on the possibility to separate isotopes by means of a ... · for the light and the heavy isotope to...
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On the possibility to separate isotopes by means of a rotatingplasma column : isotope separation with a hollow cathodedischargeCitation for published version (APA):Boeschoten, F., & Komen, R. (1977). On the possibility to separate isotopes by means of a rotating plasmacolumn : isotope separation with a hollow cathode discharge. (EUT report. E, Fac. of Electrical Engineering; Vol.77-E-72). Technische Hogeschool Eindhoven.
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On the possibility to separate isotopes by means
of a rotating plasma column
Isotope separation with a hollow cathode discharge
F. Boeschoten
R. Komen
TECHNISCHE HOGESCHOOL EINDHOVEN
NEDERLAND
AFDELING DER ELECTROTECHNIEK
GROEP ROTE~ND PLASMA
EINDHOVEN UNIVERSITY OF TECHNOLOGY
THE NETHERLANDS
DEPARTMENT OF ELECTRICAL ENGINEERING
GROUP ROTATING PLASMA
ON THE POSSIBILITY TO SEPARATE ISOTOPES BY NEANS
OF A ROTATING PLASMA COLU~
ISOTOPE SEPARATION WITH A HOLLOW CATHODE DISCHARGE
F. Boeschoten
R. Kamen
This work was performed under the terms of the agreement between the
Tecqnische Hogeschool Eindhoven and the association Euratom, to con
duct jOint research in the field of plasma physics.
TH Report 77- E- 72
Narch 1977
ISBN 90 6144 072 6
CONTENTS
ABSTRACT 1
1. INTRODUCT ION 2
2. APPARATUS AND PLASMA 5
3. DIRECT SEPARATION 9
4. PLASMA MANTLE 12
5. PERPENDICULAR INJECTION 15
6. AXIAL EFFECTS - THERMAL DIFFUSION 17
7. CONCLUSION 18
REFERENCES 20
-1-
ABSTRACT
An investigation. was made on the possibility to separate isotopes
by means of a rotating plasma column. In spite of the high rotational
velocities, direct separation in the plqsma did not seem promising because
of the high temperature of the plasma. Therefore this study was mainly
concentrated on the possibility to drive a neutral gas with the plasma
in a set up which has some similarities with an ultra centrifuge.
The poor state of plasma physics if it has to come to explanations
and predictions on the behaviour of laboratory plasmasJrequired to make an
extensive study on the rotation of a plasma. The positive c:')lumn of a holle'",,·
cathode discharge was chosen as an object of s~ch a study as it was expected
to provide for a relatively simple rotating plasma column.
It is difficult to evaluate the role of neutral particles in the
discharge. Even in r.elatively small concentrations, neutral partic3..es may
be of great importance in a highly ionized plasma. An unforeseen axial tem
perature gradient in the plasma of the hollow cathode discharge ( caused by
charge exchange collisions) leads to domination of the axial separative
effects over radial effects and to breakdown of the proposed scheme.
In pure argon gas static separation fqctors cf maximal 1.3 were
found. This is not unfavourable in comparision with other methods of isotope
separation, but not enough in view of the costs of generating the plasma.
Even if substential technical difficulties are left out of consideration
it seems that isotope separation by means of a rotating plasma is not of
practical interest.
-2-
1. INTRODUCT ION
The high rotational velocities which a,re observed in various plasma
experiments lead to the question whether rotating plasma col~mns may be used
for isotop~ sepa~ation. The same question arises as the ultimate consequence
of a method of i~otope separation I1hich was proposed by Slepian in 1942
( Lit. 1). His so called" Ionic Centrifuge H was based on the difference in
centrifugal forces which are experienced by ions of different mass,rotating
in crossed electric and magnetic fields. In the first calculations a single
particle model was used, i.e. the assumption was made that each ion moves in
the existing electric and magnetic field as if it were alone. Then the required
large ~oncentration of ions and the resul ttng collisions between the ions ..... 'ere
taken into account, leading to a fluid of ions. Loose reference was made to
electrons which are needed for neutralisation of the space charge of the ions.
But by ~dding the required electrons to the ~on fluid we automatically land
in the realm of plasma physic~ and the original scheme becomes meaningless
due to the fact that the high dielectric constant of the plasma prevents the
penetration of the required d.c. electric fields. Unjustifiable assumptions
on electric fields present in the plasma were also made by Smith, Parker and
Forrester (Lit. 2)and this fallacy has reappeared in the litterature till
the present qay ( see e.g. Lit. 3).
If plasmas are concerned it is obviously better to use the two
fluid model ( see e.g. Lit. 4) for the calculations. Instead of concentr;ting
on the life of siI'gle particles the question becorr . ....;s how different density
profiles for the isotopes may be obtained. Exte~nal electric fields which
are used in single particle models cannqt be applied at liberty. Instead
we have to deal with electrostatic space charge fields which set up auto
matically in the plasma and which may flot be controlled at will. Unfortuna
tely so far no gas discharge operated with magnetic field is studied dnd un
derstood well enough that the density profiles and the electric fields in the
plasma may be calculated theoretically. Such a knowledge is needed when the
discharge is to be used for the isotope separation.
In this context we decided to make an extensive experimental and
theoretical study of the positive column of the hollow cathode discharge.
Many of its properties are now better knmv!l and understood ( Lit. 5,6). It
-3-
turn~d out that the. relation between the measur~d values of the plasma parame
ters is properly described by the particle conservation equation (" classical
diffusion I'), the equation of motion of the ions, and the " classical II
power balance equ~tion of the ions. ~he electrons do not obey the stqtionary
power balance equation, a large power surplus ~s drained off by strong low
frequency oscillations ( Lit. 7)* The origin of the radial electric field
( which is related to the rotation of the plasma column) lies probably in
the radial gradient in the electron temperature. A theoretical expression
for the radial density profile is still lacking, but it could be shown that
the measured density profile agrees well with the angular velocity ( Lit. 8).
