Plasma Phys. Control. Fusion 39 (1997) B247–B260. Printed in the UK PII: S0741-3335(97)87238-7 High-β performance of the START spherical tokamak A Sykes, R Akers, L Appel, P G Carolan, N J Conway, M Cox, A R Field, D A Gates, S Gee, M Gryaznevich, T C Hender, I Jenkins, R Martin, K Morel, A W Morris, M P S Nightingale, C Ribeiro, D C Robinson, M Tournianski†, M Valovic, M J Walsh‡ and C Warrick UKAEA Fusion, Culham, Abingdon, Oxon, OX14 3DB, UK (UKAEA/Euratom Fusion Association) Received 13 June 1997 Abstract. Using additional heating provided by neutral-beam injection, the START spherical tokamak at UKAEA Fusion Culham has achieved high-β (ratio of volume average plasma pressure to vacuum magnetic-field pressure) values of β T  30%, more than twice the value previously obtained in a tokamak. These plasmas reach normalized beta values of β N = β%/(I/aB) ∼ 4 at values of auxiliary heating power comparable to the ohmic power. Operation at high normalized current I N = I p /aB T ∼ 8 is observed, so that the plasma current exceeds the central rod toroidal-field current for the first time in a hot tokamak. 1. Introduction The low-aspect-ratio limit of the tokamak device, known as the ‘spherical tokamak’, was predicted by Peng and Strickler in 1986 [1] to have several advantages over the conventional tokamak. These advantages include simplicity of construction, lower magnetic- field requirements and improved plasma stability. Interest in this concept is growing rapidly and the potential for a fusion power plant has recently been reported [2]. The first experimental verification of many of these properties has been provided by the START (small tight aspect ratio tokamak) at UKAEA Fusion, Culham [3, 4]. A key advantage predicted for the spherical tokamak is its ability to operate at high β , where β is the ratio of the plasma pressure to the pressure of the magnetic field required to contain the plasma, an important measure of the efficiency of a magnetic containment device. This arises from the Troyon [5] and Sykes [6] theoretical scalings, which imply that the β achievable in a tokamak should be maximized using a combination of low-aspect-ratio A (where A = R/a is the ratio of the major to minor radii of the plasma) and extreme plasma shaping (high elongation and triangularity). This relationship has been verified experimentally over a wide range of tokamaks [7, 8], and indeed the previous highest value of β was achieved on the DIII-D tokamak which combines (relatively) low-aspect-ratio A ∼ 2.8 and is capable of operation at high elongation and triangularity. The highest value achieved in DIII-D is β T = 12.6% [7], where β T = 2μ 0 VB 2 0  p dV † Also at: University of Essex, Wivenhoe Park, Colchester, UK. ‡ Also at: Walsh Scientific Ltd, Abingdon, Oxon, OX14 2RT, UK. 0741-3335/97/SB0247+14$19.50 c  1997 IOP Publishing Ltd B247