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HL
-2A
has been operated up to Ip = 400 kA, Bt = 2.6T,
line average electron density ne=5×
1013cm
-3 and discharge duration Td > 2 s with both
divertor and limiter configuration in 2005. FB
control and siliconization have been used to
improve long duration discharge reproducible and
divertor configurations have been obtained.
Several physics experimental results are as
following:
1)
Pellet injection
The long time
enhanced confinement has been realized after the
central fuelling by the pellet injection on the HL
-2A
tokamak. The evolutions of some parameters shown
in Fig. 1. are: plasma current Ip, loop voltage VL,
line average electron density ne, electron
temperature at plasma center (r =
1 cm
) and plasma edge (r =
35 cm
), intensity of the soft x ray, poloidal βp,
energy confinement time τE separately from
top to bottom. The hollow electron temperature
profiles and peaked density profile are sustained
about 200 ms. The energy confinement time increase
from 50 ms to 120 ms. The electron heat diffusion
coefficient χe decrease from
3.6m2
/s (before pellet injection) to
1.2m2
/s (after pellet injection).
Large sawteeth with long duration appear after the
pellet injection. In general, the duration of sawteeth
is more then 30ms and the maximum is about 60ms.
The magnitude of sawteeth decreases about 40%
during the crash. The MHD modes with different
frequencies have been observed using soft x ray
array and Mirnov probes. The features of the MHD
modes have been investigated. The mechanism of the
enhanced confinement during the pellet injection
may be related with both the peaked density
profile and weak (or reversed) magnetic shear in
plasma core.
2)
Electron cyclotron heating
Two 68 GHz gyrotrons
have been used during ECRH with total power of
1MW. The pulse duration is up to 1 s.Transmission
system consists of oversized wave-guides with
diameter
8 cm
and some metallic reflectors. Microwave is
launched into plasma in perpendicular to toroidal
field at the low field side as an ordinary mode
(o-mode). Both on-axis and off- axis ECRH have
been carried out. The plasma center electron
temperature increases to about 1300eV, as shown in
Fig.2. The electron temperature profile can be
changed effectively by off- axis ECRH.
3)
Zonal Flow measurement
The 3-D structure
for geodesic acoustic mode zonal flow (GAMZF) is
measured by Three Step Langmuir Probes (TSLP) at
the edge of HL
-2A
plasmas. Three arrays of TSLP are arranged
poloidally and toroidally, and the corresponding
symmetries (m = 0, n = 0) of the electric
potential and field at GAM frequency (f= 7-15kHz)
are obtained. The radial scale length of the flows
is estimated as 2.4
-4.7 cm
. The formation mechanism of the zonal flow is
identified to be nonlinear three wave coupling
between high frequency turbulent fluctuation and
the flow. The estimated ambient fluctuation and
zonal flow velocities are
1.0 km
/s and 0.26
-0.65 km
/s. The estimated shearing rate of the ZF is 8´
104 m
/s, which is lower than typical
growth rate of drift instabilities of
105 m
/s. The modulation effects of the flows on the
ambient turbulent fluctuations are also
demonstrated. Three dimensional feature of the
GAMZF is in agreement with zonal flow theory.
4)
Wall conditioning by
siliconization
By using silicon as a material for wall
coatings on HL
-2A
tokamak, the impurity fluxes released from the
wall are reduced. Fig.3 shows some results
measured by visible spectrometer. Besides the
radiation from carbon ions and oxygen ions are
suppressed significantly,, the continuum radiation
in visible region is also decreased. It’s also
found that a higher plasma density limit is
achieved by means of siliconization.. During
discharge with divertor configuration, plasma
radiation in the measured spectral range decreases
obviously.
5)
Disruption prediction
A new method has
been used for major disruption prediction by
monitoring the characters of the MHD activity.
According to the HL
-2A
experimental results, statistical analysis is
applied to investigate the relationships between
MHD activities and plasma major disruptions. Two
typical features of MHD activity can finally cause
the disruption on HL
-2A
. By introducing a new parameter,
, as the criterion of disruption precursor,
according to the statistical analysis, more than
95% major disruptions can be predicted. On the
other hand, the
>1 can be combined with other criterion
parameters in neural network predictor to predict
disruptions. Fig.4 shows statistical analysis of
MHD activities and disruptions.
6)
Condensate Molecular Beam injection
Both fuelling
experiments with condensate supersonic molecular
beam injection (CSMBI) and normal supersonic
molecular beam injection (NMBI) were carried out
on the HL
-2A
divertor plasma. The valve body was cooled to
about 87 K by liquid nitrogen. The diameter of
valve nozzle is
0.2 mm
, the hydrogen gas pressure was 1.8 MPa. The pulse
duration of the beam is less than 3 ???ms. The
pressured hydrogen cooled to liquid nitrogen
temperature provides condensated molecular beam
which may become a cluster beam.Under the same
operation parameter, the line averaged electron
density increment after CSMBI with single hydrogen
pulse is 1.35×
1019m
-3 as shown in Fig.5, which is
three times higher than that for the NSMBI with
the same pulse period.
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