similarly, with photoelectric recording techniques, a high-speed shutter, which opens after the intensity of the continuvnn has declined, should be

used to prevent overloading of the photomultiplied circuit.

As the above results have illustrated, bands

from molecular species were not observed (wavelength range studied: 3000 t9 6000 k) in the

spectra from explosions in nitrogen, argon, or vacuum. However, this result was not easy to

establish, and, before going on to the results from

other systems, the authors would like to report

some of the difficulties which should be expected

in future work of this type. Our early results

from explosions in a nitrogen environment

showed several features which appeared like

37
0

FiGUEE 54. Drum camera record and time-resolved spe

in nitrogen (60

band sequences. Figure 56 was printed from a portion of drum camera film taken during one

of these experiments. Tlie experimental conditions were identical to those of figure 5i, except

tliat the explosion chamber was made from Teflon

(rather than glass). At first, it seemed that these

bands might be from some previously unobserved

species, such as AIN. Since observation of

such species was one of our principal goals, this

would, of course, have been a most interesting

result. HoAvever, analysis of the spectrum showed

that the questionable features were C2 and CN

bands from reactions between the hot vapor

and the Teflon vessel. Explosions in vacuum Avith a Teflon vessel gave essentially the same result

C2 and CN bands predominated the spectrum,

and radiation endured for more than 2 msec. In

spite of its desirable machining and high temperature characteristics. Teflon was therefore

found not suitable for study of the spectral features from the wire and/or surrounding gas.

:-trum (at t ^ 1 msec) from exploding aluminum loire

] ,iF at H kV).

I Glass vessels with flat quartz windows gave the

j; best performance; impurities from the walls did

^ not produce features in the spectrum, and it was p possible to clean the entire vessel in hydrochloric f

acid. At the levels of energy applied, a wall : thickness of about 4 mm was required to prevent '

explosion of the vessel.

j Figure 57 illustrates another difficulty encoun- .

terecl when attempting to observe spectra from

explosions in nitrogen, argon or vacuum. The ; first portion of the drum camera record = 0 i to 300 ixsec) is nearly identical to figure 54. The ] second portion (t = 1000 to 1200 /xsec) shows 1 several band sequences which are identified , (from the AlO reference spectrum in the middle)

as the Av = -1 through +1 sequences of the AlO

^ Blue-Green System. Investigation showed that

the bands were due to a trace of oxygen from a small leak in the explosion chamber. It is clear,

j therefore, that a tightly sealed system is required

38
ALUMINUM WIRE IN ARGON (PRESSURE 76 TORR)

5890- -Nal

<<

I X

O

IJJ _l

5

4982

3944.0 & 4981.7 5890.0

ALUMINUM WIRE IN NITROGEN (PRESSURE 76 TORR)

Figure 55. Drum camera record of spectrum emitted by an exploding aluminum tvire in argon.

Time-Resolved Plate from an Aluminum Wire in Nitrogen. Pressure : 76 Torr ; Knergy : 60 iiF at 14 kV.

for studies of spectral features from the wire and

a selected environment.

It is also interesting- to see in this figure 57

that the pulsations in intensity reported earlier,

which are belieA^ed due to reflecting shock waves,

are evident for more than 200 /xsec, and that they

could not, therefore, be attributed solely to the

oscillations of the main discharge current (total

duration ^ 90 /^sec). The broadening of the spectral features, e.g., of the Nal and Mgl lines,

during the intervals of greater intensity suggests that the pressure of the radiating vapor is higher during these times. Such an effect could

very well be caused by pressure or shock waves

reflecting from the walls of the explosion

chamber.

Figure 58 is a portion of a typical drum camera record from an aluminum wire exploded in

0.5 atm oxygen. (Elxperience showed that the

features were sharper, more enduring, and more

intense at reduced pressures.) With this system,

chemical reaction occurred almost immediately

(in less than 50 jusec) and AlO bands, first in

absorption and then in emission (after about 125

/Asec), dominated the spectrum for several milliseconds. This film again illustrates the importance of time resolution in spectral studies of this

type. It is, for example, clear that intervals later

than ^ 300 /xsec are favorable for observation

of the AlO spectrum in emission, while the interval between t ^

10 and 100 fxsec is favorable for

observation of the bands in absorption.

