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A simple retropulse isolator for CO2 laser radiation
Infrared Phys. Vol. 28, No. 2, pp. 13-76, Prmted in Great Britain
A SIMPLE
1988
ooze-OS9l/S8 $3.00 + 0.00 Pergamon Press plc
RETROPULSE ISOLATOR LASER RADIATION
FOR
CO,
K. JANULEWICZ and R. JAROCKI S. Kaliski Institute of Plasma Physics and Laser Microfusion, 00-908 Warsaw 49 (Bemowo), P.O. Box 49, Poland (Received
IO June 1987; in revised form
2 July 1987)
Abstract-The design and performance of an active retropulse isolator for the infrared is described. In this device, a surface spark is used as a source of initiating plasma. For the unit tested, a retropulse attenuation of about 20dB has been achieved with laboratory air for incident energy densities above 12.8 J/cm’.
1. INTRODUCTION Large laser systems for fusion investigations have been developed for many years. One of the essential problems which occurs for all targets is to protect the optical system, especially the generator, from retropulse. The retropulse is the part of the incident energy which is reflected from the target and then amplified during propagation towards the generator. Because of the decreasing beam diameter the retropulse reaches the generator with extremely high energy densities. The most commonly applied method to protect the generator from reflected and subsequent amplified radiation is the plasma shutter inserted in the beam path between the generator and first amplifying stage. Various designs of these isolators have been described in Refs (l-4). But all of them require a high quality laser beam, a vacuum system and a complex optical layout. Because the laser beam of the LSIA-CO, laser system at IPPLM exhibits a rather moderate quality, we have decided to look for other methods to obtain an effective plasma isolator. Our attempt The plasma shutter designed by Figueira et ~1.‘~’was the main source of inspiration. to duplicate their construction failed. This was due to difficulties in focusing the beam to small and controllable patterns. Thus, we had to use a greater pinhole diameter. Effective stopping of the retropulse energy was impossible in such a pinhole of l&2.0 mm diameter. As a consequence we have decided to choose the hybrid electrical-laser spark for our isolator.
2. EXPERIMENTAL
ARRANGEMENT
The laser-spark plasma seed in Ref. (4) has been replaced by electrical surface spark from a number (four in our tested unit) of copper foil paths on fiberglass laminate substrate. The discharge from the copper strips passes through the small hole in laminate to the other side which is fully covered by the copper foil. For a large number of strips the discharge becomes similar to the plasma corona described in Ref. (4). That plasma considerably decreases the hole transmission, yet the final blocking of the retropulse energy is performed by the leading edge of the retropulse. The plasma seed improves the laser-spark development. The capacitance of the battery could be modified, but finally we used 2 x 20 nF Brooksvill’s capacitors. The high voltage was varied from 10 to 25 kV. The electrical scheme of the isolator with the view of plate design is shown in Fig. 1. Current and voltage waveform were registered during the experiment. Osciloscope traces are shown in Fig. 2. Because the circuit inductance was not minimalized, oscillations on the voltage trace can be observed on the upper trace in Fig. 2. 3. MEASUREMENTS We have registered the transmission characteristics for two different probing signals. Firstly, we have measured the transmission of the initial electricity created plasma for the chopped beam of the CWCOZ laser. The small signal transmission waveforms are presented in Fig. 3. 73
K. JANULEWICZ and R. JAROCKI (al
Fig. 1. Schematic diagram discharge plate; SG-spark
of the plasma gap; R-ballast
isolator; (a) electrical scheme; (b) construction of the surface resistors; C&capacitors: TP-triggering pulse; PHV-pulsed high voltage.
