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Production of negative lithium ions from the surface immersed in magnetic-field-free plasmas
Nuclear Instruments and Methods in Physics Research
132
B37/38 (1989) 132-135 North-Holland, Amsterdam
PRODUCTION OF NEGATIVE LITHIUM IONS FROM THE SURFACE IN MAGNETIC-FIELD-FREE PLASMAS Motoi
WADA
Department
Mamiko Insfitute
and
of Electronics,
Hiroshi Doshisha
IMMERSED
TSUDA University, Kamigyoku,
Kyoto 602, Japan
SASAO of Plasma Physics, Nagoya Uniuemty,
Chikuraky
Nagoya 464, Japan
A beam of negative lithium ions was produced from a 2.5 cm diameter Li metal-coated surface immersed in a magnetic-field-free plasma. The Li- current produced at the surface increased as the negative bias applied to the surface was increased, but did not increase in proportion to the ion current density irradiating the surface. Among He, Ne, Ar, Kr, and Xe plasmas, the Li- yield was higher for a plasma of heavier inert gas, and was also dependent upon the condition of the production surface. In particular, the Cs coverage and the oxygen adsorption on the surface seemed to play an important role to enhance the Li- sputtering yield.
2. The multicusp Li - ion source
1. Introduction
A beam of either He- or Li- with a current exceeding 10 mA is necessary [l] to perform diagnostics of alpha particles produced in a magnetically confined plasma for nuclear fusion research. Beam currents of LiC as large as 1 and 5 PA were reported to have been produced by the self-extraction source [2] and the sputter negative ion source [3], respectively. On the other hand, a self-extraction ion source that produces a H- beam current of the order of several amperes has been developed for plasma heating [4]. This kind of ion source has a large area negative ion production surface immersed in a homogeneous magnetic-field-free plasma, and this scheme was applied to produce Li- ions by immersing a tungsten surface in a Li plasma [5]. Also, LiC can be produced by volume processes in a Li plasma [6]. For the last two methods, a dense plasma of Li is necessary. However, as the neutral pressure of the Li vapor in the ion source increases, the chance to cause a failure of the electrical insulation for the beam acceleration increases. If we could produce LiC ions by plasma sputtering of a Li metal-coated surface by noble gases like Ar or Xe, then the chance that neutral Li vapor will drain out of the source could be substantially decreased. In this case, the neutral Li vapor pressure can be kept low by cooling both the chamber wall and the Li- production surface. Thus, we investigated the possibility of producing large area Li- ion beams by immersing the Li metal surface in a magnetic-field-free plasma. 0168-583X/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
A schematic diagram of the experimental arrangement is shown in fig. 1. A uniform quiescent plasma was produced in the 23 cm diameter, 17 cm long stainl&s steel chamber surrounded by 16 columns of ferrite magnets. The cusp magnetic field formed by these magnets increased the plasma confinement, but did not penetrate into the central region of the chamber. A pair of 0.02 cm diameter tungsten discharge filaments, and a 2.5 cm diameter stainless-steel target cooled with iced water were placed in the central region of the ion
errite Magnets
s Dispenser
M&i”:-i Fig. 1. The experimental
arrangement.
hf. Wada Edal. / Production of negative lithium ions
source, where virtually no magnetic field was present. A discharge voltage of 40-110 V was applied between the chamber wall anode and the hot tungsten filament cathode. He, Ne, Ar, Kr, and Xe were introduced into the chamber to sustain the arc discharge. The neutral pressure of these noble gases were usually kept at 3 x 10e4 Torr on the ionization gauge reading. A thick coating of Li metal was made on the target by evaporating Li metal from the heating cup. The heating cup was retracted from the central region of the ion source after the Li coating, and no Li was supplied to the target during the ion source discharge. The target was biased negatively with respect to the chamber wall. With this bias potential, positive ions in the plasma were accelerated across the sheath in front of the target to hit the Li metal surface, and the Li- ions formed in the process of sputtering were then accelerated back from the surface to the plasma. After traversing 6.5 cm in the plasma, the self-extracted Lii ion beam was detected by the magnetic momentum analyzer. Cesium dispensers (SAES getter) were used to introduce Cs directly into the discharge. Also, Cs was directly evaporated onto the Li surface from the dispensers positioned in front of the target mounted on a movable current feedthrough
3. Measurements In fig. 2, the mass spectrum of the extracted beam of surface-produced negative ions is shown. As shown in the figure, a large amount of H- was present, though no hydrogen gas was deliberately introduced. Also, impurity negative ions at mass 8, 9, 12, and 13 were observed on the mass spectrum. This species composition of the extracted negative ion beam is similar to the one reported by Steams and Pyle [5]. However, the spectrum
Analyzer Magnet Cur&t
( A)
Fig. 2. Mass spectrum of the negative ions formed on the Li metal surface.
