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Thermal stabilization of the discharge in a nitrogen laser
Volume
18, number
3
OPTICS COMMUNICATIONS
THERMAL STABILIZATION
August
1976
OF THE DISCHARGE IN A NITROGEN LASER*
I. BALTOG Institute of Physics of Bucharest, Bucharest, Romania
and C.B. COLLINS University of Texas at Dallas, Richardson, Received
23 March
Texas 75080,
USA
1976
The effect of cathode temperature upon power output from a transversely excited N2 laser has been studied. An increase of 60% in output power has been observed for 75°C increase in cathode temperature. This degree of thermal enhancement did not depend significantly upon pulse repetition rate so it was possible to increase the maximum average power by the same factor of 60%. As commutation of the Blumlein was accomplished without the use of spark gaps, relatively higher pulse repetition rates were accessible. By externally heating the cathode to 100°C an average output power of 150 mW was obtained at 100 Hz for a charging voltage of 15 kV.
Because of its importance as a source of pump energy for tunable dye lasers, a considerable effort in many laboratories has been applied to the development of a simple and reliable pulsed nitrogen laser. The customary objective has been to increase laser output, usually by increasing the peak power and less frequently by increasing the repetition rate. A variety of different procedures have been used which employ transverse excitation and include traveling wave excitation [l-3], preionization [4,5] , operation at atmospheric pressure [6,7] and the addition of small quantities of foreign gas such as SF, [8,9] to the otherwise pure nitrogen. A review of the results accomplished shows a wide variation of outputs ranging from 100 kW to 2.5 MW in peak power for pulses of duration varying from 1 to 15 nsec. * This work was conducted as part of the U.S.-Romania Cooperative Program in Science and Technology in association between the University of Texas at Dallas and the Institute of Physics of Bucharest. Financial support was provided in part by the U.S. National Science Foundation under Grant No. GF-443 and in part by the Comitetul de State pentru Energia Nucleara and the Consiliul National pentru Stiinta si Technologie of Romania.
282
This letter is a preliminary report of another research program aimed at increasing the output power of the transversely excited N, laser. Presented here is a versatile method for increasing the peak power by the thermal stabilization of the low inductance, distributed transverse discharge. In practice this is achieved by electrode heating. The work, to the author’s knowledge, represents the first investigation of the effect of cathode heating on N2 laser output. The experimental arrangement of the N, laser used in these investigations is shown schematically in
II
+ mirror IOO%R
output I
Fig. 1. Schematic plan of the transversely laser used in these experiments.
excited
nitrogen
Volume
18, number
3
August
OPTICS COMMUNICATIONS
1916
brass
‘dielectric
I
E=5;
d=O.l6cm
+HV
Fig. 2. Cross-sectional
view showing
fig. 1. The 0.3 X 2.5 X 100 cm laser volume was excited by a transverse discharge in the bipolar plane Blumlein strip using an EC&G thyratron as the switch. It was symmetrically placed on the pulse forming line to favor traveling wave excitation. As shown in fig. 2, the electrodes of the laser tube were made by beveling the edges of angle brass stock. The dielectric insulation between upper and lower copper plates forming the transmission lines was a 1.6 mm thick fiberglass-epoxy composition having dielectric constant e = 5. A highly reflective back mirror was found to provide the best collimation and a quartz lens was used to focus output radiation on a Hadron thermopile. Dependable operation was achieved over the l200 Hz frequency range at 15 KV with the maximum frequency being limited only by the average power rating of the HV power supply. A heating tape connected to the cathode allowed control of the cathode temperature from 25 to 100°C. Pressures of nitrogen varying from 40-80 torr were used and the results presented here were obtained at the optimum pressure of 60 torr. In operation the discharge in the laser tube was seen to be a bluish-pink glow which consisted of numerous time streamers. At low voltage the luminosity was greatest near the cathode and by increasing the applied voltage the operation could be made more visually uniform in the entire space between electrodes. The same effect was obtained at a constant voltage by lowering the pressure. In any case, the streamers emerging from the cathode were seen to have a strong tendency to go down and run across the surface of the glass plate closest to the positive HV sheet thereby degrading the output beam quality because of the
the construction
details
of the laser tube.
