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Soft x-ray diagnostics of CO2 laser-produced magnetoplasmas
Journal of Magnetism and Magnetic Materials 11 (1979) 3 7 6 - 3 7 8 © North-Holland Publishing Company
SOFT X-RAY DIAGNOSTICS OF CO2 LASER-PRODUCED MAGNETOPLASMAS * N.G. LOTER, W. HALVERSON **, B. LAX MIT Francis Bitter National Magnet Laboratory * * *, Cambridge, MA 02139, USA
and D.J. NAGEL Naval Research Laboratory, Washington, DC 20375, USA Received 29 August 1978; in revised form 115 October 1978
Gain-switched CO 2 laser pulses were focused onto planar solid targets located in an evacuated cell within a 100 kG Bitter solenoid. The resulting plasmas were studied using soft X-ray diagnostics including a TAP crystal spectrometer, fast p-i-n diodes, and a pinhole camera. Strong beam refraction effects, which depend on the target material, were observed with increasing field.
The interaction of pulsed CO 2 laser radiation with dense plasmas in magnetic fields is of considerable interest for basic plasma physics and for applications such as controlled thermonuclear fusion. In the experiments reported here, hot, dense plasmas were produced by laser irradiation of planar solid targets in vacuum. A strong dc magnetic field applied normal to the target surface partially confines the plasma in the radial direction. As the plasma expands rapidly away from the target, the laser continues to heat the plasma column at distances up to several cm from the surface. With no magnetic field, the absorption (heating) region remains near the target surface. Pulses from the Lumonics 621 CO2 laser used in these experiments have an energy of 225 J, about onethird in a 70 ns FWHM spike and the rest in a 1.5/as tail. A 2 m FL. Cassegrainian optical system focused the beam to irradiances up to 3 X 1011 W]cm 2 within the bore of a 0 - 1 0 0 kG Bitter solenoid magnet as shown in fig. 1. Targets of teflon (CF2), aluminum, and graphite were irradiated. Soft X-ray diagnostics
include a TAP crystal spectrometer, fast silicon p-i-n diodes, and a pinhole camera. The crystal spectrometer monitors simultaneously two soft X-ray lines separated by 0.1 A or more in the 13-23 A range. Time resolution obtained with a scintillator-photomultiplier combination was better than 10 ns. Single channel wavelength resolution is about 0.04 A. The spectrometer monitors line emission from a 1.5 mm wide cross-sectional slice of the plasma column which can be any distance from the target surface up to several cm. Fig. 2 shows intensities of the
SOFT X-RAY XTAL SPECTROMETER ~ LI HTGUIDES IOOKG SOLENOID
~ ] /
P======I
SPHERICAL MIRROR
I~PMT'S
LVE
* Supported by Air Force Office of Scientific Research and the National Science Foundation. ** Present address: SPIRE Corp., Bedford, MA 01730, USA. *** Supported by the National Science Foundation.
PINHOLE CAMERA
FL-2
~ ~~ ~\
Fig. 1. Experimental configuration. 376
COp LASER BEAM
N.G. Loter et aL / So[t X-ray diagnostics of Magnetoplasmas
377
ELECTRON
DENSITY
8 (0) B -- 8 0 kG
o
~-~6 I
o
o
× x x
o o
°°
°°
x
x x
x
0
v
2
>-
Z bJ
,d
o8
-
I
I
I
I
l
I
20
40
60
80
I00
120
(b)
B =
I00 kG o
Z 0
~B
-
o
o
o o
hi
x x
-J4 uJ .
.L
............
.~
.
.
.
.
.
.
.
.
.
x~x
x
.
Fig. 2. He-like F ion line emission 0.5 cm from CF 2 target surface withB = 80 kG. Upper trace: ls2ptp I --, ls 2 IS 0 (resonance) transition, 16.81 A. Lower trace: ls2p3p I --* ls 2 1SO (intercombination) transition, 16.95 A. Both traces 20 ns/division.
