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A High-gain Time-resolving Spectrograph for Diagnostics of Laboratory Simulated Re-entry Objects
A High-gain Time-resolving Spectrograph for Diagnostics of Laboratory Simulated Re-entry Objects I. D. LIU and J . R.BASKETT A . C . Electronics, Defense ResearclA Laboratories, General Motors Corporation, Santa Barbara, California, U . S . A .
INTRODWTION I n an earlier communication1 the time-resolved spectra of some hyper-velocity laboratory simulated re-entry models launched in a free-flight ballistic range have been presented. They were obtained with a large-aperture slitless spectrograph2 in conjunction with a n image converter camera. The sensitivity of this instrument using only a single-stage image converter was not sufficient either to record the spectra of the less intense far-wake region of the now, or t o resolve the molecular band systems in order that flow field parameters such as temperature and species concentration could be deduced. Increased sensitivity was made possible through the use of a recently available high-gain image intensifier of high quality. I n the initial stage of the development the output of the image converter was directly coupled t o an image intensifier by a pair of transfer lenses as described by Liu However, for reasons t o be discussed later, the overall degradation of image quality through the system was considerable so that high spectral resolution could be achieved only by using large spectral dispersion. Accordingly, the small Bass-Kessler spectrograph used previously was replaced by a larger Jarrell-Ash 0.7-m, f/6.3 Czerny-Turner type instrument. This paper describes the application of a magnetically focused image intensifier for spectroscopic data acquisition.? Temporal resolution is sufficient for the observation of transient low-intensity sources such as those encountered in hypersonic wakes. I n particular, schemes for intensity calibration and data reduction will be discussed. Examples of wake-temperature determination from the unresolved vibrational bands of C" will be given. t This work mas supported by the Atlvaiicorl Research Projects Agency as a, part of Project D e f e d e r monitored hy the U.S. Army Missile Command. I021
1022
1. D. LJLJ A N D .J. R. RASKETT
DESCRIPTION OF THE SYSTEM The intensifier chosen for this work is a 4-stage, cascade, magnetically focused tube EM1 Type 9694, described by Randall.3 This tube has sufficient gain to record a single photoelectron and its 5-cm-diameter cathode makes it convenient for use in conjunction with an optical system at 1 : 1 magnification to transfer the 5-cm output field of the RCA image converter (a TRW streak tube) to the EM1 image intensifier. The spatial resolution of the RCA image converter tube in its dynamic mode is 18lp/mm, comparable to that of the E M 1 intensifier tube. However, when the two are coupled together in
EM1 image
J-A 0.5-m
IK W
image
mnllortp.
PIC:.1. Schomatic diagram of time-resolving spectrograph system,
series the resultant resolution is much degraded by the transfer optics, the 8-distortion and, most of all, by the influence of stray magnetic fields on the irnage converter. There may also be slight signal-induced emission and contrast reduction in the intensifier. These effects reduce the overall resolution to 5 lp/mm. Improved spectral resolution can be achieved only through increased dispersion of the spectrograph and increased temporal resolution only by faster deflexion of the photoelectrons. The system is presented schematically in Fig. 1, and a photograph of the spectral recording section including the spectrograph, the image converter and the intensifier is shown in Fig. 2. The model trajectory is imaged on t o the entrance slit of the Jarrel-Ash instrument by a
HIGH-QATN TIME-RESOLVING SPECTROGRAPH
1023
simple quartz condensing lens placed at such a distmice frorn the trajectory t h a t t h e image of the model is demagnified approxiniate1~25 times while the collimator of the spect~rographis still tilled with light. This large degree of deniagnification scrves two 1)iirposes. 1 , it gives ;I sufficient depth of field t o accommodate any change in witkc trajectory from one experiment, t o another, mid 2 , it reduces t h e image of the luminous wake t o R size which is sriiall enough to yicltl the dcsircd tem1mrd resolution. Brcausc of chroniatic aberrntion it rn:iy
be necessary t o change the lens position when widely separated spectrnl regions are studied. The spectrum at the focal plane of the sp grtLph is then focuscd on t o the photocathode of the image converter. The pliotoelectrons which are emitted may be deflected across t tic phosphor screen of the image converter i i i tinirs ranging fro111 PO t o 200 p e c . The photocathode of the iriisge converter is curved witli a radius of curvature of 10 cm and the spatiitl resolution of thc tube decreases from the center to the edge of the output, pliosphor. Thus the spectrunr, which hns a flat field in the small spectral region t o be covered is sharply focused at some pointt between the cent CY and the
1024
I. D . LIU AND J. R. BASKETT
edge giving a uniform result overall, but with slightly reduced spatial resolution. The image of the time-resolved spectrum from the image converter is then relayed to the image intensifier by two Carl Meyer f / 2 transfer lenses arranged in 1 : 1 conjugate. Because of the limited space in which the instrument is located it is necessary to bend the light path between the two image tubes. This is done with a flat mirror placed between the two transfer lenses. The intensified output image is photographed either on Kodak Royal-X Pan film or on LKodak 2485 high contrast recording film with an Elgeetfll.2 objective. The films are normally developed in D-19, or in Kodak developer MX642-1 if higher y and increased sensitivity are desired.
