- Email: [email protected]
Triatomic hydrogen ion generation in a low-pressure gas discharge
Vacuum 162 (2019) 63–66
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Triatomic hydrogen ion generation in a low-pressure gas discharge a,∗
A.V. Vizir , E.M. Oks a b
a,b
a
, M.V. Shandrikov , G. Yu. Yushkov
T
a
Institute of High Current Electronics, Siberian Branch of Russian Academy of Science, 2/3 Academichesky Ave., 634055, Tomsk, Russia Tomsk State University of Control System and Radioelectronics, 40 Lenin Ave., 634050, Tomsk, Russia
A R T I C LE I N FO
A B S T R A C T
Keywords: Reflex discharge with hollow cathode Plasma ion composition Hydrogen and deuterium triatomic ions
The generation of triatomic H3+ and D3+ ions in a reflex hollow-cathode discharge using hydrogen and deuterium as the working gas, over a wide range of operating pressure, discharge current and magnetic field strength was explored. The fractional ion species composition of the extracted ion beam was monitored using a time-of-flight method. The maximum fraction of H3+ that could be obtained was as high as 95%, with similar results for D3+ ions. It was shown that the degree of ionization of the working gas is one of parameters determining the ion composition of the hollow cathode discharge plasma.
1. Introduction Hydrogen and deuterium ions and their beams are needed for nuclear fusion, medicine and other fields of science and industry. Triatomic hydrogen ions (H3+) can be used for modeling ions of rare isotopes of tritium (T+) and helium (3He+) ions in particle accelerators. Hydrogen ion implantation is crucial in semiconductor electronic and optical devices manufacturing [1], particularly, in so-called “smart cut” process for silicon-on-insulators technology [2]. H3+ ions were first observed by J.J. Thomson in 1913 [3]. Much later, H3+ ions were discovered in the diffuse interstellar medium [4,5] and in the Galactic center [6]. A number of authors [7–9] have reported on studies of specific physical and chemical properties of H3+ ion, which are important in astrophysics. Generation of H3+ in laboratory plasma using a duoplasmatron-like ion source has been demonstrated in Refs. [10,11]. H2+ and H3+ ion beam generation using an electron cyclotron resonance ion source has also been reported in Ref. [12], with the H3+ ion fraction in the beam of 43%. In prior work [13] we have shown that in a reflex discharge with hollow cathode the H3+ ion fraction reaches 70%. Stable H3+ ions are formed in the reaction [14]: H2+ + H2 → H3+ + H
(1)
This reaction is preceded by known processes of hydrogen dissociation and its ionization to H+ and H2+. The H3+ state can decay in the following ways: H3+ + e− → H2+ + H + e− H3+
∗
−
+e
→ H2 + H
+
−
+e
(2) (3)
In this work we consider the effect of the plasma parameters determined by the discharge conditions on the hydrogen and deuterium plasma ion composition. 2. Experimental An outline of the experimental setup is shown in Fig. 1. Hydrogen plasma is generated in a reflex discharge with hollow cathode 1 of inner diameter 6 mm and length 32 mm, and reflex cathode 2 located 23 mm from the output of cathode 1 and anode 3 with inner diameter of 34 mm. The discharge chamber volume is approximately 22 cm3. Another system was also used with smaller dimensions, having a discharge chamber volume of 6 cm3. All electrodes are water-cooled. The glow discharge was fed by two power supplies which can operate separately or simultaneously: a dc power supply (0–100 mA, up to 1500 V) and a pulsed power supply (0–40 A, 150–2000 V, 5–250 ms, 20–5000 pps). Both supplies are at the high accelerating voltage. If the dc power supply is on, even with very low discharge current of 1 mA, it completely eliminates the delay of the pulsed discharge current onset. An axially symmetric magnetic field is generated by a Sm-Co permanent magnet, or alternatively by solenoid 4. The magnetic field strength at the center of the 135-turn solenoid was up to 600 G; the solenoid was driven by a current of up to 20 A for a few seconds, which was sufficiently long for penetration of the field through the metal structure of the discharge system. A low-voltage (up to 20 V) power supply of the solenoid is also placed at the high accelerating voltage using isolation transformer. Cathode 2 has a central opening of 5 mm diameter serving for ion emission. Emitted ions are then accelerated to 10 keV by the positive potential applied to the anode 3. A negative potential of
Corresponding author. E-mail address: [email protected] (A.V. Vizir).
https://doi.org/10.1016/j.vacuum.2019.01.025 Received 20 November 2018; Received in revised form 11 January 2019; Accepted 16 January 2019 Available online 19 January 2019 0042-207X/ © 2019 Elsevier Ltd. All rights reserved.