F VIII resonance (16.81 It,) and intercombination (16.95 A) lines as a function o f time 5 m m from the target surface with B = 80 kG. F r o m the time o f first deflection o f the trace at this and other axial positions it is seen that the X-ray emission-front velocity is 2 - 2 . 5 X 107 cm/s regardless o f magnetic field. The second peak seen at B = 80 k G appears at earlier times with increasing magnetic field. Peak intensity o f line emission (for b o t h helium-like and hydrogen-like fluorine ions) is maximized at ~ 7 0 - 8 0 kG. The intensity ratio o f the resonance and intercombination lines was used to obtain the electron density [1 ]. This ratio varies strongly with electron density (and weakly with Te) over the density range 0 . 7 - 7 X 1018 cm - 3 for fluorine. Fig. 3 shows the intensity ratio o f these two lines, and the associated electron density versus time at positions 5 and 10 m m from the target surface for B = 60, 80, and 100 kG. A constant electron temperature o f 130 eV is assumed in fig. 3; the error thus introduced is +10% for Te < 175 eV. Measurements at the 10 m m position can be carried to later times than at 5 mm, since the X-ray signals persist at an intense level longer there than closer to the target surface. At times later than those shown, when the signal is dropping very rapidly because o f a local decrease in electron temperature, uncertainties inherent in this technique become much greater.
I
t
1
I
I
I
20
40
60
80
I00
120
TIME AFTER BREAKDOWN (ns)
Fig. 3. Intensity ratio R versus time of F VIII resonance and intercombination lines, minus 1.8. Associated electron densities [1] for Te - 130 eV shown at right: ne = (R - 1.8)/ L75 x 10 -18.
Points are shown at later times for B = 80 kG than for B = 60 o r 100 kG for similar reasons. The decrease in line intensities with increasing field above 80 k G does not translate into a decrease in electron density as well. The electron temperature o f the CF 2 plasmas was SPATIALLY - AVERAG E D ELECTRON TEMPERATURE ~20C Ld n~ }--
< 150 Ld O0
uJ }--
0
• ©
z I00
0
• 0
0 n~ }uJ
,,-J, 50
• B = 80kG o B = IOOkG
I
,
I
,
I
,
I
i
40 80 120 160 200 TIME AFTER BREAKDOWN ( n s )
Fig. 4. Electron temperature versus time, averaged over entire CF 2 plasma column, B = 80 kG and B = 100 kG.
378
N.G. Loter et al. / Soft X-ray diagnostics o f Magnetoplasrnas
measured using p-i-n detectors to record soft X.ray continuum emitted from the entire laser-heated plasma volume. Thin Be and CH2 foils were placed over the detectors to provide X-ray energy cutoffs in the range of 1.1 to 1.6 keV, and time-resolved electron temperatures were calculated from the ratios of signals with different energy cutoffs [2]. Fig. 4 shows the time history of the spatially averaged electron temperatures from a CF2 plasma in magnetic fields of 80 and 100 kG. The electron temperatures show a weak maximum at about 90 ns after the plasma formation at 80 kG; substantially higher temperatures are reached much earlier in the pulse, but large-scale fluctuations in the harder X-ray output make an accurate determination of Te very difficult. The highest X-ray yields during the tail of the laser pulse were observed at magnetic fields ~,80 kG. Spatial information on the X-ray emitting plasma was obtained with the pinhole camera. Fig. 5 shows microphotometer scans of soft X-ray pinhole photographs of the surface regions of teflon, graphite, and aluminum targets. The scans were made along the target surface (perpendicular to the magnetic field axis). At zero field the width of the X-ray image for all materials is about the same as that shown for teflon. For teflon targets, the diameter of the bright surface region (corrected for film non-linearity) expands from 0.7 mm to 1.7 mm FWHM as the magnetic field is increased from 0 to 100 kG. For aluminum targets, this effect is even more pronounced; not only does the image become dimmer and wider with increasing magnetic field, but a deep on-axis minimum also
appears in the emission profile. In contrast, no radial expansion of the image was seen for graphite targets as the magnetic field was increased to 100 kG. In fact, an earlier study [3], using a fast optical streak camera, showed that the diameter of graphite target plasmas at the surface varied as B - a l s . We have thus shown in this study that a strong magnetic field causes important qualitative changes in the interaction of CO2 laser radiation with plasmas formed from solid targets. The partial magnetic confinement provided by the field increases the density, and thus also increases the coupling of the laser radiation to the front of the plasma column. However, the pinhole camera scans at high field (fig. 5) and the second peak in the resonance line intensity (fig. 2) are indicative of strong refraction of the laser beam by the plasma column. It appears that as the plasma propagates away from the target surface and OUt of the focal volume, the magnetically steepened density gradients in the blowoff plasma refract part of the incoming beam which then falls on a considerably larger target surface area than that of the original focal spot. This would be similar to refraction effects observed from expansion of CO2-1aser-heated gas target plasmas away from the focal point in a strong homogeneous magnetic field [4]. The qualitative differences between the dynamics of different target materials and the peaking of the plasma temperature and X-ray output with increasing field indicate a competition between beam absorption and refraction in the interaction zone.