The waveforms are dependent on the battery capacitance and on the charging voltage. These parameters determine the low transmittance (LT) duration and the residual transmission. In general, an increase of the energy applied to the discharge increases the LT duration t,. A large battery capacitance quenches LT instabilities and increases t, more effectively than high voltage. On the other hand, an increase of voltage shortens the operating time top (defined as the time between the application of the voltage pulse and the appearance of a noteworthy current of &/lo, where 1, represents the peak current), decreases residual transmission level, yet mutual discharge instabilities grow. Observed tendencies are in agreement with the statements of Beverly 111’5’on surface discharge phenomena. The experimental setup for this measurement is plotted in Fig. 4(a). The arrangement for the transmission measurement of the short, high energy density pulse is shown in Fig. 4(b). In the small signal measurements the beam intensity was monitored by a HgCdTe uncooled detector connected to the low-bandpass oscilloscope. In the arrangement presented in Fig. 4(b), the short pulse of about 3 ns from the generator is amplified in UV preionized TEA amplifier of 2 m length. The hole was 2 mm in diameter. Energies were measured with two home-made pyroelectric energy meters with digital display. The plasma attenuation for short, strong signal saturates. It approaches about 23 dB as the incident energy density increases. That can be observed in Fig. 5. The attenuation without electrical discharge is shown for comparison. A very slight difference in attenuation is found for the short
Fig. 2. Voltage
(upper
trace)
and
current (lower 250 ns/div,
trace) waveform. respectively.
The time scale is IO0 ns!div
and
A simple retropulse
isolator
for CO, laser radiation
(a)
1LT_ 500ns
tL
2mns H
7
(b)
r
Fig. 3. Small signal transmission plots for: (a) C = 12 nF, V = 15kV; (b) C = 20 nF, V = 15 kV; (c) C = 28 nF, V = 15 kV; (d) C = 28 nF, V = 25 kV; (e) C = 28 nF, V = 17 kV. The pinhole diameter is 1.2mm for all plots.
pulses, which arrive at intervals of about 1 ps. According to this, the hybrid plasma shutter can compete with the result of Ref. (4) first of all, because of long LT duration, a relatively high attenuation and the simplicity of our device. However, this promising result requires a short comment. During measurements it was confirmed that less than 10% of shots undergo an attenuation down to 15 dB. We are not absolutely sure
Faraday chamber
(b)
Fig. 4. Experimental setup for (a) CWCq laser power measurement; (b) strong laser-pulse transmission measurement; D-detector, PH-photodiode, BS-beam splitter, M-mirrors, PEM-pyroelectric energy meter, PA-polyethylene attenuator, STS-synchronizing and triggering system, EDC-electrical discharge control.
16
K.
JANULEWICZ
l
30 r
and
R.
JAROCKI
The only laser spark + loser spark
~)Electrical
128
0
Incident
Fig. 5. Transmission vs incident energy isolator -0.5 ps after its triggering.
density; x -data
256 energy
density
384
(J/cm’)
O--data measured for the laser pulse arriving at the measured for the laser pulse arriving - I PCSlater.
of the reason for such behavior. The most probable cause is that it was caused by the high level of electric and acoustic noise when we were working with the most sensitive range of the output-energy meter. Earlier we had observed perturbations of our energy meters by this type of disturbances in spite of careful insulation. These measurements are not shown in Fig. 5 for a clearer picture, but they are taken into account in the extension of the error bars. 4.
SUMMARY
We have confirmed that our plasma retropulse isolator, in spite of its simplicity, shows good attenuation for relatively large hole diameters and in laboratory air atmosphere. Because of a relatively long operating time of t, - 10&l 50 ns this type of plasma shutter could work in laser systems with a generator-target distance greater than 20 m. The other important advantage of our device is that it operates without losses or distortion of the forward beam. REFERENCES I. 2. 3. 4. 5.
J. Y. R. J. R.
V. Parker. SPIE 288, 192 (1981). Kawdmura, H. Takeda, M. Matoba, S. Nakai and C. Yamanaka, Appl. Phys. Left. 33, 870 (1978). F. Benjamin, D. B. Henderson, K. B. Mitchell, M. A. Strascio and J. Thomas, Appl. Phys. Lerr. 31, 51 I (1977). F. Figueira, S. J. Czuchlewski, C. R. Phipps Jr and S. J. Thomas. Appl. opt. 20, 838 (1981). E. Beverly III. J. appl. Phys. 60, 104 (1986).
