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

Abstract

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.

Experimental scaling of divertor heat and particle flux widths in EAST L-mode plasmas

“…The measurements of divertor LPs show that the power decay length exhibits q,div = 4.97I p −0.94 ( q,div = 1.46B p −1. 15 ), in good agreement with the particle decay length js = 4.74I p −1.02 ( js = 1.34B p −1.21 ). Similar trend measurements have also been demonstrated by the reciprocating LPs.…”

“…It is demonstrated that both divertor particle and heat flux widths in EAST L-mode plasmas exhibit a strong negative dependence on I p (or B p ), i.e., js = I p −1.02 (or js = B p −1.21 ), q,div = 4.97I p −0.94 (or q,div = 1.46B p −1. 15 ), which provide more information on the prediction of particle and heat flux deposition width for ITER. Moreover, it is also demonstrated in EAST L-mode plasmas that the inverse I p (or B p ) scaling is independent of divertor configurations (LSN, DN and USN), which is good news for tokamaks operated in DN configurations, since the divertor heat flux widths do not narrow with respect to the SN case.…”

“…The divertor LPs embedded in the target plates can evaluate the divertor particle and heat flux widths simultaneously. The detailed information of divertor LP arrays on EAST before the 2014 campaign was reported in [14], the upper arrays of which have been successfully upgraded in ITER-like, water-cooled, tungsten monoblock divertor in 2014 [15]. The measurements of the profiles of upstream edge plasma parameters and the SOL flow velocity can be provided by the fast reciprocating LP system at the outer mid-plane [16].…”

Effect of ∇B drift on the H-mode power threshold in upper single null plasmas with ITER-like tungsten divertor on EAST

“…In the recent experiment of EAST, the divertor particle flux, which is directly associated with SOL parallel flow, can be readily determined from the ion saturation current measurements by the newly upgraded triple Langmuir probe with a high time resolution. 43 The ion saturation current represents the particle flux as C ¼ n t C s ¼ j s /e ¼ 6.24 Â 10 22 j s (A cm À2 ), where n t and C s are the electron density and ion sound speed at the target, respectively. Here, a calculation for the divertor particle flux is not available due to the damage of probe head in our experiment.…”

Plasma–tungsten interactions in experimental advanced superconducting tokamak (EAST)

“…The plasma current is 0.4 MA and the line-averaged plasma density is 2 × 10 19 m −3 . The distribution of the probe ion saturation current density on the target [16] was also given for comparison with the W atom flux. The vertical coordinate indicates the poloidal distance from the upper edge of the outer target schematically shown on the right.…”