Vacuum 162 (2019) 63–66
A.V. Vizir et al.
Fig. 1. Simplified schematic of the experimental setup. 1 – hollow cathode, 2 – reflex cathode, 3 – anode, 4 – permanent magnet or solenoid, 5 – beam extraction electrode, 6 – time-of-flight spectrometer gate, 7 – ion detector (secondary electron multiplier), 8 – spectrometer output to oscilloscope, 9 – vacuum vessel, 10 – turbomolecular pump, 11 – gate valve.
−300 V is applied to extraction electrode 5 for suppression of backstreaming electrons. The composition of the ion beam so formed is analyzed by the time-of-flight (TOF) spectrometer [15] comprised of deflecting gate 6, detector 7 and a drift space with length of 1.2 m. The gate consists of 6 circular gaps formed by 2 groups of concentric rings, to one of which a short (100 ns, 2 kV) deflection pulse is applied while the other group is grounded. A central plate with diameter of 6 cm prevents detecting ions when the gate pulse is not applied, and in addition serves as an ion beam current monitor. The use of a secondary electron multiplier (SEM) 7 based on 2 microchannel plates as detector allows ion beam composition measurements over a very wide range of beam current. For high beam current, a Faraday cup (FC) was used, providing confirmation that the SEM data and the FC data match. Charge exchange processes may occur within the ion drift space, and SEM could register neutral particles; but since they are not only not accelerated, but not deflected by the gate, SEM spectra represent directly the discharge plasma ion composition. Therefore, the data given below were taken by SEM, using a beam attenuator for higher currents. 0.3 m diameter, 1.7 m long vacuum vessel 9 is pumped by turbomolecular pump 10 with pumping speed of 900 l/s through gate valve 11. Pressure values given below are pressures in the vacuum chamber unless otherwise specified.
Fig. 2. Hydrogen TOF spectra. 1 – discharge current 40 mA DC, pressure 0.3 mTorr, magnetic induction 220 G, discharge system with solenoid and volume 22 cm3; 2–40 A pulsed, 0.03 mTorr, 600 G, permanent magnet, 6 cm3.
and increase of the H+ fraction up to 80%. We have shown earlier [13] that the variation of H3+ fraction with discharge current has a maximum that shifts to higher current with increasing gas pressure. With the use of a reflex discharge with hollow cathode system having volume of 6 cm3, at the working pressure of 0.02 mTorr, H3+ ion fraction increases from 25 to 70% in the discharge current range 0.8–8 mA, and then reduces down to 2% at 17 A. Further, 10-fold pressure increase up to 0.2 mTorr leads to a proportional increase of the discharge current (80 mA) at which the maximal fraction of H3+ ions (70%) is registered. New experiments show that the current dependence of the deuterium ion composition is similar to that for hydrogen. TOF spectra for deuterium are shown on Fig. 3. The maximal fraction of D3+ 67% ions was reached at 20 mA dc discharge current, and the fraction of D+ of 74% at 25 A pulsed. Since all data for deuterium (Fig. 3) were taken at one working pressure and using one discharge system (6 cm3 volume), unlike the conditions of hydrogen measurements (Fig. 2), the maximal ion fractions reached for deuterium are lower than for hydrogen. D2H+ and H+ ions are also contained in plasma, because of the presence of hydrogen in working and residual gases. Apparently, impeding radial movement of electrons, the increase of magnetic induction leads to the plasma compression to the axis of the discharge system where ions are extracted for further analysis. The use
3. Results and discussion The time-of-flight measurements show that the discharge parameters have a strong effect on the plasma ion composition. Fig. 2 shows TOF oscillograms taken at the output 8 (Fig. 1) of the spectrometer using hydrogen as a working gas, for two very different conditions, lowcurrent DC and high-current pulsed modes. In the pulsed mode, the pulse duration was 40 μs with the repetition rate of 20 pps. No noticeable change in plasma ion composition was observed through pulse duration, as well as with the repetition rate in a range of 20–100 pps. The fraction of each ion species was calculated by time integration of the corresponding oscillogram peak. As Fig. 2 (1) indicates, the H3+ ion fraction reaches 95% for the optimal combination of discharge parameters including discharge current 40 mA, working pressure 0.3 mTorr, and magnetic induction 220 G. Additionally, the use of the discharge system with larger volume of 22 cm3 compared that used earlier in Ref. [13] (6 cm3) resulted in not only to the higher H3+ ion fraction but also total extracted ion beam current which for these conditions was a few milliamperes. A thousand-fold increase in the discharge current, up to 40 A, along with a gas pressure decrease down to 0.03 mTorr, which is lowest possible for the discharge realization, and using the discharge system with smaller volume of 6 cm3 (Fig. 2 (2)), results in the almost complete absence of H3+ ions 64
Vacuum 162 (2019) 63–66
A.V. Vizir et al.