Thou<:lh of much help, a detailed knowledge of its pos.'.tive column is
still not sufficient to give an unambiguouq answer to the question how far a
continuo~sly operated hollow cathode discharge may be used for isotope sep<':!.ra-
tien. For this the cathode region must be known as well. More research in this
part of the arc may yield also interesting information on heating mechanisms
of plasma particles.
Most experiments were made with argon, b~t helium, neon, nitrogen
and hydrogen were also used. The facilities did not allow for operation of
uranium arcs. The central plasma column of the hollow cathode discharge in
argon makes up to 105
revolutions per second, which is ten times more than
what may be obtained in ultra centrifuges. However ¢ue to the high tempera
ture of the plasma and to the way that the centrifugal force is balanced, the
high rotational speed of the plasma column does not lead directly to a hlgh
separative power. The radial density profile does not differ sufficiently
for the light and the heavy isotope to make direct separation in the p[asma
attractive. The attainable separation factor is only 1.1, whereas a value of
at least 2 is required.
From the beginning our hope was directed towards the possibility to
use the high rotational velocities of the plasma for driving a neutral gas.
It was proposed (Lit. 9) to use a rotating plasma in the form of a hollow
cylinder with the neutral gas inside ( " P[asma mant[e method "). To that
,', This power loss is a great disadvantage for the economical outlook of
the isotope separation with a hollow cathode discharge.
-4-
purpose hollow cylindrical plasma columns were generated with special de
signed hollow cathodes. It turned out, however, that for cathode diameters
above about 2 em the circumferential velocity of the plasma does not increase,
but even decreases. Correspondingly the separation factors were not higher than
for the "norrQalll 1,3 em diameter cathoqe. (\180 charge exchange collisions pre
dominate over elastic collisions to such extend that the larger part of thE
rotating neutrals have s~ffered charge e~change collisionsj thus they have
not only a high rotational velocity, but also a high temperature. Moreover
the plasma mantle is not as ~'tight" i;l.S was hoped for. Additiopal experiments
made with a beam of neutral particles directed transversely at the rotating
plasma column gave further information on the charge exchange processes and
confirmed the calCufations made previously.
Contrary to what was believed to be the case, the plasma column of the
hollow cathode discharge is not homogeneQus in the direction of the magnetic
field. The measurements showed that the temperature of the plasma drops in
axial direction from about 10eV at the cathode to about leV at the anode side
of the discharge. It turned out, unexpectedly, that the separation power of
the arc is not so much dete+mined by radial forces (as was assumed in the pro
posals), but more by axial forces. Not foreseen thepmodij'fusive effects in
combination with the cuY'rent are responsible for the measured enrichment of
the light isotope at the anode side. Also it cannot be excluded complete!_y
that separative effects take place in the cathode region.
In this report the plasma physical aspects of the work are only touched
marginally. These are discussed in detail in the annual reports (Lit.S and L~t.6).
The economY requires that practically all the gas which passes through
the arc is 100 per cent enriched. In other words, separation factors of at least
two should
(maximal 10
be found (Lit. 9) _ This number folloWS directly from the gas feed 3
cm NTP/s) and the energy consumption of the discharge (about 7kW) _
As shown in Lit. 6 the high energy consumption of the discharge is not only due
to ionization qf the gas (about 1 kW), but also to heat radiation from the catho
de (about 1 kW) and to heating of the plasma with successive energy transport by
the e+ectrons (about 5kW). The magnetic field which is needed for confinement of
the plasma particles does not loose energy to the plasma; it may be generated oy
permanent magnets or by superconducting coils and has not to be incorporated tn
the energy consumption.
-5-
2. APPARATUS AND PLASMA
The hollow cathode disGharge as was used in this investigation is
an arc sustained in a vacuum environment in the p~esence of a magnetic
field. Gas - in t~is case argon- is introduced through an incandescent
tantalum tube at the cathode side and pumped away by vacuum pumps, which
maintain a pressure yarying from 10-3 Torr to 0.1 Torr along the axis. The
gas is very efficiently ionized and heated by passing the cathode region.
A picture of the set up is shown in Fig. 1; a schematic in Fig. 2. The mass
spectrometer is of the Quadrupole type (Balzers QMG lOlA). The vacuum in
the analysing chamber is sustained by a turbo molecular pump. The use of -8
gold rings provides for an ultimate pressure better than 10 Torr with
no detectable traces of parU'cles with mass 36. The gas which entered at
h d . d 1 d f 1 -5 10- 6 P' . t e ano e 81 e was ana yse at pressures a 0 to Torr. rOVl.Sl0nS
are made for freezipg of impurities on surfaces cooled with liquid nitrogen
- it turned out/ however, that this does not influence the reading of the
mass spectrometer at masses 36 and 40.
The plasma density n (r/z)/ the ion e
perature T (r,z), the mass velocity e
temperature T. (r,z), the electron tem-1
v(r/z) and the plasma potential ¢(r/z) /
were measured as function of the discharge parameters - arc current I 3
( 10 - 300A) / gas feed 12 (0.2 - 8 cm NTP / s) / magnetic field strength B ( 200-'
5400G; homogeneous within 5%)/ arc length L ( continuously variable between
25 cm and 250 cm) and core diameter d ( cathode tubes of 6/9/13/20/ 30 ~n
inner diameter and some special models). The discharge is operated continuously.
Under" standard" conditions L = 140 em, d= 1.3 em, B = 3400 G, I = 10QA 3
and Q = 4.5 cm NTP/s.
