A comprehensive investigation has been performed of the static and dynamic behaviour of detached recombining plasmas in the linear divertor plasma simulator NAGDIS-II. For stationary plasma detachment, the transition from electron-ion recombination (EIR) to molecular activated recombination (MAR) has been observed by injecting hydrogen gas into high density helium plasmas. The particle loss rate due to MAR is found to be comparable to that of EIR. Experiments have also been performed by the injection of a plasma heat pulse produced by RF heating into the detached helium plasma to demonstrate the dynamic behaviour of volumetric plasma recombination. Negative spikes in the Balmer series line emission were observed and found to be similar to the so called negative ELM observed in tokamak divertors. Observed Balmer spectra were analysed in detail using the collisional-radiative model. A rapid increase of the ion flux to the target plate was observed associated with the re-ionization of the highly excited atoms generated by EIR.

Molecular activated recombination (MAR) has been clearly observed for the first time in a divertor plasma simulator. A small amount of hydrogen gas puffing into a helium plasma strongly reduced the ion particle flux along the magnetic field, although the conventional radiative and three-body recombination processes were quenched. Careful comparison of the observed helium Balmer spectra with collisional radiative atomic and molecular data indicates that the population distribution over the atomic levels with relatively low principal quantum numbers can be well explained by taking the MAR effects into account. [S0031-9007(98)06573-9] PACS numbers: 52.40. Hf, 52.25.Ya, 52.20.Hv Recently, volumetric plasma recombinations have attracted considerable interest in detached plasmas observed in tokamak magnetic divertors and in linear divertor plasma simulators [1][2][3][4][5]. The plasma recombination is expected to play an essential role in strong reduction of ion particle flux along the magnetic field, resulting in a decrease in the heat flux to plasma-facing components [6]. Continuum and series of visible line emissions from highly excited levels were observed in detached plasmas in these devices. The analysis of these spectra shows that the radiative and three-body recombination (EIR) is important for divertor plasma conditions and gives the electron temperature T e of less than 0.4 eV in detached pure helium plasmas in linear devices [5] and around 1 eV in detached hydrogen plasmas in tokamaks [3].On the other hand, the importance of another recombination process associated with molecular reactions, that is, the molecular activated recombination (MAR) involving a vibrationally excited hydrogen molecule such as H 2 ͑y͒ 1 e ! H 2 1 H followed by H 2 1 A 1 ! H 1 A, and H 2 ͑y͒ 1 A 1 ! ͑AH͒ 1 1 H followed by ͑AH͒ 1 1 e ! A 1 H, where A 1 ͑A͒ is the hydrogen or the impurity ion (atom) existing in divertor plasmas, was pointed out in theoretical investigations and modeling [7][8][9]. MAR is expected to lead to an enhancement of the reduction of ion particle flux, and to modify the structure of detached recombining plasmas because the rate coefficient of MAR is much greater than that of EIR at relatively high T e above 0.5 eV as shown in Fig. 1 [9]. Therefore, in order to control a huge amount of ion particle and heat fluxes to the plasma-facing components in next generation fusion devices intended to have a long pulse or a steady state operation, a deep understanding of such a detached plasma regime associated with MAR effects is one of the most urgent issues in a magnetic confinement fusion research. However, no clear experimental evidence of MAR has been reported so far in a relevant plasma to the divertor condition, in which the plasma density is more than about 10 19 m 23 and the neutral gas pressure is around 10 mtorr.Studies of weakly ionized plasmas with high neutral pressure, such as discharge for gaseous lasers [10] and plasma jets [11], also show that the conversion of atomic ions into molecular one as a resul...

In the first four years of the LHD experiment, several encouraging results have emerged, the most significant of which is that MHD stability and good transport are compatible in the inward shifted axis configuration. The observed energy confinement at this optimal configuration is consistent with ISS95 scaling with an enhancement factor of 1.5. The confinement enhancement over the smaller heliotron devices is attributed to the high edge temperature. We find that the plasma with an average beta of 3% is stable in this configuration, even though the theoretical stability conditions of Mercier modes and pressure driven low-n modes are violated. In the low density discharges heated by NBI and ECR, internal transport barrier (ITB) and an associated high central temperature (>10 keV) are seen. The radial electric field measured in these discharges is positive (electron root) and expected to play a key role in the formation of the ITB. The positive electric field is also found to suppress the ion thermal diffusivity as predicted by neoclassical transport theory. The width of the externally imposed island is found to decrease when the plasma is collisionless with finite beta and increase when the plasma is collisional. The ICRF heating in LHD is successful and a high energy tail (up to 500 keV) has been detected for minority ion heating, demonstrating good confinement of the high energy particles. The magnetic field line structure unique to the heliotron edge configuration is confirmed by measuring the plasma density and temperature profiles on the divertor plate. A long pulse (2 min) discharge with an ICRF power of 0.4 MW has been demonstrated and the energy confinement characteristics are almost the same as those in short pulse discharges.

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