Acknowledgements 3O 40-(o)
CF2
--(b) GRAPHITEB = I00 kG
6O ~ 7O _~ 80 ~ 9O ~ lOG ~- 6 0 - ( c 1 70--
The author gratefully acknowledge Frank Shefton for technical assistance; C.V. Karmendy, V. Zaderej and D. Kane for assistance with the execution of these experiments; and R.F. Benjamin for supplying us with X-ray film and for many useful discussions.
References CF2
B=IOOkG
----(d)
AI
-
B =60 kG
9O I00 Fig. 5. Microphotometer scans of X-ray pinhole photos of surface region of plasmas: (a) CF2, B = 0; (b) Graphite, B = 100 kG; (c) CF2, B = 100 kG; (d) A1, B = 60 kG.
[ 1] A.H. Gabriel and C. Jordan, in: Case Studies in Atomic Collision Physics, Vol. 2, eds. E.W. McDaniel and M.R.C. McDoweU(North-Holland, Amsterdam, 1972) p. 209. [21 F.C. Jahoda, E.M. Little, W.E. Quinn, G.A. Sawyer and T.F. Stratton, Phys. Rev. 119 (1960) 843. [3] W. Halverson, N.G. Loter, W.W. Ma, R.W. Morrison and C.V. Karmendy, Appl. Phys. Lett. 32 (1978) 10. [4] H.L. Rutkowski, D.W. Scudder, Z.A. Pietrzyk and G.C. Vlases, Appl. Phys. Lett. 26 (1975) 421.
SOFT X-RAY DIAGNOSTICS OF CO2 LASER-PRODUCED MAGNETOPLASMAS * N.G. LOTER, W. HALVERSON **, B. LAX MIT Francis Bitter National Magnet Laboratory * * *, Cambridge, MA 02139, USA
and D.J. NAGEL Naval Research Laboratory, Washington, DC 20375, USA Received 29 August 1978; in revised form 115 October 1978
Gain-switched CO 2 laser pulses were focused onto planar solid targets located in an evacuated cell within a 100 kG Bitter solenoid. The resulting plasmas were studied using soft X-ray diagnostics including a TAP crystal spectrometer, fast p-i-n diodes, and a pinhole camera. Strong beam refraction effects, which depend on the target material, were observed with increasing field.
The interaction of pulsed CO 2 laser radiation with dense plasmas in magnetic fields is of considerable interest for basic plasma physics and for applications such as controlled thermonuclear fusion. In the experiments reported here, hot, dense plasmas were produced by laser irradiation of planar solid targets in vacuum. A strong dc magnetic field applied normal to the target surface partially confines the plasma in the radial direction. As the plasma expands rapidly away from the target, the laser continues to heat the plasma column at distances up to several cm from the surface. With no magnetic field, the absorption (heating) region remains near the target surface. Pulses from the Lumonics 621 CO2 laser used in these experiments have an energy of 225 J, about onethird in a 70 ns FWHM spike and the rest in a 1.5/as tail. A 2 m FL. Cassegrainian optical system focused the beam to irradiances up to 3 X 1011 W]cm 2 within the bore of a 0 - 1 0 0 kG Bitter solenoid magnet as shown in fig. 1. Targets of teflon (CF2), aluminum, and graphite were irradiated. Soft X-ray diagnostics
include a TAP crystal spectrometer, fast silicon p-i-n diodes, and a pinhole camera. The crystal spectrometer monitors simultaneously two soft X-ray lines separated by 0.1 A or more in the 13-23 A range. Time resolution obtained with a scintillator-photomultiplier combination was better than 10 ns. Single channel wavelength resolution is about 0.04 A. The spectrometer monitors line emission from a 1.5 mm wide cross-sectional slice of the plasma column which can be any distance from the target surface up to several cm. Fig. 2 shows intensities of the
SOFT X-RAY XTAL SPECTROMETER ~ LI HTGUIDES IOOKG SOLENOID
~ ] /
P======I
SPHERICAL MIRROR
I~PMT'S
LVE
* Supported by Air Force Office of Scientific Research and the National Science Foundation. ** Present address: SPIRE Corp., Bedford, MA 01730, USA. *** Supported by the National Science Foundation.