The dependence of the plasma density, n , on radius is shown in Fig. e
3 ( measured with Thomson scattering and Langmuir probes). In radial direc-
tion a distinction may be made between the core region ( O~ r <~d), the re
gular region ( ~d ~ r < rk
), and the turbulent region ( rk
~ r < R) .In the
core region the plasma carries the current; the plasma is highly ionized ~n the
core >95%) and its conductivity is found to be " normal" within a factor
of two. In the regular region the radial density profile is approximately
Gaussian~
-6-
21 2 -r q n(r)=n(o)e (1)
e e No theoretical expression for q2 is known, but from experiments we find
2 -1 -l, that q is proportional to B T. and depends only weakly on the ion
2 2 mass. In argon gas q 0 6.4 cm at B = 3400 G. The turbulent region pro-
vides for a transition of the plasma to the wall.
that n e
density
Both Thomson scattering an Langmuir probe measurements indicate
practically does not vary with z ( less than 30% per mI. The radial
profile and its logarithmic derivative dlnn lar also show little e
variation with z.
The ion temperature, T. ( derived from Doppler broading of the A II ~
4806 ~ line), is found to be constant ( within 5%) with radius ( up to r =
7 em). The electron temperature, T (measured with Thomson scattering and e
Langmuir probes) drops sharply in the region next to the core as may '·'8 seen
in Fig. 4 which was measured at z = 60 cm ( the middle of the column) . The
axial variation of T. and T. at the axis ( r = 0) is shown in Fig. 5. e ~
Approximately T (0) ~ 2 T. (0) and T. depends weakly exponentially on z. e ~ ~
The angular mass velocity ~ = ve/r ( derived from Doppler shift of
the AII R line, pendulum and directional probe measurements) is strongly
sheared Fig. 6). Like the temperat'.lre the rotational velocity decreases
with z ( Fig. 7). The
be about 6 x 104
cmls
axial component of the mass velocity Vz
,is found to
same diagnostic means): the plasma flow is direl.t:ed
from the cathode to the anode. The 2
radial componen~ of the mass velocity
is near the core v ~ r
3 x 10 cmls ( indirectly determined and apparently
in agreement with the " classical" diffusion rate) .
Fig. 8 shows the variation of T. and ~ with magnetic field strength-~ 0
up to B 0 3000 G they increase monotonously with B. Fig. 9 shows the Varia-
tion of Ti and rlo with arc current I. It is possible to vary no within certain
limits, but it has always to be kept in mind that the plasma parameters
cannot be varied independently.
In and near the core region the plasma behaves as may be expected
from the equations of the two fluid model of a plasma in the stationary state;
only the energy equation of the electrons cannot be balanced without taking
into consideration the low frequency oscillations which are present in the
-7-
plasma ( Lit. 7). The relationship between the experimental determined
d.c. values of ne
, Ti
, ve,Er
and Bz
is properly described by the equation
of motion of the ions. This is illustrated in Fig. 5 where is shown that the
relation.
( 2)
holds. Q = ve/r is the mass angular; QE
is the angular 'frequency asso
ciated with the electric drift ( c/rx(Er/B) and Qdi
is the angular frequen
cy associated with the diagmatic ion current: ( c kT. / reB x ( dlnn / 3r), ~ e
RelatIon (2) follows from the equation of motion of the ions by neglect of
the centrifugal force ( see e.g. Lit. 4). The radial electric field was de
termined from floating potential measurements with Langmuir probes ( a pro
per procedure in the region r > 2 em). Its origin lies probably in the ra
dial gradient of the electron temperature in combination with the finiteness
of the Larmor radii of the ions.
The fact that the plasma of the positive column is highly ionized
does not mean that the neutral particles do not play an important role in
the discharge. E.g. the decrease of the ion temperature along the axis is
caused by collisions which the ions suffer with an influx of cold particles
from the ,,'all. As a matter of fact we are dealing with three fluids and the
neutral gas may only be neglected in some restricted areas of the discharge.
Unfortunately it is impossible to obtain about the neutral particles "as much
information as about the plasma particles.
The density distribution of the neutral particles is very difficult
to measure as ionization gauges can only be placed far away from the plasma,
and the neutral pressure, p , which is measured at a certain place in the o
vacuum chamber does not give information on the pressure elsewhere. Besides
on the gas flow the distribution of the neutrals depends on the interaction
of the plasma with the neutral gas, like ionization ( mainly in the active
zone), recombination ( mainly at the wall and at the anode) and charge ex
change ( pumping action of the discharge). Fig. 10 shows p (z) measured at o
the various portholes at a distance of 50 cm from the centre, beyond the wall
of vacuum chamber ( R = 16 cm).
neutral particle density in the 3 part/ cm . In the neighbourhood
-4 about 7X 10 Torr and n ~ 2 x n
From the spectroscopial observations the
core region is estimated to be n (0) ~ 10. 12 n
of the wall the neutral gas pressure is 13 3 10 part/ cm . Some indication about the va-
-8-
lue of n at intermediate radii was obtained by introducing a tantalum n
tube ( ¢ = 6 mm) into the plasma and by measuring the pressure at the other
end of this tube, p , as function of the radial position of the snout"'. Fig. n
11 shows p (r), together with the most probable variation of n with r be-n n
tween its values in the core and at the wall. Outside the core n (r) drops n
exponentially with an e-folding l.ength Of about 0.6 cm. The arc clearly
acts as a" pump.
;, At r = 2 cm the snout becomes to glow and at r = 1 cm it is white hot.
