RESTRICTED

RI\"'" Pt:-- W:-j" - ~ ''---'"'' r·" .. '. If. I, "', 1I 1.- ,~ . r1 1·'i.. 'f. I.'!I .t':.- '. . =-""'*-." - "" 'I' ,",,)--~-'I ., I I' I _ ,'. I "'l III - " ....idfia..l .~ _'I>~~';~t:::::.a \;;1,,1,:.'1::-:,:, U·' '"; ~

~, ~, t

. -,..~,~~~, .. ,,~~.:~:' -

F.R. Note No. 128/1~54

I

' .......DEPARTE!,i;lTT OF SCL;:rTTI'IC Al\'D nmUS1HIAL llliSEAHCH At\'D FIRE OFFICES I COr,iMITTEE

JOINT nRE R'~Sl',ARCH ORGANIZATION

~~:-;~.::;~ -~::':~~-~~e~ ';~~~~:~:~~n~" --Ishould be considered as confidential advance !information. No reference should be made to ;it in any publication without the written iconsent of the Director, Fire Research "Station, Boreham Wood~ Herts. (Telephone:

! ELStree 1341 and 1797).L !

TIiS DEVELOPl,lCNT OF THE CARBON ARC SOURCEOF IiIGH ll-YTENSITY RADIATION

by

D. I. Lawson, R. W. Pickard and D. L. Simms

,

Surnmar-,i

A carbon arc searchlight has been modified for usc as a source ofhigh intensity radiation. Instruments have been designed to measureand monitor its output: the position of the c.rea of maximum intensitycan be observed and held constant. An electronic switch has been

,

-

'-.

. TI-;E DEVELOPiillJ!'~T OF THE CARBON ARC SOUl1CE01:' HIGH mTENSITI" RADIATION

by

D. I. Lawson, R. W. Pickard and D. 1. Simms

1. Introduction

The' amount of radiation necessary to ignite combustible ilmterir,lsis dependent upon tlle absorptivity of the aurf'ace and ~his "aries withthe wavelength of the radiation usedo In studyL~ the ignition ofmaterials by radiaUon from an atomic exp.losdon, tl:1.e quality of theradiation used in the experiments should be characteristic of such asource, otherwise corrections have to be l113.de which are difficult if notimpossible to apply, particularly as the abs9rPtivity of the surfacesurface varies continuously during charring l1). These co;rrcdions canbe obviated by using a carbon arc source since i ts radiat~on b.Qs aboutthe same spectral quality as that of an ato~uc explosion l2, 3). Inthis paper, the development of such a source is described together\1ith the auxiliary equipment.

2. Appara~

2.1. Povier supply

The power supply for the carbon arc was a standard type Lister 22 1\:8generator set but a more conv~n~ent method has been developea. using athreo phase rectifier system \4).

2.2. Carbon arc

The carbon arc source 'ilas a standard liCD 90 em },iark V searchlightsystem operating at 75 volts and 150 ampere s , burning 16 rom dLamcber-cored carbons. The carbons were fed at appr-oximabeIy their burningrates by a motor, but since these did not remain constant, uniform .feeding by the motor resulted in the arc position Yiundcrmg. For thisreason some tests wer-e carried out on a Mark III lamp. This had a.rotating positive carbon which ensured even burning end the bllrning rateof both carbons were automatically controlled by probes. The feed ~asnot co~tinuous, hoc,iever, but intermittent and it was f~lnd to beunsatisfactory. Further tests using this system sn~~ed th&t the outputradiant intensity varied in phase with the rotat~on of thp- positivecarbon. With the development. of the monitorir'

"- do this by passiT'..g the radiation throughII wire grid and 'vbaervinc«: the change in resistance of the wires tomeasure the radiation flux. This apparatus was found to be too sensitiveto draughts. Finall,Y a split beom tec9\1iq,ue was a.dopted sinHaI' to trotin use at the University of Rochester (6). A sheet of naca 9 in. x9 in. x 10/1000 in. was placed in the path of tIle beam betv/een thesecond mirror and the focus (Fisure 4). Some of the radiation wasreflected to f'orrn an Image of the focus on a sheet of frosted glass.A condensing lens placed behind the frosted glass ~irected a beam ofnearl:," uniform light upon a photocell, the output from which «t:erecorded CcutoElatically. This method had the disadvantage that itreduced the intensitz' of radiation at the focus by about, 25 per cent.

