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On magnetically confined plasmas used as neutron sources
NUCLEAR
INSTRUMENTS
AND
METHODS
I29
(1975)
27-30;
©
NORTH-HOLLAND
PUBLISHING
CO.
O N M A G N E T I C A L L Y C O N F I N E D P L A S M A S U S E D AS N E U T R O N S O U R C E S
B. L E H N E R T
Royal Institute of Technology, S-10044 Stockholm, Sweden Received 1 A u g u s t 1975 It m a y become possible to create powerful n e u t r o n sources from a n u m b e r o f m a g n e t i c p l a s m a confinement schemes being investigated in fusion research. A m o n g the one- and twoe n e r g y - c o m p o n e n t p l a s m a s discussed so far, special attention is d r a w n in this p a p e r to quasi-static systems such as the Spherator with magnetically shielded supports, as well as to rotating
p l a s m a systems such as the closed T o r n a d o bottle and open bottles. Provided that sufficiently high ion and eIectron temperatures can be reached, these systems m a y be operated at higher p l a s m a densities a n d smaller dimensions t h a n those o f mirror a n d T o k a m a k n e u t r o n sources.
1. Introduction
possible ways of creating neutron sources from present magnetic confinement schemes, with special attention to systems which can be operated with plasmas being "impermeable" to thermalized neutral particles. Some alternatives are given by fig. 1 where broken frames denote special cases of steady or quasi-steady operation which can take place in the "impermeable" ion density range. Such operation has already been demonstrated experimentally with devices of moderately large size, sometimes at two orders of magnitude larger ion densities than those so far reached in Tokamak experiments6-S).
Before reaching the final aim of a self-sustained reactor, fusion research is likely to result in the creation of powerful neutron sources, being among other things important to reactor technological research and to the development of fusion-fission hybrid reactors. In this connection considerable work has recently been devoted to the problems of two-energy-component systems in which energetic deuteron beams interact with a tritium target plasma1-5). A short discussion will be given in this paper on the
Neutron sources
I
One-energy-component plasma
Quasi-static
Two-energy-component
r R°tat'ngP lasma J C,ose, O,..
I
I
Closed and open bottles
bottles / / /
/
/
/
/
/ /
/ /
/
/
---z---Tornado
1 I I I I
J
J I bottles
F--/-----] I Special I end inI sulators I
L
/
/
]
I I
~
Neutral beam
]
Quasi-static targets
j
1
[~on beam
Rotating targets
n
~
tles
[
I I
r--l-n I I Hf i I plugs II II J [. . . . .
1
1 I i l I
r
----L--__
I Spherator
I I
1
i
I
I I [
I I J
\ \
r---L---i F---IX------I II Tornado I I Electrons Ii I I [ heated I f I I I L . . . . .
J
L
J
Fig. 1. Magnetically confined plasmas used as n e u t r o n sources. Broken frames denote special cases discussed in this paper.
