K. Nagashima

Divertor experiment on particle and energy control in neutral beam heated JT-60 discharges

The divertor characteristics in particle and energy control in neutral beam (NB) heated discharges on JT-60 have been studied with injection powers of up to 20 MW. The essential divertor functions are achieved successfully. In ohmically heated discharges, the minimum clearances between the separatrix magnetic surface and the fixed limiter for sufficient divertor action are 1.5 cm for e = 1.5 × 1019 m−3 and 2.5 to 3 cm for e = 4 × 1019 m−3. Global power balance studies show that, in NB heated divertor discharges, about 5% to 10% of the total absorbed power, PABS, is radiated from the main plasma, while 50% to 60% is radiated in the limiter discharge. For e = 6 × 1019 m−3, 50% of PABS flows to the divertor plate. The radiation loss in the divertor chamber is 15% of PABS. According to the spatial distribution of the temperature rise on the divertor plate, the half-width of the heat load is less than 1 cm for e = 1.5 to 4.4 × 1019 m−3. The neutral pressures in the divertor chamber and around the main plasma increase in proportion to e2. The compression ratio is about 45. The effectiveness of the divertor pumping system in particle control is demonstrated for NB pulses of, at least, 1 s. Reduction of evaporation by separatrix swing is also shown.

H-mode experiments with outer and lower divertors in JT-60

In JT-60, H-mode experiments with outer and lower divertors have been performed. In the outer divertor discharge, an H-mode similar to the modes observed in the lower/upper divertor discharges is obtained. Its threshold absorbed power and electron density are 16 MW and 1.8 × 1019 m−3. In the two schemes of combined heating with NB + ICRF and NB + LHRF, H-mode discharges are also obtained. Moreover, in the new configuration with the JT-60 lower divertor, H-mode phases with and without edge localized modes are obtained. The improvement in the energy confinement time in both divertor configurations is limited to values within 10%. The paper mainly presents the H-mode results of the outer divertor discharges. Also, typical results of the lower divertor discharges are shown for a comparison of the H-mode characteristics of the two configurations.

Radiation losses and global power balance of JT-60 plasmas

The radiation losses and the global power balance for Ohmic and neutral beam heated plasmas have been investigated in different JT-60 configurations. Discharges with a TiC coated molybdenum wall and with a graphite wall, with limiter, outer and lower X-point configurations have been studied by bolometric measurements, thermocouples and an infrared TV camera. In neutral beam heated outer X-point discharges with a TiC coated molybdenum first wall, the radiation loss of the main plasma was very low (10% of the absorbed power). The radiation loss due to oxygen was dominant in this case. On the contrary, in discharges with TiC coated molybdenum limiters the radiation loss was very high (>60% of the absorbed power). In the discharges with a graphite wall the radiated power from the main plasma was 20-25% for both limiter and lower X-point configurations. In lower X-point discharges the main contributor to the radiation loss was oxygen, whereas in limiter discharges the loss due to carbon was equal to the loss due to oxygen. The radiation loss from the lower X-point divertor increased with increasing electron density of the main plasma.

Current drive and confinement studies during LHRF experiments on JT-60

Results are presented of the first Lower Hybrid Current Drive (LHCD) experiments in JT-60. 2 MA of RF driven current is successfully produced for the first time in a reactor grade tokamak. The magnetic divertor works quite well in eliminating the impurities released by the current carrying fast electrons which have allowed the generation of the reactor relevant RF current in a very low density plasma. The efficiency which is defined as ηCD = eRIRF/PLH(1019 m−3 AW−1), reaches values of 0.8 to 1.7. NBI heating enhances the current drive efficiency by a factor of 1.5, and LHCD improves the confinement time of high power NBI heated plasma. The key to confinement improvement is found to be the active control of the current profile by LHCD.

A study of particle confinement properties in ohmically and neutral beam heated hydrogen plasmas in the JT-60 tokamak

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 Hα 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.

