Origin of axial current in scyllac Page: 3 of 6
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ORIGIN OF AXIAL CURRENT IN SCYLLAC
by
Kiwamu SugisakiNOTICE
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loetet pnrltetu onaed rthtABSTRACT
The origin of the axial current observed in Scyllac (a high beta
stellarator experiment) is discussed. A shaped coil and/or helical
winding produce rotational transform which links magnetic lines of force
to the plasma column and the axial current is induced electromagnetically.
This phenomenon is inherent in a pulsed high-beta stellarator. The
rotational transform produced by the induced axial current is much
smaller than that associated with the e 1, 0 equilibrium fields. The
effect of the axial current on the equilibrium and stability of the plasma
column is thus small. It is also shown that the magnetic field shear near
a plasma surface is very strong.I. INTRODUCTION
An axial plasma current of 1 ~e 1.5 kA peak
amplitude is observed in the Scylla) full torus ex-
periments.1 The amplitude of the current exceeds
the Kruskal - Shafranov limit at a radius of 1 cm
(~ 500 A). Therefore, the kink instability is pos-
sible. However, the gross behavior of the plasma
column appears to bi unchanged, irrespective of
whether the net axial current is reduced tu zero by
means of an externally induced axial current. This
effect is not inconsistent with the small theoret-
ical growth rate of the kink instability.
7n this report, the origin of the axial cur-
rent is discussed in terms of rotational transform.
Rotational transform is produced with a shaped
compression coil and/or helical windings. Magnetic
lines of force link the toroidal plasma column. If
the plasma is produced in a pulsed fashion, linkage
of the magnetic lines of force leads t, electro-
magnetic induction along the axis. The magnitude
of the linkage can be obtained from the rotational
transform, and determines the magnitude of the
axial current.
II. AXIAL LJRRENT
Rotational transform 4 i, defined as'i(r) 2n dx
d$t(1)
where X is the azimuthal magnetic flux and t is the
longitudinal flux. We assume that the longitudinal
component of the magnetic field is constant in the
plasma ani in the surrounding vacuum and is much
larger than the azimuthal component. Then the lon-
gitudinal flux 0 is, in lowest order,@ na B + n(r2 - a2) Bo
(2)
where Bi and Bo are the magnetic field in the plasma
and in the vacuum, respectively, and a is the aver-
age radius of the plasma column. The azimuthal flux
X outside the plasma column is expressed as~(r) Ho sr i(r) rdr
a(3)
The azimuthal flux X is the magnetic flux per unit
length which links the plasma column and produces
electromagnetic induction along the axis if the
magnetic field is applied abruptly. The resultant
induced axial current I has the form
_X /(b) u o Jeb i r) rdr (4)
B/It in b/a u in b/a ia r
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Sugisaki, K. Origin of axial current in scyllac, report, December 1, 1975; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc864548/m1/3/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.