CHARGE FRACTIONS AND ENERGY WIDTH OF HEAVY-ION BEAM PROBE FOR LARGE HELICAL DEVICE

A heavy ion beam probe (HIBP) has been used as a reliable method to me asure the plasma potential and its fluctuation in a magnetically confi ned fusion plasma. On the large helical device (LHD) an Au+ beam produ ced by a 3 MV tandem acceleration system will be used. It is necessary to optimize the charge fractions and minimize the energy broadening o f 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 x 10(14) cm(-2). The broadening due to electron stripping is calculated to be several elect ronvolts and does not depend on the target mass. The energy straggling due to multiple collisions at the optimum target thickness can be cal culated theoretically taking the lower limit of the impact parameter i nto account. Consequently, the total energy width of the Au+ beam prod uced by a tandem system might be about 100 eV. (C) 1997 Elsevier Scien ce S.A.