A Study of Window Focusing 

Effects on Laser Ignition for 

Medium-Caliber Systems 

by Jeffrey B. Morris and Richard A. Beyer 

ARLTR-2743 May 2002 

Approved for public release; distribution is unlimited. 

20020617 026 : 
The finclmgs in this report are not to be construed as an 

official. pepartment of the Army position unless so 

d&gnated by other authorized documents. 

Citation of manufacturer’s or trade names does not 

constitute an official endorsement or approval of the use 

thereof. 

Destroy this report when it is no longer needed. Do not 

return it to the originator. 
Army Research Laboratory 

Aberdeen Proving Ground, MD 210055066 

ARL-TR-2743 May 2002 

A Study of Window Focusing 

Effects on Laser Ignition for 

Medium-Caliber Systems 

Jeffrey B. Morris and Richard A. Beyer 

Weapons and Materials Research Directorate, ARL 

Approved for public release; distibution is unlimited. 
Abstract 

One proposed implementation of medium-caliber laser ignition employs a 

focusing ball to replace the conventional electric primer. The effects of ball size 

and igniter stand-off distance have been characterized experimentally with black 

powder pellets. Ignition threshold and delay experiments were carried out using 

4mm and &mm diameter glass beads with standoffs up to 1.5 mm. Ray tracing 

was used to model the energy distribution of a top-hat beam profile refracted 

through a focusing ball. Results of the study indicate that the tighter focus of the 

4mm bead can reduce the required laser pulse energy with little effect on the 

ignition delay. 

. 
, 

Acknowledgments 

This work was supported by the U.S. Army Environmental Quality Basic 

Research and Development (EQBRD) Program. The EQBRD program is 

managed through the Industrial Ecology Center at Picatinny Arsenal, NJ. 

. . . 

lu 
iv 
Contents 

Acknowledgments 

List of Figures 

List of Tables 

1. Introduction 

2. Experimental Setup 

3. Experimental Data 

4. Spot Size and Spatial Energy Distribution 

5. Discussion 

6. Conclusions 

7. References 

Distribution List 

Report Documentation Page 

. . . 111 

vii 

ix 

1 

2 

3 

4 

10 

12 

15 

17 

21 

V 

List of Figures 

Figure 1. Ball window configuration for laser ignition of 30-mm 

ammunition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 

Figure 2. Experimental setup for the laser ignition experiment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

Figure 3. Mapping of 20% area1 contours for a top-hat energy distribution 

(left) to ray projections (right) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 

Figure 4. Refraction of 4.76mm diameter beam passing through 6-mm 

diameter glass bead . . . . . . . . . . . . . . . . . . . . . . . . . . ..-......................................................... . . . . . . . . . . . . . 6 

Figure 5. Intensity distribution calculated as a function of stand-off 

distance behind a &run glass bead for a 5-ms, 100-mJ laser pulse. . . . . . . . . . . . . . . . . . . 7 

Figure 6. Laser spot imaging on burn paper at bead exit stand-off distances 

of O.O-, 0.5, IO-, and 1.5~mm for a 6-mm diameter bead with 4.76mm 

incident beam diameter, 5-ms pulse length, and 15-mJ average incident 

laser pulse energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 

Figure 7. Laser spot imaging on bum paper at bead exit stand-off distances 

of O.O-, 0.5-, and 1.0~mm for a 4-mm diameter bead with 3.18~mm 

incident beam diameter, 5-ms pulse length, and 12-mJ average incident 

laser pulse energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 

Figure 8. Plot of Go Fraction vs. total incident laser pulse energy for 6-mm 

bead, 4.7~mm beam diameter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 

Figure 9. Plot of Go Fraction vs. total incident laser pulse energy for 4-mm 

bead, 3.18mm beam diameter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 

Figure 10. Time-to-first-light vs. estimated laser pulse energy for the 

ignition of black powder using a 6-mm diameter bead lens and a 

4.76mm diameter laser beam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 

Figure 11. Time-to-first-light vs. estimated laser pulse energy for the 

ignition of black powder using a 4-mm diameter bead lens and a 

3.18~mm diameter laser beam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 

Vii 
INTENTIONALLY us-r BLANK. 

