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CONF-860412---12
DE86 012308
CONFINEMENT OF HIGH-DENSITY PELLET-FUELED
DISCHARGES IN TFTR *
S. L. Milora,
a
G. L. Schmidt, M. G. Bell, M. Bitter,
C.
E. Bush,
a
S. K. Combs," A. England,
0
E. Fredrickson,
R. J. Goldston, B. Grek,| L. Grisham, R. J. Hawyrluk, W. Heidebrink,
H. W. Hendel, K. W. Hillfb. Johnson, L. Johnson, P. LaMarche, R. Little,
D.
Mansfield, D. C. McCune, K. McGuire, D. M. Meade, D. Mueller,
E.
B. Nieschmidt,
6
D. K. Owens, H. Park, A. T. Ramsey, J. F. Schivell,
S. Sesnic, F. Stauffer,
c
B. L. Stratton, G. Taylor, R. M. Wieland
Plasma Physics Laboratory, Princeton University
Princeton, New Jersey 08544, U.S.A.
INTRODUCTION: TFTR pellet injection results reported by Schmidt [l] have been
extended to higher density and
TIT
in plasmas limited by a graphite inner-wall belt lim-
iter. Increased pellet penetration and larger density increases were achieved by operation
at reduced plasma current (1.6 MA), minor radius (70 cm), and major radius (235 cm).
Under these conditions, beam heating results have been extended to 7 MW.
OHMIC RESULTS: The evolution of a typical discharge for a three-pellet ohmic
sequence is shown in Fig. 1. Injection of each pellet (2.7 mm in diameter, 7 x 10
20
D°)
is accompanied by a reduction in the central electron temperature and an increase in
the surface voltage associated with the addition of cold fuel. The largest perturbation
of the central plasma density is created by the third pellet, which penetrates beyond
the nagnetic axis and increases the line-averaged density to 1.4 x 10
14
cm"
3
- This
corresponds to a Murakami parameter of 6.5 x 1O
1S
cm"
2
• T"
1
, which represents the
highest value attained on TFTR. This is more than a factor of two greater than the
disruption limit for deuterium gas fueling, which was established during operation with
the outer blade limiter. The high-density disruptions reported by Schmidt [1} are not
observed in these experiments.
The plasma density profile immmediately following the third pellet is strongly
peaked on axis, with the highest central value achieved approaching 4 x 10
14
cm"
3
.
This profile shape is maintained in time as the density decays and the central electron
temperature recovers. Sawtooth activity can be suppressed for ~1 s after the third
pellet, suggesting a broadening of the current density profile. As shown in Fig. 2a, the
plasma stored energy from the diamagnetic measurement increases after each pellet and
appears to reach a plateau of ss750 kJ, implying a gross energy confinement time of
0.4 s. This is in agreement with Goldston's empirical H-mode model [2] and lies within
the saturated region. As shown in Fig. 2b, the plasma density profile is still highly
peakec! 0.35 s after the third pellet, with a central value of 2.7 x 10
14
cm"-
3
, five times
the central density of the gas-fueled case shown for comparison. When taken with the
increased confinement time, this large central density yields a value of 1 x 10
14
cm~
3
•»
for the parameter fz
e
(0)r^(a).
Although the electron temperature for the pellet-fueled case is lower, the plasma.
pressure inside the half-radius is substantially larger than in the no-pellet case because
*Research sponsored by the Office of Fusion Energy, U.S. Dept. of Energy*
contract No. DE-AC05-84OR21400 with Martin Marietta Energy Syataaw, inc.
vadmt
of the high density. The electron temperature profiles exhibit similar shapes, but the
fiat region inside r = 25 cm in the pellet-fueled example is a consequence of higher
radiation and not sawtooth activity. The power radiated from this region is comparable
{>50%) to estimates of the ohmic input power (as 0.25 W/cm
3
on axis) as determined
from an equilibrium calculation that v^sumes a neoclassical resistivity model to match
the resistive surface voltage. About 20-40% of the radiated power from this region can
be accounted for as hydrogenic bremsstrahlung. We estimate a central Z
e
f{ « 1.5. The
principal impurities are chlorine (Z
e
fr contribution « 0.04) and carbon and oxygen [Z
e
a
contribution « 0.5).
