Development of a 1 MV ultra-fast LTD generator

A 1 MV linear transformer driver (LTD) generator has been manufactured by ITHPP and SEIV, and delivered to CEA/DIF/Polygone d´Expérimentation de Moronvilliers, in order to demonstrate the capability of this technology for flash X-ray radiography applications. This generator is composed of 10 cavities; each cavity includes 16 bricks representing two capacitors (GA 35436, 100kV, 8nF) and one multi-gaps air pressurized switch. To reduce the resistive losses in the switches, the inter-electrode gaps have been reduced from 6 mm in initial design to 4 mm, and the dry air pressure has been slightly increased. Each cavity has been tested individually on an automated testbed using a low inductive resistive load (~1.2nH) and extensive diagnostics, including a set of sixteen capacitive probes in front of each switch to control their timing sequence. The cavities deliver more than 100kV at ±100kV charging voltage into a matched load of 0.55-0.6Ω in a ~70-ns FWHM pulse. Around 300 shots have been made on each cavity (to condition the switches) before their integration into the generator. The 10 cavities are stacked in series on a stiff metal frame and can move on rails with precise and reproducible mechanical alignment. Compression of the cavities is provided by the design of this frame. The center cathode of the output MITL is a 3.5 m long straight conductor that is cantilevered and hold on a support, moving on rails, to allow easy and fast insertion and removal. All the subsidiaries are also integrated on a dedicated lateral support, so the whole generator is movable on these main rails. A high voltage trigger generator with 20 outputs (2 trigger cables per cavity) has been used to trigger the cavities synchronously with a very low jitter. The premagnetizing pulse is applied globally to all the cavities cores through the MITL cathode by using a movable contact under vacuum that is pneumatically controlled. The whole system is computer controlled and - llows easy, reliable and safe remote operation. The generator has been successfully tested with a large area ~6 Ω e-beam diode, in more than one thousand shots at full charge voltage. It proves to be very reproducible (a few percent) and has a very low jitter <;2ns, 1σ over 20 consecutive shots).