misalignment and nonuniformity of the individual filamentary arcs within the laser volume. In addition this attraction of the plasma to the walls of the discharge volume must also be considered as a source inefficiency because of the energy losses due to the electron collisions with the surfaces. As a consequence of these observations, an effort was made to detach the plasma from the lower glass wall and to stabilize it closer to the middle of the discharge volume. This was first attempted by covering the discharge space with a copper plate connected to the anode as shown in fig. 2. In this way a more symmetrical geometry of the electric field within the discharge volume was achieved with some reduction of the series inductance of the laser tube. As could be expected, it was most important that the Blumlein line impedance be reasonably well-matched with that of the discharge for a given distance between electrodes and that was accomplished by changing the impedance of the electrodes. In this configuration with the discharge cover plate the output power was found to be increased by about 20% with a corresponding increase in output beam uniformity. The same problem of avoiding the attachment of the plasma to the walls of the laser tube seems to have been resolved by Godard and Vannier [lo] through the use of a concave lower tube wall, although this must be expected to lead ultimately to an increased limiting tube inductance. That further stabilization and alignment of the individual filamentary arcs of the discharge could be achieved thermally was suggested by a systematic increase of the output power observed as the running time increased. Fig. 3 illustrates a typical varia283
Volume
18, number
3
OPTICS COMMUNICATIONS
1
I
1
.
l
. . .
100 Hz
.
I
I
20
40
RUNNING
,
60
TIME
(min)
Fig. 3. Average output power in arbitrary units as a function of the running time elapsed since the initiation of the laser discharge. discharge
Operating voltage tube was 60 torr.
was 12.5 kV and pressure
in the
tion of the output power with running time at 100 Hz and 12.5 kV. The steady value reached after about 30 min suggested a thermal equilibration of the discharge tube. Empirically this could be correlated with the cathode temperature which increased for comparable times as the laser operated. A deliberate heating of the cathode showed the same effects of increasing output power and improving homogen-
August
1976
eity of the beam. Conversely, neither a variation of anode temperature between -30°C and 80°C nor substantial preheating of the nitrogen prior to its introduction into the laser tube had any influence on output power or beam uniformity. Fig. 4 presents the dependence of the average output power (left scale) observed as a function of the cathode temperature at pulse repetition rates of 40 and 100 Hz for a pressure of 60 torr. Corresponding output pulse energies are shown by the appropriate scales to the right. As can be seen, a monotonic increase of laser output with cathode temperature is indicated. Fig. 5 shows the time dependence of the power emitted in a laser pulse during operation at the same frequencies of 40 and 100 Hz for two temperatures, room and 100°C. Data were obtained from recordings made with a Tektronix 5 19 oscilloscope directly connected to an ITT, F-4000 vacuum photodiode. From this figure it can be seen that the increase of average power shown in fig. 4 is mainly the result of an increase in peak power (solid curve of fig. 5) and not to an increase in pulse duration. The observed intensitivity of the output to both the,temperature of the anode and to the gas renders it difficult to attribute this effect to causes such as variation of the matching of impedances or variation of the refracthe index of the laser medium. More probably the explanation of the dependence of laser power on cathode temperature lies in the plasma dynamics of the filamentary discharge and in the cathode
.
.
100°tiZ .
.
. f1.2
. 40
1
,
20
T&ERAT”RE
I
I
I
60
80
100
I IPO
(“C)
Fig. 4. Dependence of the average output power as a function of cathode temperature for pulse repetition rates of 40 and 100 Hz obtained at 15 kV and 60 torr. Corresponding output energies per pulse are shown by the appropriate scales to the right.