-9-
3. DIRECT SEPARATION
At first one might think abo~t separation of the isotopes direct-
ly in the plasma. This cannot be realised so easily because of the high
temperature of the plasma. The plasma may be heavily disturbed because
of the introduction of a foreign body into the plasma and vice versa the
extraction gadget mall., go to pieces in the plasma. Only the physical aspects
of the separatiqn will be treated here and not the possible technical diffi
culties. The experiments were made with natural argon gas and the enrichment
in the 36A isotope ( natural abundance 3,4 per mil) was measured.
In order to see how far radial forces in the plasma ,nay be used for
the separation of ions of different mass, it was at first assumed that the
plasma parameters do not change along column. The axis of the plasma column
points in the direction of the magnetic field. Like in the ultracentrifuge
the separative ~ower of the plasma column is expected to depend strongly
on a difference in density profiles of the isotopes. According to eq (1)
the radial density profiles of the plasma is approximately Gaussian:
n(r) ~ n (0) exp _ r2/q2 2 -1 -~ q is found to be proportional to B T
ion mass m .. At ~
in the hydrogen
2 B = 3400 Gauss, q ~ 6.4
arc.
to depend only weakly on the 2 in the argon arc and q ~ 3.0
The fact that the ion temperature is constant over the radius, indi'::
cates that the ions at different radii are in thermodYnamic equilibrium.
This leads to a separation factor between centre and radius r:
Cl = n, (0)/n2(0) = ex r2 (! ~!) n1 (r)/n2 (r) p 2 2
ql q2
(3)
It may be expected that the light isotope (q1) is more concentrated near the
axis and the heavier one (q2) is more abundant in the outside regions. With 2 2 2 2
q2 - q1 ) = 6 q «q1 equation (3) yields approximately
2 2 4 Cl ~ exp r 6 q / q (3a)
There is some ~esemblance to the expre~sion for the ultracentrifuge where we
f ,2/ 2 d of 'q2/ q4. F t 1 (1) . d ind uDlW kT instea Ll or na ura argon gas a - loS expecte
to be about 1/10 ( r = 4 cm; 6q2 ~ 0.3 cm2).
-10-
The separative power is given by:
1f 2 :2 pDl.(a - 1) L
The perpendicular diffusioncoefficienti'
4x 10-4n Dl. ~ B2 IT
e
(4)
3 expressed in units of part/ em I B in Gauss and T in eV. With
llq2 2 e wt:ere n is
(0.-1) ~ ~ r we find: q
6u ~ 21f. 10-4 m(r2n)2
B2/T e
(4a l
Thus the separative power
tional to T~. Its maximum
is expected to be independent of Band propor
value was attained for r == q. Exp.1~ession (4a)
does not yield a promising starting point for direct separation in the
plasma. The corresponding separation factor (3a) is too low to be measu
red by the means at our disposal, permitting an accuracy in the measure
ment of a of about 10% (see also Lit. 10).
The angular frequency is not constant over the radius but is strongly
sheared (see Fig. 6). Its radial profile is related to the radial density
profile and to rk
,
~t a distance of 4
which depending on the magnetic field strength, B, lies
to 5 em from the axis (see Fig. 3). For r ~
of the plasma particles with neutrals become of importance and
rk
collisions
the transport
across the magnetic field d~ffers qualitatively from that at the centre. Cor
respondingly, the radial density profile which is approximately Gaussiap in
the regular plasma region shows a kink at radius r ~ rko It was found that
in this regiot of the arc the gas is enriched in its light component. The
dependence of the stattc separation factor on the radius is shown in Fig. 12.
In order to improve the situation one may envisage the use of a car
rier gas of much lighter mass than the isotopes which have to be separated
(Lit. 9). In the hydrogen arc the e-folding length squared has half the va-2 lue of q found in the argon arc (at the same value of B). By mixing a rela-
tivelY'small amount of argon to the hydrogen arc the separation factor for
the argon isotopes is expected to be quadrupled (4a). The increase in sepa-
* This .j.s the so called II classical II coefficient which is found in bur
experiments.
-11-
ration factor goes at the cost of the over all efficiency as the hydro
gen arc may be loaded with not more than 5% argon in order that its pro
perties are not too much changed by the presence of the argon atoms. In
deed higher separation factors are found by this method, but again in the
outside region of the arc where the theory cannot be applied (Fig. 13).
Calculations on isotope separat~on with rotating plasmas were also
made by Bonnevier (Lit. 11) and by Nathrath et al. (Lit. 12,13). The for
mula which they derived for the separation factor is basically the same
as in an ultra centrifuge:
" - exp (5 )
In experiments with a rotating neon arc at medium gas pressure (10
Torr) a values of 1.1 were found in agreement with what could be expected
(Lit. 14). The experiments of Bonnevier with hydrogen- deuterium mixtures
(Lit. 15) are harder to interprete as they were made in a complicated toroidal
geometry - the plasma rotating like a car tire. Also the pulsed operation
(duratiory about 1 msec) does not allow an interpretation in stationary state
terms, and the plasma parameters are not known locally. Similar experiments,
but with better known plasma parameters were ~ater made with neon by Cairns
(Lit. 16), who found separation factors of about 1.1.
-12-
4. PLASMA MANTLE:
Because of the low separation factors attainable directly in the
plasma, our efforts to separate isotopes with the hollow cathode dischar-
ge were mainly directed at the use of the plasma as an intermediate which
drives a neutral gas. It was expected that in this way high rotational velo
cities could be combined with relatively low temperatures of the neutral
gas. It was proposed ( Lit. 9) to generate a rotating plasma in the form
of a hollow cylinder with the neutral gas inside ( Fig. 14). The axial
magnetic field should be strong enough to compensate for the pressure of
the neutral gas and the plasma mantle must be thicker than a mean free path
length of a neutral particle in the plasma.
