The memory polynomial based digital predistorter utilizes a cluster-based particle swarm optimization technique and has an embedded model-size estimator. Each channel of the first set enables radiation from a radiating source to propagate therethrough and has an inner wall for catching debris particles. The arm can pivot as required relative to the item, to accommodate differences in shape in the item, while providing a substantially equal clamping force by both engagement members. As a result, there will be no east-west geometry mismatch between the top and bottom of the raster display.