Profiles of the Balmer lines D,(H,), Db(Hs) and D, (H,) have been measured in the scrape-off layer and within the edge of the TEXTOR (upgrade) plasma, under Ohmic conditions and with neutral-beam injection. Each line profile shows a strong Zeeman effect in the vicinity of line centre, and a marked central dip when mainly the ( I components are observed. The line core evidently originates from cold atoms in the edge plasma, excited in the course of molecular dissociation, while the broad pedestal on which the core rests is radiated by excited atoms produced through chargeexchange recombination of deuterons (protons), transported outwards from the much hotter plasma interior, and by atoms heated directly by collisions with the deuterons (protons). Core temperatures of about 0.5 eV and less are obtained from line profile analysis.

In the TEXTOR tokamak, experiments were performed to simultaneously determine the molecular, atomic and total particle flux of deuterium in front of a graphite limiter, the temperature of which can be controlled independently of the plasma conditions. With rising limiter temperatures, T TL , but constant plasma conditions an increase in Balmer emission and a decrease in Fulcherband emission were observed. This variation is associated with a change in the type of released species: molecules dominate at low temperatures (550 K < T TL < 1100 K), whereas at temperatures T TL 1100 K the direct atomic release starts to become important. The total number of deuterons remains constant for all temperatures. Since not all molecules dissociate into two potentially radiating atoms, it is necessary to take into account the ratio of atoms to molecules when deducing the total particle flux from the Balmer emission. We present a spectroscopic method which allows the determination of the atomic, molecular and total deuterium particle flux and which also gives effective conversion factors, (S/XB) eff , to deduce the total deuterium flux from Balmer-α emission alone. Analysis of the spectroscopic data of both species can be performed to determine the rotational and vibrational populations for the molecules by means of Fulcher-α spectroscopy, and the penetration depth and energy for the atoms using Balmer spectroscopy. This further analysis gives additional information about the release mechanism, showing that both species, atoms and molecules, are released predominantly as thermalized particles.

The chemical erosion of carbon in interaction with a hydrogen plasma has been studied in detail in ion beam experiments, and erosion yield values are available as a function of ion energy and surface temperature. However, the conditions in the ITER divertor cannot be simulated by ion beam experiments, especially as far as ion flux is concerned.Therefore, a joint attempt was made through the EU Task Force on plasma-wall interaction and the international tokamak physics activity involving seven different fusion devices and plasma simulators to clarify the flux dependence. For each data point the local plasma conditions were normalized to an impact energy of 30 eV, care was taken to select data for a surface temperature close to the maximum yield or room temperature and the calibration of the diagnostic was performed in situ. Through this procedure the previous large scatter was significantly reduced, revealing a clear trend for a decreasing yield with increasing ion flux, . After the attribution of an error to each data point a fit using Bayesian probability analysis was performed, yielding a decrease in the erosion yield with −0.54 at high ion fluxes.

No abstract

In a tokamak plasma the maximum achievable density is limited. A too high density will result in a violent end of a discharge. Two types of density limit disruption can be distinguished: (a) impure and moderately heated discharges, if the radiative power exceeds the input power, (b) clean, auxiliary heated discharges, where the Greenwald limit is encountered. It has been found that in TEXTOR-94 these two density limits differ by the radiative instability in the plasma boundary, which preceeds the disruption. A symmetric radiative mantle and a detachment are observed prior to the first type, while the Greenwald limit has a MARFE precursor. Control of the impurity content, edge and recycling properties prevents the growth of the MARFE and makes it possible to exceed the Greenwald limit in TEXTOR-94 by more than a factor of 2. High densities have been obtained by means of normal gas feed. Maximum central densities of ne(0) = 1.3 × 1020 m-3 have been obtained. The maximum achievable density scales with the input power and plasma current. Non-disruptive discharges, with a stationary (t > 25 τE) density a factor of 1.93 above the Greenwald limit have been produced in L mode. The radiative losses and impurity concentration have been maintained at a relatively low level during the entire high density phase.