Speaker
Sarah Elmore
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P1.1027.pdf
Scaling of the scrape-off layer width in MAST L-mode plasmas as
measured by infrared thermography
S. Elmore1, A. J. Thornton1, R. Scannell1, A. Kirk1 and the MAST Team1
1
CCFE, Culham Science Centre, Abingdon, Oxon, OX14 3DB, UK
Understanding the plasma parameters that affect the scrape off layer (SOL) width is a key
issue for future tokamaks as the power entering the SOL (of order 100 MW [1] in ITER) and
the SOL width (of order millimetres [2]) determine the heat flux to the divertor surfaces. The
Eich scaling [2] can be used to characterise the divertor heat flux profile as an exponential
decay for the SOL width and a Gaussian spreading factor due to diffusion around the last
closed flux surface (LCFS). In this work, attached, double null, L-mode infrared (IR) profiles
measured at the upper outer divertor on MAST are characterised by the fall off length, λq,
and the spreading factor, S as returned from profile fits to the data using the Eich
parameterisation for plasmas spanning an operational space of 400 kA ≤ IP ≤ 900kA, 0MW
≤ PNBI ≤ 3.1MW, 0.87 × 1019 𝑚−3 ≤ ̅̅̅
𝑛𝑒 ≤ 4.6 × 1019 𝑚−3; where IP is plasma current, PNBI
is neutral beam heating power and ̅̅̅
𝑛𝑒 is line-averaged density. Regression of the data has
shown that the strongest dependence of λq is on IP to the power of -1.04, which is consistent
with multimachine scalings of H-mode plasmas [3]. Regressions including other variables
for example, the parallel connection length (L||), will be performed to assess how they affect
λq. The parallel connection length is particularly relevant to MAST-U where it can vary by a
factor three. Midplane Thomson scattering (TS) measurements of the electron temperature
and density fall off lengths allow approximation of the heat flux width at the midplane using
either sheath or conduction limited models of the SOL. Surprisingly, the results suggest that
the sheath limited approximation gives the best agreement between the IR and TS heat flux
widths, independent of the regime; a full study will be performed in this work to investigate
this result. The best regression for the spreading parameter, S, showed a strong negative
dependence on the poloidal magnetic field, as has been seen in ASDEX Upgrade [4]. A
comparison of double and single null plasmas will be presented to investigate the dependence
of S on magnetic configuration.
[1] A. Loarte et al., Nucl Fusion 47 (2007) S203-63
[2] T. Eich et al., Phys. Rev. Lett 107 (2011) 215001
[3] T. Eich et al., Nucl. Fusion 53 (2013) 093031
[4] B. Sieglin et al., Plasma Phys Control Fusion 58 (2016) 055015