Doppler spift measurements of the solid rotating core of an argon
arc under normal operation revealed that the neutral atoms acquire indeed
the rotational velocity of the argon ions:
(6)
However, their temperature also equals the temperature of the ions:
T (r=O) '" T. (r=O) n l.
(7)
Apparently the plasma -neutral interaction is mainly determined by charge
exchange processes, which after all turned o~t to playa very important role
in the hollow cathode discharge. E.g. they are also responsible for the axial
variation of the ion temperature. (Lit. 17).
Rotating hollow plasma cylinders were realised by using cathode tubes,
consisting of two concentric cylinders (Fig. 15). Two sizes Were used: a cathode
(Cl) made of tantalum tubes ¢ 20 x 1,5 mm and ¢ 13 x 1 mm, and a cathode (C2)
with a ¢ 30 x 2 mm outer tube
the cylinders are respectively
and a ¢ 20 x 2 1.4 cm and
area of the standard cathode ¢ 13 x 1 mm.
1,5 mm inner tube. The area between
3.0 2 2
cm , compared to the 1.3 cm
Like observed before with the normal cylindrical cathodes, the ion
temperature, T., is constant with radius, but decreas~s in axial direction l.
towards the anode. The temperature of the neutrals also equals the ion tempe-
rature. The values
drical cathodes if
of T. and T are close to the values found with the cylin-l. n
operated with the same electric current density and
-13-
gas flux.
Fig. 16 shows the intensity of the A II 4806 R lime and the much
weaker A I 4201 R line as function of the radius for the large cathode,
at z = 0 cm for I = 100 A and I = 200 A ) and at z = 60 cm (I = A) .Similar profiles were found wi th the Cl cathode. The intensity pro--
files are clearly hollow, in contrast to what is found with a normal catho
de. As the intensity of the spectral line varies - ne exp - ( l/kTe) the
intensity profile would represent the n (r) profile if T were constant e e
with radius. A hollow radial intensity profile would, however, also be
found in case the electron temperature drops somewhat at the centre. A de
finite choice between these two altenatives could not be made, as we were
not able to make Thomson scattering measurements on large diameter cathodes.
The results of the Doppler shift measurements are shown in Fig. I;; ..
Inside the core region ve is not proportional to r, ( as is the case for the
normal cylindrical cathodes), but n approaches zero at the centre of the
arc column. As mentioned in the previous section, the origin of the plasma
rotation lies probably in the radial gradient of the electron temperature.
The ve (~) profile is about as expected when Te is constant with radius in~
side the cathode diameter and changes rapidly outside.
In order to increase the neutral gas density inside the hollow plas
ma'cylinder independently of the main feed, a ceramic tube with separate gas
feed was mounted at the centre of the Cl and the C2 cathodes ( Fig. 15- dot
ted lines). Additional central injection of relatively large amounts of neu
tral gas caused a decrease in the rotational velocities and in most cases
a decrease in the measured separation factors. Numerous attempts made with
various gas discharge parameters showed that the separation which may be ob
tained with rotating hollow plasma cylinders is not larger than found with
solid rotating plasma cylinders ( a ~ 1.3) .The enrichment in the light iso
tope is likewise near the anode and in the outside regions of the arc. Appa
rently.the proposed scheme ( Fig. 14) does not work because of charge exchan
ge collisions, which are also responsible for the inhomogeneity of the plas-
-14-
rna in axial direction. Moreover the plasma mantle might be somewhat
"leaky" ,-: as the m.f.p. length of a neutral particle is comparable to the
thickness of the plasma mantle.
In order to decrease the m. f.p. length for ion _ neutral collisions
some experiments were also made with a medium pressure ( Po
low cathode arc. The temperature of this arc was much lower
so were the rotational speeds. We did not manage in running
~ 1 Torr) hol
T,40 leV), but ~
the arc stably
for longer times (~ 1 min)· and no separative properties were observed.
A different proposal to use a rotating plasma for driving a neutral
gas was made by Lehnert ( Lit. 18). It is not clear, however, how in his
proposal the required d.c. electric fields may be introduced into the plas
ma ( see introduction). Moreover in Lehnerts proposal the neutral gas and the
plasma are mixed in the same volume, whereas it seems advantageous to have
the neutral gas as much as possible separated from the plasma.
,', The interaction of the neutral gas with the plasma in the hollow cathode
discharge takes place in such a multifarious and complicated way that thin
king in such terms should be regarded with the required caution.
-15-
5. PERPENDICULAR INJECTION
The findings which are described in the previous sections were ob
tained with a cylindrical plasma column and axial injection of the neutral
gas. The gas discharge ( of which the positive column is only a part) re
sults as an interplay between the applied voltage and the gas. The created
plasma does not only depend strongly on the amount of gas which is fed to
the discharge, but also on the way the gas feed ( and pumping) takes place.
Ir.l order to know more about the role of the neutrals in the discharge a beam
of neutral particles was directed transversely towards the core of the po
sitive column.
A flow channel for gas injection is easily obtained by replacing the
Langmuir probe in the probe mount shown in Fig. 8 of Lit. 5 by a 6 mm i.d.
tantalum tube. This opens the possibility of injecting a beam of neutral
particles into the arc at various radii and axial positions ( or alternati
vely to suck off neutral gas, e.g. in order to analyze it in a mass spectro
meter) •
3 Perpendicular injection at z= 60 cm of 2 cm NTP/s argon gas at a dis-
tance r=2 em from the centre of the arc causes a drop in the ion temperature
~t z= 60 cm from T. = 3.6eV to 5 ~ 5
10 rad/s to 1.2 x 10 rad/s
T = 1.7eV; in this case n drops from 2.4 x i 0
The same value of no and Ti are measured when
the gas stream is directed obliquely upstream ( remember the plasma streams 4
from the cathode towards the anode with a velocity of 6 x 10 cm/s). When di-
rected downstream the effect of the neutral particles on Ti and no is negli
gible. These measurements indicate that the ion temperature depends on ion -
neutral collisions and support the calculations which show that the axial
gradient in Ti in due to charge exchange collisions. An exact evaluation is
complicated by the fact that with extra injection of neutral particles their
density raises everywhere in the vacuum chamber.
