RF impedance measurements on the DARHT-II accelerator intercell assembly

We report upon recent experimental measurements made of RF properties of the intercell assembly of the second axis accelerator of Dual Axis Radiographic Hydrodynamic Test (DARHT) facility at LANL The intercells provide both pumping and diagnostic access to the main DARHT-II beamline. Their design includes a pumping plenum separated from the main beam pipe by return current rods together with RF shielding provided by a copper-coated stainless steel mesh. Measurements using the twin lead technique at low frequencies (f les 200 MHz) suggest a constant value for the ratio eta of the radial and azimuthal magnetic field components to which the transverse impedance is linearly related. We find that these results compare favorably to predictions from a simple analytic, lumped circuit model which includes the effects of the mesh and return current rods. We also present RF loop-to-loop frequency scans above beam pipe cutoff (~600 MHz) showing the existence of many RF modes with relatively high Q's