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The operational characteristics of A Nd disc amplifier powered from an inductive energy source
Volume 20, number 2
OPTICS COMMUNICATIONS
February 1977
THE OPERATIONAL CHARACTERISTICS OF A Nd DISC AMPLIFIER POWERED FROM AN INDUCTIVE ENERGY SOURCE G.B. GILLMAN and E.K. INALL Department of Engineering Physics, The Australian National University, Canberra Received 12 November 1976
The operation and performance of a Nd dhe amplifier powered from a homopolar generator via an inductive energy store and fast switch is presented. An increase in amplifier gain is observed with an improved flashlamp reflector design.
While the current in the coil is at a maximum and the ' switch is opening the nitrogen blast circuit breaker " (N.B.C.B.) is opened and the fusing shunts, F.S., melt. The rapid 100/as interruption of current induces a high voltage pulse of up to 4 kV when the current transfers to the load, in this case a bank of 32 flashlamps connected in 16 parallel circuits of two lamps in series. It is necessary for the lamps to conduct at the lowest possible voltage in a resistive manner thereby preventing current surges. This was achieved by sens-, ing the rising voltage across the lamps and firing a high voltage trigger to the common terminal of each pair of lamps. Current from the energy storage capacitors l~ass through the two lamps of each pair in parallel producing sufficient ionisation to ensure prompt conduction of thei main current. The amplifier consisted of 8 modules (fig. 2). Two elliptical
1. Introduction Recent progress in laser induced fusion has seen the need for large energy sources powering the laser amplifiers. Capacitor banks are so expensive that interest has been shown in inductive energy sources [1]. We describe here the characteristics of a prototype Nd disc amplifier powered from a homopolar generator (HPG) and inductive store.
2. Experiment The overall flashlamp driving circuit is shown in fig. I. Energy from the HPG is transferred to the inductor L, by means of the electrolytic switch [2]. The cycle time of the switch is approximately 1 sec.
1
2 5O~Q
L. L+ 45V
I
HPG aF
NBCB , L ,
-
4Sv
J
~
L 41/,LH
" -'1
. --
-
FS:
-
_630,_t~
i -J
16 TIMES
Fig. 1. The homopoiar generator (HPG) supplies a current of 100 kA via the nitrogen blast circuit breaker (NBCB) to the inductor L. This current is supplied to the lamp array when the NBCB opens and the shunt FS melts.
311
Volume 20, number 2
OPTICS COMMUNICATIONS
February 1977
~~//~.u~.~_ , !
?"
Fig. 2. The lamp mountings and reflectors made of gold plated copper. 1, radiation shield (inner) pyrophyllit¢; 2, insulation sleeve (quartz); 3, radiation shield (outer) pyrophyllite; 4, end plates (gold plated copper); 5, blast shield (toughened glass); 6, end cap (Ralloy); 7, laser glass disc; 8, flashlamp.
discs of 44 mm diameter aperture were pumped by four linear flashlamps. The discs were protected from flashlamp explosions by a pyrex shield. Flashlamp radiation was coupled to the discs by means of a removable gold plated reflector. It has been shown [3] that the reflector profile can significantly affect the laser performance and provision was made to confirm this. Because of the non ideal waveforms the peak fluorescence was measured as a function of time thereby allowing adjustment of the time of injection of the laser pulse to coincide with maximum inversion. Timing for the firing of the laser was taken from the voltage sensing network across the NBCB. Typical input energy to the amplifier was 1 J in 5 nsec.
3. Results Typical waveforms of the NBCB operation are shown in fig. 3. The preionisation pulse was found to significantly increase lamp life by operating the lamps in an initially resistive phase [4]. The capacitance C was found to improve the voltage waveform by damping the NBCB circuit. The circuit breaker's unique 312
'
6kA 4
2 0 i , / , 0
.
,
1
.
