Summary form only given. The linear transformer driver (LTD) is a promising technology for building a compact, high-voltage driver for radiographic applications. The 21 cavity Ursa Minor LTD at Sandia National Laboratories is designed to deliver an output voltage of 2.5 MV. This system provides the first opportunity to evaluate the effects of substantial electron flow in a many-cavity driver at the low end of the voltage range needed for radiography. Each cavity has ten bricks (two capacitors and a single switch). Improving the accuracy and reliability of trigger timing for the 210 switches is a major focus of the research effort on this system.Two PIC simulation models of this driver are being developed with the Quicksilver code at Sandia. A 2-D, azimuthally symmetric, r-z model can efficiently simulate the entire system, including the diode. Each of the 21 cavities is driven with its own external RLC circuit that can be independently triggered. The most recent work with this model has been addressing issues in the diode. In addition, a new, full-360° 3-D model is being used to address the effects of azimuthal asymmetry in the MITL. Currently, the first 14 cavities are modeled with an equivalent single coaxial feed, and each of the last seven cavities is driven by ten independently triggered RLC circuits, uniformly distributed in azimuth. Electrons are emitted from the central cathode with a conventional space-charge-limited emission model.