Ferrite-Free Stacked Blumlein Pulse Generator for Compact Induction Linacs

Stacked Blumlein pulse generators comprised of parallel-plate transmission lines are potentially a useful pulsed-power architecture for high-gradient, compact, electron-beam induction accelerators. However, like induction accelerators driven by other pulsed-power architectures, it is generally a system requirement that the multi-stage accelerator structure be enclosed in a grounded metal enclosure so that the full beam voltage is not developed on the exterior of the machine. In the past, this has been accomplished by using magnetic cores to prevent the external metal case from shorting the accelerating field. However, magnetic cores are heavy, bulky, expensive, lossy, nonlinear, and therefore generally undesirable. Various core-free pulse architectures have been reported in the past. One class uses pairs of lines with widely different dielectric constants while another class uses combinations of open-circuit lines combined with short-circuit lines. These designs are encased in metal and support stackable output pulses without the need for magnetic isolation cores. These configurations are also known as bi-polar or zero-integral configurations because they produce a positive and negative voltage pulse with a net time integral of zero. Some of these designs are inefficient leaving substantial stored energy in the lines while others have never been realized as practical accelerating structures. We present here a particular, realizable, magnetic-core-free induction linac geometry that is based on a parallel-plate, stacked Blumlein-like structure, with a symmetric bi-polar, zero-integral output pulse, and an outer metal enclosure. Our design is, in theory, 100% efficient into a matched load. We have evaluated the electromagnetic operation of this geometry by computer modeling. We present the results of this modeling.