Optimization of discharges with ion cyclotron range of frequencies using local gas injection in EAST

Abstract

In view of improving Ion Cyclotron Range of Frequencies (ICRF) wave coupling in EAST (Experimental Advanced Superconducting Tokamak) necessary to obtain high-power discharges, two series of experiments in L-mode were devoted to the use of local gas injection as a tool to increase density in front of the antennas.During the first session, the quantity of deuterium simultaneously puffed from five different poloidal positions on each side of one ICRF antenna was gradually increased. Steady gas puffing on this antenna (~8.10 20 e/s) increased antenna coupling resistance by 100%, but puffing more gas did not provoke any further improvement.Plasma central electron density was not kept constant when using local puff but kept increasing during the discharges. During the second session, the global density was well-controlled and remained constant, the distance between the separatrix and the first wall was increased by 2cm allowing, the gas to spread better, and puffing more gas this time lead to better wave coupling. The same amount of gas was then puffed from three different locations and changes in SOL (Scrape-Off Layer) parameters were measured from several locations. Extensive analysis of the edge diagnostics made it possible to observe an electron density increase and a temperature decrease caused by the ionization of neutrals in the SOL. Heat loads on the divertor targets were slightly mitigated by gas injection, which is favorable from the perspective of long pulse scenario development. Injection at mid-plane led to the best results with the highest coupling simultaneously achieved for both antennas and smallest heat loads on the divertor target (below 1MW/m²). As far as wave coupling efficiency is concerned, most experimental trends were also successfully reproduced by antenna modelling with the RAPLICASOL finite element code with experimental density profiles from reflectometry as the main input.

RF wave coupling, plasma heating and characterization of induced plasma-material interactions in WEST L-mode discharges

“…However, other synergistic mechanisms may also contribute to the beneficial effect of LHCD on the ICRF coupling observed experimentally. A similar observation was also made in EAST [25], but so far, more experiments are needed to fully identify the dominant underlying mechanism.…”

“…Further details about the ICRF system are given in the next section. All antennas (LHCD and ICRF) are actively cooled allowing for long pulse operation whilst gas can also be injected from different valves either at the top, or near antennas limiters to try to increase the density at the edge and improve the wave coupling [24,25]. Gas can be injected from over ten different valves but the one that will be used for the present study are essentially materialized by yellow circles in Fig.…”

“…Further details about the ICRF system are given in the next section. All antennas (LHCD and ICRF) are actively cooled allowing for long pulse operation, while gas can also be injected from different valves either at the top, or near the antenna limiters to try to increase the density at the edge and improve the wave coupling [24,25]. Gas can be injected from over ten different valves but the ones that will be used for the present study are essentially materialized by yellow circles in figure 2(a), respectively, with the top valve (V11) and near limiters at the low field side, either from a single point located at the mid-plane (V7 on APL) or poloidally distributed along with the object's poloidal height (ex: V9, V10).…”

Recent progress in modeling ICRF-edge plasma interactions with application to ASDEX Upgrade

“…The edge plasma density can be effectively increased by puffing the fueling gas locally in the main chamber of the machine. Recently, many experimental and numerical studies have been made of the maximization of ICRF power coupling with local gas puffing in many devices, such as AUG [43,44,[66][67][68], JET [45,69,70], DIII-D [71,72], EAST [73,74], WEST [75], ITER [46] and DEMO [76]. It is found that compared to divertor gas puffing, midplane gas puffing close to the antenna can increase the coupling resistance by ∼120% and top gas puffing by ∼20%-40%.…”

Main Challenges of Heating Plasma with Waves at the Ion Cyclotron Resonance Frequency (ICRF)

“…• Locally increase the density by fueling the plasma from valves magnetically connected to the antenna and ideally as close as possible to the middle plane [24,25] • Increase the frequency of the waves, which must remain compatible with an efficient scenario for heating the plasma (discussed in the next section) k // can be minimized at two different stages:…”