So far these experiments were only made for charge exchange studies,
but in principle it might be possible to separate isotopes in this way. Argon
particles of different mass emerge at the other side of the arc with somewhat
different directions. Due to the inhomogeneities in longitudinal direction
this effect could only be measured if two pumping baffles were placed at both
-16-
arc
beam
neutr~======~====;;;;i:~::~-----particle~
Fig. 17
sides of the region where the neutral particle beam intersects the arc ( Fig.
17). It did not seem worth while to make such efforts with the argon arc, but
it could be considered in experiments with an uranium arc. Sampling should
be relatively simple in that case.
-.1]-
6. AXIAL EFFECTS - THERMAL DIFFUSION
The tendency of the light isotope to concentrate at the anode side
of the are, together with the fact that the temperature in the arc drops
from the cathode toward the anode raises the surmise that thermo diffusive
effects could be partly responsible for the measured isotope separation.
If the suffix 1 is used to denote the light isotope 36A and 2 the
heavy one 4~, the flux density of species 1 is ( n2» n
j):
nj Vj ~ - Dt2 ( V n1 + \i n 1 V tnT) ( 8)
where v is the thermal diffusion factor ( see e.g. Lit. 19). In steady state
n 1 v j = 0 and the separation factor 0; is found to be:
in 0; = -v in (T(Z)/T(O») (9)
With the measured values of the mass separation in the argon arc v is found
to be about 0.1. ( o;.p n1
(an);! n1
(cath) ~ 1.3; T(an)! T(cath)~ 1/1Jl).
Contrary to what is generally the case in a gas, in the hollow cathode dis
charge the light isotope has the tendency to concentrate at the cold side.
This may be due to the fact that the friction with the electrons which carry
the current is larger for the light isotope (T i, e - m;>.
An effort was made to Use this effect by deviding the region around
the arc in axial direction in two parts. To this purpose a water cooled dia
phragm was mounted between two sections of the vacuum chamber ( see Fig. 18).
Indeed somewhat higher separation factors than before were measured (OC~ 1.3)
if the anode was placed close to the diaphragm.
Placing a diaphragm around the core of the arc affects inevitably the
gas discharge. The effect of the diaphragm on the plasma is described in Lit.
6. The radial electric field decreases and the rotational velocity of the arc
changes correspondingly.
-18-
7. CONCLUSION
The idea behind this work was to see whether the high rotational
speeds observed in plasma columns may be used to separate isotopes. This
seems attractive as the angular velocity is ten times larger than in an
ultra centrifuge and is not limited by mechanical problems.
It turned out that the gas is enriched considerably in the light
isotope 36A at the anode side of the hollow cathode discharge operated
with argon. The same enrichment is found with hollow plasma cylinders,
which were realized in the hope to transfer the high rotational veloci
ties of the plasma to a relatively cold neutral gas ( plasma mantle method) .
This scheme did not work as expected because of the predominating role of
charge exchange collisions. These collisions are also responsible for an
axial temperature gradient in the discharge, which obscures the effects due
to forces working in radial direction. After all it turned out that the
axial temperature gradient in combination with the current is responsible
for most of the separation which occurs in the arc.
Evaluation of the results mentioned in this report may be made by
comparision with the ultra centrifuge. Attention is paid to the physical
aspects only; other requirements, like technical feasibility, are not con
sidered. Taking the power consumption and the capital cost of the discharge
into account it is necessary that:
1. Separation factors of at least two are obtained. Assuming that
the expression for the separative power is comparable to that in an ultra
centrifuge ( eq. 4) this would correspond to ten times more separation per
unit length.
2. The separative power is attainable in practise, i.e. it must be
possible to obtain separation factors of two in at least half of the amount
of gas which flows through the arc.
·In pure argon gas separation factors higher than 1.3 were never found.
As even the first condition was not fulfilled, there where no efforts made
to check on the second. Separation factors of two could be obtained by adding
less than 5% argon to hydrogen ( carrier gas). But it is clear that in this
case the second condition is not fulfilled.
-19-
At the basis of our experience it may be stated that isotope
separation by means of a rotating plasma column does not seem to be of prac
tical interest.
ACKNOWLEDGEMENTS
The authoxs would like to acknowledge the skilful assistance of
A. van Iersel in performing the experiments, the Thomson scattering ex
periments made by W.F.H. Merck and A.F.C. Sens, and the helpful discus
sions with L.H.Th. Rietjens.
-20-
REFERENCES
1. Slepian J. in Wakerling R.K. and Guthrie A. (Eds), " Electromagnetic
separation of Isotopes in Commercial Quantities", TID - 5217 ( 1949).
2. Smith L.P., Parkins W.E. and Forrester A.T., Phys.Rev. ~, 989 (1947).
3. Hashmi M. and Van der Houven Van Oordt A.J., Int. Conf. Uranium Isoto
pe separation, paper 7, London ( 1975).
4. BraginskiI S.l., "Transport Processes in a plasma", Rev. Plasma
Physics !, Consultants Bureau, New York (1966).
5. Euratom- THE Group, "Experiments with a large sized hollow cathode dis
charge fed with argon II",TH Eindhoven - Report 75-E-59 ( 1975).
