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Heterodyne detection of CO2 laser radiation scattered from a plasma arc jet
Volume 58A, number 2
PHYSICS LETTERS
9 August 1976
HETERODYNE DETECTION OF CO2 LASER RADIATION SCATTERED FROM A PLASMA ARC JET * D. MASTERS* and B.J. RYE Department ofApplied Physics, University of Hull, Hull, HU6 7 RI, UK Received 3 May 1976 Observations have been made of CO2 laser radiation scattered incoherently from an argon arc jet plasma. The heterodyne detection technique employed is discussed.
Conventional optical resolution and detection techniques have been used routinely for measurements on laser radiation scattered incoherently from plasmas for a number of years [1]. More recently optical heterodyning has been used for detection of radiation scattered coherently (e.g., from driven waves) by a plasma [2, 3]. We report here the application of the heterodyne method to detection of CO2 laser radiation scattered incoherently from an argon arc jet plasma. The layout of our experiment is indicated in fig. 1. A triggered cathode double discharge CO2 laser was operated in the hybrid configuration [4] whereby a 3W low pressure c/w laser discharge shared the same optical cavity. The c/w section alone was used for beam alignment. The hybrid TEA laser delivered smooth reproducible pulses of energy 140 mJ and duration 2its at a repetitition rate of 4Opps. The scattering beam was focused with mirrors and salt lenses to less than 0.25 mm dia. in the plasma and then monitored with a photon drag detector. A salt beam splitter separated part of the laser output for use as a local oscillator (l.o.). After attenuation this was also focused through the plasma at the scattering angle 0 = 30°to the primary beam, the overlap of these beams defining the scattering volume. The local oscillator radiation and light scattered in the same direction was collected on a 0.25 mm dia. fast (400 MHz) Ge:Cu detector mounted in a simple d.c. bias circuit with a 50 ~2load. Under operating condi*
*
First reported at the A.P.S. Topical Conference on Diagnostics of High Temperature Plasmas, Knoxville, Tennessee, January 1976. Now at Computer Centre, University of Hull.
108
tions the pulsed l.o. power was 100 mW, reducing the detector resistance to 0.7 k&~and giving a heterodyne conversion gain [5] Ge = 0.32. Electronic and l.o. generated noise powers in this system were each 8 pW measured over 350 MHz and referred to the detector. Intermediate frequency (i.f.) output of the detector was amplified after being passed through a high pass filter with a cut-off frequency of 50 MHz to prevent pulsed l.o. signal components from saturating the amplifier and to reduce Fourier components of the l.o. pulse below the level of noise observed when using a 100 mW c/w local oscillator. A low pass filter was then used to define the resolution bandwidth before i.f. detection. The rectified output was averaged over the duration of each laser pulse and stored using boxcar integration over 1Os with the gating signal derived from the photon drag detector. Preliminary results are indicated in fig. 2. Signals were readily observed by taking a plasma in/plasma out difference. The difference between signals obtained using two different low pass filters gave the values indicated. On the frequency axis the points represent the power~weigthedmean frequency of the effective passband and the bars the i.f. bandwidth within the 3 dB points. The signal errors arose because of drifts in the boxcar baseline attributable to the presence of incipient modes in the T.E.A. laser at a level of about 10—v of the principal mode power; this is consistent with the observations of Heckenberg [4] using similar lasers. It was verified in experiments without spectral resolution that no statistically significant signal was obtained if the scattering beam was misalignedby .
.
moving its focus 1 mm above that of the local oscillator in the plasma. We have observed c/w CO2 laser
Volume 58A, number 2
PHYSICS LETTERS
1—I
L~:II~~I.1~::3
~.1_1~~,
9
August 1976
Scattering beam
Screened Room
L .J detector Ge Cu detector
I .1
A
7
er
Plasma jet
C.W. Section
,/
Local oscillator beam
Hybrid TEA laser
( ~
Attenuators
V
[•~~]
IEAft~ section
EXPERIMENTAL LAYOUT
777/777
Fig. 1. Layout of the experiment.
