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Fusion Plasma Research


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September 1997


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).


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 so far.


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 shielding.
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.