Integrated plasma physics modelling for the Culham steady state spherical tokamak fusion power plant

Abstract

Integrated modelling of important plasma physics issues related to the design of a steady-state spherical tokamak (ST) fusion power plant is described. The key is a steady-state current drive, and 92% of this is provided by a combination of bootstrap and diamagnetic currents, both of which have a substantial toroidal component in a ST. The remaining current is to be provided by either neutral beam injection or radio-frequency waves, and various schemes for providing this are discussed and quantified. The desire to achieve a high bootstrap current drives the design to high plasma pressure, β (normalized to the magnetic field pressure), and high elongation. Both these requirements have implications for ideal magneto-hydrodynamic instability which are discussed. Confinement is addressed both through comparison with the recent scaling laws developed from the conventional tokamak database and selfconsistent one-dimensional modelling of the transport processes. This modelling shows that the power required for the current drive (∼50 MW) is sufficient to heat the plasma to a regime where more than 3 GW of fusion power is produced, taking into account the dilution due to He ash and prompt α-particle losses, which are small. A preliminary study of the micro-instabilities, which may be responsible for the turbulent transport is provided. Given assumptions about the particle confinement, we make estimates of the fuelling requirements to maintain the steady state. Finally, the power loading due to the exhaust is derived using theory-based scalings for the scrape-off layer width.

Overview of physics results from MAST

“…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 barriers.…”

Two Dimensional Li Beam Imaging to Study the Magnetic Field Configuration Effects on Plasma Confinement in Spherical Tokamak CPD

“…Tokamak is one of the prospective devices to confine high temperature and high density plasma. Recently, spherical tokamak (ST) concept is also getting considerable importance in the fusion community because it has the capability of sustaining high beta plasma in a compact shape of low aspect ratio and the cost of the reactor is also cheap [1]. The major engineering difficulty associated with the ST reactor concept is to accommodate the central solenoid for plasma start-up, since STs have only a limited space at the central column of the device to ensure its low aspect ratio (< 2) property.…”

Detection of Disruptions in the High-β Spherical Torus NSTX

“…This is an effective means of driving current and supplying power to the relatively cold plasma, and can result in a substantial extension of the disruption phase, during which detection can often be easily accomplished. However, if there were no solenoid, as is common in the design of ST configurations for the FNSF/CTF [156][157][158][159], pilot plant [160], or reactor [161,162] missions, the disruption process may be much faster, and detection significantly more difficult (see Refs [163] for a description of the CTF/FNSF mission). The extensive non-inductive capabilities [164] of NSTX-Upgrade [127] should allow these studies of disruption detection in high-β, 100% non-inductive fraction discharges with solenoid-based I P feedback control disabled.…”