PINHOLE CAMERA
FL-2
~ ~~ ~\
Fig. 1. Experimental configuration. 376
COp LASER BEAM
N.G. Loter et aL / So[t X-ray diagnostics of Magnetoplasmas
377
ELECTRON
DENSITY
8 (0) B -- 8 0 kG
o
~-~6 I
o
o
× x x
o o
°°
°°
x
x x
x
0
v
2
>-
Z bJ
,d
o8
-
I
I
I
I
l
I
20
40
60
80
I00
120
(b)
B =
I00 kG o
Z 0
~B
-
o
o
o o
hi
x x
-J4 uJ .
.L
............
.~
.
.
.
.
.
.
.
.
.
x~x
x
.
Fig. 2. He-like F ion line emission 0.5 cm from CF 2 target surface withB = 80 kG. Upper trace: ls2ptp I --, ls 2 IS 0 (resonance) transition, 16.81 A. Lower trace: ls2p3p I --* ls 2 1SO (intercombination) transition, 16.95 A. Both traces 20 ns/division.
F VIII resonance (16.81 It,) and intercombination (16.95 A) lines as a function o f time 5 m m from the target surface with B = 80 kG. F r o m the time o f first deflection o f the trace at this and other axial positions it is seen that the X-ray emission-front velocity is 2 - 2 . 5 X 107 cm/s regardless o f magnetic field. The second peak seen at B = 80 k G appears at earlier times with increasing magnetic field. Peak intensity o f line emission (for b o t h helium-like and hydrogen-like fluorine ions) is maximized at ~ 7 0 - 8 0 kG. The intensity ratio o f the resonance and intercombination lines was used to obtain the electron density [1 ]. This ratio varies strongly with electron density (and weakly with Te) over the density range 0 . 7 - 7 X 1018 cm - 3 for fluorine. Fig. 3 shows the intensity ratio o f these two lines, and the associated electron density versus time at positions 5 and 10 m m from the target surface for B = 60, 80, and 100 kG. A constant electron temperature o f 130 eV is assumed in fig. 3; the error thus introduced is +10% for Te < 175 eV. Measurements at the 10 m m position can be carried to later times than at 5 mm, since the X-ray signals persist at an intense level longer there than closer to the target surface. At times later than those shown, when the signal is dropping very rapidly because o f a local decrease in electron temperature, uncertainties inherent in this technique become much greater.
I
t
1
I
I
I
20
40
60
80
I00
120
TIME AFTER BREAKDOWN (ns)
Fig. 3. Intensity ratio R versus time of F VIII resonance and intercombination lines, minus 1.8. Associated electron densities [1] for Te - 130 eV shown at right: ne = (R - 1.8)/ L75 x 10 -18.
Points are shown at later times for B = 80 kG than for B = 60 o r 100 kG for similar reasons. The decrease in line intensities with increasing field above 80 k G does not translate into a decrease in electron density as well. The electron temperature o f the CF 2 plasmas was SPATIALLY - AVERAG E D ELECTRON TEMPERATURE ~20C Ld n~ }--
< 150 Ld O0
uJ }--
0
• ©
z I00
0
• 0
0 n~ }uJ
,,-J, 50
• B = 80kG o B = IOOkG
I
,
I
,
I
,
I
i
40 80 120 160 200 TIME AFTER BREAKDOWN ( n s )
Fig. 4. Electron temperature versus time, averaged over entire CF 2 plasma column, B = 80 kG and B = 100 kG.