284
Fig. 5. Curves showing the power emitted in individual 15 kV for pulse repetition 40 Hz (right) with cathode temperature (dashed) and
time dependence of the output laser pulses during operation at frequencies of 100 Hz (left) and temperatures maintained at room at 100°C (solid).
Volume 18, number 3
OPTICS COMMUNICATIONS
processes which are quite complicated for the real state of the surface layer. For example, it can be expected that the thermal flow from the cathode would create a region of lower density near the cathode in the midplane of the laser tube thus tending to align and stabilize the arcs nearer the center of the tube. Though no definitive explanation can be offered at the present time, this preliminary work shows a new and convenient possibility for increasing the output power of transversely excited nitrogen lasers of conventional design and further studies in this direction are currently in progress.
The authors wish to express their gratitude for the generous assistance in this work provided by Dr. M.Y. Mirza of the University of Texas at Dallas.
August 1976
References [l] J.D. Shipman, Jr., Appl. Phys. Lett. 10 (1967) 3. [2] B. Godard, IEEE, J. Quantum Electronics QElO (1974) 147. [3] H. Salzmann and H. Strohwald, Opt. Commun. 12 (1974) 370. [4] N.A. Kurnit, S.J. Tubbs, K. Bidhichand, L.W. Ryan, Jr. and A. Javan, IEEE, J. Quantum Electronics QEll (1975) 174. [5] E.E. Bergmann, Appl. Phys. Lett. 28 (1976) 84. [6] H.M. von Bergmann, V. Hasson and D. Preussler, Appl. Phys. Lett. 27 (1975) 553. [7] I.N. Knyazev, V.S. Letokhov and V.G. Movshev, Opt. Commun. 6 (1972) 250. [S] C.S. Willet and D.M. Litynski, Appl. Phys. Lett. 26 (1975) 118. (91 J. Itani, K. Kagawa and Y. Kimura, Appl. Phys. Lett. 27 (1975) 503. [lo] B. Godard and M. Vannier, Opt. Commun. 16 (1976) 37.
285
18, number
3
OPTICS COMMUNICATIONS
THERMAL STABILIZATION
August
1976
OF THE DISCHARGE IN A NITROGEN LASER*
I. BALTOG Institute of Physics of Bucharest, Bucharest, Romania
and C.B. COLLINS University of Texas at Dallas, Richardson, Received
23 March
Texas 75080,
USA
1976
The effect of cathode temperature upon power output from a transversely excited N2 laser has been studied. An increase of 60% in output power has been observed for 75°C increase in cathode temperature. This degree of thermal enhancement did not depend significantly upon pulse repetition rate so it was possible to increase the maximum average power by the same factor of 60%. As commutation of the Blumlein was accomplished without the use of spark gaps, relatively higher pulse repetition rates were accessible. By externally heating the cathode to 100°C an average output power of 150 mW was obtained at 100 Hz for a charging voltage of 15 kV.
Because of its importance as a source of pump energy for tunable dye lasers, a considerable effort in many laboratories has been applied to the development of a simple and reliable pulsed nitrogen laser. The customary objective has been to increase laser output, usually by increasing the peak power and less frequently by increasing the repetition rate. A variety of different procedures have been used which employ transverse excitation and include traveling wave excitation [l-3], preionization [4,5] , operation at atmospheric pressure [6,7] and the addition of small quantities of foreign gas such as SF, [8,9] to the otherwise pure nitrogen. A review of the results accomplished shows a wide variation of outputs ranging from 100 kW to 2.5 MW in peak power for pulses of duration varying from 1 to 15 nsec. * This work was conducted as part of the U.S.-Romania Cooperative Program in Science and Technology in association between the University of Texas at Dallas and the Institute of Physics of Bucharest. Financial support was provided in part by the U.S. National Science Foundation under Grant No. GF-443 and in part by the Comitetul de State pentru Energia Nucleara and the Consiliul National pentru Stiinta si Technologie of Romania.
