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August 2004

  In this month, the experimental studies for (1) stationary sustainment of high βN plasma, (2) high beta full non-inductive current drive, (3) high density RS (reversed shear) plasma, (4) high density high βp mode plasma, (5) density limit, (6) ITB (internal transport barrier), (7) transient heat transport, (8) burning plasma simulation, (9) runaway electron control, (10) current hole, (11) CS (center solenoid)-less operation, (12) impurity reference, (13) conditioning of heating systems have been carried out. The results are shown below.

(1) Stationary sustainment of high βN plasma: Discharge scenario at Ip =0.8 MA, Bt =1.7 T and q95 ~3.6 was developed to sustain high βN plasmas. Feedback control of stored energy was employed to raise the plasma beta gradually just below the beta limit for NTM (neoclassical tearing mode) onset (βN~3). As a result, a large 3/2 NTM was avoided, and βN>2.5 was sustained for 7.9 s with H89<~1.7.
(2) High beta full non-inductive current drive: The objective of this experiment is sustainment of normalized beta βN=2.5-3.0 and bootstrap current fraction fBS >50% under full non-inductive current drive for 8 s. N-NB was injected into a target plasma (Bt =2.4T, Ip=1MA, q95 ~4.3 and δ~0.5) with an ITB and an ETB (edge transport barrier) without giant ELMs (edge localized modes), which was obtained by optimization of target density and injection pattern of main heating. As a result, βN~2.35 (βp~1.65) and H89~1.8 were sustained for ~2 s with NTMs.
(3) High density RS plasma: The plasma configuration was scanned changing the plasma volume (Vp) and the distance between the last closed flux surface and the first wall on an outer midplane (Δ16). In the configuration with larger Vp (75m3) and smaller Δ16 (16 cm), a higher central density with a lower central temperature was observed, and a higher density normalized with the Greenwald density (ne/nGW) was obtained. Pellet injection was attempted aiming at improving the fuel purity, but an ITB degraded with pellet injection. Divertor radiation was enhanced by introducing Ne and D2 gas-puff, while main radiation was decreased with decrease of NB heating power, suggesting effective impurity schielding. Following parameters were obtained; ne= 0.84nGW, HHy2 =1.1, βN=2, Pradtotal ~PNBabs+POH (main: 66%, divertor: 34%).
(4) High density high βp mode plasma: Discharge optimization was attempted with Bt ~2.5T for high confinement and high radiation at high density using high-field-side (HFS) pellet injection, Ar gas-puffing and small D2 gas-puffing. Achieved density was limited below 0.7nGW. Confinement (H89 ~1.6) was almost the same as that in the last year at low Bt (=2-2.7 T).
(5) Density limit: The dependence of density limit on the magnetic shear was studied by using plasma current ramp-down. The density limit for disruption tended to increase with the internal inductance li and high density beyond nGW (up to 1.5nGW for a tangential chord) was realized in a high li (>2) regime.
(6) ITB: In positive shear (PS) plasmas (Bt ~1.9 T and Ip = 1 MA), strong electron heating with ECRF and negative ion NB (N-NB) was attempted to form a strong Te ITB without a Ti ITB, which had not been observed before with ECRF (w/o N-NB). In the high positive ion NB (P-NB) power case (~15 MW), degradation of central Ti with electron heating was observed as before, which seems to prevent Te increase. In the low P-NB power case (~2 MW), central Te increased as ECRF power was increased stepwise, but no clear ITB structure was observed in Te profiles. In reversed shear (RS) plasmas, low-fueling heating (dominant N-NB&EC, least P-NB) was attempted to investigate if the Ti ITB was formed without a strong density gradient. With gas-puffing, increase in Ti, suggesting ion ITB formation, was observed with a small density gradient.
(7) Transient heat transport: The response of Te to an external perturbation was investigated to obtain dependence of electron thermal diffusivity χe on Te and/or Te gradient. Te perturbations were induced by using a short pulse (100 ms) off-axis (ρdep~0.4-0.6) ECRF heating in OH and H-mode plasmas. The slow heat pulse propagation from ρdep to the central region was observed in an OH plasma. The propagation time of the heat pulse was shorter in NB heated H-mode plasmas, and it decreased with the increase in NB power. The heat pulse propagation in an ITB plasma was not observed possibly due to a higher density.
(8) Burning plasma simulation: To simulate the control of a burning plasma with external heating power, a new control logic for NB power has been developed, where the neutron yield rate (Sn) proportional control for α particle heating simulation and stored energy (D2) FB control for external heating control can be applied to different NB groups simultaneously. Perpendicular NB units were controlled proportionally to the Sn and the D2 FB control was applied to tangential units. In L-mode plasmas, the stored energy and the neutron yield were successfully controlled for two different Sn proportional gains.
(9) Runaway electron control: As a target of experiment, formation of runaway current tail was attempt in intentional disruption using intense Ar gas injection with Bt ~3 T. Hard X-rays and photo neutrons, which suggested generation of runaway electrons, were observed. However, no clear runaway current tail was formed. The termination time of plasma current was shorter than that in the previous experiments with higher Bt (~3.7 T), where the runaway current tail was observed.
(10) Current hole: Te profile asymmetry in a current hole was investigated with low-field-side ECRF heating. Though an increase in Te was observed in the current hole, it was found that evaluation of the asymmetry was difficult due to non-linear response in some channels of ECE radiometer. Adjustment of gain is required.
(11) CS-less operation: A new connection of vertical field coils has been made possible where no inboard coils are used as vertical field coils. Genuine CS-less start-up without any inboard coils was demonstrated up to 100 kA (for 0.2 s) of plasma current using this connection. This also demonstrates that plasma current start-up without a field null is possible, though it requires strong ionization for plasma production (ECRF in this experiment).
(12) Impurity reference: Zeff reached the lowest level in the database of JT-60U. The oxygen and the boron content were almost zero. However, copper (Cu) might contribute to Zeff, in particular, in the H-mode phase, in which Cu line was observed clearly. Cu impurity could be generated during the LH antenna conditioning.
(13) Conditioning of heating systems: Conditioning of the heating systems (NB, LHRF and ECRF) have been progressed as follows.
P-NB: Port conditioning for low-Bt (1.7 T) was completed for 10 s high-power mode.
N-NB: Successfully injected for 4.9 s.
LHRF: Injected energy increased to 5.77 MJ. Successfully injected for 12 s with duty cycle ~0.8 and peak power ~1.2 MW.
ECRF: 0.8-1.1 MW (at gyrotron) for 1.5-3 s for all four units. Simultaneous 4-units-injection was up to 0.8 s.