The DARHT II system is a multi-pulse, single-axis flash radiography system being built at Los Alamos National Laboratory. This system is designed to be capable of taking closely spaced radiographic images so as to produce time-sequenced images of the test object. The DAMT II accelerator is nominally a 20 MeV, 2 kA , 2 (micro)s single pulse accelerator. Four pulses at the target over a 2 (micro)s window are derived from this single pulse with a fast kicker system. The electron beam interacting with the converter target (typically tantalum) generates an intense x-ray cone that produces a radiographic image on a fast detector array. As the electron beam interacts with the target surface, a plasma promptly develops. The beam eleclrons create a strong space charge field in front of the target from which ions can be extracted and accelerated in a direction opposite to the electron beam propagation. These ions partially neutraIize the beam space charge and defocusing of the beam results. The second effect results from the direct interaction of the electron beam with the target plasma on subsequent pulses. Such an interaction, depending on the interaction length and plasma density, may have an adverse effect on the beam propagation and the resultant spot on the converter target. Our on-going experimental program is to study the interaction of the electron beam with the x-ray converter target. In these experiments, we focus on the dynamics of the spot behavior measuring xray spot blur across an edge (so called 'roll-bar' technique), and 2d imaging with a gated, muhiframe, pinhole camera. Further, we are characterizing the properties of the plume by using various plasma diagnostic techniques.
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