282
This letter is a preliminary report of another research program aimed at increasing the output power of the transversely excited N, laser. Presented here is a versatile method for increasing the peak power by the thermal stabilization of the low inductance, distributed transverse discharge. In practice this is achieved by electrode heating. The work, to the author’s knowledge, represents the first investigation of the effect of cathode heating on N2 laser output. The experimental arrangement of the N, laser used in these investigations is shown schematically in
II
+ mirror IOO%R
output I
Fig. 1. Schematic plan of the transversely laser used in these experiments.
excited
nitrogen
Volume
18, number
3
August
OPTICS COMMUNICATIONS
1916
brass
‘dielectric
I
E=5;
d=O.l6cm
+HV
Fig. 2. Cross-sectional
view showing
fig. 1. The 0.3 X 2.5 X 100 cm laser volume was excited by a transverse discharge in the bipolar plane Blumlein strip using an EC&G thyratron as the switch. It was symmetrically placed on the pulse forming line to favor traveling wave excitation. As shown in fig. 2, the electrodes of the laser tube were made by beveling the edges of angle brass stock. The dielectric insulation between upper and lower copper plates forming the transmission lines was a 1.6 mm thick fiberglass-epoxy composition having dielectric constant e = 5. A highly reflective back mirror was found to provide the best collimation and a quartz lens was used to focus output radiation on a Hadron thermopile. Dependable operation was achieved over the l200 Hz frequency range at 15 KV with the maximum frequency being limited only by the average power rating of the HV power supply. A heating tape connected to the cathode allowed control of the cathode temperature from 25 to 100°C. Pressures of nitrogen varying from 40-80 torr were used and the results presented here were obtained at the optimum pressure of 60 torr. In operation the discharge in the laser tube was seen to be a bluish-pink glow which consisted of numerous time streamers. At low voltage the luminosity was greatest near the cathode and by increasing the applied voltage the operation could be made more visually uniform in the entire space between electrodes. The same effect was obtained at a constant voltage by lowering the pressure. In any case, the streamers emerging from the cathode were seen to have a strong tendency to go down and run across the surface of the glass plate closest to the positive HV sheet thereby degrading the output beam quality because of the
the construction
details
of the laser tube.
misalignment and nonuniformity of the individual filamentary arcs within the laser volume. In addition this attraction of the plasma to the walls of the discharge volume must also be considered as a source inefficiency because of the energy losses due to the electron collisions with the surfaces. As a consequence of these observations, an effort was made to detach the plasma from the lower glass wall and to stabilize it closer to the middle of the discharge volume. This was first attempted by covering the discharge space with a copper plate connected to the anode as shown in fig. 2. In this way a more symmetrical geometry of the electric field within the discharge volume was achieved with some reduction of the series inductance of the laser tube. As could be expected, it was most important that the Blumlein line impedance be reasonably well-matched with that of the discharge for a given distance between electrodes and that was accomplished by changing the impedance of the electrodes. In this configuration with the discharge cover plate the output power was found to be increased by about 20% with a corresponding increase in output beam uniformity. The same problem of avoiding the attachment of the plasma to the walls of the laser tube seems to have been resolved by Godard and Vannier [lo] through the use of a concave lower tube wall, although this must be expected to lead ultimately to an increased limiting tube inductance. That further stabilization and alignment of the individual filamentary arcs of the discharge could be achieved thermally was suggested by a systematic increase of the output power observed as the running time increased. Fig. 3 illustrates a typical varia283
Volume
18, number
3
OPTICS COMMUNICATIONS
1
I
1
.
l
. . .
100 Hz
.