PERPORMANCE OF THE SYSTEM A s mentioned above, when a slit is used, spectral resolution is limited by the overall spatial resolution of the system, and can be increased by increased dispersion in the spectrograph. Similarly, increased time resolution may be obtained by increasing the scanning rate of the TABLEI Performance of the time resolving spectrograph using a Jarrell-Ash f/6*3instrument and image converter-intensifier recording system ~~
Sensitivity with intensifier operated at 30 kV:
6x
W/A-'~terad-'cm-~ a t 4300 A
Spectral resolution with 200 pm slit:
Grating (lp/mm) Band width (A) Resolution (A)
Time resolution:
Recording time (psec) Resolution (psec)
2160 130 3.0
1200 350
200 5
8
100 2.5
600 700 16 20 0.5
Spatial field of view with 200-pm slit: 5 ~ n mx entire wake width
image converter. It has also been noticed that the Jarrell-Ash instrument has residual astigmatism amounting t o about 1 mm, which must be taken into account in estimating time resolution. The performance of this system is summarized in Table I. I n all, it yields 45 spectral resolution elements and 40 time resolution elements. Allowing for the estimated 30 intensity resolution elements, the system is therefore capable of recording about 5 x lo4 data bits.
Intensity Calibration As shown in Fig. 1, before light enters the f l 6 . 3 spectrograph, 5 % of it is reflected from a clear quartz flat and relayed into a Jarrell-Ash
Time ( p s e c )
(bi 1'100 -
Experimental points
//
/
/
i
/
1 Extrapolated values
I
Pro. 3. Intcwnity culibretion of lioduk 248.5 film with 1% xenon flash light: (a) phot,omultiplier olitpiit,, (t)) tlriisitometer tracx of t h e xe11011 qm:trum i - ~ i i ~( lc ) tho rcsulting Hurtc+r-lhiffi(~ltlciirw.
I'.E.I.D.- I J
31;
1026
I.
I). LIU ANY
J. R . BASKETT
0.5-m scanning spectrometer with photomultiplier read-out. The system is used first to determine the spectral distribution of the light emitted by a xenon flash lamp whose intensity is calibrated against a tungsten ribbon standard lamp. The xenon flash lamp is used because its decay characteristics closely resemble the luminosity decay of the wake behind a hypersonic object. The lamp? intensity is controlled by the supply voltage and it series of neutral density filters. The decay time of the discharge is similar t o the scanning time of the image converter selected for the experiment. After the experimental conditions (e.g. required radiant power gain, spectral range, spectral and time resolutions) have been selected, but prior to the actual experiment, the time-resolved spectrum of the xenon flash is photographed on the film which will be used later to record the event spectrum. At the same time the photomultiplier output from the calibration scanning spectrometer is recorded on an oscilloscope displaying the intensity of the flash versus the time for a selected element of the spectra range. After the film has been processed in accordance with the manufacturer’s recommended procedures, a microdensitometer is used t o determine the density of the xenon spectrum as a function of time. The Hurter-Driffield (H-D) characteristic curve of the film is then obtained. Figure 3 shows (a) a typical oscillographic record of the photomultiplier output through the scanning spectrometer together with (b) the densitometer trace of the xenon spectrum along the time axis and (c) the resulting H-D curve, It may be noted that the experimental H-D curve yields a y of 0 . 7 for the Kodak 2485 high-speed recording film used, while the manufacturer’s specification under the same processing procedures gives a y of 2.7, demonstrating the wellknown phenomenon of image contrast reduction when using multistage image tubes.4 Because of this, and the possibility of signalinduced emission of the image tubcs, it is desirable to use for calibration a source having a dynamic range and decay characteristics very similar to those of the event, as is the case with the xenon flash light. It may also be noted in Pig. 3, that commercially available xenon flash lamps cannot produce a dynamic range of light intensity of more than two orders of magnitude in a single flash. Therefore, in order to obtain a complete experimental H-D curve, it is necessary to photograph two spectra of the xenon flash at considerably different peak intensities. The intensity of the event spectra as a function of time, or as a function of distance measured in body diameters, is determined by a comparison of the densitomet,er traces of the event spectra with the experimental H-D curve.
