Study on H-mode access at low density with lower hybrid current drive and lithium-wall coatings on the EAST superconducting tokamak

Abstract

The first high-confinement mode (H-mode) with type-III edge localized modes at an H factor of H IPB98(y,2) ∼ 1 has been obtained with about 1 MW lower hybrid wave power on the EAST superconducting tokamak. The first H-mode plasma appeared after wall conditioning by lithium (Li) evaporation before plasma breakdown and the real-time injection of fine Li powder into the plasma edge. The threshold power for H-mode access follows the international tokamak scaling even in the low density range and a threshold in density has been identified. With increasing accumulation of deposited Li the H-mode duration was gradually extended up to 3.6 s corresponding to ∼30 confinement times, limited only by currently attainable durations of the plasma current flat top. Finally, it was observed that neutral density near the lower X-point was progressively reduced by a factor of 4 with increasing Li accumulation, which is considered the main mechanism for the H-mode power threshold reduction by the Li wall coatings.

Study on the L–H transition power threshold with RF heating and lithium-wall coating on EAST

“…In addition, it is found that the power across the separatrix P loss increases with increasing neutral density while the power threshold P thr,08 is nearly unchanged with the similar initial plasma conditions (n e , B T , S), as indicated in figure 17(b). A reduced transition power threshold was observed with reduced edge neutral density, indicating that the edge neutral density near the X-point could be a key 'hidden variable' behind the H-mode access, which is consistent with previous experimental findings in EAST with the C wall [9]. Therefore, EAST experiments indicate that the low recycling regime with reduced edge neutral density enabled by lithium wall coating facilitates H-mode access, both for the C wall and Mo/C wall.…”

“…The I p keeps the same direction. Note that, unfortunately, we have made a mistake with the field direction in the 2010 campaign (reference [9], the third paragraph on p 2). Here we have double checked the direction of B T and can deduce the field direction through the E × B flow velocity with the toroidal and poloidal rotation direction measured by the magnetic coils.…”

Study of the L–I–H transition with a new dual gas puff imaging system in the EAST superconducting tokamak

“…Experiments show strong dependence of the transition power threshold on the wall conditions and impurity concentration [33], however, the mechanism remains unclear.…”

“…Recently, there is renewed interest in the L-H transition physics and power threshold [22][23][24][25][26][27][28][29][30][31][32][33], stimulated by the ITER requirement for H-mode operation in the initial non-active 5 phases with limited power available [3], as well as remarkable progress in high-resolution diagnostic capability at the plasma edge that allows us to gain deeper insights into the physics of the L-H transition [34]. Several powerful new diagnostics, such as multichannel Doppler Reflectometry [35][36][37][38], gas puff imaging (GPI) [39][40][41], beam emission spectroscopy (BES) [42,43], and reciprocating Langmuir probes [44][45][46], have recently been employed in several tokamaks [37][38][39][40][41][42][43][44][45] and a stellarator [35,36] to study the L-H transition physics near the threshold conditions, with some experimental results comparing favorably with the L-H transition model [15].…”

Influence of Li conditioning on Lower Hybrid Current Drive efficiency in H-mode and L- mode plasmas on EAST

“…This discharge, achieved with LHCD combined with other heating methods (ICRH, ECRH and NBI), has low and steady plasma radiation thanks to the lithium coating of the plasma facing components (PFC) and in particular of the divertor tungsten tiles. Lithium coverage reduces the effective charge Z eff of the plasma by trapping the oxygen atoms [7,8]. It also reduces the particle recycling leading to a change of the n e and T e profiles at the plasma edge, including the scrape-off layer [4].…”