This thesis serves as a preliminary design study for the combination of a flight-proven ion optics system (SWIM) with a conversion surface to create a small energetic neutral atom (ENA) sensor. It is planned to use this sensor as ENA sensor for the DFP-SCIENA instrument on Comet Interceptor. Due to the nature of the Comet Interceptor mission (ESA F-class mission with a maximum launch mass of 1000 kg) the development time for a new sensor that meets the size and weight restrictions is limited. The proposed combination of SWIM with a conversion surface is based on a proven ion optics design and should result in a compact sensor design. The main goal of this thesis was to simulate different conversion surface designs and evaluate their compatibility with the SWIM instrument. During this process the different designs were optimised based on the intermediate simulation results. The simulation process was performed by using SIMION to calculate particle trajectories.  In the end, two different conversion surface designs yielded promising results. With both designs detailed simulations and data analysis were conducted to determine the different properties of the two designs. One of these designs was chosen to be further investigated for use on the Comet Interceptor mission.