. . . 

Vl.U 
List of Tables 

Table 1. Experimental data for study using 6-mm diameter bead. Incident 

laser beam diameter = 4.76 nun. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

Table 2. Experimental data for study using 4-mm diameter bead. Jncident 

laser beam diameter = 3.18 mm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

Table 3. Spot size as a function of standoff behind a 6-mm diameter glass 

bead for a 4.7~mm diameter incident laser beam. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 

Table 4. Spot size as a function of standoff behind a 4mm diameter glass 

bead for a 3.18-mm diameter incident laser beam. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 

ix 

1. Introduction 

In work described in a previous U.S. Army Research Laboratory (ARL) technical 

report, a 30-mm gun fixture was modified for laser ignition (Beyer and Hirhnger 

1999). In that report several concepts for the modification of 30-n-m ammunition 

to accept a laser ignition source were studied experimentally using single-round 

firings. Figure 1 depicts one of the more successful implementations where the 

electric primer has been replaced by a g&s focusing baII in a housing, along 

with the addition of an iwtion material specific for laser ignition and transfer to 

the flashtube. 

Housin 

Focusing 

Ball 1 

Ignition 

Material h 

Flashtl 

Pellets 

Abe 

Figure 1. Ball window configuration for laser ignition of 30-mm ammunition. 

In the configuration shown in Figure 1, the ignition material is pressed up 

against the focusing ball. We suspect that this configuration results in minimal 

focusing of the laser spot and that a smaller spot with greater intensity can be 

realized by incorporating a standoff between the focusing bail and the ignition 

material. The same effect might also be achieved through the use of a smallerdiameter ball. In this report we examine how stand-off distance and bead size 

affect the direct laser ignition of black powder pellets. While the direct ignition 

of black powder does not meet the action time* requirements (4 ms) for the 

30-mm application, these pellets are a convenient sample for this fundamental 

laboratory study and should provide some additional insight into the initial 

ignition event. 

*Action time is defined as the time from application of the firing pulse to the exit of the projectile 

from a gun barrel. 
2. Experimental Setup 

The experimental setup used for this study is shown in Figure 2. The output 

from a pulsed Nd3+ laser with a wavelength of 1.06 l.m was tnzncated using an 

adjustable iris. The laser pulse length was set to 5 ms. The truncated beam was 

directed into a glass bead that was used as a short focal-length lens. A lOO-mg 

black powder pellet was attached to a translation stage. The position of the black 

powder pellet was adjusted to provide various standoffs from the bead. Laser 

ignition experiments were carried out to determine the effect of the stand-off 

distance and bead diameter on the ignition threshold and time-to-first-light 

(TTFL). Two sets of bead/iris parameters were used. When using a 6-mm 

diameter bead, the laser beam diameter was truncated to 4.76 mm (3/W’). The 

experiments were also carried out using a 4-mm diameter bead with a truncated 

3.18~mm (l/8”) beam diameter. 

Photodiode 

Iris, 

Figure 2. Experimenta setup for the laser ignition experiment. 

Ignition threshold data were measured in groups of three data points for each 

setting of laser lamp energy and stand-off distance. An energy meter placed 

between the iris and bead was used to measure laser pulse energy before each 

experiment. The three energy measurements were averaged for each set of 

experimental conditions. Go/No-Go data were based on the ignition of black 

powder pellet. A “Go Fraction” was determined as the number of Go events 

divided by the total number of experiments for a given set of conditions (in our 

case, 3). 

The photodiode was used to generate time-to-first-light data. A transimpedance 

amplifier was used to convert current to voltage and the voltage trace was 

recorded using a digital oscilloscope. The oscilloscope was triggered using the 

leading edge of the laser pulse. A baseline light value was established for each 
trace by averaging several milliseconds in the pretrigger region. TTFL was 

defined as the time for the photodiode trace to first exceed the baseline plus 

twice the resolution of the analog-to-digital (A/D) conversion in the oscilloscope. 

