AbstractA heavy ion beam probe (HIBP) has been used as a reliable method to measure the plasma potential and its fluctuation in a magnetically confined fusion plasma. On the large helical device (LHD) an Au+ beam produced by a 3 MV tandem acceleration system will be used. It is necessary to optimize the charge fractions and minimize the energy broadening of the Au+ beam. Charge fractions can be calculated from electron loss and electron capture cross-sections. It is predicted that the optimum gas thickness for Au+ beam production is about 4 × 1014 cm−2. The broadening due to electron stripping is calculated to be several electronvolts and does not depend on the target mass. The energy straggling due to multiple collisions at the optimum target thickness can be calculated theoretically taking the lower limit of the impact parameter into account. Consequently, the total energy width of the Au+ produced by a tandem system might be about 100 eV.