JT-60U MONTHLY SUMMARY
OPERATION AND CONFINEMENT PHYSICS
Concerning the threshold heating power for the L-H transition, the initial
results showed a slightly (~10%) low threshold power compared to the open
divertor cases at a given density.
Toward steady-state high integrated performance, the discharge scenario
was optimized at plasma current Ip = 1.5-1.8MA, toroidal field Bt = 3.6T
with triangularity ~0.1 and ~0.3. Favorable performance with H-factor =1.8-2,
normalized beta =1.8-2.2 was sustained for <4 sec in the ELMy phase with
co-tangential NB for current drive and perpendicular NB for heating. The
non-inductive current fraction (NB driven current and bootstrap current)
is roughly evaluated as 70-80%.
Discharge scenario of reversed shear plasmas were optimized to obtain an
H-mode edge in addition to the internal transport barrier using a high triangularity
configuration. When triangularity was increased by keeping the plasma volume
constant, many collapses were observed due to decrease in both q95 and internal
inductance. By expanding the plasma simultaneously with increasing triangularity,
a stable discharge (3.5 T, 1.5 MA, q95 = 4.6) was obtained. However, the
discharge was finally terminated by a beta collapse with normalized beta
of 2.0 and an H factor of 2.7.
CURRENT DRIVE AND HIGH ENERGY PARTICLE PHYSICS
The conditioning of NNB progressed and the injection power of ~2 MW with
a pulse length of 700 ms was achieved at 300 keV. The NNB was injected into
a target plasma, where the loop voltage and the internal inductance were
kept almost constant for ~3 s, and a clear drop in the loop voltage was
Together with optimization of the steady state high performance plasma,
establishment of target plasma of NNB injection progressed for non-inductive
full current drive of the high performance plasma.
DIVERTOR AND BOUNDARY PHYSICS
Experiments for reducing Zeff was conducted to assess impurity control capability
of the W-shaped divertor. The minimum value of Zeff was 1.1 in ohmic discharges
and 2 in NBI heated discharges with a high power of 20 MW. Increase in
electron density was most effective for reducing Zeff. However, when electron
density reached a critical value, X-point MARFE appeared and which limited
the further reduction in Zeff.
The divertor pumping characteristics was investigated for NBI heated discharges.
The exhausted particle flux was evaluated by comparing gas puff rate necessary
to keep the electron density at a given value with or without divertor pumping.
The divertor pumping rate was estimated to be about 1% of divertor particle
flux at main electron density of 3x1019 m-3, which was as same level as
wall pumping rate.
Energy confinement of high density ELMy H-mode was investigated in radiative
discharges formed by gas puff. In ELMy H-mode discharges, strong gas puff
was needed to increase the electron density, which degraded energy confinement
characteristics. The H-factor obtained so far decreased as electron density
increased, and the value was about 1.5 at 50% of Greenwald limit. This level
of H-factor is almost similar to that obtained in the previous open divertor.