- Email: [email protected]
A concept for isotope separation in a sheet plasma by using ion cyclotron resonance heating
632
Nuclear Instruments and Methods in Physics Research B37/38 (1989) 632-635 North-Holland, Amsterdam
A CONCEPT FOR ISOTOPE SEPARATION IN A SHEET PLASMA BY USING ION CYCLOTRON RESONANCE HEATING T. NOGUCHI, T. TANIKAWA, and K. TAKAYAMA Deportment of Physics Kanagawa 259-I2, Japan
and
Institute
K. YAMAUCHI, of
Research
and
T. NIHEI Development,
*, H. WATANABE, Tokai
University,
I I I7
K. SUNAKO Kitakaname,
Hiratsuka,
The concept of a novel method for separation of isotopes, the sheet plasma method, is described. A unique feature of this method is to perform isotope separation in a special kind of magnetized plasma: a sheet plasma: ions of a desired isotopic species in this plasma are selectively energized by ion cyclotron resonance heating. Owing to the unique characteristics of sheet plasmas - (i) the guiding centers of all gyrating ions he in the vicinity of the midplane of a plasma and (ii) the plasma thickness is as thin as twice the
mean ion Larmor radius - extremely efficient separation of isotopes is possible.
1. Introduction Ion cyclotron resonance heating (ICRH) of plasmas can be applied to separate isotopes with a small fractional mass difference, AM/M [1,2]. The first successful experiment to demonstrate this idea was carried out by Takayama and his coworkers at the Institute of Plasma Physics, Nagoya [l]. They observed the effect of mass separation in helium plasmas with impurities in a line cusp. Dawson et al. [2] observed the enrichment of 41K in potassium samples collected on cooled tungsten ribbons, which were negatively biased to collect energetic ions, after selective heating of 41K ions by ICRH in a cylindrical plasma with a large cross-sectional area. However, when the plasma density is raised so as to increase the collected amount of desired isotopic species, the effect of collisions among charged particles tends to suppress heating of ions of the desired species, resulting in significant degradation of the separation efficiency. In order to overcome this problem, Takayama has proposed to perform isotope separation by ICRH in a sheet plasma which is a special type of strongly magnetized highly ionized slab plasma [3]. By definition, sheet plasmas should have the following unique characteristics: (i) the guiding centers of all gyrating ions lie in the vicinity of the midplane of a plasma and (ii) the plasma thickness in a direction perpendicular to a uniform-background dc magnetic field is as thin as twice the mean ion Larmor radius (see fig. 1). Because of these characteristics energetic ions in a sheet plasma traverse the dense plasma region only momentarily in each cyclotron gyration. As a result, the adverse effect
* Present address: Nippon Telegraph and Telephone Corporation, Japan.
0168-583X/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
of collisions to isotope separation by ICRH can be negligibly small. We call this new isotope separation technique the sheet plasma method. In this article, we describe the sheet plasma method emphasizing its high efficiency for separation of isotopes.
2. Principle of the sheet plasma method for isotope separation The sheet plasma method for isotope separation is schematically shown in fig. 2. A sheet plasma is allowed to flow between two parallel rf electrodes. The frequency of an externally applied rf electric field is chosen to match the cyclotron frequency of ions of the isotopic species to be separated (or resonant ions). The Larmor radii of resonant ions grow secularly when they travel through the region of the rf electric field, while those of nonresonant ions whose cyclotron frequencies are different from the frequency of the rf electric field remain relatively small. Typical trajectories of resonant ions are
Sheet Plasma
@P PotentId
Well
ion of Ion Guiding Centers
Fig. 1. Schematic diagram of a sheet plasma. The profile of the plasma space potential associated with the sheet plasma is also shown schematically.
