Pulsed Current Static Electrical Contact Experiment

Railguns involve both static and sliding electrical contacts, which must transmit the large transient electrical currents necessary to impart high forces onto a projectile for acceleration to hypervelocity. Static electrical contacts between metals initially take place through small asperities, or "a-spots", distributed over the contact area. The voltage developed across the interface is directly related to the contact temperature and force, the number of a-spots, the thermophysical and mechanical properties of the contacting materials, the current history, and any interfacial materials that may be present. To physically simulate some of the conditions attained within a railgun, a pulsed current static electrical contact experimental facility has been developed at the Naval Research Laboratory. This facility employs a 500-kN capacity servohydraulic load frame equipped with an electrically insulated load train to establish a contact force on interfaces between metals through which a pulsed current is transmitted. The time dependent evolutions of the voltage drops across the interfaces, as detected by probes pushed into the contacting materials, are recorded during a 40-kA peak current pulse having a 300-mus rise time with peak current densities on the order of 50 kA/cm². The interface stack is assembled from a 12-mm outside diameter annular disk of metal with a 6.3-mm hole which is compressed between two hollow pedestals of a second metal. The evolution of the voltage drop across an interface during a pulse will be described as a function of initial contact pressures, current density, and polarity for dissimilar (Al/Cu) metal contacts. Thermal effects on the surfaces, including localized melting of the interface materials, were also investigated