Ion temperature measurements of L-mode filaments in MAST by retarding field energy analyser

Abstract

Abstract. Retarding field energy analysers (RFEAs) have been used to compare the ion temperature (T i ) of large plasma filaments with the background plasma (composed of small scale filaments) at the midplane and divertor target in L mode discharges in the Mega Amp Spherical Tokamak (MAST). At low densities, at the midplane and divertor, at distances from 2 to 4cm from the separatrix the temperature of ions in large filaments was found to be 2 to 3 times larger than the background plasma. At the midplane, the electron temperature for both large filaments and background plasma was around 3 to 7 times smaller than the ion temperature and had a flat profile across the scrape off layer (SOL). At higher densities, at the midplane and divertor, both the filament and background ion temperatures were smaller than at low density. At the midplane, the filament and background ion and electron temperature profiles across the SOL were relatively flat and of comparable magnitude, ranging in temperature from 5 to 25eV.

On the role of filaments in perpendicular heat transport at the scrape-off layer

“…Instead, T i displays a strong reduction after the shoulder is formed, T i,f il becomes similar to T i,back and the e-folding length drops to λ T i 8 mm for Λ div 10. Similar evolutions have been reported in Alcator C-mod and MAST [42,22]. The ts used for each case are also displayed in the gure: for the background in the low Λ div case, a linear t is found to be the most 10 -1 10 0 10 -1 10 0…”

Divertor impurity seeding experiments at the COMPASS tokamak

“…The value of sheath heat transmission coefficient γ depends on the ratio Ti/Te and the coefficient of secondary emission [9]. We assume that the secondary emission is negligible due to the small angle of incidence of the field lines with respect to the tile top surface (1-3 o ) [10] and that due to its high collisionality the plasma is isothermal, similarly to the conditions in MAST [11]. This yields value of γ equal to 7.…”

On the relation between non-exponential scrape off layer profiles and the dynamics of filaments

“…An interesting exception is represented by the results in [4] where the temperature profiles also display a clear broadening (and an absolute decrease). Investigations of the ion temperature profile are few [17,18] and, to our knowledge, do not allow a proper characterisation of the profiles in the far SOL. This is unfortunate, since the energy of the ions impinging on the material surfaces determines their erosion.…”