An average TTFL was calculated for each data set that had a Go Fraction of 1. 

3. Experimental Data 

Table 1 shows the experimental data for the experiments using a 6-mm diameter 

bead. Each line in the table represents an average of three individual 

experiments using a common set of conditions. The pulse energy, Go Fraction, 

and TTFL are averages for each data set. Table 2 shows the corresponding data 

for the 4mm diameter bead. 

Table 1. Experimental data for study using 6-mm diameter bead. Incident laser beam 

diameter = 4.76 mm. 

3 
Table 2. Experimental data for study using Pmm diameter bead. Incident laser beam 

diameter = 3.18 rrun. 

Standoff 

(mm) 

Pulse Energy Go Fraction 

(mJ) (W 

192 1 14.7 

0.5 I 23 I 1 I 26.7 

0.5 14 1 27.9 

0.5 I 9.7 I 1 I 28.6 

0.5 7.4 0.67 

0.5 6.5 0.33 

0.5 4.6 0 

1.0 12 1 27.6 

1.0 10 1 32.0 

1.0 9.7 0.67 

1.0 7.2 0.67 - 

1.0 5.7 0.67 

1.0 4.6 0 - 

4. Spot Size and Spatial Energy Distribution 

As part of a Phase I Small Business Jnnovative Research (SBIR) contract 

(DAAD17-01-C-0061), Aurora Optics carried out preliminary ray tracing on the 

6-mrn focusing ball and determined a focal length of 1.4 mm (Burke 2001). In 

order to further investigate the refraction of a non-divergent beam with a top-hat 

profile through a sphere, we devised and implemented a ray-tracing model 

using the Microsoft Excel spreadsheet. For this model, the refractive index of air 

was taken to be 1.00 and the refractive index of the glass bead was taken to be 

1.51 for a laser wavelength of 1.064 p. The top-hat assumption is reasonable 

when one considers that an iris truncated the beam. Removing the wings from a 

Gaussian beam results in a much smaller variation in energy distribution across 

the cross-section of the resultant spot. Ray tracing was used to establish spot 

sizes and energy profiles for the 6-mm bead at various stand-off distances up to 

1.5 mm beyond the bead. Only spot sizes were calculated for the 4-n-m bead. 

Spot sizes were calculated using five sets of rays corresponding to the 20%, 40%, 

60%, 80%, and 100% energy contours across the spot size of the laser beam 

incident to the glass bead. Figure 3 shows the mapping of equal-area regions to 

radial ray positions. Figure 4 shows the refraction of these rays through a 6-mm 

diameter glass bead. Spot diameters were determined from the radial position of 
3- 

2- 

1 - 

-3 

-E O- 

* 

-1 - 

-2 - 

-3 - 

-4’ 

Figure 3. Mapping of 20% areal contours for a top-hat energy distribution (left) to ray 

projections (right). 

the ray furthest from the beam centerline at each stand-off distance beyond the 

bead. The outermost ray at each standoff defines the boundary that contains 

100% of the refracted energy. Tables 3 and 4 list the spot sizes as a function of 

standoff for the 6-mm and 4mm bead, respectively. 

The calculated diameters only define the boundaries of the spot, but yield no 

information regarding the distribution of energy. Spatial energy distributions 

were calculated for the case of the 6-mm diameter bead by repeating the raytracing procedure for 100 rays corresponding to equal-area annular regions. 

Each ray corresponds to a 1% energy contour using a top-hat spatial profile for 

the incident laser beam. The position of each ray was calculated through the 

glass bead at specific stand-off distances. A density-of-rays approach was 

employed to determine the energy distribution beyond the glass bead using an 

assumption that each ray corresponded to 1% of the total energy in the spot at 

each standoff. A minimum thickness of 10 p was used when counting the rays 

in each annular area. Figure 5 shows the calculated energy distribution at 

4 standoff distances used for the 6-n-m bead plotted as peak laser intensity for a 

5-n-~, 100-mJ pulse. These calculations indicate enhanced perimeter laser 

intensity for standoffs at 0.0 mm and 0.5 mm. The energy distribution for 

standoffs of 1.0 and 1.5 mm indicate enhanced intensity at the middle of the spot. 