,
ms
2
Fig. 3. OsciUogramsof the voltage, trace 1, and current, trace 2, for one set of lamps. design [2] has enabled currents of 100 kA to be transt~erred to the fusible shunts in 50/as and when the shunts rupture 200/as later produces 3 - 4 kV pulses with a maximum pump energy of 120 kJ. Gain measurements were made with two types of laser glass: American Optical AOLUX with 5% Nd203 and Hoya type LSG-91 H with 3.1% doping and active lengths of 60 cm and 39 cm respectively. The American
Volume 20, number 2
OPTICS COMMUNICATIONS
/4
February 1977
ment was observed although less than the computed curve (curve 4) which may be attributed to departures from ideal profiles and theoretical and experimental uncertainties.
/ss Ill i i s "/~~2
4. Conclusion
.~ 3 C.
We have demonstrated the feasibility of a large laser amplifier driven from an inductive store which is capable of being scaled to larger systems. A considerable improvement in gain Was observed with the improved design of the flashlamp reflector. I
i
20
40
I
.
60 80 Pump energy kJ
I
100
'
"I-
120
Fig. 4. Gain of the disc amplifier. Curve 1, AOLUX; curve 2, Hoya with plane reflectors; curve 3, Hoya with parabolic reflector; curve 4, theoretical improvement of gain with parabolic reflectors compared to curve 2.
Optical disks had no edge coating and therefore showed the familiar gain roll off due to parasistic oscillation [5] (fig. 4 curve 1). The Hoya discs were edge coated with black solder glass and with the higher gain gave an improved performance. The Hoya gain curve was carried out with two reflector designs, a plane reflector (curve 2) and a "cusp" parabolic reflector (curve 3). As had been predicted [4] a considerable improve-
Acknowledgements We thank Mr. L.J. Allinson and Mr. L. Dowen for their valuable assistance in the development, construction and testing of the circuits and laser system.
References
[1] [2] [3] [4] [5]
E.K. Inall, J. Phys. E: Sci. Instrum. 5 (1972) 679-85. J.W. Blamey et al., Nature 195 (1962) 113-4. M.R. Seigrist, Appl. Opt. ! 5 (1976) 2167-2171. J.H. Gonez, J. Appl. Physi 36 (1965) 742-3. J.M. McMahon, J.L. Emmett, J.F. Holzrichter and J.B. Trenholme, IEEE QE-9 (1972) 992.
313
OPTICS COMMUNICATIONS
February 1977
THE OPERATIONAL CHARACTERISTICS OF A Nd DISC AMPLIFIER POWERED FROM AN INDUCTIVE ENERGY SOURCE G.B. GILLMAN and E.K. INALL Department of Engineering Physics, The Australian National University, Canberra Received 12 November 1976
The operation and performance of a Nd dhe amplifier powered from a homopolar generator via an inductive energy store and fast switch is presented. An increase in amplifier gain is observed with an improved flashlamp reflector design.
While the current in the coil is at a maximum and the ' switch is opening the nitrogen blast circuit breaker " (N.B.C.B.) is opened and the fusing shunts, F.S., melt. The rapid 100/as interruption of current induces a high voltage pulse of up to 4 kV when the current transfers to the load, in this case a bank of 32 flashlamps connected in 16 parallel circuits of two lamps in series. It is necessary for the lamps to conduct at the lowest possible voltage in a resistive manner thereby preventing current surges. This was achieved by sens-, ing the rising voltage across the lamps and firing a high voltage trigger to the common terminal of each pair of lamps. Current from the energy storage capacitors l~ass through the two lamps of each pair in parallel producing sufficient ionisation to ensure prompt conduction of thei main current. The amplifier consisted of 8 modules (fig. 2). Two elliptical
1. Introduction Recent progress in laser induced fusion has seen the need for large energy sources powering the laser amplifiers. Capacitor banks are so expensive that interest has been shown in inductive energy sources [1]. We describe here the characteristics of a prototype Nd disc amplifier powered from a homopolar generator (HPG) and inductive store.
2. Experiment The overall flashlamp driving circuit is shown in fig. I. Energy from the HPG is transferred to the inductor L, by means of the electrolytic switch [2]. The cycle time of the switch is approximately 1 sec.
