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Development and application of KrF lasers with different excitation methods
Fusion Engineering and Design 44 Ž1999. 365]370
Development and application of KrF lasers with different excitation methods C. WuU , T. Kamiya, H. Sunami, E. Hotta, K. Kasuya Department of Energy Sciences, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Nagatsuta 4259, Midori-ku, Yokohama, Japan 226
Abstract A wire ion plasma electron gun as an X-ray source for preionizing- discharge-excited KrF laser has been built and the secondary emission electron beam flux up to a relatively low anode voltage of 6 kV has been calculated and measured. Aluminum and lithium targets are irradiated by an e-beam-pumped KrF laser and the plasma flux of each has been obtained. Q 1999 Elsevier Science S.A. All rights reserved. Keywords: Plasma flux; KrF lasers; Aluminum target; Lithium target
1. Introduction It is well-known that preionization plays an important role in stable and uniform discharge of high pressure pulsed laser such as excimer laser and the output energy and beam divergence relate closely to the discharge homogeneity and stability. A UV preionization method has been used in most of our experiments in recent years w1x. X-ray shows strong features superior to UV preionization in terms of high penetrating depth and homogeneous preionization in a large volume, high pressure gas mixture. Therefore we have developed a new wire ion plasma ŽWIP. electron source for X-ray preionization and the secondary
U
Corresponding author.
emission electron density was calculated and measured. The relation between the anode voltage, cathode voltage, gas pressure, ion beam density and electron beam density is studied, and the flux of ion and electron beam are simulated simultaneously. We used another e-beam-pumped KrF laser in this laboratory to vaporize a lithium surface and deposited it on a CaF2 plate as one of the applications. The application of using the same laser to irradiate a metal surface as an ion beam source is also in progress. 2. Development of X-ray preionization source for discharge excited KrF laser A schematic of the developed wire ion plasma electron gun is shown in Fig. 1. Helium is filled
0920-3796r99r$ - see front matter Q 1999 Elsevier Science S.A. All rights reserved. P I I: S 0 9 2 0 - 3 7 9 6 Ž 9 8 . 0 0 3 6 8 - 8
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C. Wu et al. r Fusion Engineering and Design 44 (1999) 365]370
Fig. 1. Schematic diagram of the WIP gun.
into the chamber as the working gas. Two parallel anode wires are used to increase the ion beam flux and decrease the current load of anode. Mesh connected to the chamber body is used to accelerate the secondary electrons produced by collision of ion beam and cathode. Fig. 2 shows the relation of anode discharge delay time dependent on gas pressure for different anode voltages. It is evident that delay time is decreased when either anode voltage or gas pressure is increased. High ion beam flux is the key point of getting sufficient secondary emission electrons to radiate X-ray. We use a biased ion collector to detect the ion beam flux occurring in the anode discharge region and the waveforms of the ion beam pulse, discharge current and cathode voltage are shown in Fig. 3.
Fig. 2. Discharge delay time as a function of gas pressure for different wire voltage.
The ions are accelerated to the negative high voltage cathode Žy30 kV to y70 kV. where the secondary electrons are generated by bombardment. It is very important to adjust the delay time between the ion beam reaching the cathode with the peak of negative cathode voltage to accelerate the generated secondary electrons most efficiently, as are also shown in Fig. 3. We change the inductance of the circuit to control the rela-
Fig. 3. Ion beam Župper., wire current Žmiddle. and cathode voltage Žlower..
C. Wu et al. r Fusion Engineering and Design 44 (1999) 365]370
367
Fig. 4. Secondary electron beam Župper. and wire current Žlower..
Fig. 5. Influence of gas pressure on ion beam flux for various wire voltage.
Fig. 6. Influence of wire voltage on ion beam flux for different gas pressure.