633
T. Noguchi et al. / Isotope separation in a sheet plasma
shown in figs. 2a and 2b. Since under the influence of the rf electric field the orbits of resonant and nonresonant ions in a sheet plasma can be well separated from each other in space, the resonant and nonresonant ions can be collected independently. For atoms that can deposit themselves on material surfaces, such as B, K, Cl and U, the length and separation of the rf electrodes are chosen such that resonant ions can hit the rf electrodes on which they are accumulated, while nonresonant ions simply pass through the region of the rf electric field (see fig. 2a). For atoms that cannot easily be adsorbed on material surfaces, such as Ne, Ar and Xe, resonant and nonresonant ions can be separated by using a set of separation plates placed just after the rf electrodes as shown in fig. 2b. When the resonant ions hit the outer surfaces of the separation plates, they are neutralized by capturing electrons from the separation plates. These neutrals are differentially pumped out and subsequently collected by a cryogenic collector. In summary, the advantages of the sheet plasma method for isotope separation are: (i) almost perfect separation of the desired isotopic species is possible as demonstrated by a numerical simulation [4],
Sheet Plasma
RFElectrobe
RF Oscillator
Pump & Cryogenic Collector
Fig. 2. The sheet plasma method for isotope separation. The two techniques described here are: (a) for atoms that can deposit themselves on material surfaces, such as B, K, Cl and U, and (b) for atoms that cannot be easily adsorbed on material surfaces, such as Ne, Ar and Xe. Ions of the desired isotopic species are collected on the rf electrodes in (a), while they are differentially pumped out to be collected by a cryogenic collector in (b). Typical trajectories of resonant ions are shown in (a) and (b), while those of nonresonant ions are not shown in the figures to avoid confusion.
Sheet Plasmas
dF Electrodes RF Oscillator
Fig. 3. Schematic diagram of the sheet plasma method for isotope separation with multiple sheet plasmas. The purpose of the thermionic electron emitters is to obtain the optimum plasma space potential profile in the plasma for isotope separation (see section 3 in the main text).
(ii) isotope separation of various atomic species is possible so long as they can be ionized to form plasmas, and (iii) using a plasma with density over 1Or3 cm-3 is possible without degradation of separation efficiency. However, the sheet plasma method is not immune to problems. First of all, the efficiency for isotope separation can be reduced to a quite low level if the potential well formed by the characteristic plasma space potential associated with a sheet plasma (see fig. 1) is allowed to remain intact *. The effects of this potential well are: (i) to shift the resonant frequency of ions of the desired species to the higher frequency side of the cyclotron frequency of these ions and (ii) to broaden the resonance width for these ions. As a result, the separation efficiency is degraded. In order to eliminate this potential well, we have developed a way to control the plasma space potential in a sheet plasma as described in section 3. Secondly, the collected amount of the desired isotopic species may not be so large due to the relatively small total volume of a sheet plasma even if the plasma density is very high. However, this disadvantage can be remedied by simply stacking many sheet plasmas as shown schematically in fig. 3.
* Larmor radii of ions are much larger than those of electrons, causing a significant charge separation across a sheet plasma. This makes the plasma space potential in the sheet plasma more positive in the boundary regions than in the central region of the plasma. V. NEW IMPLANTATION
EQUIPMENT
634
T. Noguchi et al. / Isotope separation in a sheet plasma
3. Sheet plasma experiment
DBo=3kG Gas : Ar
The apparatus used to produce a sheet plasma (the TU-1 machine) is shown in fig. 4. The plasma production technique is similar to that employed for the TP-D machine at Nagoya [5] and is described in detail elsewhere [6]. The typical experimental parameters in the experimental region of our device for an argon plasma are: ni = 10”-10’2 cmp3, T, = Ti = 3-6 eV and pn = 1O-4 Torr, where ni is the ion plasma density, T, the electron temperature, Ti the ion temperature and p,, the neutral pressure. The typical profiles of the ion plasma density and the plasma space potential in the y-direction are shown in fig. 5. The plasma space potential is higher in the boundary regions than in the central region of the plasma, forming a potential well. Our numerical simulation has indicated that the efficiency for isotope separation is significantly reduced due to this potential well as mentioned in section 2. Therefore, it is essential to reduce the depth of the potential well associated with a sheet plasma in order to achieve highly efficient isotope separation by the sheet plasma method. Introducing thermionic electrons into the boundary regions of the sheet plasma can make the potential well in the sheet plasma shallower since these electrons can neutralize excess positive charges in the boundary regions of the plasma. We have experimentally demonstrated the effectiveness of this scheme as shown in fig. 6. Thermionic electrons are provided by means of the oxide-coated electron emitters (see fig. 6b). It can be seen from fig. 6a that the depth of the potential well, A@r, = @r,,,,, - @r,,,,, can be reduced by increasing the amount of thermionic electrons intro-
Region Rectangular
A?ode
diagram
‘0
: ‘k//it
II -
I
Anode Width
I
1.0
0 y (cm)
1.0
Fig. 5. Profiles of the ion plasma density, a,, and the plasma space potential, 3, across an argon sheet plasma produced in the TU-1 machine. The neutral pressure, pn, in the experimental region is 1.2X10m4 Torr. duced
into
@pmax and space
,
the boundary artin
potential
regions
are the maximum of the sheet
Rectangular Magne?ic Coils \
1 CatLode ( BaO Coated 1
I’\
plasma,
of the plasma, and minimum
where plasma
respectively.
