G. Z. Deng

Upgrade of Langmuir probe diagnostic in ITER-like tungsten mono-block divertor on experimental advanced superconducting tokamak

In order to withstand rapid increase in particle and power impact onto the divertor and demonstrate the feasibility of the ITER design under long pulse operation, the upper divertor of the EAST tokamak has been upgraded to actively water-cooled, ITER-like tungsten mono-block structure since the 2014 campaign, which is the first attempt for ITER on the tokamak devices. Therefore, a new divertor Langmuir probe diagnostic system (DivLP) was designed and successfully upgraded on the tungsten divertor to obtain the plasma parameters in the divertor region such as electron temperature, electron density, particle and heat fluxes. More specifically, two identical triple probe arrays have been installed at two ports of different toroidal positions (112.5-deg separated toroidally), which can provide fundamental data to study the toroidal asymmetry of divertor power deposition and related 3-dimension (3D) physics, as induced by resonant magnetic perturbations, lower hybrid wave, and so on. The shape of graphite tip and fixed structure of the probe are designed according to the structure of the upper tungsten divertor. The ceramic support, small graphite tip, and proper connector installed make it possible to be successfully installed in the very narrow interval between the cassette body and tungsten mono-block, i.e., 13.5 mm. It was demonstrated during the 2014 and 2015 commissioning campaigns that the newly upgraded divertor Langmuir probe diagnostic system is successful. Representative experimental data are given and discussed for the DivLP measurements, then proving its availability and reliability.

In–out asymmetry of divertor particle flux in H-mode with edge localized modes on EAST

The in–out divertor asymmetry in the Experimental Advanced Superconducting Tokamak (EAST), as manifested by particle fluxes measured by the divertor triple Langmuir probe arrays, is significantly enhanced during type-I edge localized modes (ELMs), favoring the inner divertor in lower single null (LSN) for the normal toroidal field (B t) direction, i.e. with the ion B × B direction towards the lower X-point, while the in–out asymmetry is reversed when the ion B × B is directed away from the lower X-point. The plasma flow measured by the Mach probe at the outer midplane is in the ion Pfirsch–Schluter (PS) flow direction, opposite to both B × B and E × B drifts, i.e. towards the inner divertor for normal B t, and the outer divertor for reverse B t, consistent with the observed in–out divertor asymmetry in particle fluxes. Since the particle source from an ELM event is predominantly located near the outer midplane, this new finding suggests a critical role of the PS flow in driving the in–out divertor asymmetry. The divertor asymmetry during type-III ELMs exhibits a similar trend to that during type-I ELMs. Strong in–out divertor asymmetry is also present during inter-ELM and ELM-free phases for the normal field direction, i.e. with more particle flux to the lower inner divertor target, but the peak particle flux merely becomes more symmetric, or slightly reversed, for reverse B t, i.e. reversed B × B drift direction.

Design of Langmuir probe diagnostic system for the upgraded lower tungsten divertor in EAST tokamak

In order to achieve long-pulse H-mode plasma scenario over 400 s with high heating power in the Experimental Advanced Superconducting Tokamak (EAST) device, the lower graphite divertor will be upgraded into a tungsten (W) divertor with active water cooling, which consists of the W/Cu monoblock units and the W flat-tile units as the divertor plasma facing components. As a fundamental diagnostic tool, the divertor Langmuir probe (Div-LP) diagnostic system will be upgraded accordingly. This paper presents the design of two kinds of new Div-LP systems, which are planned to be utilized on the W/Cu monoblock units and the W flat-tile units for the upgraded lower tungsten divertor, respectively, including their structures and preliminary poloidal and toroidal layouts. The Div-LP diagnostic system can measure the plasma parameters with the schemes of triple-probe, double-probe, and single-probe, to obtain the spatial and temporal distribution of plasma behavior on the divertor targets, which is useful for the discharge control and operation in EAST. In addition, the thermal analysis of the two kinds of probe assemblies is also carried out by using the three-dimensional finite element code ANSYS, which is aimed to get the optimal designs to withstand the long-pulse and high-power operation in EAST future experiments.

Achieving temporary divertor plasma detachment with MARFE events by pellet injection in the EAST superconducting tokamak

A new pellet injection system has been equipped on the experimental advanced superconducting tokamak (EAST) in the 2012 campaign, with a pellet size of 2 mm × 2 mm, a frequency of 1 Hz–10 Hz and velocity of 150 m s−1–300 m s−1. The deuterium pellet is well-known for plasma fuelling as well as for triggering the edge localized mode (ELM). In the 2012 campaign, pellet injection experiments were successfully carried out on EAST. Temporary plasma detachment achieved by deuterium pellets has been observed in a double null (DN) divertor configuration, with multi-pellet injections at a repetition frequency of 2 Hz. The partial detachment of the outer divertors and complete detachment of the inner divertors was achieved after 35 ms of each pellet injection, which have a duration of 30–60 ms with the maximum degree of detachment (DOD) reaching 3.5 and 37, respectively. Meanwhile, the multifaceted asymmetric radiation from the edge (MARFE) phenomena was also observed at the high field side (HFS) near both the lower and upper X-points with radiation loss suddenly increased to about 15%–70%, which may be the main cause of divertor plasma detachment. The temporary detachment induced by pellet injection may act as a new way to study divertor detachment behaviors.

Study of plasma current effect on divertor power footprint widths through experiments and modeling in EAST L-mode plasmas

Integrated studies of the scaling of divertor power footprint widths with plasma current in Experimental Advanced Superconducting Tokamak (EAST) L-mode plasmas are carried out through experiments and modeling. The divertor power widths, which consist of the scrape-off layer power decay length λq and the heat spreading S, are calculated from the measurements of divertor Langmuir probes (div-LPs) and scrape-off layer plasma simulation (SOLPS). A strong inverse scaling of both λq (mm) and S (mm) from the div-LP with plasma current Ip (MA) is demonstrated, with the regressions being λ q = 4.37 I p − 1.04 ± 0.075 and S = 2.31 I p − 0.997 ± 0.047. However, a much weaker inverse scaling of λq with Ip is shown by the measurement of outer mid-plane reciprocating probes, which gives λ q , R P = 6.56 I p − 0.31 ± 0.056. It indicates that the connection length may have played a stronger role in the negative scaling of λq with Ip than radial transport. A similar conclusion can also be drawn from a comparison of the si...

Modeling study of radiation characteristics with different impurity species seeding in EAST

A critical issue for EAST and future tokamak machines such as ITER and China Fusion Engineering Testing Reactor is the handling of excessive heat load on the divertor target plates. As an effective means of actively reducing and controlling the power fluxes to the target plates, localized impurity (N, Ne, and Ar) gas puffing from the lower dome is investigated by using SOLPS5.0 for an L-mode discharge on EAST with double null configuration. The radiative efficiency and distribution of different impurities are compared. The effect of N, Ne, and Ar seeding on target power load, the power entering into scrape-off layer (SOL), Psep, and their concentration in SOL along the poloidal length and edge effective ion charge number (Zeff) which are closely related to core plasma performance are presented. The simulation results indicate that N, Ne, and Ar seeding can effectively reduce the peak heat load and electron temperature at divertor targets similarly. N seeding can reach the highest radiative loss fraction a...