tive delay time and get nearly perfect secondary emission electron pulse which is shown in Fig. 4. Fig. 5 shows the influence of gas pressure on ion beam flux under different discharge voltage. The pressure is varied from 15 mtorr to 100 mtorr and the ion beam flux has the maximum value with the gas pressure at 20]30 mtorr. The influence of anode wire voltage on ion beam flux is shown in Fig. 6. The ion beam flux increases with the wire anode voltage increase. When the anode voltage is higher than 6 kV, the ion beam flux becomes saturated. Changing the wire anode voltage and the gas pressure separately, the influence of the two variables on the electron beam flux are shown in Figs. 7 and 8. From these figures, another important conclusion can be drawn that the wire anode voltage and the gas pressure have little influence
on the secondary electron beam flux. Hence we fix the wire anode voltage of 6 kV and gas pressure of 20 mtorr, from which we can get the highest ion beam flux as has been concluded before, and change the cathode voltage to show the relation between the secondary electron beam and the cathode voltage. It has been proved by our experiment that the cathode voltage influences the electron beam flux notably. According to the Child-Langmuir law together with the emission efficiency of secondary electron beam and transmissivity of mesh, the flux of secondary electron beam can be calculated as the following: Ie s hg
4 9
(
2 q «0 Ž yV Ž d .. 3r2 mi d 2
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C. Wu et al. r Fusion Engineering and Design 44 (1999) 365]370
Fig. 9. Influence of cathode voltage on secondary electron beam flux with 6 kV wire voltage and 20 mtorr gas pressure. Dots are experimental results and the curve is the calculation result.
Fig. 7. Influence of wire voltage on secondary electron beam flux at the gas pressure of 20 mtorr and 100 mtorr.
Here q is the electric charge of ion, m i is the ion mass, « 0 is the vacuum dielectric constant, d is the distance between both of the electrodes and V is electric potential. Considering the electron beam divergence and focusing effect of the ion beam, coefficient 0.2 is multiplied by Ie to correct the calculation result. The experimental data fits in well with the calculated curve, as shown in Fig. 9. One of the things that influence the electron
Fig. 8. Influence of gas pressure on secondary electron beam flux at the wire voltage of 6 kV and 8 kV.
beam divergence is the configuration of the chamber. The heterogeneous electric field in the chamber causes the generation of secondary electron beam diverging significantly. With the calculation of space electric potential in the ionization cavity and speeds of ions and secondary electrons, the orbits of ion beam and secondary electron beam are simulated and shown respectively in Figs. 10 and 11 w2x. From the calculation we can get 24% of the cathode area is bombarded by the ions and about 37% of the secondary emission electrons are lost when they get to the grid to produce X-rays.
Fig. 10. Simulation of the orbit of ion beam with 6 kV wire voltage, 20 mtorr gas pressure and y80 kV cathode voltage.
C. Wu et al. r Fusion Engineering and Design 44 (1999) 365]370
369
Fig. 13. Biased ion collector signal of lithium.
Fig. 11. Simulation of the orbit of secondary electron beam with 6 kV wire anode voltage, 20 mtorr gas pressure and y80 kV cathode voltage.
3. Plasma production by e-beam-pumped KrF laser As one of the practical applications of material processing, a lithium target was irradiated by an e-beam-pumped KrF laser. Evaporated Li vapor by an electric heater was successfully deposited on a CaF2 plate as a thin film which was used as one of the targets. Now we use the same laser to create plasma to supply ion source to substitute flashover plasma.
It can produce a denser ion beam than the latter. The photo energy of the laser is 5 eV. Figs. 12 and 13 show the relative response of plasma created on the surface of two kinds of metals Žaluminum and lithium.. We use a biased ion collector to collect the induced ion beam. In this case laser energy and energy density per unit area deposited on the metals are 700 mJ and 3.2= 10 4 ŽJrcm2 ., respectively. With a rough estimation we can get the plasma flux of each metal as follows: Aluminum 1.6= 10 12 Žrm2 . Lithium 2.8= 10 12 Žrm2 . To get the ion beam from the obtained plasma is the next objective for the near future. 4. Summary The flux of secondary emission electron beam is mainly decided by the cathode voltage and the distance between the electrodes. In our case, with the cathode voltage of y70 kV and the distance of 5 cm between the electrodes, the electron beam flux is about 50 mArcm2 . We can increase the cathode voltage or shorten the distance between the electrodes to increase the flux of the secondary electron beam. Using laser to create plasma as an ion source has a bright future in its application area and we are continuing this work. Acknowledgements
Fig. 12. Biased ion collector signal of aluminum.
The present work is supported by the Grant-inAid of Scientific Research, the Ministry of Educa-
370
C. Wu et al. r Fusion Engineering and Design 44 (1999) 365]370
tion and Culture of Japan. The authors also would like to thank Dr H. Urai for his contribution in this work. References w1x K. Kasuya, et al., Quality measurements of KrF laser beams under high repetition rate operation with ad-
vanced high speed photography: preparatory experiments, Proceedings of the 10th GCL, SPIE-2502, 1995, pp. 450]457. w2x H. Sunami, Development of X-ray preionization generator for discharge pumped KrF laser, Masters Thesis, Tokyo Institute of Technology, 1997.
