Overview of physics results from MAST

Abstract

Several improvements to the MAST plant and diagnostics have facilitated new studies advancing the physics basis for ITER and DEMO, as well as for future spherical tokamaks. Using the increased heating capabilities P NBI ≤ 3.8 MW H-mode at I p = 1.2 MA was accessed showing that the energy confinement on MAST scales more weakly with I p and more strongly with B t than in the ITER IPB98(y,2) scaling. Measurements of the fuel retention of shallow pellets extrapolate to an ITER particle throughput of 70% of its original design value. The anomalous momentum diffusion, χ φ , is linked to the ion diffusion, χ i , with a Prandtl number close to P φ ≈ χ φ /χ i ≈ 1, although χ i approaches neoclassical values. New high spatially resolved measurements of the edge radial electric field, E r , show that the position of steepest gradients in electron pressure and E r are coincident, but their magnitudes are not linked. The T e pedestal width on MAST scales with the β pol rather than ρ pol . The ELM frequency for type-IV ELMs, new in MAST, was almost doubled using n = 2 resonant magnetic perturbations from a set of 4 external coils (n = 1, 2). A new internal 12 coil set (n ≤ 3) has been commissioned. The filaments in the inter-ELM and L-mode phase are different from ELM filaments, and the characteristics in L-mode agree well with turbulence calculations. A variety of fast particle driven instabilities were studied from 10 kHz saturated fishbone like activity up to 3.8 MHz compressional Alfvén eigenmodes (CAE). The damping rate of ellipticityinduced AE was measured to be 4% using the new internal coils as antennae. Fast particle instabilities also affect the off-axis NBI current drive and lead to fast ion diffusion of the order of 0.5 m 2 /s and reduce the driven current fraction from 40% to 30%. EBW current drive start-up is demonstrated for the first time in a spherical tokamak generating plasma currents up to 55 kA. Many of these studies contributed to the physics basis of a planned upgrade to MAST. Introduction: MAST [1]is one of the two leading tight aspect ratio (A = ε −1 = R/a = 0.85 m/0.65 m ∼ 1.3, I p ≤ 1.5 MA) tokamaks in the world. The hot T ≤ 3 keV, dense n e = (0.1 − 1) × 10 20 m −3 and highly shaped (δ ≤ 0.5, 1.6 ≤ κ ≤ 2.5) plasmas are accessed at moderate toroidal field B t (R = 0.7 m) ≤ 0.62 T and show many similarities to conventional aspect ratio tokamaks. Detailed physics studies using the extensive array of state of the art diagnostics and access to different physics regimes help to consolidate the physics basis for ITER and DEMO [2,3], and explore the viability of future devices based on the spherical tokamak (ST) concept such as a component test facility (CTF) [4] or an advanced power plant [5]. The challenge for today's experiments is to find an integrated scenario that extrapolates to these future devices, in particular to develop plasmas with reduced power load on plasma facing components, notably from edge localised modes (ELM), but high confinement facilitated by internal or edge transport ba...

Overview of physics results from the conclusive operation of the National Spherical Torus Experiment

“…This scaling has been reported in several tokamaks including DIII-D, C-Mod, and the MAST ST [63], but the best fit to the NSTX data indicates a significantly stronger scaling, closer to linear in β ped p (figure 9) [64]. Also, although past analysis using KBM constraints in the ballooning critical pedestal model [62] have yielded a (β has stronger shaping than MAST, and that MAST research claims that the plasmas are ballooning mode-limited [65] as opposed to NSTX which is computed to be peeling modelimited.…”

Scrape-off layer ion temperature measurements at the divertor target in MAST by retarding field energy analyser

“…The first discharges studied were in a double-null configuration with plasma current, I p = 900 kA at two neutral beam heating powers, P NBI = 1.5 MW and P NBI = 3.4 MW [16]. The discharge with P NBI = 1.5 MW has regular type III ELMs and the T i data are extracted from the inter-ELM periods of the discharge.…”

“…A similar shaped double-null discharge with a lower plasma current, I p = 600 kA, with beam heating, P NBI = 3.4 MW, and regular type I ELMs has been studied [16]. The RFEA data has been extracted from inter-ELM periods in the discharge.…”

“…Finally a lower single null discharge with I p = 600 kA, P NBI = 3.4 MW and regular type I ELMs has been studied [16]. Figure 6 shows the temperature profile of T i from RFEA and T e from LP as a function of distance from the LCFS at the target.…”

Ion energy measurements on MAST using a midplane RFEA

“…3 In this paper, the results of inter-ELM and ELM T i measurements made on MAST [11] using a RFEA mounted on a reciprocating probe at the midplane will be presented. In section 2, a brief introduction to RFEAs is given along with a description of the plasma conditions used.…”