5 
-2 

-3 

-4 

-4 -3 -2 -1 0 1 2 3 4 5 6 

X (mm) 

1 

0.5 

-1 

2.5 3 3.5 4 4.5 5 

x (mm) 

Figure 4. Refraction of 4.7~mm diameter beam passing through 6-mm diameter glass 

bead. 

6 
Table 3. Spot size as a function of standoff behind a 6-mm diameter glass bead for a 

4.76-mm diameter incident laser beam. 

Position 

mm 

3.0 

Standoff Spot Radius Spot Area 

mm) mm km2) 

0.0 0.53 t 0.00877 

3.5 0.5 0.25 0.00187 

3.7* 0.7 0.16 0.00086 

4.0 1.0 0.37 0.00431 

4.5 1.5 0.81 0.02087 

*Beam Waist 

Table 4. Spot size as a function of standoff behind a 4-mm diameter glass bead for a 

3.18~mm diameter incident laser beam. 

Position 

(mm 

2.0 

2.5* 

3.0 

Standoff Spot Diameter 

64 (mm) 

0.0 0.70 

0.5 0.20 

1.0 1.09 

Spot Area 

cm2 

0.00390 

0.00031 

0.00931 

* Beam Waist 

-D-0.5-mm Standoff 

+ 1 .O-mm Standoff 

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 

Radial position (mm) 

Figure 5. Intensity distribution calculated as a function of stand-off distance behind a 

6-mm glass bead for a 5-ms, 100-mJ laser pulse. 
Figure 6. Laser spot imaging on bum paper at bead exit stand-off distances of O.O-, 0.5-, 

l.O-, and 1.5-mm for a 6-mm diameter bead with 4.76-mm incident beam 

diameter, 5-ms pulse length, and 15-mJ average incident laser pulse energy. 
Figure 7. Laser spot imaging on burn paper at bead exit stand-off distances of O.O-, 0.5-, 

and IO-mm for a 4mm diameter bead with 3.1%mm incident beam diameter, 

5-ms pulse length, and 12-mJ average incident laser pulse energy. 

9 
Additional ray-tracing analysis shows that the individual rays come to a focus 

(i.e., cross the center line of the bead) at stand-off distances of 0.58 to 1.44 mm, 

with the rays originally at the perimeter of the incident beam being most strongly 

refracted and coming to a focus first. Our focus calculations are in excellent 

agreement with the Aurora Optics results, which were modeled using a smaller 

2.5 mm diameter beam. At stand-off distances of 0.0 and 0.5 mm, none of the 

individual rays have yet come to a focus, hence the appearance of the ring 

intensification pattern in the energy distribution. The standoff of 1.0 mm is near 

the center of this focusing region and displays a very large intensity near the 

center of the spot. The standoff at 1.5 mm is just beyond the focusing region. 

For these last two standoffs, the majority of the energy is localized in a much 

smaller spot than those predicted in Table 3. Referring to Figure 4, for standoffs 

beyond the beam waist, it is evident that the ray that corresponds to the original 

100% contour defines gross spot size. Having come to a focus close to the bead, 

this ray will diverge with a larger angle than those rays closer to the center of the 

beam that come to a focus further out. At the distance where the innermost rays 

come to a focus, the highly divergent rays distribute their energy over a large 

area, resulting in a large spot size with a fairly low intensity in the perimeter, but 

with a high intensity in the center where other rays are focused- or diverging 

much slower-in a small area. 

To validate the predicted tendencies, the experiments depicted in Figure 2 were 

repeated using laser bum paper in place of the black powder pellet. Bum 

patterns are presented in Figures 6 and 7 for 6-mm and 4-n-m beads, 

respectively. Both figures show a ring pattern at O-mm standoff with the intense 

portion of the spot burning smaller areas at the larger standoffs. Figure 6 can be 

compared directly with Figure 4. The more detailed ray tracing was not done for 

the 4-mm bead. The beam diameter was scaled down in proportion to the bead 

size relative to the parameters used for the 6-mm bead- both were reduced by 

33%. Thus, the spot sizes and energy distributions would scale down by 33% at 

standoffs scaled down by the same factor. 