t
Edgerton, Gerrrieshausen and Grkr type l3X-42A-3.
HICH-(+AIN TIME-RESOLVING SPEC'TRO(4RAPH
1027
Some Operntionnl Prarnutions When the image intensifier wils operated in a humid atmosphere, there was considerable high voltitge breitkdown or corona discharge, resulting in background radiation. 'I'liis difficultj. was eventually removed by eiiclosing the image converter and intensifier in a reasonably air-tight box and flushing it continuously with dry air during the experiment. The magnet is cooled by circulating rcfrigeratccl water a t about 20°C with a flow rate of 2 I/min. The noise lcvel of the intensifier is such t ha t the photographic film should not he exposed to i t for more than a few seconds. An electric sliu r W ~ L Sinstalled between the output screen of the intensifier and thcl tilm The shutter is actuated by an electric irnpulse just before the model is launched, and closed by another signal just after the modcl ~mssesthrough the field of view of the system. The exposure time of the film is then of the order of 1 sec. One other important precaution deals with the unexpectedly high radiant flux which may be produced either k)y inadvertently ext,ensive ablation of the model under certain cwnclitions, or by incorrect launching procedures. To protect the imagc tubes a peak detector which can be set at any desired light level is c.oiinec+tedto the image converter trigger circuit. The operation of the image c.onvertt.r is then aborted before the image intensifier is cxl)osed whcnever the radiant flux exceeds a certain predet,ermined level. R ICSC LTS A brief description of the free-flight hnllistics range facilities and of the configurations of the model under study lias been presented in an earlier conimunication.l The system described in this p i ~ p wW ~ L Snscd to study the ablating models under higher resolution so as t o permit, the determination of vibrational-band or atomic-line intensity distributions. The temperature and electron density profile of the wake flow niay be deduced from these measurements. For. reasons given tthove the resolution of the present instrument is 2 to 3 Lkw ~ h i c ~ish still inadequate for obtaining the rotational temperature of most molecules. the exceptions being those containing very light elements. This is because the rotational spacing of the molecule is inversely proportional t o its moment of inertia. A t,echnique of vibrational teniperatiire measurement based on the unresolved vibration-rotational band system hits, however, been proposed by Spier and Sniit-l\lliessen,5so th at meaningful determination of the temperature of a gas flow niay be obtained by a study of the EXPERrh.1 ENTAI,
1028
I. D. LIU AND J. R. BASKETT
partially-resolved vibrational-band profile of certain diatomic molecules. By this means the time-resolved spectra of CN and C, produced by ablation into the wake flow from 15-mm-diameter spheres having epoxy outer coatings have been obtained. Figure 4 is a low-resolution, time-integrated spectrum taken with the shutter open for the complete event, showing the very intense radiation from the shock-heated gas cap and the ablation products of CN, C,, and other molecules in the wake flow. The time-resolved spectral regions containing various CN band systems are shown under higher dispersion in Figs. 5 and 6. The scanning speed of the image converter
FIG.4. Time-integrated spectrum of an ablating sphere containing epoxy coating (pressure in the range 50 torr of N,, velority of sphere: 6.25 km/sec).
limits the length of the wake which can be observed to about 75 body diameters behind the molecule, though regions slightly further down the wake may be recorded by introducing a finite delay into the triggering circuit. I n Fig. 5, the A V = 0 sequence of the A 2 Z - XzZ band system of CN at 3883 is shown alongside the xenon flash calibration spectrum from which the H-D curve shown in Fig. 3 was obtained. The vibrational temperatures of CN along the wake flow were deduced from the A V = 0 and A V = -1 sequences by comparing the bandhead intensity ratios of the experimental spectra t o those of the synthetic spectra which were generated from a theoretical consideration of the spectral resolution of the system. The results are shown in Fig. 7. The different vibrational temperatures deduced from the two
HIGH-GAIN TIME-RESOLVING SPECTROGRAPH
1029
FIG.5. Time-resolved spectrum of the A V = 0 sequenco of the violet system of CN (prossuro in the range 50 torr of N,, velocity of sphere: 6.10 kmlsec).