6. Euratom- THE Group, " Experiments with a large sized hollow cathode
discharge III", TH Eindhoven- Report 76-E-67 ( 1976).
7. Boeschoten F., Kleyn D.J. and Komen R., Bulletin Am. Phys. Soc. Series
II, .?!. 1167 (1976).
8. Janssen P.A.E.M., to be published.
9. Boeschoten F., " Rotating Plasma", Euratom 15.~36 I XV I 69, Brussels
(1969) .
10. Boeschoten F., Int. Conf. Uranium Isotope Separation, paper 11, London
(1975) •
11. Bonnevier B., " Diffusion due to ion - ion collisions in a multicompo
nent Plasma", Arkiv for pysik~, 255 (1966).
12. McClure J.J., Nathrath N. and Schwenn R., Proc. 10th Int. Conf. in
ionization Phenomena in Gases 209 (1971).
13. Nathrath N. et aI, Int. Conf. Uranium Isotope Separation, paper B London
( 1975).
-21-
14. Heller H. and Simon M., Phys. r.etters SOA , 139 ( 1974).
15. Bonnevier B., " E~perimental evidence of element and isotope s~paration
in a rotating plasma ", Royal Institute of Technology - Report, Stockholm
( 1970).
16. Cairns J.B.S., Int. Conf. Uranium Isotope Separation, paper 9, Lon
don ( 1975).
17. Boeschoten F., Komen R. and Sens A., Proc. 12th Int. Conf. on ioniza
tion Phenomena in Gases, 98 (1975).
18. Lehnert B., Physica Scripta 2, 106 ( 1970).
19. London H. ( Ed.) " Separation of Isotopes ", George Newnes Ltd, London
( 1961).
Fig. 1. Experimental set-up
1 Vacuum vessel
2 movable cathode support
3 movable anode support
4 magnetic field coil
5 laser diagnostic equipment
6 Fabry-Perot interferometer
7 high speed camera
8 mass spectrometer
9 cdrrelator and spectrum analyser
10 Langmuir probe
r-------------------------------------------------------------------------------------------------------------------------------------------,." "
pump needle valve
l.==i)<l::::::::l flowmeter
ar
cnamoer
pump
cold t cold tra
==-ig.2 Isotope separation with to!.,· '-'8ClJU1" 3re
t"l c:: XI • '" o o '" '"
OJ C 1 r- -
4
-........ I'\.
\ \
S
---I
; _. -,
-- -- .
01 1 ..
5
--,.-
1-----
.-
--
1012
• 1 ,
d cathode
2
- -- - -_. - --
i
\ i , }r -. I I I ,
I
1 I t
:
\ ,
-
1--- --
I ~ ,
I - 1 I
I --I I
--- - . t ( I I I
4 I I
EUR.90 066
I I I B = 3400 G
---- ... _---- I = 100 A 3
Q ~ 4.5 em NTP/s
Po = 10- 3 Torr
L = 120 em
d = 13 mm
z = 60 em
f---------
. - _. -
- L-I
0 Thon son catt ring
I • Lans muir prob s
- t-- ---
I
- ---- I--------
-- -_ ... _-
~ ... - . . __ . - _ .. _.-
~ - - ---- ... .. - .• .. - -_._.-1----
s (, 7 .. r (em)
Fig.3 Radial density profile at the middle of the positive eolumn of a
hollow cathode discharge fed with Argon (standard ~onditions)
EUR.90 067
9
0 Thorn on s catt ring
"'-Lang .uir prob s •
"" X Dopp er t road ning of 1\ I 46 06)( peet al line
\
7
\ 6
,T
5
4 Ti
\
3 \ \ \
'\
~ 2
~ 1
...............
1 2 3 4 5 6
• r (em)
Fig.4 Radial profiles of electron- and ion temperature (stAnriarri condi tiona,
z=60 em).
> Qj
15
~ I , !
~ I
10
'\ I I I\. , I X : ave, age ver ~!Lm asur ment~
'\
",ITo+ ··11+ i • -- ave, age ve r p me sure ents I
--- --
tr , : star dard devi fi tiol
-i ~--t--- -- -- ~--
1"" J-~~ I
I --1- I - -- -- I I !
.. "" I I , I i ! i I , , -,-
I
"" ! I'\. I ,
i ~ ,
I i -1---. -~ t-
"" ;
~ i I
I Errc r in z I
I
~ i ,
i
'" T.
I • i ; 1 , I • ,
+--- T- -
~i 1\. i I I
~++'\~-it --~ ~+---"" ~ I , I I "-.....j----.... . '\
~~---.t-... l'\. r-~ '\ Ar,o e
5
so 100 ---..., .... z (em)
Fig.5 Axial variation of electron- and ion temperature; r =: 0; standardc.:onditions. T measurements at lower values of z, are impeded by the fact that the amount of scattered light in each wave length e "channel" becomes smaller by further broadening of the Doppler profile (6A~Te~) and finally drowns in the background fluctuations.
'" o o
'" '"
EUR.90 068
2.1)
1\ b6J1 .
x Dopp ~er hUt of A I 48 pect al LOne --
\ ---I
• Lang Inuir prob s 2
\ • Dire i::tio al L ngrnu r pr obes
\ Pend ulum +
1 .5 \ I
~ I
+-- ,
\ --- - - --l ---- --
! i
- .. - t ' . .ClE
Er or -- . .
1\
\ ~ '\ ~
.-
!\
0.5
\ '-..., ~ o
1 2 , ~
~ ; f. cr-\cm
~. ..... V .! D1
--0.5
",,-
i.-.11. - n.~
-1
Fig.6 ·Angular mass velocity • .n. at the middle of the positive column of a
hollow cathode discharge as function of radius, measured from Doppler
shift of the A;L;L 48061\ spectral line,S·,·."·\3,,d oon'l'tlun,·.; z t.,(\ ~m.
