The development of H− ion sources with performances exceeding those achieved today is a key requirement for the next generation of high power proton accelerators. The ISIS Penning surface plasma source, which routinely produces 35 mA of H− ions during a 200 μs pulse at 50 Hz for uninterrupted periods of up to 50 days, is regarded as one of the leading operational sources in the world, and should provide an excellent starting point for a development program. One goal is to produce pulse widths of 1.2 ms at 50 Hz and 2.5 ms at 50/3 Hz, thereby increasing the duty factor from 1% to as much as 10%. Increasing pulse widths will necessitate an improved cooling system to offset increased heating. The most effective cooling strategy will be determined by thermal finite element analysis of the ISIS source. The modeling will then be extended to find an optimal means of offsetting increased heat loading, and will minimize the amount of engineering required to produce an effective solution. Modeling of the ISIS sourc...