High voltage modulator family for diagnostic neutral beam injectors

For the purpose of a beam emission spectroscopy diagnostic in plasma fusion devices the Diagnostic Neutral Beam Injectors (DNBI) are used. The typical beam parameters of these DNBIs are as follows: neutral beam energy ranges from 20 kV up to 55 kV, equivalent neutral beam current is 1-3 A, beam on time varies from 50 msec up to several seconds. The pulse repetition rate is typically about one pulse per 5-20 minutes. For the above mentioned parameters, one can easily calculate that the energy carried out by this beam has a scale of 20 to 200 kJ. The high voltage modulator (HVM) family was developed in the Budker Institute of Nuclear Physics, Novosibirsk, to supply the DNBIs dedicated for this task. One type of them is intended to provide 2 sec. shots with an HVM output current up to 3 A, another one for 50 msec. shots with a current up to 10 A. Two modes of DNBI operation are used: a quasi-continuous beam and 100% amplitude modulated beam with programmable durations of time intervals of the ON and OFF states. In addition, these HVMs have relatively small energy stored by its output reactive elements and are equipped with a fast interlock system to avoid critical damages of the DNBI grids during transient processes and when an electrical breakdown between electrodes occurs. The paper describes the circuitry, design, interlocks and solutions of HVM buffering relative to the mains. In the basic design, 10 kHz IGBT inverters with PWM and matching transformers are used to power the HVMs. One of the HVMs is installed at the TCV Tokamak (EPFL, Lausanne), another one is installed at the Alcator C-mod Tokamak, (MIT, Boston).