27
28
B. L E H N E R T
2. One-energy-component systems Most conventional schemes of closed and open magnetic bottles consist of one-energy-component systems with no essential fluid motion of the plasma. In addition, the crossed-field technique provides a simple method of creating fully ionized dense rotating plasmas for which the plasma body becomes "impermeable" to neutral particles being nearly in thermal equilibrium with the plasma ions8). The main difficulty with rotating plasmas so far is the velocity limitation effect which is connected with the ionization energy and leads to a temperature limitation at some 10 s K 6). It arises during quasi-steady operation in open bottles and is associated with a strong plasma-neutral gas interaction at the end insulators. To reach temperatures of interest to neutron source operation, the possibilities described in the following subsections should be considered. 2.1. CLOSEDROTATINGPLASMABOTTLES The internal spiral coil trap " T o r n a d o " developed in Leningrad 9'1°) provides a possibility of rotating plasma operation without end insulators11). As a consequence, supercritical velocities and high plasma temperatures are expected to be reached in this type of trap. The spacing between the windings of the outer spiral which encloses the confinement region can be made large enough for the emitted neutrons to pass freely from the plasma to a sample. The mutual forces between adjacent layers of the spiral coil windings introduce a technical problem with respect to steady state operation of Tornado traps on full reactor scale. For the use merely as neutron sources on a smaller scale, this problem has to be further investigated, and pulsed operation should in any case become possible. 2.2. OPEN ROTAa-INGPLASMA BOTTLES To reach high temperatures in open rotating plasma devices, two approaches have so far been suggested: 1) The critical velocity mechanism may possibly be suppressed by means of special arrangements at the end insulator surfaces such as by concentric metal rings*2). Even if there are some experimental indications of this mechanism to be connected with the boundary conditions at the end insulators, it has so far not been possible to overcome the velocity limitation in a steady state 6,13). 2) Recently Bonnevier ~ ) has suggested that it may become possible to create a neutron source by reducing the end losses through the joint action
of the centrifugal force and high-frequency plugs. Possibly this approach has to be combined with a decrease of the plasma density, as well as with anisotropic temperature distributions.
3. Two-energy-component systems Target plasmas have so far been discussed mainly in connection with magnetic mirrors and Tokamaks~-5), partly at maximum plasma densities n ~ 102o m -3, i.e. in a range being close to the transition between permeable and impermeable plasmasS). Here we shall include confinement systems which can be operated under laboratory conditions at densities n~> 102~ m - 3 . 3.1. THE HIGH-ENERGY COMPONENT
3.1.1. The injection process Introduction of the high-energy ion component into the target region may be achieved by means of neutral particle or ion beams: 1) When a neutral beam of high injection efficiency is used 2'~5) there still remains the problem of making the beam penetrate far enough into the plasma to interact with the target ions within the desired parts of the confinement volume. The mean free paths of the beam are 2b~~ 1In,be (1 + m b Te/me Tb) ~ , 3~'b~~ l/nG'b~(1 + mu T~/mi Tb) ~, and t
~
t
1
)~bi ~ ' ~ 2/n0"bi (1 -I- Ti/Tb) ~
for ionization by electrons and ions, and for nonionizing neutral-ion collisions. Here abe, aui and o-bl are the corresponding cross-sections, n is the target density, mb ~ mi is the neutral and ion mass, To and Ti are the electron and ion target temperatures, and Tb=ed)b/k>>Ti, T~ represents a "temperature" equivalent to the beam energy e~bb. This penetration process, which has to be nearly equivalent to a free-streaming case of the injected neutrals, differs from that of the thermalized neutral gas diffusing into the plasma body from near-wall regions. Thus the penetration length of the fast component of thermalized neutrals becomes Lnr ~ [2k T/rn~(~ + ~inf)] J/F/, where ~=(~r'nW~n ) and ~inr=(O'inWln)f a r e the corresponding rates of ionization by electrons and of ion-neutral collisionsS). Since the temperature of thermalized neutrals does not exceed that of the target plasma ions, and T b ~ T i , the mean free paths 2~,~, 2~,i and 3~bi usually differ quite a lot from the penetration length Lnf which becomes t
t
M A G N E T I C A L L Y C O N F I N E D PLASMAS
L n f ~ 5 x 1018/nm -3 for hydrogen in the range 1 0 6 < T i ~ T e < 107 K. Consequently, there are ion density ranges for which a plasma target of the characteristic thickness L c becomes impermeable to thermalized neutrals, but still can be penetrated by a high-energy neutral beam. This appears to be the case for hydrogen and its isotopes at ~bb>100keV where 2bl decreases rapidly with increasing q~b and the mean free paths of the highenergy beam become substantially larger than Lnf •
2) The problems due to the penetration process of high-energy neutrals can at least be partly avoided by injecting high-energy ions into the target plasma region. However, in this case some other questions arise. Firstly, a simultaneous neutralization of the ion beam is required by injecting electrons. Secondly, the injected beams have to become trapped for a sufficiently long time. Some possibilities may be offered by non-adiabatic scattering of the injected ions into the confinement region of the target plasma, by the use of local magnetic inhomogeneities. Such scattering may be achieved in the weak-field regions of the shielded supports in closed internal ring systems, or by means of special field inhomogeneities being introduced at the end regions of open magnetic bottles. Possibly adiabatic changes of the magnetic field in time may also be used for trapping purposes. Thirdly, if neutral gas layers exist in the near-wall parts of the target plasma, the density and thickness of these layers have to be chosen small enough for the main part of the ion beam to reach the fully ionized target plasma. This condition appears to be less stringent than that imposed in section 3.1.1 (1) on the injection of high-energy neutrals, provided that the thickness of the partially ionized wall layers can be kept much smaller than that of the plasma body, and that the neutral layer density nn<~n (see e.g. refs. 6 8, and 16).
