Overview of MAST results

Abstract

Significant progress has been made on the Mega Ampere Spherical Tokamak (MAST) towards a fundamental understanding of transport, stability and edge physics and addressing technological issues for future large devices. Collaborative studies of the L-H transition with NSTX and ASDEX Upgrade confirm that operation in a connected double-null configuration significantly reduces the threshold power, P thr . The MAST data provide support for a theory for the transition based on finite β drift wave turbulence suppression by self-generated zonal flows. Analysis of low and high field side density gradients in the H-mode pedestal provides support for an analytical model of the density pedestal width dependent on the neutral penetration depth. Adding MAST data to international confinement databases has enhanced confidence in scalings for ITER by significantly expanding the range of β and ε explored and indicates a slightly stronger ε dependence than in current scalings. Studies of core transport have been conducted for well-diagnosed L-mode, H-mode and internal transport barrier (ITB) discharges using TRANSP, and microstability and turbulence studies have been carried out using GS2. Linear micro-stability analysis indicates that ITG modes are typically unstable on all flux surfaces with growth rates that are comparable to the equilibrium E × B flow shearing rate. Mixing length estimates of transport coefficients from ITG (neglecting flow shear) give diffusion coefficients that are broadly comparable with observed thermal diffusivities. Non-linear, collisionless ETG calculations have been performed and suggest radially extended electrostatic streamers up to 100ρ e across in radius. Transport from ITG could easily be suppressed in regions where the E × B shear flow rate,

Chapter 3: MHD stability, operational limits and disruptions

“…The major radii quoted are the geometric radius. plasma-facing surface in MAST [313], JET [314] and ASDEX Upgrade [315], that show evidence of disruption power fluxes outside the main divertor region. TEXTOR [316,317] is equipped with an IR camera monitoring the limiter.…”

Recent progress on the development and analysis of the ITPA global H-mode confinement database

“…Plasma rotation has also been seen to be strongly correlated with energy confinement experimentally [109][110][111][112][113][114], but the lack of rotation data in DB3 means that this cannot, as yet, be studied with this database.…”

Exhaust properties of centre-column-limited plasmas on MAST

“…Furthermore, high trapped-particle fractions and toroidicity favour large self-driven current fractions from both diamagnetic [8] and, at high normalized β N ≡ β t aB t /I p and elongation, bootstrap [9,10] components, plus stronger Ohmic heating from neo-classically enhanced resistivity [8]. All these benefits of an ST arrangement are being confirmed in current experiments [8,[11][12][13][14][15][16][17][18], especially record β values in excess of 40% [19], reinforcing confidence in its longer-term potential.…”

“…Present STs such as the Mega Ampere Spherical Tokamak (MAST, R 0 = 0.85 m, B 0 0.6 T, I p 1.4 MA, ε 0.77) [17] or National Spherical Torus Experiment (NSTX, R 0 = 0.85 m, B 0 0.5 T, I p 1.4 MA, ε 0.79) [18] typically form their plasmas in poloidal-divertor equilibria, as also used by conventional tokamaks. However, the minimum possible aspect ratio is realized when the plasma is instead limited on the centre column of the machine itself (see figure 1(a)).…”