Methods of varyin" the intensity of rdiction.-- -_._-.... -_ ....- -~ ... ...- ....~ .. -._ ..-.- - .~...,.. ..... .. . .....The forru of optical system used did not permit interpesing a

rotating sector disc in the beom to reduce the intensit:' of radiation.Instead stops '1lJre inserted in the 3\'~ttcr novomerrt so tr.at its ::engleor op::nin~; could be reduced (Fi:;;ure' 50.) and also a foramina ted~Cm ',:308 made i:,;~lich could be plG.c~:d in tl1C n!Gi.'1. be-am and 7:hichllllowed only a part of tN> radiation to pass throush (Figure 5b). Inthis way a wide range of intensities of radiation couLd be obtained.

2.8. Distribution of intensity of r~~~ticn

In order to deteIl:!ine .the intensity distrj.outic'l1 near the f'ocasa table Was built which could be adjusted in three; ",uh:allo' per-pendacuLar-directions (Plate 3.

The thermopil"c was fixed in position on the: movable tcble and'adjUlSted until the maxamum output 'las obtrdned, At the same time theoutput from the photocell corresponding to this was recorded. A seriesof readings were made using the stops and screen; the results ar-e shownin Figure 6. As the photocell has an output pr-opor-tLonr.L to theintensity of light falling 011 it the curve of Figure 6 hcs beenextre,polc.ted to the highest reading recorded on it. Thr maximum intensityof radiaticn is about 180 watts em-2 (40 cal em -2 sec.-)~ RepeatedCalibrations have agreed to within 10 per cent, a sufficiently lugh degreeof accuracY. The'position of maximum intensity of radintion'coincideswith the position of the focal spot found by. the method ~f 2.2.

.t

•

-3-

3.2. V~iation of intensity in~ th!l..f29-)_l~):a}l;e

Plate 4 shows the size of the focal spot when a photographic paperis exposed for various periods of time, from ~hich some idea of theuniformity can be obtained. Radiometric measurements of .the variationin intensity with distance across the focal plm,e for m, intensity ofabout 3 watt cm-2 (0'75 cal.cm-2 sec.-1) and .~~8 shown in Fieure (7a, b).The area of uniform r-adi.at i.on, defined as the area over whrch theintensity does not vary by more than 10 per- cene, ic less than 0'1 sq. emand the area over \"';hich the variation does net oxceed 20 per cent r isless than 0'2 sq. em.

This is shm7n plotted in Figurz 8.

3.4. Intensity distribution auay from fnou'l

As Figure 8 showed no pronounced focus exa s'ted , the areas t in.and 1 in. in front of t;'.e focus were explored. The results are shownin Figures 9a, b , 10a, b. Alt:lOugh an increased area of irradiation isobtained the loss in intensity is high (Figure 11a, b).

, I

4. The ignition of materials

Samples of oven dried oak and fibre insulating board were exposedto various intensities of radiation from the arc source. The curves,shown in Figure 12, of intensity plotted as a function of the time toignite faller. tho usuo L p"t'te-rn, the i?;nition tililiJ increasing as ·the· .intensity. is lowered. Below a certain level of intensity ignition doesnot take place ~hatever the time of irradiation. For the specimenstested this intensity level was about 50 Vlatts/cm2 and the corresJ;l0ndingignition times were 2'5 and 1·0 seconds for oak and fibre insulat~ngboard respectively.

These curves could be extended using as e. radiation source atungsten filament lamp and ellipsoidal mirror. This irradiated alarger area of the specimen and so ignition took place at ,\ lonerintensity. In spite of the fact that. the tungsten lamp operates at amuch lower temperature than that of the arc, the tVio sets of resultslie on a smooth curve.

The'experiments were repeated with bLackcned sarrpLcs of oak andfibre insulating board and curves having the same general forrn wereobtained. For ignition times .of less than 1'0 8CC. the cffect ofblackening the oak ~as small.

A curious anomaly occurred with the fit::-" i:lsalding board. Atintensitities above about 70 watts/cm2 the ignition 'cime became longand another curve was obtained. This appeared to be due to a blisterbeing formed over the irradiated area, owing no doubt to the laminarform of fibre insulati.."lg board, which shielded the subcutaneous layersfrom radiation while itself being denuded of volatiles.