Development and application of KrF lasers with different excitation methods C. WuU , T. Kamiya, H. Sunami, E. Hotta, K. Kasuya Department of Energy Sciences, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Nagatsuta 4259, Midori-ku, Yokohama, Japan 226
Abstract A wire ion plasma electron gun as an X-ray source for preionizing- discharge-excited KrF laser has been built and the secondary emission electron beam flux up to a relatively low anode voltage of 6 kV has been calculated and measured. Aluminum and lithium targets are irradiated by an e-beam-pumped KrF laser and the plasma flux of each has been obtained. Q 1999 Elsevier Science S.A. All rights reserved. Keywords: Plasma flux; KrF lasers; Aluminum target; Lithium target
1. Introduction It is well-known that preionization plays an important role in stable and uniform discharge of high pressure pulsed laser such as excimer laser and the output energy and beam divergence relate closely to the discharge homogeneity and stability. A UV preionization method has been used in most of our experiments in recent years w1x. X-ray shows strong features superior to UV preionization in terms of high penetrating depth and homogeneous preionization in a large volume, high pressure gas mixture. Therefore we have developed a new wire ion plasma ŽWIP. electron source for X-ray preionization and the secondary
U
Corresponding author.
emission electron density was calculated and measured. The relation between the anode voltage, cathode voltage, gas pressure, ion beam density and electron beam density is studied, and the flux of ion and electron beam are simulated simultaneously. We used another e-beam-pumped KrF laser in this laboratory to vaporize a lithium surface and deposited it on a CaF2 plate as one of the applications. The application of using the same laser to irradiate a metal surface as an ion beam source is also in progress. 2. Development of X-ray preionization source for discharge excited KrF laser A schematic of the developed wire ion plasma electron gun is shown in Fig. 1. Helium is filled
0920-3796r99r$ - see front matter Q 1999 Elsevier Science S.A. All rights reserved. P I I: S 0 9 2 0 - 3 7 9 6 Ž 9 8 . 0 0 3 6 8 - 8
366
C. Wu et al. r Fusion Engineering and Design 44 (1999) 365]370
Fig. 1. Schematic diagram of the WIP gun.
into the chamber as the working gas. Two parallel anode wires are used to increase the ion beam flux and decrease the current load of anode. Mesh connected to the chamber body is used to accelerate the secondary electrons produced by collision of ion beam and cathode. Fig. 2 shows the relation of anode discharge delay time dependent on gas pressure for different anode voltages. It is evident that delay time is decreased when either anode voltage or gas pressure is increased. High ion beam flux is the key point of getting sufficient secondary emission electrons to radiate X-ray. We use a biased ion collector to detect the ion beam flux occurring in the anode discharge region and the waveforms of the ion beam pulse, discharge current and cathode voltage are shown in Fig. 3.
Fig. 2. Discharge delay time as a function of gas pressure for different wire voltage.
The ions are accelerated to the negative high voltage cathode Žy30 kV to y70 kV. where the secondary electrons are generated by bombardment. It is very important to adjust the delay time between the ion beam reaching the cathode with the peak of negative cathode voltage to accelerate the generated secondary electrons most efficiently, as are also shown in Fig. 3. We change the inductance of the circuit to control the rela-
Fig. 3. Ion beam Župper., wire current Žmiddle. and cathode voltage Žlower..
C. Wu et al. r Fusion Engineering and Design 44 (1999) 365]370
367
Fig. 4. Secondary electron beam Župper. and wire current Žlower..
Fig. 5. Influence of gas pressure on ion beam flux for various wire voltage.
Fig. 6. Influence of wire voltage on ion beam flux for different gas pressure.