‘-
Gas -
Fig. 4. Schematic
?
I I L Pump
Talget
RF Electrodes
I I 1
’
Pump
of the experimental apparatus, the TU-1 machine. The rectangular used to produce a sheet plasma (see ref. [6] for details).
Pump coils and rectangular
electrodes
are
T. Noguchi et al. / Isotope separation WIa sheet plasma
a)
4. Conclusions
I
’
I
I
I
I
* -60
4.0
-20
Surface Potential of Electron Emitters
b)
635
0
2.0
I
Vb (Volts)
In this article, we have discussed a novel method to separate isotopes efficiently, the sheet plasma method, in which ICRH is employed to preferentially energize the ions of a desired isotopic species in a sheet plasma. Unique attractive features of this method have been pointed out. We have also described the experimental verification that the plasma space potential across a sheet plasma can be controlled by introducing thermionic electrons into the boundary regions of the plasma. This is essential to successfully perform isotope separation in a sheet plasma. The sheet plasma method can also be used to produce impurity-free ion beams. We gratefully acknowledge with Professor S. Kojima.
the useful discussions
References
Anode
SHEET PLASMA
[l] S. Hidekuma, S. Hiroe, T. Watari, T. Shoji, T. Sato and K.
/Therm%onlc Electron Emitters
Ir 2,
I
Fig. 6. Control of the depth of the potential well in a sheet plasma by introducing thermionic electrons into the boundary regions of the plasma. (a) Depth of the potential well, A@r,, versus the surface potential of the thermionic electron emitters, V,. The amount of thermionic electrons emitted from the emitters increases as Vb is decreased. (b) Experimental arrangement.
Takayama, Phys. Rev. Lett. 33 (974) 1537. [2] J.M. Dawson, H.C. Kim, D. Amush, B.D. Fried, R.W. Gould, L.O. Heflinger, CF. Kennel, T.E. Romesser, R.L. Stenzel, A.Y. Wong and R.F. Wuerker, Phys. Rev. Lett. 37 (1976) 1547. [3] K. Takayama and K. Sunako, in: Proc. 4th Japan-Brazil Symp. on Science and Technology (Academia Ci&ncias, S. Paulo, 1984) vol. VI, p. 29. [4] T. Nihei,S. Takeshiro and K. Takayama, in: Proc. 8th Int. Symp. on Plasma Chemistry, eds. K. Akashi and A. Kinbara (ISPC-8, Tokyo, 1987) vol. 1, p. 245. [5] M. Otsuka et al., in: Proc. 7th Int. Conf. on Phenomena in Ionized Gases (Gradjevinska Knjiga Publishing House, Beograd, 1966) vol. 1, p. 420. [6] K. Sunako, K. Yamauchi, T. Noguchi, T. Nihei, T. Tsugueda, H. Watanabe, T. Tanikawa and K. Takayama, these Proceedings (7th Int. Conf. on Ion Implantation Technology, Kyoto, Japan, 1988) Nucl. Instr. and Meth. B37/38 (1989) 636.