Plates (a) and (b) in figure 59 show portions

of the AlO Blue-Green and Ultraviolet Systems,

as observed at different times during the explosion by use of the rotating shutter. At t ^ 1 msec (a), only the most intense atomic lines re- mained, and many bands in the Av = +3 through

+5 sequences of the Blue-Green System, Avhich

could not be distinguished at t 300 /xsec (b), were observed in emission. However, the Av = +2

through -1 sequences of the A^S^-X-^* transition (which are less distinct at 1 msec) are sharply defined in (b), and the B^II-X^S* tran39
700

800

o

a>

y 900

1000

4737 5165

WAVELENGTH

5636 5890/6

FiGUEE 56. Portion of drum camera record from exploding aluminum tcire in nitrogen using a Teflon explosion chamber

(15 ixF at lit kV).

sition (which Avas not observed at 1 msec) is seen

in absorption.

PLate (c) in figure 59 shows the spectrum at

t 300 /isec Avhen a liigher energy was employed

for explosion of the wire. It was hoped that

greater energy input Avould simply increase the

intensity of the background continuum, and

thereby permit observation of the structure of

the Ultraviolet System. It was found, hoAvever,

that greater energy had several other effects:

(1) it produced and intensified more lines from

atomic and ionic species, and (2) it prolonged

the total period of radiation. Our results suggested, therefore, that higher energies are desirable if the explosion is to be utilized as an intense light soui'ce or for study of atomic and /or

ionic species. On the otlier liand, for studies of

molecular species, more energy only seems to obscure the structure of the molecular features by

j producing more intense continuum and line ra- : diation. i A portion of the spectrum from an aluminum

wire in hydrogen is given in figure 60. As with

argon and nitrogen, line radiation was i^redomi- j nant. Weak AlH bands, Avhich appeared only in ; absorption, Avere observed against the back-

| ground continuum betAveen f ^ 50 and 500 /xsec.

j

e. Spectral Results With Titanium Wires

j The radiation from titanium AA'ires Avas in gen- ! eral much more intense (by a factor of 5 to 10, as estimated from the sj^ectrograph slit Avidths) j than that from ahuninum Avires. The spectral

distributions from explosions in argon, nitrogen,

and hydrogen Avere found to be nearly identical. | Numerous lines from atomic and ionic species so

40
5890/6-

I

IXJ _jUJ5

5461-

4680-

200

AlO

REFERENCE

TIME - /^sec

Figure 57. Drum camera result from an explosion in nitrogen with trace of oxygen (60 fiF at lli. kV).

AlO A^I - X^I GREEN SYSTEM

Hg 4358

600

TIME - M sec

FiGUKE 58. D7um camera record of the AlO spectrum from exploding aluminum loirc in oxygen

(15 iiF at IJf kV; pressure 0.5 aim).

41
(b)

AIO B'n.-XT TRANSITION

(2,0) (1.0) (0.0) (0,1)

(4.0) (3,0) AIO AT- XT TRANSITION

(2,0) (1,0) (0.0) (0.1)

Mgl AJI Fel

(c)

All Cul TiE rn

Zrl CrI Till Cal

Y f n

CrI Zrl Col Znl Til

ffl k .^I.„ 1^

1 1 1 3082.8 3944.0 4981.7

WAVELENGTH A

FiGUEE 59. Time-resolved spectra from aluminum icires exploded in oxygen (pressure: 76 torr) at different intervals

and energies: (a) t 950 to 1130 ^sec, 60 uF at U kV; (b) t « 200 to 380 usee, 60 uF at U kV; and (c) t ^ 200

to 380 iisec, 60 at 18 kV.

complicated the spectra that adequate resoKition

was not possible with the present spectrograph.

Plate (a) in fignre 61 gives a time-resolved

result from a titanium wire exploded in oxygen

at a pressure of 0.76 torr. At this low pressure, a complex line spectrum, similar to those produced in argon, nitrogen and hydrogen, was produced. At higher pressures (b) and (c), the

continuum intensit}' was found to be increased,

and weaker lines could not be distinguished. At

76 torr (c), continuum radiation was predominant for several milliseconds; and, as figure 62

shows more clearly, portions of the C-n -X^II and

transitions of TiO were observed between 4900 and 5900 A. Here, as Avith A/H, the

bands appeared only in absorption.

