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,
IMPROVED CONFINEMENT WITH REVERSED MAGNETIC SHEAR IN TFTR
BY
F.M.
LEVINTON,
M.C.
ZARNSTORFF,
S.H.
BATHA, ET AL.
JULY
1995
PRINCETON
PLASMA PHYSICS
LABORATORY
-
I
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Improved Confinement
with
Reversed
Magnetic Shear
in
TFTR
F.
M.
Levintonl,
M.
C.
Zarnstorff2,
S.
H.
Bathal,
M.
Bell’,
R.
E.
Bell2,
R.
V.
Budny2,
C.
Bush’,
Z.
Chang2,
E.
F’redrickson2,
A.
Janos2,
H.
Park’,
A,
Ramsey’,
G.
L.
Schmidt’,
E.
Synakowski2,
G.
Taylor2
Fusion
Physics
and
Technology, Torrance,
CA
90503
2Princeton Plasma Physics Laboratory,
P.
0.
Box
451,
Princeton,
NJ
08543
’Oak
Ridge
National Laboratory, Oak Ridge, TN
37831
(June
7,
1995)
Abstract
Highly peaked density and pressure profiles in
a
new operating regime
have been observed on the Tokamak Fusion Test Reactor (TFTR). The
q-
profile has
a
region
of
reversed magnetic shear extending from the magnetic
axis
to
r/u
-
0.3-0.4. The central electron density rises from 0.45
x
lo2’
m-3
to
nearly 1.2
x
lo2’
m-’
during neutral beam injection. The electron particle
diffusivity drops precipitously in the plasma core with the onset of the im-
proved confinement mode and can be reduced by
a
factor
of
N
50
to near the
neoclassical particle diffusivity level.
52.55.-s, 52.55.Fa, 52.30.Bt
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OF
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UNLlMlTEb
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The economic attractiveness of the tokamak
85.8
candidate for
a
fusion reactor de-
pends on development of
a
magnetic configuration that has good confinement, stability,
and low recirculating power for steady state current drive. This requires a high fraction
of self-sustaining bootstrap current that is well aligned with an optimized current density
profile for confinement and stability. Recent studies
[l]
of
the optimization
of
the current
density profile suggest that reversed magnetic shear (i.e.
a
hollow current density pro-
file), is desirable for confinement, stability, and bootstrap alignment. Shear is defined as,
s
=
(2V/q)(dq/d$)(d$/dV)
M
(r/q)(dq/dr),
where
$
is the enclosed poloidal
flux,
V
is the
enclosed volume,
q
is the safety factor and
T
is the minor radius. Shear is thought to be
important because it can stabilize some classes of microinstabilities such
as
trapped electron
modes
[2],
a
candidate to explain the observed anomalous electron transport in tokamaks.
Reversed magnetic shear can also stabilize some magnetohydrodynamic
(MHD)
instabilities
such as ballooning modes
[3]
and resistive tearing modes.
A
further benefit, if improved
core confinement can be attained, is
a
high pressure gradient that would’generate
a
strong
off-axis bootstrap current, and sustain the hollow current density profile. This scenario
may lead to an attractive concept for
a
steady state tokamak reactor
[4].
Most tokamaks
operate with inductive current drive which will normally produce peaked current density
profiles
at
the magnetic axis due to the strong dependence of the plasma conductivity
on
the electron temperature. Only by non-inductive current drive or transient techniques can
a
hollow current density profile be generated. This has been done in several experiments
such as Tore-Supra
[5]
using lower hybrid current drive
(LHCD)
and JET
[6,7]
using pellet
injection. These experiments have reported improved performance that could be attributed
to reversed shear.
Recent experiments on the Tokamak Fusion Test Reactor
(TFTR)
[8]
have demonstrated
a
reversed shear configuration. With measurements of
q(R,t)
from the motional Stark ef-
fect
(MSE)
[9,10]
diagnostic providing good temporal and spatial resolution,
a
correlation
of
the effect of magnetic shear on improved transport and stability has been observed and will
be reported in this Letter. Two different discharge startup scenarios have been developed
2