~i (diamagnetism of ions)
.nE
(electric drift) from Langmuir probe measurements.
n=A +AE Di
n (105rad/s) o
5
It "-"- "-" "- '" r.......
" ......
~ ~o 3
2 ~ ~ .........
i'-.. " I'---~ ~ 1
". - - ..
Fig.? Dependence of an~lar frequencyJlo
' and ion temperRture Ti
• on
axial distance. Normal operation of the arc.
T. (eV) 1.
8 1
6
It
2
';:'
t>l ~ ., '" o o 00
'"
I EUR.90 091
If T (. IT) •
.] .0.
0 ( 05r die .,/'
,....
".\ / l
V 1 ,
lA( ~ 2 2
./' • ./
/ V
1 1
/ / - B(56 ~ G)
, I ( 1p
, ~. D", ~ .. ,,~. "". bf .. , ... ,,. . .. fr, ,auer icvn .an. ior til" b .. ra' ,ure
T i on m ~gn .. ie ield str n~h B. ~orm 1 0 erat ~on f
.n. arc.
11.( 05r dis 0 T (e V' .
~ ... ./ 6
~ /-" 5 ~
V ,-~
nJ-l ~ 4 2
~ V ~ 1'1 • '.
~ ~
1 2
l1 ICA)
5 1 0 150 2C 0 2 0 3PO
~ig. De. ende ~c. f ar 19u1a r fr lquer c,..n •• an iOI t ... " .. ra lire
Ti cn a Ire: c rrel t I. !lor! al. c p.r. ion of t IBe a re.
,-I
EUR.90 0 1
.-...-r'" ~ r p2 P-"3 .. CI thOd"~ _
t tAnodl ... - . _.- --- ---r ., I
I II 1
-
j ! I
I I I I
p (Torr) 10- 1 ,-, --- --f--_i(
4
2
1
-4 '0
- -
L-I i
--------lC
T
--_._---- -- I~ --
-I
-----_.
P1 n~. 10a. A,.~on gas
P2 P, ~
i
,/
/ /
10 20
/
pr.""",,. .. m .... sured :.ith i(')niz .. tion ~"up;es)
at vari(,)UR plac .. s in the vacuum ch .. mber. G .. ~ reed
through the cathode; RtRnd .. rd con~ttions.
/ /'
T(A)
30 40 50
F't;.10b.Arll:On lI;as pressure inaide hollow anod., all function of lire
current I. G •• reed throu!l;h cathode; standard conditions.
, ---
,--- - ---r-
0.2
0.1
3
2
1
EUR.90 090
v-- .......... z=1 bOcm
Erro lr 7 / ./ """ '''= c;r .~
17 7 /
[7 7 i 1 2 3 4 5 6 r( em)
Fi~.12 Static enrichment in 36A isotope as function of
radius. Pure argon arc;Q=4.5cm3NTP/s;L=120 cm.
z=1 pOcm
1F. .... n~ V V
/" z=5 pcm
V V 1.--
V I
1 2 4 5 6 r(cm)
Fig.13 Static enrichment in 36A isotope as function of
rsdius. Hydrogen arc + argon. Q=10 cm3NTP/s hydro
gen + 0.5 cm3NTP/s argon; L=120 cm.
-------------i .... magnetic field gas feed for the arc
.. ROtATING HOLLOW PLASMA CYLINDER
_.~ ______ gas depleted
gas to be ---:::~ .. ~:.:::::::---separated
........ -----in light component
.. cathode j gas depleted
in light
component
Fig.1!, Scheme of "PlaRma Mantle" method
gas enriched
--~===~.: in light component
.~ectl0n A and B
eu insert
'fa shield; Tungsten wi re Breed
reed
wtI_::=H:~-~-----~ A B
~u wed
Fig.15 The C2 cathode with additional injection of neutral gas in the centre.
PI c:
"" • '" o a 00 C7\
(::~:::~~~t-':!LIl:""'i It-'-:!..S.II ~.J.ln't"IX: •• _~ i- jr1 _ ... t~-+-"-t---I-----t-~\---,.-+, -+--·+--+D+, -t-+--+-+-
120~ __ +-__ b-~~r+ __ ~ __ ~~+-+-+-__ ~~ __
~. I I
10<UU"'r--t--lHt
T
---l\H--t-i ~--tJ!;~JJ-----lrIf\ -tt .. -+! --+1--
~: v - .. 4 \ \\
2 + .. ~. --.----+.~~-+--~f---+~\..-+-
V . 1. ___ ~ __ +_~+---+~~+ ____ I_;\:---+--+-•
40 5n 60 70 110 seannin~ distanc~ (mm)
e ! ?()(': A; z..;..!' em;T; d;.P. eV.
, !=10f" A;z 7 O cm:T, ' 3.f. eV. •
~ r:~ '0(\ A· z.": hO"'m· T _-.., • ., '-' , i . eV.
--.-- lntensHy Af 4201 ~ line(x10n);T =3.1) eV. n
~i~.16h. Hadial prD~i Ie of th~ azimuthal mass
velocity, Vt' of an arp:on arc operated
w1th tr.p C? cathode.
[email protected]. Radial profil .. of the int .. nsity of the All 4Qo{.~ lin .. in an ar~on arc operated with th .. C2 cathode
••
Q; .r.
IL<!--=< ___ -". ---- . ... a .. Cu ~ n"ert I> ., I>
~. E II .... '" .. I>
" " cooHnl\' ....
stair.leS5 step.
unp;ste" p1ate
radiation shie:<i
(to prntp.ct O-r'in~)
I
-B
o
\
-f
o