Transverse impedance measurements of the DARHT-2 accelerator cell

A new technique was developed to measure the transverse interaction impedance of the DARHT-2 induction cells, designed to accelerate a 2-4 kA, 2 μsec electron beam pulse. An extensive campaign to minimize the transverse resistive impedance of these massive metglas-filled structures lead to a design with thin ferrite tiles in the form of a ring placed along one wall of the radial line connecting the pulseline feeds to the accelerator gap. This ferrite ring heavily damped all transverse modes. To measure the broadband m=1 impedance, one cell is excited by a matched twinlead on the axis of the beamtube. The ratio of the radial RF magnetic field in the accelerator gap to the azimuthal RF magnetic field at the beamtube wall (far away from the gap) determines the complex impedance, as explained in the paper. This technique is much simpler than the TSD approach for these very large bore systems. It also covers the full frequency band including the reactive impedance at low frequency related to the "image displacement instability". Results with the 10 inch diameter bore "standard cell" indicate a peak resistive impedance of 280 ohms per meter around 575 MHz (Q=6)