I
I
20
40
RUNNING
,
60
TIME
(min)
Fig. 3. Average output power in arbitrary units as a function of the running time elapsed since the initiation of the laser discharge. discharge
Operating voltage tube was 60 torr.
was 12.5 kV and pressure
in the
tion of the output power with running time at 100 Hz and 12.5 kV. The steady value reached after about 30 min suggested a thermal equilibration of the discharge tube. Empirically this could be correlated with the cathode temperature which increased for comparable times as the laser operated. A deliberate heating of the cathode showed the same effects of increasing output power and improving homogen-
August
1976
eity of the beam. Conversely, neither a variation of anode temperature between -30°C and 80°C nor substantial preheating of the nitrogen prior to its introduction into the laser tube had any influence on output power or beam uniformity. Fig. 4 presents the dependence of the average output power (left scale) observed as a function of the cathode temperature at pulse repetition rates of 40 and 100 Hz for a pressure of 60 torr. Corresponding output pulse energies are shown by the appropriate scales to the right. As can be seen, a monotonic increase of laser output with cathode temperature is indicated. Fig. 5 shows the time dependence of the power emitted in a laser pulse during operation at the same frequencies of 40 and 100 Hz for two temperatures, room and 100°C. Data were obtained from recordings made with a Tektronix 5 19 oscilloscope directly connected to an ITT, F-4000 vacuum photodiode. From this figure it can be seen that the increase of average power shown in fig. 4 is mainly the result of an increase in peak power (solid curve of fig. 5) and not to an increase in pulse duration. The observed intensitivity of the output to both the,temperature of the anode and to the gas renders it difficult to attribute this effect to causes such as variation of the matching of impedances or variation of the refracthe index of the laser medium. More probably the explanation of the dependence of laser power on cathode temperature lies in the plasma dynamics of the filamentary discharge and in the cathode
.
.
100°tiZ .
.
. f1.2
. 40
1
,
20
T&ERAT”RE
I
I
I
60
80
100
I IPO
(“C)
Fig. 4. Dependence of the average output power as a function of cathode temperature for pulse repetition rates of 40 and 100 Hz obtained at 15 kV and 60 torr. Corresponding output energies per pulse are shown by the appropriate scales to the right.
284
Fig. 5. Curves showing the power emitted in individual 15 kV for pulse repetition 40 Hz (right) with cathode temperature (dashed) and
time dependence of the output laser pulses during operation at frequencies of 100 Hz (left) and temperatures maintained at room at 100°C (solid).
Volume 18, number 3
OPTICS COMMUNICATIONS
processes which are quite complicated for the real state of the surface layer. For example, it can be expected that the thermal flow from the cathode would create a region of lower density near the cathode in the midplane of the laser tube thus tending to align and stabilize the arcs nearer the center of the tube. Though no definitive explanation can be offered at the present time, this preliminary work shows a new and convenient possibility for increasing the output power of transversely excited nitrogen lasers of conventional design and further studies in this direction are currently in progress.
The authors wish to express their gratitude for the generous assistance in this work provided by Dr. M.Y. Mirza of the University of Texas at Dallas.
August 1976
References [l] J.D. Shipman, Jr., Appl. Phys. Lett. 10 (1967) 3. [2] B. Godard, IEEE, J. Quantum Electronics QElO (1974) 147. [3] H. Salzmann and H. Strohwald, Opt. Commun. 12 (1974) 370. [4] N.A. Kurnit, S.J. Tubbs, K. Bidhichand, L.W. Ryan, Jr. and A. Javan, IEEE, J. Quantum Electronics QEll (1975) 174. [5] E.E. Bergmann, Appl. Phys. Lett. 28 (1976) 84. [6] H.M. von Bergmann, V. Hasson and D. Preussler, Appl. Phys. Lett. 27 (1975) 553. [7] I.N. Knyazev, V.S. Letokhov and V.G. Movshev, Opt. Commun. 6 (1972) 250. [S] C.S. Willet and D.M. Litynski, Appl. Phys. Lett. 26 (1975) 118. (91 J. Itani, K. Kagawa and Y. Kimura, Appl. Phys. Lett. 27 (1975) 503. [lo] B. Godard and M. Vannier, Opt. Commun. 16 (1976) 37.
285