The effect of an applied toroidal magnetic field upon the rotamak  configuration has been investigated.  It has been found that the configuration can be maintained for at least several milliseconds but it has not been unambiguously established  whether the addition of the toroidal field is beneficial or detrimental to the rotamak discharge

Durance, G

Hogg, GR

Jones, IR

Tendys, J

Trowse, J

ANSTO Publications Online

AUSTRALIAN ATOMIC ENERGY COMMISSION
RESEARCH ESTABLISHMENT
LUCAS HEIGHTS RESEARCH LABORATORIES
THE EFFECT OF AN APPLIED TOROIDAL MAGNETIC FIELD
ON ROTAMAK DISCHARGES
by
J. TROWSE*
G. DURANCE
G.R. HOGG
I.R.JONES*
J. TENDYS
l-liiulcrs Univers i ty of South Australia
DECEMBER 1984
I S B N O 642598126
AAEC/E617
AUSTRALIAN ATOMIC ENERGY COMMISSION
RESEARCH ESTABLISHMENT
LUCAS HEIGHTS RESEARCH LABORATORIES
THE EFFECT OF AN APPLIED TOROIDAL MAGNETIC FIELD
ON ROTAMAK DISCHARGES
by
J. TROWSE*
G. DURANCE
G.R. HOGG
I.R. JONES*
J. TENDYS
ABSTRACT
The effect of an applied toroidal magnetic field upon the rotamak configuration has been investigated.
It has been found that the configuration can be maintained for at least several milliseconds, but it has not
been unambiguously established whether the addition of the toroidal field is beneficial or detrimental to the
rotamak discharge.
Flinders University of South Australia
National Library of Australia card number and ISBN 0 642 59812 6
The following descriptors have been selected from the INIS Thesaurus to describe the subject content of
this report for information retrieval purposes. For further details please refer to IAEA-INIS-12 (INIS:
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Atomic Energy Agency.
PLASMA; TOKAMAK DEVICES; COMPACT TORUS; MAGNETIC FIELDS; CONFINEMENT TIME;
ELECTRIC CURRENTS; ELECTRIC DISCHARGES; HYDROGEN; EXPERIMENTAL DATA
CONTENTS
!. INTRODUCTION I
2. EXPERIMENTAL EQUIPMENT 1
3. EXPERIMENTS AND RESULTS 1
4. DISCUSSION 2
5. CONCLUSIONS 2
6. ACKNOWLEDGEMENTS 2
7. REFERENCES 3
Figure 1 The magnetic field configuration of a compact torus device 5
Figure 2 The applied toroidal and vertical magnetic fields 6
Figure 3 The radial distribution of the z-component of the poloidal field during the 7
discharge, both with (solid circles) and without (open circles) the applied
toroidal field at time t = 10 ms
Figure 4 The radial position of the separatrix at z = 0, both with and without the 7
applied toroidal field
Figure 5 The radial distribution of the toroidal magnetic field in vacua (open circles) 3
and during a plasma discharge (closed circles)
Figure 6 The quantity r \B§lasma (r,0) - Bfiac (r,0\ vs r 8
Figure 7 The penetration of the RF rotating field into the plasma both (a) without and 9
(b) with an applied toroidal magnetic field
Figure 8 Estimates of the RF power input into the plasma, both with and without the 10
toroidal field
1. INTRODUCTION
The rotamak is a compact torus device in which the toroidal current is driven by a rotating magnetic
field. The resulting steady magnetic field consists of a combination of closed and open poloidal field lines,
as indicated in figure 1. It is believed that the inclusion of a toroidal magnetic field is unnecessary from a
stability viewpoint, consequently nearly all the rotamak research undertaken so i'ar has been performed
without such a field. However, a toroidal field may offer other benefits such as improved particle and
energy confinement.
Hugrass et al. (1980] performed a preliminary investigation in the early high power, short duration
rotamak experiments using argon as the filling gas. The limited .duration of these discharges made it
difficult to establish the merits of including a toroidal field. The present experiments extend the earlier
investigation to long duration (~ 15 ms) hydrogen discharges.
2. EXPERIMENTAL EQUIPMENT
The discharge vessel, vacuum system. Helmholtz coils and radio-frequency (RF) current sources have
been described by Durance et al. |1984a]. However, for the present experiment the frequency of the rotating
field has been lowered from 1.85 to 1.0 MHz. The diagnostics described by Durance el al. (1984a.bj have
been used in the present studies.
The toroidal field is generated by passing a current (~ 500 A) through a metal conductor (3.2 mm
diameter) located along the z-axis. The return path is made up of four straight conductors arranged
symmetrically about the spherical discharge vessel and at a distance of ~ 19 cm from the central axis. The
risetimc and duration of the current pulses to the toroidal and vertical magnetic field coils are identical, but
