Wendelstein 7-X (W7-X), the largest advanced stellarator, is built to demonstrate high power, high performance quasi-continuous operation. Therefore, in the recent campaign, experiments were performed to prepare for long pulse operation, addressing three critical issues: the development of stable detachment, control of the heat and particle exhaust, and the impact of leading edges on plasma performance. The heat and particle exhaust in W7-X is realized with the help of an island divertor, which utilizes large magnetic islands at the plasma boundary. This concept shows very efficient heat flux spreading and favourable scaling with input power. Experiments performed to overload leading edges showed that the island divertor yields good impurity screening. A highlight of the recent campaign was a robust detachment scenario, which allowed reducing power loads even by a factor of ten. At the same time, neutral pressures at the pumping gap entrance yielded the particle removal rate close to the values required for stable density control in steady-state operation.

HELicon Experiment for Negative ion source (HELEN-I) with single driver is developed with a focus on the production of negative hydrogen ions. In the Helicon wave heated plasmas, very high plasma densities (~10 !" !! ) can be attained with electron temperatures as low as ~ 1 eV in the downstream region. These conditions favor the production of negative hydrogen ions. In HELEN-I device at IPR, helicon plasma is produced using Hydrogen gas in a diverging magnetic field, created by a permanent ring magnet. RF Power (P RF ) of 800-1000W at 13.56 MHz frequency is applied to a Nagoya-III antenna to excite m = 1 helicon mode in the plasma. The plasma is confined by a multi-cusp field configuration in the expansion chamber. The transition from inductively coupled mode to Helicon mode is observed near P RF ~ 700W with plasma density ~ 10 18 m -3 and electron temperature ~ 5 eV in the driver and ~ 1eV in the expansion volume. Line integrated negative hydrogen ion density is measured in the expansion chamber by employing an Optical Emission Spectroscopy (OES) diagnostic technique using ! / ! ratio and Laser photo-detachment based Cavity Ring Down spectroscopic (CRDS) diagnostic technique. The measured value of negative hydrogen ion density is in the order of 10 16 m -3 at 6 mTorr pressure and does not vary significantly with power in the helicon mode, pressure and downstream axial magnetic field variation. The negative ion density measurements are compared with theoretically estimated values calculated using particle balance method considering different reaction rates responsible for negative hydrogen ion creation and destruction. It is to be noted that at present Caesium (Cs) is not injected in the plasma discharge to enhance ! ion density.In surface process, H-ion are produced on a low work-function surface due to surface conversion of energetic H atoms (H 0 * ) or hydrogen ions (H n + ) in the plasma [9];

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Drifts affect particle, momentum, and energy transport in the scrape-off layer (SOL) of tokamaks and stellarators, altering plasma flows and creating asymmetries between divertors. To understand how drifts affect SOL transport in the W7-X island divertor, an experiment was performed to compare plasmas with matched core parameters but opposite magnetic field directions, and therefore opposite drift transport directions. Parallel flow measurements made with coherence imaging spectroscopy are interpreted with the aid of a diagnostic forward model and a 1D simple SOL model that includes the E × B drift. In low-density plasmas (n e < 2 × 1019 m−3), the poloidal E × B drift induces a large poloidal density asymmetry within the island SOL, as measured by divertor Langmuir probes. This in turn causes the parallel flow stagnation point to shift from the position halfway between targets to the X-point in the drift direction, leading to near-unidirectional flow throughout the SOL. As density increases, the effects of the poloidal E × B drift decrease substantially, resulting in a smaller density asymmetry and the development of a counter-streaming flow pattern. For the entire density range probed in this experiment (n e = 1.5–6 × 1019 m−3), the experimental observations are more consistent with the effects of the poloidal E × B drift than the radial E × B drift.