A novel plasma diagnostic technique for simultaneous measurements of internal magnetic fields, ion temperature, and plasma potential is described. This technique consists of the injection of a 120-keV, 5-mA (current equivalent) neutral xenon beam into a magnetized plasma and the detection of radiation and xenon ions, which result from interaction of xenon atoms with hydrogen ions. This system is shown to be particularly suitable for devices like the Princeton Large Torus and other mainline machines.