discharge was ignited. Supposed that, evidently, the discharge ignites at the same pressure inside the discharge system in both cases, we found that the pressure in the discharge system in regular gas flow mode with open gate valve 11 (Fig. 1) is 70 times greater than in the vacuum vessel. For the conditions of Fig. 2 (1), assuming that the working gas is at room temperature, the neutral molecule density is 7·1014 cm−3. Because the cathode current is ion saturation current, we can consider the cathode as a probe. Using the Bohm equation for ion current density,
j = 0.8(1 + γ ) en
2kTe , Mi
(4)
where γ is the secondary ion-electron emission coefficient, e the electron charge, k Boltzmann's constant, Te the electron temperature, and Mi the ion mass, we can estimate the plasma density n. Assuming, for the conditions of Fig. 2 (1), j = 3 mA/cm2, typical for low-pressure glow discharge values of γ = 0.1 and Te = 6 eV, and Mi = 3 amu, the plasma density is about 1010 cm−3. Thus a rough estimate of the degree of ionization for maximum H3+ ion fraction is 1.5·10−5. Correspondingly, for the conditions for Fig. 2 (2), the hydrogen ionization degree is 0.15. It is expected that during the high-current plasma pulse, relatively high gas ionization degree leads to intense dissociation of H3+ and D3+ ions according Eqs. (2) and (3) as well as of molecular ions. Uncontemplated is the fact that at the certain gas ionization degree which is determined by the specific discharge conditions, plasma ions are almost completely represented by triatomic ions. We suppose that these conditions provide maximal rate for reaction (1) and minimal rates for reactions (2) and (3).
Fig. 3. Deuterium TOF spectra. Discharge system with permanent magnet and volume of 6 cm3. Pressure is 0.03 mTorr. 1 – discharge current 20 mA dc; 2–10 A pulsed.
of the solenoid allows us to study the effect on the plasma ion composition of magnetic induction variation, as illustrated in Fig. 4. Fig. 4 (1) shows the results for the near-optimal values of gas pressure and discharge current for maximizing the H3+ ion fraction. Thus, initially the H3+ ion fraction increases slightly with increasing magnetic field due to magnetic compression of the plasma and increased plasma density. Conversely, Fig. 4 (2) illustrates conditions far from H3+ maximum: much higher current and lower gas pressure. Monatomic ions then prevail even at low magnetic field strength, and have increasing fractional content with increasing magnetic field strength. These results, along with the pressure and the discharge current dependencies of ion composition given in our previous work [13] allow us to suppose that an optimum exists for maximizing the H3+ ion fraction with respect to plasma density and neutral gas density. A simple estimate can be made of the optimum ratio of plasma density to neutral gas density. Evidently, since the gas is fed into the discharge system, a pressure drop occurs at the openings in electrodes 2 and 5 (Fig. 1). The pressure of hydrogen inside the discharge system was determined in the following way. First the minimal gas flow rate required for discharge ignition was experimentally found and the pressure p1 in the vacuum vessel was registered contemporaneously with the discharge ignition. Then the gate valve 11 (Fig. 1) was closed and hydrogen was fed into the vessel slowly so as to avoid any significant pressure drop. The pressure p2 was registered at which the
4. Conclusion The ion composition of the plasma in the hollow-cathode reflex discharge with hydrogen and deuterium as working gases is determined by the specific discharge parameters including discharge current, gas pressure, magnetic field strength and discharge system dimensions. For maximal fraction of H3+ ions, with discharge current 40 mA, pressure 0.3 mTorr, magnetic field 220 G, plasma volume 24 cm3, the degree of ionization is about 1.5·10−5 and the H3+ fraction in the plasma is 95%. Increasing the ionization degree up to approximately 0.15 by changing the discharge parameters results in a monatomic ion fraction of up to 80% with the remainder being molecular H2+ ions. The behavior of deuterium plasma ion composition dependencies on the discharge parameters are similar to that for hydrogen.
