Achievements of actively controlled divertor detachment compatible with sustained high confinement core in DIII-D and EAST

Abstract

The compatibility of efficient divertor detachment with high-performance core plasma is vital to the development of magnetically controlled fusion energy. The joint research on the EAST and DIII-D tokamaks demonstrates successful integration of divertor detachment with excellent core plasma confinement quality, a milestone towards solving the critical Plasma-wall-interaction (PWI) issue and core-edge integration for ITER and future reactors. In EAST, actively controlled partial detachment with Tet,div ~ 5 eV around the strike point and H98 > 1 in different H-mode scenarios including the high βP H-mode scenario have been achieved with ITER-like tungsten divertor, by optimizing the detachment access condition and performing detailed experiments for core-edge integration. For active long pulse detachment feedback control, a 30s H-mode operation with detachment-control duration being 25s has been successfully achieved in EAST. DIII-D has achieved actively controlled fully detached divertor with low plasma electron temperature (Tet,div ≤ 5 eV across the entire divertor target) and low particle flux (degree of detachment, DoD >3), simultaneously with very high core performance (βN ~3, βP >2 and H98~1.5) in the high βP scenario being developed for ITER and future reactors. The high-βP high confinement scenario is characterized by an internal transport barrier (ITB) at large radius and a weak edge transport barrier (ETB, or pedestal), which are synergistically self-organized. Both the high-βP scenario and impurity seeding facilitate divertor detachment. The detachment access leads to the reduction of ETB, which facilitates the development of an even stronger ITB at large radius in the high βP scenario. Thus, this strong large radius ITB enables the core confinement improvement during detachment. These significant joint DIII-D and EAST advances on the compatibility of high confinement core and detached divertor show a great potential for achieving a high-performance core plasma suitable for long pulse operation of fusion reactors with controllable steady-state PWIs.

Comparison of partial and deep energy detachment behaviors with Ar seeding on EAST new corner slot divertor

“…As shown in figure 1, the lower outer (LO) divertor with corner slot geometry is more closed than the upper divertor. With the lower closed divertor and LO strike point near the divertor corner, the line-averaged midplane density threshold for detachment by increasing density is ∼10% lower than that with upper open divertor [19], which is consistent with the results in DIII-D [20] and TCV [21]. In addition, with the upper open divertor, the partial energy detachment with Ar seeding is usually accompanied with confinement degradation of ∼10% [16], and the deep energy detachment has not been achieved.…”

Integrated RMP-based ELM-crash-control process for plasma performance enhancement during ELM crash suppression in KSTAR

“…One of the potential candidates is the KSTAR divertor plasma detachment control based on the simplified real-time modeling [66], reducing the heat load on PFCs. However, the impurity seeding, the primary actuator of the detachment control, has a side effect on core plasma performance [67] and affects conditions to access ELM crash suppression by changing electron density and recycling conditions [68,69]. Therefore, incorporating the detachment control requires upgrading the integrated process to balance plasma detachment, ELM crash suppression, and plasma performance enhancement.…”

Closed corner divertor with B × ∇B away from the divertor: a promising divertor scenario for tokamak power exhaust

“…Partly for this reason, ITER is designed to operate with downwards B × ∇B direction. In addition, a relatively lower H 98 factor is usually obtained with B × ∇B directed away from the divertor compared with B × ∇B towards the divertor [22], but impurity exhaust capability is usually stronger, which is more important for steady-state high-performance operations in a tokamak with full-metal plasma-facing components. As the controlling of heat flux and impurity concentration are more serious challenges for future fusion reactors, they should have higher priorities than other factors, thus this divertor scenario may be a solution that deserves serious considerations.…”