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Effect of composition distribution of ABC linear terpolymers on the formation of periodic structures was investigated. Five poly(isoprene-b-styrene-b-2-vinylpyridine) (ISP) triblock terpolymers with almost constant molecular weights of ca. 130k and with similar center-block fraction at around 0.55, were blended variously. It has been found that tricontinuous gyroid structures gradually transform into a cylindrical structure whose rectangular cylinders are packed tetragonally if composition distribution increases. Further experiments by 3D-TEM observation on binary equimolar mixtures of two molecules with similar molecular weights of 122k and 124k, giving the average composition of φI/φS/φP = 0.23/0.59/0.18, has verified to show more evident rectangular-shaped cylinders with 4-fold symmetry. This new structure, having periodic surfaces with nonconstant mean-curvature, could be formed due to the systematic localization of component polymer chains along the domain interfaces.

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Non-inductive plasma current start-up by EC and RF power was carried out on the TST-2 device.Low frequency RF (21 MHz) sustainment was demonstrated, and the obtained high β p spherical tokamak configuration has similar equilibrium values as the EC (2.45 GHz) sustained plasma. Equilibrium analysis revealed the detailed information on three discharge phases: (i) In the initial current formation phase, linearity between the plasma current and the stored energy was confirmed. (ii) In the current jump phase, the initial closed flux surfaces cause a change in the current increasing rate, but the stored energy does not show such a change. (iii) The current sustained plasma is characterised by the fraction of the current inside the last closed flux surface to the total current, and the fraction seems to determine the ratio of the plasma current to the external vertical field. MHD instabilities often terminate the RF sustained plasma, but no such phenomenon was observed in the EC sustained plasma. IntroductionKey issues in spherical tokamak (ST) research are plasma current I p start-up and formation of the ST configuration without the use of a central solenoid (ohmic coil). Successful current generation, ST formation and sustainment have been achieved by injecting RF power (usually in the EC frequency range) to a configuration with a toroidal field and a weak vertical field. This scenario was developed in CDX-U using EC waves [1], and similar experiments were performed in the ST devices: LATE [2,3], TST-2@K [4], TST-2 [5], CPD [6], MAST [7]. A clear transition from open field line configuration to ST configuration, accompanied by a rapid increase in I p (so called current jump), was found in LATE. This phenomenon was also observed in TST-2 and in CPD. Experiments in these devices suggest that a higher vertical field B z and a higher EC power are preferable to achieve a higher I p as long as a current jump occurs. However, the current formation mechanism and the ST formation mechanism are still not clearly understood. Especially, equilibrium reconstruction was performed for only one case [4], and the time evolution and the variation in different operations remain unknown. The mechanisms for ST formation and the features of the plasma should be identified to extrapolate present results to next step ST devices. In TST-2, effects of various operational parameters are studied in [8]. It was found that the sustained current is roughly proportional to the vertical field strength B z , but dependences on other parameters are very weak. On the other hand, the initial current ramp-up rate depends on several parameters, and a scaling law was obtained. A current jump occurs when the initial current reaches a value proportional to B z , and the value is consistent with the condition to form closed flux surfaces. Recently, a low frequency RF source (21MHz) was used, and ST configuration was sustained by a non-EC heating method for the first time [9]. There is a threshold in the RF power, and the threshold for deuterium plasma was lower...

The characteristics of particle confinement and recycling of ohmically and neutral beam (NB) heated hydrogen plasmas in the JT-60 tokamak are investigated by using the global particle balance equation. The total number of ionization events is estimated by employing a simple formula for the H a radiation profile, and the gas fuelling efficiency is determined from the time derivatives of the electron density before and after gas feed termination. The global particle confinement time in the Ohmic heating divertor plasma decreases nearly inversely proportionally to the electron density, because of the enhancement of recycling in the plasma periphery. NB injection deteriorates the particle confinement time by a factor of about two. The effective particle confinement time closely related to the density control increases proportionally to the electron density in the Ohmic plasma. In contrast, the effective particle confinement time of the NB heated plasma is independent of the electron density although it decreases with the NB injection power. These results show that the plasma density can be controlled by the gas puff in both ohmically and NB heated hydrogen discharges.