Dependence of upstream SOL density shoulder on divertor neutral pressure observed in L-mode and H-mode plasmas in the EAST superconducting tokamak

Abstract

Upstream density profiles in the scrape-off layer (SOL) have been examined in low-confinement mode (L-mode) and high-confinement mode (H-mode) plasmas in the EAST superconducting tokamak. A weak density shoulder forms in the near SOL region in upper single-null configurations when the neutral pressure measured at the lower divertor exceeds a threshold value of 2 × 10−2 Pa in L-mode plasmas. When the neutral pressure is below this threshold, the weak density shoulder is absent and the sidebands of the lower hybrid waves associated with SOL parametric instabilities are reduced. Active detachment control with neon–deuterium seeding demonstrate that the weak density shoulder can form before the onset of the outer divertor detachment as long as the neutral pressure is above the threshold. Furthermore, no remarkable expansion of a shoulder is observed during divertor detachment, suggesting that divertor detachment is not a necessary condition for the formation or growth of a density shoulder. Through the increase in neutral pressure in the lower divertor by an order of magnitude, the weak shoulder was observed to expand into the far SOL and reach the leading edge of the limiter. The results in L-mode discharges identified the neutral pressure in the lower divertor as a primary factor for the formation of an SOL density shoulder in the upper single-null discharges. For the type-I ELMy H-mode plasmas, a similar density shoulder was detected during the inter-ELM phase when the neutral pressure in the lower divertor exceeded a threshold value of 4 × 10−2 Pa. On the other hand, the shoulder was absent when the divertor neutral pressure went below this threshold even though the plasma discharge was conducted with a higher core line-averaged density and divertor collisionality. This is consistent with the observations in L-mode plasmas. The neutral particle ionization of the working gas is thus believed to play a key role during the formation of the SOL density shoulder in the EAST tokamak.

Effects of recycling neutral on density shoulder formation in tokamak plasmas

“…Although collisionality seems to be the key factor, there are still some experimental phenomena that deviate from the expected dependence of λ n on Λ div [42], which implies that Λ div might be a necessary but not sufficient condition to obtain a flatter density profile [22,23,42]. As an important complement of the previous studies, which centered on upstream SOL turbulence, more recent reports on JET [9], AUG [24] and EAST [25] plasmas emphasized that divertor recycling or neutral pressure may be the sole process involved in SOL density shoulders, where JET plasmas in particular indicated that the character of shoulder formation has completely changed for different divertor configurations (vertical target (VT) or horizontal target (HT)). So, in short, the three main processes (again, the ionization source, parallel loss and perpendicular transport) are not independent, but affect each other intricately, which brings challenges to revealing the physical mechanism behind shoulder formation.…”

“…One of the important reasons for concern is that the density shoulder influences the cross-field particle fluxes to the main chamber surfaces [7,[10][11][12][13] and affects the design of future devices [14][15][16]. Although shoulder formation of the upstream SOL density profile has been experimentally studied on several tokamaks in a variety of confinement regimes [7,9,11,[17][18][19][20][21][22][23][24][25], the corresponding understanding still remains in a * Authors to whom any correspondence should be addressed.…”

“…plasma blobs) are found to have strong interactions with the neutrals [36][37][38]. Moreover, since the density shoulder usually occurs at high density and low current [8,9,18,25], a common conjecture has been concluded that the filament characteristics could be changed via electrical disconnection from the divertor target sheath corresponding to the high divertor collisionality Λ div [39] for a high plasma density, and ultimately the advective transport could be improved [27,40,41]. Although collisionality seems to be the key factor, there are still some experimental phenomena that deviate from the expected dependence of λ n on Λ div [42], which implies that Λ div might be a necessary but not sufficient condition to obtain a flatter density profile [22,23,42].…”

“…Inspired by the analyses of neutrals on JET [9], AUG [24] and EAST [25] plasmas, the aim of this paper is also to explore the formation of density shoulders. But for simplicity disentangled from complex convective turbulent transport, the main content is to model a very simple idea as shown in figure 1, which is that, without additional radial convective transport, the density profile can be flattened in a certain SOL region where the effective particle source S eff is dominant over the parallel loss L SOL .…”

The influence of full drifts on density shoulder formation at the midplane by numerical modeling

“…JET's experiment found that shoulder formation is dominated by ionization at the midplane in a high-density discharge [2]. The density shoulder is also influenced by neutral ionization in the divertor region during L-mode discharge in EAST [15]. The EMC3-EIRENE simulation showed that reduction of the mean free path of neutral particles at the midplane during shoulder formation can enhance particle recycling and reinforce the ion sources in front of the first wall, thus influencing the shoulder [16].…”