V. NEW IMPLANTATION
EQUIPMENT
Nuclear Instruments and Methods in Physics Research B37/38 (1989) 632-635 North-Holland, Amsterdam
A CONCEPT FOR ISOTOPE SEPARATION IN A SHEET PLASMA BY USING ION CYCLOTRON RESONANCE HEATING T. NOGUCHI, T. TANIKAWA, and K. TAKAYAMA Deportment of Physics Kanagawa 259-I2, Japan
and
Institute
K. YAMAUCHI, of
Research
and
T. NIHEI Development,
*, H. WATANABE, Tokai
University,
I I I7
K. SUNAKO Kitakaname,
Hiratsuka,
The concept of a novel method for separation of isotopes, the sheet plasma method, is described. A unique feature of this method is to perform isotope separation in a special kind of magnetized plasma: a sheet plasma: ions of a desired isotopic species in this plasma are selectively energized by ion cyclotron resonance heating. Owing to the unique characteristics of sheet plasmas - (i) the guiding centers of all gyrating ions he in the vicinity of the midplane of a plasma and (ii) the plasma thickness is as thin as twice the
mean ion Larmor radius - extremely efficient separation of isotopes is possible.
1. Introduction Ion cyclotron resonance heating (ICRH) of plasmas can be applied to separate isotopes with a small fractional mass difference, AM/M [1,2]. The first successful experiment to demonstrate this idea was carried out by Takayama and his coworkers at the Institute of Plasma Physics, Nagoya [l]. They observed the effect of mass separation in helium plasmas with impurities in a line cusp. Dawson et al. [2] observed the enrichment of 41K in potassium samples collected on cooled tungsten ribbons, which were negatively biased to collect energetic ions, after selective heating of 41K ions by ICRH in a cylindrical plasma with a large cross-sectional area. However, when the plasma density is raised so as to increase the collected amount of desired isotopic species, the effect of collisions among charged particles tends to suppress heating of ions of the desired species, resulting in significant degradation of the separation efficiency. In order to overcome this problem, Takayama has proposed to perform isotope separation by ICRH in a sheet plasma which is a special type of strongly magnetized highly ionized slab plasma [3]. By definition, sheet plasmas should have the following unique characteristics: (i) the guiding centers of all gyrating ions lie in the vicinity of the midplane of a plasma and (ii) the plasma thickness in a direction perpendicular to a uniform-background dc magnetic field is as thin as twice the mean ion Larmor radius (see fig. 1). Because of these characteristics energetic ions in a sheet plasma traverse the dense plasma region only momentarily in each cyclotron gyration. As a result, the adverse effect
* Present address: Nippon Telegraph and Telephone Corporation, Japan.
0168-583X/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
of collisions to isotope separation by ICRH can be negligibly small. We call this new isotope separation technique the sheet plasma method. In this article, we describe the sheet plasma method emphasizing its high efficiency for separation of isotopes.
2. Principle of the sheet plasma method for isotope separation The sheet plasma method for isotope separation is schematically shown in fig. 2. A sheet plasma is allowed to flow between two parallel rf electrodes. The frequency of an externally applied rf electric field is chosen to match the cyclotron frequency of ions of the isotopic species to be separated (or resonant ions). The Larmor radii of resonant ions grow secularly when they travel through the region of the rf electric field, while those of nonresonant ions whose cyclotron frequencies are different from the frequency of the rf electric field remain relatively small. Typical trajectories of resonant ions are
Sheet Plasma
@P PotentId
Well
ion of Ion Guiding Centers
Fig. 1. Schematic diagram of a sheet plasma. The profile of the plasma space potential associated with the sheet plasma is also shown schematically.