Fig. 4. Effect of magnetic induction on plasma ion composition. 1 – dc discharge current 40 mA, pressure 0.25 mTorr; 2 – pulsed discharge current 14 A, pressure 0.15 mTorr. 65
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[5] B.J. McCall, T.R. Geballe, K.H. Hinkle, T. Oka, Detection of H3+ in the diffuse interstellar medium toward Cygnus OB2 No. 12, Science 279 (1998) 1910–1913. [6] T.R. Geballe, B.J. McCall, K.H. Hinkle, T. Oka, Detection of H3+ in the diffuse interstellar medium: the Galactic center and Cygnus OB2 number 12, Astrophys. J. 510 (1999) 251–257. [7] T. Oka, The H3+ ion, in: T.A. Miller, V.E. Bondybey (Eds.), Book Molecular Ions: Spectroscopy, Structure and Chemistry, North Holland Publishing Company, Amsterdam, The Netherlands, 1983, pp. 73–90. [8] T. Oka, Chemistry, astronomy and physics of H3+, Phil. Trans. Roy. Soc. A 370 (2012) 4991–5000. [9] T. Oka, Interstellar H3+, Proc. Natl. Acad. Sci. U.S.A. 103 (33) (2006) 12235–12242. [10] L.P. Veresov, O.L. Veresov, Ion source with a cold magnetron cathode and magnetic plasma compression, Tech. Phys. 48 (2003) 1338–1345. [11] L.P. Veresov, O.L. Veresov, A.F. Chachakov, Hydrogen ion source with a cold magnetron cathode and magnetic plasma compression, Tech. Phys. 51 (2006) 130–133. [12] Yuan Xu, Shixiang Peng, Haitao Ren, Jie Zhao, Chen Jia, Ailin Zhang, Tao Zhang, Zhiyu Guo, Jia’er Chen, High current H2+ and H3+ beam generation by pulsed 2.45 GHz electron cyclotron resonance ion source, Rev. Sci. Instrum. 85 (2014) 02A943. [13] A.V. Vizir, E.M. Oks, M.V. Shandrikov, G. Yu Yushkov, The influence of discharge parameters on the generation of ions H3+ in the source based on reflective discharge with hollow cathode, Tech. Phys. 62 (2017) 380–383. [14] E. Herbst, The astrochemistry of H3+, Phil. Trans. Roy. Soc. Lond. 358 (1774) (2000) 2523–2534. [15] I.G. Brown, J.E. Galvin, R.A. MacGill, R.T. Wright, Improved time-of-flight charge state diagnostic, Rev. Sci. Instrum. 58 (1987) 1589–1592.
Acknowledgments Test experiments with deuterium were supported by Russian Science Foundation, Grant # 18-19-00069. The authors greatly appreciate Dr. Ian Brown (Berkeley Lab) for helpful discussion as well as for English correction. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.vacuum.2019.01.025. References [1] A. Szekeres, S. Alexandrova, A. Paneva, Effect of hydrogen ion implantation on the physical properties of SiO2/Si system, Vacuum 58 (2000) 166–173. [2] Suet To, Emil V. Jelenković, Lyudmila V. Goncharova, Sing Fai Wong, Mechanical characteristics of hydrogen-implanted crystalline silicon after post-implantation annealing, Vacuum 152 (2018) 40–46. [3] J.J. Thomson, Rays of positive electricity, Proc. Roy. Soc. A. London 89 (1913) 1–20. [4] T.R. Geballe, T. Oka, Detection of H+ 3 in interstellar space, Nature 384 (1996) 334–335.
