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A low-energy multipole line cusp ion source with two-grid extraction system has been developed and characterized for studying the role of the charge exchange processes on ion beam extraction, transport, and neutralization. The ion source is operated for extraction of Ar+ ion beam of current 50–240 mA at energies 850–1650 eV. The characterization of the ion beam is carried out at various axial positions along a 200–815-mm beam path with a background pressure of 4– $20\times10$ −5 Torr. An 11-channel Faraday cup array is used to measure the radial profiles of ion current density and angular beam divergence at these positions. The beam divergences are found to be in the range of 6°–15° for beam perveance of 0.50– $3.75\times10$ −9 AV−3/2. The attenuation of ion beam current along the beam path is observed due to the charge exchange neutralization of the energetic ions in the presence of high background pressure. Since ion current density along the beam path can be reduced by both the charge exchange processes and beam divergence, the accounting of beam particles, i.e., energetic ions and neutrals, at larger distances is difficult by the measurements of ion current density using the Faraday cup alone. The flux of energetic particles (energetic ions and fast neutrals) is therefore obtained by a thrust measurements on a beam target fixed at a distance of 800 mm from the ion source. The neutralization of the ion beam obtained by the emission of electrons from a hot filament neutralizer is also investigated in the presence of the charge exchange processes. Space-charge neutralization is provided initially by the secondary electrons emitted from the vessel walls or by the charge exchange processes even without the neutralizer operating at the desired temperature. However, the current neutralization is achieved only by the electrons emitted from the hot filament neutralizer operating in a space-charge-limited mode.

Research in ITER grade magnetic confinement fusion uses high power radio frequency sources and accelerated neutral particles to heat the plasma to fusion temperature. These devices require high voltage DC power supplies in the range of several tens of kilovolts at a power level of few megawatts. Pulse step modulation technique is widely used to generate high voltage by cascading bulk numbers of low voltage switched power modules, offers a minimum ripple with microsecond order transient response and low let-through energy in case of load fault. However, the power quality and reliability of these systems still need enhancement for compatibility with an industrial environment. In a conventional diode rectifier-based switched power module, current drawn from the multisecondary transformer is highly distorted affecting transformer performance, leading to deploy higher capacity with enhanced thermal management. A novel concept of pulsewidth modulated (PWM) rectifiers in pulse step modulated power supplies is attempted for the first time with comparative analysis over conventional diode rectifiers, quoting an example of dual output high voltage power supply. Articles also discuss the development of a laboratory-scale prototype of the switched power supply module utilizing a PWM rectifier.

Regulated high-voltage power supplies (RHVPSs) are important power sources in high-power heating and current drive systems for a tokamak. Along with high power at output, these power supplies meet the requirements of fast dynamics (on the order of few microseconds), low ripple factor, and good output regulation. A development overview is presented for RHVPS ranging from 14 to 80 kV at several-megawatt power with typical output results. Having extensive tests performed with an 80-kV 75-A RHVPS, it is being regularly used for a neutral beam injector (NBI) at the Institute for Plasma Research (IPR). A large number of identical and series-connected switched power modules (SPMs) are fed from multisecondary transformers. Pulse step modulation control applied to an insulated-gate bipolar transistor switch within SPMs regulates output. Different rise and fall times of voltage can be set from few microsecond to millisecond range. Fast detection of instantaneous overcurrent reduces turnoff time within 2 μs. Due to higher frequency of ripple at higher voltages, output filter component value is of smaller order, which, in turn, limits fault energy below 10 J. The development of RHVPS at IPR started by making a functional prototype at lower rating (14 kV; 35 A) to have an experience in testing design concepts. This was divided in various activities like selection of topology, selection and design of different subparts, e.g., control, switching power converters with protections, and multisecondary transformers. Having successful results and testing all functional parameters on prototype, a series of RHVPSs at higher ratings (up to 80 kV), are developed and installed for NBI and radio-frequency heating system. The design details for each subsystem along with results are discussed.

Pulse step modulation (PSM)-based high-voltage power supplies have played a significant role in fusion research. In many cases, multisecondary transformers are used for generation of tens of kilovolt range outputs with fast (microsecond-order) transient response. In this scheme, large numbers of isolated voltage sources are connected in series to generate the output. Isolated voltage sources can be achieved by a large number of separate transformers or by a single unit of multisecondary transformer. Naturally, a transformer having numbers of secondary windings (40) on a single core is the preferred solution for PSM topology due to its space and cost consideration. For design and simulation analysis of such a power supply, the model of a multisecondary transformer poses special problem to any circuit analysis software. While many simulation software packages provide transformer models with limited number (3-6) of secondary windings, they still fail to predict the actual observed results from a manufactured unit. This paper discusses the difficulties in modeling, step by step with representative schemes of multisecondary transformer, and advantages/disadvantages of each scheme with simulation results. Finally, a model using flux-coupled cores as basic building blocks is proposed; this model is able to simulate actual transformers very close to its observed parameters in test and actual usage.