FIG.6. Time-resolved spectrum of the A V - 1 sequence of tho violet system of CN (pressure in the range 50 torr of N,, velocity of sphere: 6.28 km/sec). :
1030
I. D. I J I J AND J. R. BASKETT
7000
I
I I I r I Vibrational temperature of the near wake from the band height ratios of CN
I
I
2c'c
I
6000-
e!
e
t
.O e
5000-
0 0
4000-
e
% e
4
3000-
I 4
2000 -
1
I000 0
/
Appleton and Yang (1967) calculated for pD.375 i n a i r and v=6.2krn/sec I
10
I 20
I 30
I 40
I
50
I
60
I 70
1
80
Body diameier (crn)
FIa. 7. Vibrational temperature of hypersonic wake determined from time-resolved spectra of CN and the calculated values.E
FIG.8. Time-resolved spectrum for a titanium ablating sphere (pressure in the range 50 torr, velocity of sphere: 6.28 km/sec).
HIGH-GAIN TII\ZE-RESOLVINC Sl’EC‘TRO(4KAPH
d V sequences are attributed t o grrlater srlf-nbsorptioii of the A V
1031 =
0
sequence. For the other niolecdrs, R similar technique may be employed t o deduce the vibrational teinl)eratures of the g,zs flow, provided t ha t the assumptions rnadc in applying the technique are fulfilled. The time-resolved spectra for some iitomic lines were also obtained by impregnating the epoxj’ eoatiiigs of tlw model meiitionrd a1)ovc. with api)roprittte salts. A typical wsult is resented in Fig. 8. ‘I’tic electronic excitation trml)eratures of the neutral ;tnd ionized spcciw may be determined from tlic intensity distribution of the lines. If’ thermal equilibrium is ntt;iintd, t hc c~lt~ctron deiisity of the flow may also be deducied from a cornparison of the neutIrd and the ion line intensities. Detailed results will be presented in R s e p r a t e report. REP E R E N C HS
1 . Liii, I. I)., .I. A p p l . O p t ~ c s6, 1195 (1967). 2. rAlli, r. D., .I. A ~ ~ optics U I . 1, (i75 ( i ~ i q . 3. Randall, R. P., In “Atlvitnec.b i n Elt~*tioriicsuntl Elrct r o n Physics”, ocl. J. D. Mc(:rc, D. McMnllm and E. K t i h m 1 , Vol 22A, p. 8 7 . Acrtdt~~nic Pie London (1966). 4. Livingston, W. C., I n “A~lvuncch i n Elrctronics ant1 IiX.ctron Physics”, ed. by L. Murton, Vol. 23, 11. 490. Aciitlt.niic Picw, Neir York (1907). 5 . Spicr, *J.L. arid Smit-Miessen, M. M., P / / ~ S Z 9, C L422 L (1942). 6. Appleton, .J. .mtl Y‘itng, C. C., T’rivatt~ COiiiiiiiiriieiition ( 1 908). IhSCITSSJON
You iisrtl Iiodak 2485 filni, u t i \ ththitb it 1 t t i - g ~yirltl cwinpnrrtl n i t h other filmh such Kotlltk 2475 filin! I. r). LILT: The only film othor than Kotlitl, 2485 which has twen used is Kotlulc tlr\xclopetl in 1)-19 for 15 rnm itt, Royal-X Pm Rrcording FiLrn. Whtm both 2O”C, the sensitivity of Ttoyd-X Pan filni appmrs to be coniptirat)lt~ w i t h that of‘ Kodali 2485 film. Howevw, tintic+r cxttmdcd tk\doping t h c Itoyitl-x Pun has niuch greater hiwkgroitnd fog, which srvc~c~ly it.diiccs t h r contrast. W11c.n Kodtik 2485 is developed in Kocliik Drvc4opcr MX 642-1 for 4 min at 32”C, its scwsitivity is thwc to four t i t m i g i w t r r t h a n Roy.tl-X Pan tlcvrloped in D- 19. ,Tudging from the rnnnlificturcr’s spfTificat ion\, thc sensitivity anti contrnit of Kodak 2475 film 5hoiiltl tw some\\htit t)thtv o t m t h o w of’Kodiik 2485 kind Royul-X Pan filins R . GIESE:
.