A SIMPLE
1988
ooze-OS9l/S8 $3.00 + 0.00 Pergamon Press plc
RETROPULSE ISOLATOR LASER RADIATION
FOR
CO,
K. JANULEWICZ and R. JAROCKI S. Kaliski Institute of Plasma Physics and Laser Microfusion, 00-908 Warsaw 49 (Bemowo), P.O. Box 49, Poland (Received
IO June 1987; in revised form
2 July 1987)
Abstract-The design and performance of an active retropulse isolator for the infrared is described. In this device, a surface spark is used as a source of initiating plasma. For the unit tested, a retropulse attenuation of about 20dB has been achieved with laboratory air for incident energy densities above 12.8 J/cm’.
1. INTRODUCTION Large laser systems for fusion investigations have been developed for many years. One of the essential problems which occurs for all targets is to protect the optical system, especially the generator, from retropulse. The retropulse is the part of the incident energy which is reflected from the target and then amplified during propagation towards the generator. Because of the decreasing beam diameter the retropulse reaches the generator with extremely high energy densities. The most commonly applied method to protect the generator from reflected and subsequent amplified radiation is the plasma shutter inserted in the beam path between the generator and first amplifying stage. Various designs of these isolators have been described in Refs (l-4). But all of them require a high quality laser beam, a vacuum system and a complex optical layout. Because the laser beam of the LSIA-CO, laser system at IPPLM exhibits a rather moderate quality, we have decided to look for other methods to obtain an effective plasma isolator. Our attempt The plasma shutter designed by Figueira et ~1.‘~’was the main source of inspiration. to duplicate their construction failed. This was due to difficulties in focusing the beam to small and controllable patterns. Thus, we had to use a greater pinhole diameter. Effective stopping of the retropulse energy was impossible in such a pinhole of l&2.0 mm diameter. As a consequence we have decided to choose the hybrid electrical-laser spark for our isolator.
2. EXPERIMENTAL
ARRANGEMENT
The laser-spark plasma seed in Ref. (4) has been replaced by electrical surface spark from a number (four in our tested unit) of copper foil paths on fiberglass laminate substrate. The discharge from the copper strips passes through the small hole in laminate to the other side which is fully covered by the copper foil. For a large number of strips the discharge becomes similar to the plasma corona described in Ref. (4). That plasma considerably decreases the hole transmission, yet the final blocking of the retropulse energy is performed by the leading edge of the retropulse. The plasma seed improves the laser-spark development. The capacitance of the battery could be modified, but finally we used 2 x 20 nF Brooksvill’s capacitors. The high voltage was varied from 10 to 25 kV. The electrical scheme of the isolator with the view of plate design is shown in Fig. 1. Current and voltage waveform were registered during the experiment. Osciloscope traces are shown in Fig. 2. Because the circuit inductance was not minimalized, oscillations on the voltage trace can be observed on the upper trace in Fig. 2. 3. MEASUREMENTS We have registered the transmission characteristics for two different probing signals. Firstly, we have measured the transmission of the initial electricity created plasma for the chopped beam of the CWCOZ laser. The small signal transmission waveforms are presented in Fig. 3. 73
K. JANULEWICZ and R. JAROCKI (al
Fig. 1. Schematic diagram discharge plate; SG-spark
of the plasma gap; R-ballast
isolator; (a) electrical scheme; (b) construction of the surface resistors; C&capacitors: TP-triggering pulse; PHV-pulsed high voltage.
