Speaker
Peter Francis Buxton
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P5.1058.pdf
Merging/Compression start-up in ST40:
Comparison between the first experimental results and numerical model
P.F. Buxton1 , O. Asunta1 , M.P. Gryaznevich1 , B Huang1 , S. McNamara1 , J.M. Wood1
1 Tokamak Energy Ltd., 120A Olympic Avenue, Milton Park OX14 4SA, UK
Tokamak Energy Ltd. is presently commissioning ST40 [1], a relatively small (R = 0.4 m)
spherical tokamak which has been designed to operate with a high toroidal field (BT ∼ 3 T)
and high current density (I p ∼ 2 MA). The aim of this commissioning has been to test all sub-
systems and to integrate them into the Plasma Control System (PCS). During this phase ST40
operated with a toroidal field of 0.7 T at R = 0.4 m (Irod = 1.38 MA) and have achieved a plasma
current of I p ∼ 300 kA.
Start-up in ST40 uses a technique called Merging/Compression which involves [2, 3]: in-
ductively forming plasma around two internal poloidal field coils, when the current within the
internal poloidal coils is close to zero the two plasma rings are attracted towards each other and
merge, and through magnetic reconnection ∼ 5% of the poloidal flux is converted into thermal
energy.
To manage forces within ST40 the vacuum vessel is relatively thick, consequently, substantial
eddy currents (up to 700 kA) are induced into the vessel during Merging/Compression start-
up. This adds two complications: firstly, it makes magnetic reconstruction more challenging
and secondly, the eddy currents have a large impact on the Merging/Compression technique -
therefore they must be accounted for when designing Merging/Compression start-up scenarios.
In this presentation both of these issues are discussed and addressed. During this commissioning
phase ST40 had the following magnetic diagnostics: 5 Rogowski coils, 36 flux loops and 70
poloidal field pickup probes, and we have developed and tested a new magnetic reconstruction
code in which the vessel is approximated by the 20 longest lived eigenmode excitations and the
plasma is approximated by a set of orthogonal basis functions. Later, this code will be integrated
into the PCS to allow real-time control of the plasma current, plasma current centroid position
(RI p and ZI p ) and plasma shape. We have included the effects of these eddy currents in our
predictive modeling of the Merging/Compression start-up technique, and find good agreement
with experimental results.
References
[1] M.P. Gryaznevich, ST Path to Fusion: First Results from ST40, EPS (2018)
[2] P.F. Buxton, M.P. Gryaznevich, Merging compression start-up predictions for ST40, Fus. Eng. Des. (2017)
[3] M.P. Gryaznevich, A. Sykes, Merging-compression formation of high temperature tokamak, Nuc. Fus. (2017)
