Estimation of inter-conductor stray capacitance for HVDC transmission line of negative neutral beam injector

Neutral beam injectors inject multi megawatt neutral beams, several tens of amperes and energies from few 100 kV to MV, into the tokamak for heating and diagnostic purposes. The neutral beams are produced through the route of neutralization of ion beams. The ion beams of machines like ITER shall use large area RF based negative ion sources, for plasma production, coupled to multi-grid (3-7), extractor and accelerator systems. Depending on the energy requirements and the beam optics the gaps between the extractor and accelerator stages can range between a few mm to few tens of mm. The multi-aperture multi-grid extractor accelerator systems also provide the route for the gas being fed in the ion source for the plasma production to escape to the surroundings. As a result, the gas density in the gaps is high and can lead to breakdowns often referred to as Paschen breakdowns. A major source of stored energy could be the inter conductor stray capacitance of the high voltage transmission line. These breakdowns could lead to damage of the grid segments and thereby considerable down time of the injector. One of the possible routes to reduce the stored energy could be to reduce the inter conductor stray capacitance by increasing the distance between the conductor and the outer ground cover. This will result in a transmission line with a complex geometry and direct estimation of inter conductor stray capacitance of such complex geometry is difficult. Hence a technique is proposed to estimate the inter conductor stray capacitance of a complex geometry transmission line. A study has been carried out to estimate the inter conductor stray capacitance for various configurations of the transmission line using the method of stored energy in the COMSOL platform. The estimates for one such configuration have been validated experimentally from measured values of capacitance for a 1 m long prototype element. The results of these studies and the experimental observations shall be presented and discussed.