5. Discussion 

Two important issues for laser ignition are ignition delay and threshold energy. 

Ignition delay can be especially critical for a system such as the 30-mm M230 

chain gun, whose ballistic cycle allows only a very short time in battery. The 

threshold energy requirement affects the volume and weight of the laser package 

required for effective ignition. 

10 
Figure 8 shows a plot of “Go Fraction” vs. total laser pulse energy incident to the 

6-mm glass bead. The data show the lowest energy thresholds for laser ignition 

of black powder occur at stand-off distances of 1.0 to 1.5 mm. Figure 9 shows 

similar data for the experiments using the 4-mm bead. For these data, the lowest 

energy thresholds occur at stand-off distances of 0.5 to 1.0 mm. Comparison of 

Figures 8 and 9 show Iaser ignition thresholds for the 4-mm bead to be 

substantially lower than those observed for the 6-n-m bead at similar stand-off 

distances (see also Tables 1 and 2). At O-r-run standoff, the ignition threshold 

energy using the 4-mm bead is half that for the 6-mm bead. The tighter focus of 

the 4-mm bead results in smaller spot sizes with greater intensity relative to the 

6-mm bead. The results of this effect can be seen in the burn patterns shown in 

Figures 6 and 7. 

1.0 

0.8 

0.2 

0.0 

a[D q 00 0 

A 

A 0 

13 0.5mm Standoff 

I 0 O.O-mm Standoff 

0 1 .O-mm Standoff 

A 1 J-mm Standoff 

0 20 40 60 80 100 120 140 160 180 200 

Laser Pulse Energy (mJ) 

Figure 8. Plot of Go Fraction vs. total incident laser pulse energy for 6-mm bead, 4.76-mm 

beam diameter. 

1.0. mEI 0 0 c 

0.8 

E jj 0.6 

2 

u. 

0 (3 0.4 

CD 

III co 

0.2 •I 0.5-mm Standoff 

0 1 .O-mm Standoff 

0.0 6 

0 20 40 60 80 100 120 140 160 180 200 

Laser Pulse Energy (mJ) 

Figure 9. Plot of Go Fraction vs. total incident laser pulse energy for 4-mm bead, 3.1%mm 

beam diameter. 

11 
Ignition delay is characterized by TTFL. A tradeoff exists between energy 

threshold and ignition delay. At low-laser intensities, energy absorption, 

melting, and heat transfer govern the ignition event. As the laser intensity is 

increased, eventually the incident energy flux exceeds the capacity for absorption 

and heat transfer, and the excess energy is converted into kinetic energy through 

ablation of the target material. In experiments where a laser beam with a top-hat 

profile and a fixed spot size is used to ignite a propellant, one typically observes 

large ignition delays near ignition threshold, with shorter delays as the intensity 

is increased until ablation becomes significant. The ignition delay increases at 

higher ablative laser intensities. Given enough information about the propellant 

system, it is possible to model the ignition delay as a function of the energy flux, 

at least in the pre-ablative region (Cohen and Beyer 1993). 

As a first approximation one could convert the pulse energies (mJ) listed in 

Tables 1 and 2 into intensity (KW/ cm2) using the 5-ms pulse width and the gross 

spot areas listed in Tables 3 and 4. However, the data presented in Figures 5,6, 

and 7 reveal very nonuniform energy distributions once the laser pulse has been 

refracted by the focusing ball. We observe two different types of energy 

distributions as a function of stand-off distance. At short standoff, much of the 

energy is concentrated in a ring at the spot perimeter. At long standoff, a 

majority of the energy is focused in a spot that is much smaller than the gross 

spot areas indicated in Tables 3 and 4. While some shot-to-shot variation in the 

laser pulse energy is observed, the burn patterns presented in Figures 6 and 7 are 

repeatable. 

