Thomson scattering measurements in low-density plasmas in the TST-2 spherical tokamak

Fully non-inductive plasma start-up with lower-hybrid waves using the outboard-launch and top-launch antennas on the TST-2 spherical tokamak

“…2). Because of the low density, the same discharge was repeated and the measurements were ensemble averaged to bring the Thomson scattering signal out of the noise floor [19]. In fully non-inductive plasma current rampup, current holes due to back EMF with little particle and energy confinement have been observed elsewhere [20].…”

A simulation environment to simulate lower-hybrid-wave-driven plasmas efficiently

“…The viscosity and diffusivity in the simulation are set to be ν = ν n = 10 −6 v A R 0 ≈ 1 m 2 /s and the resistivity η = 10 −7 µ 0 v A R 0 ≈ 1•10 −6 Ω•m, where v A is the Alfvén velocity at magnetic axis. For grid study purposes the number of grid points of the simulations are set to (24,16,24), (32, 16, 32), (64, 16, 64), and (128, 16, 128) for the cylindrical coordinates (R, φ, z), denoted as very coarse, coarse, fine, and very fine, respectively. The bulk plasma for the MEGA simulation is initialized using a simple fit of the experimentally measured density profile as well as constant bulk pressure as initial conditions.…”

“…Furthermore, recent measurements showed that the bulk temperature profile is hollow, which also implies that the toroidal current density profile might also be hollow. Furthermore, in the TST-2 experiments, a small fraction of electrons at high energy are thought to carry almost the entire plasma current [24].…”

“…Furthermore, some plasma parameters were tuned artificially to obtain converged simulations. A more sophisticated simulation including density and temperature profiles measured by Thomson scattering diagnostic [24] was presented recently [26], where a ray-tracing solver GENRAY [17] and a bounced-averaged Fokker-Planck solver CQL3D [4] were coupled using EFIT [9] equilibrium field for peaked distributions. It was observed that the current carrying electrons do not penetrate to the core region for a fixed magnetic field geometry such that the force balance of the plasma was not solved self-consistently.…”

“…Therefore, it can handle arbitrary distribution functions, and hence, energetic electron density, pressure, and current density profiles self consistently. Since the contribu-tion of the bulk plasma to the total pressure is about two orders of magnitude lower compared to the energetic particle contribution [24], the focus of this study is set to the significant modifications of the magnetic field configuration due to the presence of the energetic electrons.…”