Performance assessment of advanced long-leg divertor geometries using the ARC reactor concept M. Wigram1, B. LaBombard2, M.V. Umansky3

A. Q. Kuang2, T. Golfinopoulos2, D. Brunner2, J.L. Terry2, M.E. Rensink3, D.G. Whyte2

ARC is a compact demonstration fusion power plant design, producing ITER levels of fusion power (525MW) at a size comparable to JET (major radius R ~ 3m).

This is possible through employing REBCO superconducting TF coils, allowing high magnetic fields to be achieved (Pfus/Swall RB4).[1] The HTS magnets support resistive joints/demountable coils, enabling a modular inner vessel design.[2,3]

The reduced size/cost of ARC aims to realise fusion power in a faster time frame.

ARC (Affordable, Robust, Compact) Reactor

References [1] Sorbom, B. et al, Fusion Engineering and Design, 100, 2015, pp378-405. [2] Hartwig, Z. et al, Fusion Engineering and Design, 87 (3), 2012, p201. [3] Barnard, H. et al, Fusion Engineering and Design, 87 (3), 2012, pp248-262. [4] Kuang, A.Q. et al, Conceptual design study for heat exhaust management in the ARC fusion pilot plant, submitted to Fusion Engineering and Design. [5] Eich, T. et al, Nuclear Fusion, 53(9), 2013, 093031. [6] Umansky, M. et al, Physics of Plasmas, 24, 2017, 056112.

UEDGE SOL Physics Model

Ongoing and future work

Work is ongoing to further study the performance of the X-point target geometry in ARC, in particular with separations between X-points on the order of ~1 λq, and with a Ne impurity seeding, to maximise the design power handling performance.

In addition, sensitivity to code input parameters will be explored, such as the impurity model/species, upstream density, and the cross-field transport models. Will seek to move to a full domain grid, modelling up-down asymmetries.

SXD produces stable detachment up to 108MW

SXD grid: Double-null configuration, modelling half-domain. Power scan performed using described physics model.

No impurity seeding - detachment power window of 32-40 MW obtained.

0.5 % Ne impurity - Bifurcation of solutions (Fig 5): • Cold branch - only accessible by

maintaining a detached solution. Detachment window P = 80-108 MW.

• Hot branch - accessible approaching from a high plate-Te, attached solution. No detachment.

Transport model:

• Parallel transport: flux-limited Braginskii fluid equations.

• Radial transport: diffusive thermal transport, particle transport given by:

• Values of D, vconv and χe,i (Fig. 3)tuned to produce expected ARC I-mode midplane profiles (Figs. 4).

Boundary conditions:

• Fixed core input power/density. • Extrapolated wall boundaries Te,i, ni. • Plasma sheath target plates. • 100% recycling on target/wall

boundaries (steady-state operation).

X-point Target Divertor implemented in ARC [4]

ARC divertor challenge: reactor-scale power in compact machine. • ARC divertor has to exhaust 105 MW

of power (assuming ~30% core radiation).

• Eich scaling: λq = 0.4mm - High,

intense divertor power loading.[5] • Compact design - smaller surface area

to dissipate exhaust plasma loading. • Advanced divertor geometry: double-

null, long-legged, secondary X-point target divertor (XPTD).

Simulations of the ADX divertor test tokamak show improved performance of long-legged geometries, in particular the X-point target.[6] Goal to model the X-point target divertor design for ARC in UEDGE, to determine the power loading/detached regime.

1York Plasma Institute, Department of Physics, University of York, Heslington, York, YO10 5DD, UK. 2MIT PSFC, Cambridge, MA 02139, USA. 3LLNL, Livermore, CA 94550, USA

XPTD attains detachment at 90MW without seeding

Figure 1: ARC reactor CAD model cross-section.[1]

Figure 2: Proposed ARC X-point target divertor

geometry, showing closed (blue) and open SOL

(green) magnetic flux surfaces.[3]

Figure 5: Peak outer target Te vs input power P (full

domain), with 0.5% Ne impurity seeding.

Figure 6: Te plot for detached P=105MW case,

with combined plasma/radiation peak power

flux densities to boundaries.

Figure 4: Midplane ni and Te,i profiles produced for

ARC I-mode model. λn~ 5.5mm, λq~ 0.6mm.

Figure 3: vconv and D/χe,i profiles. Transport barrier

implemented by drop in χe,i around separatrix.

Fusion power:

525MW

TF coils:

B0=9.2T

Vacuum vessel: single,

replaceable component

Magnetic joints allow

vertical maintenance.

Figure 7: ARC XPTD grid in UEDGE

(0.56mm X-point separation). Closest

approach between divertor X-point and

primary X-point flux surface ~0.1m.

Analysing the P = 105 MW, 0.5% Ne detached case (cold branch), acceptable power loading are found for all wall boundaries - peak power flux density to outer target = 6.4 MW/m2 (Fig 6).

Impurities: Fixed-fraction impurity model, specified % of local electron density. Neutrals: Fluid-diffusive model.

Figure 8: Peak outer target Te vs input power P, for SXD

(red), and XPTD grids with 1.6mm (green), 0.9 (purple)

and 0.56mm (blue) X-point separations. The XPTD

outer target is taken as the lower plate in the X-point

region (all other plates fully detached).

Work has begun to study the XPTD in the ARC concept setting (Fig 7). Core power scans are performed for the SXD grid and XPTD grids with primary and divertor X-point separations of 1.6mm, 0.9mm and 0.56mm (mapped to midplane).

First results (Fig 8): significant performance benefits over SXD, but only when x-points are closely mapped. For 0.56mm separation of the X-points, the detachment power threshold increases up to 90MW (approaching 105MW requirement, no impurity seeding). Relating to λq, results indicate the X-point separation needs to be within 1-2 λq for significant performance benefit to be realised.

Acknowledgements: This work is supported by the UK Engineering and Physical Sciences Research Council (EPSRC) under training grant number EP/LO1663X/1, and this poster by the Institute of Physics (IOP) Student Conference Fund.