Status of CERN Chopper Driver and the Solid State Alternative

The Superconducting H- Linac under study at CERN requires a high performance 3 MeV chopper. It has to establish the required beam time-structure by dumping part of the beam exiting the RFQ in 0.6 ms bursts of 1 ns bunches spaced by 2.84 ns and repeated at 50 Hz. For maximum flexibility, the system must be able to remove any number of consecutive bunches with a minimum of 3 and up to a few thousand. The amplifier driving the chopper structure has thus to provide 500 V on 50 omega, with rise and fall times below 2 ns and repetition rate as high as 45 MHz. To achieve the very wide frequency response required for this application a first proposal based on the idea of generating the low and high frequency part of the required spectrum with distinct amplifiers has been followed. In-depth studies and prototyping have proven the principle but also shown its fragilities. Distortions, saturation effects in the ferrite or in the driving circuits, vacuum tube ageing, etc. are difficult to keep within the limits required for proper and long term operation. To overcome these problems and relax the frequency response requirements, an alternative scheme based on solid state devices has been devised. The principle, the prototype results and following improvements are described in this paper.