66
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Triatomic hydrogen ion generation in a low-pressure gas discharge a,∗
A.V. Vizir , E.M. Oks a b
a,b
a
, M.V. Shandrikov , G. Yu. Yushkov
T
a
Institute of High Current Electronics, Siberian Branch of Russian Academy of Science, 2/3 Academichesky Ave., 634055, Tomsk, Russia Tomsk State University of Control System and Radioelectronics, 40 Lenin Ave., 634050, Tomsk, Russia
A R T I C LE I N FO
A B S T R A C T
Keywords: Reflex discharge with hollow cathode Plasma ion composition Hydrogen and deuterium triatomic ions
The generation of triatomic H3+ and D3+ ions in a reflex hollow-cathode discharge using hydrogen and deuterium as the working gas, over a wide range of operating pressure, discharge current and magnetic field strength was explored. The fractional ion species composition of the extracted ion beam was monitored using a time-of-flight method. The maximum fraction of H3+ that could be obtained was as high as 95%, with similar results for D3+ ions. It was shown that the degree of ionization of the working gas is one of parameters determining the ion composition of the hollow cathode discharge plasma.
1. Introduction Hydrogen and deuterium ions and their beams are needed for nuclear fusion, medicine and other fields of science and industry. Triatomic hydrogen ions (H3+) can be used for modeling ions of rare isotopes of tritium (T+) and helium (3He+) ions in particle accelerators. Hydrogen ion implantation is crucial in semiconductor electronic and optical devices manufacturing [1], particularly, in so-called “smart cut” process for silicon-on-insulators technology [2]. H3+ ions were first observed by J.J. Thomson in 1913 [3]. Much later, H3+ ions were discovered in the diffuse interstellar medium [4,5] and in the Galactic center [6]. A number of authors [7–9] have reported on studies of specific physical and chemical properties of H3+ ion, which are important in astrophysics. Generation of H3+ in laboratory plasma using a duoplasmatron-like ion source has been demonstrated in Refs. [10,11]. H2+ and H3+ ion beam generation using an electron cyclotron resonance ion source has also been reported in Ref. [12], with the H3+ ion fraction in the beam of 43%. In prior work [13] we have shown that in a reflex discharge with hollow cathode the H3+ ion fraction reaches 70%. Stable H3+ ions are formed in the reaction [14]: H2+ + H2 → H3+ + H
(1)
This reaction is preceded by known processes of hydrogen dissociation and its ionization to H+ and H2+. The H3+ state can decay in the following ways: H3+ + e− → H2+ + H + e− H3+
∗
−
+e
→ H2 + H
+
−
+e
(2) (3)
In this work we consider the effect of the plasma parameters determined by the discharge conditions on the hydrogen and deuterium plasma ion composition. 2. Experimental An outline of the experimental setup is shown in Fig. 1. Hydrogen plasma is generated in a reflex discharge with hollow cathode 1 of inner diameter 6 mm and length 32 mm, and reflex cathode 2 located 23 mm from the output of cathode 1 and anode 3 with inner diameter of 34 mm. The discharge chamber volume is approximately 22 cm3. Another system was also used with smaller dimensions, having a discharge chamber volume of 6 cm3. All electrodes are water-cooled. The glow discharge was fed by two power supplies which can operate separately or simultaneously: a dc power supply (0–100 mA, up to 1500 V) and a pulsed power supply (0–40 A, 150–2000 V, 5–250 ms, 20–5000 pps). Both supplies are at the high accelerating voltage. If the dc power supply is on, even with very low discharge current of 1 mA, it completely eliminates the delay of the pulsed discharge current onset. An axially symmetric magnetic field is generated by a Sm-Co permanent magnet, or alternatively by solenoid 4. The magnetic field strength at the center of the 135-turn solenoid was up to 600 G; the solenoid was driven by a current of up to 20 A for a few seconds, which was sufficiently long for penetration of the field through the metal structure of the discharge system. A low-voltage (up to 20 V) power supply of the solenoid is also placed at the high accelerating voltage using isolation transformer. Cathode 2 has a central opening of 5 mm diameter serving for ion emission. Emitted ions are then accelerated to 10 keV by the positive potential applied to the anode 3. A negative potential of
Corresponding author. E-mail address: [email protected] (A.V. Vizir).
https://doi.org/10.1016/j.vacuum.2019.01.025 Received 20 November 2018; Received in revised form 11 January 2019; Accepted 16 January 2019 Available online 19 January 2019 0042-207X/ © 2019 Elsevier Ltd. All rights reserved.
Vacuum 162 (2019) 63–66
A.V. Vizir et al.