INTRODWTION I n an earlier communication1 the time-resolved spectra of some hyper-velocity laboratory simulated re-entry models launched in a free-flight ballistic range have been presented. They were obtained with a large-aperture slitless spectrograph2 in conjunction with a n image converter camera. The sensitivity of this instrument using only a single-stage image converter was not sufficient either to record the spectra of the less intense far-wake region of the now, or t o resolve the molecular band systems in order that flow field parameters such as temperature and species concentration could be deduced. Increased sensitivity was made possible through the use of a recently available high-gain image intensifier of high quality. I n the initial stage of the development the output of the image converter was directly coupled t o an image intensifier by a pair of transfer lenses as described by Liu However, for reasons t o be discussed later, the overall degradation of image quality through the system was considerable so that high spectral resolution could be achieved only by using large spectral dispersion. Accordingly, the small Bass-Kessler spectrograph used previously was replaced by a larger Jarrell-Ash 0.7-m, f/6.3 Czerny-Turner type instrument. This paper describes the application of a magnetically focused image intensifier for spectroscopic data acquisition.? Temporal resolution is sufficient for the observation of transient low-intensity sources such as those encountered in hypersonic wakes. I n particular, schemes for intensity calibration and data reduction will be discussed. Examples of wake-temperature determination from the unresolved vibrational bands of C" will be given. t This work mas supported by the Atlvaiicorl Research Projects Agency as a, part of Project D e f e d e r monitored hy the U.S. Army Missile Command. I021
1022
1. D. LJLJ A N D .J. R. RASKETT
DESCRIPTION OF THE SYSTEM The intensifier chosen for this work is a 4-stage, cascade, magnetically focused tube EM1 Type 9694, described by Randall.3 This tube has sufficient gain to record a single photoelectron and its 5-cm-diameter cathode makes it convenient for use in conjunction with an optical system at 1 : 1 magnification to transfer the 5-cm output field of the RCA image converter (a TRW streak tube) to the EM1 image intensifier. The spatial resolution of the RCA image converter tube in its dynamic mode is 18lp/mm, comparable to that of the E M 1 intensifier tube. However, when the two are coupled together in
EM1 image
J-A 0.5-m
IK W
image
mnllortp.
PIC:.1. Schomatic diagram of time-resolving spectrograph system,
series the resultant resolution is much degraded by the transfer optics, the 8-distortion and, most of all, by the influence of stray magnetic fields on the irnage converter. There may also be slight signal-induced emission and contrast reduction in the intensifier. These effects reduce the overall resolution to 5 lp/mm. Improved spectral resolution can be achieved only through increased dispersion of the spectrograph and increased temporal resolution only by faster deflexion of the photoelectrons. The system is presented schematically in Fig. 1, and a photograph of the spectral recording section including the spectrograph, the image converter and the intensifier is shown in Fig. 2. The model trajectory is imaged on t o the entrance slit of the Jarrel-Ash instrument by a
HIGH-QATN TIME-RESOLVING SPECTROGRAPH
1023
simple quartz condensing lens placed at such a distmice frorn the trajectory t h a t t h e image of the model is demagnified approxiniate1~25 times while the collimator of the spect~rographis still tilled with light. This large degree of deniagnification scrves two 1)iirposes. 1 , it gives ;I sufficient depth of field t o accommodate any change in witkc trajectory from one experiment, t o another, mid 2 , it reduces t h e image of the luminous wake t o R size which is sriiall enough to yicltl the dcsircd tem1mrd resolution. Brcausc of chroniatic aberrntion it rn:iy
be necessary t o change the lens position when widely separated spectrnl regions are studied. The spectrum at the focal plane of the sp grtLph is then focuscd on t o the photocathode of the image converter. The pliotoelectrons which are emitted may be deflected across t tic phosphor screen of the image converter i i i tinirs ranging fro111 PO t o 200 p e c . The photocathode of the iriisge converter is curved witli a radius of curvature of 10 cm and the spatiitl resolution of thc tube decreases from the center to the edge of the output, pliosphor. Thus the spectrunr, which hns a flat field in the small spectral region t o be covered is sharply focused at some pointt between the cent CY and the
1024
I. D . LIU AND J. R. BASKETT
edge giving a uniform result overall, but with slightly reduced spatial resolution. The image of the time-resolved spectrum from the image converter is then relayed to the image intensifier by two Carl Meyer f / 2 transfer lenses arranged in 1 : 1 conjugate. Because of the limited space in which the instrument is located it is necessary to bend the light path between the two image tubes. This is done with a flat mirror placed between the two transfer lenses. The intensified output image is photographed either on Kodak Royal-X Pan film or on LKodak 2485 high contrast recording film with an Elgeetfll.2 objective. The films are normally developed in D-19, or in Kodak developer MX642-1 if higher y and increased sensitivity are desired.
