JT-60U MONTHLY SUMMARY
OPERATION AND CONFINEMENT PHYSICS
A reversed shear discharge (H-factor of 1.7 and normalized beta of 1.5)
with an internal transport barrier was sustained for 4.3 seconds in a high
triangularity configuration (triangularity ~ 0.3, toroidal field Bt= 3.5
T, plasma current Ip = 1.5 MA, safety factor q95 = 4.6) with an ELMy H mode
edge. Beta collapses were successfully avoided by beam power feedback using
the neutron emission rate.
Toward steady-state high integrated performance, discharge scenarios were
optimized at Ip = 1.5, Bt = 3.6T with triangularity ~0.1 and ~0.3. At triangularity
~0.1, an ELMy H-mode with H-factor ~ 1.7 and central ion temperature ~ 9keV
was successfully sustained for 9 sec under a high heating power of 20 -25
MW. In this discharge, the time integrated NB heating power (the total energy
input) reached 203MJ. Even with such a high energy input, no increase in
carbon and recycling particles was observed. Before the divertor modification,
large increase in carbon and recycling severely degraded the discharge performance
at the total energy input of ~70-80MJ. Such improvement in lifetime of high
performance ELMy H-mode is expected to accelerate physics understanding
related to parameters with long time constants such as the current profile
and particle recycling etc.
At triangularity of ~0.3, which has a better pressure limit at the edge
than the low triangularity, a favorable performance with H-factor ~ 2, normalized
beta ~ 2, poloidal beta ~ 1.4 was sustained for 4 - 5 sec in the ELMy phase
with co-tangential NB for current drive and perpendicular NB for heating.
In this case, the lifetime of the high beta H-mode is limited by heat capacity
of the poloidal coils used to increase triangularity. The non-inductive
current fraction (NB driven current and bootstrap current) is roughly evaluated
as 70-80%. We injected NNB into this kind of discharges aiming at the full
noninductive current drive (see the next section).
CURRENT DRIVE AND HIGH ENERGY PARTICLE PHYSICS
The NNB of 2-3 MW at 350 keV was injected into the high performance plasma
(Ip=1.5MA, H-factor ~ 2) to achieve full noninductive current drive. During
the NNB injection of 0.8 s, a loop voltage decreased down to ~0 V.
TAE modes in the frequency range of 40-130 kHz with low toroidal mode number
of n=1-3 were observed with the NNB injection (350 keV, ~3 MW) for the first
time. The TAE modes show bursting activities similar to those observed with
tangential neutral beams in TFTR and DIII-D. A small drop in neutron emission
rate (2-3 %) indicates that the loss of NNB injected ions is not significant
DIVERTOR AND BOUNDARY PHYSICS
To evaluate how carbon impurity generation in the divertor was affected
by the divertor modification, intensities of CII, CD band and bremsstrahlung
were measured with two dimensional visible spectrometers(60 channels for
top view and 42 channels for side view) under various discharge conditions.
At the same time, SOL profile was measured with a newly installed divertor
reciprocating Langmuir probe. Based on these data, detailed analyses of
impurity generation rate and impurity transport are in progress. By this
study, it is expected to clarify the geometrical effect of dome on carbon
impurity generation by chemical sputtering in the private flux region,
effects of gas puff location(main gas puff/divertor gas puff), divertor
pumping and plasma configurations(X-point height, safety factor) on impurity
SOL characteristics study was started using the three sets of Langmuir probes;
the horizontal reciprocation Langmuir probe for mid plane SOL profile measurement,
the divertor reciprocating Langmuir probe for X-point vicinity SOL profile
measurement and the 16 ch Langmuir probe imbedded to the divertor tiles
for target SOL profile measurement. It has been confirmed that the plasma
pressure in SOL is nearly constant along field lines in attached plasmas.
With increasing electron density, pressure drops at the downstream of the
separatrix, which shows development of detachment. In addition, an X-point
height scan was carried out with fixed other parameters to obtain a two
dimensional distribution of SOL plasmas in the divertor region with the
divertor reciprocating Langmuir probe.
The experiment to get divertor radiation enhancement with neon gas was done
in negative shear plasmas to study a steady-state tokamak scenario. The
radiating power was increased by a combination of neon gas puff and deuterium
gas puff, but the internal transport barrier of negative shear discharge
was weak and disappeared when the strong gas puff was injected. More optimization
of discharges is necessary.