3.1.2. The energy balance To obtain a practically interesting fusion reaction yield, a number of conditions have to be satisfied for the high-energy ion component: 1) The beam energy should be chosen high enough for the fusion reaction rate to be optimized in respect to the loss by thermalizing Coulomb collisions with the target ions. This leads to values of q~b~ ~, 100 keV 1,2).
29
2) The temperature of the target electrons has at least to be in the keV range, to avoid a strong drain of ion beam energy by the corresponding Coulomb collisions1'2). This condition is likely to be satisfied in closed magnetic bottles, but there may arise difficulties in open bottles at high target densities, such as in rotating plasmas. Possibly the strong interaction between the ion beam and the target electrons may heat the latter to a temperature To>>Ti. The loss of rotating target plasma from an open bottle may still be kept at a moderately low level, provided that the target ions can be efficiently confined at a rather low temperature T~, at the same time as the end loss of hot target electrons is reduced by an ambipolar or an imposed electric field. 3) The magnetic confinement time of the ion beam should be comparable to or larger than the time of beam-target ion collisions. 3.2. THE
L O W - E N E R G Y COMPONENT
3.2.1. Quasi-static plasma targets Several quasi-static two-energy-compop.ezat systems such as Tokamaks and mirror devices have been extensively discussed so far. To these we add internal ring devices with magnetically shielded supports, such as the Spherator. The latter has the advantage of a closed bottle which can be run in a steady state by an electrodeless high-frequency discharge, at required power levels of the order of 1 MW and densities being at least of the order of n = 2 x 1021 m - 3 according to present experimentsT). Also the high-energy ions in the 100 keV range can be prevented from reaching the support surfaces in Spherator systems of a moderately large sizeT). 3.2.2. Rotating plasma targets The rotating plasma technique provides simple means of creating and sustaining fully ionized target plasmas with densities at least up to some n = 1022 m -3, at power levels of the order of 1 MW 6). Keeping in mind the possible technical difficulties in running a steady electrode discharge at these power levels, there are the following alternatives: 1) A closed bottle for rotating plasma targets is provided by the Tornado trap. If the critical velocity phenomenon can be avoided in this trap, electron temperatures in the keV range may be reached, and an efficient reaction yield be established.
30
B. L~HNERT 2) O p e n bottles for r o t a t i n g p l a s m a targets are p r o v i d e d b y a n u m b e r o f devices being o p e r a t e d so far. Here further investigations are required on the h e a t i n g o f the t a r g e t electrons.