NEUTRAL BEAM HEATING RESULTS: A comparison of ohmically heated and 7-
MW neutral beam (80-kV D°
—•
D
+
) injection results is shown in Fig. 3. A stored energy
increase of ~300 kJ is achieved by the addition of auxiliary heating. This corresponds
to a global energy confinement time of 0.13 s which is «30% higher than Goldston L-
mode scaling [2]. Because beam penetration at this high density is poor, the central
input power remains low (~ 2x ohmic). Under these conditions, although the fraction
of total input power radiated is «s35%, strong central radiation from carbon and oxygen
(>100%
of local input power) produces a hollow temperature profile during neutral
injection, and the confinement properties of the plasma core cannot be evaluated.
The confinement properties of 80-cm, 2.2-MA, beam-heated plasmas limited by
the outer blade limiter [l] have been studied in detail using a one-dimensional time-
dependent transport code, TRANSP. These discharges have beam power deposition
profiles similar to those of the discharge in Fig. 3, but they are at lower density and
do not exhibit such strong central radiation. Figure
"4
compares the calculated power
deposition profiles for gas-fueled and pellet-fueled discharges at 5.7 MW. In the pellet
case,
the beam power deposition is peaked off axi.-;, with roughly 50% of the total power
absorbed outside the q — 1 surface. The global confinement times and the temporal
evolution of the plasma pressure profiles are comparable, suggesting a different radial
dependence in the local energy confinement times. This is illustrated in Fig. 5a, where
we compare values of
TE{T)
at 2.6 and 2.8 s (0.2 and 0.4 s after the last pellet and the
start of beam heating). These findings are consistent with the results of off-axis heating
experiments reported by Speth [3] and Goldston et al. at this conference. Accompa-
nying the improvement in central confinement with pellet injection is a decrease in the
electron thermal diffusivity, as shown in Fig. 5. At small radii (r ss 30 cm), values of
Xe that are comparable to the neoclassical ion heat diffusivity are inferred.
SUMMARY: High-density plasmas with peak-to-average values greater than 2 have
been produced by pellet injection in TFTR. In ohmically heated plasmas a* .6 MA, nr
values of 1 x 10
14
cm"
3
-s have been achieved. Neutral beam injection in thu-e plasmas
is characterized by strong edge heating, but global confinement does not deteriorate
relative to central heating conditions.
ACKNOWLEDGMENTS: The authors acknowledge the support and continuing
interest of M. W. Rosenthal, H. P. Furth, and D. J. Grove. This work was supported
by the U.S. Department of Energy, Office of Fusion Energy, under Contract No. DE-
AC02-76-CHO-3073 with Princeton University and Contract No. DE-AC05-84OR21400
with Martin Marietta Energy Systems, Inc.
REFERENCES:
[l]
G. L.
Schmidt
et
al.,
p.
874
in
Proceedings
of
the 12th European Conference
on Controlled Fusion and Plasma Physics, Budapest, Hungary, September 2-6, 1985:
Contributed Papers, Vol.
II,
European Physical Society, 1986.
[2]
R.
J.
Goldston, Plasma Phys. Controlled Fusion 26, 87-99 (1984).
[3]
E.
Speth
et
al
M
p.
284
in
Proceedings
of
the 12th European Conference
on
Con-
trolled Fusion
and
Plasma Physics, Budapest, Hungary, September
2-6,
1985:
Con-
tributed Papers, Vol.
II,
European Physical Society, 1986.
a
Oak Ridge National Laboratory, Oak Ridge, Tennessee
37831,
U.S.A.
6
EG&G Idaho.
c
University
of
Maryland.
10
I*
a
>•»•
>"
0
n
*
o
2
u
n
n
i
(
/
a~
-R-
»)
rK
»«
Pellet injection
.