633
T. Noguchi et al. / Isotope separation in a sheet plasma
shown in figs. 2a and 2b. Since under the influence of the rf electric field the orbits of resonant and nonresonant ions in a sheet plasma can be well separated from each other in space, the resonant and nonresonant ions can be collected independently. For atoms that can deposit themselves on material surfaces, such as B, K, Cl and U, the length and separation of the rf electrodes are chosen such that resonant ions can hit the rf electrodes on which they are accumulated, while nonresonant ions simply pass through the region of the rf electric field (see fig. 2a). For atoms that cannot easily be adsorbed on material surfaces, such as Ne, Ar and Xe, resonant and nonresonant ions can be separated by using a set of separation plates placed just after the rf electrodes as shown in fig. 2b. When the resonant ions hit the outer surfaces of the separation plates, they are neutralized by capturing electrons from the separation plates. These neutrals are differentially pumped out and subsequently collected by a cryogenic collector. In summary, the advantages of the sheet plasma method for isotope separation are: (i) almost perfect separation of the desired isotopic species is possible as demonstrated by a numerical simulation [4],
Sheet Plasma
RFElectrobe
RF Oscillator
Pump & Cryogenic Collector
Fig. 2. The sheet plasma method for isotope separation. The two techniques described here are: (a) for atoms that can deposit themselves on material surfaces, such as B, K, Cl and U, and (b) for atoms that cannot be easily adsorbed on material surfaces, such as Ne, Ar and Xe. Ions of the desired isotopic species are collected on the rf electrodes in (a), while they are differentially pumped out to be collected by a cryogenic collector in (b). Typical trajectories of resonant ions are shown in (a) and (b), while those of nonresonant ions are not shown in the figures to avoid confusion.
Sheet Plasmas
dF Electrodes RF Oscillator
Fig. 3. Schematic diagram of the sheet plasma method for isotope separation with multiple sheet plasmas. The purpose of the thermionic electron emitters is to obtain the optimum plasma space potential profile in the plasma for isotope separation (see section 3 in the main text).
(ii) isotope separation of various atomic species is possible so long as they can be ionized to form plasmas, and (iii) using a plasma with density over 1Or3 cm-3 is possible without degradation of separation efficiency. However, the sheet plasma method is not immune to problems. First of all, the efficiency for isotope separation can be reduced to a quite low level if the potential well formed by the characteristic plasma space potential associated with a sheet plasma (see fig. 1) is allowed to remain intact *. The effects of this potential well are: (i) to shift the resonant frequency of ions of the desired species to the higher frequency side of the cyclotron frequency of these ions and (ii) to broaden the resonance width for these ions. As a result, the separation efficiency is degraded. In order to eliminate this potential well, we have developed a way to control the plasma space potential in a sheet plasma as described in section 3. Secondly, the collected amount of the desired isotopic species may not be so large due to the relatively small total volume of a sheet plasma even if the plasma density is very high. However, this disadvantage can be remedied by simply stacking many sheet plasmas as shown schematically in fig. 3.
* Larmor radii of ions are much larger than those of electrons, causing a significant charge separation across a sheet plasma. This makes the plasma space potential in the sheet plasma more positive in the boundary regions than in the central region of the plasma. V. NEW IMPLANTATION
EQUIPMENT
634
T. Noguchi et al. / Isotope separation in a sheet plasma
3. Sheet plasma experiment
DBo=3kG Gas : Ar
The apparatus used to produce a sheet plasma (the TU-1 machine) is shown in fig. 4. The plasma production technique is similar to that employed for the TP-D machine at Nagoya [5] and is described in detail elsewhere [6]. The typical experimental parameters in the experimental region of our device for an argon plasma are: ni = 10”-10’2 cmp3, T, = Ti = 3-6 eV and pn = 1O-4 Torr, where ni is the ion plasma density, T, the electron temperature, Ti the ion temperature and p,, the neutral pressure. The typical profiles of the ion plasma density and the plasma space potential in the y-direction are shown in fig. 5. The plasma space potential is higher in the boundary regions than in the central region of the plasma, forming a potential well. Our numerical simulation has indicated that the efficiency for isotope separation is significantly reduced due to this potential well as mentioned in section 2. Therefore, it is essential to reduce the depth of the potential well associated with a sheet plasma in order to achieve highly efficient isotope separation by the sheet plasma method. Introducing thermionic electrons into the boundary regions of the sheet plasma can make the potential well in the sheet plasma shallower since these electrons can neutralize excess positive charges in the boundary regions of the plasma. We have experimentally demonstrated the effectiveness of this scheme as shown in fig. 6. Thermionic electrons are provided by means of the oxide-coated electron emitters (see fig. 6b). It can be seen from fig. 6a that the depth of the potential well, A@r, = @r,,,,, - @r,,,,, can be reduced by increasing the amount of thermionic electrons intro-
Region Rectangular
A?ode
diagram
‘0
: ‘k//it
II -
I
Anode Width
I
1.0
0 y (cm)
1.0
Fig. 5. Profiles of the ion plasma density, a,, and the plasma space potential, 3, across an argon sheet plasma produced in the TU-1 machine. The neutral pressure, pn, in the experimental region is 1.2X10m4 Torr. duced
into
@pmax and space
,
the boundary artin
potential
regions
are the maximum of the sheet
Rectangular Magne?ic Coils \
1 CatLode ( BaO Coated 1
I’\
plasma,
of the plasma, and minimum
where plasma
respectively.