133
shown in fig. 2 exhibits substantially higher peaks of O- and OH- compared with that reported by Stearns and Pyle. The reason for this difference is probably due to the presence of the oxygen adlayer on the Li surface. In the experiment done by Stearns and Pyle, the Li evaporator was kept hot to produce Li plasma in front of the tungsten metal surface. This Li vapor could have acted as the getter to pump oxygen out of the chamber, so the amount of oxygen on their surface would be less than that of the present experiment. In our experiment, the O- and OH- peaks gradually decreased while the Li surface was bombarded with inert gas ions for a long time with a high discharge power. Meanwhile, the Lipeak had also decreased, in accordance with the decreasing O- and OH- peaks, and eventually all negative ion current diminished below the detection limit of the measurement system. When the arc had been turned off at this condition, the base pressure in the ion source chamber measured with the ion gauge was less than 5 X lo-’ Torr, while the ion gauge reading of the ion source chamber was usually 1.2 x lo-” Torr. From this, it can be inferred that the fresh Li sputtered from the surface had coated the inner wall of the ion source chamber and pumped out the residual gas. Some portion of the residual gas should have been oxygen and/or other electronegative elements. The work function of the Li surface may become lower when it adsorbs electronegative elements, and the negative ion sputtering yield is reported to be higher for a lower work function surface [7]. Thus, the higher work function of the well-sputtered Li target surface in a low residual gas environment could have resulted the low Lii yield. Because of the above stated phenomena, it was occasionally observed that Li decreased in the course of the experiment. To avoid this gradual drift of Lip signal, a small amount of pure oxygen gas (4 x 10m6 Torr on the ion gauge reading) was intentionally introduced into the ion source chamber to maintain the oxygen adsorption on the surface. With this treatment, the neutral pressure reading by the ion gauge had been constant at about 1.2 X 10m6 Torr after shutting off the gas flow and the arc power supply. Fig. 3 shows the dependence of the surface-produced Lii current detected by the mass analyzer through a 1 mm diameter collimator on the target current, when the Li surface was coated with Cs and maintained at - 700 V with respect to the ion source chamber wall. The Lii current increased with the target current up to some critical value. When the yield is defined by the detected Li- current density divided by the target current density, the maximum yield is observed for every kind of gas near the knee of each line in fig. 3. The lower yield at the higher target current is probably due to the higher work function of the surface cleaned by plasma bombardments. Fig. 4 shows the dependence of the Lii yield on the target bias at the target current that gives a II. ION SOURCES
M. Wada et al. / Production
134
of negative lithium ions r
Target
Current
(mA)
Fig. 3. Dependence of Li- ion current on the target current. The target was biased -700 V with respect to the ion source anode.
maximum Li- yield for the corresponding gas. As shown in the figure, the Li- yield increased as the negative bias to the target was increased. This dependence of Liyield on the target bias was the same for all kinds of gases, but it seemed slightly more pronounced for heavier gases. When the target current was adjusted to give the maximum Li- yield for the corresponding plasma, the yield usualiy showed a dependence nearly proportional to the mass number of the gas. Fig. 5 is an
v He 0 Ne b Ar (3 Kr 0 Xe
Fig. 5. Li- ion yield plotted as a function of the mass of the plasma gas. The target was biased - 700 V with respect to the ion source anode, and the target current was adjusted to give the maximum Li- yield.
example of this dependence on the mass of the gas for Li- emission from the surface, maintained at -700 V with respect to the plasma. This effect may be due to the higher sputtering yields of Li by heavier gases. The occurrence of the direct arc between the target and the chamber wall was observed occasionally, when the discharge power or the target bias voltage was high, and in particular, when Cs was directly introduced into the discharge. For this reason, the Li- yield had not been optimized by varying the Cs density in a steady state discharge. The Li surface was first coated with a thick layer of Cs, and then the Li- current was observed, while the Cs layer was removed by the plasma particle bombardment. With this procedure, a maximum current density of about 0.01 PA/cm’ was obtained for Xe discharge. This value was observed at 0.2 mA target current, and can probably be increased by properly adjusting the surface conditions.