tive delay time and get nearly perfect secondary emission electron pulse which is shown in Fig. 4. Fig. 5 shows the influence of gas pressure on ion beam flux under different discharge voltage. The pressure is varied from 15 mtorr to 100 mtorr and the ion beam flux has the maximum value with the gas pressure at 20]30 mtorr. The influence of anode wire voltage on ion beam flux is shown in Fig. 6. The ion beam flux increases with the wire anode voltage increase. When the anode voltage is higher than 6 kV, the ion beam flux becomes saturated. Changing the wire anode voltage and the gas pressure separately, the influence of the two variables on the electron beam flux are shown in Figs. 7 and 8. From these figures, another important conclusion can be drawn that the wire anode voltage and the gas pressure have little influence
on the secondary electron beam flux. Hence we fix the wire anode voltage of 6 kV and gas pressure of 20 mtorr, from which we can get the highest ion beam flux as has been concluded before, and change the cathode voltage to show the relation between the secondary electron beam and the cathode voltage. It has been proved by our experiment that the cathode voltage influences the electron beam flux notably. According to the Child-Langmuir law together with the emission efficiency of secondary electron beam and transmissivity of mesh, the flux of secondary electron beam can be calculated as the following: Ie s hg
4 9
(
2 q «0 Ž yV Ž d .. 3r2 mi d 2
368
C. Wu et al. r Fusion Engineering and Design 44 (1999) 365]370
Fig. 9. Influence of cathode voltage on secondary electron beam flux with 6 kV wire voltage and 20 mtorr gas pressure. Dots are experimental results and the curve is the calculation result.
Fig. 7. Influence of wire voltage on secondary electron beam flux at the gas pressure of 20 mtorr and 100 mtorr.
Here q is the electric charge of ion, m i is the ion mass, « 0 is the vacuum dielectric constant, d is the distance between both of the electrodes and V is electric potential. Considering the electron beam divergence and focusing effect of the ion beam, coefficient 0.2 is multiplied by Ie to correct the calculation result. The experimental data fits in well with the calculated curve, as shown in Fig. 9. One of the things that influence the electron
Fig. 8. Influence of gas pressure on secondary electron beam flux at the wire voltage of 6 kV and 8 kV.
beam divergence is the configuration of the chamber. The heterogeneous electric field in the chamber causes the generation of secondary electron beam diverging significantly. With the calculation of space electric potential in the ionization cavity and speeds of ions and secondary electrons, the orbits of ion beam and secondary electron beam are simulated and shown respectively in Figs. 10 and 11 w2x. From the calculation we can get 24% of the cathode area is bombarded by the ions and about 37% of the secondary emission electrons are lost when they get to the grid to produce X-rays.
Fig. 10. Simulation of the orbit of ion beam with 6 kV wire voltage, 20 mtorr gas pressure and y80 kV cathode voltage.
C. Wu et al. r Fusion Engineering and Design 44 (1999) 365]370
369
Fig. 13. Biased ion collector signal of lithium.
Fig. 11. Simulation of the orbit of secondary electron beam with 6 kV wire anode voltage, 20 mtorr gas pressure and y80 kV cathode voltage.
3. Plasma production by e-beam-pumped KrF laser As one of the practical applications of material processing, a lithium target was irradiated by an e-beam-pumped KrF laser. Evaporated Li vapor by an electric heater was successfully deposited on a CaF2 plate as a thin film which was used as one of the targets. Now we use the same laser to create plasma to supply ion source to substitute flashover plasma.
It can produce a denser ion beam than the latter. The photo energy of the laser is 5 eV. Figs. 12 and 13 show the relative response of plasma created on the surface of two kinds of metals Žaluminum and lithium.. We use a biased ion collector to collect the induced ion beam. In this case laser energy and energy density per unit area deposited on the metals are 700 mJ and 3.2= 10 4 ŽJrcm2 ., respectively. With a rough estimation we can get the plasma flux of each metal as follows: Aluminum 1.6= 10 12 Žrm2 . Lithium 2.8= 10 12 Žrm2 . To get the ion beam from the obtained plasma is the next objective for the near future. 4. Summary The flux of secondary emission electron beam is mainly decided by the cathode voltage and the distance between the electrodes. In our case, with the cathode voltage of y70 kV and the distance of 5 cm between the electrodes, the electron beam flux is about 50 mArcm2 . We can increase the cathode voltage or shorten the distance between the electrodes to increase the flux of the secondary electron beam. Using laser to create plasma as an ion source has a bright future in its application area and we are continuing this work. Acknowledgements
Fig. 12. Biased ion collector signal of aluminum.
The present work is supported by the Grant-inAid of Scientific Research, the Ministry of Educa-
370
C. Wu et al. r Fusion Engineering and Design 44 (1999) 365]370
tion and Culture of Japan. The authors also would like to thank Dr H. Urai for his contribution in this work. References w1x K. Kasuya, et al., Quality measurements of KrF laser beams under high repetition rate operation with ad-
vanced high speed photography: preparatory experiments, Proceedings of the 10th GCL, SPIE-2502, 1995, pp. 450]457. w2x H. Sunami, Development of X-ray preionization generator for discharge pumped KrF laser, Masters Thesis, Tokyo Institute of Technology, 1997.