In conclusion, the fact that several molecular

species (AIO in the ultraviolet region, AlH and

TiO) were observed only in absorj^tion suggests

that these molecules were produced only in the

ground or some other long-lived electronic state. It Avas concluded, therefore, that their spectral

features might be studied more effectively by

using a secondarj' continuum source for absorption studies later in the explosion, after most of

the complex line radiation has ceased. (Absorption studies are not possible at these times Avithout use of a secondary backarround source, be- cause the continuum radiation emitted b}^ the

wire itself is A^ery weak in the later stages of the

explosion.) Apparatus suitable for such studies

Avas therefore cleA'eloped and used for time-resolved, high-resolution studies of the AIO spectrum, as described in the next section.

4.4. Time-Resolved Observations of the

Spectrum of A/O

a. Introduction

The results from absorption studies of the species produced by scA^eral different exploding wire

systems are presented in this section. Particular

emphasis is giA^en to the AIO spectra, since the

explosions (under selected conditions) proA-ed to

be excellent sources for study of this molecule.

Over the course of the iuA-estigations, a considerable number of ncAv bands Avere observed in the

ultraviolet region. The observed Avavelengths and

the methods deA-ised for obtaining the absorption

spectra are giA^en in the folloAving pages.

b. Apparatus and Procedure

The 60 fjF capacitor bank, at voltages betAveen

5 and 20 kV, was used for these experiments.

Time-resolution of the spectra Avas achieved by

use of the shutter disk as described in the last

section. Absorption spectra Avere obtained by

flashing radiation from an intense, pulsed light source through the exploding Avire mixture under study as shoAvn in figure 63.

A photographic plate of the spectrum, with

features from the species in the system appearing in absorption against the strong background

continuum of the secondary source, Avas thus obtained at the focal plane of the spectrograph.

The intervals of the explosion spectnmi photographed for stud}^ AA-ere selected mainly on the

basis of the results reported in the last section.

42
AIH A' TT-X'S"" TRANSITION

o

CO

I

200

300-

AIH BANDS (1,0) (0,0) (1,1) (0,1)

III CrI All

3578.7 4533.2

WAVELENGTH -a

FiGUEE 60. Drum camera record and integrated plate of spectrum from aluminum wire exploded in hydrogen.

Pressure : 0.1 and 0.5 atm, respectively ; Energy : 60 yiiF at 14 kV and 15 ^iF at 14 kV, respectively.

The secondary flash source was synchronized

Avith the selected interval and Avith the arrival

of the shutter slot at the entrance slit of the

spectrograph by use of an additional delay generator, as shown in figure 63. With the present

setup, it was possible to record the spectrum for

any 180 /Asec interval from t = Q to t ^ '^O msec

after initiation of the discharge current.

In some experiments, an exploding titanium

wire (0.14 mm diam., 6.2 cm length), enclosed

in a sealed glass vessel with a quartz window,

10 Suprasil windows and lenses are believed to be preferable, because of their high transmission characteristics in the ultraviolet region.

was used as the secondary source. The vessel was

charged with oxygen (pressure V2 a.tm). In

other experiments, a Lyman fiashtube [56] was

used as the secondary source. In general, the con- tinuum radiation emitted by the latter was less

intense than that from the titanium wire. However, because of the simplicity of its operation

(no reinstallation of wires was required), the

fiashtube was used for most of the experiments.

Its use was also desirable because very few

atomic lines and no bands appeared in its sDectruni; confusion of spectral features from the

lamp with those from the explosion mixture un43
0.76 Torr

Til

25 Torr flllll

Cul Mol Nal Nal Til

All Fel Til

76 Torr

Nal

3944.0/61.5 4981.7 5890.0/95.9

WAVELENGTH - A

.Figure 61. Time-resolved plates (t ^ 150 to 330 usee) of spectrum from titanium wires exploded

in, oxygen at various pressures. Energy : 60 nF at 14 kV.

der study was therefore less likely. However,

maximum intensity was required for observations

at wavelengths shorter than 2500 A, because the

explosion vapor scattered or absorbed most of

the radiation in this region. In fact, several se- quences (at shorter wavelengths) of the ultraviolet system of AlO could be observed only

during the fii'st 400 /^sec of the main explosion,

when the initial background continuum radiation

from the wire under study supplemented that

from the secondary source. In such cases, the

titanium wire source was used, because of its

high intensity.

The feasibility of absorption studies by these

methods was first investigated in some exploratory experiments with the 600 1/mm grating installed in the spectrograph. Some typical results

from experiments, with aluminum wires immersed in a hydrogen, a nitrogen, and an oxygen

medium (preexplosion pressure = 76 torr), re- spectively, are given in figure 64. In these initial

experiments, an exploding titanium wire was

used as a secondary source. All of the plates were

taken about 1 msec after initiation of the main

discharge. In spite of the emission lines from the

titanium wire, band features from the All!