the current amplitudes are independently variable.
3. EXPERIMENTS AND RESULTS
All experiments were performed with an initial hydrogen filling pressure of ~ 1.2 mtorr. In conventional
rotamak discharges, it has been observed that the magnitude of the driven toroidal current is related linearly
to the amplitude of the externally applied vertical field. Such behaviour also occurs when a toroidal
magnetic field is applied: this has enabled the poloidal field to be varied with respect to the toroidal field by
simply varying the currents through the vertical field coils relative to the current through the axial
conductor. From such studies, two general observations can be made: first, for the same vertical field, the
driven toroidal current is significantly smaller in the presence of a toroidal field; second, when a toroidal
field is present, the driven toroidal current is observed to 'cut off abruptly before the termination of either
the rotating magnetic field or the applied vertical field.
For the present measurements, a relatively small value of the driven toroidal current (~ 150 A) has been
chosen to maximise the effect of the toroidal magnetic field relative to the poloidal field. The driven
toroidal current, with and without a toroidal field, is shown in figure 2. Also shown is the vertical field
which has been kept constant in both cases. Figure 3 shows the radial distribution of the z-component of
the poloidal field during the discharge at time t = 10 ms for the two cases. A small correction has been
applied to account for pick-up of the toroidal field (estimated to be 1.3 per cent of the toroidal field) arising
from imperfect alignment of the probe. When such results taken over the whole discharge are integrated,
the radial position of the separatrix can be determined (see figure 4).
The radial distributions at z = 0 of the toroidal field in vacua and during a plasma discharge were
measured with a Hall effect probe; the results are shown in figure 5. The toroidal field decreases slightly
during the plasma discharge implying that a diamagnetic current is generated within the plasma. It is
possible to calculate the magnitude of this poloidal current as follows. The resultant toroidal field. Bg . due
to these poloidal plasma currents must be given by
Bfa 0) = BPelasma (r, 0) - Blac(r, 0),
where Bp9lasma (r.O) and Bfc (r.O) are the measured toroidal fields with and without a plasma. From
Ampere's Law. it follows that the poloidal current (Ipol) inside the radius r is given by
Ipoi(r) = —Mo
- ~> -
The quanti ty r\BPelasma (r.O) - Bfc (r.O)J is plotted in figure 6. From the maximum value of this
quantity the total poloidal current is estimated to be ~ 75 A. The poloidal current is relatively large
compared with the driven toroidal current of ~ 150 A.
The penetration of the rotating RF field into the plasma was investigated using a small \virc-\vounJ
probe: the results are shown in figure 7. Since the probe output is proportional to dB/dt, the slowly varying
toroidal field is negligible compared to the rapidiy varying RF component.
Estimates of the power input to the plasma were obtained by measuring the r.m.s. voltage V(l, the r.m.s.
current /„ and their relative phase 00 in the matching circuit during a plasma discharge. These quantities
were also measured during a vacuum shot and the effective circuit resistance. R, derived. The input power,
P, is thus
P =V0I0cos(<l>0)-I0R2.
The results are shown in figure 8.
All these results were unaffected by reversal of the direction of the toroidal field in the discharge.
4. DISCUSSION
From the experimental results a number of observations can be made:
(a) A compact torus plasma with an embedded toroidal magnetic field has been maintained for at
least ~ 6 ms.
(b) Since the toroidal field is non-zero outside the plasma, the resulting plasma/field configuration
resembles a compact tokamak rather than a spheromak.
(c) A diamagnetic poloidal current has been generated. This contrasts with the paramagnetic
current reported for the earlier short duration experiments with an argon plasma [Hugrass et al
1980],
(d) The driven toroidal current is smaller than that found in the equivalent experiment without
a toroidal field.
(e) A simple calculation shows that the net Jfol X Blor force is outwards for a weakly diamagnetic
current. This is consistent with the observation that the separatrix radius is slightly larger when
the toroidal field is present.
(0 The penetration of the rotating magnetic field into the plasma improves when the toroidal field is