4. Summary A beam of Li- was produced by a sputtering process from a Li metal surface immersed in a magnetic-fieldfree plasma. Li- current density can be enhanced by increasing the negative bias to the production surface, but it does not increase in proportion to the current Rowing into the Li surface. This can be attributed to the change of the Li surface condition, so the control of the surface condition is important to the further enhancement of the Li- yield. Target
Bias (V)
Fig. 4. Dependence of the Li- ion yiefd on the target bias. The target current was adjusted to give maximum Li- yield.
This work was performed under the Collaborating Research Program at the Institute of Plasma Physics, Nagoya University, Japan.
M. Wada et al. / Production
References
[I] M. Sasao and K.N. Sato, Fusion Technol. 10 (1986) 1. [2] P. Tykesson, H.H. Andersen and J. Heinemeier, IEEE Trans. Nucl. Sci. NS-23 (1976) 1104. [3] R. Middleton, IEEE Trans. Nucl. Sci. NS-23 (1976) 1098. [4] J.W. Kwan, G.D. Ackerman, O.A. Anderson, C.F. Chan, W.S. Cooper, G.J. de Vries, A.F. Lietzke, L. Soroka and W.F. Steele, Rev. Sci. Instr. 57 (1986) 831;
ofnegative lithium ions
135
K.W. Ehlers and K.N. Leung, Rev. Sci. Instr. 51 (1980) 721. [5] J.W. Steams and R.V. Pyle, J. Vat. Sci. Technol. A3 (1985) 1243. [6] S.R. Walther, K.N. Leung and W.B. Kunkel, Appl. Phys. Lett. 51 (1987) 566. [7] M.L. Yu, Phys. Rev. Lett. 49 (1978) 574.
II. ION SOURCES
132
B37/38 (1989) 132-135 North-Holland, Amsterdam
PRODUCTION OF NEGATIVE LITHIUM IONS FROM THE SURFACE IN MAGNETIC-FIELD-FREE PLASMAS Motoi
WADA
Department
Mamiko Insfitute
and
of Electronics,
Hiroshi Doshisha
IMMERSED
TSUDA University, Kamigyoku,
Kyoto 602, Japan
SASAO of Plasma Physics, Nagoya Uniuemty,
Chikuraky
Nagoya 464, Japan
A beam of negative lithium ions was produced from a 2.5 cm diameter Li metal-coated surface immersed in a magnetic-field-free plasma. The Li- current produced at the surface increased as the negative bias applied to the surface was increased, but did not increase in proportion to the ion current density irradiating the surface. Among He, Ne, Ar, Kr, and Xe plasmas, the Li- yield was higher for a plasma of heavier inert gas, and was also dependent upon the condition of the production surface. In particular, the Cs coverage and the oxygen adsorption on the surface seemed to play an important role to enhance the Li- sputtering yield.