A^II-X^S and the AlO A^X-X^X transitions

and line features from atomic species (e.g., All,

Nal, Cul, etc.) in the explosion mixtures are

clearly evident in absorption. The 2160 1/mm

grating was then installed in the spectrograph

for further studies at higher resolution of the

spectrum of the AlO molecule. The results of

these studies are given in the following paragraphs.

c. The A^S - X^S Blue-Green System

The visible bands of AlO have been studied

extensively. Tyte and Nichols [57] have recently

collected and correlated the results of the most

significant studies in the first section of their

Identification Atlas Series.

In the present work, the Blue-Green System

was distinct and well developed. However, examination of our spectra indicated that significant

additions to the extensive observations of Shimauchi [58] and Becart and Mahieu [59] w^ere

not likely. The details of the spectra were therefore not' carefully measured, and attention was

directed to the observations made in the ultraviolet region.

d. Ultraviolet Systems

In 1941, Coheur and Rosen [60] reported observation of three systems of bands which they

attributed to the AlO molecule: (1) a complex

group of bands between 3322 and 2879 A; (2) a group of three broad bands between 2879 and

2686 A; and (3) a weak, complicated system between 2686 and 2390 A. As in the present study,

they worked with an exploding wire source and

44
TiO BANDS- (l,0)a {0,0)a (0J)« (Op)>ff (l,3)«

I 1 I 1 I

Til

4981.7

Nal

5890.0/95.9

WAVELENGTH -&

Figure 62. Drum camera record and time-resolved plate (t «s 150 to 330 /isec) of spectrum from titanium wir&

exploded in oxygen.

Energy : 60 iiF at 14 kV ; Pressure : 76 torr.

with rather low dispersion instruments. In 1959,

Goodlet and Innes [61] studied a few of the

more intense bands of the system near 3000 A

with higher resolution, using a hollow-cathode

source. They also observed a fragmentary system

near 2500 A. In both of these cases, the bands

were observed in emission.

In the present work, a rather extensive, complex series of bands was observed in absorj^tion

in the region from 2200 to 3400 A. Comparison

of our results Avith those of Coheur and Eosen

showed that there is good agreement between the

Avavelengths of the moi^e intense bands. Hoavever, our observations shoAved many more bands

than have been previously reported.

Because of the good agreement betAveen our

measurements and those of Coheur and Rosen,

extended Deslandres tables AA-ere calculated from

the formulae they deriA^ed for representation of

the band origins of the systems near 3000 A and

2500 A. These formulae are

V = 33085 + 845'y' - - 97lv" + 72v"^

and

V = 38554 + 715^' - 9Q0v" + lOv"^,

respectively. The calculated Deslandres arrays

Avere then used as guides in interpreting the absorption spectra obtained in the present Avork.

By this procedure, it Avas evident that most of

the observed bands did indeed fit into the schemes

predicted by the above formulae, the maximum

deviation of the observed AvaA'elengths from the

calculated values being less than 1.5 A.

45
SECONDARY

LIGHT SOURCE 60 /xF DISCHARGE CIRCUIT SHUTTER DISC

QUARTZ

LENS

SPECTROGRAPH

Q—PM TUBE

EXPLODING WIRE UNDER STUDY

PM SIGNALS (-2v)

OSCILLOSCOPE

(TRIGGER IN

SINGLE SWEEP)

GATE OUT (+20v)

WAVEFORM

GENERATOR

MAIN EXPLOSION

*• DELAY GENERATOR - SECONDARY

SOURCE

DELAY

TRIGGER EXPLODING WIRE

TRIGGER

• SECONDARY SOURCE

FiGXJBE 63. Block diagram of setup used for absorption studies.

(1,0) (0,0) (1,1)

All

L

Cal

AIH A'tt - X'Z TRANSITION

CrI Til Cul

I JJIWpijHpWiW

A! 0 A^Z"^ - X TRANSITION

(3,0) (2,0) (1,0) (0,0) (0,1) (0,2)

3944.0 4842.0

WAVELENGTH -S

Hal Hal

(0,3)

IT

Cul

Figure 64. Absorption spectra from exploding aluminum wires in various gases.

Preesplosion pressure : 76 torr.