4. Conclusions In a d d i t i o n to earlier discussed alternatives for c r e a t i n g n e u t r o n sources f r o m m a g n e t i c a l l l y confined one- a n d t w o - e n e r g y - c o m p o n e n t plasmas, this p a p e r suggests some systems in which steady target p l a s m a s m a y be sustained at higher densities a n d smaller d i m e n s i o n s t h a n those so far being considered in T o k a m a k s a n d m i r r o r machines: 1) A m o n g the o n e - e n e r g y - c o m p o n e n t systems the r o t a t i n g p l a s m a t e c h n i q u e p r o v i d e s closed bottles o f the T o r n a d o type, a n d p o s s i b l y o p e n bottles in which sufficiently high t e m p e r a t u r e s c o u l d be achieved b y e l i m i n a t i o n o f the critical velocity p h e n o m e n o n at the end i n s u l a t o r s or by highfrequency plugs. 2) A m o n g the t w o - e n e r g y - c o m p o n e n t systems there exist quasi-static p l a s m a targets which can be confined a n d sustained in the closed S p h e r a t o r b o t t l e b y electrodeless high-frequency discharges. F u r t h e r , there exist r o t a t i n g p l a s m a targets o f the closed T o r n a d o type, and possibly o f the open type p r o v i d e d t h a t the p l a s m a target electrons o f this t y p e can b e k e p t at a sufficiently high temperature.
T h e a u t h o r is i n d e b t e d to D r B. Bonnevier for v a l u a b l e discussions.
References 1) R. F. Post, Rev. Mod. Phys. 28 (1956) 338. 2) j. M. Dawson, H. P. Furth and F. H. Tenney, Phys. Rev. Letters 26 (1971) 1156. 3) R. F. Post, T. K. Fowler, J. Killeen and A. A. Mirin, Phys. Rev. Letters 31 (1973) 280. 4) H. P. Furth and D. L. Jassby, Phys. Rev. Letters 32 (1974) 1176. 5) H. L. Berk, H. P. Furth, D. L. Jassby, R. M. Ku|srud, C. S. Lin, M. N. Rosenbluth, P. H. Rutherford and F. H. Tenney, 5th Conf. on Plasma physics and controlled nuclear fusion research, Tokyo 1974 (IAEA, Vienna, 1974) paper G2-3. 6) B. Lehnert, Nucl. Fusion 11 (1971) 485. v) B. Lehnert, J. Bergstr0m, M. Buret, E. Tennfors and B. Wilner, Plasma physics and controlled nuclear ]hsion research (IAEA, Vienna, 1971) vol. 1, p. 59. 8) B. Lehnert, Royal Institute of Technology, Stockholm, TRITA-EPP-75-06 (1975); Phys. Scripta (in press). 9) K. B. Abramova, G. A. Galechyan and B. P. Peregood, Zh. Techn. Fiz. 36 (1966) 1426. lo) B. P. Peregood and A. A. Semenov, Zh. Techn. Fiz. 41 (1971) 2297. 11) B. Lebnert, Phys. Scripta (1975, in press). 12) B. Lehnert, Phys. Scripta 9 (1974) 189. 13) j. Bergstr0m and T. Hellsten, Royal Institute of Technology, Stockholm, TRITA-EPP-73-06 (1973). ~4) B. Bonnevier, unpublished work (1974). 15) R. F. Post, in Proc. Conf. on Nuclear fusion reactors (British Nuclear Energy Society, London, 1969) p. 88. 16) B. ! ehnert, Nucl. Fusion 8 (1968) 173.
INSTRUMENTS
AND
METHODS
I29
(1975)
27-30;
©
NORTH-HOLLAND
PUBLISHING
CO.
O N M A G N E T I C A L L Y C O N F I N E D P L A S M A S U S E D AS N E U T R O N S O U R C E S
B. L E H N E R T
Royal Institute of Technology, S-10044 Stockholm, Sweden Received 1 A u g u s t 1975 It m a y become possible to create powerful n e u t r o n sources from a n u m b e r o f m a g n e t i c p l a s m a confinement schemes being investigated in fusion research. A m o n g the one- and twoe n e r g y - c o m p o n e n t p l a s m a s discussed so far, special attention is d r a w n in this p a p e r to quasi-static systems such as the Spherator with magnetically shielded supports, as well as to rotating
p l a s m a systems such as the closed T o r n a d o bottle and open bottles. Provided that sufficiently high ion and eIectron temperatures can be reached, these systems m a y be operated at higher p l a s m a densities a n d smaller dimensions t h a n those o f mirror a n d T o k a m a k n e u t r o n sources.