PERPORMANCE OF THE SYSTEM A s mentioned above, when a slit is used, spectral resolution is limited by the overall spatial resolution of the system, and can be increased by increased dispersion in the spectrograph. Similarly, increased time resolution may be obtained by increasing the scanning rate of the TABLEI Performance of the time resolving spectrograph using a Jarrell-Ash f/6*3instrument and image converter-intensifier recording system ~~
Sensitivity with intensifier operated at 30 kV:
6x
W/A-'~terad-'cm-~ a t 4300 A
Spectral resolution with 200 pm slit:
Grating (lp/mm) Band width (A) Resolution (A)
Time resolution:
Recording time (psec) Resolution (psec)
2160 130 3.0
1200 350
200 5
8
100 2.5
600 700 16 20 0.5
Spatial field of view with 200-pm slit: 5 ~ n mx entire wake width
image converter. It has also been noticed that the Jarrell-Ash instrument has residual astigmatism amounting t o about 1 mm, which must be taken into account in estimating time resolution. The performance of this system is summarized in Table I. I n all, it yields 45 spectral resolution elements and 40 time resolution elements. Allowing for the estimated 30 intensity resolution elements, the system is therefore capable of recording about 5 x lo4 data bits.
Intensity Calibration As shown in Fig. 1, before light enters the f l 6 . 3 spectrograph, 5 % of it is reflected from a clear quartz flat and relayed into a Jarrell-Ash
Time ( p s e c )
(bi 1'100 -
Experimental points
//
/
/
i
/
1 Extrapolated values
I
Pro. 3. Intcwnity culibretion of lioduk 248.5 film with 1% xenon flash light: (a) phot,omultiplier olitpiit,, (t)) tlriisitometer tracx of t h e xe11011 qm:trum i - ~ i i ~( lc ) tho rcsulting Hurtc+r-lhiffi(~ltlciirw.
I'.E.I.D.- I J
31;
1026
I.
I). LIU ANY
J. R . BASKETT
0.5-m scanning spectrometer with photomultiplier read-out. The system is used first to determine the spectral distribution of the light emitted by a xenon flash lamp whose intensity is calibrated against a tungsten ribbon standard lamp. The xenon flash lamp is used because its decay characteristics closely resemble the luminosity decay of the wake behind a hypersonic object. The lamp? intensity is controlled by the supply voltage and it series of neutral density filters. The decay time of the discharge is similar t o the scanning time of the image converter selected for the experiment. After the experimental conditions (e.g. required radiant power gain, spectral range, spectral and time resolutions) have been selected, but prior to the actual experiment, the time-resolved spectrum of the xenon flash is photographed on the film which will be used later to record the event spectrum. At the same time the photomultiplier output from the calibration scanning spectrometer is recorded on an oscilloscope displaying the intensity of the flash versus the time for a selected element of the spectra range. After the film has been processed in accordance with the manufacturer’s recommended procedures, a microdensitometer is used t o determine the density of the xenon spectrum as a function of time. The Hurter-Driffield (H-D) characteristic curve of the film is then obtained. Figure 3 shows (a) a typical oscillographic record of the photomultiplier output through the scanning spectrometer together with (b) the densitometer trace of the xenon spectrum along the time axis and (c) the resulting H-D curve, It may be noted that the experimental H-D curve yields a y of 0 . 7 for the Kodak 2485 high-speed recording film used, while the manufacturer’s specification under the same processing procedures gives a y of 2.7, demonstrating the wellknown phenomenon of image contrast reduction when using multistage image tubes.4 Because of this, and the possibility of signalinduced emission of the image tubcs, it is desirable to use for calibration a source having a dynamic range and decay characteristics very similar to those of the event, as is the case with the xenon flash light. It may also be noted in Pig. 3, that commercially available xenon flash lamps cannot produce a dynamic range of light intensity of more than two orders of magnitude in a single flash. Therefore, in order to obtain a complete experimental H-D curve, it is necessary to photograph two spectra of the xenon flash at considerably different peak intensities. The intensity of the event spectra as a function of time, or as a function of distance measured in body diameters, is determined by a comparison of the densitomet,er traces of the event spectra with the experimental H-D curve.
t
Edgerton, Gerrrieshausen and Grkr type l3X-42A-3.