Due to the large variation in the laser intensity distribution patterns at different 

stand-off distances, there is no meaningful way to plot ignition delay vs. laser 

intensity. As an alternate presentation, Figures 10 and 11 show plots of TTFL vs. 

laser energy incident to the glass bead. We note that the average TTFL values at 

ignition threshold are similar for the two data sets at similar standoffs: 19-20 ms 

at 0 mm, 27-29 ms at 0.5 mm, and 32 ms at 1.0 mm. The similar ignition delays 

would indicate similar ignition threshold laser intensities. With the tighter focus 

of the 4-n-m bead, it is possible to achieve similar laser intensity using less 

incident energy than would be required for the 6-mm bead and without a 

significant change in ignition delay. 

6. Conclusions 

The objectives of this study were to determine the potential benefit to mediumcaliber laser ignition by incorporating a standoff between the focusing ball and 

the material being ignited. In addition, we also investigated the differences 

between using a 6-mm and 4-mm diameter focusing ball. The experiments were 

limited to black powder pellets. Our conclusions are summarized as follows: 
35. 

30. s 

0 0.5mm Standoff 

0 1 .O-mm Standoff 

G 25. 

z 

.E. 20. 0 E: E 15. Q P 

IOm 

5- 

OV 

0 20 40 60 80 100 120 140 160 180 200 

Laser Pulse Energy (mJ) 

Figure 10. Time-to-first-light vs. estimated laser pulse energy for the ignition of black 

powder using a 6-mm diameter bead lens and a 4.76-mm diameter laser 

beam. 

40- 

35. 

q 0.5-mm Standoff 

30. 0 1 .O-mm Standoff 

j$25- Q 

E 20. 

i;i 4 

E 15 

10 

I 

Q 

5 I o! . 

0 20 40 60 80 100 120 140 160 180 200 

Laser Pulse Energy (mJ) 

Figure 11. Time-to-first-light vs. estimated laser pulse energy for the ignition of black 

powder using a 4-mm diameter bead lens and a 3.Wmm diameter laser 

beam. 

l A laser pulse with a top-hat energy distribution refracting through a glass 

ball will have a nonuniform energy distribution that changes dramatically 

as a function of stand-off distance from the ball. At a short stand-off 

distance, much of the energy is concentrated in a ring at the perimeter of 

the spot. At longer standoffs, the majority of the energy is concentrated in 

a much smaller spot. 

l Ignition threshold energy decreases as standoff is increased in the range of 

oto1.5 nun. 

13 
l Ignition delay at threshold increases as standoff is increased in the range of 

0 to 1.5 mm. 

l With the tighter focus of the 4-mm bead, it is possible to achieve similar 

ignition delays at threshold using less incident energy than would be 

required for the 6-nun bead. 

We also note that the minimum ignition delays for both sets of data using the 

4-mm and 6-nun beads were minimized at O-mm standoff, using a pulse-energy 

substantially greater than the ignition threshold. However, experiments at 

standoff distances greater than 0 mm were carried out only at pulse energies near 

ignition threshold. It is likely that increased pulse energies at nonzero standoffs 

would result in reduced ignition delays. We cannot conclude that the minimum 

ignition delay at 0 mm is a global minimum for all standoffs or that a similar 

ignition delay requiring less energy could not be achieved at a nonzero standoff. 

Recommendations: 

l Due to its tighter focus and lower energy thresholds, the smaller 4-mm 

focusing ball should be implemented. 

l The efficacy of this new configuration should be validated through 

additional firings using a Mann barrel. 

The decrease in energy threshold can result in the implementation of a laser 

system with less volume and mass. A smaller system would positively impact 

engineering efforts to retrofit a laser ignition source to an existing gun system. 

14 
7. References 

Beyer, R. A., and J. M. Hirlinger. “Laser Ignition in a Medium-Caliber Gun: A 

Study of Igniter Influence on Action Time.” ARL-TR-1864, U.S. Army 

Research Laboratory, Aberdeen Proving Ground, MD, January 1999. 

Burke, G. Personal communication. Aurora Optics Inc., Hanover, NH, 

September 2001. 