‘-
Gas -
Fig. 4. Schematic
?
I I L Pump
Talget
RF Electrodes
I I 1
’
Pump
of the experimental apparatus, the TU-1 machine. The rectangular used to produce a sheet plasma (see ref. [6] for details).
Pump coils and rectangular
electrodes
are
T. Noguchi et al. / Isotope separation WIa sheet plasma
a)
4. Conclusions
I
’
I
I
I
I
* -60
4.0
-20
Surface Potential of Electron Emitters
b)
635
0
2.0
I
Vb (Volts)
In this article, we have discussed a novel method to separate isotopes efficiently, the sheet plasma method, in which ICRH is employed to preferentially energize the ions of a desired isotopic species in a sheet plasma. Unique attractive features of this method have been pointed out. We have also described the experimental verification that the plasma space potential across a sheet plasma can be controlled by introducing thermionic electrons into the boundary regions of the plasma. This is essential to successfully perform isotope separation in a sheet plasma. The sheet plasma method can also be used to produce impurity-free ion beams. We gratefully acknowledge with Professor S. Kojima.
the useful discussions
References
Anode
SHEET PLASMA
[l] S. Hidekuma, S. Hiroe, T. Watari, T. Shoji, T. Sato and K.
/Therm%onlc Electron Emitters
Ir 2,
I
Fig. 6. Control of the depth of the potential well in a sheet plasma by introducing thermionic electrons into the boundary regions of the plasma. (a) Depth of the potential well, A@r,, versus the surface potential of the thermionic electron emitters, V,. The amount of thermionic electrons emitted from the emitters increases as Vb is decreased. (b) Experimental arrangement.
Takayama, Phys. Rev. Lett. 33 (974) 1537. [2] J.M. Dawson, H.C. Kim, D. Amush, B.D. Fried, R.W. Gould, L.O. Heflinger, CF. Kennel, T.E. Romesser, R.L. Stenzel, A.Y. Wong and R.F. Wuerker, Phys. Rev. Lett. 37 (1976) 1547. [3] K. Takayama and K. Sunako, in: Proc. 4th Japan-Brazil Symp. on Science and Technology (Academia Ci&ncias, S. Paulo, 1984) vol. VI, p. 29. [4] T. Nihei,S. Takeshiro and K. Takayama, in: Proc. 8th Int. Symp. on Plasma Chemistry, eds. K. Akashi and A. Kinbara (ISPC-8, Tokyo, 1987) vol. 1, p. 245. [5] M. Otsuka et al., in: Proc. 7th Int. Conf. on Phenomena in Ionized Gases (Gradjevinska Knjiga Publishing House, Beograd, 1966) vol. 1, p. 420. [6] K. Sunako, K. Yamauchi, T. Noguchi, T. Nihei, T. Tsugueda, H. Watanabe, T. Tanikawa and K. Takayama, these Proceedings (7th Int. Conf. on Ion Implantation Technology, Kyoto, Japan, 1988) Nucl. Instr. and Meth. B37/38 (1989) 636.
V. NEW IMPLANTATION
EQUIPMENT