HICH-(+AIN TIME-RESOLVING SPEC'TRO(4RAPH
1027
Some Operntionnl Prarnutions When the image intensifier wils operated in a humid atmosphere, there was considerable high voltitge breitkdown or corona discharge, resulting in background radiation. 'I'liis difficultj. was eventually removed by eiiclosing the image converter and intensifier in a reasonably air-tight box and flushing it continuously with dry air during the experiment. The magnet is cooled by circulating rcfrigeratccl water a t about 20°C with a flow rate of 2 I/min. The noise lcvel of the intensifier is such t ha t the photographic film should not he exposed to i t for more than a few seconds. An electric sliu r W ~ L Sinstalled between the output screen of the intensifier and thcl tilm The shutter is actuated by an electric irnpulse just before the model is launched, and closed by another signal just after the modcl ~mssesthrough the field of view of the system. The exposure time of the film is then of the order of 1 sec. One other important precaution deals with the unexpectedly high radiant flux which may be produced either k)y inadvertently ext,ensive ablation of the model under certain cwnclitions, or by incorrect launching procedures. To protect the imagc tubes a peak detector which can be set at any desired light level is c.oiinec+tedto the image converter trigger circuit. The operation of the image c.onvertt.r is then aborted before the image intensifier is cxl)osed whcnever the radiant flux exceeds a certain predet,ermined level. R ICSC LTS A brief description of the free-flight hnllistics range facilities and of the configurations of the model under study lias been presented in an earlier conimunication.l The system described in this p i ~ p wW ~ L Snscd to study the ablating models under higher resolution so as t o permit, the determination of vibrational-band or atomic-line intensity distributions. The temperature and electron density profile of the wake flow niay be deduced from these measurements. For. reasons given tthove the resolution of the present instrument is 2 to 3 Lkw ~ h i c ~ish still inadequate for obtaining the rotational temperature of most molecules. the exceptions being those containing very light elements. This is because the rotational spacing of the molecule is inversely proportional t o its moment of inertia. A t,echnique of vibrational teniperatiire measurement based on the unresolved vibration-rotational band system hits, however, been proposed by Spier and Sniit-l\lliessen,5so th at meaningful determination of the temperature of a gas flow niay be obtained by a study of the EXPERrh.1 ENTAI,
1028
I. D. LIU AND J. R. BASKETT
partially-resolved vibrational-band profile of certain diatomic molecules. By this means the time-resolved spectra of CN and C, produced by ablation into the wake flow from 15-mm-diameter spheres having epoxy outer coatings have been obtained. Figure 4 is a low-resolution, time-integrated spectrum taken with the shutter open for the complete event, showing the very intense radiation from the shock-heated gas cap and the ablation products of CN, C,, and other molecules in the wake flow. The time-resolved spectral regions containing various CN band systems are shown under higher dispersion in Figs. 5 and 6. The scanning speed of the image converter
FIG.4. Time-integrated spectrum of an ablating sphere containing epoxy coating (pressure in the range 50 torr of N,, velority of sphere: 6.25 km/sec).
limits the length of the wake which can be observed to about 75 body diameters behind the molecule, though regions slightly further down the wake may be recorded by introducing a finite delay into the triggering circuit. I n Fig. 5, the A V = 0 sequence of the A 2 Z - XzZ band system of CN at 3883 is shown alongside the xenon flash calibration spectrum from which the H-D curve shown in Fig. 3 was obtained. The vibrational temperatures of CN along the wake flow were deduced from the A V = 0 and A V = -1 sequences by comparing the bandhead intensity ratios of the experimental spectra t o those of the synthetic spectra which were generated from a theoretical consideration of the spectral resolution of the system. The results are shown in Fig. 7. The different vibrational temperatures deduced from the two
HIGH-GAIN TIME-RESOLVING SPECTROGRAPH
1029
FIG.5. Time-resolved spectrum of the A V = 0 sequenco of the violet system of CN (prossuro in the range 50 torr of N,, velocity of sphere: 6.10 kmlsec).