46
AlO B ^Tl- - X^X* TRANSITION

(3,0) (2,0) (1,0) (0,0) (0,1) (0,2)

Cul

Ail Ain All Nil Cul

2840.0 3082.2 3247.5

WAVELENGTH, A

Figure 65. Absorption spectrum of the B^n—X^s* transition of AlO as observed in an exploding wire experiment.

The measured bands are given in the Deslan- 2500 A). The bands observed by Coheur and

dres Tables I (for the system near 3000 A: the Rosen are enclosed in parentheses. In many

B^n-X^S transition) and II (for the system near cases, a descriptive remark, as per the table key,

Table 1

Deslandres Table of Observed Band Head Wavelengths (in air) (A) and o Band Head Wavenumbers (in vacuum) (cm~l) for System of Bands with (0,0) Sequence at 3021. 6A \ 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

0 33085

(3021.6) T

33933

(2946.1)

34755

(2876.4)

D

35560 2811.3

(10,6)

36399

2746.5

37223 2685.7

(2,3)

38011 2630.0

38807

2576.1 An

(3,2)

39578 2525.940356

2477.2

(14,4)

41123

2431.0

D

41898

2386.0

(6,1)

42654

2343.7

D

43403

2303.3

D

44148

2264.4

44843 2229.3

(14,4)

1

32122 . (3112.2)

(9,9) (11,11)

32963

(3032.8) D

33799

(2957.8)

34619

(2887.7)

T

35444 2820.5

D

36245 2758.237047 2698.5

(3,4)

37852

2641.1

38618 2588.7

(4,3)

39408

2536.8

T

40174

2488.4 409342442 .

D

41682

2398.4

(7,2)

42430

2356.1

(13,6) OR

43155

2316.5

43894

2277.5

(15,6)

2 31174

(3206.9)

D

32016

(3122.5)

32847

(3043.5) T

33673

(2968.9)

34482

(2899.2) 352982832.2

36098 2769.436891 2709.9

(4,5)

37675 2653.5 A£I38459 2599.4

39221 2548.9

39983 2500.3

(2,0)

40736

2454.1

(3,0) (14,9)

41495

2409.2

(8,3)

42215

2368.1

42953 2327.4

3

30251 3304.7 W,H

31082

(3216.4)

31913

(3132.6)

(15,15)

32748

(3052.7) D

33548

(2979.9)

T

34367

(2908.9)

(4,8)

35169 2842.635950 2780.836746

2720.6 All

Cal

37522 2664.338287 2611.1

(6,5)

39052 2559.939792 2512.3

40547

2465.5

41282

2421.6

(9,4)

42016

2379.3

A

29320 3409.7

W

30157

3315.0 W.H

30991

(3225.8)

31813 3142.5

H

32623

(3064.4)

D

33443 2989.3

D

34238 2919.935039 2853.135832 2790.036600 2731.437373 2674.938119 2622.6

(2,2)

38882 2571.1

(8,6)

39634

2522.3

40362

2476.8

(9,0)

41092

2432.8

5

30086

(3322.9)

30906

(3234.7)

31717

(3152.0)

32541 3072.2

D

33340

2998.5

D

34130

2929.1

T

34908

2863.8

35700

2800.3

36472

2741.0

(1,3)

37225

2685.6

(2,3)

38729 2581.3

40185

2487.7

(10,1)

(11,7)

6

29200

3423.7

Nil(?)

30012

3331.0

30827

(3243.0)

31647

(3158.9)

32444

3081.3

All

33242 3007.434021 2938.5

D

34807 2872.1

35568 2810.7

(3,0)

36325

2752.1

(11,2)

37082 2695.9

(14,14)

(11,11)

38564

2592.3

(0,0)

39213 2549.4

(10,2)

(14,11)

7

29944 3338.6

30765

(3249.5)

31571

(3166.5)

32354

3089.9

I

33,156 3015.233890 2949.9

H

34689 2881.9

35452

2819.9

(4,1)

36195 2762.0

36951

2705.5

W

37687 2652.7

Ail

38415 2602.4

8

30695

(3256.9)

H

31502

(3173.5)

32280

3097.0

T

33059 3024.0

D

33821 2955.934584 2890.735335 2829.236087 2770.3

(6,2)

36827 2714.6

W

9

10

11

12

13

14

15

29851 3349.0

Till, Cul

30652

(3261.5) Mnl

31440 3179.7

(1,9)

