Wide-Bandwidth Test Fixture for Electromagnetic Beam Sensors

The Fusion Materials Irradiation Test (FMIT) accelerator will provide an intense deuteron beam at 35 MeV (ß = 0.19). The beam, following a stripping reaction on a lithium target, will supply the neutron flux required for studying materials that may be used in a fusion environment. The diagnostic measurement instrumentation, which will characterize the accelerator beam, must be noninterceptive because of the beam's power density. Instrumentation also must be fully functional for start up of the FMIT accelerator. To this end, a test facility was needed to examine signals from diagnostic probes that sense electromagnetic fields emanating from the charged particle beam. The test facility also will help evaluate the probes' corresponding measurement systems before final assembly into the beamline. Three types of test facility were proposed: (1) a low-energy electron accelerator, (2) a large "electron-gun assembly," and (3) a coaxial structure that produces electromagnetic fields similar to that of the proposed FMIT accelerator. The third type was chosen because the design and fabrication could be done at Los Alamos and Types (1) and (2) would be more costly. The coaxial structure can carry the impulse currents required but cannot model the FMIT accelerator charged-particle bunch velocities accurately. However, one may introduce a highly resistive dielectric material into the coaxial test fixture so that its electromagnetic-wave velocities match those of the accelerator's particle-bunch velocities. This paper describes the design and some experimental results of the coaxial test fixture.