FIG.6. Time-resolved spectrum of the A V - 1 sequence of tho violet system of CN (pressure in the range 50 torr of N,, velocity of sphere: 6.28 km/sec). :
1030
I. D. I J I J AND J. R. BASKETT
7000
I
I I I r I Vibrational temperature of the near wake from the band height ratios of CN
I
I
2c'c
I
6000-
e!
e
t
.O e
5000-
0 0
4000-
e
% e
4
3000-
I 4
2000 -
1
I000 0
/
Appleton and Yang (1967) calculated for pD.375 i n a i r and v=6.2krn/sec I
10
I 20
I 30
I 40
I
50
I
60
I 70
1
80
Body diameier (crn)
FIa. 7. Vibrational temperature of hypersonic wake determined from time-resolved spectra of CN and the calculated values.E
FIG.8. Time-resolved spectrum for a titanium ablating sphere (pressure in the range 50 torr, velocity of sphere: 6.28 km/sec).
HIGH-GAIN TII\ZE-RESOLVINC Sl’EC‘TRO(4KAPH
d V sequences are attributed t o grrlater srlf-nbsorptioii of the A V
1031 =
0
sequence. For the other niolecdrs, R similar technique may be employed t o deduce the vibrational teinl)eratures of the g,zs flow, provided t ha t the assumptions rnadc in applying the technique are fulfilled. The time-resolved spectra for some iitomic lines were also obtained by impregnating the epoxj’ eoatiiigs of tlw model meiitionrd a1)ovc. with api)roprittte salts. A typical wsult is resented in Fig. 8. ‘I’tic electronic excitation trml)eratures of the neutral ;tnd ionized spcciw may be determined from tlic intensity distribution of the lines. If’ thermal equilibrium is ntt;iintd, t hc c~lt~ctron deiisity of the flow may also be deducied from a cornparison of the neutIrd and the ion line intensities. Detailed results will be presented in R s e p r a t e report. REP E R E N C HS
1 . Liii, I. I)., .I. A p p l . O p t ~ c s6, 1195 (1967). 2. rAlli, r. D., .I. A ~ ~ optics U I . 1, (i75 ( i ~ i q . 3. Randall, R. P., In “Atlvitnec.b i n Elt~*tioriicsuntl Elrct r o n Physics”, ocl. J. D. Mc(:rc, D. McMnllm and E. K t i h m 1 , Vol 22A, p. 8 7 . Acrtdt~~nic Pie London (1966). 4. Livingston, W. C., I n “A~lvuncch i n Elrctronics ant1 IiX.ctron Physics”, ed. by L. Murton, Vol. 23, 11. 490. Aciitlt.niic Picw, Neir York (1907). 5 . Spicr, *J.L. arid Smit-Miessen, M. M., P / / ~ S Z 9, C L422 L (1942). 6. Appleton, .J. .mtl Y‘itng, C. C., T’rivatt~ COiiiiiiiiriieiition ( 1 908). IhSCITSSJON
You iisrtl Iiodak 2485 filni, u t i \ ththitb it 1 t t i - g ~yirltl cwinpnrrtl n i t h other filmh such Kotlltk 2475 filin! I. r). LILT: The only film othor than Kotlitl, 2485 which has twen used is Kotlulc tlr\xclopetl in 1)-19 for 15 rnm itt, Royal-X Pm Rrcording FiLrn. Whtm both 2O”C, the sensitivity of Ttoyd-X Pan filni appmrs to be coniptirat)lt~ w i t h that of‘ Kodali 2485 film. Howevw, tintic+r cxttmdcd tk\doping t h c Itoyitl-x Pun has niuch greater hiwkgroitnd fog, which srvc~c~ly it.diiccs t h r contrast. W11c.n Kodtik 2485 is developed in Kocliik Drvc4opcr MX 642-1 for 4 min at 32”C, its scwsitivity is thwc to four t i t m i g i w t r r t h a n Roy.tl-X Pan tlcvrloped in D- 19. ,Tudging from the rnnnlificturcr’s spfTificat ion\, thc sensitivity anti contrnit of Kodak 2475 film 5hoiiltl tw some\\htit t)thtv o t m t h o w of’Kodiik 2485 kind Royul-X Pan filins R . GIESE:
.