scatteringat relatively high intensities from metal partides flowing in the jet when it runs unstably, but this gave rise to Doppler shifts and transit line broadening of less than 15 MHz in the present beam geometry. The signal power levels reported here are typically about a third of those expected for a thermal plasma (possibly due to heterodyning inefficiencies) which suggests that the plasma is not strongly unstable. The electron density measured using Langmuir probes passed through the plasma was 1.5 X 1017 cm3 and the temperature can be obtained as approximately 2 eV from the apparent peak in the spectrum. These values are consistent with Saha equation calculations based on the argon flow rate. Assuming the ions to be in thermal equilibrium with the electrons the ion
mean free path for Coulomb collisions is less than the scattering scale length, but that for ion-neutral collisions is an order of magnitude longer. Some authors [6, 8] have predicted that the effect of Coulomb collisions on the ion feature is to enhance the ion-acoustic sideband while others [9—il] expect the spectrum to be narrowed. The differences between these predictions have been attributed by Leonard and Osborn [7] to use in the latter case of a collision model which omits momentum conserving terms. In fig. 2, curve (I) is constructed from equations (3.10) and (3.11) of Dubois and Gilinsky [6], obtained in their approximation (m/M)1!2 W~Ir~ ~ 1 where here the electron to ion mass ratio (rn/N)1!2 = 3.7 X 1O~and the ratio of the ion acoustic frequency ~iL to the ion collision ~
109
Volume 58A, number 2
PHYSICS LETTERS
Intensity (Arbitary Units)
17
1
‘
I I I
/
~e
\
15 10 cm
3
T~:Tj~22,5O0K
~
]~ II
I
It
I
2 T
/
/
\
/
‘S.
,
0
iS
100
200
I 300
9 August 1976
use the results of Rice [15] to show that after halfwave linear i.f. detection we have (a) if P5 ~ (S/N)power 1’N then P,~then (S/N) owes = 2nBr and (b) if P5 ~ = ~(F~’P~)nBr;here B is the i.f. bandwidth, r is the post-detector integration time (limited to the duration of the laser pulse) and n is the number of pulses. Similar formulae have been obtained in different contexts alsewhere [14, 16]. Averaging techniques are therefore necessary for accurate measurement of mean signal level. In this experiment our main problem has been the drift mentioned earlier; we are attempting to eliminate this by use of a d/w local oscillator, and intend to report the result of this later in a fuller publication. The technical assistance of P. Lowsley and support of this
400
FREQUENCY SHIFT (MHz)
work by the Science Research Council are gratefully acknowledged.
Fig. 2. Results obtained with pulsed local oscillator.
References frequency r’ 11is 0.33; curve (II) is plotted from the equations of Grewal [9] for his parameter p1 = ~ ~L) = 3.5. Previous attempts [12, 13] to resolve the ion feature in scattering from an argon arc jet plasma mdicated only that the spectrum was narrower than the instrumental linewidth of the Fabry-Perot interferometer employed. In addition to providing higher spectral resolution use of the heterodyne technique in plasma scattering experiments removes the problem of stray laser light which is only required to be less than the l.o. level. However, it has two drawbacks when used for detection of incoherently scattered radiation. Firstly, the available etendue of At a heterodyne 2 [141. lO~mand detecunder tor is limited to A~2 X experiment the improvethe high resolution of this ment in etendue of about an order of magnitude to be obtained by using an interferometer did not outweigh the advantages of heterodyne detection in amplifying the signal and ease of alignment, although as in all etendue-limited scattering systems the diameter of the scattering volume had to be minimised. Secondly, and more importantly, the signal to noise ratio attamable is degraded at the i.f. detector. In terms of —.
the electrical signal power, F 5, and the noise power,
~N’ referred to the detector, and assuming that both
are broadband, that the statistics of each are Gaussian, and that the filter spectral profile is squire, we can 110
[1] D.E. Evans and J. Katzenstein, Rep. Prog. Phys. 32(1969) 135. [2] C.M. Surko, R.E. Slusher, D.R. Moles and M. Porkolab, 29 (1972) 81. [3] D.R. Baker, N.R. Heckenberg and J. Meyer, Pliys. Lett. 51A (1975) 185. [4] N.R. Heckenberg, Garching report IPP IV!83 (1975). [5] F.R. Axams, E.W. Sard, B.J. Peyton and F.P. Pace, IEEE I. Quant. Elects., QE-3 (1967) 484. [6] D.F. Dubois and V. Gilinsky, PhyS. Rev. 133 (1964) Al 317. [7] E.M. Leonard and R.H. Osborn, Phys. Rev. A4 (1973) 2021. [8] E.J. Linnebur and J.J. Duderstadt, Phys. Fluids 16 665. Phys. Rev. 134 (1964) A86. [9] (1973) M.S. Grewal, [10] E.C. Taylor and G.C. Comisai, Phys. Rev. 132 (1963) 2379. [11] C.S. Cheng and J.A. MacLennan, Phys. Fluids 15 (1972) 1285. [12] M. Decroisette and F. Cabannes, J.Q.S.R.T. 9 (1968) 619. [13] A.A. Offenberger and R.D. Kerr, Phys. Lett. 37A (1971) 435. [14] H.Z. Cummins and H.L. Swinney, Progr. in Opt. 8 (1970) 135. [15] S.O. Rice, Bell System Tech. J. 23 (1944) 282; 24 (1948) 46; 27 (1948) 109. [16] E. Jakeman, C.J. Oliver and E.R. Pike, Adv. in Phys. 24 (1975) 349.