2. The multicusp Li - ion source
1. Introduction
A beam of either He- or Li- with a current exceeding 10 mA is necessary [l] to perform diagnostics of alpha particles produced in a magnetically confined plasma for nuclear fusion research. Beam currents of LiC as large as 1 and 5 PA were reported to have been produced by the self-extraction source [2] and the sputter negative ion source [3], respectively. On the other hand, a self-extraction ion source that produces a H- beam current of the order of several amperes has been developed for plasma heating [4]. This kind of ion source has a large area negative ion production surface immersed in a homogeneous magnetic-field-free plasma, and this scheme was applied to produce Li- ions by immersing a tungsten surface in a Li plasma [5]. Also, LiC can be produced by volume processes in a Li plasma [6]. For the last two methods, a dense plasma of Li is necessary. However, as the neutral pressure of the Li vapor in the ion source increases, the chance to cause a failure of the electrical insulation for the beam acceleration increases. If we could produce LiC ions by plasma sputtering of a Li metal-coated surface by noble gases like Ar or Xe, then the chance that neutral Li vapor will drain out of the source could be substantially decreased. In this case, the neutral Li vapor pressure can be kept low by cooling both the chamber wall and the Li- production surface. Thus, we investigated the possibility of producing large area Li- ion beams by immersing the Li metal surface in a magnetic-field-free plasma. 0168-583X/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
A schematic diagram of the experimental arrangement is shown in fig. 1. A uniform quiescent plasma was produced in the 23 cm diameter, 17 cm long stainl&s steel chamber surrounded by 16 columns of ferrite magnets. The cusp magnetic field formed by these magnets increased the plasma confinement, but did not penetrate into the central region of the chamber. A pair of 0.02 cm diameter tungsten discharge filaments, and a 2.5 cm diameter stainless-steel target cooled with iced water were placed in the central region of the ion
errite Magnets
s Dispenser
M&i”:-i Fig. 1. The experimental
arrangement.
hf. Wada Edal. / Production of negative lithium ions
source, where virtually no magnetic field was present. A discharge voltage of 40-110 V was applied between the chamber wall anode and the hot tungsten filament cathode. He, Ne, Ar, Kr, and Xe were introduced into the chamber to sustain the arc discharge. The neutral pressure of these noble gases were usually kept at 3 x 10e4 Torr on the ionization gauge reading. A thick coating of Li metal was made on the target by evaporating Li metal from the heating cup. The heating cup was retracted from the central region of the ion source after the Li coating, and no Li was supplied to the target during the ion source discharge. The target was biased negatively with respect to the chamber wall. With this bias potential, positive ions in the plasma were accelerated across the sheath in front of the target to hit the Li metal surface, and the Li- ions formed in the process of sputtering were then accelerated back from the surface to the plasma. After traversing 6.5 cm in the plasma, the self-extracted Lii ion beam was detected by the magnetic momentum analyzer. Cesium dispensers (SAES getter) were used to introduce Cs directly into the discharge. Also, Cs was directly evaporated onto the Li surface from the dispensers positioned in front of the target mounted on a movable current feedthrough
3. Measurements In fig. 2, the mass spectrum of the extracted beam of surface-produced negative ions is shown. As shown in the figure, a large amount of H- was present, though no hydrogen gas was deliberately introduced. Also, impurity negative ions at mass 8, 9, 12, and 13 were observed on the mass spectrum. This species composition of the extracted negative ion beam is similar to the one reported by Steams and Pyle [5]. However, the spectrum
Analyzer Magnet Cur&t
( A)
Fig. 2. Mass spectrum of the negative ions formed on the Li metal surface.
133
shown in fig. 2 exhibits substantially higher peaks of O- and OH- compared with that reported by Stearns and Pyle. The reason for this difference is probably due to the presence of the oxygen adlayer on the Li surface. In the experiment done by Stearns and Pyle, the Li evaporator was kept hot to produce Li plasma in front of the tungsten metal surface. This Li vapor could have acted as the getter to pump oxygen out of the chamber, so the amount of oxygen on their surface would be less than that of the present experiment. In our experiment, the O- and OH- peaks gradually decreased while the Li surface was bombarded with inert gas ions for a long time with a high discharge power. Meanwhile, the Lipeak had also decreased, in accordance with the decreasing O- and OH- peaks, and eventually all negative ion current diminished below the detection limit of the measurement system. When the arc had been turned off at this condition, the base pressure in the ion source chamber measured with the ion gauge was less than 5 X lo-’ Torr, while the ion gauge reading of the ion source chamber was usually 1.2 x lo-” Torr. From this, it can be inferred that the fresh Li sputtered from the surface had coated the inner wall of the ion source chamber and pumped out the residual gas. Some portion of the residual gas should have been oxygen and/or other electronegative elements. The work function of the Li surface may become lower when it adsorbs electronegative elements, and the negative ion sputtering yield is reported to be higher for a lower work function surface [7]. Thus, the higher work function of the well-sputtered Li target surface in a low residual gas environment could have resulted the low Lii yield. Because of the above stated phenomena, it was occasionally observed that Li decreased in the course of the experiment. To avoid this gradual drift of Lip signal, a small amount of pure oxygen gas (4 x 10m6 Torr on the ion gauge reading) was intentionally introduced into the ion source chamber to maintain the oxygen adsorption on the surface. With this treatment, the neutral pressure reading by the ion gauge had been constant at about 1.2 X 10m6 Torr after shutting off the gas flow and the arc power supply. Fig. 3 shows the dependence of the surface-produced Lii current detected by the mass analyzer through a 1 mm diameter collimator on the target current, when the Li surface was coated with Cs and maintained at - 700 V with respect to the ion source chamber wall. The Lii current increased with the target current up to some critical value. When the yield is defined by the detected Li- current density divided by the target current density, the maximum yield is observed for every kind of gas near the knee of each line in fig. 3. The lower yield at the higher target current is probably due to the higher work function of the surface cleaned by plasma bombardments. Fig. 4 shows the dependence of the Lii yield on the target bias at the target current that gives a II. ION SOURCES
M. Wada et al. / Production
134
of negative lithium ions r
Target
Current
(mA)
Fig. 3. Dependence of Li- ion current on the target current. The target was biased -700 V with respect to the ion source anode.