Fig. 1. Simplified schematic of the experimental setup. 1 – hollow cathode, 2 – reflex cathode, 3 – anode, 4 – permanent magnet or solenoid, 5 – beam extraction electrode, 6 – time-of-flight spectrometer gate, 7 – ion detector (secondary electron multiplier), 8 – spectrometer output to oscilloscope, 9 – vacuum vessel, 10 – turbomolecular pump, 11 – gate valve.
−300 V is applied to extraction electrode 5 for suppression of backstreaming electrons. The composition of the ion beam so formed is analyzed by the time-of-flight (TOF) spectrometer [15] comprised of deflecting gate 6, detector 7 and a drift space with length of 1.2 m. The gate consists of 6 circular gaps formed by 2 groups of concentric rings, to one of which a short (100 ns, 2 kV) deflection pulse is applied while the other group is grounded. A central plate with diameter of 6 cm prevents detecting ions when the gate pulse is not applied, and in addition serves as an ion beam current monitor. The use of a secondary electron multiplier (SEM) 7 based on 2 microchannel plates as detector allows ion beam composition measurements over a very wide range of beam current. For high beam current, a Faraday cup (FC) was used, providing confirmation that the SEM data and the FC data match. Charge exchange processes may occur within the ion drift space, and SEM could register neutral particles; but since they are not only not accelerated, but not deflected by the gate, SEM spectra represent directly the discharge plasma ion composition. Therefore, the data given below were taken by SEM, using a beam attenuator for higher currents. 0.3 m diameter, 1.7 m long vacuum vessel 9 is pumped by turbomolecular pump 10 with pumping speed of 900 l/s through gate valve 11. Pressure values given below are pressures in the vacuum chamber unless otherwise specified.
Fig. 2. Hydrogen TOF spectra. 1 – discharge current 40 mA DC, pressure 0.3 mTorr, magnetic induction 220 G, discharge system with solenoid and volume 22 cm3; 2–40 A pulsed, 0.03 mTorr, 600 G, permanent magnet, 6 cm3.
and increase of the H+ fraction up to 80%. We have shown earlier [13] that the variation of H3+ fraction with discharge current has a maximum that shifts to higher current with increasing gas pressure. With the use of a reflex discharge with hollow cathode system having volume of 6 cm3, at the working pressure of 0.02 mTorr, H3+ ion fraction increases from 25 to 70% in the discharge current range 0.8–8 mA, and then reduces down to 2% at 17 A. Further, 10-fold pressure increase up to 0.2 mTorr leads to a proportional increase of the discharge current (80 mA) at which the maximal fraction of H3+ ions (70%) is registered. New experiments show that the current dependence of the deuterium ion composition is similar to that for hydrogen. TOF spectra for deuterium are shown on Fig. 3. The maximal fraction of D3+ 67% ions was reached at 20 mA dc discharge current, and the fraction of D+ of 74% at 25 A pulsed. Since all data for deuterium (Fig. 3) were taken at one working pressure and using one discharge system (6 cm3 volume), unlike the conditions of hydrogen measurements (Fig. 2), the maximal ion fractions reached for deuterium are lower than for hydrogen. D2H+ and H+ ions are also contained in plasma, because of the presence of hydrogen in working and residual gases. Apparently, impeding radial movement of electrons, the increase of magnetic induction leads to the plasma compression to the axis of the discharge system where ions are extracted for further analysis. The use
3. Results and discussion The time-of-flight measurements show that the discharge parameters have a strong effect on the plasma ion composition. Fig. 2 shows TOF oscillograms taken at the output 8 (Fig. 1) of the spectrometer using hydrogen as a working gas, for two very different conditions, lowcurrent DC and high-current pulsed modes. In the pulsed mode, the pulse duration was 40 μs with the repetition rate of 20 pps. No noticeable change in plasma ion composition was observed through pulse duration, as well as with the repetition rate in a range of 20–100 pps. The fraction of each ion species was calculated by time integration of the corresponding oscillogram peak. As Fig. 2 (1) indicates, the H3+ ion fraction reaches 95% for the optimal combination of discharge parameters including discharge current 40 mA, working pressure 0.3 mTorr, and magnetic induction 220 G. Additionally, the use of the discharge system with larger volume of 22 cm3 compared that used earlier in Ref. [13] (6 cm3) resulted in not only to the higher H3+ ion fraction but also total extracted ion beam current which for these conditions was a few milliamperes. A thousand-fold increase in the discharge current, up to 40 A, along with a gas pressure decrease down to 0.03 mTorr, which is lowest possible for the discharge realization, and using the discharge system with smaller volume of 6 cm3 (Fig. 2 (2)), results in the almost complete absence of H3+ ions 64
