Speaker
Description
The voltage breakdown in the accelerator of Neutral Beam Injectors for Plasma Heating and Current Drive (H&CD) has a very frequent occurrence, due to the harsh environment of the accelerator. The beams formed by accelerated defocused ions and electrons escaped from magnetic traps hit the electrodes (e.g. the acceleration grids and the vessel) melting the surfaces with massive production of gas, metal debris and intense Xray radiation: in such an environment, the breakdown occurrences increase dramatically. Moreover, due to the fact the p.d (pressure x distance) product inside the vessel locally can be very close to left branch of the Paschen curve, Townsend-like discharges have a great probability to occur.
For this reasons, the voltage breakdown in the NBI systems is regarded as an operating condition more than a fault one.
In MITICA - the prototype of the ITER NBI under construction in Padova - this situation is expected to be more and more severe, being its parameters magnitude larger than any other existing NBI device. Basically, the voltage breakdowns in MITICA (up to 1 MV in the case of the direct breakdown of the ion source to ground) produce huge and very fast transient currents. The detrimental effects can be grouped into three categories: i) permanent damages to the grids surface, ii) electrical stresses to the alumina insulators in vacuum, to the Power Supply components (Accelerator and Ion Source Power Supplies), to the Ion Source diagnostic and iii) safety/EMI effects due to current induced in the surrounding ground.
This presentation describes the solutions adopted to mitigate such effects, as well as the transient computer model of the overall system, which will be essential during the next commissioning phase and during the operation. Particular emphasis will be given to the presence of a huge current limiting device (40 tons magnetic core snubber - the key component to reduce the grid damage during breakdown) and to the grounding techniques adopted to keep under control the fast current flow.