maximum Li- yield for the corresponding gas. As shown in the figure, the Li- yield increased as the negative bias to the target was increased. This dependence of Liyield on the target bias was the same for all kinds of gases, but it seemed slightly more pronounced for heavier gases. When the target current was adjusted to give the maximum Li- yield for the corresponding plasma, the yield usualiy showed a dependence nearly proportional to the mass number of the gas. Fig. 5 is an
v He 0 Ne b Ar (3 Kr 0 Xe
Fig. 5. Li- ion yield plotted as a function of the mass of the plasma gas. The target was biased - 700 V with respect to the ion source anode, and the target current was adjusted to give the maximum Li- yield.
example of this dependence on the mass of the gas for Li- emission from the surface, maintained at -700 V with respect to the plasma. This effect may be due to the higher sputtering yields of Li by heavier gases. The occurrence of the direct arc between the target and the chamber wall was observed occasionally, when the discharge power or the target bias voltage was high, and in particular, when Cs was directly introduced into the discharge. For this reason, the Li- yield had not been optimized by varying the Cs density in a steady state discharge. The Li surface was first coated with a thick layer of Cs, and then the Li- current was observed, while the Cs layer was removed by the plasma particle bombardment. With this procedure, a maximum current density of about 0.01 PA/cm’ was obtained for Xe discharge. This value was observed at 0.2 mA target current, and can probably be increased by properly adjusting the surface conditions.
4. Summary A beam of Li- was produced by a sputtering process from a Li metal surface immersed in a magnetic-fieldfree plasma. Li- current density can be enhanced by increasing the negative bias to the production surface, but it does not increase in proportion to the current Rowing into the Li surface. This can be attributed to the change of the Li surface condition, so the control of the surface condition is important to the further enhancement of the Li- yield. Target
Bias (V)
Fig. 4. Dependence of the Li- ion yiefd on the target bias. The target current was adjusted to give maximum Li- yield.
This work was performed under the Collaborating Research Program at the Institute of Plasma Physics, Nagoya University, Japan.
M. Wada et al. / Production
References
[I] M. Sasao and K.N. Sato, Fusion Technol. 10 (1986) 1. [2] P. Tykesson, H.H. Andersen and J. Heinemeier, IEEE Trans. Nucl. Sci. NS-23 (1976) 1104. [3] R. Middleton, IEEE Trans. Nucl. Sci. NS-23 (1976) 1098. [4] J.W. Kwan, G.D. Ackerman, O.A. Anderson, C.F. Chan, W.S. Cooper, G.J. de Vries, A.F. Lietzke, L. Soroka and W.F. Steele, Rev. Sci. Instr. 57 (1986) 831;
ofnegative lithium ions
135
K.W. Ehlers and K.N. Leung, Rev. Sci. Instr. 51 (1980) 721. [5] J.W. Steams and R.V. Pyle, J. Vat. Sci. Technol. A3 (1985) 1243. [6] S.R. Walther, K.N. Leung and W.B. Kunkel, Appl. Phys. Lett. 51 (1987) 566. [7] M.L. Yu, Phys. Rev. Lett. 49 (1978) 574.
II. ION SOURCES