Using SOLPS to confirm the importance of total flux expansion in Super-X divertors

TY - JOUR

T1 - Using SOLPS to confirm the importance of total flux expansion in Super-X divertors

AU - Moulton, D.

AU - Harrison, J.

AU - Lipschultz, B.

AU - Coster, D.

N1 - © 2017, IOP Publishing Ltd. This is an author-produced version of the published paper. Uploaded in accordance with the publisher’s self-archiving policy. Further copying may not be permitted; contact the publisher for details.

PY - 2017/5/8

Y1 - 2017/5/8

N2 - We show that a central characteristic of Super-X divertors, total flux expansion f R (defined as the ratio of the elementary area normal to the magnetic field at the target to that at the X-point), significantly changes the characteristics of the target plasma for fixed upstream conditions. To isolate the effect of total flux expansion from other effects, we utilise SOLPS-5.0 simulations of an isolated slot divertor leg in a minimally complex, rectangular geometry. The grid is rotated outwards about a fixed X-point in order to perform a scan in which only the total flux expansion increases, by means of a decrease in the target magnetic field at higher major radius. We find that if the SOL remains in the attached, conduction-limited regime throughout the scan, the target electron density scales approximately as , while the target electron temperature scales approximately as , in good agreement with the modified two-point model presented in Petrie et al (2013 Nucl. Fusion 53 113024). If, however, the SOL transitions from the sheath-limited regime to the conduction-limited regime during the scan, the simulated scalings of target electron temperature and density are weaker than predicted by the modified two-point model. The upstream density for transition from sheath- to conduction-limited regimes is found to scale approximately with , in agreement with the modified two-point model. Assessing upstream-density-driven detachment onset, we find that the target electron temperature at which target density rollover occurs (∼0.6 eV) is independent of f R. Given this, the modified two-point model predicts a halving of the upstream (and target) densities at which rollover occurs when f R is doubled, in good agreement with the simulation results.

AB - We show that a central characteristic of Super-X divertors, total flux expansion f R (defined as the ratio of the elementary area normal to the magnetic field at the target to that at the X-point), significantly changes the characteristics of the target plasma for fixed upstream conditions. To isolate the effect of total flux expansion from other effects, we utilise SOLPS-5.0 simulations of an isolated slot divertor leg in a minimally complex, rectangular geometry. The grid is rotated outwards about a fixed X-point in order to perform a scan in which only the total flux expansion increases, by means of a decrease in the target magnetic field at higher major radius. We find that if the SOL remains in the attached, conduction-limited regime throughout the scan, the target electron density scales approximately as , while the target electron temperature scales approximately as , in good agreement with the modified two-point model presented in Petrie et al (2013 Nucl. Fusion 53 113024). If, however, the SOL transitions from the sheath-limited regime to the conduction-limited regime during the scan, the simulated scalings of target electron temperature and density are weaker than predicted by the modified two-point model. The upstream density for transition from sheath- to conduction-limited regimes is found to scale approximately with , in agreement with the modified two-point model. Assessing upstream-density-driven detachment onset, we find that the target electron temperature at which target density rollover occurs (∼0.6 eV) is independent of f R. Given this, the modified two-point model predicts a halving of the upstream (and target) densities at which rollover occurs when f R is doubled, in good agreement with the simulation results.

KW - simplified geometry

KW - SOLPS

KW - Super-X

UR - http://www.scopus.com/inward/record.url?scp=85019407593&partnerID=8YFLogxK

U2 - 10.1088/1361-6587/aa6b13

DO - 10.1088/1361-6587/aa6b13

M3 - Article

AN - SCOPUS:85019407593

SN - 0741-3335

VL - 59

JO - Plasma Physics and Controlled Fusion

JF - Plasma Physics and Controlled Fusion

IS - 6

M1 - 065011

ER -