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  • WIGRAM et al.

    1

    PERFORMANCE ASSESSMENT OF TIGHTLY-BAFFLED LONG-LEG

    DIVERTOR GEOMETRIES IN THE ARC REACTOR CONCEPT

    M.R.K. WIGRAM

    York Plasma Institute, University of York

    York, United Kingdom

    Email: mrkw500@york.ac.uk

    B. LABOMBARD, A.Q. KUANG, T. GOLFINOPOULOS, J. TERRY, D. BRUNNER, D.G. WHYTE

    MIT Plasma Science and Fusion Center

    Cambridge, MA 02139, USA

    M.V. UMANSKY, M.E. RENSINK

    Lawrence Livermore National Laboratory

    Livermore, CA 94550, USA

    Abstract

    A means to handle the extreme power exhaust from tokamak-based fusion power reactors remains to be demonstrated.

    Advanced divertor configurations have been proposed as potential solutions, including double-nulls, long-legs and magnetic

    field flaring with secondary X-points. Modelling of tightly-baffled, long-leg divertor geometries in the divertor test tokamak

    concept ADX has shown the potential to access passively stable, fully detached divertor regimes over a broad range of

    parameters. The question remains as to how these advanced divertor configurations may perform in a reactor setting. To

    explore this, numerical simulations are performed of these configurations in the context of the ARC reactor concept. The ARC

    design has been recently updated to include a tightly-baffled, long-leg divertor with an X-point target. ARC provides an

    appropriate reactor test scenario for advanced divertor configurations, with a projected scrape-off-layer (SOL) heat flux width

    of 0.4 mm and total power exhaust requirement of 105 MW. Using the divertor geometry and magnetic equilibrium from the

    updated ARC design, simulations of the ARC edge plasma and divertor are carried out with UEDGE. The anticipated radial

    plasma profiles at the outer midplane are specified and power exhaust from the core is scanned over a wide range. Results

    indicate that for a Super-X Divertor configuration with 0.5% fixed-fraction neon impurity radiation there exists a passible

    stable detached divertor regime for power exhaust in the range of 80 to 108 MW. For an X-point target divertor geometry,

    significant performance benefits are observed over the Super-X, but only when radial separation of X-point flux surfaces are

    small. With separations within 2 SOL heat flux widths, the detachment exhaust power threshold increases up to 74 MW without

    impurity seeding. Pushing simulation grids to achieve even smaller X-point separations could potentially achieve detachment

    at even greater exhaust power.

    1. INTRODUCTION

    The ARC reactor (Affordable, Robust, Compact reactor) is a conceptual tokamak design for a reduced size, cost

    and complexity demonstration fusion pilot power plant (200-250 MWe), designed to operate at a comparable

    fusion power to ITER (~500 MW), but at a compact size (R0 = 3.3 m) comparable to JET [1]. To achieve this

    fusion power at a compact size, the design employs REBCO (Rare Earth Barium Copper Oxide) superconducting

    tape for the toroidal field (TF) coils [2] to allow for high magnetic field operation (B0 = 9.2T). An added benefit

    FIG 1: (a) 3D ARC reactor design projection, with demountable toroidal magnetic field coils [1]. (b) Schematic diagram

    of the proposed ARC long-legged X-point target divertor [4], with closed (blue) and open SOL (green) magnetic field

    lines shown. [Permissions for use of figures obtained]

  • IAEA-CN-258-TH/P7-20

    of the superconducting REBCO tape material is that it supports the use of resistive joints, enabling the TF coils

    to be demountable [2], which allows for easy inner vessel replacement, as well as for the poloidal field coil set to be placed inside the TF coils while still being sufficiently shielded by the blanket to neutron damage. The reduced

    size and cost of this novel design makes it more economical, with potentially shorter development timeframes

    than other fusion reactor concepts. These factors make the ARC concept an interesting design to study. A 3D

    design projection for the ARC concept is given in Fig. 1(a).

