This paper describes direct experimental investigation of the region of plasma perturbed by a probe. Two adjacent probes have been mounted on the DITE tokamak. The large 'disturber' probe is a rectangular graphite plate that creates the main particle flow pattern and can be biased with respect to the torus. The smaller 'search' Langmuir probe is located 0.5 m away on the same magnetic field line. It can be swept back and forth mechanically through the shadow of the main probe many times during each discharge. Through measurements of ion saturation current at many distinct radii, a complete map of the perturbed region has been built up and transformed into radial and poloidal coordinates. A numerical solution of the two-dimensional diffusion equations is compared with experimental data in the limiter shadow. Two zones of substantially different interconnection length are included in the model. The assumption of equal radial and poloidal cross-field diffusion coefficients (Dperpendicularto = 0.18 m2 s-1) is found to give a good fit with the data. (orig.)
The authors discuss the impurity control limiter (ICL) which has an inverted geometry. The ICL shape is designed to direct the impurities towards the wall. They present the results from a two-dimensional neutral particle code which maps the ionisation of carbon physically sputtered by deuterons from a carbon limiter. This ionisation source is coupled to a one-dimensional impurity transport code which calculates the implied central impurity density. The results demonstrate that the ICL achieves impurity control in two ways. Firstly, many of the sputtered impurities directed towards the wall are not ionised and return to the wall as neutrals. Secondly, much of the ionisation which does occur is located in the scrape-off layer. They conclude that a reduction in central impurity density of a factor of 10 is possible in a Tokamak such as DITE provided that the limiter is the main source of impurities.
Experimental measurements of the energy distributions of ions and electrons arriving at a surface in the boundary layer of a tokamak have been made with a retarding field analyser. The analyser uses a 9 μm entrance slit which is less than the Debye length for the plasma conditions investigated. The electron energy distributions are Maxwellian and the ion energy distributions are displaced Maxwellians, as expected. The sheath potential is derived from the ion energy distribution and found to be ∼2kTe, slowly decreasing with electron temperature. Measurements of secondary electron emission yield have been made on materials exposed in a tokamak as functions of incident electron energy and angle. This data have been integrated to obtain the effective yield of a full three-dimensional Maxwellian distribution. Using these results the theoretical sheath potentials have been calculated as functions of electron temperature and have been found in good agreement with the values that were measured directly.
Measurements of secondary-electron-emission (SEE) yield are reported for samples of clean 5890 PT graphite and for the same material exposed for some months in the JET Tokamak. The samples exposed in the JET Tokamak give substantially higher values than the clean samples. Controlled experiments show that both implantation with H2+ ions and deposition of thin metal films increase the SEE yield. The experimental data have been integrated over a 3D Maxwellian distribution to obtain the values of effective SEE yield expected from a plasma as a function of electron temperature. From these SEE yields, theoretical values of sheath potential and sheath energy transmission factor have been calculated. The implications for ion sputtering and power loss are discussed.