Improvement of the density limit with an external helical field on JFT-2M tokamak

Abstract The density limit is increased by the application of an external helical field in the JFT-2M tokamak. The effect of the magnetic stochasticity due to the external field is investigated to study the mechanism of the improved density limit related to the edge plasma behaviour. The improvement is correlated with the retardation of the increase in the plasma inductance. At the improved density limit, local radiation loss is modified by the helical field, in which that from the vicinity of separatrix X-point is remarkably reduced, while that from outboard edge is slightly increased. The formation of a positive radial electric field at the plasma edge is also observed in the presence of the helical field.

Particle and heat balance analysis in scrape-off and divertor regions of the JFT-2M tokamak

Abstract Particle and heat balance in the scrape-off layer and the divertor region were studied in the JFT-2M tokamak. Using particle and energy conservation laws, particle and heat diffusivities perpendicular to the flux surface were evaluated just outside the magnetic separatrix. It was found that the particle diffusivity decreases with increasing electron density in the scrape-off layer and decreases by a factor of 2–3 in the H-mode phase as compared with that in L-mode. The heat diffusivity has almost the same dependence on the electron density. The ratio of the heat diffusivity to the particle diffusivity is about 2.

Simultaneous heating by high power lower hybrid waves and neutral beams in the JT-60 tokamak

By injection of high power lower hybrid (LH) waves of up to 6.0 MW into neutral beam heated (20 MW) plasmas in the medium electron density regime (e ≤ 3.5 × 1019 m-3) the plasma energy content is increased at the same incremental energy confinement time as it is increased with neutral beam heating alone. When LH heating is used in addition to thermal electron and ion heating, the beam ions are accelerated to energies considerably higher than the beam injection energy. In contrast to LH injection into ohmically heated plasmas in the same density regime, where substantial high energy electron production is observed, there are much less high energy electrons in the case of combined heating. The heating efficiency of LH waves during combined heating tends to decrease as the electron density is increased. Ray tracing analysis suggests that the accessibility condition prevents effective heating in a high density plasma. Calculation of wave damping, taking into account the beam component of the ion velocity distribution function, indicates that the waves are absorbed by beam ions before they are absorbed by electrons.

High ion temperatures and transport characteristics of high Zeff hydrogen plasmas in JT-60 limiter discharges

Ion temperatures around 10 keV were obtained in JT-60 limiter plasmas by high power neutral beam heating (H0 → H+), which is much higher than the maximum value of 6.6 keV obtained in JT-60 diverted plasmas. The experiment was performed in the parameter region of Ip = 1−2 MA, e (before heating) = (0.7−1.5)×1019 m−3, e (end of heating) = (2.5−4.1)×1019 m−3 and = 19−21.8 MW. The Zeff values of the limiter plasmas were high (i.e. 4 to 7) compared with those of the diverted plasmas (i.e. 1 to 2). Transport analysis indicates that, since the deposition power to electrons, Pbe, increases with Zeff, high Zeff is effective in raising the electron temperature. Furthermore, it is shown that since the equipartition loss is reduced by both the increase in electron temperature and the decrease in the effective ion density, np + nzZ2mp/mz, with increasing Zeff, high ion temperatures are obtained in limiter plasmas with high Zeff.

Investigation of edge localized mode in the JFT-2M Tokamak

The edge localized mode (ELM) was investigated in detail on the JFT-2M tokamak. Spatially, a pivot point was observed inside the separatrix flux surface. Temporally, an ELM event starts from the precursor oscillation, of which the growth rate was about 40 mu s. Another high frequency oscillation with about 300 kHz was found during the magnetic fluctuation burst. Just after the burst, a poloidal electric current (ELM current) was induced in the scrape-off layer. During the ELM, the particle flux to the divertor plates was transiently enhanced by a factor of about 4. The profile of the induced flux was 2-3 times broader than that in the ELM-free period. The total particle flux induced by one ELM was evaluated to be about 5% of the total plasma particles. The ELM current is characterized by its direction and its large fluctuation level. Furthermore, it was found that a large opposite current exists near the strike points on both divertor plates