The waveforms are dependent on the battery capacitance and on the charging voltage. These parameters determine the low transmittance (LT) duration and the residual transmission. In general, an increase of the energy applied to the discharge increases the LT duration t,. A large battery capacitance quenches LT instabilities and increases t, more effectively than high voltage. On the other hand, an increase of voltage shortens the operating time top (defined as the time between the application of the voltage pulse and the appearance of a noteworthy current of &/lo, where 1, represents the peak current), decreases residual transmission level, yet mutual discharge instabilities grow. Observed tendencies are in agreement with the statements of Beverly 111’5’on surface discharge phenomena. The experimental setup for this measurement is plotted in Fig. 4(a). The arrangement for the transmission measurement of the short, high energy density pulse is shown in Fig. 4(b). In the small signal measurements the beam intensity was monitored by a HgCdTe uncooled detector connected to the low-bandpass oscilloscope. In the arrangement presented in Fig. 4(b), the short pulse of about 3 ns from the generator is amplified in UV preionized TEA amplifier of 2 m length. The hole was 2 mm in diameter. Energies were measured with two home-made pyroelectric energy meters with digital display. The plasma attenuation for short, strong signal saturates. It approaches about 23 dB as the incident energy density increases. That can be observed in Fig. 5. The attenuation without electrical discharge is shown for comparison. A very slight difference in attenuation is found for the short
Fig. 2. Voltage
(upper
trace)
and
current (lower 250 ns/div,
trace) waveform. respectively.
The time scale is IO0 ns!div
and
A simple retropulse
isolator
for CO, laser radiation
(a)
1LT_ 500ns
tL
2mns H
7
(b)
r
Fig. 3. Small signal transmission plots for: (a) C = 12 nF, V = 15kV; (b) C = 20 nF, V = 15 kV; (c) C = 28 nF, V = 15 kV; (d) C = 28 nF, V = 25 kV; (e) C = 28 nF, V = 17 kV. The pinhole diameter is 1.2mm for all plots.
pulses, which arrive at intervals of about 1 ps. According to this, the hybrid plasma shutter can compete with the result of Ref. (4) first of all, because of long LT duration, a relatively high attenuation and the simplicity of our device. However, this promising result requires a short comment. During measurements it was confirmed that less than 10% of shots undergo an attenuation down to 15 dB. We are not absolutely sure
Faraday chamber
(b)
Fig. 4. Experimental setup for (a) CWCq laser power measurement; (b) strong laser-pulse transmission measurement; D-detector, PH-photodiode, BS-beam splitter, M-mirrors, PEM-pyroelectric energy meter, PA-polyethylene attenuator, STS-synchronizing and triggering system, EDC-electrical discharge control.
16
K.
JANULEWICZ
l
30 r
and
R.
JAROCKI
The only laser spark + loser spark
~)Electrical
128
0
Incident
Fig. 5. Transmission vs incident energy isolator -0.5 ps after its triggering.
density; x -data
256 energy
density
384
(J/cm’)
O--data measured for the laser pulse arriving at the measured for the laser pulse arriving - I PCSlater.
of the reason for such behavior. The most probable cause is that it was caused by the high level of electric and acoustic noise when we were working with the most sensitive range of the output-energy meter. Earlier we had observed perturbations of our energy meters by this type of disturbances in spite of careful insulation. These measurements are not shown in Fig. 5 for a clearer picture, but they are taken into account in the extension of the error bars. 4.
SUMMARY
We have confirmed that our plasma retropulse isolator, in spite of its simplicity, shows good attenuation for relatively large hole diameters and in laboratory air atmosphere. Because of a relatively long operating time of t, - 10&l 50 ns this type of plasma shutter could work in laser systems with a generator-target distance greater than 20 m. The other important advantage of our device is that it operates without losses or distortion of the forward beam. REFERENCES I. 2. 3. 4. 5.
J. Y. R. J. R.
V. Parker. SPIE 288, 192 (1981). Kawdmura, H. Takeda, M. Matoba, S. Nakai and C. Yamanaka, Appl. Phys. Left. 33, 870 (1978). F. Benjamin, D. B. Henderson, K. B. Mitchell, M. A. Strascio and J. Thomas, Appl. Phys. Lerr. 31, 51 I (1977). F. Figueira, S. J. Czuchlewski, C. R. Phipps Jr and S. J. Thomas. Appl. opt. 20, 838 (1981). E. Beverly III. J. appl. Phys. 60, 104 (1986).