32207 3104.0

32978 3031.4

33743 2962.734500 2897.735232 2837.535980

2778.5

36702 2723.8 W

Key:

Remar

Wavenumber

Wavelength

Remark

ks: DT

30604

3266.6

31376

3186.2

H

32142 3110.3

32898 3038.833659 2970.1

(3,2) (3,8)

34403 2905.9

35143 2844.735870 2787.0

Doublet

Triplet

30558

3271.5

31317 3192.232094

3114.9

(0,1)

32839 3044.3

(9,15)

33583 2976.834319 2913.0

35052

2852.1

(v',v") - Indicates overlapping bands

W - Weak

H - Hazy

30517 3275.931291 3194.9

T

32034

3120.8

D

32784 3049.4

(8,14)

33510

2983.3

(Wavelength) - Our values also observed by Coheur and Rosen [60] 30490 3278.8

31250

3199.1

T

31990

3125.1

D

32725 3054.9

T

33455 2988.2

30471 3280.9

31202 3204.0

H

31945 3129.5

D

32680

3059.1

T 30454

3282.7

31183 3205.9

H

31918 3132.1

(2,3)

47
Table II Deslandres Table of Observed Band Head Wavelengths (in air) (X) and Band Head Wavenumbers (in vacuum) (cm"-'-) for System of Bands with (0,0) Sequence at 2592. 3A.

V \ 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

0

38564

(2592.3) T

39276

(2545.3) T

39973

(2500.9) D

40706

(2455.9) D

41419 2413.6

D

42128

2373.0

42843 2333.4

T

43567

2294,6

I

44273

2258.0 44970

2223.0

D

45713

2186.9

1

37591 2659.4 Ml38331

(2608.1) T

39034

(2561.1) T

39749

(2515.0)

40449

(2471.5)

D

41180

2427.6

T

41898

2386.0

D

42604

2346.5

T

43335

2306.9

D

44043 2269.8 W

44757

2233.6

45467 2198.7

46192

2164.2

2

36674

2725.9

37394

(2673.4)

T

38119 2622.6

T

38822

2575.1 AXI

39548 2527.8

T

40257

2483.3

D

40954

2441.0

Cul

41682 2398.4

(12,1)

42381

2358.8

T

43116

2318.6

T

43831

2280.8

44538

2244.6

45247 2209.445982

2174.1

3

35761 2795.5

Mnl

36477 2740.6

T

37194

(2687.0)

(5,0)

37892 2638.3

T

38618 2588.7

39333 2541.6

T

40050

2496.1

T

40756

2452,9

T

41495

2409.2

T

42192

2369.4

T

42905

2330.0

43626

2291.5 45043

2219.4 457902183.2

4

34877 2866.435582 2809.636299 2754.1

T

37020 2700.437730 2649.6

All

38444

2600.3

39161 2552.8

39889 2506,2

T

40577

2463.7

T

41299

2420.6

(14,3)

42025

2378.8

42727

2339.7

43452

2300.7 448432229.3 45696

2192.5

5

33993 2940.934724 2879.035444

2820.5

D

36150 2765.436881 2710.6

(7,2)

37577 2660.4

A^I

38278

2611.7

39013 2562,5

T

39724

2516.6

T

40426

2472.9

(4,1)

41155

2429.1

(6.1)

42587 2347.4

(7.1)

43286

2309.5

44016

2271.2 44729

2235.0

6

33155 3015.2

T

33864

2952.1

T

34583 2890.7

D

35287

2833.1

(5,2)

36014

2775.9

36727 2722.0

(8,3)

37446 2669.7 W38165 2619.438882

2571.1

T

39578 2525.9

40302

2480.5

(8,0)

41005 2438.0 41734

2395.4

42452 2354.9

43155 2316.5

43894

2277.5

(15,1)

7

32336 3091.6

T

33040

3025.7

D

33758 2961.4

D

34459

2901.1

35187

2841,1

T

35905

2784.3

T

36600 2731.4

(9,4)

37322 2678.6

T

38048 2627.5

D

38748 2580.039484

2531,9 401902487.4

40894

2444.6

41628

2401.5

42324

2362.0

(8.2)

8

31519 3171.8

32226 3102.2

T

23990 3030.3

D

33659

2970.1

(3,2)

34374

2008 .