1. Introduction
possible ways of creating neutron sources from present magnetic confinement schemes, with special attention to systems which can be operated with plasmas being "impermeable" to thermalized neutral particles. Some alternatives are given by fig. 1 where broken frames denote special cases of steady or quasi-steady operation which can take place in the "impermeable" ion density range. Such operation has already been demonstrated experimentally with devices of moderately large size, sometimes at two orders of magnitude larger ion densities than those so far reached in Tokamak experiments6-S).
Before reaching the final aim of a self-sustained reactor, fusion research is likely to result in the creation of powerful neutron sources, being among other things important to reactor technological research and to the development of fusion-fission hybrid reactors. In this connection considerable work has recently been devoted to the problems of two-energy-component systems in which energetic deuteron beams interact with a tritium target plasma1-5). A short discussion will be given in this paper on the
Neutron sources
I
One-energy-component plasma
Quasi-static
Two-energy-component
r R°tat'ngP lasma J C,ose, O,..
I
I
Closed and open bottles
bottles / / /
/
/
/
/
/ /
/ /
/
/
---z---Tornado
1 I I I I
J
J I bottles
F--/-----] I Special I end inI sulators I
L
/
/
]
I I
~
Neutral beam
]
Quasi-static targets
j
1
[~on beam
Rotating targets
n
~
tles
[
I I
r--l-n I I Hf i I plugs II II J [. . . . .
1
1 I i l I
r
----L--__
I Spherator
I I
1
i
I
I I [
I I J
\ \
r---L---i F---IX------I II Tornado I I Electrons Ii I I [ heated I f I I I L . . . . .
J
L
J
Fig. 1. Magnetically confined plasmas used as n e u t r o n sources. Broken frames denote special cases discussed in this paper.
27
28
B. L E H N E R T
2. One-energy-component systems Most conventional schemes of closed and open magnetic bottles consist of one-energy-component systems with no essential fluid motion of the plasma. In addition, the crossed-field technique provides a simple method of creating fully ionized dense rotating plasmas for which the plasma body becomes "impermeable" to neutral particles being nearly in thermal equilibrium with the plasma ions8). The main difficulty with rotating plasmas so far is the velocity limitation effect which is connected with the ionization energy and leads to a temperature limitation at some 10 s K 6). It arises during quasi-steady operation in open bottles and is associated with a strong plasma-neutral gas interaction at the end insulators. To reach temperatures of interest to neutron source operation, the possibilities described in the following subsections should be considered. 2.1. CLOSEDROTATINGPLASMABOTTLES The internal spiral coil trap " T o r n a d o " developed in Leningrad 9'1°) provides a possibility of rotating plasma operation without end insulators11). As a consequence, supercritical velocities and high plasma temperatures are expected to be reached in this type of trap. The spacing between the windings of the outer spiral which encloses the confinement region can be made large enough for the emitted neutrons to pass freely from the plasma to a sample. The mutual forces between adjacent layers of the spiral coil windings introduce a technical problem with respect to steady state operation of Tornado traps on full reactor scale. For the use merely as neutron sources on a smaller scale, this problem has to be further investigated, and pulsed operation should in any case become possible. 2.2. OPEN ROTAa-INGPLASMA BOTTLES To reach high temperatures in open rotating plasma devices, two approaches have so far been suggested: 1) The critical velocity mechanism may possibly be suppressed by means of special arrangements at the end insulator surfaces such as by concentric metal rings*2). Even if there are some experimental indications of this mechanism to be connected with the boundary conditions at the end insulators, it has so far not been possible to overcome the velocity limitation in a steady state 6,13). 2) Recently Bonnevier ~ ) has suggested that it may become possible to create a neutron source by reducing the end losses through the joint action
of the centrifugal force and high-frequency plugs. Possibly this approach has to be combined with a decrease of the plasma density, as well as with anisotropic temperature distributions.