Vacuum 162 (2019) 63–66
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discharge was ignited. Supposed that, evidently, the discharge ignites at the same pressure inside the discharge system in both cases, we found that the pressure in the discharge system in regular gas flow mode with open gate valve 11 (Fig. 1) is 70 times greater than in the vacuum vessel. For the conditions of Fig. 2 (1), assuming that the working gas is at room temperature, the neutral molecule density is 7·1014 cm−3. Because the cathode current is ion saturation current, we can consider the cathode as a probe. Using the Bohm equation for ion current density,
j = 0.8(1 + γ ) en
2kTe , Mi
(4)
where γ is the secondary ion-electron emission coefficient, e the electron charge, k Boltzmann's constant, Te the electron temperature, and Mi the ion mass, we can estimate the plasma density n. Assuming, for the conditions of Fig. 2 (1), j = 3 mA/cm2, typical for low-pressure glow discharge values of γ = 0.1 and Te = 6 eV, and Mi = 3 amu, the plasma density is about 1010 cm−3. Thus a rough estimate of the degree of ionization for maximum H3+ ion fraction is 1.5·10−5. Correspondingly, for the conditions for Fig. 2 (2), the hydrogen ionization degree is 0.15. It is expected that during the high-current plasma pulse, relatively high gas ionization degree leads to intense dissociation of H3+ and D3+ ions according Eqs. (2) and (3) as well as of molecular ions. Uncontemplated is the fact that at the certain gas ionization degree which is determined by the specific discharge conditions, plasma ions are almost completely represented by triatomic ions. We suppose that these conditions provide maximal rate for reaction (1) and minimal rates for reactions (2) and (3).
Fig. 3. Deuterium TOF spectra. Discharge system with permanent magnet and volume of 6 cm3. Pressure is 0.03 mTorr. 1 – discharge current 20 mA dc; 2–10 A pulsed.
of the solenoid allows us to study the effect on the plasma ion composition of magnetic induction variation, as illustrated in Fig. 4. Fig. 4 (1) shows the results for the near-optimal values of gas pressure and discharge current for maximizing the H3+ ion fraction. Thus, initially the H3+ ion fraction increases slightly with increasing magnetic field due to magnetic compression of the plasma and increased plasma density. Conversely, Fig. 4 (2) illustrates conditions far from H3+ maximum: much higher current and lower gas pressure. Monatomic ions then prevail even at low magnetic field strength, and have increasing fractional content with increasing magnetic field strength. These results, along with the pressure and the discharge current dependencies of ion composition given in our previous work [13] allow us to suppose that an optimum exists for maximizing the H3+ ion fraction with respect to plasma density and neutral gas density. A simple estimate can be made of the optimum ratio of plasma density to neutral gas density. Evidently, since the gas is fed into the discharge system, a pressure drop occurs at the openings in electrodes 2 and 5 (Fig. 1). The pressure of hydrogen inside the discharge system was determined in the following way. First the minimal gas flow rate required for discharge ignition was experimentally found and the pressure p1 in the vacuum vessel was registered contemporaneously with the discharge ignition. Then the gate valve 11 (Fig. 1) was closed and hydrogen was fed into the vessel slowly so as to avoid any significant pressure drop. The pressure p2 was registered at which the
4. Conclusion The ion composition of the plasma in the hollow-cathode reflex discharge with hydrogen and deuterium as working gases is determined by the specific discharge parameters including discharge current, gas pressure, magnetic field strength and discharge system dimensions. For maximal fraction of H3+ ions, with discharge current 40 mA, pressure 0.3 mTorr, magnetic field 220 G, plasma volume 24 cm3, the degree of ionization is about 1.5·10−5 and the H3+ fraction in the plasma is 95%. Increasing the ionization degree up to approximately 0.15 by changing the discharge parameters results in a monatomic ion fraction of up to 80% with the remainder being molecular H2+ ions. The behavior of deuterium plasma ion composition dependencies on the discharge parameters are similar to that for hydrogen.
Fig. 4. Effect of magnetic induction on plasma ion composition. 1 – dc discharge current 40 mA, pressure 0.25 mTorr; 2 – pulsed discharge current 14 A, pressure 0.15 mTorr. 65
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