378
N.G. Loter et al. / Soft X-ray diagnostics o f Magnetoplasrnas
measured using p-i-n detectors to record soft X.ray continuum emitted from the entire laser-heated plasma volume. Thin Be and CH2 foils were placed over the detectors to provide X-ray energy cutoffs in the range of 1.1 to 1.6 keV, and time-resolved electron temperatures were calculated from the ratios of signals with different energy cutoffs [2]. Fig. 4 shows the time history of the spatially averaged electron temperatures from a CF2 plasma in magnetic fields of 80 and 100 kG. The electron temperatures show a weak maximum at about 90 ns after the plasma formation at 80 kG; substantially higher temperatures are reached much earlier in the pulse, but large-scale fluctuations in the harder X-ray output make an accurate determination of Te very difficult. The highest X-ray yields during the tail of the laser pulse were observed at magnetic fields ~,80 kG. Spatial information on the X-ray emitting plasma was obtained with the pinhole camera. Fig. 5 shows microphotometer scans of soft X-ray pinhole photographs of the surface regions of teflon, graphite, and aluminum targets. The scans were made along the target surface (perpendicular to the magnetic field axis). At zero field the width of the X-ray image for all materials is about the same as that shown for teflon. For teflon targets, the diameter of the bright surface region (corrected for film non-linearity) expands from 0.7 mm to 1.7 mm FWHM as the magnetic field is increased from 0 to 100 kG. For aluminum targets, this effect is even more pronounced; not only does the image become dimmer and wider with increasing magnetic field, but a deep on-axis minimum also
appears in the emission profile. In contrast, no radial expansion of the image was seen for graphite targets as the magnetic field was increased to 100 kG. In fact, an earlier study [3], using a fast optical streak camera, showed that the diameter of graphite target plasmas at the surface varied as B - a l s . We have thus shown in this study that a strong magnetic field causes important qualitative changes in the interaction of CO2 laser radiation with plasmas formed from solid targets. The partial magnetic confinement provided by the field increases the density, and thus also increases the coupling of the laser radiation to the front of the plasma column. However, the pinhole camera scans at high field (fig. 5) and the second peak in the resonance line intensity (fig. 2) are indicative of strong refraction of the laser beam by the plasma column. It appears that as the plasma propagates away from the target surface and OUt of the focal volume, the magnetically steepened density gradients in the blowoff plasma refract part of the incoming beam which then falls on a considerably larger target surface area than that of the original focal spot. This would be similar to refraction effects observed from expansion of CO2-1aser-heated gas target plasmas away from the focal point in a strong homogeneous magnetic field [4]. The qualitative differences between the dynamics of different target materials and the peaking of the plasma temperature and X-ray output with increasing field indicate a competition between beam absorption and refraction in the interaction zone.
Acknowledgements 3O 40-(o)
CF2
--(b) GRAPHITEB = I00 kG
6O ~ 7O _~ 80 ~ 9O ~ lOG ~- 6 0 - ( c 1 70--
The author gratefully acknowledge Frank Shefton for technical assistance; C.V. Karmendy, V. Zaderej and D. Kane for assistance with the execution of these experiments; and R.F. Benjamin for supplying us with X-ray film and for many useful discussions.
References CF2
B=IOOkG
----(d)
AI
-
B =60 kG
9O I00 Fig. 5. Microphotometer scans of X-ray pinhole photos of surface region of plasmas: (a) CF2, B = 0; (b) Graphite, B = 100 kG; (c) CF2, B = 100 kG; (d) A1, B = 60 kG.
[ 1] A.H. Gabriel and C. Jordan, in: Case Studies in Atomic Collision Physics, Vol. 2, eds. E.W. McDaniel and M.R.C. McDoweU(North-Holland, Amsterdam, 1972) p. 209. [21 F.C. Jahoda, E.M. Little, W.E. Quinn, G.A. Sawyer and T.F. Stratton, Phys. Rev. 119 (1960) 843. [3] W. Halverson, N.G. Loter, W.W. Ma, R.W. Morrison and C.V. Karmendy, Appl. Phys. Lett. 32 (1978) 10. [4] H.L. Rutkowski, D.W. Scudder, Z.A. Pietrzyk and G.C. Vlases, Appl. Phys. Lett. 26 (1975) 421.