    Like all tokamak power plant designs, ARC must tackle the divertor heat flux issue – where peak heat fluxes in

    the scrape-off-layer (SOL) can exceed the limits that materials components can withstand. At first glance, divertor

    concerns appear to be even greater for ARC, where the high magnetic field leads to an Eich H-mode scaling power

    decay width of λq|| ~ 0.4 mm [3]. However, ARC’s high magnetic field allows it to attain the areal power density

    needed for a reactor (~2.5 MW/m2) based on economic considerations. At the same time, its reduced major radius

    decreases the total power output required as R2 (scaling with first wall surface area). The net effect is that the

    parallel heat flux entering into the divertor is expected to be similar to that of larger, low field devices that achieve

    similar areal power loading, despite the smaller λq||. This, combined with the total exhaust power approaching 105

    MW (assuming a ~30% radiation fraction in the core) defines the power exhaust challenge for ARC.

    To attempt to cope with the high divertor power loads, the ARC design has been recently updated to include a

    tightly-baffled, long-leg divertor with an X-point target design [4] – with no impact on core plasma volume or tritium breeding ratio. Whilst tightly-baffled long-legged configurations like these have not yet been

    experimentally studied at reactor relevant parameters (only low power density, unbaffled experiments in TCV [5]), modelling of these configuration in application to the ADX design has shown the potential to access passively

    stable, fully detached divertor regimes over a broad range of parameters [6]. A factor of 10 enhancement in peak

    power handling compared to conventional divertors has been obtained in some cases. This paper presents work

    that has been performed to model the ARC SOL and divertor design using the UEDGE code to assess the thermal

    loading challenge for ARC and determine appropriate operating conditions for which passively stable detached

    regimes can be achieved.

    This paper is structured as follows: Section 2 describes the UEDGE physics model used for the ARC study.

    Section 3 applies this model to a Super-X divertor setup and presents the results for input power scans both with

    and without impurity seeding. Section 4 presents the results applying the same model and power scans to an X-

    point target divertor geometry without any impurity seeding. Discussion of the results is presented in Section 5.

    2. UEDGE ARC SOL PHYSICS MODEL

    UEDGE is a well established edge fluid simulations code [7–9], that has been extensively used for interpretation

    of tokamak edge data [10–12] and for modelling of advanced divertors [13]. Most recently, UEDGE has been

    applied to modelling X-point target divertors in the ADX concept [6], making it an ideal tool for extending the

    study of X-point target divertors to ARC.

    FIG 2: Schematic diagram of UEDGE ARC SOL/divertor grid mapped over magnetic ARC magnetic geometry (left), with

    the location of the reactor first wall given by the blue line. Simulation grid plots for the SXD (middle) and X-point target

    (right) geometries.

  • WIGRAM et al.

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    ARC employs an upper- and lower-divertor configuration for double-null operation (Fig. 1(b)). Magnetic

    equilibrium data was used to implement a half-domain ARC geometry into UEDGE for two divertor setups: a)

    Super-X Divertor (SXD), and b) secondary X-point target divertor. Fig. 2 shows the UEDGE grids for each case.

    Both configurations are considered in these modelling studies to see how they will compare with each other for

    ARC.

    The ARC design paper [1] as well as knowledge from previous data provided motivation for the physics model.

    ARC is designed to operate in I-mode — an improved energy confinement regime with the combined high energy

    confinement of H-mode and low particle confinement of L-mode [14]. The thermal and particle transport models

    were therefore tuned to reproduce the expected midplane profiles expected for the ARC design. In the UEDGE

    model of ARC used here, the radial particle transport is represented by a combination of diffusion and pinch

    velocity, given by the equation:

    𝛤 = 𝐷∇𝑛 + 𝑣𝑐𝑜𝑛𝑣𝑛 (1)

    where 𝛤 is the radial particle flux, D is the diffusion coefficient and vconv is the convective pinch velocity. This form of a convection velocity for anomalous radial transport has been previously used in UEDGE modelling

    studies [15]. The diffusion coefficient D was set to a typical value of 0.025 m2s-1 throughout the domain, and a

    profile for vconv was determined to reproduce the midplane n profile expected for ARC (based on I-mode data):

    nLCFS ~ 1020 m-3, falling with a decay length of λn ~ 5.5 mm, and a flattened density shoulder at 10 mm radial

    distance into the SOL. The vconv profile was mapped uniformly along the magnetic flux surfaces on the low-field-

    side (LFS), from the outer midplane down to the divertor target plate. On the high-field-side (HFS), this value

    was set to zero throughout the SOL, as no density shoulder or convective radial flux is observed experimentally

    on the HFS [16]. The v