3. Two-energy-component systems Target plasmas have so far been discussed mainly in connection with magnetic mirrors and Tokamaks~-5), partly at maximum plasma densities n ~ 102o m -3, i.e. in a range being close to the transition between permeable and impermeable plasmasS). Here we shall include confinement systems which can be operated under laboratory conditions at densities n~> 102~ m - 3 . 3.1. THE HIGH-ENERGY COMPONENT
3.1.1. The injection process Introduction of the high-energy ion component into the target region may be achieved by means of neutral particle or ion beams: 1) When a neutral beam of high injection efficiency is used 2'~5) there still remains the problem of making the beam penetrate far enough into the plasma to interact with the target ions within the desired parts of the confinement volume. The mean free paths of the beam are 2b~~ 1In,be (1 + m b Te/me Tb) ~ , 3~'b~~ l/nG'b~(1 + mu T~/mi Tb) ~, and t
~
t
1
)~bi ~ ' ~ 2/n0"bi (1 -I- Ti/Tb) ~
for ionization by electrons and ions, and for nonionizing neutral-ion collisions. Here abe, aui and o-bl are the corresponding cross-sections, n is the target density, mb ~ mi is the neutral and ion mass, To and Ti are the electron and ion target temperatures, and Tb=ed)b/k>>Ti, T~ represents a "temperature" equivalent to the beam energy e~bb. This penetration process, which has to be nearly equivalent to a free-streaming case of the injected neutrals, differs from that of the thermalized neutral gas diffusing into the plasma body from near-wall regions. Thus the penetration length of the fast component of thermalized neutrals becomes Lnr ~ [2k T/rn~(~ + ~inf)] J/F/, where ~=(~r'nW~n ) and ~inr=(O'inWln)f a r e the corresponding rates of ionization by electrons and of ion-neutral collisionsS). Since the temperature of thermalized neutrals does not exceed that of the target plasma ions, and T b ~ T i , the mean free paths 2~,~, 2~,i and 3~bi usually differ quite a lot from the penetration length Lnf which becomes t
t
M A G N E T I C A L L Y C O N F I N E D PLASMAS
L n f ~ 5 x 1018/nm -3 for hydrogen in the range 1 0 6 < T i ~ T e < 107 K. Consequently, there are ion density ranges for which a plasma target of the characteristic thickness L c becomes impermeable to thermalized neutrals, but still can be penetrated by a high-energy neutral beam. This appears to be the case for hydrogen and its isotopes at ~bb>100keV where 2bl decreases rapidly with increasing q~b and the mean free paths of the highenergy beam become substantially larger than Lnf •
2) The problems due to the penetration process of high-energy neutrals can at least be partly avoided by injecting high-energy ions into the target plasma region. However, in this case some other questions arise. Firstly, a simultaneous neutralization of the ion beam is required by injecting electrons. Secondly, the injected beams have to become trapped for a sufficiently long time. Some possibilities may be offered by non-adiabatic scattering of the injected ions into the confinement region of the target plasma, by the use of local magnetic inhomogeneities. Such scattering may be achieved in the weak-field regions of the shielded supports in closed internal ring systems, or by means of special field inhomogeneities being introduced at the end regions of open magnetic bottles. Possibly adiabatic changes of the magnetic field in time may also be used for trapping purposes. Thirdly, if neutral gas layers exist in the near-wall parts of the target plasma, the density and thickness of these layers have to be chosen small enough for the main part of the ion beam to reach the fully ionized target plasma. This condition appears to be less stringent than that imposed in section 3.1.1 (1) on the injection of high-energy neutrals, provided that the thickness of the partially ionized wall layers can be kept much smaller than that of the plasma body, and that the neutral layer density nn<~n (see e.g. refs. 6 8, and 16).