5. Conclusions

The carbon I'rc r0.diation source developed is capable c£ gavangvery high radiation i~tensities over a small area. The output can bemonitored adequately and the position of the focus held constant.Specimens can be exposed to radiation for known periods by means of anelectronic switch operating a venetian blind shutter. The intensity ofradir.tion to ..rhach specimens are exposed can be v"I"ied by inser·tingstops in the shutter movement which alters the angle of opening, andby placing perforated screens in the path of the beam.

Tests on the spontaneous ignition of oak and fibrc insulatingboard both blackened and unblackcned have shmvn that this apparatusrates woods in the same order for case of ignition as other methodsusing sources of l~ner temperature. This may howover not be true·forother materials.

"'4-

References

(1 )

(s)

(6)

ERR.

Simms, D. L., Pickard, R. ¥/.. and LaVl. M.. Fac+'crs Inf'Iuencangthe ignition of mnteriols (to be publisheQ).The effects of atQmic'~eepons. McGr~~-lilll &Co., P. 183.Fiat Final Report 1052, The hi;~h current carbon arc. P. 72.Pickard, R. W. A .proposed pav.r~r SL1PP2.y for a ~:igh currentcarbon arc. F.R. No. 114/15'54.,Simms, D. L. and Pdckard, R~ 'i{, ~1'.0rrLj0~·)~Jcl:~ :~\1"..'" J1ien~:n.:,.l'lw..ghigh intensity rud:i.Elr~iona F .R., No., C:/';S:-1L...!)alvis, T. P., Krolak, L. J. and !nGb;.:~3~., 'FI .. h~ ~b,lrl~.es of flashburns: the carbon arc source, Uj::.:l7:;.r~:~·~:"T .):~ ROC:-hJSt·3:':' a'cotai.cenergy project, project report :No" li}:···2?6 ':1 ~)2.LLawson, D. I. and Simms, D. 1.. ~l:..e ig{:i.:i.:t:::'G:'l of' wood. by radia.tion.Brit. J. App; Phys , .3, P:t:.. 228-··252..Carslew, H. S. and Jaeger, J. C. CC~Quctio~ of hc&t in solids.;;:to 57, Section 25, Equation 9, Chap., i.r,Ibid. P. 56, Section 25, Equation 8, Cr~p. II.

i,....1

I

~ .. ".;.

APPEI~1)IX A

The solenoid operating the shutter was energiRed by means of an80 volt D.C. supply independent of the sea:cchlizh'c supply. The timein"i;erval for which t'·,(; shutter WE'.3 held open 'iii:!.::; contrro.l.Led 'by aflip-flop trigger circuit (l!'igure 2).~ The -:;ir.lc i,:~t0:""raJ. -t:~::-YI".)e,n themaking and breaking of the oper-at ing rc:i.8f I;::ii"t.>::.,:l:,S 'i1'1B ·,'b.:ci,::d by meansof the 2 02 1Ul. potentiometer ("3'igure 2) e ::'}~.:.: :.:?·~.:-'t].OJ.) ":",:1:.-, 'I";~l: -the timeinterval and the pcaent.Lcme'ccr scale r:>~:::;'r..:i ~..:..; ::'~;lX!:.! ::."

Since the intensity rises linearlY with time

- ..,J1ft:, -- '"T') J- i /,..- --'" t r. .'" I •l- I ' J, ,\

J'V,. 1

whore I is thE) reaximum intonsity of rr\2.5_~'.t~(,:.'"

Eence

n......,I

. ,If'.- .".

If the radiation is of' constant intensity I from t = 0 until t = t cr

" then

Thus if B= Qthen.-:.tc =4:t1 "9

Hence a linearly increasing intensity of radiation for time t1 reachinga maximum value of I is equivalent to a constant intensity of radiationI for a period ~ t1-

It is unIike~ that ignition of a specimen will take place ~uringthe closing of the shutter and the correction for the time taken forthis has not been calculated.

Using t:le result of Appendix A the time for which the specimen iseffectiveJs' exposed to the maximum intensity of l~die.tion is given by

=,.

where"/ is the time interval of the operating -relay. This resultshows that the time interval of the operating relay may be taken asthe exposure time.

