Numerical simulation and experimental verification of the IMS modular divertors

One of the advantages of the stellarator for plasma magnetic confinement lies in its naturally occurring divertor structure. Modularization of the divertor structure is attractive in a fusion environment in terms of device maintainability and ease of impurity control and refueling. Experiments performed on the Interchangeable Module Stellarator (IMS) confirm the existence of ''modular'' divertors predicted by a magnetic field line following code. Two-dimensional ion and electron saturation current profiles indicate a particle flux bundle width which is about a factor of two larger than the width of the predicted field line bundles. Electron temperature and density in the divertor region are about 6 eV and 5 x 1010 cm-3 as compared to central plasma electron parameters of approx. = 15 eV and 3 x 1011 cm-3. The field lines which focus into each of the modular divertor footprints have been found to originate from localized regions of the launch surface outside the separatrix. It has also been found that the modulation of the magnetic field lines in IMS outside the separatrix going into the divertors has similar ripple structure as that just inside the confinement region. Experiments have also been performed while biasing particle collection plates in the divertor regions. Density-decay time increases, and ion saturation current increases have been observed in the central plasma as a result of the bias to these particle collection plates. Electric field measurements in the central plasma, using an emissive probe, are currently being conducted to gain insight into the mechanisms responsible for the changes observed when biases are applied to the divertor shields