Cohen, A., and R. A. Beyer. “Laser Ignition of Solid Propellants: I. Ignition 

Delays.” ARL-TR-162, U.S. Army Research Laboratory, Aberdeen Proving 

Ground, MD, July 1993. 

15 
INTENTIONALLY LEE-~ BLANK. 

16 
NO. OF 

COPIES ORGANIZATION 

2 DEFENSE TECHNICAL 

INFORMATION CENTER 

DTIC OCA 

8725 JOHN J KINGMAN RD 

STE 0944 

FL’ BELVOIR VA 22060-6218 

1 HQDA 

DAM0 FDT 

400 ARMY PENTAGON 

WASHINGTON DC 20310-0460 

1 OSD 

OUSD(A&T)/ODDR&E(R) 

DR R J TREW 

3800 DEFENSE PENTAGON 

WASHINGTON DC 20301-3800 

1 COMMANDING GENERAL 

US ARMY MATERIEL CMD 

AMCRDA TF 

5001 EISENHOWER AVE 

ALEXANDRIA VA 22333-0001 

1 INST FOR ADVNCD TCHNLGY 

THE UNIV OF TEXAS AT AUSTIN 

3925 W BRAKER LN STE 400 

AUSTIN TX 78759-5316 

1 US MILITARY ACADEMY 

MATH SC1 CTR EXCELLENCE 

MADN MATH 

THAYER HALL 

WEST POINT NY 109961786 

1 DIRECTOR 

US ARMY RESEARCH LAB 

AMSRL D 

DR D SMITH 

2800 POWDER MILL RD 

ADELPHI MD 20783-1197 

NO. OF 

COPIES ORGANIZATION 

3 DIRECTOR 

US ARMY RESEARCH LAB 

AMSRL CL LL 

2800 POWDER MILL RD 

ADELPHI MD 20783-1197 

3 DIRECTOR 

US ARMY RESEARCH LAB 

AMSRL CI IS T 

2800 POWDER MILL RD 

ADELPHI MD 20783-1197 

ABERDEEN PROVING GROUND 

2 DIR USARL 

AMSRL CI LP (BLDG 305) 