T

35087 2849.2

T

35799 2792.5

T

36509

2738.2

D

37251 2683.7

T

37948 2634.4

D

38669 2585.3

(9,1)

40094

2493.4

40807

2449.8

41536

2406.8

42245 2366.4

9

30731 3253.1

31440 3179.7

(8,9)

32151 3109.4

T

32859 3042.4

T

33589 2976.3

D

34301 2914,5

D

35012

2855,3

T

35729 2798.0

T

36439 2743.5

D

37165 2689.9 W37879 2639.2

(3,3)

38594 2590,339309 2543.240028

2497.5

40727 2454.6

(3,0)

41454

2411.6

10

30667 3259.9

(9.10) (0,1)

32818 3046.2

33531 2981.434238

2919.9

(6,4)

34968

2858.9 Cul (4,0)

37113

2693.7

(0,0)

39245 2547.339973 2500.9

'2,0)

40681 2457.4

11 32068 3117.5

32776

3050.1 Ail

33500 2984,234197 2923.434927 2862.3

(8.5)

37082

2695.9

38506

2596.2

(0.0)

39221 2548.9

(10.2)

39929

2503.7

(11.2)

12 32047 3119.5 Wl.Nil

(5,11) (6,12)

33477 2986.234194 2923.6

(8,5) CR

37052

2698.1

37757 2647.7

(0,0)

39192 2550.8

13

14

15

Key: Wavenumber

Wavelength

Remark

32769 3050.833487 2985.434202 2922.9

(8,5),CR CR (0,0) 32792 3048.6

33510

2983.3

CR

37082 2695.9

I - Triplet

(v',v") - Indicates overlapping bands

CR - Cannot resolve (0,1)

32839 3044.3

33548 2979.9

(4,3)

34274

2916.8

CR

37137

2691 .9

W - Weak

H - Hazy

(Wavelength) - Our values also observed by Coheur and Rosen [60]

is include(i beneath the measured value. It is interesting to see that the two systems overlap

considerably, so much so that it Avas in some

cases (indicated by CR in the tables) impossible

to resolve the overlapping features with the

present instrument.

A portion of the B-H-X^S transition is shown

in ligure 65. The spectrum from an iron arc in

air was superimposed on the AlO spectrum for

reference purposes. In printing of the s])ectral

plates, several different exposures were used in

order to bring out the details of the various se- quences. The spectra were enlarged to the scale

of the Vatican Tables of Iron Line Spectra [62],

in order to permit direct comparison of the observed spectra with the reference Fel lines. The

wavelength measurements are believed to be accurate to ±0.5 A.

The lower wavelength portions of both systems were quite difficult to observe, principally

because the continuum radiation from the sec- ondary source was either scattered or absorbed

by the exploding wire vapor. It was therefore

possible to observe features below about 2800 A

for only about 400 /xsec after initiation of the

explosion-discharge. Observations after 400

/xsec showed no background continuum radiation and thus no absorption features from atomic

or molecular species. Prior to 400 /^sec, however,

when the initial continuum radiation from the

exploding wire itself supplemented that from

the secondary source, it was possible to distinguish sharp "All lines and numerous bands. The

quality of the spectrum was however reduced by

the relatively low intensity of the continuum

radiation and by emission lines from atomic

species (All, Cul, etc.) which are very intense

in the early stages of the explosion. The number

48
of lines from impurities was also increased, because, as mentioned earlier, it was necessary to use an exploding titanium wire as the secondary

flash source. Til lines from the titanium wire

were therefore superimposed on the spectrum.

It was not possible to distinguish any features

below 2100 A, because no radiation below this

wavelength was passed by the present system.

e. Conclusions

Detailed analysis of the observed data was not

possible. Higher resolution instruments than

those available for the present investigation are required to distinguish the overlapping features

more clearly and to establish the symmetry of

the upper state in the system near 2500 A. (The

authors have learned that such studies are now

in progress in India [63].) In addition, a secondary source of higher intensity should be employed. However, since the present observations

revealed a consiclerable number of bands which

have not been reported previously, it is felt that

these data shovdd be recorded. It is hoped that

this work and the techniques applied will be

helpful in future spectroscopic studies of hightemperature species.

The authors are grateful to I. R. Bartky, A. M.

Bass, F. R. Kotter, H. Cones, R. C. Thompson,

and A. Cezairliyan for helpful suggestions and

discussions; to William A. Bagley and William

F. Barnard for technical assistance; and to Miss

Mary King and Miss Rosemary Selapak for

typing of the manuscript.

5. References

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