2.7 mm O
f
iili...
i
70
cm
.
•
_
hi
^^
b)
R-240cm
J
if
-J—
r
A
1 Pellet injection
2
Gas
injection
l.G
MA
Ohmic
•-70
cm
200 250 300
/
j»
i
TTT«3.3«
2
Seconds
Fig.
1
200
250 300
Major Radius (cm)
Fig.
2
Fig.
1.
Injection
of
three pellets into
an
ohmic discharge, showing surface voltage
V
f
, plasma current
/
p
,
line-averaged density
ft
e
,
and central electron temperature T«.
Fig.
2.
Comparison
of
plasma stored energy and electron density and temperature
profiles
for
pellet injection and gas fueling cases.
1
h
injection (IB/14) JU g
.
j.- - J j
w
u
Ul
1 Ohmic (t'J0Z3)
I
2 Neutral
31
J>
7 -
2
,
-
- i
b)
\.
. . .
3&0
200 ZSO
Major Radius (cm)
Fig.
3
10
Z0 30 40 SO 60 70 80
E
I
1
2
1
0
-
^
2
Bs
- pi OH
-"-
c
1
1 1
Gas fueling
n
r
- 3
3X1O
1
*.
P.
-S
•a.
•-*
pi
CL
-
M
1
:m*
T
MW I
l
r
«2
2 MA
•-8
rn
IM -
0 ci
rn
0
10 20 30 40 SO 60 70 80
10
2
0
10 20 30 40 SO 60 70 80
Plasma Radius (cm)
Fig.
5
.2
1
u
a
2
a
b)
. Pellet fueling
•
n
r
- 7 SxlO
11
P
lF
..-5 7 MW
;,.
-
p
o£
•
----
p«
1
q-2
cm*
OH
J
pTB
••."li.:;.
i
y
-
f
N
\
\
\
\
» \
\
\
0 L_-_-.
0 10 20 30 40 bO 60 70 80
Plasma Radius (cm)
Fig.
4
Fig.
3.
Comparison
of
plasma stored
energy
and
Thomson scattering profiles
for
three pellet-fueled ohmic
and
neutral beam dis-
charges.
J
p
= 1.6
MA,
a = 70 cm.
Fig.
4.
Power deposition profiles calculated
by TRANSP
for
gas-fueled
and
pellet-fueled
beam-heated discharges. POH
=
ohmic power,
PgJS
= ion
heating,
P|'
M
=
electron heating,
Pgki
—
beam
ion
thermalization power.
Fig.
5.
Comparison
of
radial profiles
of en-
ergy confinement time
and
electron thermal
dif-
fusivity calculated from TRANSP
for the
cases
shown
in Fig. 4.
Error bars represent uncer-
tainties
in the
electron temperature measure-
ment.
The
upper
and
lower limits
of x« cor-
respond
to
calculations based
on ion
heat
con-
ductivity values
of 1 and 3 x
neoclassical,
re-
spectively.
CONFINEMENT OF HIGH DENSITY
PELLET FUELED DISCHARGES IN TFTR
Presented by
S.
L MILORA
Oak Ridge National Laboratory
Oak Ridge, Tennessee, USA
DISCLAIMER
This report was prepared as an account of work sponsored by an agency of the United States
Government. Neither the United States Government nor any agency thereof, nor any of their
employees, makes aay warranty, express or implied, or assumes any legal liability or responsi-
bility for the accuracy, completeness, or usefulness of any information, apparatus, product, or
process disclosed, or represents that its use would not infringe privately owned rights. Refer-
ence herein to any specific commercial product, process, or service by trade name, trademark,
manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom-
mendation, or favoring by the United States Government or any agency thereof. The views
and opinions of authors expressed herein do not necessarily state or reflect thoK of the
United States Government or any agency
t'r.reof.
13th European Conference on
Controlled Fusion and Plasma Heating
Schliersee, Federal Republic of Germany
April 14-18, 1986
