fixtures and methods for clamping workpieces in a workplace to enable the optimized exposure thereof to a stream or flow of a supercritical fluid . provided is a rotatably indexable chuck or locator mounting the workpiece and enabling orientating the latter in specific static pitch position within a high pressure vessel in order to subject the workpiece to a full frontal exposure to the supercritical fluid stream within the vessel . this mounting arrangement facilitates an optimum positioning of the workpiece being processed in the flow path of the supercritical fluid stream while oriented in selectively indexed rotational positions .