present
(g) Within experimental errors, the RF power input to the plasma remains essentially unchanged by
the presence of the toroidal field. This suggests that the additional ohmic dissipation introduced
by the poloidal current is largely compensated for by a decrease in the ohmic dissipation from
the toroidal and screening currents.
(h) The reason for the abrupt termination of the configuration is not known; the most likely
explanations are either that a plasma instability occurs or that insufficient power is available to
sustain the equilibrium.
5. CONCLUSIONS
It is concluded that a compact torus configuration with a toroidal magnetic field can be maintained, for
several milliseconds at least, in a rotamak device. Within the limited scope of these experiments, it has not
been possible to establish unambiguously whether the addition of the toroidal field has had a beneficial or
detrimental effect on the rotamak discharges. Such an assessment must await detailed energy and particle
confinement measurements.
6. ACKNOWLEDGEMENTS
One of the authors (J. Trowse) wishes to acknowledge the assistance of the Australian Institute of
Nuclear Science and Engineering in the form of a research studentship.
-3 -
7. REFERENCES
Durance. G. |1983] - The magnetic field structure of rotamak discharges. PhD Thesis. The Flinders
University of South Australia.
Durance, G., Hogg, G.R., Tendys. J. [1984a] - The AAEC rotamak experiment: description and preliminary
results at low input power. AAEC/E602.
Durance. G.. Hogg. G.R.. Tendys, J. [1984b] - The effects of rotating field frequency on the rotamak
configuration. AAEC/E603.
Hugrass. W.N.. Jones. I.R.. McKenna. K.F. [1980] - A compact torus configuration generated by a rotating
magnetic field : the rotamak. Proc. Third Symp. on Physics and Technology of Compact Toroids
in the Magnetic Fusion Energy Program, 2-4 December. Los Alamos, New Mexico. LA-8700-c.
pp.105-108.
Hugrass, W.N., Kirolous, H. [1984] - The steady currents driven in a conducting sphere placed in a rotating
magnetic field. Aust. J. Phys., 37:521-529.
Neutral Poinh
Separatrix
MagneMc
Axis
Figure 1 The magnetic field configuration of a compact torus device. Bp is the poloidal magnetic
field ; Bg is the toroidal magnetic field. The origin of the cylindrical coordinate system (r,Qj.)
is also indicated
40
§ 30
5- 20
c
CD
CD 10
1 I I I T"
J L I I _ [ 1 . 1 _ 1_J _ I _ I I I - D 1 - 1- 1- L
Toroidal
Magnetic
Field
Be
N
ca
T~I—i—j'-i -T~I—r—i—i—r
1 1 1 1 I I I I I 1 1 N , . ! . , , . I .1
Vertical
Magnetic
Field
200
~ 100
i—i—i——i—i—i—i——i—i—i—i—|—i—i—i—i—r
Toroidal
Plasma
Current
Be=0
Figure 2
<
CD
200
100
i—i—i——i—i—i—i——i—i—i—i—i—i—i—i—i—r
i I i r
Toroidal
Plasma
Current
B0/0
5 ' 1 0 1 5
TIME(ms)
The applied toroidal and vertical magnetic fields are shown. The toroidal plasma current is
also shown for the cases with and without the toroidal magnetic field. The two overlying
current traces are shown to indicate their reproducibility
Figure 3
Figure 4
10
ID
"o
c_r
co1
-5
10
— 00
o
• i
8 .
o
0
o
a
o §
j I
10
r ( c m )
15
The radial distribution of the z-component of the poloidal field during the discharge, both
with (solid circles) and without (open circles) the applied toroidal field at time t = 10 ms
LJ
X
C£.
1—
 :
o:
Q-
UJ
00
U_
O
O
\—
00
o
Q.
_J
5
cc
14
12
10
8
6
4
2
1 1 1 1 1 1 1 1 1 | 1 1 1 1 1 1 1 I 1
I I I
Q
. -t^^_ Bft^O
"^ ^ —
13 ** )^
o
Be=° -
_ _
— —
— —
— —
-
— —
, , , , 1 , , , , | , , , , 1 , , , ,
5 10
TIME (ms)
15
The radial position of the separatrix at z = 0, both with and without the applied toroidal
field
Figure 5
30 r- •
o
_J
LU
20
10 -
o
i—i—i—i—i—i—i—i—i—i—r
o
o
o
o o
• 8
I I I 1 I I
5 10 15
r (cm)
The radial distribution of the toroidal magnetic field in vacua (open circles) and during a
plasma discharge (closed circles)
>DQ T
CD
c_
r
J I 1—J_
5 10 15
r(cm)
Figure 6 The quantity r[Bj'asma (rf>) - B$ac (rff)\ vs r
S 4
CD
t=4ms
~
CQ
= 6ms
t=8ms
t=10ms
5 10 15
r(cm)
(b) 60*0
Figure 7 The penetration of the RF rotating field into the plasma both (a) without and (b) with an
applied toroidal magnetic field. The solid line represents the distribution of the field with no
plasma
~ 5
o
Q.
10
TIMEdmsper div.)
Figure 8 Estimates of the RF power input into the plasma, both with (solid line) and
without (dotted line) the toroidal field


Effect of an applied toroidal magnetic field on ROTAMAK discharges.

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Effect of an applied toroidal magnetic field on ROTAMAK discharges.

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