M. Keilhacker

Measurements on the particle balance in diverted ASDEX discharges

Abstract In the ASDEX divertor we have measured the flux of neutral particles to the wall, the plasma flux to the neutralizer plates and the electron temperature and density. After a description of our apparatus we give some results for ohmically heated discharges and discuss them in connection with the particle and power balance and the dominant fueling mechanism in diverted ungettered discharges. We estimate from our results ionization rate and power fluxes in the divertor. The ionization rate is in reasonable agreement with the plasma flux to the neutralizer plates and the power fluxes transferred by neutrals and photons to the wall are not in disagreement with bolometer measurements.

Divertor efficiency in ASDEX

Abstract The divertor efficiency in ASDEX is discussed for ohmically heated plasmas. The parameters of the boundary layer both in the torus midplane and the divertor chamber have been measured. The results are reasonably well understood in terms of parallel and perpendicular transport. A high pressure of neutral hydrogen builds up in the divertor chamber and Franck-Condon particles recycle back through the divertor throat. Due to dissociation processes the boundary plasma is effectively cooled before it reaches the neutralizer plates. The shielding property of the boundary layer against impurity influx is comparable to that of a limiter plasma. The transport of iron is numerically simulated for an iron influx produced by sputtering of charge exchange neutrals at the wall. The results are consistent with the measured iron concentration. First results from a comparison of the poloidal divertor with toroidally closed limiters (stainless steel, carbon) are given. Diverted discharges are considerably cleaner and easier to create.

Test of a toroidal large area limiter in the ASDEX tokamak

Abstract A toroidal large area limiter was installed in the divertor tokamak ASDEX. The limiter basically consists of a plane annular stainless steel sheet which is tangential to the plasma and which is placed in the bottom of the main plasma chamber. In a second series of experiments this limiter was covered with graphite plates. With the steel limiter clean discharges could not be obtained. Energy deposition on the limiter was only 12% of the input. With the graphite limiter Z eff was below 3, energy deposition on the limiter was 25% (up to 40% in low density operation). Energy deposition was toroidally symmetrical. Results, including some experiments with a single null divertor are presented.

Comparison of different refuelling methods with respect to plasma contamination

Abstract A large contribution to plasma contamination is due to sputtering on the vessel wall by charge-exchange neutrals. In a divertor tokamak, the flux and energy of cx neutrals hitting the wall, and hence their contribution to plasma contamination, depend on how the plasma loss into the divertor is replaced. One of the most important features of the different refuelling methods in this respect is the radial deposition profile of the refuelled hydrogen. It affects — via the plasma temperature profile — the energy of the cx neutrals and thereby their sputtering coefficient and — via the radial dependence of the particle confinement time — the refuelling flux necessary for maintaining a given density. This paper compares cold gas inlet, pellet injection and cluster injection in this respect. Cold gas inlet results in the lowest plasma contamination per hydrogen ion created, the reason being the small penetration depth, which results in low energy cx neutrals. But, as the confinement time for ions created in the inner plasma region is expected to be larger than that for ions created in the outer plasma layer, smaller particle fluxes are needed for the other refuelling methods, which deposit the fuel deeper in the plasma. This beneficial effect is calculated for different refuelling fluxes.

Conditioning and Optimization of Discharges in ASDEX

Abstract In the first experimental phase the divertor tokamak ASDEX was run with a closed SS-limiter. The cleaning procedure for the vessel consisted of baking to 120°C, carbon removal by glow discharge in hydrogen, and oxygen removal with a continuous low power 50 Hz AC discharge. Wall contact of the plasma was reduced by carefully positioning the plasma with a feedback system. Discharges with plasma currents up to 280 kA and a disruption-free duration of up to 1 s were reliably produced with a filling pressure of 5 × 10 −5 mbar and a programmed current rise. No change in start-up conditions and discharge behaviour was observed in material limiter discharges with superimposed divertor field.

Aspects of Plasma-Wall Interaction in the ASDEX Divertor Experiment

Abstract ASDEX (Axially SymmetriC Divertor Experiment) is a large tokamak now under construction at IPP Garching. The main parameters are: major radius 1.65 m, plasma radius 0.4 m, toroidal field on axis 2.8 T and plasma current 500 kA. The experiment is scheduled to go into operation in about two years. The first aim of the experiment is to test the divertor action, i.e. plasma stability without material limiter and reduction of impurity influx. Since the divertor will essentially be of the unload-type, the latter problem should be solved by reducing wall bombardment. On the premises of sufficient stability the plasma-wall interaction will occur on the divertor slits, in the main divertor chamber and (with refuelling) on the wall owing to charge exchange neutrals. Plans for surface studies are being discussed.

Divertor Pumping System of ASDEX

For the divertor pumps in ASDEX speeds for H 2 of several 10 6 1/sec are required. Initially two alternativ concepts were persued: Volume getter pumps based on ST101 and Ti sublimation pumps with cryogenic surfaces. Though both concepts turned out to be feasible, preference was finally given to Ti sublimation pumping, because its efficiency is less dependent on the plasma conditions in the divertor. For this reason the following paper renders a more detailed description for the Ti sublimation pumps only.

Design and Manufacture of the ASDEX Toroidal Field Coils

Each coil consists of a vacuum impregnated winding embedded in a casing. The coils are supported against the centripetal forces by a central column. Forces due to the poloidal field are taken up by the casing. The D-shape minimizes bending caused by the self-forces of the coil system. These forces are taken up by the copper itself. The finite element program STRUDL was used to calculate stresses in winding and casing. It was checked that the design is insensitive to the variations of the elastic properties of the windings.