"-

".

Q

DO NOT LOOK AT THE ARC;.r j~ ~Mf\.=t.;.n::"" .... :..a

"

PLATE. I. GENERAL VIEW OF THE APPARATUS

PLAT.E.2. TYPICAL INTENSITY TlME CURVETIME INTER.VAL -I SECOND

\,. \.'

.:

PLATE.3. ADJ.l:JSTABLE TABLE

o

--

..~I

10

EXPOSURE - i sec

EXPOSURE- t sec

EXPO 5 UR E - I sec

PLATE. 4. VARIATION IN SIZE OF FOCAL SPOT

L!S82657/1/114 60 1!/.s.,Cl.

ParabolicRczfltctor.

.r

Standard Signalling Shuttczr.

------H--~--Ca ... bon Arc.~:....:......:..~

LJ

IIIIII ~

'"I)

II1- . ... _0_- __ . .__.

~...../

Scr een."

.:/

\ 0

\

\

/

, I 1 1. '

r-

............."

~----ParabolicRdlczctor.

--FIG.I. OPTICAL SYSTEM EMPLOYED. TOGETHER WITH RADIATION

REDUCING DEViCES.

------------------ - --

r------......~....---.....-----__t~- ....--......------300V

. '22K

EFSO

4K

lOOK

E FSO

5

lOOK

ISOK

56K

5 Operating switch .R Relay operutlnq shuUtrp Potentiornete r to vary limit or operation

,.oJ,"

FIG.2. CIRCUIT DIAGRAM OF TIMING DEVICE

)

_----.--------,----- 0

CD

l.LJU->l.LJ0

L?-e Z-e ~Z -- t-.-.-Ln W U.tI)

0...J4:(

Z-0 0~t-«a::co-...J-cU.

('Y').L?-u..

~--....L....--~--_____f---....L...--___1~o.

;)3S - 3~ 1.1

-~-... -.

Photoc~1I

ULensk------~---

\ ': Frosted-'~- ql c ss- \,

----. \ . --'- \ /

" /- \..' "~-

/~i~;-~71l Specimen. // ...' ----;;/---:

.". /. /---

J

'\,

, jf j

I

! If[] rl' j

1 j. i !r [ I rII '

Ij

.~.\ .\\I.

zwn,o>-..J..J:::>I.L

a:uJt-t-:::>:rV'J

~..J-ct-ex:

IV'J

(f)

C-O....(f)

u,ot--Uwu,u,W

w~a:~ .::>-I..J

2«a::t-'«o

owt/)

a..Ju

eLI)

o-u,

---------------,--------IL -./t , • • I

'60.-------...,.---------T""-----------,.-------,..------~7"'_r"------~

--..l65

Ir----------I

43PHOTOCELL OUTPUT- mv

2

~----- - ---------_.. _-- ---- - -- - ,- - - ---

o O~------~------~--------J.-------,....---...1.-------~------~

20

40-·--

120t---------t-----------+---

140

'"~~

e~I 80r---------;-----------+------:;.,..-:------+------,-+------------- --~----_t

>-I-lJlZ 60wI-Z

(OJ

I

Eu 100-'

FIG.6. INTENSITY OF RADIATION AT THE FOCUS AS A FUNCTION OF PHOTOCELL OUTPUT------~----~---~----------------

II'4

05

III..e

4 :)>0,'

"'"·0"'"...,.-

3.l)

"'"0,2 ~... -

(/)

ZUJt-Z

·35I

·30I

·25I

·20I

·15I

·10I

·05I

·05I

·10 ·15 ·20 ·25 '30 ·35

01 STANCE FROM FOCAL POI NT - in..".,.",

FIG.7a. INTENSITY DISTRIBUTION IN FOCAL. PLANE. ( HORIZONTAL)-c::A

._ _-------------

0P'l·()

IIIN

0~

0 WN ....· (/)0 >-

(./)

Vt u;_.· 00

c- (/)0 -- )(· ..... 4:0 z-

0· d)0 \aJ -U a:

Z t-oe('Q ~ (./)C/) -· 00 -0

>-III ....-· -0 (/)

ZW

0 t-N Z·0 .

a)

lJ"l.

