Refinement of Transition-Edge Sensor Dimensions for the X-Ray Integral Field Unit on ATHENA

At NASA Goddard Space Flight Center, we have previously demonstrated a kilo-pixel array of transition-edge sensor (TES) microcalorimeters capable of meeting the energy resolution requirements of the future X-ray Integral Field Unit (X-IFU) instrument that is being developed for the Advanced Telescope for High ENergy Astrophysics (ATHENA) observatory satellite. The TES design in this array was a square device with side length of50 \, m. Here, we describe studies of TES designs with small variations of the dimensions, exploring lengths, parallel to the current direction, ranging from75 \, mto50 \, mand widths, perpendicular to the current direction, ranging from50 \, mto15 \, m. We describe how these changes impact transition properties, thermal conductance and magnetic field sensitivity. In particular, we show that using a TES with a length of50 \, mand width of30 \, mmay be a promising route to reduce the maximum time-derivative of the TES current in an X-ray pulse and reduce the sensitivity of the TES to magnetic field.