Profile measurements in the plasma edge of mega amp spherical tokamak using a ball pen probe

Abstract

The ball pen probe (BPP) technique is used successfully to make profile measurements of plasma potential, electron temperature, and radial electric field on the Mega Amp Spherical Tokamak. The potential profile measured by the BPP is shown to significantly differ from the floating potential both in polarity and profile shape. By combining the BPP potential and the floating potential, the electron temperature can be measured, which is compared with the Thomson scattering (TS) diagnostic. Excellent agreement between the two diagnostics is obtained when secondary electron emission is accounted for in the floating potential. From the BPP profile, an estimate of the radial electric field is extracted which is shown to be of the order ∼1 kV/m and increases with plasma current. Corrections to the BPP measurement, constrained by the TS comparison, introduce uncertainty into the ER measurements. The uncertainty is most significant in the electric field well inside the separatrix. The electric field is used to estimate toroidal and poloidal rotation velocities from E × B motion. This paper further demonstrates the ability of the ball pen probe to make valuable and important measurements in the boundary plasma of a tokamak.

Quiescence near the X-point of MAST measured by high speed visible imaging

“…In general it is unclear how a filament may act when only one of it's dimensions is sub-gyro scale (i.e a thin ribbon structure) however for circular filaments, FLR effects can be important in gyro-scale filaments [36]. Additionally strong shear flows are known to be driven in the vicinity of the separatrix due, at least in part, to the variation in the sheath potential [37]. This shear flow may interact strongly with filaments close the X-point, where the cross-section begins to distort, either causing the filament to break up or spread the filament poloidally thereby further reducing its coherency Finally, it is well known that in any realistic plasma there is a degree of stochasticity to the magnetic field in the X-point region [38].…”

Multi-code analysis of scrape-off layer filament dynamics in MAST

“…While indirect experimental estimates of the magnetic shear are available through magnetic reconstruction, the velocity profile in the SOL is considerably more difficult to quantify. However, previous measurements [50] suggest that the velocity shear might be significant in the proximity of the separatrix but it quickly decays further out, where our filaments were tracked and simulated. The study of these effects is therefore left for future studies.…”

Numerical investigation of isolated filament motion in a realistic tokamak geometry

“…If the electrostatic potential is initialized as 0 or an odd function, This serves as an approximate temperature above which 3D effects can be expected to play a role in filament dynamics. Given that near SOL temperatures (and in some cases far SOL temperatures) in MAST are universally above 4eV [47] it is important to assess the role of these 3D effects. To investigate the role of 3D effects on filament motion and the resulting particle transport scans in both electron temperature and filament radius have been conducted in a geometry representative of the SOL in MAST.…”