C"\l o· -0 u,

0P'l·0IIIIf')

·0

I I I I I0'35 0·30 0·25 0·20 0-15

I J0'10 0·05

I I I I I I I0·05 0-10 0'15 0-20 0·25 0-30 0·35

,)

FfG.9b. INTENSITY. DISTRIBUTION

DISTANCE FROM CENTRAL AXIS-InI .,

IN PLANE ""2 IN FRONT OF FOCAL PLANE (VERTICAL)

0·40

0-36

II'l...C

0·32:;,

>.~

DL.-

0·28 .D~D

I>-

0-24 t--'"ZuJt-

0·20z

0·35 0·30 0-25 0-20 0-15 0-10 0-05 a 0·05 0-10 0-15 0-20 0-25 0-30 0-35DISTANCE FROM CENTRAL AXIS-In

FIG.9o. INTENSITY DISTRIBUTION IN PLANE f' IN FR.ONT OF FOCAL PLANE (t-fORrZONTAL)

0~5 0·4

:l.-c:~

>..L.

0'16. E~.-.DLC

Io ·14-1 ~

0-10

0·08

0·3 0·2 0·1 0 0·1 0·2 0-3

DISTANCE FROM CENTRAL AXIS - in

0·4 o-s

FIG.IOb. INTENSITY DISTRIBUTION IN PLANE I" IN FR.ONT OF FOCAL PLANE (VERTICAL)~ ~,~ ~p

"o

e 0·15

'J

0·35I I

0·30 0 ·25

0·1 '". ..c~

>-L

0·1 c~...0L

e

0·1 I~tJ')

Z0·0 W·

I-Z-

I I I I l I I0-20 0·15 0-10 a-os a 0·05 0·10

DISTANCE FROM CENTRAL AXIS - in

0·15 0-25 0·30 0·35

.',FIG. 100. rNTENSITY DISTRIBUTION IN PLANE r" IN FR.ONT OF FOCAL PLANE (HORIZONTAL)

,-----_._~ -~---~

I

- -- --- -~,--.- - ---- ------i

, 0I

I

-------------..,- ----------~1 ,

4f-----.,---- ------- - -- ----------- ----- -.------~.

26.------.-------,---------,..-----r-----,

241-------+---

22

201--------->--------

16 ---- --- ------

8

18

6-

l~ 12 ---- - - ------- ---, -enzUJt-Z 10---

I2 14 16

"FIG .110. INTENSITY IN PLANE t' IN FRONT OF

FOCAL PLANE

III

. I

I,I

III

II--,IIl!~I

I

1

II , I I--r--------T-I 1II

IIII

II

12 14 16

'I

I

8

I.j.iI

---------

,.

I6

---------J--------t.----------I4II

I

II4

3

1.2 ------f- ---·-1---------II

4 -" . 1- ---.. _ ....·-~·_------t------··---+----4. I '

II

12 --' .. -'--'--.- -.- ---

13,.-------------r--------,-------~------__;____,

10 ------.--t- -- .

9

N

IEucI'I e ... n~____•.......0~

7-

>-~

V)

z 6LLl~

Z-5

PHOTOCELL OUTPUT - mv

FIG.llb. INTENSITY IN PLANE I" IN FRON,T OFFOCAL PLAN E

502010

• •---- .

Maximum i'nl~nsity_ --.Lfrom_lamp sourc~_

•

5

•

32

---

0·50·2I0 L--- ..1...._ --l. .l.- ...L......._--I..._-U:~ ..1....__~¥_.....J..... ___l

0-1

50 t-------t------4----u

30 t-------+--'----'--~

200~-----r------.-----r------,----..-----r------r------r----------.

LLo>t-tl)

Zwt-Z

zot-oe:(-o-ca:

In..,c; I00 1-------f-------'='~--_4'~

TIME TO IGNITE -s~c

LAMP SOURCE

o Oako Oak (blackenetd)

CARBON ARC· SOURCE,

6 Fibr~board

F· b b d r I I I d'• i re 00 r l DIOCI((Z.nrt Iv Oakx Oak (black~ned)

+ Fibr~boardI\. Fi b ret boa rd

I ••• . ,tor ocxe nee /

FIG. 12. SPONTANEOUS IGNITION OF OAK AND FIBREBOA.RD