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Fusion Plasma Research


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March 1996


A negative-ion-based neutral beam (NNB) was successfully injected into plasmas for the first time. The Initial injection power was about 100 kW with the energy of 200 keV and the D- beam current of 3.2 A. The pulse duration of the beam reached 0.47 s. The neutron yield was slightly increased in response to the NNB injection. The NNB injected ions were measured with the tangentially viewing charge-exchange neutral particle analyzer, which was developed under the collaboration between the Ioffe Institute in Russia and JAERI. The decay time of neutron emission produced by the NNB injection is consistent with that evaluated from the classical slowing-down time, suggesting the NNB injected ions could be confined well as expected. Boronization by a glow discharge was carried out for three days. Decaborane (B10H14) of 75 g was consumed by the glow discharge. Mixed gas of helium (60%) and deuterium (40%) was used as working gas of the glow discharge. Deuterium gas was used to reduce hydrogen-deuterium dilution in NB heated discharges after the boronization. As a result, the hydrogen/deuterium ratio was rapidly reduced less than 0.1 after one day NB heated operation. Oxygen concentration was decreased to below 0.02% and 1-1.5% during OH and NB phases, respectively.


High performance campaign was conducted for two weeks just after the intense boronization with high triangularity configurations. Enhanced performance on both confinement and stability has been successfully demonstrated with increasing triangularity at Ip=0.9-2 MA with high power NB injection up to 28 MW. At Ip=1.5 and 1.7 MA (triangularity=0.3-0.35), the highest stored energy for these current plasmas in JT-60U was obtained (6.9 MJ at 1.5 MA and 7.2 MJ at 1.7 MA) with high-beta-p-H-mode confinement. In particular at 1.7MA/4T, highly integrated performance with a quasi-steady ELMy phase was obtained: Wdia=7.2 MJ, neutron yield=3x1016 n/s, Ti(0)=30 keV, ne(0)=5-6x1019m-3, H-factor=2.5 and betaN=2.5. At Ip=1.5 MA and Bt=3.6 T (qeff=5 or q95=4), beta-N limit was increased up to 3.3. For ITER physics R&D, beta-limit and non-dimensional transport experiments were also carried out with an ITER-like triangular configuration.


High density operation with high triangularity configuration was carried out to investigate the density limit. Line-averaged density of 0.9 x nGr ( nGr[10 20 m-3]P / pai a2[MA m-2]; the Greenwald density limit ) was obtained in Ip=1.0 and 1.5 MA discharges with the triangularity of 0.4 and NBI power of 28 MW. The density was 3.7 and 5.5 x 1019 m-3, respectively. Density feedback control was used to increase the density by D2 gas puff. Disruption did not occur yet at the density. H-factor=2.0 was obtained at the density of 0.5 x nGr. MARFE occurred at the density of 0.7 x nGr and H-factor was reduced to 1.2-1.4. L-mode discharges with H-factor=0.9-1.0 was observed at the density of 0.9 x nGr.