1 DIRECTOR 

US ARMY RESEARCH LAB 

AMSRL CI AI R 

2800 POWDER MILL RD 

ADELPHI MD 20783-1197 

17 
NO. OF 

COPIES ORGANIZATION 

ABERDEEN PROVING GROUND 

31 DIR USARL 

AMSRL WM BD 

W R ANDERSON 

R A BEYER 

A BIRK 

A L BRANT 

SW BUNTE 

C F CHABALOWSKI 

L M CHANG 

T I’ COFFEE 

J COLBURN 

P J CONROY 

R A FIFER 

B E FORCH 

B E HOMAN 

S L HOWARD 

P J KASTE 

A J KOTLAR 

C LEVERITT 

K L MCNESBY 

M MCQUAID 

M S MILLER 

T C MINOR 

A W MIZIOLEK 

J B MORRIS 

J A NEWBERRY 

M J NUSCA 

R A PESCE-RODRIGUEZ 

G I’ REEVES 

B M RICE 

R C SAUSA 

J A VANDERHOFF 

A W WILLIAMS 

18 
NO. OF 

COPIES ORGANIZATION 

1 PROGRAM EXECUTIVE 

OFFICE 

AVIATION BLDG 

5681 WOODS RD 

REDSTONE ARSENAL AL 35898 

1 PM APACHE 

N BRUCHMAN 

REDSTONE ARSENAL AL 35898 

1 COMMANDER 

TACOM ARDEC 

AMSTA AR CCL B 

W BUNTING 

PICATINNY ARSENAL NJ 

07806-5000 

3 COMMANDER 

TACOM ARDEC 

AMSTA AR CCL C 

J HTRLINGER 

PICATINNY ARSENAL NJ 

07806-5000 

1 COMMANDER 

TACOM ARDEC 

AMSTA AR CCL D 

J GUELPH 

PICATINNY ARSENAL NJ 

078065000 

1 COMMANDER 

TACOM ARDEC 

AMSTA AR WEE 

T DORIS JR 

PICATINNY ARSENAL NJ 

07806-5000 

2 COMMANDER 

TACOM ARDEC 

AMSTA AR WET 

T SACHAR 

D TOLLIVER 

PICATINNY ARSENAL NJ 

07806-5000 

NO. OF 

COPIES ORGANIZATION 

1 AURORA OPTICS INC 

PO BOX 493 

G BURKE 

HANOVER NH 03755 

2 CDR USAATC 

STEAC TS PC 

I’ MARSH 

A RAUNIG 

BLDG 363 

AI’G MD 21005-5059 

7 DIR USARL 

AMSRL WM B 

A HORST 

AMSRL WM M 

J PLUMER 

AMSRL WM MC 

R ADLER 

AMSRL WM TB 

I’ BAKER 

R SJSAGGS 

L VANDE KIEFT 

W HILLSTROM 

1 MEGAWATT LASERS 

18 HUNTER RD, SUITE 6 

S HAMLIN 

HILTON HEAD ISLAND SC 

29926 

19 

I REPORT DOCUMENTATION PAGE 1 Form Approved 

OMB No. 0704-0188 I 

. 

I 

PU~IIC reporting burden forths connection of information is estimated to average 1 hour per response. n~ludn~g the time for rewewing instrucf!ons. S%rChtng existing data 50~rC85. 

gafhorfng and mainfaining the data needed. and completing and reviewing the tollcction of information. Send comments regarding this burden estimate or any otheraspeti of this 

collection of Information, including suggestions for reducing this burden. to Washington Headquarter5 Services, Directorate for Information Operations and Repot% 1215 Jeffor~on 

Davis Hiqhway. Suite 1204. Arlinqton. VA 222024302. and to the Oftice of Management and Budqet. Paperwork Reduction Proiect(070401881, Washinufon. DC 20503. 

1. AGENCY USE ONLY (Leave blank) 12. REPORT DATE 13. REPORT TYPE AND DATES COVERED 

May 2002 Final, October 2001 - March 2002 

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS 

A Study of Window Focusing Effects on Laser Ignition for Medium-Caliber 611102.~67 

Systems 

6. AUTHOR(S) 

Jeffrey B. Morris and Richard A. Beyer 

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS 

U.S. Army Research Laboratory 

ATTN: AMSRL-WM-BC 

Aberdeen Proving Ground, MD 2 1005-5066 

8. PERFORMING ORGANIZATION 

REPORT NUMBER 

ARL-TR-2743 

9. SPONSORING/MONITORING AGENCY NAMES(S) AND ADDRESS 

TACOM-ARDEC 

ATTN: AMST-AR-WET 

Picatinny Arsenal, NJ 07806-5000 

10.SPONSORING/MONITORlNG 

AGENCY REPORT NUMBER 

11. SUPPLEMENTARY NOTES 

12a. DISTRIBUTION/AVAILABILITY STATEMENT 

Approved for public release; distribution is unlimited. 

12b. DISTRIBUTION CODE 

13. ABSTRACT(Maximum 200 words) 

One proposed implementation of medium-caliber laser ignition employs a focusing ball to replace the conventional 

electric primer. The effects of ball size and igniter stand-off distance have been characterized experimentally with black 

powder pellets. Ignition threshold and delay experiments were carried out using 4-mm and 6-mm diameter glass beads 

with standoffs up to 1.5 mm. Ray tracing was used to model the energy distribution of a top-hat beam profile refracted 

through a focusing ball. Results of the study indicate that the tighter focus of the 4-mm bead can reduce the required 

laser pulse energy with little effect on the ignition delay. 

14. SUBJECT TERMS 15. NUMBER OF PAGES 

laser ignition, black powder, medium-caliber gun 25 

16. PRICE CODE 

17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACT 

OF REPORT OF THIS PAGE OF ABSTRACT 

UNCLASSIFIED UNCLASSIFIED UNCLASSIFIED UL 

NSN 7540-01-280-5500 

21 

Standard Form 298 (Rev. 2-89) 

Prescribed by ANSI Std. 239-18 298-l 02 
22