PHYSICS LETTERS
9 August 1976
HETERODYNE DETECTION OF CO2 LASER RADIATION SCATTERED FROM A PLASMA ARC JET * D. MASTERS* and B.J. RYE Department ofApplied Physics, University of Hull, Hull, HU6 7 RI, UK Received 3 May 1976 Observations have been made of CO2 laser radiation scattered incoherently from an argon arc jet plasma. The heterodyne detection technique employed is discussed.
Conventional optical resolution and detection techniques have been used routinely for measurements on laser radiation scattered incoherently from plasmas for a number of years [1]. More recently optical heterodyning has been used for detection of radiation scattered coherently (e.g., from driven waves) by a plasma [2, 3]. We report here the application of the heterodyne method to detection of CO2 laser radiation scattered incoherently from an argon arc jet plasma. The layout of our experiment is indicated in fig. 1. A triggered cathode double discharge CO2 laser was operated in the hybrid configuration [4] whereby a 3W low pressure c/w laser discharge shared the same optical cavity. The c/w section alone was used for beam alignment. The hybrid TEA laser delivered smooth reproducible pulses of energy 140 mJ and duration 2its at a repetitition rate of 4Opps. The scattering beam was focused with mirrors and salt lenses to less than 0.25 mm dia. in the plasma and then monitored with a photon drag detector. A salt beam splitter separated part of the laser output for use as a local oscillator (l.o.). After attenuation this was also focused through the plasma at the scattering angle 0 = 30°to the primary beam, the overlap of these beams defining the scattering volume. The local oscillator radiation and light scattered in the same direction was collected on a 0.25 mm dia. fast (400 MHz) Ge:Cu detector mounted in a simple d.c. bias circuit with a 50 ~2load. Under operating condi*
*
First reported at the A.P.S. Topical Conference on Diagnostics of High Temperature Plasmas, Knoxville, Tennessee, January 1976. Now at Computer Centre, University of Hull.
108
tions the pulsed l.o. power was 100 mW, reducing the detector resistance to 0.7 k&~and giving a heterodyne conversion gain [5] Ge = 0.32. Electronic and l.o. generated noise powers in this system were each 8 pW measured over 350 MHz and referred to the detector. Intermediate frequency (i.f.) output of the detector was amplified after being passed through a high pass filter with a cut-off frequency of 50 MHz to prevent pulsed l.o. signal components from saturating the amplifier and to reduce Fourier components of the l.o. pulse below the level of noise observed when using a 100 mW c/w local oscillator. A low pass filter was then used to define the resolution bandwidth before i.f. detection. The rectified output was averaged over the duration of each laser pulse and stored using boxcar integration over 1Os with the gating signal derived from the photon drag detector. Preliminary results are indicated in fig. 2. Signals were readily observed by taking a plasma in/plasma out difference. The difference between signals obtained using two different low pass filters gave the values indicated. On the frequency axis the points represent the power~weigthedmean frequency of the effective passband and the bars the i.f. bandwidth within the 3 dB points. The signal errors arose because of drifts in the boxcar baseline attributable to the presence of incipient modes in the T.E.A. laser at a level of about 10—v of the principal mode power; this is consistent with the observations of Heckenberg [4] using similar lasers. It was verified in experiments without spectral resolution that no statistically significant signal was obtained if the scattering beam was misalignedby .
.
moving its focus 1 mm above that of the local oscillator in the plasma. We have observed c/w CO2 laser
Volume 58A, number 2
PHYSICS LETTERS
1—I
L~:II~~I.1~::3
~.1_1~~,
9
August 1976
Scattering beam
Screened Room
L .J detector Ge Cu detector
I .1
A
7
er
Plasma jet
C.W. Section
,/
Local oscillator beam
Hybrid TEA laser
( ~
Attenuators
V
[•~~]
IEAft~ section
EXPERIMENTAL LAYOUT
777/777
Fig. 1. Layout of the experiment.