3.1.2. The energy balance To obtain a practically interesting fusion reaction yield, a number of conditions have to be satisfied for the high-energy ion component: 1) The beam energy should be chosen high enough for the fusion reaction rate to be optimized in respect to the loss by thermalizing Coulomb collisions with the target ions. This leads to values of q~b~ ~, 100 keV 1,2).
29
2) The temperature of the target electrons has at least to be in the keV range, to avoid a strong drain of ion beam energy by the corresponding Coulomb collisions1'2). This condition is likely to be satisfied in closed magnetic bottles, but there may arise difficulties in open bottles at high target densities, such as in rotating plasmas. Possibly the strong interaction between the ion beam and the target electrons may heat the latter to a temperature To>>Ti. The loss of rotating target plasma from an open bottle may still be kept at a moderately low level, provided that the target ions can be efficiently confined at a rather low temperature T~, at the same time as the end loss of hot target electrons is reduced by an ambipolar or an imposed electric field. 3) The magnetic confinement time of the ion beam should be comparable to or larger than the time of beam-target ion collisions. 3.2. THE
L O W - E N E R G Y COMPONENT
3.2.1. Quasi-static plasma targets Several quasi-static two-energy-compop.ezat systems such as Tokamaks and mirror devices have been extensively discussed so far. To these we add internal ring devices with magnetically shielded supports, such as the Spherator. The latter has the advantage of a closed bottle which can be run in a steady state by an electrodeless high-frequency discharge, at required power levels of the order of 1 MW and densities being at least of the order of n = 2 x 1021 m - 3 according to present experimentsT). Also the high-energy ions in the 100 keV range can be prevented from reaching the support surfaces in Spherator systems of a moderately large sizeT). 3.2.2. Rotating plasma targets The rotating plasma technique provides simple means of creating and sustaining fully ionized target plasmas with densities at least up to some n = 1022 m -3, at power levels of the order of 1 MW 6). Keeping in mind the possible technical difficulties in running a steady electrode discharge at these power levels, there are the following alternatives: 1) A closed bottle for rotating plasma targets is provided by the Tornado trap. If the critical velocity phenomenon can be avoided in this trap, electron temperatures in the keV range may be reached, and an efficient reaction yield be established.
30
B. L~HNERT 2) O p e n bottles for r o t a t i n g p l a s m a targets are p r o v i d e d b y a n u m b e r o f devices being o p e r a t e d so far. Here further investigations are required on the h e a t i n g o f the t a r g e t electrons.
4. Conclusions In a d d i t i o n to earlier discussed alternatives for c r e a t i n g n e u t r o n sources f r o m m a g n e t i c a l l l y confined one- a n d t w o - e n e r g y - c o m p o n e n t plasmas, this p a p e r suggests some systems in which steady target p l a s m a s m a y be sustained at higher densities a n d smaller d i m e n s i o n s t h a n those so far being considered in T o k a m a k s a n d m i r r o r machines: 1) A m o n g the o n e - e n e r g y - c o m p o n e n t systems the r o t a t i n g p l a s m a t e c h n i q u e p r o v i d e s closed bottles o f the T o r n a d o type, a n d p o s s i b l y o p e n bottles in which sufficiently high t e m p e r a t u r e s c o u l d be achieved b y e l i m i n a t i o n o f the critical velocity p h e n o m e n o n at the end i n s u l a t o r s or by highfrequency plugs. 2) A m o n g the t w o - e n e r g y - c o m p o n e n t systems there exist quasi-static p l a s m a targets which can be confined a n d sustained in the closed S p h e r a t o r b o t t l e b y electrodeless high-frequency discharges. F u r t h e r , there exist r o t a t i n g p l a s m a targets o f the closed T o r n a d o type, and possibly o f the open type p r o v i d e d t h a t the p l a s m a target electrons o f this t y p e can b e k e p t at a sufficiently high temperature.
T h e a u t h o r is i n d e b t e d to D r B. Bonnevier for v a l u a b l e discussions.
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