1
2 5O~Q
L. L+ 45V
I
HPG aF
NBCB , L ,
-
4Sv
J
~
L 41/,LH
" -'1
. --
-
FS:
-
_630,_t~
i -J
16 TIMES
Fig. 1. The homopoiar generator (HPG) supplies a current of 100 kA via the nitrogen blast circuit breaker (NBCB) to the inductor L. This current is supplied to the lamp array when the NBCB opens and the shunt FS melts.
311
Volume 20, number 2
OPTICS COMMUNICATIONS
February 1977
~~//~.u~.~_ , !
?"
Fig. 2. The lamp mountings and reflectors made of gold plated copper. 1, radiation shield (inner) pyrophyllit¢; 2, insulation sleeve (quartz); 3, radiation shield (outer) pyrophyllite; 4, end plates (gold plated copper); 5, blast shield (toughened glass); 6, end cap (Ralloy); 7, laser glass disc; 8, flashlamp.
discs of 44 mm diameter aperture were pumped by four linear flashlamps. The discs were protected from flashlamp explosions by a pyrex shield. Flashlamp radiation was coupled to the discs by means of a removable gold plated reflector. It has been shown [3] that the reflector profile can significantly affect the laser performance and provision was made to confirm this. Because of the non ideal waveforms the peak fluorescence was measured as a function of time thereby allowing adjustment of the time of injection of the laser pulse to coincide with maximum inversion. Timing for the firing of the laser was taken from the voltage sensing network across the NBCB. Typical input energy to the amplifier was 1 J in 5 nsec.
3. Results Typical waveforms of the NBCB operation are shown in fig. 3. The preionisation pulse was found to significantly increase lamp life by operating the lamps in an initially resistive phase [4]. The capacitance C was found to improve the voltage waveform by damping the NBCB circuit. The circuit breaker's unique 312
'
6kA 4
2 0 i , / , 0
.
,
1
.
,
ms
2
Fig. 3. OsciUogramsof the voltage, trace 1, and current, trace 2, for one set of lamps. design [2] has enabled currents of 100 kA to be transt~erred to the fusible shunts in 50/as and when the shunts rupture 200/as later produces 3 - 4 kV pulses with a maximum pump energy of 120 kJ. Gain measurements were made with two types of laser glass: American Optical AOLUX with 5% Nd203 and Hoya type LSG-91 H with 3.1% doping and active lengths of 60 cm and 39 cm respectively. The American
Volume 20, number 2
OPTICS COMMUNICATIONS
/4
February 1977
ment was observed although less than the computed curve (curve 4) which may be attributed to departures from ideal profiles and theoretical and experimental uncertainties.
/ss Ill i i s "/~~2
4. Conclusion
.~ 3 C.
We have demonstrated the feasibility of a large laser amplifier driven from an inductive store which is capable of being scaled to larger systems. A considerable improvement in gain Was observed with the improved design of the flashlamp reflector. I
i
20
40
I
.
60 80 Pump energy kJ
I
100
'
"I-
120
Fig. 4. Gain of the disc amplifier. Curve 1, AOLUX; curve 2, Hoya with plane reflectors; curve 3, Hoya with parabolic reflector; curve 4, theoretical improvement of gain with parabolic reflectors compared to curve 2.
Optical disks had no edge coating and therefore showed the familiar gain roll off due to parasistic oscillation [5] (fig. 4 curve 1). The Hoya discs were edge coated with black solder glass and with the higher gain gave an improved performance. The Hoya gain curve was carried out with two reflector designs, a plane reflector (curve 2) and a "cusp" parabolic reflector (curve 3). As had been predicted [4] a considerable improve-
Acknowledgements We thank Mr. L.J. Allinson and Mr. L. Dowen for their valuable assistance in the development, construction and testing of the circuits and laser system.
References
[1] [2] [3] [4] [5]
E.K. Inall, J. Phys. E: Sci. Instrum. 5 (1972) 679-85. J.W. Blamey et al., Nature 195 (1962) 113-4. M.R. Seigrist, Appl. Opt. ! 5 (1976) 2167-2171. J.H. Gonez, J. Appl. Physi 36 (1965) 742-3. J.M. McMahon, J.L. Emmett, J.F. Holzrichter and J.B. Trenholme, IEEE QE-9 (1972) 992.
313