scatteringat relatively high intensities from metal partides flowing in the jet when it runs unstably, but this gave rise to Doppler shifts and transit line broadening of less than 15 MHz in the present beam geometry. The signal power levels reported here are typically about a third of those expected for a thermal plasma (possibly due to heterodyning inefficiencies) which suggests that the plasma is not strongly unstable. The electron density measured using Langmuir probes passed through the plasma was 1.5 X 1017 cm3 and the temperature can be obtained as approximately 2 eV from the apparent peak in the spectrum. These values are consistent with Saha equation calculations based on the argon flow rate. Assuming the ions to be in thermal equilibrium with the electrons the ion
mean free path for Coulomb collisions is less than the scattering scale length, but that for ion-neutral collisions is an order of magnitude longer. Some authors [6, 8] have predicted that the effect of Coulomb collisions on the ion feature is to enhance the ion-acoustic sideband while others [9—il] expect the spectrum to be narrowed. The differences between these predictions have been attributed by Leonard and Osborn [7] to use in the latter case of a collision model which omits momentum conserving terms. In fig. 2, curve (I) is constructed from equations (3.10) and (3.11) of Dubois and Gilinsky [6], obtained in their approximation (m/M)1!2 W~Ir~ ~ 1 where here the electron to ion mass ratio (rn/N)1!2 = 3.7 X 1O~and the ratio of the ion acoustic frequency ~iL to the ion collision ~
109
Volume 58A, number 2
PHYSICS LETTERS
Intensity (Arbitary Units)
17
1
‘
I I I
/
~e
\
15 10 cm
3
T~:Tj~22,5O0K
~
]~ II
I
It
I
2 T
/
/
\
/
‘S.
,
0
iS
100
200
I 300
9 August 1976
use the results of Rice [15] to show that after halfwave linear i.f. detection we have (a) if P5 ~ (S/N)power 1’N then P,~then (S/N) owes = 2nBr and (b) if P5 ~ = ~(F~’P~)nBr;here B is the i.f. bandwidth, r is the post-detector integration time (limited to the duration of the laser pulse) and n is the number of pulses. Similar formulae have been obtained in different contexts alsewhere [14, 16]. Averaging techniques are therefore necessary for accurate measurement of mean signal level. In this experiment our main problem has been the drift mentioned earlier; we are attempting to eliminate this by use of a d/w local oscillator, and intend to report the result of this later in a fuller publication. The technical assistance of P. Lowsley and support of this
400
FREQUENCY SHIFT (MHz)
work by the Science Research Council are gratefully acknowledged.
Fig. 2. Results obtained with pulsed local oscillator.
References frequency r’ 11is 0.33; curve (II) is plotted from the equations of Grewal [9] for his parameter p1 = ~ ~L) = 3.5. Previous attempts [12, 13] to resolve the ion feature in scattering from an argon arc jet plasma mdicated only that the spectrum was narrower than the instrumental linewidth of the Fabry-Perot interferometer employed. In addition to providing higher spectral resolution use of the heterodyne technique in plasma scattering experiments removes the problem of stray laser light which is only required to be less than the l.o. level. However, it has two drawbacks when used for detection of incoherently scattered radiation. Firstly, the available etendue of At a heterodyne 2 [141. lO~mand detecunder tor is limited to A~2 X experiment the improvethe high resolution of this ment in etendue of about an order of magnitude to be obtained by using an interferometer did not outweigh the advantages of heterodyne detection in amplifying the signal and ease of alignment, although as in all etendue-limited scattering systems the diameter of the scattering volume had to be minimised. Secondly, and more importantly, the signal to noise ratio attamable is degraded at the i.f. detector. In terms of —.
the electrical signal power, F 5, and the noise power,
~N’ referred to the detector, and assuming that both
are broadband, that the statistics of each are Gaussian, and that the filter spectral profile is squire, we can 110
[1] D.E. Evans and J. Katzenstein, Rep. Prog. Phys. 32(1969) 135. [2] C.M. Surko, R.E. Slusher, D.R. Moles and M. Porkolab, 29 (1972) 81. [3] D.R. Baker, N.R. Heckenberg and J. Meyer, Pliys. Lett. 51A (1975) 185. [4] N.R. Heckenberg, Garching report IPP IV!83 (1975). [5] F.R. Axams, E.W. Sard, B.J. Peyton and F.P. Pace, IEEE I. Quant. Elects., QE-3 (1967) 484. [6] D.F. Dubois and V. Gilinsky, PhyS. Rev. 133 (1964) Al 317. [7] E.M. Leonard and R.H. Osborn, Phys. Rev. A4 (1973) 2021. [8] E.J. Linnebur and J.J. Duderstadt, Phys. Fluids 16 665. Phys. Rev. 134 (1964) A86. [9] (1973) M.S. Grewal, [10] E.C. Taylor and G.C. Comisai, Phys. Rev. 132 (1963) 2379. [11] C.S. Cheng and J.A. MacLennan, Phys. Fluids 15 (1972) 1285. [12] M. Decroisette and F. Cabannes, J.Q.S.R.T. 9 (1968) 619. [13] A.A. Offenberger and R.D. Kerr, Phys. Lett. 37A (1971) 435. [14] H.Z. Cummins and H.L. Swinney, Progr. in Opt. 8 (1970) 135. [15] S.O. Rice, Bell System Tech. J. 23 (1944) 282; 24 (1948) 46; 27 (1948) 109. [16] E. Jakeman, C.J. Oliver and E.R. Pike, Adv. in Phys. 24 (1975) 349.